31

Renin-angiotensin-aldosterone system (RAS)

31

Renin-angiotensin-aldosterone system (RAS)

31

Renin-angiotensin-aldosterone system (RAS)

31

Renin-angiotensin-aldosterone system (RAS)

  31 Renin-angiotensin-aldosterone system (RAS)

Lothar Thomas

31.1 Hypertension

The European Society of Cardiology and the European Society of Hypertension retain the threshold value of ≥ 140/90 mmHg for the definition of high blood pressure. At least three measurements should be made on each of several days, with 1–2 minutes between measurements and with a 3–5 minute pause before blood pressure is measured with the patient sitting /1/. Elevated blood pressure may affect as many as 1 billion individuals worldwide. In Germany, approximately 13% of women and 18% of men have uncontrolled high blood pressure /2/. The World Health Organization estimates that 54% of strokes and 47% of cases of ischemic heart disease are the direct consequences of high blood pressure /3/. Suboptimal blood pressure control is considered the number one attributable for death worldwide, and early treatment of blood pressure may reduce the incidence, as well as the long term consequences of hypertension /4/.

Most patients with elevated blood pressure have essential hypertension and there are no tests to investigate the processes involved. Secondary types of hypertension such hyperaldosteronism, low renin hypertension or pheochromocytoma induced increased levels of catecholamines must be considered in the differential diagnosis.

The ESH/ESC guidelines advocate blood pressure treatment of ≥ 140/90 mmH in the age 65-79 years, the ACC/AHA advocates ≥ 130/80 mmHg. The 1999–2000 National Health and Nutrition Examination Survey (NHANES) found that only 31% of hypertensive patients had blood pressure controlled to < 140/90 mmHg /5/. Data from NHANES III suggest that the prevalence of hypertension increases progressively with increasing body mass index (BMI) from about 15% among people with a BMI less than 25 kg/m2 to approximately 40% among those with a BMI of 30 kg/m2 or greater /6/.

Up to 30% of hypertensives have a low or suppressed renin. The phenotype of low renin hypertension may be the manifestation of inherited genetic syndromes, acquired somatic mutations, or environmental exposures. Activation of the mineralocorticoid receptor is a common final mechanism for the development of low renin hypertension /7/.

An early onset of anti-hypertensive treatment provides in addition to clinical benefits (e.g., lower risk of cardiovascular events) psychological benefits to the patient.

References

1. Williams B, Mania G, Spiering W, Agabiti Rosei E, Azizi M, Burnier M, et al. 2018 ESC/ESH guidelines on hypertension.Eur Heart J 2018; 39: 3021–3104.

2. Jordan J, Kurschat C, Reuter H. Arterial hypertension. Dtsch Arztebl Int 2018; 115: 557–68.

3. Lawes CM, Vander Hoorn S, Rodgers A. Global burden of blood pressure related disease 2001. Lancet 2008; 371: 1513–8.

4. Basile JN, Chrysant S. The importance of early antihypertensive efficacy: the role of angiotensin II receptor blocker therapy. J Human Hypertens 2006; 20: 169–75.

5. Hajjar J, Kotchen TA. Trends in the prevalence, awareness, treatment, and control of hypertension in the United States, 1988–2000. JAMA 2003; 290: 199–206.

6. Malik S, Wong ND, Franklin SS, Kamath TV, L’Italien GJ, Pio JR, et al. Impact on the metabolic syndrome on mortality from coronary heart disease, cardiovascular disease, and all causes in the United States adults. Circulation 2004; 110: 1245–50.

7. Baudrand R, Vaidya A. The low-renin hypertension phenotype: genetics and the role of the mineralocorticoid receptor. Int J Mol Sci 2018; 19: 546; https://doi.org/10.3390/ijms19020546.

31.2 Renin-angiotensin-aldosterone system

The renin-angiotensin-aldosterone system (RAS) plays a key role in the salt and water retention regulation, the extracellular fluid volume, and the regulation of blood pressure. Refer to:

31.2.1 Function of renin

Expression of the Renin gene leads to /12/:

  • Synthesis of prorenin in the afferent arteriole of the glomerulus (e.g., juxtaglomerular apparatus) of the kidney
  • Its circulating substrate, synthesized in the liver, is the protein angiotensinogen
  • From angiotensinogen the renin generates the decapeptide angiotensin I
  • Angiotensin I in turn is converted to the octapeptide angiotensin II by angiotensin converting enzyme (ACE)
  • Angiotensin II is the principal effector molecule of the RAS, whose main actions are to stimulate the angiotensin II receptor type 1 (AT1) on arteries and the adrenal cortex to cause vasoconstriction and stimulation of the aldosterone secretion
  • The AT1 also facilitates noradrenalin release from sympathetic nerves.

Angiotensin II mediates the following effects /3/:

  • Vasoconstriction by binding alternatively to G-protein coupled angiotensin II receptors type 1 or angiotensin II receptors type 2. Angiotensin II exerts its classic vasopressive effect through the angiotensin II receptors type 1, and the opposite effect through the angiotensin II receptors type 2.
  • Stimulation of aldosterone production and thus an increase in salt and water retention. The adrenal response to angiotensin II takes only minutes, reflecting the fast conversion of aldosterone precursors to aldosterone.

The rate limiting step in the RAS is renin activity, since all other components of the cascade are normally present in excess amounts.

The synthesis and release of renin by the juxtaglomerular apparatus of the kidney depend on:

  • The perfusion pressure of the afferent arterioles and thus the blood pressure
  • The distal tubular Na+ concentration at the level of the macula densa and thus the sodium chloride supply
  • The sympathetic nervous system and thus its activation
  • The negative feedback regulation by angiotensin II.

Angiotensin II exerts a negative feedback control on renin secretion. Thus, inhibition of the reaction cascade by ACE inhibitors or AT1 antagonists leads to reduced production of angiotensin II and increased production of renin.

Unadjusted activation or over activation of the RAS, be it systemic or localized, leads to increased production of angiotensin II, which can cause salt and volume retention, an increase in blood pressure and, through activation of transforming growth factor-β, inflammatory vascular hypertrophy and organ fibroses. Increased production of angiotensin II can also be due to mutations in the genes for renin (REN), angiotensinogen (AGT), angiotensin- I-converting enzyme (ACE), angiotensin II receptor type 1 (AT1) and aldosterone synthase (CYP11B2).

31.2.2 Function of aldosterone

Aldosterone is produced in the zona glomerulosa of the adrenal cortex and regulates the body’s electrolyte and volume balance via its mineralocorticoid effect /4/. The regulators of adrenal aldosterone biosynthesis are the angiotensin II produced by the RAS, the extracellular concentration of K+, and ACTH. The effect of each agonist is modified by prevailing Na+ and K+ status. Aldosterone biosynthesis is acutely sensitive to small changes in serum K+ concentration. Increased K+ levels, increase aldosterone secretion, thereby restoring the K+ homeostasis. The effects of extracellular K+ concentration and angiotensin II are synergistic. Aldosterone secretion is also influenced by factors, such as atrial natriuretic peptide, serotonin, and adrenomodullin. ACTH stimulates renal blood flow and contributes moderately to the synthesis of aldosterone by interacting directly with G-protein coupled receptors in the zona glomerulosa of the adrenal cortex.

Aldosterone is synthesized from cholesterol via a series of hydroxylations and oxidations. The final steps of this pathway, the conversion of 11-deoxycorticosterone to aldosterone, require conversion via the intermediates 18 hydroxydeoxycorticosterone or corticosterone and 18-hydroxycorticosterone. The enzymes involved in these reactions are mostly members of the cytochrome P450 super family.

Refer to Fig. 31.3-2 – Synthesis of aldosterone.

The effect of aldosterone is mediated by the cytoplasmic mineralocorticoid receptor (MR), particularly in cells of the renal collecting duct. The receptor belongs to the nuclear receptor super family of proteins and consists of an N-terminal domain, a DNA-binding domain and a C-terminal ligand binding domain. The binding of aldosterone to this domain induces a conformational change in the MR. As a result, the MR dissociates from heat shock proteins, dimerizes and trans locates to the nucleus where it binds to the hormone responsive element of genes responsible for aldosterone and activates gene transcription.

In the epithelia, aldosterone regulates the reabsorption of Na+, which also effects the transport of water, K+ and H+ across the cell membrane. An electrochemical gradient permits the passage of Na+ from the lumen into the epithelial cell via the amiloride sensitive epithelial Na+ channel (ENaC). From the cytoplasm, Na+ is actively transported by the Na+-K+-ATPase across the basolateral membrane from the epithelial cell into the bloodstream, while simultaneously excreting K+; water follows the movement of the Na+ /5/.

Refer to Fig. 8.8-2 – Reabsorption of solutes in the collecting duct.

The effect of aldosterone on Na+ reabsorption consists in modulating the activity of ENaC by inducing the α-, β- and γ-subunit expression although a major effect is achieved by increasing the number of channels in the plasma membrane.

31.2.3 Mineralocorticoid excess

The RAS has a major role in the control of extracellular volume in Na+ and K+ homeostasis, and in the regulation of blood pressure. Disorders of the RAS are divided into those with mineralocorticoid excess and those with mineralocorticoid deficiency. The former are generally associated with hyperaldosteronism and the latter with hypoaldosteronism.

Refer to Tab. 31.2-1 – Disorders of the renin-angiotensin-aldosterone system.

Disorders of mineralocorticoid excess are differentiated into:

  • Primary disorders (primary aldosteronism), which are due to an adrenal adenoma or adrenal hyperplasia with autonomous aldosterone production
  • Secondary disorders of the RAS (secondary hyperaldosteronism), which are due to renin producing tumors or systemic diseases such as renal artery stenosis, arteriosclerosis of both renal arteries, or renal and cardiac diseases.

31.2.4 Mineralocorticoid deficiency

Mineralocorticoid deficiency is due to lack of aldosterone synthesis or action. While normally 99.5% of the filtered Na+ is reabsorbed by the kidneys with the aid of mineralocorticoids, only 98.5% is reabsorbed if there is a lack of aldosterone effect. This causes a daily loss of approximately 1 mol of Na+, corresponding to about 28 g of sodium chloride. Chronic Na+ loss leads to reduction of the extracellular volume and hyponatremia. Plasma osmolality decreases, the cell water content increases, and there is hypotonic dehydration in the extracellular space. Since the aldosterone dependent transport of Na+ is coupled with the exchange of K+ , the renal excretion of K+ is reduced, resulting in hyperkalemia. Since both K+ and H+ excretion is insufficient, hyperkalemic acidosis develops. As in other electrolyte shifts, the Mg++ behave analogously to the K+.

Mineralocorticoid deficiency can be primary and is due to reduced aldosterone synthesis in:

A more common form of mineralocorticoid deficiency is secondary deficiency, which is due to reduced target organ responsiveness to aldosterone, such as in pseudo hypoaldosteronism.

Disorders of mineralocorticoid deficiency can be due to:

  • A defect in the hypothalamic-pituitary-adrenal axis
  • Isolated hypoaldosteronism
  • Reduced target organ response to aldosterone, as is the case in pseudo hypoaldosteronism.

31.2.5 Testing for disorders of the RAS

Biomarkers for diagnosing and differentiating disorders of the RAS include Na+, K+, renin, aldosterone, aldosterone/renin ratio (ARR), 18-hydroxycorticosterone, and 18-hydroxy cortisol.

The ARR is the most reliable means for screening of hyperaldosteronism. The main indications are:

  • Hypertension and suspected primary aldosteronism; the ARR is increased
  • Hypertension and suspected secondary aldosteronism; the ARR is normal
  • A low arterial blood volume or chronic renal insufficiency; renin and aldosterone are increased.

A high ARR should be confirmed by functional tests.

The determination of steroid hormones of the mineral corticoid pathway with negligible mineral corticoid activity such as plasma 18-hydroxycorticosterone (18-OHB) and urinary 18-hydroxy cortisol (18-OHF) excretion can be markedly elevated in primary aldosteronism.

31.2.6 Biochemistry and pathophysiology of RAS

The RAS plays an important role in maintaining blood pressure and the balance of water, Na+ and K+ metabolism (Fig. 31.2-2 – The role of the RAS in maintaining blood pressure and Na+ balance in the case of decreased table salt intake). The release of renin from the juxtaglomerular apparatus of the kidney is stimulated by a decrease in blood volume, blood pressure, and renal perfusion. Renin converts angiotensinogen, which is produced in the liver, into angiotensin I. Pulmonary angiotensin converting enzyme (ACE) cleaves angiotensin I to produce angiotensin II, which leads to a rise in blood pressure through vasoconstriction and stimulates the adrenal cortex to produce aldosterone. The rise in aldosterone causes Na+ and water retention with an increase in extracellular fluid volume. Thus, there is no further stimulus for excessive renin secretion and normal renin secretion is resumed; negative feedback mechanism.

Aldosterone secretion may also be stimulated by decreased Na+ or elevated K+ plasma levels as well as by ACTH.

Aldosterone is metabolized rapidly, having a biological half life of only 31 minutes. Its metabolism occurs primarily in the liver, where it is converted to tetrahydroaldosterone (approximately 40%), and secondarily in the kidney, where it is converted to aldosterone 18-glucuronide (approximately 10%). About 0.2–0.5% is excreted into the urine as free aldosterone.

Aldosterone binds to the mineralocorticoid receptor (MR) which belongs to the super family of steroid receptors. In the tissues, the MR is coexpressed with the enzyme 11β-hydoxysteroid dehydrogenase 2 (11β-HSD2), which metabolizes cortisol to cortisone. Cortisol is another potential ligand of the MR and competes with aldosterone for binding to the receptor. Although both have approximately the same receptor affinity, cortisol inhibits the binding of aldosterone, since its tissue concentration is at least 10-fold higher and its plasma concentration 100-fold higher than that of aldosterone. The MR is thus protected from binding by aldosterone. However, when cortisol is metabolized to inactive cortisone by 11β-HSD2, the receptor is free for aldosterone. Inactivating mutations of 11β-HSD2 result in apparent mineralocorticoid excess syndrome.

The precursor of renin, prorenin, circulates in plasma at a concentration 10-fold higher than the renin level. While several tissues are capable of producing prorenin, only the juxtaglomerular cells of the kidneys are capable of the controlled release of prorenin from the storage vesicles. Prorenin and renin have high binding affinity for a receptor known as prorenin receptor. Binding of renin to the receptor increases its activity by a factor of 5. Free prorenin, which normally does not exhibit enzymatic activity, is activated by binding to the receptor, and as a result has the same activity as free renin.

Mutations in the proteins of the RAS are playing an increasingly important role in the search for causes of aldosterone producing adenoma. Approximately one third of patients with adenoma are found to have mutations in the KCNJ5 gene, which encodes the cell-specific K+ channel. Mutations are also found in ATPase genes, which encode the α-subunit of the Na+-K+-ATPase in aldosterone producing adenoma /6/.

An overview of the genetic determinants of the RAS is shown in Tab. 31.2-2 – Genetic determinants of the RAS and diseases influenced by them.

References

1. Sparks MA, Crowley SD, Gurley SB, Mirotsou M, Coffman TM. Clasical renin-angiotensin system in kidney physiology. Compr Physiol 2014; 4: 1201–28.

2. Yang T, Xu C. Physiology and pathophysiology of the intrarenal renin-angiotensin system: an update. J Am Soc Nephrol 2017; 28: 1040–9.

3. Brown MJ. Renin: friend or foe? Heart 2007; 93: 1026–33.

4. Basile JN, Chrysant S. The importance of early antihypertensive efficacy: the role of angiotensin II receptor blocker therapy. J Human Hypertens 2006; 20: 169–75.

5. Fuller PJ. Adrenal diagnostics: an endocrinologist’s perspective focused on hyperaldosteronism. Clin Biochem Rev 3013; 34: 111–5.

6. Hubl W, Thomas L. Renin-Angiotensin-Aldosteron-System. In: Thomas L, ed. Labor und Diagnose, S 1406–24. Frankfurt, TH-Books 2008.

31.3 Renin

Renin and sodium are the two main factors in blood pressure control and renin levels vary conversely with sodium load. Renin maintains blood pressure through vasoconstriction when there is adequate sodium to maintain volume. Blood pressure control requires a combination of natriuresis and blocking the consequential increase in renin activity /1/.

31.3.1 Indication

Patients with hypertension and suspicion of low-renin hypertension.

31.3.2 Method of determination

Renin is synthesized as the inactive zymogen, prorenin. Prorenin contains a pro segment that masks the active site, thereby preventing access by angiotensinogen the renin substrate. Cleavage of the pro segment converts prorenin to renin. The plasma concentration of prorenin is approximately 10-fold higher than for renin. Prorenin exists in two different conformations. More than 98% of prorenin is in a closed conformation in which the pro segment masks the binding site for angio­tensinogen; the molecule in this conformation has no enzymatic activity. Less than 2% of plasma prorenin has an open conformation in which the pro segment no longer masks the active site. The open conformation is accessible to angiotensinogen and is enzymatically active (active prorenin) /2/.

Two different methods are used to determine renin:

  • Renin activity; the activity of renin to generate angiotensin I is determined. The renin activity assay is still considered the gold standard
  • Renin mass; the concentration of renin (with or without including prorenin) in the plasma is determined.

Both methods provide different information. Whereas activity assays measure only active renin, immunoassays measure both active and inhibited renin (e.g. inhibited by renin inhibitors). Different conformations of renin and prorenin are measured using activity assays and immunoassays (Fig. 31.3-1 – Conformations of renin and prorenin).

Plasma renin activity measurement

The assay comprises two steps. First, angiotensin I is generated by renin acting on endogenous angiotensinogen in plasma incubated at 37 °C. Inhibitors of angiotensinase and angiotensin converting enzyme (ACE) are added to the sample to prevent the degradation of angiotensin I and its conversion to angiotensin II. The concentration of angiotensin I is measured with a radioimmunoassay /2/. Angiotensin I can also be quantified by liquid chromatography tandem mass spectrometry using online solid phase extraction (XLS-MS/MS) /3/. Enzyme activity is expressed as ng (μg) angiotensin I generated per mL (liter) of plasma per hour. Expression as pmol or nmol per liter of plasma and hour is also common. Pre analytical factors influencing the result of the renin activity assay are shown in Tab. 31.3-1 – Influence factors causing activation of prorenin and recommendations for specimen handling.

Plasma renin concentration measurement

Immunoassays are commercially available that measure /4/:

  • Renin and active prorenin (prorenin in open conformation). Total renin (renin and prorenin in the active form) is determined using sandwich immunoassays or competitive immunoassays.
  • Renin without prorenin (prorenin in closed conformation). For the determination of renin it is essential to prevent inadvertent conversion of plasma prorenin from a closed to an open conformation during sample preparation and during the assay itself. This reportedly occurs to 5% if the immunoassay is performed at 22 °C for 24 h, but not at 37 °C for 6 h /2/.

Calibration is performed to International Standard 68/356, in which 1 U is generally equivalent to 0.6 μg of active renin. The functional sensitivity of the assays is at the lower reference interval value and ranges between 2 and 4 mU/L, depending on the assay. The detection limit is 1 ng/L (1.7 mU/L) /5/. Results are reported in mU/L, ng/L, or nmol/L.

31.3.3 Specimen

EDTA blood: 5 mL

Centrifuge within 30 minutes, preferably within 10 minutes; freeze plasma, if assay is not performed immediately.

31.3.4 Reference interval

Refer to Tab. 31.3-2 – Reference intervals for renin.

31.3.5 Clinical significance

The physiologic activation of the renin-angiotensin-aldosterone system (RAS) is characterized by renin induced aldosterone increase and serves to maintain the intravascular volume and the blood pressure /6/.

Low renin can be caused by physiological suppression of renin, in the context of intravascular volume expansion or a pathologic condition of aldosterone excess as described in primary aldosteronism.

Besides primary aldosteronism conditions that manifest with low renin and hypertension (low renin hypertension) can result from mineralocorticoid excess without aldosterone excess resulting from increased production of corticosteroids in the synthetic pathway of aldosterone from 11-deoxycorticosterone. Although these corticosteroids only have a mild mineralocorticoid effect, they have a clinical impact if produced in large amounts /7/.

Refer to:

All drugs in use of hypertension have an influence on plasma renin and other components of the RAS /1/. Refer to Tab. 31.3-4 – Impact of different classes of antihypertensive agents on renin mass and plasma renin activity.

31.3.6 Comments and problems

Before measurement of renin pre analytical conditions are required to prevent unfolding and cleavage of the pro segment of prorenin (Tab. 31.3-1 – Measurement of renin by activity assay or immunoassay).

Method of determination

The detection of prorenin in the open conformation is a problem especially in the diagnosis of diseases in which low renin levels are expected, such as in primary aldosteronism.

Renin assays used for determining the ARR should be sufficiently sensitive to measure levels as low as 0.2–0.3 ng/mL/h; renin concentration assays should have a functional sensitivity of at least 2 mU/L /6/.

Renin activity

In low renin states plasma renin activity may not ensure enough sensitivity in many patients. In a study /14/ a two step procedure is proposed: a brief 1.5 h enzymatic reaction time followed by a prolonged reaction (18 h) only if PRA is below 0.2 ng/mL/h.

Renin concentration

The results obtained with assays from different diagnostics manufacturers are not comparable. Therefore, the conversion factors used to convert renin activity in ng/mL/h to renin concentration in ng/L or mU/L also differ. For example, in one commercial assay, a plasma renin activity level of 1 ng/mL/h converts to a direct renin concentration of 12 mU/L (7.6 ng/L) /13/.

In patients with renin inhibitor therapy immunoassay measurement of renin can result in false results because binding of the renin inhibitor to the active site of prorenin molecules with an open conformation prevents refolding of the pro segment. Thereby the amount of prorenin recognized by the renin immunoassay is increased /2/.

Stability

Samples may show a fast angiotensin I degradation activity during the incubation period for determination of plasma renin activity mediated by calcium. In a study most samples had normal calcium concentrations indicating that serum or heparinized but not EDTA plasma samples had been send to the laboratory /15/.

References

1. Brown MJ. Renin: friend or foe? Heart 2007; 93: 1026–33.

2. Campbell DJ, Nussberger J, Stowasser M, Danser AHJ, Morganti A, Frandsen E, et al. Activity assays and immunoassays for plasma renin and prorenin: information provided and precautions necessary for acute measurement. Clin Chem 2009; 867–77.

3. Carter S, Owen LJ, Kerstens MN, Dullaart RPF, Keevil BG. A liquid chromatography tandem mass spectrometry assay for plasma renin activity using online solid-phase extraction. Ann Clin Biochem 2012; 49: 570–9.

4. Deinum J, Derkx FHM, Schalekamp MADH. Improved immunoradiometric assay for plasma renin. Clin Chem 1999; 45: 847–54.

5. Ferrari P, Shaw SG, Nicod J, Saner E, Nussberger J. Active renin versus plasma renin activity to define aldosterone-to-renin ratio for primary aldosteronism J Hypertens 2004; 22: 377–81.

6. Funder JW, Carey RM, Mantero F, Murad HM, Reincke M, Shibata H, et al. The management of primary aldosteronism: case detection, diagnosis, and treatment: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2016, 101: 1889–1916.

7. Baudrand R, Vaidya A. The low-renin hypertension phenotype: genetics and the role of the mineralocorticoid receptor. Int J Mol Sci 2018; 19: 546; https://doi.org/10.3390/ijms19020546.

8. Hajjar J, Kotchen TA. Trends in the prevalence, awareness, treatment, and control of hypertension in the United States, 1988–2000. JAMA 2003; 290: 199–206.

9. Malik S, Wong ND, Franklin SS, Kamath TV, L’Italien GJ, Pio JR, et al. Impact on the metabolic syndrome on mortality from coronary heart disease, cardiovascular disease, and all causes in the United States adults. Circulation 2004; 110: 1245–50.

10. Corry DB Tuck ML. Secondary aldosteronism. Endocrinol Metab Clin North Am 1995; 24: 511–27.

11. Gupta V. Mineralocorticoid hypertension. Indian J Endocrinol 2011; 15 (Suppl 4): S298–S312.

12. Eisenhofer G, Dekkers T, Peitzsch M, Dietz AS, Bidlingmaier M, Treitl M, et al. Mass spectrometry-based adrenal and peripheral venous steroid profiling for subtyping of primary aldosteronism. Clin Chen 2016; 62: 514–24

13. Fischer E, Reuschl S, Quinkler M, Rump LC, Hahner S, Bidlingmaier M, et al. Assay characteristics influence the aldosterone to renin ratio as a screening tool for primary aldosteronism: results of the German Conn’s Registry. Horm Metab Res 2013; 45: 526–31.

14. Brossaud J, Corcuff JB. Pre-analytical and analytical considerations for the determination of plasma renin activity. Clin Chim Acta 2009; 410: 90–2.

15. Clarke NJ, Reitz RE, McPhaul MJ, Wu Z. Plasma renin activity: the impotance of correct sample type. Clin Chem 2016; 62: 408–9.

31.4 Aldosterone

Mineralocorticoid excess syndrome results from excess adrenal production of aldosterone and is the most frequent cause of secondary arterial hypertension. The inappropriate increased production of aldosterone is associated with sodium retention, hypervolemia. Suppression of plasma renin, increased potassium excretion, which may lead to hypokalemia, and cardiovascular disease /1/.

Mineralocorticoid deficiency is due to lack of aldosterone synthesis of the adrenal cortex or decreased action of aldosterone. The inappropriate decreased effect of aldosterone is associated with low aldosterone levels, sodium depletion, increase of plasma renin, decreased potassium excretion, which may lead to hyperkalemia , and hypovolemia.

31.4.1 Indication

Suspected excess of mineralocorticoids:

  • Treatment-resistant hypertension
  • Primary aldosteronism

Suspected mineralocorticoid deficiency (e.g., in hyperkalemia without renal failure).

31.4.2 Method of determination

HPLC tandem mass spectrometry

This is the reference method. Principle and detailed procedure see Ref. /2/.

Immunoassay

Radioimmunoassay and assays with non-radioactive tracers are used.

Measurement of urinary aldosterone

The major proportion of urinary aldosterone is tetrahydroaldosterone-3-glucuronide and aldosterone-18-glucuronide. In most cases, however, free aldosterone is measured, which only accounts for 0.2% of total aldosterone. To measure free aldosterone, the urine sample is subjected to acid hydrolysis, buffered, and then analyzed by gas chromatography mass spectrometry.

31.4.3 Specimen

Serum or plasma (blood is collected in recumbent or seated position): 1 mL

24 h urine collection (neutral).

31.4.4 Reference interval

The reference interval for aldosterone in plasma and the different forms of aldosterone in urine are shown in Tab. 31.4 -1– Reference intervals for aldosterone.

31.4.5 Clinical assessment

The RAS is often involved in the pathophysiology of hypertension, be it primary or secondary. The prevalence of primary aldosteronism increases with the severity of hypertension, from 2% in patients with low grade hypertension to 20% among resistant hypertensives.

Mineralocorticoid hypertension comprises the etiological spectrum listed in Tab. 31.4-2 – Mineralocorticoid hypertension disorders.

Indications for testing for mineralocorticoid hypertension are listed in Tab. 31.4-3 – Case detection of primary aldosteronism.

31.4.5.1 Primary aldosteronism

Primary aldosteronism (PA) is a group of disorders in which aldosterone production is inappropriately high for Na+ status and non-suppressible by Na+ loading. PA is relatively autonomous for the most frequent regulators of the RAS. PA is the most frequent cause of secondary hypertension and is responsible for 5–15% of cases of increased blood pressure among hypertensive population. Laboratory findings are increased level of aldosterone, decreased plasma renin, and hypokalemia (9–37%) /3/.

The two forms of PA are:

  • Unilateral aldosterone secretion caused by excessive aldosterone producing adenoma (APA) and treated by adrenalectomy
  • Bilateral adrenal hyperplasia (BAH) resulting from hyperplasia of the zona glomerulosa, also known as idiopathic hyperaldosteronism. BAH is treated with mineralocorticoid receptor antagonists.

Rare subtypes of PA are:

  • Adrenocortical carcinoma
  • Familial aldosteronism type II
  • Ectopic aldosterone production
  • Unilateral primary adrenal hyperplasia
  • Glucocorticoid remediable aldosteronism.

Refer to Tab. 31.4-4 – Primary aldosteronism, mineralocorticoid excess and hypertension.

31.4.5.1.1 Diagnosis of primary aldosteronism

It is important to diagnose PA, since aldosterone producing adenoma induced hypertension is treated surgically while bilateral hyperplasia induced hypertension can only be treated with mineralocorticoid receptor antagonists. Patients with PA have a higher incidence of cardiovascular events than those with essential hypertension. The Endocrine Society Clinical Practice Guideline of the USA recommends for the diagnosis of PA three diagnostic steps /3/:

  • Case detection
  • Case confirmation
  • Subtype classification.

The plasma aldosterone/renin ratio (ARR) is recommended to detect possible cases of PA /3/.

Case detection test: Aldosterone/renin ratio (ARR)

Drugs that may lead to false-positive or false-negative ARR results should be discontinued.

Refer to Tab. 31.4-5 – Medications to be discontinued prior to testing for mineralocorticoid induced hypertension

The ARR test is most sensitive when samples are collected in the morning after patients have been out of bed for at least 2 hours, usually after they have been seated for 5–15 minutes /3/. A detailed approach is shown in Ref. /3/.

The ARR is the best screening test for suspected PA-induced hypertension and its differentiation from essential hypertension. The ARR can be used under random conditions because it is less affected by diurnal variations, posture and gender. Prior to blood collection the K+ level should be normal. The diagnostic sensitivity of the ARR for PHA is 64–100%, with a specificity of 87–100% /4/. Influencing factors include medication, hormonal contraceptives, antidepressants, hypokalemia, and increased table salt intake. The cutoff values are depending on the renin assay used /3/ and are shown in Tab. 31.4-6 – Aldosterone/renin-ratio (ARR) cutoff values depending on the renin assay.

Limitations of the ARR are that in the presence of very low renin levels (e.g., renin below 0.1 ng/mL/h) the ARR may be elevated even when aldosterone is also low (e.g., 4 ng/dL or 110 pmol/L) /3/.

Case confirmation of primary aldosteronism

Patients with a positive ARR should undergo one or more confirmatory tests to definitively confirm or exclude the diagnosis of PA. However in the setting of spontaneous hypokalemia, plasma renin below detection levels, plus aldosterone > 20 ng/dL (550 pmol/L) the Endocrine Society /3/ suggests that there may be no need for further confirmation. There is no gold standard confirmatory test.

For confirmatory tests refer to Tab. 31.4-7 – Primary aldosteronism: confirmatory tests.

Subtype classification of primary aldosteronism (PA)

For treatment strategy subtype classification of PA is needed independent of etiology of PA. Lateralization of the source of the excessive aldosterone production is critical to guide the management of PA. The two main forms of PA the unilateral aldosterone producing adenoma and the bilateral adrenal hyperplasia are differentiated by imaging. However imaging cannot reliably visualize micro adenoma or distinguish non-functioning incidentaloma from aldosterone producing adenoma. Adrenal vein sampling is the most accurate means of differentiating unilateral from bilateral forms of PA. Distinguishing between unilateral and bilateral abnormalities is critical in deciding whether surgical intervention is indicated, because unilateral adrenalectomy in patients with adenoma, unilateral hyperplasia, adrenal carcinoma, ectopic ACTH production and renin- and 11-deoxycorticosterone producing tumors results in normalization of K+ levels in 30–60% of the hypertensive patients. In bilateral idiopathic aldosteronism and glucocorticoid remediable hyperaldosteronism, unilateral and bilateral adrenalectomy rarely corrects the hypertension, and a conservative approach is necessary.

Adrenal venous sampling (AVS) is the most accurate means of differentiating unilateral from bilateral forms of PA. The diagnostic sensitivity and specificity of AVS for detecting unilateral aldosterone excess are 95% and 100%, respectively. However, in patients younger than 35 years with marked PA (e.g., spontaneous hyperkalemia, aldosterone > 30 ng/dL, 831 pmol/L) and solitary unilateral apparent adenoma on CT scan, a case can be made to proceed directly to unilateral adrenalectomy without prior AVS /3/.

The protocol for adrenal venous sampling (AVS) test is shown in Tab. 31.4-8 – Adrenal venous sampling test.

An algorithm for the workup of hypertension in the case of a suspected mineralocorticoid related etiology is shown in Fig. 31.4-1 – Diagnostic approach for suspected primary aldosteronism and possible diagnoses.

31.4.5.2 Secondary aldosteronism

Secondary aldosteronism, occurs in states of low effective blood volume, which activates the RAS. The resultant increase in renin and aldosterone stimulates the distal reabsorption of Na+ by the kidney to restore blood volume /5/. This Na+ retaining effect occurs in association with an increase in K+ excretion. Because hyponatremia and hypovolemia increase renin release the concentrations of aldosterone and renin are increased in secondary aldosteronism. The aldosteronism results in hyperkaluresis, which also is present when there is hypokalemia.

Secondary aldosteronism is a common finding in both normotensive and hypertensive patients. The primary stimulus is caused by hypovolemia and hyponatremia that stimulate renin release, which acts as proteolytic enzyme on angiotensinogen to produce angiotensin I /5/. In edema, for example, water and Na+ are conserved at the expense of K+. This is also the case in pregnancy, although there is no loss of K+, since the effect of aldosterone is overridden by pregnancy hormones. In chronic kidney disease, the secondary aldosteronism counteracts hyperkalemia. Secondary aldosteronism is most common in renal artery stenosis, diuretic abuse, or renin secreting tumors /5/.

Findings in different forms of secondary aldosteronism are shown in Tab. 31.4-9 – Secondary hyperaldosteronism, mineralocorticoid excess and hypertension.

31.4.5.3 Pseudo hyperaldosteronism

Pseudo hyperaldosteronism is a condition of mineralocorticoid excess and normal aldosterone characterized by hypertension, hypokalemic alkalosis, and suppressed renin /6/. Some of the cases include Liddle’s syndrome, Cushing’s syndrome, congenital adrenal hyperplasia, and 11β-hydroxysteroid dehydrogenase deficiency.

Refer to Tab. 31.3-3 – Disorders associated with low renin hypertension.

31.4.5.4 Primary hypoaldosteronism

Laboratory findings are hyponatremia, hyperkalemia, hypermagnesemia, metabolic acidosis, significantly reduced aldosterone, significantly elevated renin.

Hypoaldosteronism in primary adrenal insufficiency

Laboratory findings are reduced cortisol and aldosterone, and elevated renin. Normalization of renin can be a sensitive indicator for monitoring the treatment of adrenal insufficiency.

Aldosterone synthase type I deficiency with 18-hydroxylase deficiency

Laboratory findings are reduced aldosterone, elevated renin, reduced levels of the mineralocorticoids corticosterone, 11-deoxycorticosterone, and 18-hydroxycorticosterone. As a further step, the ACTH test should be performed to check that cortisol synthesis is intact.

Aldosterone synthase type II deficiency with 18-oxidase deficiency

Findings are elevated 18-hydroxycorticosterone levels in relation to aldosterone and/or an increased ratio of 18-OH-tetrahydroaldosterone to tetrahydroaldosterone excretion in urine /6/. The diagnosis can be confirmed by molecular genetic investigations. Patients have low aldosterone, hyperkalemia, renal salt loss, and metabolic acidosis.

Besides the different 18-hydroxycorticosterone concentrations, the ratio of 18-hydroxycorticosterone to aldosterone is also important in the differential diagnosis of aldosterone synthase deficiency. Ratios less than 10 are found in type I, ratios greater than 100 in type II.

31.4.5.5 Secondary hypoaldosteronism

Secondary mineralocorticoid deficiency is associated with low renin and low aldosterone levels and occurs in Liddle’s syndrome, congenital adrenal hyperplasia, apparent mineralocorticoid excess syndrome, and licorice abuse.

Refer to Tab. 31.3-3 – Disorders associated with low renin hypertension.

31.4.5.6 Pseudo hypoaldosteronism

Pseudo hypoaldosteronism represents a disorder associated with functional secretion of aldosterone however, clinical symptoms are comparable to hypoaldosteronism. Pseudo hypoaldosteronism is a group of hereditary or acquired disorders due to a defect in the loop of Henle or the distal tubule. Secondary forms of pseudo hypoaldosteronism are seen in kidney diseases, such as diabetic nephropathy and analgesic nephropathy.

Disorders

Clinical findings

Manifestation in childhood, failure to thrive, and hypotension.

Laboratory findings

High renal sodium chloride excretion, hyperkalemia, dramatically increased aldosterone and renin.

31.4.6 Comments and problems

Because ARR is mathematically highly dependent on renin, assays for the determination of renin should be sufficiently sensitive to measure levels as low as 0.2–0.3 ng/mL/h or 2 mU/l using immunoassays /3/.

The available radioimmunoassays for aldosterone overestimate levels by 50–100% in the range below 200 pmol/l /3/.

31.4.7 Biochemistry

Aldosterone is the most important naturally occurring mineralocorticoid. Aldosterone is produced exclusively in the zona glomerulosa of the adrenal cortex from cholesterol via a series of hydroxylations and oxidations (Fig. 31.3-2 – Synthesis of aldosterone).

The final steps of this pathway, the conversion of 11-deoxycorticosterone (DOC) to aldosterone, require conversion via the intermediates to 18-hydroxy-DOC or corticosterone and 18-hydroxycorticosterone /5/. Aldosterone is present in serum in the isomeric forms through formation of a cyclic hemiacetal bond between the positions C11 and C18.

Refer to Fig. 31.4-2 – Isomeric forms of aldosterone in plasma.

The enzyme P450aldo (aldosterone synthase, CYP11B2) catalyzes the last three steps (11β-hydroxylation, 18-hydroxylation and 18-oxidation) of the aldosterone biosynthesis pathway. The enzyme is encoded by the gene CYP11B2.

Aldosterone is excreted into the urine as aldosterone-18-glucuronide, tetrahydroaldosterone, and free aldosterone. Aldosterone is glucuronidated in the liver and kidneys at position C18. Only about 0.2% of secreted aldosterone is excreted as free aldosterone. The remainder is converted in the liver to tetrahydroaldosterone by reduction of the ring A, and glucuronidated at position C3 (Fig. 31.4-3 – Conjugation of aldosterone to aldosterone 18-glucuronide and tetrahydroaldosterone 3-glucuronide).

References

1. Dick SM, Queiroz M, Bernardi BL, Dall´Agnol A, Brondani LA Silveiro SP. Update in diagnosis and management of primary aldosteronism. Clin Chem Lab Med 2018; 56: 360–72.

2. Taylor PJ, Cooper DP, Gordon RD, Stowasser M. Measurement of aldosterone in human plasma by semiautomated HPLC-tandem mass spectrometry. Clin Chem 2009; 55: 1155–62.

3. Funder JW, Carey RM, Mantero F, Murad HM, Reincke M, Shibata H, et al. The management of primary aldosteronism: case detection, diagnosis, and treatment: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2016, 101: 1889–1916.

4. Gupta V. Mineralocorticoid hypertension. Indian J Endocrinol 2011; 15 (Suppl 4): S298–S312.

5. Corry DB Tuck ML. Secondary aldosteronism. Endocrinol Metab Clin North Am 1995; 24: 511–27.

6. Hauffa BP, Solyom J, Glaz E, Shackleton CHL, Wambach G, Vecsei P, Stolecke H, Homoki J. Severe hypoaldosteronism due to corticosterone methyl oxidase type II deficiency in two boys: metabolic and gas chromatography-mass spectrometry studies. Eur J Pediatr 1991; 150: 149–53.

7. Sayer JA, Pearce SAS. Diagnosis and clinical biochemistry of inherited tubulopathies. Ann Clin Biochem 2001; 38: 459–70.

8. Hubl W. Aldosterone. In: Bergmeyer HU. Methods of enzymatic analysis, Vol VIII. Weinheim: VCH, 1985: 256–66.

9. Taylor PJ, Cooper DP, Gordon RD, Stowasser M. Measurement of aldosterone in human plasma by semiautomated HPLC-tandem mass spectrometry. Clin Chem 2009; 55: 1155–62.

10. Demers LM, Whitley RJ. Function of the adrenal cortex. In: Burtis CA, Ashwood ER, eds. Tietz Textbook of Clinical chemistry. Philadelphia; Saunders 1998: 1561.

11. Abdelhamid S, Vecsei P, Haak D, Gless KH, Walb D, Fiegel P, Lichtwald K. Elevated free 18-OH-corticosterone excretion as a possible indicator for early diagnosis of primary aldosteronism. J Steroid Biochem 1981; 14: 913.

12. Abdelhamid S, Thomas L, Neusel U, Lorenz H, Rückel A, Bönhof JA, Müller-Lobeck H. Studies on the diagnostic value of aldosterone precursor 18-hydroxycorticosterone in the diagnosis and differential diagnosis of primary hyperaldosteronism. Lab Med 1994; 18: 275–84.

13. Kohl KH, Vecsei P, Abdelhamid S. Radioimmunoassay of tetrahydroaldosterone (TH-aldo) in human urine. Acta Endocrinol 1978; 87: 596–608.

14. Miller WL, Auchus RJ. The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders. Endocr Rev 2011; 32: 81–151.

15. Fuller PJ. Adrenal diagnostics: an endocrinologist’s perspective focused on hyperaldosteronism. Clin Biochem Rev 2013; 34: 111–5.

16. Funder JW, Carey RM, Mantero F, Murad HM, Reincke M, Shibata H, et al. The management of primary aldosteronism: case detection, diagnosis, and treatment: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2016, 101: 1889–1916.

17. Mulatero P, Morra di Cella S, Monticone S, Schiavone D, Manzo M, Mengozzi G, et al. 18-hydroxycorticosterone, 18-hydroxycortisol, and 18-oxocortisol in the diagnosis of primary aldosteronism and its subtypes. J Clin Endocrinol Metab 2012; 97: 881–9.

18. Jin S, Wada N, Takahashi Y, Hui SP, Sakurai T, Fuda H, et al. Quantification of urinary 18-hydroxycortisol using LC-MS/MS. Ann Clin Biochem 2013; 50: 450–6.

19. Eisenhofer G, Dekkers T, Peitzsch M, Dietz AS, Bidlingmaier M, Treitl M, et al. Mass spectrometry-based adrenal and peripheral venous steroid profiling for subtyping of primary aldosteronism. Clin Chen 2016; 62: 514–24.

20. Wilson M, Morganti AA, Zervoudakis I, et al. Blood pressure, the renin-aldosterone system and sex steroids throughout normal pregnancy. Am J Med 1980; 68: 97.

21. Nicod J, Bruhin D, Auer L, et al. A biallelic gene polymorphism of CYP11B2 predicts increased aldosterone to renin ratio in selected hypertensive patients. J Clin Endocrinol Metab 2003; 88: 2495–500.

22. Dworkin LD, Cooper CJ. Renal-artery stenosis. N Engl J Med 2009; 361: 1972–8.

23. Amirlak I, Dawson KP. Bartter syndrome: an overview. Q J Med 2000; 93: 207–15.

31.5 18-Hydroxycortisol

18-hydroxycortisol (18-OHF) is known as hybrid steroid, because it has structural characteristics of both cortisol and aldosterone (Fig. 31.3-2 – Synthesis of aldosterone). 18-OHF is synthesized by aldosterone synthase using 11-deoxycortisol as substrate. Small amounts are also synthesized by 11β-hydroxylase. Because aldosterone synthase expression is normally limited to the zona glomerulosa of the adrenal cortex and 17α-hydroxylase and 11β-hydroxylase necessary for cortisol synthesis occur in the zona fasciculate, production of 18-OHF is normally very low /1/.

31.5.1 Indication

Diagnosis of primary aldosteronism.

31.5.2 Method of determination

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) /2/.

31.5.3 Specimen

Collect urine over 24 hours, keep sample frozen until it is analyzed.

31.5.4 Reference interval

Urine: 28–485 nmol/L /2/.

31.5.5 Clinical significance

In patients with elevated aldosterone/renin ratio (ARR) the determination of 18-OHF urinary excretion is recommended. Levels below 130 ug/24 h exclude primary aldosteronism and are secreted from patients with non secreting cortical adrenal tumors e.g., in patients with essential hypertension. In the range of 130–510 ug/24 h no assessment is possible and confirmatory tests are needed. Levels higher than 510 ug/24 h indicate primary aldosteronism, however differentiation between adenoma and glucocorticoid remediable aldosteronism is not possible /1/.

References

1. Mulatero P, Morra di Cella S, Monticone S, Schiavone D, Manzo M, Mengozzi G, et al. 18-hydroxycorticosterone, 18-hydroxycortisol, and 18-oxocortisol in the diagnosis of primary aldosteronism and its subtypes. J Clin Endocrinol Metab 2012; 97: 881–9.

2. Jin S, Wada N, Takahashi Y, Hui SP, Sakurai T, Fuda H, et al. Quantification of urinary 18-hydroxycortisol using LC-MS/MS. Ann Clin Biochem 2013; 50: 450–6.

31.6 18-Hydroxycorticosterone

18-Hydroxycorticosterone (18-OHB) is formed by 18-hydroxylation of corticosterone. 18-OHB is an intermediate precursor in aldosterone biosynthesis that originates from the conversion of corticosterone by the aldosterone synthase (Fig. 31.3-2 – Synthesis of aldosterone). Although small amounts may be produced by the 11β-hydroxylase, 18-OHB has only low affinity for the mineralocorticoid receptor.

31.6.1 Indication

Differentiation of aldosterone producing adenoma and bilateral adrenal hyperplasia.

31.6.2 Method of determination

Radioimmunoassay /1/, HPLC or gas chromatography-tandem mass spectrometry.

31.6.3 Specimen

Heparinized and EDTA plasma: 1 mL

Urine: collect urine (neutral) over 24 hours, take sample to laboratory or measure volume and ship in 10 mL container.

31.6.4 Reference interval

  • Plasma: 115–550 ng/L (317–1.418 pmol/L)
  • Urine: 1.5–6.5 μg/24 h /2/.

31.6.5 Clinical significance

Patients with aldosterone producing adenoma generally have recumbent 18-OHB levels higher than 1,000 ng/L at 8.00 a.m., whereas patients with idiopathic hyperaldosteronism have 18-OHB levels that are usually below 1,000 ng/L /34/.

References

1. Abdelhamid S, Vecsei P, Haak D, Gless KH, Walb D, Fiegel P, Lichtwald K. Elevated free 18-OH-corticosterone excretion as a possible indicator for early diagnosis of primary aldosteronism. J Steroid Biochem 1981; 14: 913.

2. Hornung J, Gless KH, Abdelhamid S, Vielhauer W, Vecsei P. Radioimmunoassay of free urinary 18-hydroxy-deoxycorticosterone (18-OH-DOC) in patients with essential hypertension. Clin Chim Acta 1978; 87: 181.

3. Biglieri EG, Schambelan M. The significance of elevated levels of plasma 18-hydroxycorticosterone in patients with primary aldosteronism. J Clin Endocrinol Metab 1979; 49: 87–91.

4. Miller WL, Auchus RJ. The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders. Endocr Rev 2011; 32: 81–151.

Table 31.2-1 Disorders of the renin-angiotensin-aldosterone system

Hyperaldosteronism

I. Primary aldosteronism. This disorder is characterized by the autonomous hypersecretion of aldosterone by the adrenal cortex. The following types are distinguished:

  • Adenoma, or rarely cancer of the adrenal cortex (Conn’s syndrome)
  • Unilateral or bilateral adrenal hyperplasia (idiopathic aldosteronism).

II. Monogenetic forms of hypertension

  • Glucocorticoid-remediable aldosteronism
  • Familial aldosteronism type II
  • Apparent mineralocorticoid excess
  • Liddle’s syndrome.

III. Secondary aldosteronism with hypertension. Stimulation of renin and secondarily of aldosterone by an extra adrenal signal

  • Malignant hypertension, renin producing tumors, renovascular hypertension
  • Edema (e.g., in conjunction with heart failure, liver cirrhosis, nephrotic syndrome).

IV. Secondary aldosteronism without hypertension.

  • Sodium losing kidney diseases, renal tubular acidosis, Bartter syndrome
  • Physiological aldosteronism due to volume or sodium depletion (e.g., as in the use of diuretics and laxatives or as a result of sweating, vomiting or diarrhea).

Hypoaldosteronism

Pseudo hyperaldosteronism

Isolated hypoaldosteronism:

  • Primary, hyperreninemic hypoaldosteronism due to 18-hydroxylase deficiency
  • Secondary hyporeninemic hypoaldosteronism.

Table 31.2-2 Genetic determinants of the RAS and diseases influenced by them /4/

Protein

Disease

Gene

Locus

Aldosterone synthase (P450c11Aldo)

Familial aldosteronism type I: GRA, glucocorticoid-remediable aldosteronism, fusion with 11β-hydroxylase promoter gene CYP11B1

CYP11B2

8q24.3

Aldosterone synthase (P450c11Aldo),18-hydroxylase

Aldosterone synthase type I deficiency (formerly CMO I deficiency)

CYP11B2

8q24.3

Aldosterone synthase (P450c11Aldo),18-oxidase

Aldosterone synthase type II deficiency (formerly CMO II deficiency)

CYP11B2

8q24.3

Mineralocorticoid receptor

Pseudo hypoaldosteronism

type I

MLR

4q31.1

11β-hydroxysteroid dehydrogenase type 2

Mineralocorticoid excess syndrome

HSD11B2

16q22

Epithelial Na+ channel (ENaC), β-subunit

Liddle’s syndrome, PHA1

SCNN1B

16p13-p12

Epithelial Na+ channel (ENaC), γ-subunit

Liddle’s syndrome, PHA1

SCNN1G

16p13-p12

NaK2C1 cotransporter

Bartter syndrome type 1

SLC12A1

5q15-q21.1

Potassium channel, ROMK

Bartter syndrome type 2

KCNJ1

11q24

Chloride channel B

Bartter syndrome type 3

CLCNKB

1p36

Angiotensinogen

Essential hypertension,
cardiovascular
disease (CVD)
preeclampsia

AGT

1q42-q43

Renin

 

REN

1q32

Angiotensin converting enzyme (ACE)

Diabetic ephropathy,cardiac hypertrophy,
CVD

DCP1, ACE

17q23

Table 31.3-1 Influence factors causing activation of prorenin and recommendations for specimen handling /2/

Cooling and low pH promote unfolding and cleavage of the prorenin pro segment.

Refolding of the pro segment is promoted at physiological pH and 37 °C.

Prorenin is activated by cryoactivation; activation is about 5%.

Spontaneous activation of prorenin occurs within 8 hours if the plasma is left at room temperature, or within 6 hours if it is incubated at 37 °C.

Recommendation on the handling of samples: blood should be centrifuged within 30 min. (preferably within 10 min.) of collection, and the plasma should be rapidly frozen if renin cannot be determined immediately. Frozen plasma should be rapidly thawed just before the assay, and the plasma should be frozen only once.

Table 31.3-2 Reference intervals for renin

Plasma renin activity (PRA)

Renin and Prorenin /8/: 1.5 (0.7–2.2) ng/mL/h

Values expressed are median and 2.5th and 97.5th percentiles

Renin and Prorenin (XLS-MS/MS) /9/: 0.12–1.75 nmol/L/h

Values expressed are 2.5th and 97.5th percentiles

Renin concentration (immunoassay)

23 (3–116) mU/L /9/

Renin plus prorenin concentration (immunoassay)

202 (123–344) mU/L /8/

Values expressed are median and 2.5th and 97.5th percentiles

Conversion renin concentration: 1 pmol/L = 1.296 ng/L; 1 ng/L = 1.7 mU/L

Conversion renin plus prorenin: 1 ng/mL/h PRA = 12 mU/L renin = 7.6 ng/L renin, measured with the automated commercially available immunoassay /10/

Table 31.3-3 Low-renin hypertension due to non-aldosterone mineralocorticoids /1112/

Clinical and laboratory findings

Hypercortisolism

Endogenous hypercortisolism without or with overt manifestations of Cushing’s syndrome can result in chronic stimulation of the glucocorticoid receptor and also potentially the mineralocorticoid receptor (MR) with consequent development of hypertension, insulin resistance, diabetes and cardiovascular disease. Cortisol and aldosterone are potent MR agonists, but cortisol is inactivated to cortisone by 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2), thereby protecting the renal MR from stimulation. However, in states of severe hypercortisolism, excess cortisol can overwhelm 11βHSD2 and result in direct cortisol mediated MR activation and subsequent intravascular volume expansion. Endogenous hypercortisolism is most commonly due to Cushing’s disease (benign ACTH secreting tumor).

Laboratory findings: the phenotype with endogenous hypercortisolism is characterized by suppression of renin and aldosterone, hypertension, and low serum K+ and increased urinary excretion of K+ /7/.

Congenital adrenal hyperplasia (11β-hydroxylase deficiency)

Congenital adrenal hyperplasia results from defective synthesis of cortisol and has an incidence of 1 per 15,000 live births. It is more prevalent in the white population. Causes are /12/:

  • 11β-hydroxylase (P450c11β) catalyzes the conversion of 11-deoxycortisol to cortisol, 11-deoxycorticosterone to corticosterone, corticosterone to 18-hydroxy-corticosterone, but has no aldosterone synthase activity. 11β-hydroxylase deficiency accounts for 5–8% of congenital adrenal hyperplasia cases in Europeans and for 15% of adrenogenital syndrome (AGS) cases in the Muslim and Jewish populations of the Middle East. It has an autosomal recessive mode of inheritance. Deficiency of 11β-hydroxylase results in reduced production of glucocorticoids (11-deoxycortisol is not converted to cortisol) and overproduction of metabolites proximal to the block. Plasma renin is suppressed and ACTH elevated leading to bilateral adrenal hyperplasia with increased accumulation of 11-deoxycorticosterone, 11-deoxycortisol, 17-hydroxyprogesterone, progesterone, and sex steroids. Since 11-deoxycorticosterone has a mineralocorticoid effect, salt retention (rarely hypernatremia), hypertension, hypokalemia and alkalosis result. Hypertension only occurs in late childhood and adolescence; precocious puberty in males and masculinization in females.
  • Deficiency in 21-hydroxylase (P450c21) is responsible for 90–95% of cases of congenital adrenal hyperplasia. The enzyme catalyzes the 21-hydroxylation of progesterone to 11-deoxycorticosterone and 17-OH progesterone to 11-deoxycortisol in the biosynthesis of mineralocorticoids and glucocorticoids, respectively.

Refer to Fig. 31.3-2 – Synthesis of aldosterone, cortisol and androstendione.

17α-hydroxylase deficiency

17α-hydroxylase (P450c17) hydroxylates both pregnenolone and progesterone to 17-OH pregnenolone and 17-OH progesterone, respectively /12/. Deficiency in this enzyme results in reduced synthesis of cortisol, dehydroepiandrostendione, androstendione, and its downstream products, as also an overproduction of proximal to 17α-hydroxylase (excess pregnenolone, progesterone) and metabolites of the mineralocorticoid axis including (deoxycorticosterone, corticosterone, 18-hydroxylated metabolites, and aldosterone). The reduced cortisol levels lead to increased ACTH secretion and adrenal hyperplasia; the hyperaldosteronism suppresses renin and causes salt retention and hypertension. The lack of sex steroids can lead to tallness, delayed skeletal maturation and osteoporosis during adulthood. In males under androgenization and absent or reduced masculinization is typical; females may have irregular menstruation or amenorrhea.

Refer to Fig. 31.3-2 – Synthesis of aldosterone, cortisol and androstendione.

Laboratory findings: hyperkalemia, reduced renin, elevated aldosterone, elevated ACTH, elevated 11-deoxycorticosterone.

Aldosterone synthase (AS) deficiency

The final step in the synthesis of aldosterone is catalyzed by AS (P450c11AS). The conversion of 11-deoxycorticosterone to aldosterone requires three catalytic steps: 11β-hydroxylase, 18-hydroxylase, and 18-methyl oxidase. AS is capable of catalyzing all three reactions involved in the production of aldosterone. Mutations in the CYP11B2 gene can lead to complete loss of AS activity and aldosterone production.

Refer to Fig. 31.3-2 – Synthesis of aldosterone, cortisol and androstendione.

Ectopic ACTH syndrome

Lung carcinoma, pancreas carcinoma, foregut carcinoid and thymus carcinoma can secrete ACTH. The extent of cortisol production correlates directly with the severity of the hypertension and hypokalemia. The mineralocorticoid effect is thought to be caused by the high cortisol production rather than by the inhibition of the 11β-hydroxylase.

Laboratory findings: elevated ACTH, elevated cortisol excretion in 24-h urine, hypokalemia.

Liddle’s syndrome

This disorder is a rare autosomal dominant form of hypertension with early penetrance and heart problems. It is caused by mutations in the epithelial Na+ channel (ENaC) of the collecting duct of the nephron. The ENaC consist of the three subunits α, β and γ. Due to a mutation in the β-subunit of the amiloride-sensitive ENaC, there is increased reabsorption of Na+ and volume expanded hypertension results.

Refer to Fig. 8.8-2 – Reabsorption of solutes in the collecting duct.

Laboratory findings: hypokalemia, low renin and normal (pseudo hyperaldosteronism).

Syndrome of apparent mineralocorticoid excess (AME)

The conversions of cortisol to cortisone and of corticosterone to 11-deoxycorticosterone are mediated by two isoenzymes of 11β-hydroxy steroid dehydrogenase (11βHSD). Both enzymes have oxidase and reductase activity. AME is an autosomal recessive disorder and results from a loss of function mutation of the gene of 11β-hydroxy steroid dehydrogenase type 2 (11βHSD2), which transforms cortisol to cortisone. AME and the consumption of licorice allow cortisol to occupy the mineralocorticoid receptor, which is normally activated by aldosterone. Since the serum concentration of cortisol is 10 to 100-fold higher than that of aldosterone, a mineralocorticoid effect results, even though aldosterone is suppressed. Affected children may present with hypertension, intrauterine growth retardation, cerebrovascular episodes, nephrogenic diabetes insipidus and rhabdomyolysis during childhood and may develop nephrocalcinosis due to the hypokalemia.

Laboratory findings: hypokalemic alkalosis, low renin, low aldosterone.

Familial glucocorticoid resistance syndrome (FGR)

Inactivating mutations of the glucocorticoid receptor gene (NR3C1) in chromosome 5q31-q32 cause familial glucocorticoid resistance. As a result, there is peripheral resistance to glucocorticoids, which leads to increased ACTH secretion with increased production of cortisol, mineralocorticoids and sex steroids. Clinical symptoms include hypertension, fatigue, and various forms of hyperandrogenism.

Refer to Fig. 31.3-2 – Synthesis of aldosterone, cortisol and androstendione.

Laboratory findings: hyperkalemia, elevated aldosterone, decreased renin, elevated ACTH and cortisol.

Gordon syndrome

Rare familial hypertension syndrome that presents with low renin and hyperkalemia. Mutations in WNK1, WNK4, CUL3 and KLH3 genes have been identified. The genes interact with the thiazide sensitive Na+/Cl co transporter (NCC) in the distal nephron. The co transporter is responsible for Na+ reabsorption.

Mineralocorticoid receptor (MR) activating mutations

The MR activating mutation is secondary to a gain of function mutation, by a substitution of leucine for serine at codon 810 in the MR gene. In males and non-pregnant females, cortisone and 11-dehydrocorticosterone can activate the mutant MR and result in increased absorption of Na+ /12/. Low renin hypertension is worsened in pregnancy because progesterone activates the mutated MR receptor.

Tabelle 31.3-4 Impact of antihypertensive agents on renin concentration and renin activity /1/

Treatment

Renin
level

Renin
activity

Angio-
tensin I

Angio-
tensin II

ACE inhibitor

I 3

I 3

I 3

D 2

Angiotensin
receptor blocker

I 3

I 3

I 3

I 3

Beta blocker

D 2

D 2

D 2

D 2

Calcium channel
blocker

I 1

I 1

I 1

I 1

Thiazide diuretic

I1

I 1

I 1

I 1

Potassium sparing
diuretic

I 2

I 2

I 2

I 2

Direct renin inhibitor

I 3

D 2

D 2

D 2

Legend: I, increase; D, decline; 1= low; 2 = moderate; 3 = significant

Table 31.4-1 Reference intervals for aldosterone

Aldosterone in plasma and serum

Adults (measurement with immunoassay) /8/

Recumbent

29–145 ng/L

80–400 pmol/L

Seated

65–285 ng/L

180–790 pmol/L

Adults (reference method) /9/

Seated

< 25–229 ng/L

< 69.4–635 pmol/L

Neonates and children (recumbent) /8/

Age

ng/L

pmol/L

12 hours

343–1,253

950–3,470

24 hours

217–1,054

600–2,920

2 days

191–1,123

531–3,110

3 days

90–913

250–2,531

4 days

83–921

231–2,550

5 days

72–831

200–2,310

6–31 days

69–812

190–2,250

1–12 months

69–552

190–1,531

1–2 years

61–495

170–1,310

2–6 years

40–271

110–750

6–14 years

31–148

85–410

Aldosterone in urine

Total aldosterone /10/

Adults

3–19 μg/24 h (8–51 nmol/24 h)

1.5–20 μg/g creatinine

Children 4–10 years

1–8 μg/24 h (3–22 nmol/24 h)

4–22 μg/g creatinine

Neonates

0.5–5 μg/24 h (1–14) nmol/24 h

20–140 μg/g creatinine

Free aldosterone (adults) /11/

0.1–0.4 μg/24 h

Aldosterone-18-glucuronide (adults) /12/

3,5–17,5 μg/24 h (6.3–32 nmol/24 h)

Tetrahydroaldosterone (adults) /13/

10–70 μg/24 h (28–190) nmol/24 h

Conversion of aldosterone: ng/L × 2.77 = pmol/L; μg/L × 2.77 = nmol/L

Table 31.4-2 Mineralocorticoid hypertension disorders /14/

Renin producing etiologies

  • Renin secreting tumors
  • Malignant hypertension
  • Renal artery stenosis

Aldosterone-producing etiologies

  • Primary aldosteronism (Conn’s syndrome)
  • Familial aldosteronism types 1, 2 and 3

Mineralocorticoid etiologies without excess aldosterone production

  • Apparent mineralocorticoid excess syndrome
  • Liddle’s syndrome
  • Deoxycorticosterone secreting tumors
  • Ectopic ACTH synthesis
  • Congenital adrenal syndrome

Medications with mineralocorticoid effect

  • Carbenoxolone therapy, licorice

Glucocorticoid-remediable aldosteronism

Table 31.4-3 Case detection of primary aldosteronism /3/

Sustained blood pressure above 150/100 mmHg on each three measurements obtained on different days with hypertension (BP > 140/90 mmHg) resistent to three conventional antihypertensive drugs (including diuretics), or controlled BP (> 140/90 mmHg) on four or more antihypertensive drugs

Hypertension and spontaneous or diuretic-induced hypokalemia

Hypertension and adrenal incidentaloma

Hypertension and sleep apnea

Hypertension and a family history of early hypertension or cerebrovascular events (at age < 40)

All hypertensive first degree relatives with primary aldosteronism

The plasma aldosterone/renin ratio (ARR) is recommended to detect possible cases of primary aldosteronism

Tab. 31.4-4 Primary aldosteronism, mineralocorticoid excess and hypertension /34/

Clinical and laboratory findings

Primary aldosteronism (PA)

PA accounts for up to 10% of hypertensive patients and is the most common cause of secondary hypertension. Its prevalence increases with the severity of hypertension, from 2% in patients with stage 1 hypertension to 20% among resistant hypertensives. Only 9–37% of patients with PA associated hypertension are hypokalemic. In relation to sodium balance and the RAS, PA is associated with an inadequately high aldosterone concentration and low renin levels. A screening test result of ARR > 30 (20) is indicative of PA when renin activity is assayed. Using the test kit of a specific manufacturer (concentration of direct renin in mU/L and aldosterone in ng/L) to determine the ARR, a threshold ≥ 11.5 differentiates:

  • Normotensive patients from PA with a sensitivity of 97.1% at a specificity of 89.9%
  • Essential hypertensives from PA with a sensitivity of 97.1% at a specificity of 84.1%.

To prevent false positive results of ARR, some investigators use a formal aldosterone cutoff of 150 ng/L (416 pmol/L) in patients with lower aldosterone for diagnosing PA. Against this are the findings of several studies which have reported aldosterone concentrations of 70–160 ng/L (250–440 pmol/L) in patients with PA and an elevated ARR /15/.

Aldosterone-producing adenoma (APA)

APA is associated with moderate to severe hypertension and is responsible for two thirds of cases with severe secondary hypertension. Tumors with a diameter > 2.5 cm have a higher tendency to malignancy. Patients are usually less than 40 years of age.

Laboratory findings: serum K+ is decreased in one-third of cases, and there is alkalosis. The ARR is increased. Alternative to the ARR other markers are recommended. One study /16/ reported 24-h urine excretion of 18-OHF of 725 ± 451 nmol, 102 ± 68 nmol and 88 ± 76 nmol in patients with APA, bilateral adrenal hyperplasia (BAH) and essential hypertension (EH), respectively. APA can be differentiated by measuring serum 18-OHB. In one study, the levels (at rest) for APA, BAH and EH were 1090 (792–1324) ng/L, 654 (513–835) ng/L and 567 (462–695) ng/L, respectively /16/. For adenoma localization and differentiation of unilateral from bilateral adenoma, aldosterone and cortisol are measured from adrenal vein blood samples.

Idiopathic hyperaldosteronism (IHA), bilateral adrenal hyperplasia (BAH)

Unilateral adrenal hyperplasia is biochemically very similar to APA. Adrenal vein catheterization findings are similar to those in APA, but aldosterone levels are generally lower. Adrenal imaging shows no evidence of a tumor.

BAH is more common than unilateral adrenal hyperplasia. BAH is associated with mild to moderate hypertension and is found in the majority of patients newly diagnosed with hyperaldosteronism. Patients are often > 40 years of age. The hyperplasia may be smooth, micro nodular or macro nodular. Surgical therapy is not indicated. BHA is treated conservatively using mineralocorticoid receptor antagonists.

Laboratory findings: normokalemia, alkalosis, elevated aldosterone, decreased renin, normal 18-OHF. In one study /17/, APA could be clearly differentiated from BAH by measuring 18-OHF in 24 hour urine samples. Excretions were 407 (290–435) μg/24 h in APA, but only 160 (106–258) μg/24 h in BAH (values expressed as medians and 25th and 75th percentiles).

Familial hyperaldosteronism type I (FH-I), glucocorticoid remediable aldosteronism (GRA)

In FH-I, also known as glucocorticoid-remediable aldosteronism (GRA), aldosterone secretion can be suppressed by dexamethasone. GRA is an autosomal dominant inherited form of mineralocorticoid hypertension which accounts for less than 1% of all cases of primary aldosteronism and is correctable by a small dose of glucocorticoid. Aldosterone is moderately overproduced and regulated by ACTH rather than by the RAS. Clinical manifestations are variable. Some patients are normotensive, others have early hypertension with elevated aldosterone and low renin. FH-I has a high prevalence among young adults with severe resistant hypertension or a family history of early stroke.

FH-I is caused by a chimeric gene duplication resulting from an unequal crossing over between the highly homologous 11β-hydoxylase and aldosterone genes, such that the chimeric gene represents a fusion of the 11β-hydoxylase gene (normally expressed in the cortisol producing zona fasciculata of the adrenal cortex) and the 3’ coding sequences of the aldosterone synthase gene (normally expressed in the aldosterone-producing zona glomerulosa). The fusion results in the ectopic over expression of the chimeric Zona fasciculata gene, which is under the control of ACTH /4/.

Laboratory findings: hypokalemia, normal or elevated aldosterone, low renin, increased aldosterone/renin ratio, plasma aldosterone can be suppressed to less than 40 ng/L by dexamethasone. 18-OHF levels are 10 to 20 times higher than normal, e.g. 759 (476–1167) μg/24 h /1718/.

Familial hyperaldosteronism type II (FH-II)

FH-II is a familial genetic disorder. Unlike FH-I, aldosterone is not suppressible by dexamethasone. Approximately 7% of primary aldosteronism patients have FH-II. Children of these patients have adenoma or idiopathic hyperaldosteronism.

Familial hyperaldosteronism type III (FH-III)

FH-III is characterized by severe hypertension in early childhood and causes severe end organ damage. The adrenal glands are greatly enlarged and show diffuse hyperplasia of the zona fasciculata and atrophy of the zona glomerulosa.

Laboratory findings: hypokalemia, marked hyperaldosteronism, significant increase in adrenal corticosteroids (10–1,000 fold). Paradoxical increase in aldosterone in the dexamethasone test.

Table 31.4-5 Medications to be discontinued prior to testing for mineralocorticoid induced hypertension /3/

Two weeks prior: β-blockers, ACE inhibitors, angiotensin receptor blockers, renin inhibitors, calcium antagonists of the dihydropyridine type, central α2-antagonists.

Four weeks prior: spironolactone, eplerenone, amiloride, triamterene, loop diuretics.

Dis continuation not possible: if dis continuation is not possible for medical reasons, at least spironolactone and eplerenone should be stopped.

Direct renin inhibitors, amiloride, peripheral α-blockers (doxazosin) and calcium antagonists other than those of the dihydroperidine type (verampamil) may be continued.

Influence of medications: a renin activity level below 1 ng/mL/h during treatment with ACE inhibitors or angiotensin receptor blockers is highly suggestive of primary aldosteronism.

Amlodipine and irbesartan can lead to false negative results in 1.8% and 23.5% of cases, respectively.

Even if renin secretion is suppressed by β-blockers and central α2-antagonists (both are adrenergic inhibitors), aldosterone secretion is also suppressed and the aldosterone/renin ratio remains relatively unaffected.

Table 31.4-6 Aldosterone/renin ratio (ARR) cutoffs depending on the renin assay based on renin activity or renin concentration /3/

Aldosterone

Renin activity
(ng/mL/h)

Renin activity
(pmol/L/min.)

Renin conc.
(mU/L)

Renin conc.
(ng/L)

ng/dL

20

1.6

2.4

3.8

30

2.5

3.7

5.7

40

3.1

4.9

7.7

pmol/L

750

60

91

144

1,000

80

122

192

Values shown are on the basis of a conversion factor of 8.2 for renin activity assay to renin immunoassay. The most common cutoffs are 30 for conventional units and 750 for SI units.

Table 31.4-7 Primary aldosteronism: confirmatory tests /3/

Test

Clinical and laboratory findings

Oral saline loading tests, saline infusion test

Principle: sodium chloride loading suppresses aldosterone synthesis. This is not the case in adenoma.

Oral saline loading test

Patients should increase their sodium intake to over 200 mmol/d (approximately 6 g/d) for 3 days, verified by 24-h urine Na+ content. Patients should receive adequate slow-release potassium chloride supplementation to maintain plasma K+ in the normal range. Urinary aldosterone is measured in 24 hour urine collection from the morning of day 3 to the morning of day 4.

Clinical significance: primary aldosteronism is unlikely if aldosterone-18-glucuronide excretion is lower than 10 μg/24 h (< 28 nmol/24 h). Elevated excretion above 12 μg/24 h (> 33 nmol/24 h) confirms primary aldosteronism.

Saline infusion test 

Two liters of physiologic NaCl are infused over 4 hours, starting between 8 a.m. and 9.30 a.m. Blood is sampled before and after the NaCl infusion to determine aldosterone and K+. During the infusion, the heart rate and blood pressure are monitored at regular intervals.

Clinical significance: post infusion plasma aldosterone concentrations below 5 ng/dL (140 pmol/L) make the diagnosis of primary aldosteronism unlikely, whereas levels higher than 10 ng/dL (280 pmol/L) are a sign of very probable primary aldosteronism.

Restrictions: the sodium loading tests should not be performed in patients with severe, uncontrollable hypertension, renal failure, heart failure, arrhythmia, and severe hypokalemia. In renal failure, aldosterone excretion is impaired.

Captopril challenge test

Principle: captopril normally suppresses aldosterone secretion.

Procedure: patients receive 25–50 mg of captopril orally after sitting or standing for at least 1 hour. Blood samples are drawn for measurement of plasma renin and plasma aldosterone, and cortisol at time zero and at 1 or 2 h after challenge, with the patient remaining seated during this period.

Clinical significance: plasma aldosterone is normally suppressed by captopril (over 30%). In patients with primary aldosteronism, aldosterone remains elevated and renin remains suppressed. Differences may be seen between patients with adrenal adenoma and those with idiopathic hyperaldosteronism (IHA), in that some decrease of aldosterone levels is occasionally seen in IHA.

Fludrocortisone suppression test

Principle: in the fludrocortisone suppression test, the response of plasma aldosterone to a Na+ retaining steroid is tested. The fludrocortisone test is the most sensitive functional test for the diagnosis of primary aldosteronism and is considered the gold standard.

Procedure: patients receive 0.1 mg fludrocortisone oral every 6 h for 4 days together with slow release KCl supplements (every 6 h at doses sufficient to keep plasma K+, measured four times a day, close to 4.0 mmol/L). Patients should be given slow release NaCl supplements, 30 mmol (1.75 g table salt) 3 times daily, and sufficient dietary salt to maintain a urinary Na+ excretion rate of at least 3 mmol/kg body weight over 200 mmol/d). On day 4, blood is collected for plasma aldosterone and renin measurement at 10 a.m. with the patient in the seated position, and blood is collected for plasma cortisol measurement at 7 a.m. and 10 a.m.

Clinical significance: upright aldosterone level above 6 ng/dL (110 nmol/L) on day 4 at 10 a.m. confirms primary aldosteronism, provided that plasma renin is suppressed to less than 1 ng/mL/h and the cortisol level at 10 a.m. is lower than the value obtained at 7 a. m. (to excludes a confounding ACTH effect).

Table 31.4-8 Adrenal venous sampling (AVS) test /13/

Procedure and results

The two main forms of primary aldosteronism are either aldosterone producing adenoma (APA), best treated by adrenalectomy or bilateral adrenal hyperplasia (BAH) treated with mineralocorticoid receptor antagonists. Adrenal venous sampling is used to differentiate unilateral from bilateral sources of excess aldosterone. The criteria used to determine the lateralization of aldosterone hypersecretion depend on whether the sampling is done under cosyntropin (a synthetic derivative of ACTH) administration. Before the procedure, medications should be withdrawn for at least 3 weeks, 6 weeks for mineralocorticoid receptor antagonists. Hypokalemia should be corrected with oral or intravenous potassium before AVS, because hypokalemia may lead to a decrease in aldosterone secretion and potentially mask a unilateral APA. Variable techniques of the adrenal vein sampling test are used:

  • Simultaneous or sequential bilateral catheterization without stimulation
  • Unstimulated sequential or simultaneous bilateral catheterization followed by bolus cosyntropin stimulated sequential or simultaneous bilateral AVS (250 ug of cosyntropin)
  • Continuous cosyntropin stimulation (infusion of 50 ug/h, 30 min before catheterization and during the procedure) with sequential bilateral AVS.

Principle

A standard AVS procedure depends on comparisons of plasma aldosterone in each adrenal vein, normalized to concentrations of cortisol, to establish presence or absence of asymmetric aldosterone secretion. Important parameters in the diagnostic performance of the AVS are:

  • The selectivity index (SI): the index allows to asses the selectivity of adrenal vein catheterization (e.g., the ratio between the cortisol concentration in the adrenal vein and in the inferior vena cava).
  • The lateralization index determines whether a lateralized aldosterone excess exists (e.g., aldosterone/cortisol on the dominant side related to aldosterone/cortisol on the non dominant side). The plasma concentrations of aldosterone from each side are divided by their respective cortisol concentration in order to avoid dilution effects.

Procedure

  • The adrenal veins are catheterized via the percutaneous femoral vein approach and the position of the catheder tip is verified by radiography after injecting an amount of non ionic contrast medium
  • Blood is collected from both adrenal veins. The peripheral sample should come from a cubital or iliac vein. The samples are used for the determination of aldosterone and cortisol.
  • The venous sample from the left side is obtained with the catheder tip at the junction of the inferior phrenic and left adrenal vein.
  • The right adrenal vein is difficult to cathederize, because it is short and enters the inferior vena cava at an acute angle.

Results

Selectivity Index

The adrenal/peripheral vein cortisol ratio is typically more than 2 : 1 without cosyntropin use and more than 5 : 1 with the continuous cosyntropin infusion.

Lateralization index

  • Some investigators consider a cortisol corrected aldosterone lateralization ratio of more than 2 : 1 in the absence of cosyntropin stimulation as consistent with unilateral disease (APA or unilateral aldosteronism).
  • With continuous cosyntropin administration a cutoff of the cortisol corrected aldosterone ratio from high side to low side of more than 4 : 1 indicates unilateral aldosterone excess (diagnostic sensitivity 95% and specificity 100%) /19/. A ratio of below 3 : 1 is suggestive of BAH. Patients with ratios between 3 : 1 and 4 : 1 may have either unilateral or bilateral disease.
  • Other groups rely primarily on comparing the aldosterone/cortisol ratios to those in a simultaneously collected venous sample. When the aldosterone/cortisol ratio from an adrenal vein is at least 2.5 times greater than that of the peripheral vein, and the aldosterone/cortisol ratio in the contralateral adrenal vein is not higher than peripheral vein, this is interpreted as showing lateralization.

Table 31.4-9 Secondary aldosteronism, mineralocorticoid excess and hypertension

Clinical and laboratory findings

Secondary aldosteronism

Secondary aldosteronism occurs in conditions with low effective arterial blood volume, resulting in activation of the RAS with an increase in aldosterone and increased distal renal tubular reabsorption of Na+ to restore normal blood volume. The Na+ retaining effect causes increased excretion of K+. The loss of K+ in the presence of normal kidney function is always caused by the presence of hyperaldosteronism, even if serum K+ is reduced /5/.

Laboratory findings: secondary aldosteronism leads to hypokalemia and elevated renin and aldosterone levels.

Pregnancy

In pregnancy, the secondary aldosteronism maintains volume expansion. Primarily, there is vasodilation, resulting in reduced blood pressure, increased cardiac output, non osmotic stimulation of the thirst mechanism, and activation of the RAS /5/. Renin levels are about two times higher than before pregnancy at 8 weeks and four times higher at 20 weeks’ gestation. Plasma aldosterone levels increase from 62 ± 11 ng/L at baseline to 164 ± 47 ng/L at 8 weeks and then remain stable before reaching a peak of 594 ± 139 ng/L in the last trimester /20/. There is a direct correlation between the concentrations of aldosterone, progesterone and estradiol. The effect of the hyperaldosteronism on the blood pressure and the K+ balance appears to be compensated by other hormones and hemodynamic mechanisms.

Laboratory findings: aldosterone and renin are mildly to moderately elevated.

Essential hypertension

Essential hypertensives with long term use of diuretics may develop demonstrable secondary aldosteronism as a result of Na+ deficiency. Some patients with essential hypertension and an increased aldosterone/renin ratio have increased aldosterone synthase (CYP11B2) activity due to gain-of-function mutants in the form of a biallelic gene polymorphism of CYP11B2. This gene polymorphism predicts an increased aldosterone/renin ratio in patients with essential hypertension /21/.

Malignant hypertension

These patients have Na+ and volume depletion and in many there is an inverse relationship between the extent of reduction of the extracellular volume and the hypertension /5/. There is renal loss of Na+ and water despite marked hyperaldosteronism. A vicious circle of volume depletion, renal ischemia and increasing renin and aldosterone production develops, since the short regulatory circuit for the inhibition of renin secretion by angiotensin II no longer works.

Laboratory findings: hypokalemia, elevated renin and aldosterone.

Renal artery stenosis

Renal artery stenosis is defined as a narrowing of one or both renal arteries or their branches is most commonly caused by atherosclerosis. The reported prevalence is 0.5% in the Medicare population and 5.5% in patients with chronic kidney disease. Atherosclerotic renal artery stenosis is a common cause of secondary hypertension /521/.

The hypertension results from renal ischemia caused by intrinsic or extrinsic narrowing of one or both renal arteries or their segmental branches or is due to fibromuscular dysplasia. The hypertension develops relatively quickly and is often resistant to anti-hypertensives. Patients with renal artery stenosis have increased rates of chronic kidney disease (25% vs. 2% in patients without renal artery stenosis), coronary artery disease (67% vs. 25%), stroke (37% vs. 12%), and peripheral vascular disease (56% vs. 13%). Patients are usually over 50 years of age and have a blood pressure above 160/100 mmHg /22/.

Unilateral renal artery stenosis: the Na+ retention in unilateral renal artery stenosis depends on the function of the kidney with non stenosed artery. The increased production of angiotensin II in the latter causes vasoconstriction with reduction of the glomerular filtration rate and reduced proximal Na+ reabsorption. This leads to pressure diuresis with an increase in Na+ delivered to the distal convoluted tubule and increased Na+ excretion in the urine. As a result, the natriuretic response by the kidney with the non stenosed artery to changes in the Na+ supply will be inadequate. In the kidney with the stenosed artery, the pressure distal to the stenose remains reduced, which is a continuous stimulus for renin secretion.

Bilateral renal artery stenosis: initially, the increased secretion of renin occurs in response to reduced renal perfusion, similar to unilateral artery stenosis. The increased renin secretion leads to Na+ retention, volume expansion, restoration of normal post stenotic perfusion and thus a reduction in the release of renin.

Laboratory findings: hypokalemia is found in 10–20% of patients. Baseline plasma aldosterone is only moderately elevated and renin may also still be normal. One hour after the captopril test, renin activity increases by a factor of 4.2 ± 0.6 while in essential hypertension it increases only by a factor of 0.3 . The reported diagnostic sensitivity of the captopril test is 100%, with a specificity of 78% /21/.

Chronic kidney disease

Patients with chronic kidney disease may develop mild hyperaldosteronism with hypertension, especially during the final stage of the disease. The hyperaldosteronism is either due to stimulation of the RAS by reduced excretion of Na+ or due to hyperkalemia, which is the driving force for aldosterone release and begins to develop when the glomerular filtration rate is below 20 [ml × min–1 × (1.73 m2)–1]. Since aldosterone causes increased intestinal excretion of K+ in these patients, many of them will not develop hyperkalemia if, additionally, they are on a diet restricted in K+ /5/.

Renin producing tumors

Renin producing tumors are responsible for primary hyperreninemia, a rare form of hypertension which mainly occurs in renal tumors such as renal cell carcinoma, hemangiopericytoma, and Wilms’ tumor. Secondary hyperreninemia is seen in patients with ovarian Sertoli cell tumors or pheochromocytoma. In this tumor, increased catecholamine secretion stimulates the juxtaglomerular cells to secrete renin /5/.

Laboratory findings: since the tumors produce renin and prorenin, a high prorenin/renin ratio can help differentiate the tumors from essential hypertension. However, renin levels are generally very high and above 50 ng/mL/h /5/. Patients have relatively constant hypokalemia, which can be less than 2.0 mmol/L.

Polyarteritis nodosa

These patients have hypertension caused by increased renin secretion due to narrowing of the renal arteries with consecutive renal ischemia. The renin producing cells of the juxtaglomerular apparatus are hyperplastic.

Laboratory findings: hypokalemia, mildly elevated renin and aldosterone.

Bartter syndrome

Rare congenital disorder characterized by hyperreninemic hyperaldosteronism, metabolic alkalosis and hypokalemia. Bartter syndrome usually manifests before the age of 25 with clinical symptoms such as myasthenia and polyuria. Despite the hyperaldosteronism, there is no hypertension /23/.

Laboratory findings: hypokalemia, metabolic alkalosis, hypomagnesemia, hyponatremia, hypochloremia, hyperreninemia, hyperaldosteronemia, hypocalcuria, abnormal platelet aggregation.

Iatrogenic secondary aldosteronism

Surgical procedures during which gastrointestinal tissue is used and resected can lead to disorders with secondary aldosteronism. This is the case, for example, in colon resection, if more than half of the colon is removed, resulting in the enteral loss of Na+. Ileostomates have a high stoma related loss of Na+, which leads to Na+ depletion with hyperaldosteronism and a Na+ excretion in spot urine of less than 20 mmol/L. Similar abnormalities are seen in patients who have had a cystectomy and ureterocolostomy /5/.

Cyclosporine therapy

Depending on the dose, cyclosporine therapy can cause nephrotoxicity with reduction of the renal blood flow, the glomerular filtration rate and renal Na+ excretion. This leads to constriction of the afferent glomerular arterioles with an increase in the vascular resistance, and hypertension. Due to the reduced renal excretion of Na+, the RAS is activated /5/.

Figure 31.2-1 Renin-angiotensin-aldosterone (RAS) system.

Fall in plasmasodium, decrease in renal blood flow Juxtaglomerularcells Renin Angiotensinogen Angiotensin I Angiotensin II Sodiumreabsorption Aldosterone Vaso-constriction Adrenalreceptors

Figure 31.2-2 The role of the RAS in maintaining blood pressure and Na+ balance in the case of decreased sodium chloride (table salt) intake. The kidney synthesizes renin; angiotensin II, which is produced in subsequent reaction steps, causes elevated blood pressure by arteriolar vasoconstriction and the aldosterone stimulated retention of Na+.

Angiotensin I (high) Angiotensinogen Angiotensin II (high) Renin high Low Blood Pressure Distal tubule low NaCl Low effective blood volume Elevation of blood pressure Aldosterone high High effective blood volume Vasoconstriction Low Na-Absorption Renal sodium retention

Figure 31.3-1 Different conformations of renin and prorenin, and specificities of the different activity assays and assays measuring the concentration (e.g., immunoassays). Assays 1 and 2 are commonly used in routine diagnostics. Modified from Ref. /2/.

Active prorenin(Pro segmentopenconfirmation) Renin Inactive prorenin(Pro segmentclosedconfirmation) Immunoassay: Total renin concentration(Renin + Prorenin) Activity: Total renin (Renin + Prorenin) Activity:Renin + active prorenin Concentration (Immuno-assay): Renin + activeprorenin Bindingsite 1. 2. 3. 4.

Figure 31.3-2 Aldosterone is synthesized from cholesterol, as are the other adrenal steroids. The enzymes catalyzing each of the synthetic reactions are printed in italics. The dotted arrows indicate those biochemical transformations that are catalyzed by each of the corresponding enzymes. AS, aldosterone synthase (P450aldo, CYP11B2). The steroid molecule depicted in the right lower portion shows the biologically important positions on the steroid molecule.

Cholesterol desmolase(CYP11A) 21-Hydroxylase(CYP21) 3β-Hydroxysteroiddeydrogenase Cholesterol Pregnenolone Progesterone 18-Hydroxylase(CYP11B2) 18-OH Corticosterone Aldosterone 17α-Hydroxylase (CYP17) 11β-Hydroxylase(CYP11B2) 17-OH pregnenolone 11-Desoxycortisol Cortisol Dehydroepiandrosterone Androstenedione Deoxycorticosterone 18-Oxidase(CYP11B2) Corticosterone 17,20-Lyase 17-OH progesterone 11β-Hydroxylase(CYP11B1) 17β-Hydroxysteroiddehydrogenase 5α-Reductase Testosterone Dihydrotestosterone O CH 3 3 11 HO 17 CH 2 HC = O 21 CH 2 OH O CH 3 3 17 CH 3 OH O CH 3 3 11 HO CH HC = O CH 2 OH 21 O 18 O CH 3 CH 3 HC CH 3 CH 2 CH 2 CH 2 CH CH 3 CH 3

Figure 31.4-1 Diagnostic approach for suspected primary aldosteronism (PA) and possible diagnoses. Modified from Ref. /3/. A positive aldosterone/renin ratio (ARR) must be confirmed by functional tests. Adrenal venous sampling (AVS) is used to identify the subtype of PA. PAC, concentration of aldosterone in plasma; CT, computer tomography; MR, mineralocorticoid receptor.

Patients with hypertension that are at increased risk for PA Primary diagnostics: ARR pos. K + Renin ↓↓↓ PAC > 20 ng/dL No need for confirmatory testing Marked PA, young age, pos. CT AVS PA unlikely Neg. Confirmatory testing: pos. PA unlikely unilateral bilateral If surgery not desired If surgerydesired Neg. Adrenal CT: pos. Treat with MR antagonist Treatment with laparoscopic adrenalectomy + additional results

Figure 31.4-2 Isomeric forms of aldosterone in plasma.

CH2OH C O O O HO C CH2OH C O O O HO C

Figure 31.4-3 Conjugation of aldosterone to aldosterone 18-glucuronide and tetrahydroaldosterone 3-glucuronide. Both forms are excreted via the kidneys.

CH2OH C O O O HO Aldosterone Aldosterone 18-glucuronide CH CH2OH C O O O O Gluc CH Tetrahydroaldosterone 3-glucuronide CH2OH C O O O HO CH Gluc
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