Aldosterone Synthesis

Aldosterone biosynthesis occurs at the zona glomerulosa, the outermost layer of the adrenal cortex, via the action of several enzymes: cholesterol desmolase [also known as cholesterol side-chain cleavage enzyme] (CYP11A1), 3β-hydroxysteroid dehydrogenase (HSD3B2), 21-hydroxylase (CYP21A2), 11-hydroxylase (CYP11B1) and aldosterone synthase (CYP11B2) (Figure1). The first four enzymes are also expressed in the zona fasciculata and are involved in cortisol biosynthesis. Defects in any of these enzymes may lead to combined aldosterone/cortisol deficiencies as part of the syndromes seen in Congenital Adrenal Hyperplasia. Aldosterone synthase encoded by CYP11B2, the last enzymatic step in aldosterone biosynthesis, is expressed only at the zona glomerulosa and genetic defects in this gene result in isolated aldosterone deficiency (Figure 1).

Aldosterone synthesis involves two steps. The first includes the 18-hydroxylation of corticosterone to form 18-hydroxycorticosterone (18OH corticosterone) and the second is the 18-oxidation of 18OH corticosterone to form aldosterone. Although it was previously considered that these two steps are catalyzed by two different enzymes, it is now known to involve the same enzyme, aldosterone synthase (5). Based on the two final steps in aldosterone synthesis, two subtypes of aldosterone synthase deficiency (ASD) have been described; however, with further clarification of the enzymatic process this is now thought to be an overlapping clinical spectrum, depending on the degree of enzyme deficiency (5).

Aldosterone Regulation and Action

Serum potassium concentrations and the Renin, Angiotensin, Aldosterone (RAA) axis are the main regulators of aldosterone synthesis. Hyperkalemia has a direct stimulating effect independent of RAA axis (2). The RAA axis is a feedback loop that regulates sodium, potassium, water, fluid volume, and blood pressure (2). The cells in the macula densa of the juxtaglomerular apparatus are triggered to release renin in response to a drop in perfusion. Angiotensinogen is a protein produced from the liver that is cleaved to angiotensin I (Ang I) by renin (2). Angiotensin-converting enzyme (ACE) in vascular endothelium rapidly converts Ang I to Angiotensin II (Ang II). Ang II is the most potent stimulus for aldosterone production and release (2). Of note, tissue and plasma peptidase inactivate angiotensin within minutes and circulating renin levels are the rate-limiting factor of this process (1).

Aldosterone mediates its effects by binding to the mineralocorticoid receptor (MR; aka NR3C2) at the distal convoluted tubules and collecting duct epithelial cells of the kidneys (Figure 2). The MR is a member of the nuclear receptor family, and along with the glucocorticoid and androgen receptors, forms the steroid receptor subfamily. In its unliganded state, the MR is located in the cytoplasm. Upon binding with its ligand, MR is translocated into the nucleus, where it modulates the transcription of several genes, such as those that encode the ENaC subunits (1). Mutations that inactivate the MR result in aldosterone resistance or pseudo-hypoaldosteronism type 1 (PHA1).

Aldosterone, 11-deoxycorticosterone (DOC), and cortisol are all endogenous agonists of the MR. Specifically, cortisol and aldosterone have an equal affinity for the mineralocorticoid receptor (2); however, selectivity of MR receptor for aldosterone is ensured in epithelial target tissues by 11βHSD2 enzyme that converts active cortisol to inactive cortisone (1) (Figure 1). This is of particular importance as cortisol circulates at concentrations 100 to 1,000-fold higher than aldosterone. Loss-of-function mutations of the kidney 11βHSD2 result in excessive cortisol-dependent MR activation and cause an autosomal recessive form of familial hypertension called apparent mineralocorticoid excess (6).

After binding to MR, aldosterone activates ENaC gene transcription, decreases ENaC degradation, and activates Na-K ATPase pump. ENaC, located at the apical membrane of epithelial cells, plays a crucial role in sodium reabsorption, potassium secretion, and subsequent volume expansion. ENaC consists of 3 subunits (a, b, and g) that are encoded by unique genes (SCNN1A, SCNN1B, SCNN1G, respectively) (1). Defects in these genes can impair ENaC function and lead also to aldosterone resistance or pseudo hypoaldosteronism type Ib. In addition to the epithelial cells of the distal convoluted tubule, ENaC is expressed at the epithelial cells of other tissues that are involved in salt conservation, such as colon, sweat glands, and lungs. Dysfunction of ENaC, therefore, has systemic manifestations from muti-organ water and salt loss.

Figure 2.

Physiology of aldosterone secretion and action. The figure demonstrates the renin-angiotensin-aldosterone (RAA) system and its effects on sodium and potassium homeostasis, and blood pressure. Aldosterone secretion is regulated by decreased blood volume and hyponatremia via activation of the RAA axis, and indirectly, by hyperkalemia. Aldosterone then binds to the mineralocorticoid receptor (MR; aka NR3C2) at the distal convoluted tubules and collecting duct of the kidneys. Upon binding with aldosterone, MR translocate into the nucleus, where it modulates the transcription of the genes that encode the epithelial sodium channel (ENaC). ENaC is a sodium-selective ion channel that plays a crucial role in sodium reabsorption. Aldosterone action results in urinary potassium excretion and sodium reabsorption, and thus, increased blood volume.

Aldosterone Secretion in the Newborn

There are limited studies in infants investigating the interaction between water, sodium, and the renin-angiotensin-aldosterone system. Various changes related to water turnover, sodium metabolism, and kidney adaptation to extrauterine life occur in the neonatal period (1). The immediate postnatal phase in the first week of life is characterized by oliguria followed by a diuretic phase with extracellular contraction and net loss of sodium and water (1). Maximum weight loss occurs during this period (up to 10% of birth weight is considered normal). Kidneys in the neonate exhibit tubular immaturity resulting in sodium wasting and impaired ability to reabsorb water (1). Additionally, aldosterone and renin concentrations are higher in the newborn period, whereas expression of renal MR is reduced, leading to transient renal resistance to aldosterone (7). In very preterm infants, there is decreased activity of 11β-hydroxylase (CYP11B1) and low aldosterone synthase (CYP11B2) activity, possibly due to immaturity of these enzymes in the fetal adrenal cortex, leading to deficient aldosterone secretion (8, 9). After the first week of life, water losses decrease, and positive sodium balance is important for growth (1). It is essential to acknowledge these physiologic changes when evaluating mineralocorticoid function in the neonatal period.

Defective Aldosterone Synthesis

This refers to hyperreninemic hypoaldosteronism in which the renin production is intact, and the defect is at the level of the adrenal gland. The etiology of defective aldosterone synthesis can be separated into congenital and acquired causes. It is important to note that aldosterone deficiency due to defect in synthesis can be the first presenting sign of adrenal cortex failure and later progress to involve insufficient cortisol production. For descriptions of disorders involving adrenal cortical failure such as congenital adrenal hyperplasia and Addison’s disease, see Endotext.org chapter: Adrenal Insufficiency in Children (3).

Aldosterone Resistance

This refers to impaired action of aldosterone at the level of the target tissue and can further be categorized into congenital and acquired causes.

Diminished Stimulation

Decreased renin or angiotensin II results in decreased aldosterone production due to diminished adrenal stimulation. When this hyporeninemic hypoaldosteronism occurs with hyperchloremia and non-anion gap metabolic acidosis, it is called Type 4 RTA. In adults, this is most often associated with nephropathy (diabetes, autonomic neuropathy, sickle cell disease, HIV, SLE) and medications (beta blockers, ACE inhibitors) (26). In children, Gordon Syndrome (Familial Hyperkalemic hypertension or pseudo-hypoaldosteronism type II [PHA2]) is rare and associated with low renin/low aldosterone (or inappropriately normal for degree of hyperkalemia) state with normal glomerular filtration. This is thought to be due to abnormal thiazide-sensitive sodium-chloride co-transporter in the distal nephron (mutations in WNK1, WNK4, CUL3, or KLHL3 genes) (27). The increased sodium and chloride reabsorption leads to hypertension, volume expansion, and decreased potassium and hydrogen excretion resulting in hyperkalemia and metabolic acidosis. In contrast to PHA1, PHA2 does have a biochemical profile that aligns with Type IV RTA including hyponatremic, hyperkalemic, and hyperchloremic non-anion gap metabolic acidosis (28). These causes of defective aldosterone stimulation are rare in the pediatric population, so when identified, affected children should be referred to the appropriate subspecialities such as nephrology for evaluation and treatment. Given the rare nature and lack of a primary endocrine etiology, these causes are not reviewed in the table below.

Differential Diagnosis and Laboratory Evaluation

The first step in the evaluation of a child with suspected mineralocorticoid deficiency is to determine whether there is associated adrenal insufficiency (figure 3). The evaluation for adrenal insufficiency includes measurement of serum cortisol (ideally morning level depending on the clinical scenario and age of patient), ACTH, 17-hydroxyprogesterone (17OHP), and possible provocative testing (ACTH stimulation test). Plasma renin activity and serum aldosterone should be measured to evaluate for mineralocorticoid deficiency. If there is global adrenal dysfunction resulting in both cortisol and aldosterone deficiency, the differential diagnosis can be narrowed to causes of primary adrenal insufficiency (see Endotext: Adrenal Insufficiency in Children) (3). It is critical to identify primary adrenal insufficiency, especially in infants, and promptly treat with hydrocortisone to avoid adrenal crisis.

If an isolated aldosterone defect is considered, the second step is to evaluate whether the defect is at the level of the adrenal glands or kidneys. High renin and low aldosterone points to a defect at the level of the adrenal glands (defective aldosterone synthesis). High renin and high aldosterone points to a defect at the level of the kidneys causing resistance to aldosterone. The various causes of aldosterone resistance are detailed above in the section “Etiology”. Briefly, these include congenital (mutations in MR or ENaC channel) and acquired causes (medications, transient resistance in the setting of UTI, or renal tubular dysfunction). Low renin and low aldosterone states do not commonly occur in children, as they are often the consequence of chronic illness causing type IV RTA (i.e. in adults with diabetic nephropathy); however, there is also a genetic form, Gordon Syndrome or pseudo-hypoaldosteronism type 2 which is characterized by hypertension, hyperkalemia, and metabolic acidosis.

As stated above, measurement of renin and aldosterone at the time of hyponatremia and hyperkalemia are important biochemical markers to differentiate the etiology of hypoaldosteronism. Furthermore, if there is suspicion for aldosterone synthase deficiency, corticosterone and 18-hydroxycorticosterone measurements can be useful (see figure 1). Values need to be interpreted according to age of the patient. Hemolyzed lblood may result in a falsely elevated potassium level and must be repeated to ensure accuracy of test values.

Figure 3.

A proposed approach in the differential and diagnostic evaluation of children with suspected aldosterone deficiency.

Genetic Testing

In addition to biochemical evaluation, genetic testing is an invaluable tool to help guide treatment and prognosis, especially in infanta and children where the clinical manifestations of aldosterone defects vary widely (29). Genetic testing including whole exome sequencing or gene panels (for pseudo-hypoaldosteronism) may clarify the diagnosis, treatment, and prognosis. Genes associated with hypoaldosteronism and pseudo-hypoaldosteronism include CYP11B2, NR3C2, SCNN1A, SCNN1B, SCNN1G, WNK1, WNK4, CUL3, KLHL3 (2).

Primary Hypoaldosteronism

Children with primary hypoaldosteronism (including those with adrenal insufficiency such as Addison’s Disease or CAH) should start mineralocorticoid replacement (fludrocortisone 0.05-0.2 mg/day). Infants and young children usually need higher doses of fludrocortisone in addition to sodium chloride supplementation due to renal resistance and general diet that is lower in salt. Sodium chloride is weaned over time as renin activity normalizes, and salt is incorporated into the diet. Fludrocortisone is continued for mineralocorticoid replacement and titrated based on normalization of blood pressure, electrolytes, and renin levels.

Aldosterone Resistance

Children with autosomal recessive (PHA1b) and autosomal dominant (PHA1a) pseudo-hypoaldosteronism are usually treated with high dose sodium chloride supplementation. Those who have PHA1a (mild form only affecting the kidneys) usually need lower doses of salt supplementation with gradual clinical improvement (typically no need for salt supplementation by 1-3 years of age) (30). Infants and children with the severe/systemic form (PHA1b) are more difficult to manage given the need for higher doses of salt supplementation, potassium lowering agents, and potential for recurrent pulmonary infections (31). Some of these children might need gastrostomy tubes to allow for consistent high dose salt supplementation which is not always tolerated by mouth. Sodium bicarbonate is another medication used to improve metabolic acidosis which can impact growth and development if acidosis persists. Given the rarity of pseudo-hypoaldosteronism, the doses of sodium chloride and sodium bicarbonate are not well established and must be titrated based on serum sodium and bicarbonate concentrations.