Aldosterone

1. Overview

Aldosterone is a steroid hormone that plays a vital role in regulating blood pressure, fluid balance, and electrolyte levels in the body. It belongs to the class of hormones known as mineralocorticoids and is the primary mineralocorticoid produced by the adrenal cortex. Aldosterone acts mainly on the kidneys to promote sodium retention, potassium excretion, and water reabsorption, thereby influencing blood volume and systemic arterial pressure.

2. Location

Aldosterone is synthesized and secreted by the zona glomerulosa, the outermost layer of the adrenal cortex. The adrenal glands are located atop each kidney in the retroperitoneal space. Its secretion is primarily regulated by the renin-angiotensin-aldosterone system (RAAS), along with influences from serum potassium levels and adrenocorticotropic hormone (ACTH).

3. Structure

Aldosterone is a steroid hormone derived from cholesterol. It has a molecular formula of C21H28O5 and a molecular weight of approximately 360.4 g/mol. Structurally, it is characterized by a cyclopentanoperhydrophenanthrene nucleus typical of steroids, with hydroxyl and aldehyde functional groups that contribute to its biological activity. Aldosterone is lipid-soluble, allowing it to pass through cell membranes and bind to intracellular receptors.

4. Function

Aldosterone’s primary function is to regulate electrolyte and fluid balance, especially in the distal nephron of the kidney:

• Increases sodium reabsorption in the distal convoluted tubules and collecting ducts of the nephron.

• Promotes potassium excretion by enhancing its secretion into the urine.

• Facilitates water retention indirectly through osmotic effects following sodium reabsorption.

• Stimulates hydrogen ion excretion to help regulate acid-base balance.

These actions collectively lead to an increase in blood volume and blood pressure.

5. Physiological role(s)

Aldosterone plays several key roles in maintaining homeostasis:

• Blood pressure regulation: By promoting sodium and water reabsorption, aldosterone helps maintain intravascular volume and arterial pressure.

• Electrolyte balance: Controls plasma sodium and potassium concentrations, which are critical for neuromuscular function.

• Renin-angiotensin-aldosterone system (RAAS): Renin from the kidneys initiates a cascade that leads to aldosterone release during hypotension or volume depletion.

• Response to hyperkalemia: Elevated potassium levels directly stimulate aldosterone secretion to promote potassium excretion.

• Acid-base balance: Aids in the excretion of hydrogen ions, thus playing a role in preventing metabolic acidosis.

6. Clinical Significance

Disorders of aldosterone secretion can lead to significant clinical syndromes:

• Primary Hyperaldosteronism (Conn’s Syndrome): Caused by an adrenal adenoma or bilateral adrenal hyperplasia. Characterized by hypertension, hypokalemia, metabolic alkalosis, and low plasma renin activity. It is a potentially curable cause of secondary hypertension.

• Secondary Hyperaldosteronism: Occurs due to increased renin levels, often seen in conditions like renal artery stenosis, congestive heart failure, or cirrhosis. Aldosterone is elevated in response to perceived volume depletion.

• Hypoaldosteronism: May result from adrenal insufficiency (e.g., Addison’s disease), congenital defects in aldosterone synthesis, or diabetic hyporeninemic hypoaldosteronism. Symptoms include hyperkalemia, hypotension, hyponatremia, and acidosis.

• Pseudohypoaldosteronism: A condition where renal tubules are resistant to aldosterone despite high hormone levels. Patients present with salt-wasting, hyperkalemia, and hypotension, often from infancy.

• Drug effects: Medications like ACE inhibitors, ARBs, aldosterone antagonists (e.g., spironolactone), and potassium-sparing diuretics can alter aldosterone activity for therapeutic purposes.