Aldosterone: Definition, Function, Synthesis, Mechanism of Action and Abnormal Levels

It is a steroid hormone secreted by the adrenal glands.

Aldosterone serves as the main regulator of water and salt balance in the body and, therefore, is classified as a mineralocorticoid. It also has a small effect on the metabolism of fats, carbohydrates and proteins, which has an effect on blood pressure.

Aldosterone has a plasma half-life of less than 20 minutes.

Function of aldosterone

It plays a central role in regulating blood pressure mainly by acting on organs such as the kidney and colon.

It is essential for the preservation of sodium in the kidney , the salivary glands and the sweat glands.

It plays a central role in the homeostatic regulation of blood pressure, plasma levels of sodium and potassium. It does so mainly by acting on the mineralocorticoid receptors in the distal tubules and the collecting ducts of the nephron.

Aldosterone is responsible for the reabsorption of approximately 2% of the filtered sodium in the kidneys, which is almost equal to the total sodium content in human blood under normal glomerular filtration rates .

It also influences the excretion of potassium from the kidney, which indirectly influences the retention or loss of water, blood pressure and blood volume.

Synthesis

Aldosterone is synthesized in the body from corticosterone, a steroid derived from cholesterol. The production of aldosterone in the glomerulosa zone of the adrenal cortex is regulated by the renin-angiotensin system.

Renin is secreted from the kidneys in response to variations in blood pressure and volume and plasma levels of sodium and potassium.

Renin acts on a protein circulating in the plasma called angiotensinogen, which divides this substance into angiotensin I. Angiotensin I is subsequently converted to angiotensin II, which stimulates the release of aldosterone from the adrenal glands .

Mechanism of action

It acts by binding and activating a receptor in the cytoplasm of renal tubular cells. The activated receptor then stimulates the production of ion channels in renal tubular cells, which increases the reabsorption of sodium in the blood and increases the excretion of potassium in the urine.

How is aldosterone controlled?

Medications that interfere with the secretion or action of aldosterone are used as antihypertensives, such as lisinopril, which lowers blood pressure by blocking the angiotensin-converting enzyme , leading to decreased secretion of aldosterone.

These medications stimulate the excretion of sodium and water in the urine, while blocking the excretion of potassium.

Abnormal levels

When deregulated, aldosterone is pathogenic and contributes to the development and progression of cardiovascular and renal disease.

What happens if there are high levels of aldosterone?

The most common cause of high levels of aldosterone is overproduction, often of a small benign adrenal tumor ( primary hyperaldosteronism ). Symptoms include high blood pressure , low levels of potassium in the blood and an abnormal increase in blood volume.

What happens if there are low levels of aldosterone?

Low levels of aldosterone are found in a rare condition called Addison’s disease.

In Addison’s disease, there is a general loss of adrenal function that results in low blood pressure , lethargy and an increase in potassium levels in the blood (for more information, see the article on Addison’s disease).