Give an account of the regulation of aldosterone secretion.

Outline:

·        Aldosterone actions

·        Regulators of secretion:

- ACTH

- Renin and angiotensin II

- electrolytes

- atrial natriuretic peptide

Essay:

            Aldosterone is a steroid hormone synthesized by the zona glomerulosa of the adrenal cortex. The principal function of aldosterone is to sustain extracellular fluid (ECF) volume by conserving body sodium. Hence, aldosterone is largely secreted in response to signals that arise from the kidney when a reduction in circulating fluid volume is sensed. This is essential in the maintaining the ECF volume within a narrow physiological permissible range that is optimal for the functioning of the various tissues and organs of the body.

            When body sodium is depleted, the fall in ECF and plasma volume decreases renal arterial blood flow and pressure. The juxtaglomerular cells of the kidney respond to this change by secreting the enzyme renin into the peripheral circulation. Renin acts on its substrate, angiotensinogen to form the decapeptide angiotensin I which is further cleaved by an angiotensin converting enzyme to the octapeptide angiotensin II. This potent vasoconstrictor binds to specific adrenal zona glomerulosa cell plasma membrane receptors; through G-protein linkages, calcium and phosphatidylinositol products are generated as second messengers. Protein kinase C is translocated to the plasma membrane and is activated. The early action is on the conversion of cholesterol to pregnenolone, and the late action is on the conversion of corticosterone to aldosterone. However, changes in plasma sodium itself have little effect on regulation of aldosterone secretion since the plasma sodium concentration must drop about 20 meq/L to produce stimulation.

            Aldosterone also participates in vital negative feedback relationship with potassium. The hormone facilitates the clearance of potassium from the ECF, and in physiological concordance, potassium acts to stimulate aldosterone secretion. Potassium stimulates the conversion of cholesterol to pregnenolone and the conversion of corticosterone to aldosterone.

            ACTH is another regulator of aldosterone secretion. The physiological role of ACTH in maintaining aldosterone output is a tonic one; when ACTH is deficient, the zona glomerulosa is less able to respond to its primary stimulus of sodium depletion. ACTH stimulates the secretion of deoxycorticosterone and 18-OH-deoxycorticosterone. Atrial natriuretic peptide inhibits renin secretion and decreases the responsiveness of the zona glomerulosa to angiotensin II.