Veterinary Physiology-III

MINERALOCORTICOIDS

• Aldosterone (electrocortin), deoxycorticosterone and 17‑hydroxy‑11 deoxycorticosterone are naturally occurring mineralocorticoid of adrenal gland. 

• Aldosterone is very potent and contributes 95% of all mineralocorticoid activity. Deoxy corticosterone is 1/5^thas active as aldosterone and secreted in very less amounts. Corticosterone has very slight mineralocorticoid activity.
• 9 alpha fluorocortisol is synthetic compound having potent activity than aldosterone.
• Cortisol and cortisone have sight mineralocorticoid activity.
• Electrolyte balance and blood pressure homeostasis are the principle physiological effects of mineralocorticoids.
• It promotes sodium retention and potassium and hydrogen ions secretions at the level of the distal tubules of kidney. Mineralocorticoids stimu­lates the synthesis of a protein to increase Na⁺K⁺ ATPase activity in the tubular cells thus increases the permeability of luminal cell surface to sodium influx from the renal filtrate.
• Aldosterone increases the sodium retention in the extra renal tissues like sweat glands, salivary glands and intestine.

Regulation of the Mineralocorticoids

• Aldosterone secretion is primarily regulated by (1) angiotensin-II and (2) K⁺ concentration in the extracellular fluid
• Fall in systemic arterial blood pressure or increase in sympathetic nerve activity to kidney increases secretion of the enzyme renin from the juxtaglomerular cells of the kidney which is the most important factor in regulating mineralocorticoid secretion
• In the blood renin acts on angiotensinogen, an alpha-2- globulin produced by the liver and converts it to angiotensin-I. It is hydrolyzed by the enzyme angiotensin-converting enzyme (ACE) present in lungs and endothelial cells to angiotensin-II which directly acts on adrenal cortex to release aldos­terone.
• Aldosterone promotes Na retention and K and H secretions in the distal tubules of kidney.
• Hypovolumia increases the output of renin and aldosterone.
• An increase in K⁺ stimulates aldosterone secretion. Macula densa, a group of specialized cells at the origin of distal tubules of the kidney detects Na ions level and exerts control on aldosterone through the renin‑angiotensin system.
• A decrease of Na ion concentration in the body fluids increases renin release.
• β ‑adrenergics, some PGs, high levels of K ions in the ECF and under some conditions of stress ACTH also stimulates aldosterone release.
• In contrast, natriuretic hormone reduces sodium retention by the kidney by its inhibitory effect on the production of mineralo­corticoids and renin. ACTH produces a transient increase in mineralocorticoid secretion.

Mechanism of Action

• Aldosterone acts on the distal renal tubules and collecting ducts and causes
• More numbers of Na channels for Na ion transport via electro‑chemical gradient.
• Activation of Na⁺‑K⁺ ATPase