Veterinary Physiology-III

ADRENOCORTICOTROPIC HORMONE (ACTH)

• Adrenocorticotropin is a polypeptide containing 39 amino acids in a straight chain in sheep, pig, cow and man.
• It stimu­lates synthesis and immediate release of corticosteriods.
• It helps the uptake of cholesterol by the adrenal cells, choles­terol ester hydrolysis, cholesterol transfer to mitochondria, the rate‑limiting desmolase reaction and the critical 11 ‑ hydroxylation steps in cortisol synthesis.
• The primary physiological function of ACTH is to stimulate the secretion of the adrenal cortex, especially of cortisol and corticosterone.
• However the mineralocorticoids show lesser effects to ACTH. During haemorrhage it causes aldosterone release.
• Only in the bird ACTH is needed for aldosterone output. The specific action of ACTH in the adrenal gland is stimulation of cyclic AMP production leading to the production of energy for corticoid biogenesis.
• Stressful stimuli such as haemorrhage, temperature, pain, toxin and emotional states, influence the release of ACTH by stimulating the hypothalamus to release CRH.
• ACTH secre­tion is influenced by circadian light/darkness cycle. In human and horses ACTH secretion is high during morning hours (6.00 to 8.00 A.M.), in pigs two peaks occur, one in morning and one in late afternoon. In dogs, there is no peak level.
• By long loop servomechanism, the glucocorticoid cortisol regulates CRH release from the hypothalamus, thus decreases ACTH formation in the adenohypophysis.
• Arginine, vasopressin also stimulates ACTH release.
• By short-loop negative feed back action ACTH inhibits CRH release.