ACTH belongs to the lipotrophin family of peptides which includes alpha-MSH and beta-endorphins. It is synthesised by corticotroph cells in the anterior and intermediate lobes of the pituitary from a large precursor molecule POMC - pro -opiomelanocortin.
Release of ACTH is controlled by:
- stimuli - corticotrophin-releasing factor, vasopressin, IL-1, catecholamines, physical or mental stress
- inhibitor - cortisol, by both a feedback loop to hypothalamus and pituitary
Patterns of ACTH secretion:
- 24 - hour rhythm - lowest levels during the early hours of sleep; peak levels just prior to waking
- episodes of secretion at meal times and every 1-3 hours
Effects of ACTH include:
- glucocorticoid production by the innermost layers of the adrenal cortex:
- increases cAMP within cells that:
- decreases cholesterol breakdown and increases cholesterol uptake from the blood
- activates the rate-limiting enzymes at the start of the steroid biosynthetic pathway
- cortisol production commences within minutes of an intravenous dose
- minimal melanocyte stimulating hormone activity and has the ability to stimulate melanocytes if secreted in excess
- minimal release of aldosterone and sex steroids from adrenal cortex