Adeno-cortical stimulating hormone or Corticotropin is a 39 amino acid peptide hormone secreted by the anterior pituitary. It is cleaved from the large precursor molecule, POMC.

ACTH is rapidly degraded in the circulation, with a half life of 10 minutes.This allows for rapid activation and termination of this component of the stress response.

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Secretion and Action ACTH is secreted in irregular bursts throughout the day and plasma cortisol rises and falls in repsonse to those bursts, which are most frequent during sleep in the early morning (before waking up) and least frequent in the evening. The duirnal ACTH rhythm is maintained by the suprachiasmatic nuclei of the hypothalamus.

Durning stress, the amount of ACTH concentration exceeds the amount necessary to produce maximal glucocorticoid output. However, prolonged exposure to high ACTH levels cause adrenal hypertrophy and increases maximal cortisol output.

Increases in ACTH secretion to meet emergency situiations are mediated through the release of CRH from the hypothalamus.

High levels of free glucocorticoids inhibit CRH, ACTH and thus cortisol secretion and the degree of inhibition is porportionate to the glucocorticoid level.

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ACTH binds to high-affinity receptors on the plasma membrane of adrenocortical cells. This is coupled to the adenylate cyclase effector via a G protein. The acute effect is increased production of free cholesterol and increased conversion of cholesterol to pregnolone through enhanced side chain cleavage. The chronic effects of ACTH include increased synthesis of most other enzymes of the steriogenic pathways and stimulated adrenocortical cell growth.

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