Corticotropin (ACTH)

What is a Tropic hormone or Tropin?

“A tropin or tropic hormone is the one which influences the activities of the other endocrine glands, principally those involved in stress and reproduction”.
These are carried by the blood to the other target glands.
The pituitary tropins are under the positive & negative control of peptide factors from hypothalamus.

Corticotropin (ACTH) is secreted as a large precursor molecule, known as Pro-opio-Melanocortin (POMC), with a molecular weight of 32 kD.
It is then cleaved to give about 30 different endorphins & several hormones.
The active adrenocorticotropic hormone (ACTH) is a polypeptide with 39 amino acids.
The secretion of POMC is under the control of CRH (Cortricotropin Releasing Hormone).
It is a stress hormone which is released from the pituitary in a pulsatile manner with a definite diurnal rhythm.
Its secretion is highest in the early morning and minimum at midnight.
Factors that increase ACTH secretion include stress such as pain, cold exposure, acute hypoglycemia, trauma, depression, and surgery.
ACTH binds to specific receptors on the adrenal gland.
After binding to the receptors it activates Adenylate cyclase which in turn activates cAMP.
The cAMP level is raised as a result of this activation.
ACTH induces adrenocortical steroidogenesis through the melanocortin-2 receptor.
Steroid hormones in turn, cause feedback inhibition of HPA (hypothalamopituitary adrenal axis).

The principal actions of corticotropin are exerted on the adrenal cortex and extra-adrenal tissue.
ACTH increases the synthesis of corticosteroids by the adrenal cortex and also stimulates their release from the gland.
The Total protein synthesis is increased as a result.
ACTH produces both:

ACTH stimulates the synthesis and secretion of glucocorticoids.
It increases the transfer of cholesterol from plasma lipoproteins into the fasciculata cells.
It induces rise in cAMP, brings about phosphorylation and activation of cholesterol esterase. The enzyme action ultimately makes a large pool of free cholesterol.
It leads to increased ketogenesis.
It helps in lowering of blood glucose.
It causes Glycogen deposition.
It increases glucose tolerance.