Melanocyte-Stimulating Hormone MSH

intermedin; in animals and man, a hormone, secreted by the posterior lobe and pars intermedia of the pituitary, that participates in the formation of pigments in the integuments and the retina.

MSH is a polypeptide that occurs in two forms: ±-MSH, consisting of 13 amino acid radicals, and ²-MSH, consisting of 18. The former has the same structure in all animal species. Horse and bull ²-MSH differ from ²-MSH in monkeys and apes in two amino acids; swine ²-MSH differs from the monkey form in one. In man, ²-MSH consists of 22 radicals, of which a section of 18 amino acids corresponds to the ²-MSH characteristic of monkeys and apes. All forms of MSH include a section of seven radicals (heptapeptides) responsible for the hormone’s activity. This section is also part of the polypeptide chain of adrenocorticotropic hormone (ACTH), accounting for the latter’s melanocyte-stimulating activity.

In lower vertebrates, MSH dilates the melanophores of the skin. The injection of fish and amphibians with synthetic MSH results in the formation of cutaneous pigments. The function of MSH in birds and mammals is still somewhat obscure, although the hormone seems to stimulate melanin synthesis in the skin of mammals by activating the enzyme tyrosinase. The secretion of MSH is regulated by the pituitary, which elaborates special peptide substances that stimulate or suppress the release of MSH into the blood.

MSH preparations are used to increase visual acuity, improve adaptation to the dark, and treat certain eye diseases (for example, pigmentary degeneration of the retina).