Alpha-melanocyte stimulating hormone increases plasma levels of glucagon and insulin in rabbits

Abstract

Intravenous injections of 25 and 2.5 μg alpha-melanocyte stimulating hormone (α-MSH) increased plasma levels of glucagon, insulin and free fatty acids in fasted and fed rabbits. 45 μg beta-melanocyte stimulating hormone (β-MSH) had similar effects, whereas 22 μg gamma-2-melanocyte stimulating hormone (γ-MSH) was inactive. The α-MSH-induced increases in the plasma levels of glucagon, insulin and free fatty acids were not inhibited by alpha- or beta-adrenergic blocking drugs. The α-MSH-induced increases in the plasma levels of insulin were, however, augmented by phentolamine (an alpha-adrenergic receptor blocking drug). The plasma levels of glucose were increased by 25 μg α-MSH in fed rabbits, only, and were decreased by α-MSH during alpha-receptor blockade. The acute in vivo effects of α-MSH and β-MSH on the plasma levels of glucagon, insulin and free fatty acids were rather similar to those previously reported for corticotropin (ACTH). It is possible that the 4–10 ACTH sequence, present in α-MSH, β-MSH and ACTH, but not in γ-MSH, is a message sequence for the observed effects. However, ORG 2766, a 4–9 ACTH analogue, was inactive. The mechanism by which α-MSH increased the plasma levels of glucagon and insulin in rabbits remains to be determined. It is possible, that the effects were mediated by both a central nervous action and a direct action on the endocrine pancreas.