Coordinated Action of Calcium Ion and Adenosine 3′,5′-Monophosphate upon the Release of α-Melanocyte-Stimulating Hormone from the Intermediate Lobe of the Rat Pituitary Gland

Abstract

Stimulation of the release of αMSH from dispersed rat melanotrophs by L-isoproterenol, 8-bromo-cAMP, or cholera toxin requires calcium ion (Ca⁺⁺) in the incubation medium; the stimulatory effect of each of these agents is attenuated by D-600, a Ca⁺⁺ antagonist. In contrast, stimulation of the formation of cAMP by L-isoproterenol, isobutyl methylxanthine, or cholera toxin does not require Ca⁺⁺ in the incubation medium. Ca⁺⁺ diminishes the amount of cAMP formed by cholera toxin-treated melanotrophs. Ca⁺⁺ inhibits adenylate cyclase activity and enhances cyclic nucleotide phosphodiesterase activity in cell-free homogenates of intermediate lobe tissue. A23187, a calcium ionophore, increases the accumulation of ⁴⁵Ca by melanotrophs and enhances the release of αMSH. Furthermore, when tested upon cholera toxin-treated melanotrophs, A23187 potentiates the Ca⁺⁺-induced inhibition of cAMP formation. The results indicate that Ca⁺⁺ is essential for the release of αMSH, and that cAMP in some way enhances the effects of Ca⁺⁺ upon the release process.