title: Calcium requirement for alpha-MSH action on melanophores: studies with forskolin.
abstract: alpha-MSH-induced pigment dispersion in melanophores shows an absolute requirement for extracellular Ca2+. To localize Ca2+ sites involved in the mechanism of action of alpha-MSH we studied the effects of Ca2+ deprivation on alpha-MSH and forskolin-induced melanophore responses. In an in vitro melanophore system employing ventral tailfins of Xenopus tadpoles, melanophore responses were assayed in terms of pigment dispersion and the phosphorylation state of a 53 kDa melanophore-specific protein. In the same melanophore system alpha-MSH has been shown to specifically increase the phosphorylation of this 53 kDa protein. Forskolin induces a dose-dependent pigment dispersion (EC50 7 X 10(-7) M). In contrast to the dispersion induced by alpha-MSH forskolin-induced dispersion does not require extracellular Ca2+. Moreover, in a Ca2+-free medium melanophores with permanently activated MSH-receptors aggregate, but can be redispersed by the addition of forskolin. Forskolin increases 53 kDa phosphorylation in a dose-dependent manner. Maximal stimulation with forskolin (10(-5) M) is four-fold and equals maximal 53 kDa phosphorylation obtainable with alpha-MSH. The MSH-induced increase in 53 kDa phosphorylation is inhibited by Ca2+ deprivation, whereas the forakolin-induced increase is unaffected. Our results suggest that alpha-MSH and forskolin stimulate melanophores through a common pathway and confirm that cAMP is a second messenger in alpha-MSH action in this system. We conclude that the Ca2+ sites in the mechanism of alpha-MSH action on melanophores precede adenylate cyclase activation.
mesh_terms: D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D005576:Colforsin; D000242:Cyclic AMP / Q000494:pharmacology; D004224:Diterpenes / Q000494:pharmacology*; D007814:Larva; D009074:Melanocyte-Stimulating Hormones / Q000494:pharmacology*; D008547:Melanophores / Q000187:drug effects* / Q000378:metabolism; D010766:Phosphorylation; D012880:Skin Pigmentation / Q000187:drug effects*; D014982:Xenopus laevis
chemical_list: D004224:Diterpenes; D005576:Colforsin; D003994:Bucladesine; D009074:Melanocyte-Stimulating Hormones; D000242:Cyclic AMP; D002118:Calcium