de Graan P N, van de Kamp A J, Hup D R, Gispen W H, van de Veerdonk F C
J Recept Res. 1984;4(1-6):521-36. doi: 10.3109/10799898409042571.
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.
α-促黑素（α-MSH）诱导黑素细胞中的色素分散显示对细胞外Ca2+有绝对需求。为了定位参与α-MSH作用机制的Ca2+位点，我们研究了Ca2+剥夺对α-MSH和福斯可林诱导的黑素细胞反应的影响。在一个使用非洲爪蟾蝌蚪腹侧尾鳍的体外黑素细胞系统中，根据色素分散和一种53 kDa黑素细胞特异性蛋白的磷酸化状态来测定黑素细胞反应。在同一黑素细胞系统中，α-MSH已被证明能特异性增加这种53 kDa蛋白的磷酸化。福斯可林诱导剂量依赖性的色素分散（EC50为7×10(-7) M）。与α-MSH诱导的分散不同，福斯可林诱导的分散不需要细胞外Ca2+。此外，在无Ca2+培养基中，具有永久激活的MSH受体的黑素细胞会聚集，但可通过添加福斯可林重新分散。福斯可林以剂量依赖性方式增加53 kDa磷酸化。用福斯可林（10(-5) M）进行最大刺激时是四倍，且等于用α-MSH可获得的最大53 kDa磷酸化。Ca2+剥夺会抑制MSH诱导的53 kDa磷酸化增加，而福斯可林诱导的增加不受影响。我们的结果表明，α-MSH和福斯可林通过共同途径刺激黑素细胞，并证实cAMP是该系统中α-MSH作用的第二信使。我们得出结论，α-MSH作用于黑素细胞的机制中的Ca2+位点先于腺苷酸环化酶激活。
