Photoaffinity labelling of MSH receptors on Anolis melanophores was used as a tool for studying the effects of catecholamines, calcium and forskolin on hormone-receptor interaction and receptor-adenylate cyclase coupling. Covalent attachment of photoreactive alpha-MSH to its receptor was suppressed in calcium-free buffer but was hardly influenced by catecholamines or forskolin. The longlasting signal generated by the covalent MSH-receptor complex was readily and reversibly abolished by adrenaline, noradrenaline, dopamine or clonidine or by the absence of calcium. The suppression of pigment dispersion by catecholamines was blocked by the simultaneous presence of yohimbine but not prazosin, indicating that the catecholamines antagonize the alpha-MSH signal by inhibitory action on the adenylate cyclase system through an alpha-2 receptor. Forskolin, which stimulates melanophores by direct action on the catalytic unit of the adenylate cyclase and at about the same speed as alpha-MSH, produced a slower and weaker response in the presence of noradrenaline. If MSH receptors were covalently labelled and then exposed to noradrenaline, the characteristics of the forskolin-induced response were identical to those of unlabelled cells that had not been exposed to noradrenaline. This may point to a partial restoration of receptor-adenylate cyclase coupling by forskolin. The results show that the longlasting stimulation of Anolis melanophores by photoaffinity labelling proceeds via a permanently stimulated adenylate-cyclase system whose coupling to the receptor depends on calcium and is abolished by alpha-2 receptor agonists. Calcium is also essential for hormone-receptor binding.
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