Photoaffinity labeling of MSH receptors on Anolis melanophores was used as a tool for studying the effects of catecholamines, Ca and forskolin on hormone-receptor interaction and receptor-adenylate cyclase coupling. Covalent attachment of photoreactive .alpha.-MSH to its receptor was suppressed in Ca-free buffer but was hardly influenced by catecholamines or forskolin. The long-lasting signal generated by the covalent MSH-receptor complex was readily and reversibly abolished by adrenaline [epinephrine, E], noradrenaline [norepinephrine, NE], dopamine or clonidine or by the absence of Ca. 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 NE. If MSH receptors were covalently labeled and then exposed to NE, the characteristics of the forskolin-induced response were identical to those of unlabeled cells that had not been exposed to NE. This may point to a partial restoration of receptor-adenylate cyclase coupling by forskolin. Apparently, the long-lasting stimulation of Anolis melanophores by photoaffinity labeling proceeds via a permanently stimulated adenylate-cyclase system whose coupling to the receptor depends on Ca and is abolished by .alpha.-2 receptor agonists. Ca is also essential for hormone-receptor binding.
