Evidence for separate receptors for melanophore stimulating hormone and catecholamine regulation of cyclic AMP in the control of melanophore responses

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Abstract
1 . Skins of the lizard Anolis carolinensis darken in vitro in response to melanophore stimulating hormone (MSH), a peptide hormone, as well as to catecholamines. These hormones darken Anolis skins by dispersing the subcellular organelle, the melanosome, out into the dendritic processes of the dermal melanophores. 2 . Dibutyryl cyclic AMP and methylxanthines also darken skins. In addition, methylxanthines are synergistic with both catecholamines and MSH in causing skin darkening. These data suggest that the dispersion of melanosomes within melanophores in response to both MSH and catecholamines is mediated by cyclic AMP. 3 . α-Adrenoceptor blocking agents inhibit MSH-induced darkening but potentiate catecholamine-induced darkening. β-Adrenoceptor blocking agents, in contrast, inhibit catecholamine-induced darkening but have no effect on MSH-induced darkening. This selective blockade of one receptor while the functional integrity of the other receptor is maintained suggests that MSH and catecholamines increase cyclic AMP levels through different receptors. 4 . Catecholamines exert their action through β-adrenoceptors; MSH darkens skins through what appears to be a component of the β-adrenoceptor. β-Adrenoceptor stimulation may stimulate adenyl cyclase to increase cyclic AMP levels whereas MSH may inhibit cyclic AMP phosphodiesterase thereby preventing cyclic AMP breakdown.