Integrated Nature of Chromatophore Responses in the in Vitro Frog Skin Bioassay

A cytological analysis of the in vitro response of frog skin chromatophores to hormonal and pharmacological agents is presented. The significance of the diverse chromatophore responses to stimulation is discussed in light of interpretations that might erroneously be drawn from objective photometric measurements as in the frog skin bioassay. Melanophorestimulating hormone (MSH) disperses melanin granules within both dermal and epidermal melanophores; the previously dispersed reflecting platelets within iridophores become aggregated. Acetylcholine lightens MSH-darkened skin to a near pre-MSH level by reversing the action of MSH on all chromatophores. Melatonin and norepinephrine only weakly reverse the darkening action of MSH on frog skin as not all chromatophores respond to these agents. Melatonin causes melanin granule aggregation within only a small number of localized dermal melanophores and is without effect on iridophores. Norepinephrine, unlike melatonin, but like acetylcholine, redisperses the reflecting platelets within iridophores. This catecholamine may aggregate melanin granules within dermal interspotinterspot melanophores but is without effect on spot melanophores. Neither melatonin nor norepinephrine reaggregates melanin granules within epidermal melanophores. Preincubation of skins in acetylcholine inhibits the action of MSH on all chromatophores of those skins sensitive to this agent. Preincubation of skins in very high concentrations of norepinephrine may prevent the action of MSH on the dermal interspot melanophores and iridophores but not its action on either spot melanophores or epidermal melanophores. Skins either preincubated or not in melatonin respond to MSH by darkening to almost the same degree. Neither melatonin nor norepinephrine lightened skins darkened by caffeine. Dermal spot melanophores are not physiologically equivalent to dermal interspot melanophores as they show differential responses to hormonal agents. These observations emphasize that in the in vitro photometric assay similar reflectance values obtained from the use of different hormones or pharmacological agents may result from dissimilar cytological responses. (Endocrinology84: 69, 1969)