Receptor mechanisms in fish chromatophores--VIII. Mediated by beta adrenoceptors, catecholamines always act to disperse pigment in siluroid melanophores.

The sympathomimetic amines tested, including those of alpha and beta type, were all ineffective in arousing melanosome aggregation within dermal and epidermal melanophores of the siluroid catfish, Parasilurus asotus. Conversely, these amines unfailingly gave rise to a dispersion of the pigment. While alpha-adrenergic blocking agents had only a little influence, beta agents exhibited a strong inhibitory effect on the pigment-dispersing action of the amines. Electrical nervous stimulation failed to bring about a melanosome dispersion. It was concluded that the adrenoceptors possessed by dermal and epidermal melanophores of this species seemed to be solely of the beta-adrenergic type, mediating the pigment dispersion, and that the endogenous amines involved in the darkening reaction of animals may originate in adrenal chromaffin cells, and thus are not derived from the peripheral nervous elements.