The beta adrenergic receptors of chromatophores of the frog, Rana pipiens

The isolated skin of Rana pipiens was found to be a suitable model for the quantitative study of chromatophore beta adrenergic receptors uninfluenced by prejunctional phenomena. Cumulative concentration-response curves for adrenergic agonists were obtained in preparations in which effective alpha adrenergic blockade had been produced with phenoxybenzamine. The beta adrenergic agonists darkened the preparation, as did melanocyte-stimulating hormone, but the maximum effects differed. The maximum of the l-isoproterenol cumulative concentration-response curve was approximately 50% less than that of melanocyte-stimulating hormone, while the maxima for l-epinephrine and l-norepinephrine were significantly less than that for isoproterenol. Microscopic examination revealed a qualitative difference: while maximal darkening produced by melanocyte-stimulating hormone was associated with maximal changes in both interspot melanophores and iridophores, maximal adrenergic-induced darkening was associated with maximal iridophore granule concentration only. No qualitative differences could be observed in the darkening caused by the three adrenergic agonists. The beta adrenergic potencies of l-norepinephrine and l-isoproterenol relative to l-epi...Continue Reading