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1 Department of Biological Sciences and Division of Pharmacology, College of Pharmacy, University of Arizona, Tucson, Arizona
Catecholamines cause both melanin granule aggregation (skin lightening) and dispersion (skin darkening) within melanophores of the lizard, Anolis carolinensis. In vitro experiments were designed to demonstrate and characterize the nature of the adrenergic receptors mediating these responses. Isoproterenol (ISO) darkened skins in Ringer's solution, whereas phenylephrine (PE) lightened them. Norepinephrine (NE) and epinephrine (E) also lightened the skins, but in some experiments they caused darkening. These sympathomimetic agents could be ranked as to their relative effectiveness in lightening skins in Ringer's solution in the following order: PE > NE > E > ISO. These agents could also be ranked as to their ability to lighten skins darkened by melanocyte-stimulating hormone in the following order: NE 
E » PE ISO, again a demonstration of an alpha receptor-mediated response. Ergotamine prevented sympathomimetic induced lightening of skins darkened by melanocyte-stimulating hormone.
Dibenamine and phentolamine prevented skin lightening in response to sympathomimetic stimulation. This blockade resulted in NE and E causing an intense darkening of skins due to a classical "epinephrine reversal" resulting from beta stimulation by these agonists. After alpha blockade the sympathomimetic agents darkened skins in the following order of relative effectiveness: ISO > E > NE > PE, the ranking for characterizing a beta adrenergic receptor-mediated response. These agonists have a similar order with respect to their darkening of skins maintained solely in Ringer's solution. Dichloroisoproterenol prevented skin darkening in response to sympathomimetic stimulation. These results reveal that alpha receptors mediate melanin granule aggregation and that beta receptors control melanin granule dispersion within Anolis melanophores in vitro in response to sympathomimetic stimulation.
Submitted on May 16, 1968
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