UNUSUAL ALPHA-ADRENERGIC RECEPTOR POTENCY OF METHYLDOPA METABOLITES ON MELANOCYTE FUNCTION

  • 1 January 1977
    • journal article
    • research article
    • Vol. 201  (3) , 622-626
Abstract
Catecholamines possessing .alpha.-adrenergic receptor agonist properties induce lightening or reverse melanocyte stimulating hormone darkening of frog skin in vitro. The capacity to activate this .alpha.-receptor by the methyldopa metabolites methyldopamine and methylnorepinephrine was compared with the capacity of the naturally occurring dopa metabolites, dopamine and norepinephrine. Melanocyte stimulating hormone-induced darkening or dispersion of granules was reversed by each of these metabolites. Methylnorepinephrine was 10 times as potent as norepinephrine, and methyldopamine was 30- to 100-fold more potent than the naturally occurring dopamine. These inhibitory effects on melanocyte stimulating hormone could be blocked or partially impaired using the .alpha.-adrenergic blocker, phentolamine. They were not affected by pretreatment of frogs with the monoamine oxidase inhibitor pheniprazine (Catron) nor by application of pheniprazine, angiotensin or serotonin in vitro. This neuroendocrine model has .alpha.-adrenergic receptor relationships analogous to those described in the CNS for methyldopa metabolites.