Abstract
Although the melanosome-aggregating mediation of catecholamines through alpha -adrenoceptors is well established for teleost melanophores, the regulation of the dispersive process is not as clearly understood. The melanosome-aggregating effect of high concentrations of catecholamines in vitro is reversed at low concentrations with melanophores of the winter flounder, Pleuronectes americanus. In vitro incubation in low concentrations of isoproterenol ( <<= 10-7M) and noradrenaline ( <<= 10-8M) enhances Na+-induced melanosome dispersion in balanced salt solution, which can be depressed by propranolol (10-4M), indicating beta -adrenoceptor mediation in pigment dispersion. The subtype of this adrenoceptor appears to be the beta2conformation, since the beta2-adrenoceptor agonist terbutaline (>=>3.15 times 10-6M) reverses the melanosome-aggregating effect associated with higher concentrations of noradrenaline and with electrical stimulation. It is concluded that in this species there is adrenergic neuronal control of melanosome aggregation through alpha -adrenoceptors on release of noradrenaline and of dispersion through beta2-adrenoceptors during a subsequent decrease in concentration.
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