BETA(2)-ADRENOCEPTORS MEDIATE MELANOSOME DISPERSION IN WINTER FLOUNDER (PLEURONECTES-AMERICANUS)

Although the melanosome-aggregaling mediation of catecholamines throug h a-adrenoceptors is well established for teleost melanophores, the re gulation of the dispersive process is not as clearly understood. The m elanosome-aggregating effect of high concentrations of catecholamines in vitro is reversed at low concentrations with melanophores of the wi nter flounder, Pleuronectes americanus. In vitro incubation in low con centrations of isoproterenol (less than or equal to 10(-7) M) and nora drenaline (less than or equal to 10(-8) M) enhances Na+-induced melano some dispersion in balanced salt solution, which can be depressed by p ropranolol (10(-4) M), indicating beta-adrenoceptor mediation in pigme nt dispersion. The subtype of this adrenoceptor appears to be the beta (2) conformation, since the beta(2)-adrenoceptor agonist terbutaline ( greater than or equal to 3.15 x 10(-6) M) reverses the melanosome-aggr egating 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 cc-adrenoceptors on release of noradrenaline and of dispersion throug h beta(2)-adrenoceptors during a subsequent decrease in concentration.