Beta2-adrenoceptors mediate melanosome dispersion in winter flounder (Pleuronectes americanus))
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 ( <<= 10-7 M) and noradrenaline ( <<= 10-8 M) enhances Na + -induced melanosome dispersion in balanced salt solution, which can be depressed by propranolol (10-4 M), indicating beta -adrenoceptor mediation in pigment dispersion. The subtype of this adrenoceptor appears to be the beta 2 conformation, since the beta 2-adrenoceptor agonist terbutaline (>=>3.15 times 10-6 M) 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 beta 2-adrenoceptors during a subsequent decrease in concentration.