Inhibition of adenylate cyclase activity in the goldfish melanophore is mediated by α₂-adrenoceptors and a pertussis toxin-sensitive GTP-binding protein.

The signal-transduction system that mediates the melanosome-aggregating response in melanophores of the black-moor goldfish, Carassius auratus, was investigated by examining the inhibition of adenylate cyclase activity mediated by α-adrenoceptors in cultured cells. When the melanophores were incubated with 1 mmol·1⁻¹ 3-isobutyl-1-methylxanthine for 5 min, the intracellular level of cyclic adenosine-3',5'-monophosphate increased two- to three-fold. Norepinephrine at 100 nmol· 1⁻¹ and naphazoline at 1 μmol· 1⁻¹ inhibited the 3-isobutyl-1-methylxanthine-induced accumulation of cyclic adenosine-3',5'-monophosphate in the cells in both the presence and the absence of isoproterenol, a beta-adrenergic agonist. Methoxamine and phenylephrine also reduced the extent of accumulation of cyclic adenosine-3',5'-monophosphate, but only when they were present at relatively high concentrations (above 100 μmol·1⁻¹). The range of concentrations at which norepinephrine inhibited the accumulation of cyclic adenosine-3',5'-monophosphate was consistent with the range at which it induced the aggregation of melanosomes. Pretreatment of the cells with pertussis toxin (1 μg·m1⁻¹) for 15 h or treatment with 100 nmol·1⁻¹ yohimbine (an α₂-adrenergic antagonist) inhibited the effects of the α-adrenergic agonists on both the aggregation of melanosomes and the 3-isobutyl-1-methylxanthine-induced accumulation of cyclic adenosine-3',5'-monophosphate, but prazosin (an α₁-adrenergic antagonist) at 100 nmol·1⁻¹ was not inhibitory. These results indicate that the melanosome-aggregating response of the goldfish melanophore is induced mainly via inhibition of the activity of adenylate cyclase,which occurs as a result of stimulation of a pathway that involves α₂-adrenoceptors and a inhibitory GTP-binding protein.