INHIBITION OF ADENYLATE-CYCLASE ACTIVITY IN THE GOLDFISH MELANOPHORE IS MEDIATED BY ALPHA-2-ADRENOCEPTORS AND A PERTUSSIS-TOXIN-SENSITIVE GTP-BINDING PROTEIN

The signal-transduction system that mediates the melanosome-aggregatin g response in melanophores of the black-moor goldfish, Carassius aurat us, was investigated by examining the inhibition of adenylate cyclase activity mediated by alpha-adrenoceptors in cultured cells. When the m elanophores were incubated with 1 mmol.1(-1) 3-isobutyl-1-methylxanthi ne for 5 min, the intracellular level of cyclic adenosine-3',5'-monoph osphate increased two- to three-fold. Norepinephrine at 100 nmol.1(-1) and naphazoline at 1 mu mol.1(-1) inhibited the 3-isobutyl-1-methylxa nthine-induced accumulation of cyclic adenosine-3',5'-monophosphate in the cells in both the presence and the absence of isoproterenol, a be ta-adrenergic agonist. Methoxamine and phenylephrine also reduced the extent of accumulation of cyclic adenosine-3',5'-monophosphate, but on ly when they were present at relatively high concentrations (above 100 mu mol.1(-1)). The range of concentrations at which norepinephrine in hibited the accumulation of cyclic adenosine-3',5'-monophosphate was c onsistent with the range at which it induced the aggregation of melano somes. Pretreatment of the cells with pertussis toxin (1 mu g.ml(-1)) for 15 h or treatment with 100 nmol.1(-1) yohimbine (an alpha(2)-adren ergic antagonist) inhibited the effects of the alpha-adrenergic agonis ts on both the aggregation of melanosomes and the 3-isobutyl-1-methylx anthine-induced accumulation of cyclic adenosine-3',5'-monophosphate, but prazosin (an alpha(1)-adrenergic antagonist) at 100 nmol.1(-1) was not inhibitory. These results indicate that the melanosome-aggregatin g response of the goldfish melanophore is induced mainly via inhibitio n of the activity of adenylate cyclase, which occurs as a result of st imulation of a pathway that involves alpha(2)-adrenoceptors and a inhi bitory GTP-binding protein.