THE EFFECTS OF ACTIVATORS OF G-PROTEINS, MASTOPARAN AND COMPOUND-48 80, ON THE G-PROTEIN/ADENYLATE CYCLASE SYSTEM IN CULTURED MELANOPHORES OF THE BLACK MOOR GOLDFISH, CARASSIUS-AURATUS/

To investigate the functions of GTP-binding protein(s) in the melanoso me-aggregating response in fish melanophores, the effects of activator s of G-proteins, namely, mastoparan and compound 48/80, were examined in cultured melanophores of the balck-moor goldfish, Carassius auratus . Both mastoparan and compound 48/80 induced an approximately 40% incr ease in the GTP-hydrolyzing activity in the melanophore membranes comp ared to the basal level. In intact melanophores, these compounds inhib ited the effect of 3-isobutyl-1-methylxanthine, which induced the accu mulation of intracellular cAMP. Pretreatment of melanophores with pert ussis toxin at 1 mu g . ml(-1) for 15 h attenuated the inhibitory effe ct of mastoparan on the accumulation of cAMP. However, pretreatment wi th the toxin only slightly attenuated the inhibitory effect of compoun d 48/80 on the accumulation of cAMP. In addition, compound 48/80 at 1 mg . ml(-1) induced full aggregation of the melanosomes in melanophore s, though mastoparan at 5 mu mol . l(-1) induced only 10-20% aggregati on of melanophores. These results suggest that mastoparan and compound 48/80 can each activate the inhibitory G-protein in goldfish melanoph ores, which results in inhibition of adenylate cyclase activity. This signal-transduction pathway is involved in the aggregation of melanoso mes in these cells.