G-PROTEINS AND ADENYLYL-CYCLASE IN OVARIAN GRANULOSA-CELLS OF AMAGO SALMON (ONCORHYNCHUS-RHODURUS)

The involvement of guanine nucleotide-binding regulatory proteins (G-proteins) and adenylyl cyclase in the gonadotropin stimulation of cAMP was investigated using crude membrane fractions from granulosa cells of amago salmon (Oncorhynchus rhodurus) postvitellogenic ovarian follicles. Although cholera toxin-catalyzed ADP ribosylation occurred in 45- and 50-kDa proteins, only the former was recognized by an antibody against the alpha-subunit of Gs. With pertussis toxin, only the 41-kDa protein was ADP-ribosylated. This 41-kDa protein was recognized by an antibody against the alpha-subunit of Gi. Partially purified chum salmon gonadotropin (SGA) stimulated adenylyl cyclase activity in crude membrane preparations of granulosa cells only in the presence of pertussis toxin in the incubation medium. Adenosine inhibited adenylyl cyclase in the presence of GTP and pertussis toxin reversed it. Unlike SGA, forskolin, which acts upon adenylyl cyclase without G-protein interaction, markedly stimulated adenylyl cyclase activity in the absence of pertussis toxin. These results provide evidence that both stimulatory (Gs) and inhibitory (Gi) regulation by adenylyl cyclase operates in the granulosa cells of amago salmon postvitellogenic ovarian follicles. It is possible that, although a stimulatory receptor interacts with Gs, its activity is influenced by the functional state of Gi.