GI PROTEIN MEDIATES ADENYLATE-CYCLASE INHIBITION BY NEUROHYPOPHYSEAL HORMONES IN FISH GILL

Adenylate cyclase activity was measured on membrane fractions from the gill epithelium of rainbow trout Salmo gairdneri. Basal and glucagon-stimulated activities responded negatively to homologous neurohypophyseal peptides (arginine-vasotocin and isotocin). This inhibitory effect was totally abolished in the presence of pertussis toxin (IAP). The guanine nucleotide dependence of the enzyme was further explored by using GTP, GDP, and their stable analogs Gpp(NH)p, GTPgammaS, and GDPbetaS. The results suggest that neurohypophyseal peptides at low concentrations inhibit the adenylate cyclase system directly by way of a G(i)-protein, thus implying the intervention of a new type of membrane receptor for these hormones in fish gills.