SOMATOSTATIN SELECTIVELY COUPLES TO G(0-ALPHA) IN HIT-T15 CELLS

Previously, we have demonstrated that somatostatin mediates all of its inhibitory effects on glucose-induced insulin secretion from the HIT-TIE cell through pertussis toxin-sensitive G-proteins and that the membrane fraction of this clonal line of pancreatic beta-cells contains six such proteins: G(i alpha 1), G(i alpha 2), G(i alpha 3), and three forms of G(o alpha). To determine the specificity of somatostatin receptor-G-protein coupling in HIT-TIE cells, we examined the ability of antisera specific for the COOH-terminus of G(alpha) subtypes to inhibit somatostatin-induced augmentation of membrane GTPase activity. GTPase activity increased in membranes as a function of GTP. At all concentrations of GTP studied, 1 mu mol/l somatostatin stimulated GTPase activity. Pertussis-torrin pretreatment prevented the effects of somatostatin. Antisera selective for G(o alpha) subtypes reduced the effects of somatostatin on GTPase activity (GTPase activity in absence of antisera, 125 +/- 3% of control; in the presence of antisera 976, 110 +/- 2% of control; n = 13, P < 0.001), whereas antisera directed against G(i alpha 1), G(i alpha 2), G(i alpha 3), and G(s alpha) were without effect. Somatostatin also significantly prevented cyclic AMP accumulation during perifusion with 11.1 mmol/l glucose through a pertussis toxin-sensitive mechanism. These data indicate that the somatostatin receptor couples to G(o alpha) in the HIT-T15 cell and suggest that G(o alpha) may link somatostatin to cyclic AMP metabolism in pancreatic beta-cells.