FUNCTIONAL-CHARACTERIZATION OF SOMATOSTATIN RECEPTORS EXPRESSED ON HAMSTER GLUCAGONOMA CELLS

We characterized somatostatin receptors expressed in hamster glucagonoma INR1G9 cells and the effects of somatostatin on glucagon secretion, proglucagon gene expression, and the adenosine 3',5'-cyclic monophosphate (cAMP)-dependent signal-transduction cascade. I-125-labeled somatostatin was displaced by somatostatin-14 and somatostatin-28 with a dissociation constant of 2 nmol/l. Stable GTP analogues decreased binding of I-125-somatostatin to its receptors, suggesting an interaction of somatostatin receptors with G proteins. Chemical cross-linking of I-125-somatostatin to its receptor revealed a molecular mass of the ligand-receptor complex of 47 kDa. Somatostatin inhibited forskolin-stimulated activation of adenylate cyclase [2.5 mu M forskolin (161%) + 1 mu M somatostatin (128%); P < 0.05] and protein kinase A [10 mu M forskolin (143%) + 1 mu M somatostatin (114%); P < 0.05] but did not influence basal activities of these enzymes. Forskolin-induced stimulation of cAMP generation was reduced by somatostatin [2.5 mu M forskolin (306%) + 1 mu M somatostatin (145%); P < 0.05]. Somatostatin inhibited forskolin, theophylline, and arginine stimulation of glucagon secretion. Basal as well as forskolin-, theophylline-, and isobutyl methylxanthine-induced proglucagon gene expression was significantly reduced by somatostatin. Our data show that, in INR1G9 cells, somatostatin receptors are at least in part coupled to the adenylate cyclase system. Somatostatin is a potent negative regulator of both basal and forskolin-stimulated proglucagon gene expression, The interaction with forskolin occurs at the level of adenylate cyclase. The effect of somatostatin on basal proglucagon gene transcription is most probably mediated by an unrelated second messenger system. Somatostatin may influence several functions of the pancreatic A cell.