Somatostatin(5) receptor-mediated [S-35]guanosine-5'-O-(3-thio)triphosphate binding: Agonist potencies and the influence of sodium chloride on intrinsic activity

We studied the activation of the human somatostatin, receptor recombinantly expressed in CHO-K1 cells by using some newly available agonists and antagonists. Somatostatin-28 bound to this receptor with a higher affinity than somatostatin-14 and was more potent in increasing [S-35]guanosine-5'-O-(3-thio)triphosphate (S-35]GTP gamma S) binding. Somatostatin-14-induced [S-35]GTP gamma S binding to membranes from this cell line was decreased in a concentration-related manner by increasing concentrations of GDP and sodium chloride. At 50 mM (low) sodium, agonist EC50 values for stimulating [S-35]GTP gamma S binding were lower than those at 150 mM (high) sodium and were closer to their respective affinity estimates (dissociation equilibrium constants) for binding to the receptor in the absence of sodium. Both agonist binding to the high affinity state of the receptor and agonist-induced [S-35]GTP gamma S binding were abolished by pertussis toxin pretreatment. The putative somatostatin(5) receptor-selective ligand L-362,855, unlike somatostatin-14 and somatostatin-28, showed differential intrinsic activity for stimulation of [S-35]GTP gamma S binding, behaving as a partial agonist in high sodium and a full agonist in low sodium. In contrast, BIM-23056 did not behave as an agonist under any conditions studied but was able to antagonize somatostatin-14-induced [S-35]GTP gamma S binding. We conclude that measurement of [S-35]GTP gamma S binding mediated by somatostatin receptor activation in the presence of different concentrations of sodium chloride provides a useful functional assay for assessing the relative agonist efficacies of novel ligands identified from radioligand binding studies.