Characterization of human recombinant somatostatin sst(5) receptors mediating activation of phosphoinositide metabolism

1 We have functionally characterized the human recombinant somatostatin (SRIF) sst(5) receptor in Chinese hamster ovary-K1 (CHOsst(5)) cells by measuring total [H-3]-inositol phosphate ([H-3]-InsPx) accumulation, in the presence of 10 mM LiCl, in cells labelled with [H-3]-myo-inositol. 2 In CHOsst(5) cells, SRIF, SRIF-28 and the cyclic hexapeptide, L-362,855, produced time- and concentration-related increases in [H-3]-InsPx accumulation, with similar potency (pEC(50) values of 6.5, 6.8 and 7.2, respectively). L-362,855 behaved as a partial agonist, producing approximately 30% of the SRIF maximum response. The other peptide analogues of SRIF, BIM-23027 and BIM-23056, were inactive as agonists. 3 Increasing concentrations of L-362,855 increased [H-3]-InsPx accumulation and simultaneously produced rightward shifts of SRIF concentration-effect curves, with an estimated pK(p) value of 7.6, confirming that it was acting as a partial agonist. 4 BIM-23056, but not BIM-23027, potently antagonized SRIF-induced [H-3]-InsPx accumulation, with an estimated pK(B) value of 7.4. BIM-23056 did not antagonize [H-3]-InsPx accumulation induced by uridine 5'-triphosphate (UTP). 5 SRIF- but not UTP-induced [H-3]-InsPx accumulation was inhibited by increasing concentrations of pertussis toxin (0.01-100 ng ml(-1)), indicating the involvement of pertussis toxin-sensitive G-proteins. 6 These findings show that the human recombinant sst(5) receptor, when stably expressed in CHO-K1 cells, is able to mediate activation of phosphoinositide metabolism in a pertussis toxin-sensitive manner. In this system L-362,855 behaved as a partial agonist while BIM-23056 was a specific antagonist. These agents should provide useful tools for functionally characterizing endogenous SRIF receptors.