Characterization of human recombinant somatostatin sst5 receptors mediating activation of phosphoinositide metabolism

1. We have functionally characterized the human recombinant somatostatin (SRIF) sst₅ receptor in Chinese hamster ovary-K1 (CHOsst₅) cells by measuring total [³H]-inositol phosphate ([³H]-InsPx) accumulation, in the presence of 10 mM LiCl, in cells labelled with [³H]-myo-inositol.
2. In CHOsst₅ cells, SRIF, SRIF-28 and the cyclic hexapeptide, L-362,855, produced time- and concentration-related increases in [³H]-InsPx accumulation, with similar potency (pEC₅₀ 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 [³H]-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 [³H]-InsPx accumulation, with an estimated pK_B value of 7.4. BIM-23056 did not antagonize [³H]-InsPx accumulation induced by uridine 5′-triphosphate (UTP).
5. SRIF- but not UTP-induced [³H]-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 sst₅ 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.