SOMATOSTATIN RECEPTOR SUBTYPE 3 MEDIATES THE INHIBITORY-ACTION OF SOMATOSTATIN ON GASTRIC SMOOTH-MUSCLE CELLS

Previous functional studies show that somatostatin (SS) interacts with specific receptors to inhibit relaxation in gastric smooth muscle cells. There are no ligand binding studies, and it is unknown which of the five subtypes of SS receptors mediates the action. Dispersed gastric smooth muscle cells from guinea pig bound both (125)-labeled SS-14 and I-125-D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Cys-Nal-NH2 (where Nal indicates N-naphthylalanine) (cyclo-SS-8), a synthetic peptidase-resistant octapeptide SS analogue. SS-28 and SS-14, cyclo-SS-8, and SS analogue D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-ol [SMS-(201-995) (octreotide)] inhibited I-125-cyclo-SS-8 binding with relative potencies of SS-28 = cyclo-SS-8 = SMS-(201-995) (octreotide), and the binding was not affected by the addition of protease inhibitors. SS-14 caused inhibition only in the presence of protease inhibitors. Ligand analysis demonstrated a two-binding-site model. Analysis of the relationship between biological function and binding suggested the high-affinity sites mediated the relaxant action of SS. 5'-Guanylylimidodiphosphate [Gpp(NH)p?] inhibited binding by reducing the affinity of the high-affinity site. Six SS-8 analogues that distinguish SS subtypes showed that I-125-SS-14 bound to somatostatin receptor subtype 3 (SSTR(3)). The results demonstrated that gastric smooth muscle cells possess distinct receptors for SS of the SSTR(3) subtype. Occupation of these sites inhibits relaxation in gastric smooth muscle cells. Modulation between the high- and low-affinity binding states of SSTR(3) is at least partially mediated by activation of guanine nucleotide regulatory proteins.