Actions of somatostatins on gastric smooth muscle cells

The effects of somatostatin-28, somatostatin-14, and a synthetic somatostatin octapeptide analogue, D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Cys-Nal-NH₂ (cyclo SS-8) were examined on contraction of dispersed gastric smooth muscle cells from guinea pigs. The somatostatins did not cause contraction of gastric smooth muscle cells, nor did they inhibit carbachol-stimulated contraction. However, they reversed vasoactive intestinal peptide (VIP)- induced inhibition (relaxation) of carbachol-stimulated contraction. Somatostatin-28 had a half-maximal effect (EC₅₀) at 1.6 ± 0.8 nM, cyclo SS-8 at 0.6 ± 0.3 nM, but somatostatin-14 had no effect even when used in concentrations as high as 1 μM. Incubation of muscle cells with peptidase inhibitors phosphoramidon (1 μM) plus amastatin (10 μM) had no effect on the EC₅₀ of somatostatin-28 or cyclo SS-8 but increased the potency of somatostatin-14 >1,000-fold. When peptides were incubated with muscle cells and the products applied to high-performance liquid chromatography, cyclo SS- 8 was not degraded, but somatostatin-14 was rapidly degraded when present alone, and the addition of peptidase inhibitors partially inhibited the degradation. Cyclo SS-8 had its maximal effect at 0.5-1 min and inhibited relaxation induced by VIP, isoproterenol, glucagon, or dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP). Cyclo SS-8 partially inhibited the increase in VIP-stimulated cAMP. Preincubation with pertussis toxin blocked the inhibitory action of cyclo SS-8 on VIP or DBcAMP-induced relaxation. These results indicate that gastric smooth muscle cells rapidly degrade somatostatin-14 and suggest that muscle cell peptidases could have a major effect on the actions of somatostatin-14. Furthermore, gastric muscle cells possess somatostatin receptors, occupation of which inhibits relaxation induced by a variety of agents. Somatostatins act both through the guanine nucleotide regulatory protein G(i) to inhibit adenylate cyclase and at sites distal to the generation of cAMP.