Somatostatin inhibits bombesin-stimulated Gi-protein via its own receptor in rabbit colonic smooth muscle cells.

The effect of somatostatin on Bombesin-induced contraction of isolated rabbit colonic smooth muscle cells was examined. Preincubation of muscle cells with somatostatin 10(-6) M inhibited bombesin-induced contraction. To characterize somatostatin receptors, muscle cells (10(5) cells/tube) were incubated at 24 degrees C with 125I-Tyr0-SS-28. Binding reached a plateau at 60 sec and was reversible by addition of excess synthetic SS-28. Scatchard analysis revealed high and low affinity bindings sites (Ka = 0.48 +/- 0.01 and 40 +/- 13 (nM +/- S.E.), 1830 +/- 433 and 65820 +/- 13183 receptors/cell +/- S.E.). Inhibition of 125I-Tyr0-SS-28 binding was possible with biologically active analogs of somatostatin, indicating the specificity of the receptors to somatostatin. Binding of 125I-Tyr0-SS-28 was inhibited by GTP gamma s, a nonhydrolysable analog of guanosine 5'-triphosphate, whereas adenosine 5'-triphosphate at a high concentration (100 microM) slightly inhibited the binding. Further, pretreatment of muscle cells with pertussis toxin at 37 degrees C abolished binding of 125I-Tyr0-SS-28, although pretreatment of cells with cholera toxin had no effect. Inasmuch as Gi protein is postulated as a signal protein, muscle cells were labeled with 3H-methionine, before stimulation with Bombesin (10(-6) M), in the presence and absence of somatostatin (10(-6) M). The cells were then lysed and Gi was precipitated by a Gi specific antibody. Gi synthesis was stimulated by bombesin at 60 sec and somatostatin inhibited it (6114 +/- 986 vs. 2998 +/- 841 cpm +/- S.E., P < .05). These data suggest that colonic smooth muscle cells contain specific receptor for somatostatin-28 and that somatostatin reverses bombesin-induced contraction regulated by Gi-type G protein.