Somatostatin excites canine ileum ex vivo: role for nitric oxide?

Isolated perfused segments of canine ileum have no spontaneous motor activity and release large quantities of vasoactive intestinal polypeptide (VIP) continuously. Somatostatin perfusion was shown to decrease VIP release, accompanied by increased contractions and amplification of responses to low-frequency electrical field stimulation. After perfusion of higher somatostatin concentrations, the VIP output did not recover but quiescence returned. The actions of somatostatin on motor activity were not modified by hexamethonium, slightly reduced by atropine, and markedly reduced by tetrodotoxin. Inhibition of VIP output was not the major determinant of motor activity in the ileum because 1) a second infusion of somatostatin had similar motor effects despite markedly reduced VIP output, 2) abolition of tonic VIP output did not prevent induction of motor activity by somatostatin, and 3) artificial restoration of VIP levels did not prevent or antagonize somatostatin-induced ileal contractions. In contrast, the increment in motor responses induced by somatostatin was not apparent after N omega-nitro-L-arginine methyl ester, an inhibitor of nitric oxide (NO) synthase, but recovered after reversal by L-arginine. We conclude that the mode of somatostatin activation of intestinal motor activity involves reduced NO output, enhanced excitatory mediator action or release, a direct action on smooth muscle, and possibly inhibition of VIP output. Of these, reduced NO output plays the most important role.