Effects of intra-arterial bradykinin and substance P on isolated, blood-perfused small intestine of the rat.

Experiments were conducted on rat isolated, small intestine perfused at a fixed flow rate through the superior mesenteric artery with arterial blood from a donor rat, to determine the responses of the ileum to different peptides. Drugs were closely injected into the superior mesenteric artery. Single injections of bradykinin produced monophasic fast contractions of the ileum preceded by an initial fall and a subsequent rise of tone. The fast contraction was abolished by tetrodotoxin (TTX), morphine and hexamethonium (C6), but was resistant to blockade by atropine or mepyramine. Changes in ileal tone induced by bradykinin remained evident even in the presence of these blocking agents, thereby suggesting a direct action on smooth muscle fibers of the ileum. The fast contraction in response to substance P was not influenced by either TTX, morphine, C6, atropine or mepyramine. The present results indicate that bradykinin induces the fast contraction of the ileum by excitation of myenteric neuronal elements involving cholinergic interneurons, while substance P produces the contraction by a direct stimulation of smooth muscle fibers of the ileal region.