The mediators of non‐adrenergic non‐cholinergic (NANC) relaxation of the longitudinal muscle of rat proximal, middle and distal colon were examined in vitro. Electrical transmural stimulation (TMS) of proximal, middle and distal segments of rat colon induced NANC relaxations which were inhibited by tetrodotoxin (1 μm), but not by atropine (1 μm) or guanethidine (4 μm). In the proximal colon, l‐nitro‐arginine (N5‐nitroamidino‐l‐2,5‐diaminopentanoic acid) inhibited the TMS‐induced NANC relaxation and l‐arginine (1 mm) reversed this inhibition. Nitric oxide (0.3–10 μm) induced relaxation of the proximal segment. NANC relaxation of the proximal segments was still evident after desensitization to vasoactive intestinal peptide (VIP). A VIP antagonist (VIP 10–28, 10 μm) had no effect on the TMS‐induced NANC relaxation, which was also resistant to α‐chymotrypsin (2 units ml−1) and a substance P antagonist ([d‐Pro2, d‐Trp7,9]substance P, 1 μm). In the middle colon, l‐nitro‐arginine did not inhibit the TMS‐induced NANC relaxation in 6 of 9 preparations tested and partially inhibited the relaxation in the other 3 preparations. l‐Arginine did not reverse the partial inhibition. Complete desensitization to VIP was not achieved in the middle colon. The VIP antagonist had no effect on the TMS‐induced NANC relaxation. After α‐chymotrypsin treatment of the segment, desensitization of the segments to substance P, or in the presence of the substance P antagonist, the TMS‐induced NANC relaxation was augmented. In the distal colon, l‐nitro‐arginine did not have any significant effect on the TMS‐induced relaxation and nitric oxide did not induce relaxation. The VIP antagonist significantly inhibited TMS‐induced NANC relaxation. α‐Chymotrypsin‐treatment of the distal segments resulted in significant inhibition of NANC relaxation. No desensitization to substance P was achieved. Treatment with the substance P antagonist had no effect. These results suggest that nitric oxide is the mediator of the NANC inhibitory response in the proximal region of rat colon; in the middle colon, substance P acts as an excitatory neurotransmitter, antagonizing the NANC relaxation caused by the mediator of the response, which is still uncertain. Our results also suggest that VIP is the most likely candidate as a NANC transmitter in the distal colon.

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ABSTRACT-The role of cyclic GMP (cGMP) in nonadrenergic, noncholinergic (NANC) relaxation of the longitudinal muscle of rat proximal and distal colon was examined. Electrical field stimulation (EFS) of preparations of longitudinal muscle from the proximal region significantly increased the cGMP content. Nitro-L-arginine inhibited this increase, and L-arginine reversed the inhibitory effect of nitro-L-arginine. Exogenously added nitric oxide (NO) and atrial natriuretic peptide (ANP) also increased the cGMP content of preparations of the proximal colon and induced muscle relaxation. From these and our previous findings suggesting an essential role of NO in NANC inhibition in the proximal colon, we conclude that the mechanism of NANC inhibition in the proximal region of rat colon involves NO and a cGMP generating system. In contrast, although exogenously added NO and ANP increased the cGMP content in the distal colon to the same extent as in the proximal colon, they did not induce any muscle relaxation. Vasoactive intestinal peptide (VIP), the most likely candidate as a NANC neurotransmitter in rat distal colon, did not increase the cGMP content in this region. Furthermore, no participation of NO in the NANC inhibitory response was observed in the distal region, but EFS increased the cGMP content significantly. Thus we con clude that relaxation of longitudinal smooth muscle in the distal portion of rat colon is not associated with a change in the cGMP content.Keywords: Cyclic GMP, Nitric oxide (NO), Vasoactive intestinal peptide (VIP), Nonadrenergic, noncholinergic (NANC) inhibition, Colonic motility Several lines of evidence suggest that nitric oxide (NO) is a mediator of the nonadrenergic, noncholinergic (NANC) inhibitory response in various parts of the gas trointestinal tract, including the caecum (1) and stomach (2, 3) of guinea pigs; the gastric fundus (4, 5), duodenum (6) and proximal colon (7) of rats; and human ileum (8).The release of NO was suggested to occur during nerve stimulation of the canine ileocolonic junction (9) and rat gastric fundus (10). The high immunoreactivity of NO synthase in the myenteric plexus throughout the gut (11, 12) also supports the idea that NO is involved in the NANC inhibitory response. At present, the exact mechanism of the action of NO is unknown. Some reports suggest that NO and other nitrovasodilators induce relaxation of vascular smooth muscle cells by increasing the cyclic GMP (cGMP) con tent (13 16). There is also recent evidence that cGMP for mation is involved in the non-vascular smooth muscle relaxation elicited by NO or NO-producing compounds in preparations of taenia coli (17), corpus cavernosum (18, 19), tracheal (20) and ileal (21) smooth muscle. In these preparations, the relaxant effects of NO or NO-producing compounds were closely associated with an elevation of the intracellular cGMP level. NO was reported to activate soluble guanylate cyclase and increase the cGMP level in various tissue preparations (22). These observations strongly support the idea that ...

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