Mediators of nonadrenergic, noncholinergic inhibition in the proximal, middle and distal regions of rat colon

Abstract

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.

Mediators of Nonadrenergic, Noncholinergic Relaxation in Sprague Dawley Rat Intestine: Comparison with the Mediators of Other Strains.

“…Drugs: Apamin, VIP fragment VIP [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and N G -nitro-Larginine (N 5 -nitro-amidino-L-2,5-diamino-pentanoic acid: L-NOARG) were purchased from Sigma Chemical Co., ST. Louis, U.S.A. VIP, charybdotoxin, PACAP and its fragment PACAP were purchased from The Peptide Institute, Osaka, Japan. Gaseous nitric oxide was dissolved in Tyrode solution just before experiments, as described by Gillespie and Sheng [4], and added to the organ bath in volumes of 0.3-300 µl.…”

“…However, participation of the peptides as well as nitric oxide in NANC relaxation does not seem to be uniform throughout the gastrointestine. For example, neither role of VIP in longitudinal muscle of the proximal and mid colon [24], and circular muscle of the rectum [19] of Wistar-ST rats, nor role of PACAP in circular muscle of the rectum of Wistar-ST rats [19] was suggested. We also found differences of participation of VIP among strains of the rat: VIP partially mediates the relaxation of longitudinal muscle of the distal colon in Wistar-ST but not in Sprague Dawley rats and relaxation of circular muscle in Sprague Dawley but not in Wistar-ST rats [21].…”

“…That is, an essential role of nitric oxide in the relaxation was suggested in circular [11] and longitudinal [24,26] muscles of the proximal colon, and in circular muscle of the rectum [27], while only minor or no role was suggested in longitudinal muscle of the jejunum [19] or the distal colon [16,24], respectively. In addition to regional difference, we recently found a strain difference in the mediator of NANC relaxation of the distal colon of rats between the Sprague Dawley and Wistar-ST strains: nitric oxide was suggested to partially mediate NANC relaxation of the Sprague Dawley but not Wistar-ST strain [21].In addition to nitric oxide, vasoactive intestinal peptide (VIP) is another candidate for the mediator of NANC relaxation in several gastrointestinal regions: lower oesophageal sphincter of opossums [5] and rabbits [2], stomach of dogs [1], guinea pigs [6, 10] and rats [13], taenia coli of guinea pigs [7,10], and colon of rats [9,24]. Pituitary adenylate cyclase activating peptide (PACAP) was also suggested to mediate NANC relaxation in some regions: lower oesophageal sphincter of cats and humans [20], stomach of guinea pigs [14], and colon of guinea pigs [12] and rats [8,15].…”

“…Wistar-ST rat intestine is a case in point. That is, an essential role of nitric oxide in the relaxation was suggested in circular [11] and longitudinal [24,26] muscles of the proximal colon, and in circular muscle of the rectum [27], while only minor or no role was suggested in longitudinal muscle of the jejunum [19] or the distal colon [16,24], respectively. In addition to regional difference, we recently found a strain difference in the mediator of NANC relaxation of the distal colon of rats between the Sprague Dawley and Wistar-ST strains: nitric oxide was suggested to partially mediate NANC relaxation of the Sprague Dawley but not Wistar-ST strain [21].…”

“…In addition to nitric oxide, vasoactive intestinal peptide (VIP) is another candidate for the mediator of NANC relaxation in several gastrointestinal regions: lower oesophageal sphincter of opossums [5] and rabbits [2], stomach of dogs [1], guinea pigs [6, 10] and rats [13], taenia coli of guinea pigs [7,10], and colon of rats [9,24]. Pituitary adenylate cyclase activating peptide (PACAP) was also suggested to mediate NANC relaxation in some regions: lower oesophageal sphincter of cats and humans [20], stomach of guinea pigs [14], and colon of guinea pigs [12] and rats [8,15].…”