The role of the nitric oxide pathway in peristalsis in the mouse colon A. El-Majdoub, R.J. Naylor & B.R. Tuladhar, Bradford School of Pharmacy, University of Bradford, Bradford, BD7 1DP.	Print Abstract Search PubMed for: El-Majdoub A Naylor RJ Tuladhar BR

Nitric oxide is an established inhibitory neurotransmitter in the gastrointestinal tract (Sanders & Ward, 1992). However, it has been shown to have dual excitatory and inhibitory role in peristalsis in the guinea-pig ileum (Holzer et al., 1997). In the present study, we investigated the role of the nitric oxide pathway for peristalsis in the mouse isolated colon.

Segments of proximal colon (3cm in length, next to the caecum) were obtained from BKW mice of either sex (38-42g), cannulated at the oral and anal ends and secured horizontally in a water-jacketed bath containing Krebs' solution at 37ºC, gassed with 95% O₂ and 5% CO₂. Regular peristalsis was obtained with intraluminal pressures of 3-4 cm of water as described previously (Tuladhar & Naylor, 2002). N^G-Nitro-L-arginine (L-NNA) was added cumulatively by doubling the concentration of L-NNA until peristalsis was abolished. The concentration at which peristalsis was abolished was expressed as a geometric mean with a 95% confidence limit. The changes in the resting tone of the tissues due to L-NNA were also measured. The experiments were repeated in the presence of D-arginine (2 mM) and L-arginine (2 mM). All drugs were added to the serosal side. L- and D-Arginine were equilibrated for 15 min before the addition of L-NNA. Statistical comparisons between the different treatments were made using one way ANOVA followed by Bonferroni Dunnett's t test.

The cumulative addition of L-NNA (1-32 µM) caused concentration-related decreases in both the rate of peristalsis and the volume ejected by each peristaltic wave until finally, the peristalsis was abolished. The mean concentration at which peristalsis was abolished by L-NNA was 10.1 µM (95% confidence limit 5.6-18.3 µM, n=6). The presence of D-arginine in the buffer had no significant effect on the concentration of L-NNA required to abolish peristalsis (18.0 µM, 95% confidence limit 8.8-36.7 µM, n=6). But an approximately 18 fold higher concentration of L-NNA was required to abolish peristalsis in the presence of L-arginine (187.9 µM, 95% confidence limit 98.8-357.4 µM, n=6, P<0.0001). In all tissues, the circular muscle exhibited contraction when peristalsis was abolished by L-NNA (the volume expelled to the anal side at maximum contraction: 0.12±0.04, 0.16±0.03 and 0.06±0.02 ml (n=6, all) with L-NNA alone, L-NNA with D-arginine and L-NNA with L-arginine respectively).

The abolition of peristalsis with L-NNA accompanied by the generalized contraction of the circular muscle indicates that the nitric oxide pathway plays an essential role in the peristaltic reflex in the mouse colon. The effect of L-NNA observed in the present study is comparable to that of tetrodotoxin in this preparation (Tuladhar & Naylor, 2002), suggesting that a removal of the neuronal nitric oxide tone plays a major role in triggering contraction of the circular muscle during peristalsis in the mouse colon.

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