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Nitric oxide modulates cholinergic reflex pathways to the longitudinal and circular muscle in the isolated guinea-pig distal colon

Terence K. Smith and Sarah L. McCarron

1. The involvement of nitric oxide (NO) in enteric neural pathways underlying reflex responses of the longitudinal muscle (LM) and circular muscle (CM) layers activated by mucosal stimulation was examined in the isolated guinea-pig distal colon.

2. A segment of colon spanned two partitions (10 mm apart), which divided the organ bath into three chambers: a recording chamber where LM and CM tension was measured; a stimulation chamber where mucosal stimulation was applied; and a middle chamber separating them.

3. Brushing the mucosa anal and oral to the recording site evoked simultaneous oral contraction and anal relaxation of both the LM and CM.

4. N -nitro-L-argininel-NA; 100 µM) or N -nitro-L-arginine methyl ester (L-NAME; 100 µM) applied to the middle chamber or stimulation chamber decreased the oral contractile response of the LM and CM (by about 30-40 %), but increased the anal relaxation (> 600 %) and exposed an anal contraction (> 1000 % increase) of both muscles. The addition of L-NA to the recording chamber reduced the anal relaxation of the LM and CM and the anal contraction of the LM, but slightly increased the anal contraction of the CM.

5. S-Nitroso-N-acetylpenicillamine (SNAP; 10 µM), an NO donor, reversed the effects of L-NA in the middle or stimulation chambers.

6. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 µM), a soluble guanylate cyclase inhibitor, mimicked the effects of L-NAin the middle chamber or stimulation chamber, but these effects were not reversed by SNAP.

7. The oral contractile responses, and the anal relaxation and contractile responses of the LM and CM produced by L-NA in the stimulation or middle chambers, were blocked by hexamethonium (300 µM) in any chamber. Atropine (1 µM) in the recording chamber reduced the contractile responses of the LM and CM.

8. In conclusion, endogenous NO facilitates and depresses release of acetylcholine from interneurons in ascending and descending nervous pathways, respectively. These NO effects are mediated through soluble guanylate cyclase in cholinergic interneurons

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