Nitric oxide‐like activity in guinea pig colon as determined by effector responses, bioassay and chemiluminescence analysis

The role of nerve‐induced release of nitric oxide (NO) as a modulator of neuroeffector transmission was studied in the longitudinal muscle of the guinea pig colon. The biological activity of a vascular relaxing factor released by nerve stimulation was examined in a bioassay cascade system. Furthermore, biochemical measurements of nerve‐induced release of the NO metabolite nitrite (NO₂^‐) were made with a chemiluminescence technique. Transmural nerve stimulation elicited contractile responses that were partly blocked by atropine and further inhibited after additional application of the tachykinin receptor antagonist CP‐96, 345. The NO‐synthase inhibitor N^w‐nitro‐L‐arginine (NOARG) enhanced the nerve‐induced contractions and concomitantly increased the basal degree of contraction (‘tone’). The relaxations obtained by nerve stimulation after treatment with atropine and histamine were inhibited by NOARG. Electrical stimulation of the guinea pig colon released a non‐adrenergic non‐cholinergic (NANC) vascular relaxing factor into the tissue superfusate. The half‐life of this factor down the cascade was the same as that observed with exogenous application of NO NOARG and tetrodotoxin (TTX) inhibited the release of the relaxing factor. During transmural nerve stimulation there was a significant increase in NO/NO₂^‐ release. This increase was inhibited by TTX and N^w‐nitro‐L‐arginine methyl ester (L‐NAME). In conclusion, pharmacological analysis as well as bioassay and biochemical measurements suggest that NO is released during nerve stimulation in the guinea pig colon, where it mediates smooth muscle relaxation.