The Role of Nitric Oxide in the Acetylcholine-Induced Relaxation of the Feline Internal Anal Sphincter, in Vitro

Background: The relaxatory effect of acetylcholine was investigated on the feline internal anal sphincter (IAS), in vitro. Results: Acetylcholine (10, 30, 100, and 1000 μM) caused a concentration-dependent relaxation of the same magnitude in strips from the proximal and distal IAS. The antagonist of nitric oxide synthase, N$oM-nitro-L-arginine (L-NNA; 1, 10, and 100 μM), in a concentration-dependent and stereospecific manner, blocked the acetylcholine-induced relaxation, leaving a residual response of 10-30%. The blocking effect of L-NNA (100 μM) could not be shown in tissues that had been incubated with the substrate for nitric oxide synthase, L-arginine (1 mM). Conclusions: The present results suggest that the acetylcholine-induced relaxation of the IAS to a major extent is due to an activation of nitrergic, inhibitory motor neurons to the IAS.