Ng-nitro-L-arginine reduces nonadrenergic, noncholinergic relaxations of human gut.

The aim of this study was to determine whether nonadrenergic, noncholinergic inhibitory neurotransmission in human circular sigmoid colonic and internal anal sphincter muscle involves release of a nitric oxide-like substance. Colonic and sphincter muscle respond to electrical field stimulation by giving nonadrenergic, noncholinergic relaxations. After-contractions always occur in colonic muscle but only sometimes in sphincter muscle. Ng-Nitro-L-arginine abolished relaxations of sphincter muscle and partially reduced those of colonic muscle. After-contractions were undiminished as were relaxations of sphincter muscle to sodium nitroprusside. The effects of Ng-nitro-L-arginine were reversed by L-arginine. The results suggest that nitric oxide is possibly the neurotransmitter mediating nonadrenergic, noncholinergic relaxations of the human internal anal sphincter muscle.