Nitrergic inhibitory innervation of porcine rectal circular smooth muscle

This study was performed to determine whether nitric oxide contributes to inhibitory non-adrenergic non-cholinergic (NANC) neurotransmission in the circular smooth muscle layer of pig rectum. Smooth muscle strips were mounted for isometric tension recording in superfusion organ baths. In the presence of atropine sulphate and guanethidine, transmural electrical field stimulation (EFS) produced frequency-dependent relaxation of precontracted muscle strips. N omega-nitro-L-arginine (L-NOArg), a powerful competitive antagonist of nitric oxide synthase, reduced the relaxant response in dose-dependent fashion; this response was maximally reduced by mean(s.e.m.) 86.5(2.6) per cent (P = 0.0003) at a concentration of 3 x 10(-5) mol/l L-NOArg. The response was restored by the addition of L-arginine. Sodium nitroprusside, an exogenous donor of nitric oxide, mimicked the relaxant response. Responses to EFS were abolished by tetrodotoxin. These results suggest that inhibitory NANC neurotransmission in this tissue is mediated, at least in part, by nitric oxide. The failure of L-NOArg to completely abolish the relaxant response suggests that additional neurotransmitters may be involved.