Nitrergic innervation and relaxant response of rectal circular smooth muscle

PURPOSE: This study was designed to investigate whether nitric oxide mediates inhibitory innervation in human rectal circular smooth muscle. METHODS: Tissue was obtained from the midrectum of patients undergoing anterior resection for carcinoma. Adjacent strips of circular muscle were dissected and mounted in superfusion organ baths for isometric tension recording and initially loaded with 1 g of weight. Strips were continuously bathed with standard Krebs solution (37°C, bubbled with 97 percent O ₂ /3 percent CO ₂ ) containing 3×10 ^?6 M guanethidine and 3×10 ^?6 M atropine sulfate to block adrenergic and muscarinic cholinergic neurotransmission. After equilibration, strips had no intrinsic tone, and reproducible and stable tension was, therefore, induced by the addition of 3×10 ^?6 M histamine for five-minute “test” periods, during which electrical field stimulation (EFS) and additional drugs were applied. RESULTS: EFS elicited frequency-dependent, neurogenic (tetrodotoxin-sensitive) relaxations of precontracted strips. Addition of N-ω-nitro- l -arginine, a powerful competitive inhibitor of nitric oxide synthase, reduced the relaxant response to EFS in a dose-dependent fashion, an effect reversed by addition of 3×10 ^?4 M l -arginine but not by D-arginine. Addition of exogenous nitric oxide (sodium nitroprusside) mimicked the relaxant response induced by EFS. CONCLUSIONS: Human rectal circular smooth muscle receives an intrinsic inhibitory innervation mediated by nitric oxide. The presence of a residual response following blockade of the enzyme nitric oxide synthase suggests the involvement of additional neurotransmitters.