The inhibitory role of nitric oxide in the control of porcine and human sphincter of Oddi activity

Abstract

Background—The possible role of nitric oxide in the regulation of the sphincter of Oddi is not known in species with a resistor-like sphincter of Oddi such as humans and pigs.Methods—Sphincter of Oddi perfusion manometry and simultaneous electromyography (EMG) were recorded transduodenally in eight anaesthetised pigs. Acetylcholine (4 μg/kg) was given intra-arterially, with or without sodium nitroprusside (10–100 μg/kg), an exogenous nitric oxide donor. For in vitro studies the sphincter was removed from the eight pigs and from six patients undergoing pancreaticoduodenectomy, cut into rings, and the amplitude of contraction was measured in an ex vivo bath. Each ring was stimulated with acetylcholine (100 μM) and KCl (125 mM). The stimulation was repeated after incubation with l-NAME (a stereospecific competitive inhibitor of nitric oxide synthase), withl-NAME plus l-arginine (a substrate for nitric oxide synthase), and with sodium nitroprusside. The sphincter rings were then submersed in liquid nitrogen and stored. Immunohistochemical analysis was used to localise nitric oxide synthase in the pig and human sphincter specimens.Results—In vivo EMG revealed 2–3 phasic bursts per minute with the basal pressure variation 6–40 mm Hg. Acetylcholine induced a large electrical burst and the pressure increased by (mean (SE)) 20 (10) mm Hg (p <0.01) for 17 (4) seconds. After sodium nitroprusside (10 μg/kg) acetylcholine did not induce pressure changes and electrical activity was almost abolished. In vitro, l-NAME increased the KCl induced sphincter contraction in both pig and human specimens (p<0.01). In human, but not in pig, specimens l-NAME increased the amplitude of acetylcholine induced contraction (p<0.01). l-Arginine partly reversed the effect of l-NAME in both pig and human specimens. Sodium nitroprusside decreased the acetylcholine and KCl induced contractions in both pig and human specimens. Immunohistochemical studies localised nitric oxide synthase to rich plexi of nerve fibres in the mucosa and the muscle in both pig and human sphincter of Oddi.Conclusions—The sphincter of Oddi in both pigs and humans has endogenous nitric oxide synthase activity and immunoreactivity. Inhibition of endogenous nitric oxide production enhances contractility while exogenous nitric oxide decreases sphincter contractility and electrical activity.