EFFECT OF NITRIC-OXIDE ON CIRCULAR MUSCLE OF THE CANINE SMALL-INTESTINE

1. Experiments were designed to determine in circular muscle of the canine jejunum whether exogenous nitric oxide (NO) mimics the non-adrenergic, non-cholinergic inhibitory junction potential (NANC IJP), and whether changes in the availability of endogenous NO affects IJP amplitude. 2. Mechanical and intracellular electrical activity were recorded simultaneously from circular muscle of the canine jejunum. Electrical field stimulation evoked NANC IJPs and inhibited spontaneous contractions. 3. Infusions of NO solutions evoked immediate dose-dependent and transient hyperpolarizations and transiently inhibited spontaneous contractions. NO-evoked hyperpolarizations were unaffected by atropine, propranolol, phentolamine and tetrodotoxin. 4. The maximum IJP amplitude and the maximum amplitude of NO-evoked hyperpolarization were similar. 5. N(G)-Mono-methyl-L-arginine (L-NMMA), which inhibits synthesis of NO from L-arginine, reduced IJP amplitudes but did not reduce the response to exogenous NO. L-Arginine, but not D-arginine, reversed the effect of L-NMMA on IJP amplitude. 6. Oxyhaemoglobin, which binds and inactivates NO, reduced IJP amplitude and abolished the response to exogenous NO. 7. Exogenous NO mimicked the effects of NANC inhibitory input. Reducing the availability of endogenous NO reduced NANC inhibitory input. 8. It was concluded that NO mediates NANC neural inhibition and may act as a NANC inhibitory neurotransmitter in the canine jejunum.