ROLE OF NITRIC-OXIDE AS AN INHIBITORY NEUROTRANSMITTER IN THE CANINE PYLORIC SPHINCTER

Experiments were performed to test the hypothesis that enteric inhibit ory neurotransmission in pyloric muscles is mediated by NO. Junction p otentials were recorded with intracellular microelectrodes from cells near the myenteric and submucosal surfaces of the circular muscle laye r. Inhibitory junction potentials (IJPs) were apamin sensitive and wer e reduced by arginine analogues [N(G)-nitro-L-arginine-methyl ester (L -NAME) and N(G)--monomethyl-L-arginine (L-NMMA)]. The effects of argin ine analogues were reversed by L-arginine. Inhibition of IJPs unmasked excitatory JPs (EJPs) in the myenteric region and increased excitabil ity of cells in the submucosal region. IJPs were also reduced by oxyhe moglobin. As with arginine analogues, reduction in IJPs increased EJP amplitude. Combination Of L-NAME and oxyhemoglobin completely blocked IJPs, suggesting that NO, or an NO-containing compound, mediated the e nteric inhibitory nerve responses. Exogenous NO hyperpolarized membran e potential, and these responses were also reduced by apamin. The magn itude of the responses to a given dose of NO was similar in cells of t he myenteric and submucosal regions, suggesting that relatively smalle r IJPs in submucosal cells may be due to a lower density of enteric in hibitory innervation in the submucosal region. The effects of NO were mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate (cGMP) and M & B 22948, a specific cGMP phosphodiesterase inhibitor, suggesting that the hyperpolarization response to NO may be mediated by enhanced prod uction of cGMP. IJPs were also prolonged by M & B 22948. IJPs and NO d isrupted normal electrical rhythmicity in cells in the myenteric regio n. This may provide a basis for inhibitory effects of enteric inhibito ry nerve stimulation on sphincter pressure in pyloric canal in vivo. T he results are consistent with the hypothesis that at least a portion of the enteric inhibitory neural regulation of pyloric sphincter is du e to NO synthesis and release.