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

Experiments were performed to test the hypothesis that enteric inhibitory neurotransmission in pyloric muscles is mediated by NO. Junction potentials were recorded with intracellular microelectrodes from cells near the myenteric and submucosal surfaces of the circular muscle layer. Inhibitory junction potentials (IJPs) were apamin sensitive and were reduced by arginine analogues [N(G)-nitro-L-arginine-methyl ester (L-NAME) and N(G)--monomethyl-L-arginine (L-NMMA)]. The effects of arginine analogues were reversed by L-arginine. Inhibition of IJPs unmasked excitatory JPs (EJPs) in the myenteric region and increased excitability of cells in the submucosal region. IJPs were also reduced by oxyhemoglobin. 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 enteric inhibitory nerve responses. Exogenous NO hyperpolarized membrane potential, and these responses were also reduced by apamin. The magnitude of the responses to a given dose of NO was similar in cells of the myenteric and submucosal regions, suggesting that relatively smaller IJPs in submucosal cells may be due to a lower density of enteric inhibitory 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 production of cGMP. IJPs were also prolonged by M & B 22948. IJPs and NO disrupted normal electrical rhythmicity in cells in the myenteric region. This may provide a basis for inhibitory effects of enteric inhibitory nerve stimulation on sphincter pressure in pyloric canal in vivo. The results are consistent with the hypothesis that at least a portion of the enteric inhibitory neural regulation of pyloric sphincter is due to NO synthesis and release.