EVIDENCE FOR NITRIC-OXIDE AS MEDIATOR OF NONADRENERGIC, NONCHOLINERGIC RELAXATIONS INDUCED BY ATP AND GABA IN THE CANINE GUT

1 The effects of haemoglobin, and the nitric oxide (NO) biosynthesis-inhibitors N(G)-monomethyl-L-arginine (L-NMMA), its enantiomer D-NMMA, and N(G)-nitro-L-arginine (L-NNA) were investigated on non-adrenergic non-cholinergic (NANC)-mediated relaxation of circular muscle strips of the canine terminal ileum and ileocolonic junction induced by electrical stimulation, adenosine 5'-triphosphate (ATP), gamma-aminobutyric acid (GABA) and NO. 2 Tetrodotoxin, L-NMMA and L-NNA, but not D-NMMA, inhibited the relaxations induced by electrical stimulation, ATP and GABA, but not those in response to NO. 3 The inhibitory effect of L-NMMA and L-NNA was prevented by L-arginine, but not by D-arginine. L-Arginine did not potentiate any of the NANC relaxations. 4 Haemoglobin reduced the relaxation induced by electrical stimulation, ATP and GABA, and abolished those in response to NO. 5 Our results demonstrate that the ATP- and GABA-induced relaxations resulting from stimulation of intramural NANC neurones, in addition to those induced by electrical impulses, are mediated by NO or a NO releasing substance and thus provide further evidence in support of the proposal that NO is the final inhibitory NANC neurotransmitter in the canine terminal ileum and ileocolonic junction.