ROLE OF NITRIC-OXIDE IN NONADRENERGIC, NONCHOLINERGIC INHIBITORY JUNCTION POTENTIALS IN CANINE ILEOCOLONIC SPHINCTER

1 Electrical field stimulation causes neurally-mediated relaxation of the ileocolonic sphincter that is due to activation of non-adrenergic and non-cholinergic (NANC) nerves. Recent studies have suggested that nitric oxide (NO) is the neurotransmitter that mediates relaxation. 2 Using intracellular recording techniques, we have tested whether NANC inhibitory junction potentials (i.j.ps) in the canine ileocolonic sphi ncter are also mediated by NO. 3 Electrical field stimulation elicited excitatory and inhibitory junction potentials: e.j.ps were blocked by atropine (10(-6) M) and tetrodotoxin (TTX; 10(-6) M); i.j.ps were als o blocked by TTX and partially blocked by apamin (10(-6) M). I.j.ps we re unaffected by atropine, phentolamine and propranolol (all at 10(-6) M). 4 The arginine analogues, L-N(G)-nitroarginine methyl ester (L-NA ME) and N(G)-monomethyl-L-arginine (L-NMMA), decreased the amplitude o f i.j.ps and L-arginine, but not D-arginine, partially restored the i. j.ps. 5 I.j.ps were also inhibited by oxyhaemoglobin (1%), but not by methaemoglobin. 6 Exogenous NO (10(-7) M to 3 x 10(-5) M) caused conce ntration-dependent hyperpolarizations that were similar in amplitude t o the NANC nerve-evoked i.j.ps. Hyperpolarizations to NO were unaffect ed by L-NAME, but were blocked by oxyhaemoglobin. 7 Tetrodotoxin, L-NA ME and oxyhaemoglobin all caused depolarization of resting membrane po tential. 8 The specific guanosine 3':5'-cyclic monophosphate phosphodi esterase inhibitor, M&B 22948, caused hyperpolarization, increased the maximum level of hyperpolarization reached during i.j.ps, and increas ed the duration of i.j.ps. 9 These data further support the hypothesis that NANC neurotransmission in the ileocolonic sphincter is mediated by NO or an NO-releasing compound. The data also suggest that tonic re lease of NO, possibly from spontaneous firing of NANC nerves, may regu late resting membrane potential and tone in this sphincter.