Role of nitric oxide in non‐adrenergic, non‐cholinergic inhibitory junction potentials in canine ileocolonic sphincter

Abstract

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 sphincter 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 also blocked by TTX and partially blocked by apamin (10⁻⁶ m). I.j.ps were unaffected by atropine, phentolamine and propranolol (all at 10⁻⁶ m). 4 The arginine analogues, l-N^G-nitroarginine methyl ester (l-NAME) and N^G-monomethyl-l-arginine (l-NMMA), decreased the amplitude of 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 × 10⁻⁵ m) caused concentration-dependent hyperpolarizations that were similar in amplitude to the NANC nerve-evoked i.j.ps. Hyperpolarizations to NO were unaffected by l-NAME, but were blocked by oxyhaemoglobin. 7 Tetrodotoxin, l-NAME and oxyhaemoglobin all caused depolarization of resting membrane potential. 8 The specific guanosine 3′:5′-cyclic monophosphate phosphodiesterase inhibitor, M&B 22948, caused hyperpolarization, increased the maximum level of hyperpolarization reached during i.j.ps, and increased 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 release of NO, possibly from spontaneous firing of NANC nerves, may regulate resting membrane potential and tone in this sphincter.