CONTRIBUTION OF BOTH NITRIC-OXIDE AND A CHANGE IN MEMBRANE-POTENTIAL TO ACETYLCHOLINE-INDUCED RELAXATION IN THE RAT SMALL MESENTERIC-ARTERY

1 Acetylcholine stimulated repolarization and relaxation in isolated s egments of rat small mesenteric artery (D-100, = 325 +/- 9 mu m) in wh ich the smooth muscle cells were depolarized and contracted by submaxi mal concentrations of noradrenaline (0.75-2.5 mu M). There was no sign ificant difference either in the time taken to initiate relaxation or hyperpolarization, or for these parameters to reach maximum in respons e to acetylcholine. 2 The nitric oxide synthase inhibitor, N-G-nitro L -arginine methyl ester (L-NAME, 100 mu M) reduced the pD(2) for acetyl choline-induced relaxation from 7.5 to 7 and depressed the maximum rel axation from 89% to 68% in tissues stimulated with noradrenaline. The pD(2) for smooth muscle repolarization in these experiments was also r educed (7.4 to 6.6) but the maximum change in membrane potential in re sponse to acetylcholine was unaltered. The increase in potential now c learly preceded relaxation by 3.7 s (to initiation) and 4.7 s (to maxi mum). 3 In the presence of noradrenaline and a raised potassium concen tration (25 mM), the repolarization to acetylcholine was markedly atte nuated. Simultaneous tension measurements also revealed a marked reduc tion in the maximal relaxation to acetylcholine, but the pD(2) was unc hanged at 7.4. 4 The residual relaxation recorded in the absence of ma rked repolarization (in the presence of noradrenaline and 25 mM potass ium) was abolished by the addition of 100 mu M L-NAME. 5 Nitric oxide gas in solution (0.2-2.2 mu M; NOg) relaxed artery segments precontrac ted with noradrenaline. The magnitude of relaxation to NOg was not alt ered in the presence of noradrenaline and 25 mM potassium. 6 These dat a provide additional evidence that acetylcholine-evoked endothelium-de pendent increases in membrane potential provide a major mechanism for smooth muscle relaxation in the mesenteric artery. They also show that voltage-dependent and independent (initiated by NO) mechanisms can bo th contribute to relaxation, and suggest that NO may modulate the incr ease in membrane potential or the release of a hyperpolarizing factor.