THE RELATIVE IMPORTANCE OF NITRIC-OXIDE AND NITRIC OXIDE-INDEPENDENT MECHANISMS IN ACETYLCHOLINE-EVOKED DILATATION OF THE RAT MESENTERIC BED

1 The relative contribution of nitric oxide (NO) to acetylcholine-indu ced smooth muscle relaxation was investigated in the rat perfused mese nteric vasculature and in isolated segments of second, third and fourt h order arterial branches. 2 The EC(50) values and maximal relaxation to acetylcholine were not significantly different in the sequential ar terial branches, being approximately 0.05 mu M and 85%, respectively. 3 The NO synthase inhibitor L-N-G-nitro-L-arginine methyl ester (L-NAM E; 100 mu M) reduced acetylcholine-evoked endothelium-dependent dilata tion and relaxation in the perfused mesenteric bed and in isolated art erial segments. The maximum response to acetylcholine in both preparat ions was reduced by between 35% to 40% while the EC(50) values were in creased by 5-6 fold. L-NAME had no effect on basal smooth muscle tone in either case. 4 In contrast, endothelium-dependent dilatation of the perfused mesenteric bed evoked by A23187 (0.002-20 nmol), was unaffec ted by exposure to L-NAME. The EC(50) values and maximal responses eli cited by A23187 (20 nmol) before and after exposure to L-NAME were 0.9 6 +/- 0.5 nmol and 67.0 +/- 7.0% (n = 4), and 0.7 +/- 0.4 nmol and 70. 0 +/- 5.0% (n = 4; P>0.01), respectively. 5 Perfusion of the isolated mesenteric bed with raised K+-Krebs buffer (25 mM) had no effect on ba sal tone, but reduced the amplitude of both acetylcholine- and A23187- evoked dilatation. The maximum responses to acetylcholine (2 mu mol) a nd A23187 (20 nmol) were reduced from 67.5 +/- 7.3% and 65.4 +/- 8.2% to 18.9 +/- 11.0% (n = 5; P<0.01) and 13.5 +/- 12.0% (n = 4; P<0.01), respectively. 6 Exposure of the mesenteric bed to L-NAME in the presen ce of raised K+-Krebs further reduced the maximal response elicited by acetylcholine to only 8.9 +/- 2.8% (n = 4; P<0.01). 7 These results i ndicate that acetylcholine-evoked vasodilatation of the rat mesenteric vasculature is mediated by both NO-dependent and -independent mechani sms. The relative contribution made by these mechanisms does not appea r to differ in sequential branches of the mesenteric artery. In contra st, A23187-evoked vasodilatation appears to be mediated predominantly by a NO-independent mechanism which is sensitive to increases in the e xtracellular potassium concentration and may reflect the action of end othelium-derived hyperpolarizing factor (EDHF).