MULTIPLE PATHWAYS UNDERLYING ENDOTHELIUM-DEPENDENT RELAXATION IN THE RABBIT ISOLATED FEMORAL-ARTERY

1 In isolated segments of the rabbit femoral artery stimulated with no radrenaline, both acetylcholine (1 nM-10 mu M) and the calcium ionopho re A23187 (1 nM-100 mu M) evoked endothelium-dependent smooth muscle r elaxation and hyperpolarization while bradykinin (0.01-100 nM) had no effect. 2 The nitric oxide synthase inhibitors, N-G-nitro-L-arginine ( L-NOARG; 100 mu M; 20 min) or N-G-nitro-L-arginine methyl ester (L-NAM E; 100 mu M; 20 min) each abolished the hyperpolarization and the majo rity of the relaxation to acetylcholine (maximal response reduced from 96.8 +/- 2.3% to 2.0 +/- 1.4%). 3 The potassium channel blocker, glib enclamide (10 mu M; 10 min) also abolished the change in membrane pote ntial to acetylcholine but did not modify the smooth muscle relaxation . 4 In contrast, neither L-NAME nor glibenclamide modified the compara ble responses of the femoral artery to A23187, which were also unaffec ted by the cyclo-oxygenase inhibitor, indomethacin (10 mu M). 5 In art ery segments stimulated with potassium chloride (25 mM), the maximal c hange in tension and membrane potential evoked by A23187 (100 mu M) wa s significantly reduced from 95.0 +/- 4.5% and 23.0 +/- 2.0 mV to 69.0 +/- 10.1% and 12.0 +/- 1.5 mV, respectively. Under these conditions L -NAME further reduced the relaxation but not the accompanying hyperpol arization to A23187. 6 Endothelium-denuded arterial segments sandwiche d with endothelium-intact 'donor' segments gave qualitatively similar relaxant responses to those described above for acetylcholine and A231 87. 7 Exogenous nitric oxide (0.5-10 mu M) stimulated a transient rela xation in pre-contracted artery segments, which at concentrations abov e 5 mu M was accompanied by smooth muscle hyperpolarization (maximum 8 .5 +/- 3.2 mV; n = 4). The hyperpolarization but not the relaxation to nitric oxide was abolished by either glibenclamide or 25 mM potassium . 8 These data indicate that in the femoral artery, acetylcholine-indu ced relaxation can be attributed solely to the release of nitric oxide from the endothelium, which then stimulates relaxation independently of a change in smooth muscle membrane potential. In contrast, both the relaxation and hyperpolarization evoked by A23187 appear to be mediat ed predominantly by nitric oxide-independent pathways which appear to involve a diffusible factor released from the endothelium. The results suggest that this diffusible hyperpolarizing factor can be released f rom endothelial cells in the femoral artery by A23187 but not by acety lcholine.