Multiple pathways underlying endothelium-dependent relaxation in the rabbit isolated femoral artery

1. In isolated segments of the rabbit femoral artery stimulated with noradrenaline, both acetylcholine (1 nM-10 microM) and the calcium ionophore A23187 (1 nM-100 microM) evoked endothelium-dependent smooth muscle relaxation and hyperpolarization while bradykinin (0.01-100 nM) had no effect. 2. The nitric oxide synthase inhibitors, NG-nitro-L-arginine (L-NOARG; 100 microM; 20 min) or NG-nitro-L-arginine methyl ester (L-NAME; 100 microM; 20 min) each abolished the hyperpolarization and the majority of the relaxation to acetylcholine (maximal response reduced from 96.8 +/- 2.3% to 2.0 +/- 1.4%). 3. The potassium channel blocker, glibenclamide (10 microM; 10 min) also abolished the change in membrane potential to acetylcholine but did not modify the smooth muscle relaxation. 4. In contrast, neither L-NAME nor glibenclamide modified the comparable responses of the femoral artery to A23187, which were also unaffected by the cyclo-oxygenase inhibitor, indomethacin (10 microM). 5. In artery segments stimulated with potassium chloride (25 mM), the maximal change in tension and membrane potential evoked by A23187 (100 microM) was 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 hyperpolarization to A23187. 6. Endothelium-denuded arterial segments sandwiched with endothelium-intact 'donor' segments gave qualitatively similar relaxant responses to those described above for acetylcholine and A23187. 7. Exogenous nitric oxide (0.5-10 microM) stimulated a transient relaxation in pre-contracted artery segments, which at concentrations above 5 microM 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-induced 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 mediated 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 from endothelial cells in the femoral artery by A23187 but not by acetylcholine.