Inhibition Of Ca(2+)-ATPase Of Vascular Smooth Muscle Sarcoplasmic Reticulum By Reactive Oxygen Intermediates
Effects of reactive oxygen intermediates generated by hypoxanthine plus xanthine oxidase on the Ca(2+)-adenosinetriphosphatase (ATPase) of sarcoplasmic reticulum from bovine aortic smooth muscle were studied. Exogenous hypoxanthine (0.1-100 microM) plus xanthine oxidase (10 mU/ml) produced an hypoxanthine concentration-dependent inhibition of the Ca(2+)-ATPase. The inhibition could be completely blocked by superoxide dismutase (100 U/ml) but not by either mannitol (20 mM) or deferoxamine (100 microM). Direct addition of hydrogen peroxide in the micromolar range did not cause significant inhibition. These results suggest that superoxide is the primary damaging species. Cysteine blocked this inhibition, suggesting possible involvement of sulfhydryl groups in the inhibition mechanism. Additionally, 1.16 +/- 0.17 mU/g wet wt of xanthine oxidase activity was detected in the postnuclear supernatant of bovine aortic smooth muscle, suggesting the existence of a possible intracellular source of superoxide. This value was calculated to be approximately 5 mU/ml by using a usual value of vascular smooth muscle cellular volume. Thus the level of endogenous xanthine oxidase in vascular smooth muscle is comparable with the level of exogenous xanthine oxidase used in the present study. These findings suggest a potential role of xanthine oxidase-generated superoxide in oxidative damage to vascular smooth muscle during a number of pathophysiological conditions.