The effect of oxygen free radicals on calcium permeability and calcium loading at steady state in cardiac sarcoplasmic reticulum

Abstract

It has been proposed that oxygen free radical production is an important mediator of the myocardial dysfunction during the course of acute ischemia. We tested this hypothesis by characterizing the pathway of calcium efflux across sarcoplasmic reticulum (SR) membranes affected by oxygen free radicals. The effect of oxygen free radicals on the steady state calcium load, calcium permeability, and Ca,Mg-ATPase activity of isolated canine cardiac SR vesicles was investigated at pH 7.0. In vitro generation of oxygen free radicals by xanthine oxidase (0.09 units/ml), acting on xanthine in doses up to 50 microM as a substrate, increased the permeability of the SR vesicles to calcium, determined by measuring net efflux of calcium after stopping pump-mediated fluxes, and decreased total intravesicular calcium and free intravesicular calcium with no effect on Ca,Mg-ATPase activity. The effect of oxygen free radicals on calcium permeability was calcium gradient-dependent. Xanthine alone or xanthine plus denatured xanthine oxidase had no effect on this system. Superoxide dismutase (SOD, 56 units/ml), but not denatured SOD, significantly inhibited the effect of xanthine-xanthine oxidase reaction. The calcium permeability of the SR membrane decreased with decreasing calcium load. In addition, inasmuch as extravesicular calcium exerts only a slight effect on calcium permeability, the decrease in the permeability with calcium load is specifically related to the calcium load. Oxygen free radical-induced increase in calcium permeability was unaffected by Mg concentration between 2.1 and 21 mM. In summary, our data reveal that .O2- can produce a diminished level of accumulated calcium, which is reflected by the decreased calcium load and an increase in passive calcium permeability, and that the decreased calcium accumulation in the presence of the xanthine-xanthine oxidase system may not be mainly due to an inhibited calcium pump but due to an increased calcium permeability. Our results also suggest that increased SR membrane passive calcium permeability induced by oxygen free radicals is not carrier mediated. It is postulated that, with the oxygen free radical-mediated progressive increase in calcium permeability, free cytosolic calcium concentrations would increase in ischemic myocardium.