The effect of ryanodine on oxygen free radical-induced dysfunction of cardiac sarcoplasmic reticulum

The effect of the plant alkaloid ryanodine on the cardiac sarcoplasmic reticulum (SR) function, which plays a major role in the regulation of intracellular calcium and thereby in the generation of force, was studied by determining oxalate-supported calcium uptake, steady-state calcium load, calcium permeability, intravesicular-free calcium and Ca,Mg-adenosine triphosphatase (ATPase) activity of "heavy" vesicles in the presence or absence of the oxygen-free radical-generating system. In vitro generation of oxygen-free radicals by xanthine oxidase (0.09 u/ml), acting on xanthine (25 microM) as a substrate, increased the permeability of the vesicles to calcium, determined by measuring net efflux of calcium after stopping pump-mediated fluxes, and decreased oxalate-supported calcium uptake and steady-state calcium load with no effect on Ca,Mg-ATPase activity. This effect of oxygen-free radicals was inhibited completely by superoxide dismutase, which eliminated completely superoxide anion radical production and caused an anticipated increase in hydrogen peroxide from the xanthine-xanthine oxidase reaction in our system. The xanthine-xanthine oxidase reaction decreased intravesicular-free calcium. The diminished level of intravesicular-free calcium, which was reflected by the decreased steady-state calcium load induced by oxygen-free radicals, was prevented by specific closure of the SR calcium release channel by ryanodine under established optimal conditions; under the same conditions, ryanodine also prevented superoxide dismutase-inhibitable reduction of calcium uptake induced by oxygen-free radicals in the presence or absence of oxalate. Ryanodine was without effect on Ca,Mg-ATPase activity by itself and had no effect on any of the changes in calcium permeability mediated by the generation of oxygen-free radicals under the experimental conditions used.(ABSTRACT TRUNCATED AT 250 WORDS)