Mediation of Sarcoplasmic Reticulum Disruption in the Ischemic Myocardium: Proposed Mechanism by the Interaction of Hydrogen Ions and Oxygen Free Radicals
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Acute myocardial ischemia results in a decrease in developed tension and an increase in resting tension. A breakdown of the excitation-contraction coupling system can explain the behavior of the ischemic muscle at a subcellular level. We have identified a specific defect in the sarcoplasmic reticulum (SR) from the ischemic myocardium; i.e., the uncoupling of calcium transport from ATP hydrolysis. The mediators of this excitation-contraction uncoupling process have not been identified. It is now established that the intracellular pH of the ischemic myocardium is in the range of 6.4 but the role of protons and potential role of free radicals have not been identified. We have hypothesized that protons and free radicals may interact to produce the excitation-contraction uncoupling of the ischemic myocardium. Cardiac SR was isolated from the wall of canine left ventricle and calcium uptake velocity and Ca2+ stimulated-Mg2+ dependent ATPase activity determined. Increasing proton concentration between pH 7.0 and 6.4 significantly reduced calcium uptake rates (pH 7.0 = 0.95 ± 0.02; 6.4 = 0.50 ± 0.02 µmoles Ca2+/ mg-min; p<0.01) with no effect on ATPase activity. Calculated coupling ratios (µmoles Ca2+ /µmoles Pi) decreased from 0.87 ± 0.06 at pH 7.0 to 0.51 ± 0.05 at pH 6.4. At pH 7.0, the generation of exogenous free radicals from the xanthine-xanthine oxidase system significantly depressed both calcium uptake rates (Control = 0.95 ± 0.02; X+XO = 0.15 ± 0.02) and ATPase activity (Control = 1.05 ± 0.02; X+XO + 0.30 ± 0.01 ymoles Pi/mg-min; p<0.01). The decreases in calcium uptake and in ATPase activity were completely reversible with superoxide dismutase (SOD). At pH 6.4 in the presence of xanthine and xanthine oxidase, there is a further depression of calcium uptake rates (Control = 0.50 + 0.02; X+XO = 0.11 ± 0.01; p<0.05) but there is no SOD reversible component. The addition of SOD + 20mM mannitol normalized calcium transport at pH6.4. The calculated coupling ratio at pH 6.4 in the presence of free radicals was 0.13. In contrast sarcoplasmic reticulum isolated from ischemic myocardium demonstrated a significant depression of calcium uptake rates at pH 7.1 which was further accentuated at pH 6.4. Ca2+-ATPase was significantly depressed at pH 7.1 but there was no accentuation at pH 6.4. It is concluded that no single species of free radical can explain the intarcellular excitation-contraction uncoupling of the ischemic myocardium. The system can be explained by the interaction of hydrogen ions and superoxide anions producing both injury to the sarcoplasmic reticulum and the formation of lipid free radicals with hydroxyl-like activity.
KeywordsATPase Activity Sarcoplasmic Reticulum Calcium Uptake Xanthine Oxidase Calcium Transport
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