COMPENSATORY UP-REGULATION OF CARDIAC SR CA2+-PUMP BY HEAT-SHOCK COUNTERACTS SR CA2+-CHANNEL ACTIVATION BY ISCHEMIA/REPERFUSION

We tested the hypothesis that heat-shock protected myocardialCa2+-cycling by sarcoplasmic reticulum from ischemia and reperfusion (I/R)injury. Twenty-four hours after increasing body temperature to 42°C for15 min, rat hearts were isolated, Langendorff-perfused, and subjected to 30min ischemia then 30 min reperfusion. Left ventricles were homogenized andtheir ionized Ca2+ concentration monitored with indo-1 during Ca2+-uptake inthe presence and absence of the Ca2+-release channel (CRC) modulatorryanodine. Tissue content of heat-shock protein 72 (HSP 72) was analyzed.Exposure to I/R resulted in a 37% enhancement of CRC activity but noeffect on Ca2+-pumping activity, resulting in 25% decreased netCa2+-uptake activity. Pre-exposure to heat-shock resulted in a 10-foldincrease in HSP 72, and a 25% enhancement of maximal Ca2+-pumpingactivity which counteracted the effect of I/R on CRC and net Ca2+-uptakeactivities. This protection of SR Ca2+-cycling was associated with partialprotection of myocardial physiological performance. Net Ca2+-uptake activitywas correlated with the left ventricular developed pressure and its rate ofchange. We conclude that one of the mechanisms by which heat-shock protectsmyocardium from I/R injury is to upregulate SR Ca2+-pumping activity tocounteract the enhanced SR Ca2+-release produced by I/R.