Effects of sustained low-flow ischemia and reperfusion on Ca2+ transients and contractility in perfused rat hearts

We investigated changes in cytoplasmic Ca2+ concentration ([Ca2+](i)) and i n left ventricular contractility during sustained ischemia and reperfusion in isolated beating rat hearts. Hearts from male Sprague-Dawley rats were p erfused retrogradely and were loaded with 4 muM fura-2. Low-flow global isc hemia was induced by reducing perfusion flow to 10% and by electric pacing. The hearts were exposed to ischemia for 10 min or 30 min and then reperfus ed. [Ca2+](i) was measured by monitoring the ratio of 500 nm fluorescence e xcited at 340 and 380 nm while simultaneously measuring left ventricular pr essure (LVP). To determine diastolic [Ca2+](i), background autofluorescence was subtracted. LVP rapidly decreased from 82.3 +/- 8.2 to 17.1 +/- 2.9 mm Hg , whereas the amplitude of the Ca2+ transient did not change significant ly during the first 1 min of ischemia. After 10 min of ischemia, the amplit ude decreased to 60.8 +/- 10.6% (p < 0.05) and diastolic [Ca2+](i) increase d by 26.3 +/- 2.9% (p < 0.001) compared with the pre-ischemic value (n = 8) . When the hearts were reperfused after 10 min of ischemia, the amplitude o f the Ca2+ transient and LVP recovered to 79.0 +/- 7.2% and 73.2 +/- 7.5 mm Hg, respectively. Whereas diastolic [Ca2+](i) decreased to the pre-ischemic value. In the hearts exposed to 30 min of ischemia (n = 10), diastolic [Ca 2+](i) increased even further by 32.7 +/- 5.3% at the end of ischemia and c ontinued increasing during the 10 min of reperfusion by 42.6 +/- 15.6%. Six of 10 hearts developed ventricular fibrillation (VF) and intracellular Ca2 + overload after reperfusion. Recovery of LVP after reperfusion was signifi cantly smaller in the hearts exposed to 30 min of ischemia than in the hear ts exposed to 10 min of ischemia (58.9 +/- 11.7 vs. 97.2 +/- 3.0% of pre-is chemic value, p < 0.05). Diastolic [Ca2+](i) also increased under hypoxic c onditions (N-2 bubbling) in this model. These results suggest that increase s in diastolic [Ca2+](i) might play an important role in myocardial contrac tile dysfunction and viability in ischemia-reperfusion injury.