Objective: We have recently shown that spontaneous Ca2+ oscillations elicit irreversible hypercontracture of cardiomyocytes during reoxygenation. The aim of this study was to investigate whether influx of exterior Ca2+ through the reverse mode of the Na+/Ca2+ exchanger (NCE) contributes to the development of these oscillations and, therefore, to reoxygenation-induced hypercontracture.
Methods: Isolated cardiomyocytes and hearts from rats were used as models. Cardiomyocytes were exposed to 60 min simulated ischemia (pH(o) 6.4) and 10 min reoxygenation (pH(o) 7.4). During reoxygenation cardiomyocytes were superfused with medium containing 1 mmol/l Ca2+ (control), with nominally Ca2+-free medium or with medium containing 10 micromol/l KB-R 7943 (KB), a selective inhibitor of the reverse mode of the NCE.
Results: In reoxygenated cardiomyocytes rapid Ca2+ oscillations occurred which were reduced under Ca2+-free conditions or in presence of KB. Hypercontracture was also significantly reduced under Ca2+-free conditions or in presence of KB. After 30 min of normoxic perfusion isolated rat hearts were subjected to 60 min global ischemia and reperfusion. KB (10 micromol/l) was present during the first 10 min of reperfusion. LVEDP, LVdevP and lactate dehydrogenase (LDH) release were measured. Presence of KB reduced post-ischemic LVEDP and improved left ventricular function (LVdevP). In KB treated hearts the reperfusion induced release of LDH was markedly reduced from 81.1 +/- 9.9 (control) to 49.3 +/- 8.8 U/60 min/g dry weight.
Conclusion: Our study shows that inhibition of the reverse mode of the NCE, during reperfusion only, protects cardiomyocytes and whole hearts against reperfusion injury.