Cytosolic Ca2+ concentration during Ca2+ depletion of isolated rat hearts

Reperfusion of isolated mammalian hearts with a Ca2+-containing solution after a short Ca2+-free period at 37 degrees C results in massive influx of Ca2+ into the cells and irreversible cell damage: the Ca2+ paradox. Information about the free intracellular, cytosolic [Ca2+] ([Ca2+]i) during Ca2+ depletion is essential to assess the possibility of Ca2+ influx through reversed Na+/Ca2+ exchange upon Ca2+ repletion. Furthermore, the increase in end-diastolic pressure often seen during Ca2+-free perfusion of intact hearts may be similar to that seen during ischemia and caused by liberation of Ca2+ from intracellular stores. Therefore, in this study, we measured [Ca2+]i during Ca2+-free perfusion of isolated rat hearts. To this end, the fluorescent indicator Indo-1 was loaded into isolated Langendorff-perfused hearts and Ca2+-transients were recorded. Ca2+-transients disappeared within 1 min of Ca2+ depletion. Systolic [Ca2+]i during control perfusion was 268 +/- 54 nM. Diastolic [Ca2+]i during control perfusion was 114 +/- 34 nM and decreased to 53 +/- 19 nM after 10 min of Ca2+ depletion. Left ventricular end-diastolic pressure (LVEDP) significantly increased from 13 +/- 4 mmHg during control perfusion after Indo-1 AM loading to 31 +/- 5 mmHg after 10 min Ca2+ depletion. Left ventricular developed pressure did not recover during Ca2+ repletion, indicating a full Ca2+ paradox. These results show that LVEDP increased during Ca2+ depletion despite a decrease in [Ca2+]i, and is therefore not comparable to the contracture seen during ischemia. Furthermore, calculation of the driving force for the Na+/Ca2+ exchanger showed that reversed Na+/Ca2+ exchange during Ca2+ repletion is not able to increase [Ca2+]i to cytotoxic levels.