Intracellular free Ca2+, Na+, and H+ concentrations in the isolated perfused rat heart during the Ca2+ paradox

Enzyme release from the normothermic perfused rat heart was determined to evaluate myocardial cell damage during the perfusion sequence of the Ca2+ Ca2+ paradox. In addition sarcosolic free Ca2+ and monovalent cation concentrations were measured using ion-selective microelectrodes (ISMEs). A reduction of the extracellular Ca2+ concentration, [Ca2+]e, from 1.0 mmol/l to 0.3 or 0.1 mmol/l, respectively, during the re-perfusion period markedly decreased the rate of enzyme release. Slow Ca2+ channel blockers were less (verapamil) or not at all (nifedipine) effective in providing protection. The sarcosolic free Ca2+ and Na+ concentrations, [Ca2+]i and [Na+]i, were significantly (P < 0.01) elevated during Ca(2+)-free perfusion, [H+]i was not significantly changed, while the membrane potential became continuously more positive. Addition of verapamil to the perfusion medium increased [Na+]i, but did not further increase [Ca2+]i. The critical [Ca2+]i for cell damage was between 12 and 18 mumol/l. It could be demonstrated, that Ca2+ entry during re-perfusion via Na/Ca exchange is thermodynamically unlikely to elevate [Ca2+]i to critical values. It is concluded therefore that the predisposition of the rat heart for the Ca2+ paradox is brought about by membrane leaks, which form during Ca(2+)-free perfusion and that Ca2+ influx into the sarcosol proceeds mainly through these leaks.