Protection of isolated rat heart against the Ca2+ paradox. Are gap junction channels involved?

Enzyme release from isolated Langendorff-perfused rat hearts was studied under various protective conditions against the Ca2+ paradox. In addition sarcosolic free cation concentrations and the membrane potential were measured employing ion-selective microelectrode techniques during Ca(2+)-free perfusion. Low temperature (18 degrees C), low pH (6.5 or 6.1), and polyethylene glycol (9%) during Ca(2+)-free perfusion all protected isolated hearts against the Ca2+ paradox. Protection could only be afforded if the protective agent was continuously present from the beginning of the Ca(2+)-free perfusion period. A 10 min "normal" Ca(2+)-free pre-perfusion was sufficient to abolish the protective potency of the subsequent perfusion phase in the presence of the protective agent. The gap junction channel blocker heptanol (2 mmol/l) markedly decreased enzyme release during re-perfusion, but did not afford protection. Sarcosolic free cation concentrations were measured during Ca(2+)-free acidic perfusion. [Na+]i was markedly increased to about 44 mmol/l without predisposing to cell damage under these conditions. A marked reduction of cell damage was also afforded under conditions of hypoxia during Ca(2+)- and substrate-free perfusion. Acidosis (pHe = 6.5) under these conditions prevented a lethal increase of [Ca2+]i (2 mumol/l) and partially preserved a negative membrane potential. It is concluded that the predisposition to the Ca2+ paradox is produced by a permeabilisation of gap junction channels at low [Ca2+]e and that upon re-elevation of [Ca2+]e a serious Ca2+ influx proceeds through these leaks.