Metabolic recovery of isolated adult rat cardiomyocytes after energy depletion: existence of an ATP threshold?

The question was investigated whether cardiomyocytes can be resuscitated after extreme energy depletion, i.e. after loss of ATP >70%. Isolated ventricular cardiomyocytes of the adult rat were exposed to metabolic inhibition with dinitrophenol and cyanide (DNP 0.2 mm; KCN 2 mm). After rapid energy depletion, cells were "reoxygenated" by wash-out of DNP and KCN. Intracellular calcium, cell length, ATP and creatine phosphate (CrP) of the cardiomyocytes were monitored. Metabolic inhibition resulted in a depletion of the stores of ATP and CrP by more than 95% of the normoxic values and caused a cytosolic Ca2+ overload. Parameters of metabolic recovery were: (i) resynthesis of CrP; (ii) recovery of a normal cytosolic Ca2+ control; and (iii) the elicitation of energy-dependent hypercontracture. "Reoxygenation", i.e. wash-out of metabolic inhibitors, reactivated oxidative phosphorylation. Consecutively, CrP levels recovered to 76.0+/-7.3%, ATP levels recovered to 10. 4+/-2.3% (means+/-s.dn=10) of the initial normoxic values, a normoxic intracellular calcium level was re-established and hypercontracture was elicited. Prolongation of metabolic inhibition with KCN (2 mm) or inhibition of the Na+/K+ pump with ouabain (0.5 mm) disabled the cardiomyocytes to recover from cytosolic Ca2+ overload and prevented hypercontracture. It is concluded that even after extensive energy depletion metabolic resuscitation of the myocardial cell remains possible and a critical range of ATP for recovery, i.e. a "threshold" of a 70% loss of ATP, does not exist.