Hibernation in Noncontracting Mammalian Cardiomyocytes

Background—The aim of the present study was to establish whether isolated neonatal mammalian cardiomyocytes were capable of downregulating energy-using processes other than contraction while maintaining metabolic stability when oxygen availability was reduced. Methods and Results—Metabolic response of cardiomyocytes was investigated under moderate (5 to 6 &mgr;mol/L) and severe (2 to 3 &mgr;mol/L) forms of hypoxia. Cells exposed to oxygen concentrations of 5 to 6 &mgr;mol/L exhibited rates of oxygen consumption, which were decreased to 64% of normoxic rates. Rates of cellular energy usage were decreased because this reduced rate of oxygen consumption was not associated with either decreased intracellular ATP and phosphocreatine concentrations or a compensatory switch to glycolysis. When cells were exposed to oxygen concentrations of 2 to 3 &mgr;mol/L, rates of oxygen consumption decreased to 9% of normoxic rates. This decreased rate of oxygen consumption was associated with energetic stress, because a significant switch to glycolysis occurred and intracellular phosphocreatine concentrations were decreased by 40%. Rates of cellular energy usage were further decreased as indicated by stable intracellular ATP concentrations. Conclusions—Our results suggest that isolated cardiomyocytes are capable of downregulating energy-consuming processes other than contraction when oxygen supply is decreased. Regions of myocardial tissue are also capable of downregulating metabolic activity during ischemia by shutting down contractile activity (myocardial hibernation). We suggest that metabolic downregulation associated with myocardial hibernation may not be exclusively due to reduced rates of contractile activity. Other energy-using processes (eg, protein synthesis, mRNA synthesis, ion channel activity, and proton leak) may also be shut down.