Short-term hibernation in adult cardiomyocytes is Po 2 dependent and Ca2+mediated
The mechanism of myocardial hibernation, the reversible downregulation of contractile activity on reduction of coronary flow with unchanged cardiac energetics, is presently not understood. The oxygen consumption (V˙o 2), shortening fraction (Δ L), energy status [phosphocreatine (PCr), ATP, and adenosine and lactate release], and free intracellular Ca2+ concentration ([Ca2+]i) were measured in isolated rat cardiomyocytes at precisely controlled ambient Po 2 (Oxystat). When Po 2was reduced from 25 to 6 mmHg, V˙o 2decreased by 50%, while Δ L was downregulated from 11.2 ± 4.1 to 7.6 ± 4.0%, and energy status was unchanged in the steady state (observation time 12 min). Only transiently PCr decreased, and lactate and adenosine release increased. Further reduction of Po 2 (to 3 mmHg) reducedV˙o 2 by 80%, decreased PCr by 35%, moderately increased adenosine and lactate release, and progressively reduced Δ L by 50% (to 5.6 ± 3.3%). All parameters fully recovered during reoxygenation. Po 2-dependent downregulation of Δ Lwas accompanied by a progressive reduction in systolic [Ca2+]i (from 512 ± 110 to 357 ± 91 nmol/l at 6 mmHg and to 251 ± 69 nmol/l at 3 mmHg), whereas diastolic free [Ca2+]i remained unchanged. Therefore, the mechanism of the reversible, Po 2-dependent downregulation of contractile activity (myocardial hibernation) involves a substantial reduction of systolic calcium.