Phosphorylation potential, adenosine formation, and critical PO2 in stimulated rat cardiomyocytes

This study investigated the relationship between O2 consumption (VO2) and energy status in isolated rat cardiomyocytes using a system in which O2 supply (PO2) was maintained constant. For this purpose, VO2, phosphocreatine, ATP, intracellular pH, and adenosine of quiescent and stimulated cardiomyocytes were measured while the ambient PO2 was clamped between 0.1 and 120 mmHg. In quiescent cardiomyocytes (VO2: 7.9 +/- 1.2 nmol.min-1.mg protein-1), the threshold below which respiration decreased (critical PO2) was 1.4 mmHg. Above this value, energy status remained constant; below 1 mmHg, both free ADP and adenosine increased. Stimulation increased VO2 threefold and shifted the critical PO2 to 10 mmHg. Above this value, free ADP and adenosine remained unchanged; between 10 and 5 mmHg. VO2 was reduced but this did not change free ADP or adenosine. These findings demonstrate that 1) under well-oxygenated conditions (PO2 > 10 mmHg), VO2 is not controlled by ADP; 2) similarly, the adenosine formation is independent of VO2; a PO2 < 5 mmHg is a prerequisite for enhanced adenosine formation; and 3) when O2 supply becomes limiting, ATP consumption is downregulated without measurable changes in energy status (hibernation).