Oxygen sensitivity of mitochondrial metabolic state in isolated skeletal and cardiac myocytes

In striated muscle the coupling of blood flow to changes in tissue metabolism is hypothesized to be dependent in part on release of vasodilating metabolic by-products generated when mitochondrial metabolism becomes O₂ limited. Cytochrome oxidase, the terminal step in oxidative phosphorylation, is half-maximally saturated at <1 mmHg PO₂ in isolated mitochondria. However, blood flow is regulated at tissue PO₂ of ~20 mmHg. If the affinity of mitochondrial respiration for O₂ were higher in vivo than in vitro, O₂ limitation of mitochondrial metabolism near mean tissue levels could occur. In the present study the PO₂ at which mitochondrial metabolism becomes inhibited (critical PO₂) was measured for cardiac myocytes in suspension (1.1 ± mmHg) and single cells (1.0 ± 0.22 and 1.25 ± 0.22 mmHg in cardiac myocytes and rat spinotrapezius cells, respectively). These measurements are consistent with those from isolated mitochondria, indicating that vasodilators produced when oxidative phosphorylation becomes inhibited may be important for regulating blood flow only in highly glycolytic muscles or under conditions of severe O₂ limitation.