Intracellular PO2 in individual cardiac myocytes in dogs, cats, rabbits, ferrets, and rats

Myoglobin (Mb) saturation in individual subepicardial myocytes was determined by cryospectroscopy in dogs, cats, ferrets, rabbits, and rats. Mb saturation within 800 microns of the epicardium is not affected by quick freezing or absorption of light by cytochromes. The PO2 in equilibrium with Mb (PMbO2) was calculated from the Mb oxydissociation curve. The minimum PMbO2 found among the 1,000 cells examined was 2.5 Torr, at least five times the critical PO2 for cytochrome turnover in myocardium. The maximum PMbO2 found was about one-half that in subepicardial venules, suggesting a large change in PO2 between capillaries and the cytosol. PMbO2 was the same in right and left ventricles and was unchanged by moderate hemodynamic stress. Median PMbO2 was remarkably uniform among species (range, 4.3-7.0 Torr in 20 animals), even though left ventricular work per minute varied approximately 200-fold, heart rate about fivefold, and arterial O2 content about twofold. Relatively uniform Mb saturation below venous PO2 should accelerate release of O2 from capillaries, promote Mb-facilitated O2 diffusion, and minimize diffusive O2 shunting.