Is resting muscle oxygen uptake controlled by oxygen availability to cells?

To estimate oxidative capacity of noncontracting rat skeletal muscle, the isolated gracilis muscle was perfused at various high flow rates with high-PO2 (88 kPa) saline-albumin solution and simultaneously perifused at either low (6.3 kPa) or high PO2 in a calorimeter at 28 degrees C. Under low-PO2 perifusion, specific O2 consumption and heat production rates (MO2 and E, respectively) were flow-rate dependent. E values were all larger than those obtained on blood-perfused preparations at 28 degrees C. MO2 reached 0.47 mumol.min-1.g muscle-1 and E reached 4 mW/g. Normalized to 36 degrees C by means of activation energies determined from 30 and 36 degrees C measurements on nonperfused gracilis strips, these maxima correspond to three times the largest MO2 measured by other authors in blood-autoperfused gracilis. Increasing perifusion PO2 from 6.3 to 88 kPa sharply decreased MO2. These results confirm that MO2 of blood-perfused skeletal muscles in vitro (and a fortiori in vivo) is kept much below its maximum for a noncontracting organ; they also suggest that this maximum MO2 is not necessarily an effect of unphysiologically high PO2 in the tissue cells.