Oxygen regulation and limitation to cellular respiration in mouse skeletal muscle in vivo

In skeletal muscle, intracellular PO2 can fall to as low as 2 - 3 mmHg. This study tested whether oxygen regulates cellular respiration in this range of oxygen tensions through direct coupling between phosphorylation potential and intracellular PO2. Oxygen may also behave as a simple substrate in cellular respiration that is near saturating levels over most of the physiological range. A novel optical spectroscopic method was used to measure tissue oxygen consumption ((M) over dot O-2) and intracellular PO2 using the decline in hemoglobin and myoglobin saturation in the ischemic hindlimb muscle of Swiss-Webster mice. P-31 magnetic resonance spectroscopic determinations yielded phosphocreatine concentration ([PCr]) and pH in the same muscle volume. Intracellular PO2 fell to <2 mmHg during the ischemic period without a change in the muscle [ PCr] or pH. The constant phosphorylation state despite the decline in intracellular PO2 rejects the hypothesis that direct coupling between these two variables results in a regulatory role for oxygen in cellular respiration. A second set of experiments tested the relationship between intracellular PO2 and (M) over dot O-2. In vivo (M) over dot O-2 in mouse skeletal muscle was increased by systemic treatment with 2 and 4 mg/kg body wt 2,4-dinitrophenol to partially uncouple mitochondria. (M) over dot O-2 was not dependent on intracellular PO2 above 3 mmHg in the three groups despite a threefold increase in (M) over dot O-2. These results indicate that (M) over dot O-2 and the phosphorylation state of the cell are independent of intracellular PO2 throughout the physiological range of oxygen tensions. Therefore, we reject a regulatory role for oxygen in cellular respiration and conclude that oxygen acts as a simple substrate for respiration under physiological conditions.