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1 Department of Radiology, University of Washington Medical Center, Seattle, WA, USA
2 Children's Hospital and Regional Medical Center, Seattle, WA, USA
3 Department of Radiology, University of Washington Medical Center, Seattle, WA, USA; Department of Physiology and Biophysics and Bioengineering, University of Washington Medical Center, Seattle, WA, USA; Children's Hospital and Regional Medical Center, Seattle, WA, USA
4 Children's Hospital and Regional Medical Center, Seattle, WA, USA; Department of Pediatrics, Anesthesiology, and Bioengineering, University of Washington Medical Center, Seattle, WA, USA; Children's Hospital and Regional Medical Center, Seattle, WA, USA
* To whom correspondence should be addressed. E-mail: dmarc{at}u.washington.edu.
In skeletal muscle intracellular PO2 can fall to as low as 2 - 3 mm Hg. This study tests 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 O2) and intracellular PO2 using the decline in hemoglobin and myoglobin saturation in ischemic hindlimb muscle of Swiss-Webster mice. 31P magnetic resonance spectroscopic determinations yielded [PCr] and pH in the same muscle volume. Intracellular PO2 fell to <2 mm Hg during the ischemic period without 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 O2. In vivo M O2 in mouse skeletal muscle was increased by systemic treatment with 2 and 4 mg/kg 2,4-dinitrophenol to partially uncouple mitochondria. M O2 was not dependent on intracellular PO2 above 3 mm Hg in the three groups, despite a three-fold increase in M O2. These results indicate that M O2 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.
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