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1 Department of Radiology, University of Washington, Seattle, WA, USA
2 Department of Pediatrics, University of Washington, Seattle, WA, USA; Department of Anesthesiology, University of Washington, Seattle, WA, USA; Department of Bioengineering, University of Washington, Seattle, WA, USA
3 Children's Hospital and Regional Medical Center, Seattle, WA, USA
4 Department of Radiology, University of Washington, Seattle, WA, USA; Department of Physiology and Biophysics, University of Washington, Seatle, WA, USA; Department of Bioengineering, University of Washington, Seattle, WA, USA
* To whom correspondence should be addressed. E-mail: dmarc{at}u.washington.edu.
The coupling of mitochondrial ATP synthesis and oxygen consumption (P/O) plays a central role in cellular bioenergetics. Reduced P/O values are associated with mitochondrial pathologies that can lead to reduced capacity for ATP synthesis and tissue degeneration. Previous work has found a wide range of values for the P/O in normal mitochondria. To measure mitochondrial coupling under physiological conditions, we have developed a procedure for determining the P/O of skeletal muscle in vivo. This technique measures ATPase and oxygen consumption rates during ischemia, using 31P magnetic resonance and optical spectroscopies, respectively. This novel approach allows the independent quantitative measurement of ATPase and oxygen flux rates in intact tissue. The quantitative measure of oxygen consumption is made possible by our ability to independently measure the saturations of Hb and Mb from optical spectra. Our results indicate that the in vivo P/O in skeletal muscle of the mouse hindlimb is 2.16 ± 0.24. The theoretical P/O ratio for resting muscle is 2.33. Systemic treatment with 2,4-dinitrophenol to partially uncouple mitochondria does not affect the ATPase rate in the mouse hindlimb, but nearly doubles the rate of oxygen consumption, reducing the in vivo P/O to 1.37 ± 0.22. These results indicate that only a small fraction of the oxygen consumption in resting mouse skeletal muscle is non-phosphorylating under physiological conditions, suggesting that mitochondria are more tightly coupled than previously thought.
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