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J. Biol. Chem., Vol. 275, Issue 50, 39279-39286, December 15, 2000

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¹³C/³¹P NMR Assessment of Mitochondrial Energy Coupling in Skeletal Muscle of Awake Fed and Fasted Rats
RELATIONSHIP WITH UNCOUPLING PROTEIN 3 EXPRESSION^*

Beat M. Jucker, Jianming Ren^§, Sylvie Dufour^¶, Xueying Cao^§, Stephen F. Previs, Kevin S. Cadman, and Gerald I. Shulman^¶

From the ^¶ Howard Hughes Medical Institute and the Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520 and the ^§ Department of Metabolic Research, Bristol-Myers Squibb Co., Princeton, New Jersey 08543

To examine the relationship between mitochondrial energy coupling in skeletal muscle and change in uncoupling protein 3 (UCP3) expression during the transition from the fed to fasted state, we used a novel noninvasive ³¹P/¹³C NMR spectroscopic approach to measure the degree of mitochondrial energy coupling in the hind limb muscles of awake rats before and after a 48-h fast. Compared with fed levels, UCP3 mRNA and protein levels in the gastrocnemius increased 1.7- (p < 0.01) and 2.9-fold (p < 0.001), respectively, following a 48-h fast. Tricarboxylic acid cycle flux measured using ¹³C NMR as an index of mitochondrial substrate oxidation was 212 ± 23 and 173 ± 25 nmol/g/min (p not significant) in the fed and 48-h fasted groups, respectively. Unidirectional ATP synthesis flux measured using ³¹P NMR was 79 ± 15 and 57 ± 9 nmol/g/s (p not significant) in the fed and 48-h fasted groups, respectively. Mitochondrial energy coupling as expressed by the ratio of ATP synthesis to tricarboxylic acid cycle flux was not different between the fed and fasted states. To test the hypothesis that UCP3 may be involved in the translocation of long chain free fatty acids (FFA) into the mitochondrial matrix under conditions of elevated FFA availability, [U-¹³C]palmitate/albumin was administered in a separate group of rats with (+) or without () etomoxir (an inhibitor of carnitine palmitoyltransferase I). The ratio of glutamate enrichment ((+) etomoxir/() etomoxir) in the hind limb muscles was the same between groups, indicating that UCP3 does not appear to function as a translocator for long chain FFA in skeletal muscle following a 48-h fast. In summary, these data demonstrate that despite a 2-3-fold increase in UCP3 mRNA and protein expression in skeletal muscle during the transition from the fed to fasted state, mitochondrial energy coupling does not change. Furthermore, UCP3 does not appear to have a major role in FFA translocation into the mitochondria. The physiological role of UCP3 following a 48-h fast in skeletal muscle remains to be elucidated.

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