OBJECTIVE: The new uncoupling proteins, UCP2 and UCP3, are thought to play a role in energy efficiency in humans. Endurance training has been suggested to have effects on resting metabolic rate and energy efficiency. We therefore determined UCP2 and UCP3 mRNA levels in skeletal muscle of trained and untrained male subjects.

METHODS: Using reverse transcription-polymerase chain reaction (RT-PCR), expression of UCP2, UCP3L and UCP3S mRNA were measured in muscle biopsies from the quadriceps femoris in eight trained (23.9±1.6 y; 70.6±3.1 kg; 14±3% body fat; maximal power output (W max): 5.6±0.4 W/kg; mean±sd) and 10 lean, untrained (22.1±2.9 y; 72.0±7.9 kg; 18±4% body fat; W max: 3.9±0.4 W/kg; mean±sd) subjects. In six of the trained subjects, UCP2 and UCP3 mRNA were measured before and after an exercise bout to exhaustion. To correct for differences in mitochondrial content, levels of UCP2 and UCP3 mRNA were expressed relative to cytochrome-b, a marker of mitochondrial content.

RESULTS: Acute exercise had no effect on the expression of UCP3L or UCP3S, but in five out of six subjects UCP2 expression decreased after exercise, although the difference was not statistically significant (P= 0.11). Trained subjects had significantly reduced mRNA levels of UCP3L (P= 0.028) and UCP3S (P= 0.031). VO 2max expressed per kg of fat-free mass was negatively correlated with UCP3L (r=− 0.61, P= 0.009) and UCP3S (r=− 0.52, P= 0.028). Mechanical efficiency correlated negatively with UCP3L (r=− 0.56, P= 0.019), UCP3S (r=− 0.47, P= 0.048) and tended to correlate with UCP2 (r=− 0.46, P= 0.06).

CONCLUSION: The lower levels of UCP3 mRNA in trained subjects and the inverse relationship of UCP3 expression and mechanical efficiency suggest that exercise training produces an adaptive physiological response in skeletal muscle improving mechanical efficiency.