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Excess recovery heat production by isolated muscles from mice overexpressing uncoupling protein-3

N. A. Curtin, J. C. Clapham * and C. J. Barclay §

Contractile and energetic performance of bundles of muscle fibres from the soleus of mice overexpressing uncoupling protein 3 (UCP-3tg) were compared with the performance of bundles from wild-type mice. Force and heat production were measured during a series of thirty 0.2 s isometric tetani at L_o, the length optimal for force. UCP-3tg fibres were as strong as the wild-type and maintained force in the series equally well; in the first tetanus force was 116.9 ± 15.1 and 133.3 ± 19.7 mN mm^-2 respectively (all values means ± S.E.M., n = 6 for UCP-3tg and n = 5 for wild-type). Heat production was partitioned into initial heat (due to contractile ATPases and the creatine kinase reaction) and recovery heat (due to other ATP-supplying processes) and expressed relative to the first cycle total heat. Initial heat production was similar for the UCP-3tg and wild-type fibres, decreasing during the series from 0.799 ± 0.052 to 0.661 ± 0.061 relative units (UCP-3tg), and from 0.806 ± 0.024 to 0.729 ± 0.039 relative units (wild-type). In both types the recovery heat was small at the start of the series and increased as the series progressed. At the end of the series, recovery heat production by UCP-3tg fibres, 1.575 ± 0.246 relative units, was twice that of the wild-type fibres, 0.729 ± 0.072 relative units. The extra recovery heat represents inefficient recovery in UCP-3tg fibres. This is the first direct evidence of enhanced energy dissipation as heat when UCP-3tg is overexpressed.

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