1. ATP consumption and force development were determined in single skinned muscle fibres of the rat at 12°C. Myofibrillar ATPase consumption was measured photometrically from NADH oxidation which was coupled to ATP hydrolysis. Myosin heavy chain (MHC) and light chain (MLC) isoforms were identified by gel electrophoresis. 2. Slow fibres (n = 14) containing MHCI and fast fibres (n = 18) containing MHCIIB were compared. Maximum shortening velocity was 1.02 ± 0.63 and 3.05 ± 0.23 lengths s-1, maximum power was 1.47 ± 0.22 and 9.59 ± 0.84 W l-1, and isometric ATPase activity was 0.034 ± 0.003 and 0.25 ± 0.01 mM s-1 in slow and in fast fibres, respectively. 3. In fast as well as in slow fibres ATP consumption during shortening increased above isometric ATP consumption. The increase was much greater in fast fibres than in slow fibres, but became similar when expressed relative to the isometric ATPase rate. 4. Efficiency was calculated from mechanical power and free energy change associated with ATP hydrolysis. Maximum efficiency was larger in slow than in fast fibres (0.38 ± 0.04 versus 0.28 ± 0.03) and was reached at a lower shortening velocity. 5. Within the group of fast fibres efficiency was lower in fibres which contained more MLC(3f). We conclude that both MHC and essential MLC isoforms contribute to determine efficiency of chemo-mechanical transduction.
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