CHEMOMECHANICAL ENERGY TRANSDUCTION IN RELATION TO MYOSIN ISOFORM COMPOSITION IN SKELETAL-MUSCLE FIBERS OF THE RAT

1. ATP consumption and force development were determined in single ski nned muscle fibres of the rat at 12 degrees C. Myofibrillar ATPase con sumption was measured photometrically from NADH oxidation which was co upled 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 c ompared, 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 abo ve isometric ATP consumption. The increase was much greater in fast fi bres than in slow fibres, but became similar when expressed relative t o the isometric ATPase rate. 4. Efficiency was calculated from mechani cal power and free energy change associated with ATP hydrolysis. Maxim um efficiency was larger in slow than in fast fibres (0.38 +/- 0.04 ve rsus 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 MLC3f. We conclude that both MHC and essential MLC isof orms contribute to determine efficiency of chemo-mechanical transducti on.