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ENVIRONMENTAL, EXERCISE, AND RESPIRATORY PHYSIOLOGY

Measurement of activation energy and oxidative phosphorylation onset kinetics in isolated muscle fibers in the absence of cross-bridge cycling

Division of Physiology, Department of Medicine, University of California, San Diego, La Jolla, California

Submitted 23 September 2005 ; accepted in final form 16 January 2006

This study utilized N-benzyl-p-toluene sulfonamide (BTS), a potent inhibitor of cross-bridge cycling, to measure 1) the relative metabolic costs of cross-bridge cycling and activation energy during contraction, and 2) oxygen uptake kinetics in the presence and absence of myosin ATPase activity, in isolated Xenopus laevis muscle fibers. Isometric tension development and either cytosolic Ca²⁺ concentration ([Ca²⁺]_c) or intracellular PO₂ (P) were measured during contractions at 20°C in control conditions (Con) and after exposure to 12.5 µM BTS. BTS attenuated tension development to 5 ± 0.4% of Con but did not affect either resting or peak [Ca²⁺]_c during repeated isometric contractions. To determine the relative metabolic cost of cross-bridge cycling, we measured the fall in P (P; a proxy for O₂) during contractions in Con and BTS groups. BTS attenuated P by 55 ± 6%, reflecting the relative ATP cost of cross-bridge cycling. Thus, extrapolating P to a value that would occur at 0% tension suggests that actomyosin ATP requirement is 58% of overall ATP consumption during isometric contractions in mixed fiber types. BTS also slowed the fall in P (time to 63% of overall P) from 75 ± 9 s (Con) to 101 ± 9 s (BTS) (P < 0.05), suggesting an important role of the products of ATP hydrolysis in determining the O₂ onset kinetics. These results demonstrate in isolated skeletal muscle fibers that 1) activation energy accounts for a substantial proportion (42%) of total ATP cost during isometric contractions, and 2) despite unchanged [Ca²⁺]_c transients, a reduced rate of ATP consumption results in slower O₂ onset kinetics.

oxygen consumption; N-benzyl-p-toluene sulfonamide; sarcoplasmic reticulum; sarco(endo)plasmic reticulum calcium-adenosine triphosphatase; adenosine 5'-triphosphate

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