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Am J Physiol Cell Physiol 284: C816-C825, 2003. First published November 27, 2002; doi:10.1152/ajpcell.00291.2002
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Vol. 284, Issue 4, C816-C825, April 2003

Effect of an ADP analog on isometric force and ATPase activity of active muscle fibers

Christina Karatzaferi1, Kathryn H. Myburgh2, Marc K. Chinn1, Kathleen Franks-Skiba1, and Roger Cooke1

1 Department of Biochemistry & Biophysics, Cardiovascular Research Institute, University of California, San Francisco, California 94143; and 2 Department of Physiological Sciences, University of Stellenbosch, Matieland, Stellenbosch 7602, South Africa

The role played by ADP in modulating cross-bridge function has been difficult to study, because it is hard to buffer ADP concentration in skinned muscle preparations. To solve this, we used an analog of ADP, spin-labeled ADP (SL-ADP). SL-ADP binds tightly to myosin but is a very poor substrate for creatine kinase or pyruvate kinase. Thus ATP can be regenerated, allowing well-defined concentrations of both ATP and SL-ADP. We measured isometric ATPase rate and isometric tension as a function of both [SL-ADP], 0.1-2 mM, and [ATP], 0.05-0.5 mM, in skinned rabbit psoas muscle, simulating fresh or fatigued states. Saturating levels of SL-ADP increased isometric tension (by P'), the absolute value of P' being nearly constant, ~0.04 N/mm2, in variable ATP levels, pH 7. Tension decreased (50-60%) at pH 6, but upon addition of SL-ADP, P' was still ~0.04 N/mm2. The ATPase was inhibited competitively by SL-ADP with an inhibition constant, Ki, of ~240 and 280 µM at pH 7 and 6, respectively. Isometric force and ATPase activity could both be fit by a simple model of cross-bridge kinetics.

skeletal muscle; tension; fatigue; cross-bridge modeling


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