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Articles in PresS, published online ahead of print November 27, 2002
Am J Physiol Cell Physiol, 10.1152/ajpcell.00291.2002
Submitted on June 24, 2002
Accepted on November 21, 2002
1 Biochemistry and Biophysics, UCSF, San Francisco, California, USA
2 Physiological Sciences, University of Stellenbosch, Stellenbosch, Matieland, South Africa
3 Biochemistry and Biophysics, UCSF, San Francisco, California, USA; Physiological Sciences, University of Stellenbosch, Stellenbosch, Matieland, South Africa
* To whom correspondence should be addressed. E-mail: ck{at}itsa.ucsf.edu.
The role played by ADP in modulating cross-bridge function has been difficult to study, as 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.mm-2, 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.mm-2. The ATPase was inhibited competitively by SL-ADP with an inhibition constant, Ki, of approx. 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.
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