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Originally published In Press as doi:10.1074/jbc.M007371200 on March 21, 2001

J. Biol. Chem., Vol. 276, Issue 23, 20245-20251, June 8, 2001
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Modulation of Contractile Activation in Skeletal Muscle by a Calcium-insensitive Troponin C Mutant*

Carl A. MorrisDagger §, Larry S. Tobacman, and Earl HomsherDagger

From the Dagger  Department of Physiology, School of Medicine, University of California, Los Angeles, CA 90095 and the  Departments of Internal Medicine and Biochemistry, University of Iowa, Iowa City, IA 52252

Calcium controls the level of muscle activation via interactions with the troponin complex. Replacement of the native, skeletal calcium-binding subunit of troponin, troponin C, with mixtures of functional cardiac and mutant cardiac troponin C insensitive to calcium and permanently inactive provides a novel method to alter the number of myosin cross-bridges capable of binding to the actin filament. Extraction of skeletal troponin C and replacement with functional and mutant cardiac troponin C were used to evaluate the relationship between the extent of thin filament activation (fractional calcium binding), isometric force, and the rate of force generation in muscle fibers independent of the calcium concentration. The experiments showed a direct, linear relationship between force and the number of cross-bridges attaching to the thin filament. Further, above 35% maximal isometric activation, following partial replacement with mixtures of cardiac and mutant troponin C, the rate of force generation was independent of the number of actin sites available for cross-bridge interaction at saturating calcium concentrations. This contrasts with the marked decrease in the rate of force generation when force was reduced by decreasing the calcium concentration. The results are consistent with hypotheses proposing that calcium controls the transition between weakly and strongly bound cross-bridge states.

* This work was supported in part by National Institutes of Health Grants AR-30988 (to E. H.) and HL38834 (to L. S. T.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

§ Partially supported by National Institutes of Health Predoctoral Training Grant GM08496. To whom correspondence should be addressed: Dept. of Physiology, Pennsylvania Muscle Institute, University of Pennsylvania, Philadelphia, PA 19104. Tel: 215-898-0046; Fax: 215-573-8871; E-mail: camorris@mail.med.upenn.edu.

Copyright © 2001 by The American Society for Biochemistry and Molecular Biology, Inc.
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