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J. Biol. Chem., Vol. 276, Issue 23, 20245-20251, June 8, 2001
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From the 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.
Modulation of Contractile Activation in Skeletal Muscle by a
Calcium-insensitive Troponin C Mutant*
§,
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
*
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.
Copyright © 2001 by The American Society for Biochemistry and Molecular Biology, Inc.
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