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Ca²⁺ Regulation of Rabbit Skeletal Muscle Thin Filament Sliding: Role of Cross-Bridge Number

Bo Liang ^*, Ying Chen , Chien-Kao Wang ^* , Zhaoxiong Luo ^* , Michael Regnier , Albert M. Gordon ^* and P. Bryant Chase 

Departments of ^* Physiology and Biophysics, Radiology, and Bioengineering, University of Washington, Seattle, Washington 98195; and Department of Biological Science, Florida State University, Tallahassee, Florida 32306

Correspondence: Address reprint requests to P. Bryant Chase, Ph.D., F.A.H.A., Florida State University, Dept. of Biological Science and Program in Molecular Biophysics, Biology Unit One, Tallahassee, FL 32306-4370. Tel.: 850-644-0056; Fax: 850-644-0481; E-mail: chase{at}bio.fsu.edu.

We investigated how strong cross-bridge number affects sliding speed of regulated Ca²⁺-activated, thin filaments. First, using in vitro motility assays, sliding speed decreased nonlinearly with reduced density of heavy meromyosin (HMM) for regulated (and unregulated) F-actin at maximal Ca²⁺. Second, we varied the number of Ca²⁺-activatable troponin complexes at maximal Ca²⁺ using mixtures of recombinant rabbit skeletal troponin (WT sTn) and sTn containing sTnC(D27A,D63A), a mutant deficient in Ca²⁺ binding at both N-terminal, low affinity Ca²⁺-binding sites (xxsTnC-sTn). Sliding speed decreased nonlinearly as the proportion of WT sTn decreased. Speed of regulated thin filaments varied with pCa when filaments contained WT sTn but filaments containing only xxsTnC-sTn did not move. pCa₅₀ decreased by 0.12–0.18 when either heavy meromyosin density was reduced to 60% or the fraction of Ca²⁺-activatable regulatory units was reduced to 33%. Third, we exchanged mixtures of sTnC and xxsTnC into single, permeabilized fibers from rabbit psoas. As the proportion of xxsTnC increased, unloaded shortening velocity decreased nonlinearly at maximal Ca²⁺. These data are consistent with unloaded filament sliding speed being limited by the number of cycling cross-bridges so that maximal speed is attained with a critical, low level of actomyosin interactions.

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