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Received November 1, 2006
Revised November 27, 2006
Accepted after revision December 18, 2006
1 University of Washington
2 University of Washington (Current: University of Guelph)
3 Florida State University
* To whom correspondence should be addressed. E-mail: mregnier{at}u.washington.edu.
Thin filament regulation of isometric force redevelopment (ktr) was examined in rabbit psoas fibres by substituting native TnC with either cardiac TnC (cTnC), a site I-inactive skeletal TnC mutant (xsTnC), or mixtures of native purified skeletal TnC (sTnC) and a site I & II-inactive skeletal TnC mutant (xxsTnC). Reconstituted maximal Ca2+-activated force (rFmax) decreased as the fraction of sTnC in sTnC:xxsTnC mixtures was reduced, but maximal ktr was unaffected until rFmax was < 0.2 of pre-extracted Fmax. In contrast, reconstitution with cTnC or xsTnC reduced maximal ktr to 0.48 and 0.44 of control (p < 0.01), respectively, with corresponding rFmax of 0.68 ± 0.03 and 0.25 ± 0.02 Fmax. The ktr-pCa relation of fibres containing sTnC:xxsTnC mixtures (rFmax > 0.2 Fmax) was little effected, though ktr was slightly elevated at low Ca2+ activation. The magnitude of the Ca2+ dependent increase in ktr was greatly reduced following cTnC or xsTnC reconstitution because ktr at low levels of Ca2+ was elevated and maximal ktr was reduced. Solution Ca2+ dissociation rates (koff) from whole Tn complexes containing sTnC (26 ± 0.1 s-1), cTnC (38 ± 0.9 s-1) and xsTnC (50 ± 1.5 s-1) correlated with ktr at low Ca2+ levels and were inversely related to rFmax. At low Ca2+ activation, ktr was similarly elevated in cTnC reconstituted fibres with ATP or when cross-bridge cycling rate was increased with 2-deoxy-ATP. Our results and model simulations indicate little or no requirement for cooperative interactions between thin filament regulatory units in modulating ktr at any [Ca2+] and suggest Ca2+ activation properties of individual troponin complexes may influence the apparent rate constant of cross-bridge detachment.
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