Thin-filament regulation of force redevelopment kinetics in rabbit skeletal muscle fibres

Abstract

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- and 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 ± 1.2 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. © 2007 The Authors. Journal compilation © 2007 The Physiological Society.