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

Thin-filament regulation of isometric force redevelopment (k(tr)) 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 (rF(max)) decreased as the fraction of sTnC in sTnC: xxsTnC mixtures was reduced, but maximal k(tr) was unaffected until rF(max) was < 0.2 of pre-extracted F-max. In contrast, reconstitution with cTnC or xsTnC reduced maximal k(tr) to 0.48 and 0.44 of control (P < 0.01), respectively, with corresponding rF(max) of 0.68 +/- 0.03 and 0.25 +/- 0.02 F-max. The k(tr)-pCa relation of fibres containing sTnC: xxsTnC mixtures (rF(max) > 0.2 F-max) was little effected, though k(tr) was slightly elevated at low Ca2+ activation. The magnitude of the Ca2+-dependent increase in k(tr) was greatly reduced following cTnC or xsTnC reconstitution because k(tr) at low levels of Ca2+ was elevated and maximal k(tr) was reduced. Solution Ca2+ dissociation rates (k(off)) 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 k(tr) at low Ca2+ levels and were inversely related to rF(max). At low Ca2+ activation, k(tr) 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 k(tr) at any [Ca2+] and suggest Ca2+ activation properties of individual troponin complexes may influence the apparent rate constant of cross-bridge detachment.