Cardiac troponin C (TnC) and a site I skeletal TnC mutant alter Ca2+ versus crossbridge contribution to force in rabbit skeletal fibres

We studied the relative contributions of Ca2+ binding to troponin C (TnC) and myosin binding to actin in activating thin filaments of rabbit psoas fibres. The ability of Ca2+ to activate thin filaments was reduced by replacing native TnC with cardiac TnC (cTnC) or a site I-inactive skeletal TnC mutant (xsTnC). Acto-myosin (crossbridge) interaction was either inhibited using N-benzyl-p-toluene sulphonamide (BTS) or enhanced by lowering [ATP] from 5.0 to 0.5 mm. Reconstitution with cTnC reduced maximal force (F-max) by similar to1/3 and the Ca2+ sensitivity of force (pCa(50)) by 0.17 unit (P < 0.001), while reconstitution with xsTnC reduced F-max by similar to2/3 and pCa(50) by 0.19 unit (P < 0.001). In both cases the apparent cooperativity of activation (n(H)) was greatly decreased. In control fibres 3 mum BTS inhibited force to 57% of F-max while in fibres reconstituted with cTnC or xsTnC, reconstituted maximal force (rF(max)) was inhibited to 8.8% and 14.3%, respectively. Under control conditions 3 mum BTS significantly decreased the pCa(50), but this effect was considerably reduced in cTnC reconstituted fibres, and eliminated in xsTnC reconstituted fibres. In contrast, when crossbridge cycle kinetics were slowed by lowering [ATP] from 5 to 0.5 mm in xsTnC reconstituted fibres, pCa(50) and nH were increased towards control values. Combined, our results demonstrate that when the ability of Ca2+ binding to activate thin filaments is compromised, the relative contribution of strong crossbridges to maintain thin filament activation is increased. Furthermore, the data suggest that at low levels of Ca2+, the level of thin filament activation is determined primarily by the direct effects of Ca2+ on tropomyosin mobility, while at higher levels of Ca2+ the final level of thin filament activation is primarily determined by strong cycling crossbridges.