Regulation of contraction kinetics in skinned skeletal muscle fibers by calcium and troponin C

The influences of [Ca2+] and Ca2+ dissociation rate from troponin C (TnC) on the kinetics of contraction (kCa) activated by photolysis of a caged Ca2+ compound in skinned fast-twitch psoas and slow-twitch soleus fibers from rabbits were investigated at 15 °C. Increasing the amount of Ca2+ released increased the amount of force in psoas and soleus fibers and increased kCa in a curvilinear manner in psoas fibers ∼5-fold but did not alter kCa in soleus fibers. Reconstituting psoas fibers with mutants of TnC that in solution exhibited increased Ca2+ affinity and ∼2- to 5-fold decreased Ca2+ dissociation rate (M82Q TnC) or decreased Ca2+ affinity and ∼2-fold increased Ca2+ dissociation rate (NHdel TnC) did not affect maximal kCa. Thus the influence of [Ca2+] on kCa is fiber type dependent and the maximum kCa in psoas fibers is dominated by kinetics of cross-bridge cycling over kinetics of Ca2+ exchange with TnC.