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

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