Regulation of striated muscle contraction: Effects of cross-bridge kinetics on calcium(2+) binding to troponin C

Striated muscle contraction is initiated by Ca2+ binding to troponin C (TnC). This Ca2+ binding causes a series of conformational changes in the regulatory proteins on the thin filament, leading to the formation of force generating cross-bridges, consequently force generation and/or shortening. Therefore, Ca2+ binding regulates cross-bridge formation; In turn, the cross-bridges may influence Ca2+ binding to TnC.; In the present study, the hypothesis tested is that the cross-bridge kinetics influences Ca2+ binding to TnC in striated muscle. Mechanical length perturbations are used to change the cross-bridge kinetics, and their effects on Ca2+ binding to TnC are investigated by three different measurements; the intracellular Ca2+ changes, the Ca2+ sensitivity of force and ATPase activity, and the Ca2+ sensitivity of cTnCbadan fluorescence. When the dissociation rate of force generating cross-bridges is increased by mechanical length perturbations, a transient increase in the intracellular Ca 2+ in intact muscle, a reduced Ca2+ sensitivity of force and ATPase activity in skinned fibers, and a reduced Ca2+ sensitivity of the Ca2+ activated cTnCbadan fluorescence in cTnCbadan reconstituted fibers are observed. This suggests that the cross-bridge kinetic change can alter Ca2+ binding to TnC in striated muscle. A three-state model developed can explain all the results obtained in this thesis work.