Varying concentrations of Pi and Ca2+ showed that the rate of tension transient is independent of [Ca2+], while the rate of tension redevelopment is sensitive to [Ca2+].
Calcium concentration does not directly affect the phosphate release or force-generating steps of the cross-bridge cycle in rabbit skeletal muscle fibers.
The effect of varying concentrations of Pi and Ca2+ on isometric force and on the rate of force development in skinned rabbit psoas muscle fibers has been investigated. Steady-state results show that the three parameters that define the force-pCa relation (Po, pK, and n) all vary linearly with log Pi. As Pi increases, Po and pK decrease while n increases. The kinetics of force generation in isometrically contracting fibers were studied by laser flash photolysis of caged phosphate. The observed rate of the resulting tension transient, kPi, is 23.5 +/- 1.7 s-1 at 10 degrees C, 0.7 mM Pi, and is independent of Ca2+ over the range pCa 4.5-7.2. By contrast, kTR, the rate of tension redevelopment following a period of isotonic shortening, is sensitive to Ca2+ and is slower than kPi (kTR = 13.6 +/- 0.2 s-1 at pCa 4.5, 0.7 mM Pi). The results show that Ca2+ does not directly affect the Pi release or force-generating steps of the cross-bridge cycle and show that the observed rate of force development depends on how the measurement is made. The data can be interpreted in terms of a model in which strong cross-bridges activate the thin filament, this activation being modulated by Ca2+ binding to troponin.
Millar et al. (Thu,) reported a other. Varying concentrations of Pi and Ca2+ was evaluated on Isometric force and rate of force development. Varying concentrations of Pi and Ca2+ showed that the rate of tension transient is independent of [Ca2+], while the rate of tension redevelopment is sensitive to [Ca2+].