Determinants of relaxation rate in rabbit skinned skeletal muscle fibres

The influence of Ca(2+)-activated force, the rate of dissociation of Ca(2+) from troponin C (TnC) and decreased crossbridge detachment rate on the time course of relaxation induced by flash photolysis of diazo-2 in rabbit skinned psoas fibres was investigated at 15 degrees C. The rate of relaxation increased as the diazo-2 chelating capacity (i.e. free [diazo-2]/free [Ca(2+)]) increased. At a constant diazo-2 chelating capacity, the rate of relaxation was independent of the pre-photolysis Ca(2+)-activated force in the range 0.3-0.8 of maximum isometric force. A TnC mutant that exhibited increased Ca(2+) sensitivity caused by a decreased Ca(2+) dissociation rate in solution (M82Q TnC) also increased the Ca(2+) sensitivity of steady-state force and decreased the rate of relaxation in fibres by approximately twofold. In contrast, a TnC mutant with decreased Ca(2+) sensitivity caused by an increased Ca(2+) dissociation rate in solution (NHdel TnC) decreased the Ca(2+) sensitivity of steady-state force but did not accelerate relaxation. Decreasing the rate of crossbridge kinetics by reducing intracellular inorganic phosphate concentration ([P(i)]) slowed relaxation by approximately twofold and led to two phases of relaxation, a slow linear phase followed by a fast exponential phase. In fibres, M82Q TnC further slowed relaxation in low [P(i)] conditions by approximately twofold, whereas NHdel TnC had no significant effect on relaxation. These results are consistent with the interpretation that the Ca(2+)-dissociation rate and crossbridge detachment rate are similar in fast-twitch skeletal muscle, such that decreasing either rate slows relaxation, but accelerating Ca(2+) dissociation has little effect on relaxation.