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 °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.