Effect of temperature on crossbridge force changes during fatigue and recovery in intact mouse muscle fibers

Abstract

Repetitive or prolonged muscle contractions induce muscular fatigue, defined as the inability of the muscle to maintain the initial tension or power output. In the present experiments, made on intact fiber bundles from FDB mouse, fatigue and recovery from fatigue were investigated at 24°C and 35°C. Force and stiffness were measured during tetani elicited every 90 s during the pre-fatigue control phase and recovery and every 1.5 s during the fatiguing phase made of 105 consecutive tetani. The results showed that force decline could be split in an initial phase followed by a later one. Loss of force during the first phase was smaller and slower at 35°C than at 24°C, whereas force decline during the later phase was greater at 35°C so that total force depression at the end of fatigue was the same at both temperatures. The initial force decline occurred without great reduction of fiber stiffness and was attributed to a decrease of the average force per attached crossbridge. Force decline during the later phase was accompanied by a proportional stiffness decrease and was attributed to a decrease of the number of attached crossbridge. Similarly to fatigue, at both 24 and 35°C, force recovery occurred in two phases: the first associated with the recovery of the average force per attached crossbridge and the second due to the recovery of the pre-fatigue attached crossbridge number. These changes, symmetrical to those occurring during fatigue, are consistent with the idea that, i) initial phase is due to the direct fast inhibitory effect of [Pi]i increase during fatigue on crossbridge force; ii) the second phase is due to the delayed reduction of Ca(2+) release and /or reduction of the Ca(2+) sensitivity of the myofibrils due to high [Pi]i.

Figure 1. Typical force and length records from an experiments at 24 and 35°C.

A, fiber length and superimposed tetani at 24 and 35°C (dashed and continuous line, respectively). A portion of the burst of length and force oscillations at 6.5 kHz visible in A is shown at fast time base in B. Note the absence of phase shift between force and length sinusoids. These records were used to measure fiber stiffness. Length oscillation amplitude was 1.90 µm (p-p), corresponding to 0.18% l _f.

Figure 2. Relative average tension and stiffness changes occurring during fatigue.

Tension and stiffness at 24 and 35°C are expressed relatively to plateau control values. Data represent mean ± SEM (n = 9). SEM not visible when smaller than symbols. Note that stiffness falls less than tension at both temperatures.

Figure 3. Average tension fall during the two phases of fatigue.

Temperature increase reduced the fall of force during initial phase but increased that occurring during later phase. At the end of fatigue the loss of tension was the same at 24 and 35°C. Asterisks indicate statistically significant changes (P < 0.05) respect to 24°C.

Figure 4. Half-time of tetanic tension rise during fatigue.

Measures were made at control tetanus, 18^th and 105^th tetanus. Asterisks indicate statistically significant changes (P < 0.05) respect to control values.

Figure 5. Average time course of tension and stiffness during fatigue and recovery.

Experiments were made on a group of bundles (n = 8) different from that used for Figure 2. Empty symbols, stiffness; filled symbols, tension. Note the much faster recovery at 35°C.

Figure 6. Comparison of the stiffness-tension relations during fatigue and tetanic tension rise.

Stiffness and tension are expressed relatively to their plateau values before fatigue. Data during fatigue are the same of Figure 2. The dashed straight line indicates the direct proportionality between tension and stiffness. Measurements on the tetanus rise (filled symbols) and fatigue (empty symbols) were made on the same fiber.

Figure 7. Tension, crossbridge stiffness and average force per crossbridge during fatigue.

Note (left) that the period during which crossbridge stiffness remains almost constant in spite of the fall in tension (continuous and dashed lines, same data of Figure 2), is much longer at 35°C (triangles) than at 24°C (circles). During this phase (right) at both temperatures almost all the drop of force continuous and dashed lines) is accounted for by a reduction of the average force per crossbridge (open symbols).

Figure 8. Time course of average force per crossbridge, crossbridge stiffness and tension during recovery at 24 and 35°C.

Note that the recovery of the average force per crossbridge is already complete 90 s after the end of fatigue at both temperatures whereas the recovery of attached crossbridge number requires a much longer time.