The contribution of [Ca2+]i to the slowing of relaxation in fatigued single fibres from mouse skeletal muscle.

1. The contribution of Ca2+ vs. cross-bridges to the slowing of relaxation in fatigue was studied in intact, single fibres dissected from a mouse foot muscle. Fatigue was produced by repeated 350 ms tetani. The free myoplasmic Ca2+ concentration ([Ca2+]i) was measured with indo-1. 2. The rate of [Ca2+]i decline after a tetanus was reduced in fatigue but this reduction appeared smaller than the decrease in relaxation speed. 3. Under control conditions, steady-state [Ca2+]i-tension curves were constructed from measurements of tetani at various stimulus frequencies. Measurements from the final part of fatigue runs, where both tetanic [Ca2+]i and tension fell relatively fast, were used to construct similar curves in fatigue. A comparison of these curves revealed a reduction of both the maximum Ca(2+)-activated tension and the myofibrillar Ca2+ sensitivity in fatigue. 4. Calcium-derived tension records were generated by converting [Ca2+]i signals during tetani into tension by means of the steady-state [Ca2+]i-tension relation. These records would represent a situation where tension responds to changes of [Ca2+]i without delays due to, for instance, cross-bridge attachment or detachment. The relaxation speed of the calcium-derived tension was similar in control and fatigue; the lag between the calcium-derived and the real tension was about twice as long in fatigue. 5. Both the decline of [Ca2+]i and tension were markedly faster after a 10 s pause elicited during fatigue runs at the time of maximum slowing of relaxation. The relaxation of the calcium-derived tension was also faster after the pause. 6. The function of the sarcoplasmic reticulum (SR) Ca2+ pumps was analysed by plotting the rate of [Ca2+]i decline vs. [Ca2+]i during the tail of elevated [Ca2+]i after tetani. This analysis showed approximately a sevenfold reduction of the pump rate in fatigue. 7. In conclusion, the rate of [Ca2+]i decline after a tetanus is reduced in fatigue probably due to impaired SR Ca2+ pumping. This is not the cause of the slowed relaxation because it is counteracted by a reduced myofibrillar Ca2+ sensitivity. Thus, the slowing of relaxation in fatigued mouse muscle fibres would reflect slowed cross-bridge kinetics.