Slowing of relaxation during fatigue in single mouse muscle fibres.

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Abstract
1. In the preceding paper we analysed the force decline in fatigue of isolated mouse muscle fibres. Using the same preparation and stimulation scheme we have now examined another aspect of muscle fatigue, namely slowing of relaxation. 2. Relaxation at the end of a tetanic contraction can usually be divided into two phases, an initial nearly linear force decline, followed by an exponential phase. We have analysed the initial phase in terms of decline rate and duration. In rested fibres the decline rate after a 350 ms tetanus was not affected by a 30% reduction of tetanic tension (obtained by decreasing the stimulation frequency). 3. Relaxation became gradually slower during fatiguing stimulation with a maximum reduction at the time when tetanic tension was reduced to about 75% of the original (end of phase 2, see preceding paper). At this time the decline rate of the linear phase had fallen to 55.2 +/- 2.9% (mean +/- S.E.M., n = 25) and its duration had increased to 151.3 +/- 14.2% of the control (unfatigued 350 ms, 70 Hz tetanus). 4. Short rest periods (duration = 10s), given at various times during fatiguing stimulation, resulted in a clear, but partial, normalization of the relaxation parameters; at the time of maximum slowing the mean decline rate increased from 50.3 to 58.7% and the duration decreased from 167.9 to 144.0% of the control (n = 14). 5. The influence of intracellular acidosis on relaxation was studied by exposing rested fibres to 30% CO2 instead of the normal 5%. This resulted in a decline rate of 43.0 +/- 2.6% and a duration of 221.1 +/- 13.1% of the control (n = 7). 6. In amphibian muscle relaxation is known to become gradually slower during a single, prolonged tetanus. The existence of such an effect also in the present preparation was studied by giving 'interrupted' tetani with a total duration of about 2 s. In rested fibres the mean rate of relaxation was found to fall from 140.9 to 71.8% (n = 11) of the control (end of 350 ms stimulation) with a time constant of about 0.5 s. Thus, a marked slowing during a long tetanus occurs also in mammalian muscle. 7. A distinct slowing of relaxation during prolonged tetani was observed also in the fatigued and in the acidified state. Under these two conditions the mean rate of relaxation fell from 87.0 to 34.0% (n = 3) and from 74.0 to 23.0% (n = 5) of the control, respectively, with time constants similar to that in the rested state.(ABSTRACT TRUNCATED AT 400 WORDS)

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