Force and membrane potential during and after fatiguing, continuous high‐frequency stimulation of single Xenopus muscle fibres

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Abstract
‘High‐frequency fatigue’ has been studied in isolated, short toe muscle fibres of Xenopus with recordings of tension and membrane potential during the development of fatigue and during recovery. With 70 Hz, continuous stimulation (22.5 oC) tension fell to 20% of the original in 11.6 ±4.9 s. Tension recovery occurred in two phases: an initial, fast phase to 45–80% of the original with fi of about 1 s, and a second, much slower phase. The membrane potential preceding action potentials (Em) fell gradually to about – 50 mV during stimulation and returned to a level of – 66 to – 70 mV with a time‐constant of about 10 ms when the stimulation was stopped. Action potentials towards the end of stimulation were reduced in amplitude, had a slow time‐course, and showed no signs of an early negative after‐potential (EAP). Increasing[K]o to 14 mM (m.p. – 56 to – 52 mV) caused similar changes in action potential configuration, except that an EAP seemed to be present. Those action potentials were associated with potentiated twitch response reaching 70% of original tetanic tension. It is concluded that mechanical fatigue is unlikely to be due to ‘fatigue’ of surface action potentials per se; a more likely mechanism would be impaired t‐tubule function, associated with membrane depolarization (Em becoming more positive) and possibly compounded by ionic changes of the t‐tubule fluid.