Action potential fatigue in single skeletal muscle fibres of Xenopus

Action potential fatigue has been studied in single short toe muscle fibres of Xenopus under three different conditions: in rested fibres which produced maximum tension, in fibres during post-contractile depression (PCD), a state of depressed tension generation but seemingly normal membrane properties, and in fibres de-tubulated by glycerol treatment. The fibres were stimulated continuously at 70 Hz (22.5 degrees C) and membrane potential was measured throughout the stimulation period with an intracellular microelectrode. Rested and PCD fibres exhibited similarities in the development of action potential fatigue during a 30 s stimulation period; the amplitude was reduced by 86 and 70 mV, respectively, and the duration, measured at a level of one-third of the peak amplitude, was increased from 1.1 to 4.2 and 1.3 to 3.7 ms, respectively. De-tubulated fibres were more resistant to action potential fatigue; the amplitude decreased by only 20 and 35 mV during 30 and 60 s of stimulation, respectively, and the duration was increased from 1.1 to 2.7 ms. It is concluded that action potential fatigue in skeletal muscle fibres is primarily caused by failing regenerative activity in the t-tubules, which is reflected in an altered shape of conventionally recorded action potentials.