Force and membrane potential during and after fatiguing, intermittent tetanic stimulation of single Xenopus muscle fibres.

Tension and membrane potential have been measured in isolated, short toe muscle fibres of Xenopus during development of fatigue produced by intermittent, tetanic stimulation (0.5 s long 70 Hz trains repeated at 0.3-0.8 Hz) and during recovery. Fibres could be divided into three groups on the basis of their fatigue resistance. In the first group (type 1 fibres) tension fell to 40% of the original after about 70 tetani, in the second group (type 2 fibres) after about 300 tetani and in the third group (type 3 fibres) after about 600 tetani. Recovery was slow in type 1 and 2 fibres and faster in type 3. The former also displayed post-contractile depression (PCD), a 10-40 min period of severely reduced force production, eventually followed by complete recovery. As a result of fatiguing stimulation the membrane potential fell to -70 to -60 mV. It returned to the pre-stimulation value more rapidly than did tension. At the time of maximum PCD most fibres had repolarized to at least -75 mV and action potentials of normal configuration could be elicited. The results of the present experiments underline the importance of defining fibre types in studies of fatigue, also in amphibian muscle. Together with the results of a previous study (Lännergren & Westerblad 1986) they also demonstrate that both the decline phase and the recovery phase are markedly different when different modes of fatiguing stimulation are used.