Stretch- and stimulation frequency-induced changes in extracellular action potentials of muscle fibres during continuous activity.

The present investigation aimed to use the extracellular action potentials (ECAPs) of muscle fibres for evaluation of the membrane functional state during long-lasting activity. Repetitive stimulation during 3 min trials at three different frequencies (5, 6.7 and 10 Hz) was applied to isolated frog muscle fibres at four different rates of stretch (up to 35% of the initial length). The evoked changes in the ECAPs were estimated by alterations in: the peak-to-peak amplitude (A); the time intervals between the positive and the negative maxima (T1) and between cross-points of the potential rise and fall with the base line (i.e. the negative phase duration) (T0); as well as in the propagation velocity (PV) of excitation. Due to the significantly shorter duration of continuous activity of the fast muscle fibres (FMFs), at the time-point of potential failure the decrease of PV and the increase in T1 and T0 were more pronounced in the slow muscle fibre (SMF) potentials than in FMF potentials. The amplitude decrease in most of the trials for both fibre types was similar. Up until the end of the trials, the activity of both fibre types was intermittent, and in the majority of trials the percentage changes in the potential parameters held the values reached during the continuous activity. Only the time parameters gradually increased throughout the trial at maximal stretch and 5 Hz stimulation frequency, as for the FMF potentials they were more prolonged than those of the SMF potentials. T1 and T0 reflect the slowing of the depolarization phase and rapid repolarization of the intracellular potential. Hence the duration of the ECAPs was the parameter most affected by the maximal stretch. The changes in the ECAP parameters and PV induced by repetitive stimulation and fibre stretch reflect changes in ionic currents and muscle fibre membrane conductivity.