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1 Institute of Physiology and Biophysics, University of Aarhus, Århus, Denmark
In skeletal muscle, the intracellular recording of trains of action potentials is difficult owing to the movement of the muscle upon stimulation. A potential tool for the removal of muscle movement is the cross-bridge cycle blocker, N-benzyl-p-toluene sulphonamide (BTS), although the effects of BTS on the passive and active membrane properties of intact muscle fibres are not known. Rat extensor digitorum longus (EDL) muscle was used to show that 50 µM BTS reduced tetanic force to 
10% of control force, without markedly altering muscle excitability. Incubation with BTS did not alter intracellular K+ content or Na+–K+ pump activity, but produced minor decreases in intracellular Na+ content (7%), resting 22Na+ influx (14%) and excitation-induced 22Na+ influx (29%). Despite these alterations to Na+ fluxes, BTS did not impair muscle excitability, since membrane conductance, resting membrane potential (RMP), rheobase current and the amplitude, overshoot and maximum rate of depolarization of the action potential were all unaltered. However, BTS did induce a small (8%) decrease in the maximum rate of repolarization of the action potential and an increase in the refractory period. The minor effects of BTS on muscle membrane properties did not compromise the ability of the muscle to propagate action potentials, even during tetanic stimulation. In conclusion, BTS can be used successfully to reduce contractility, allowing the intracellular recording of action potentials during both twitch and tetanic contraction of nerve-stimulated muscle, thus making it an excellent tool for the study of electrophysiology in fast-twitch skeletal muscle.
(Received 23 June 2005;
accepted after revision 15 July 2005; first published online 19 July 2005)
Corresponding author W. Macdonald: Institute of Physiology and Biophysics, University of Aarhus, DK-8000, Århus C, Denmark. Email: wmd{at}fi.au.dk
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