Components of charge movement in rabbit skeletal muscle: the effect of tetracaine and nifedipine.

The effects of tetracaine and nifedipine on asymmetric charge movement in rabbit muscle fibres were examined to investigate whether mammalian charge movement could be subdivided into several components. Tetracaine (0.05-0.2 mM) stopped contraction in every sternomastoid fibre examined (n = 9) and reduced the asymmetric charge (moved by depolarizing steps to 0 mV) by 15% (S.E. of mean 3%). Tetracaine had little effect on the charge moved at potentials more negative than the threshold potential (established in the absence of the drug). Application of the Ca2+ channel blocker nifedipine (2 or 10 microM), reduced the mean maximum asymmetric charge to 50% (+/- 4) of the control value in twenty-three sternomastoid fibres and to 32% (+/- 5) in four soleus fibres. Increasing the concentration of nifedipine to 120 microM had little further effect. The charge moved at potentials more negative than -60 mV was unaffected by nifedipine. A similar result was found with 30 microM-D600 (two fibres). 10 microM-nifedipine completely blocked Ca2+ currents (external [Ca2+] = 8 mM), but 0.15 microM-nifedipine only had a small effect on either the Ca2+ current or charge movement in the four fibres examined. Contractions could no longer be elicited in eleven of eighteen fibres within 6 min of the application of 2 or 10 microM-nifedipine. However, in the remaining seven fibres contractions could be elicited with unchanged thresholds over 30 min, even in the presence of 50 microM-nifedipine. Nifedipine did not noticeably effect q gamma. It is suggested that nifedipine might prevent contraction only when, for other reasons, the normal release of Ca2+ from the sarcoplasmic reticulum has been disrupted and contraction is dependent on the inflow of external Ca2+. The amount of asymmetric charge moved by depolarizing steps was about 50% greater with a holding potential of -110 mV than with one of -90 mV. This 'extra' charge was not suppressed by nifedipine.