Asymmetric charge movement in polarized and depolarized muscle fibres of the rabbit.

1. The Vaseline-gap technique was used to record asymmetric charge movement in single white sternomastoid fibres of the rabbit, both when the fibres were normally polarized (holding potential, -90 mV) and when they were subjected to prolonged depolarization (holding potential, 0 mV). 2. In normally polarized fibres, 10 microM-nifedipine suppressed substantial charge movement (charge 1) without any prior depolarization, showing that the asymmetric charge does not have to be activated for the drug to exert its action. 3. Examination of the charge moved over a potential range from -140 to +10 mV confirmed that nifedipine affects only that charge generated at potentials more positive than -60 mV. 4. It was shown that there was charge movement in fibres subjected to prolonged depolarization (charge 2), with the greatest movement of charge occurring at about -71 mV, and that this charge was unaffected by nifedipine. 5. The total capacitive charge, that is, the sum of 'linear' capacitive and asymmetric charges, moved between -90 and -80 mV, was the same in polarized and depolarized fibres. Furthermore, the amount of asymmetric charge moved between potentials in the range -140 to -70 mV was the same in polarized and depolarized fibres. These observations indicate that the asymmetric charge moved between -140 and -70 mV in polarized fibres (charge 1) and depolarized fibres (charge 2) are in fact the same charge movement. 6. These results are inconsistent with the hypothesis that prolonged depolarization or nifedipine can cause a transition between charge 1 and charge 2. The results also suggest that the charge movement involved in calcium release may be quite different from that usually assumed.