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J. Gen. Physiol.,
Volume 113, Number 5, May 1, 1999 679-694
From the Department of Biomedical Sciences and Consiglio Nationale delle Rícerche Center of Biomembranes, University of Padova,
35121 Padova, Italy
Single channel patch-clamp recordings show that embryonic rat spinal motoneurons express anomalous L-type calcium channels, which reopen upon repolarization to resting potentials, displaying both short and
long reopenings. The probability of reopening increases with increasing voltage of the preceding depolarization
without any apparent correlation with inactivation during the depolarization. The probability of long with respect
to short reopenings increases with increasing length of the depolarization, with little change in the total number
of reopenings and in their delay. With less negative repolarization voltages, the delay increases, while the mean
duration of both short and long reopenings decreases, remaining longer than that of the openings during the
preceding depolarization. Open times decrease with increasing voltage in the range 
60 to +40 mV. Closed times
tend to increase at V > 20 mV. The open probability is low at all voltages and has an anomalous bell-shaped voltage dependence. We provide evidence that short and long reopenings of anomalous L-type channels correspond
to two gating modes, whose relative probability depends on voltage. Positive voltages favor both the transition
from a short-opening to a long-opening mode and the occupancy of a closed state outside the activation pathway within each mode from which the channel reopens upon repolarization. The voltage dependence of the probability of reopenings reflects the voltage dependence of the occupancy of these closed states, while the relative probability of long with respect to short reopenings reflects the voltage dependence of the equilibrium between modes. The anomalous gating persists after patch excision, and therefore our data rule out voltage-dependent block by
diffusible ions as the basis for the anomalous gating and imply that a diffusible cytosolic factor is not necessary for
voltage-dependent potentiation of anomalous L-type channels.
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