Modulation Produced by Nifedipine of the Unitary Ba Current of Dispersed Smooth Muscle Cells of the Rabbit Ileum

Using a whole-cell clamp technique on longitudinal smooth muscle cells of the rabbit ileum, it was found that the rate of decay of the macroscopic Ba current evoked by depolarization (to +20 mV) was not modified by changing the holding potential from -60 mV to -30 mV. Cadmium (20 microM) left only a small transient inward current. Using the patch clamp technique with cell-attached configuration, only one type of unitary Ba current, with conductance of 25 pS was obtained in 100 mM Ba solution. The conductance depended on the external Ba concentration, had a dissociation constant of 19 mM and maximum conductance of 42 pS, suggesting that the Ca channel is of the L-type. Depolarizing pulses (to 0 mV and 150 ms duration) delivered at a frequency of 0.5 Hz (high frequency) evoked a unitary Ba current with low open probability (p less than 0.3). However 65-75% of depolarizing pulses evoked no unitary Ba current ("blank" sweep). On the other hand, depolarizing pulses at 0.033 Hz (low frequency) reduced the number of "blank" sweeps (20-50%), and increased the fraction of sweeps with high open probability (p greater than 0.5). Thus, the channel activity may depend on the stimulus frequency. Nifedipine, in both stimulus conditions, reduced the open probability of the channel due to an increase in the fraction of "blank" sweeps to a greater extent than to a decrease in the time constant of open-times.