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Laboratoire de Neurobiologie Cellulaire et Moléculaire, CNRS, Gif-sur-Yvette, France.

1. The intramembrane charge movement was recorded in freshly dissociated Purkinje cells from 14- to 18-day-old mouse cerebellum using the whole-cell voltage clamp technique. 2. After pharmacological elimination of all ionic currents, a depolarizing pulse from a holding potential of -80 mV revealed a transient capacitive outward current at the onset and a transient inward current at the end of the pulse. The amount of charge transferred at the onset (Qon) was equivalent to that moved at the end of the pulse (Qoff). The decay time course of Qon can be fitted by a single exponential curve with a maximum time constant of 1.89 +/- 0.35 ms at 20 mV (n = 11). 3. The charge movement had an S-shaped dependence on test membrane potential, according to a two-state Boltzmann function. The maximum amount (Qmax) of Qon that could be moved was 17.46 +/- 0.83 nC muF-1; the membrane potential at which half the charge movement occurred (V) was 13.48 +/- 2.20 mV and the slope factor (k) was 16.83 +/- 0.84 mV (n = 27). 4. Phenylglyoxal (2 mM), an arginine-specific modifying reagent, reduced Qmax to 60% of control after 20 min treatment. 5. The charge movement was partially immobilized by nifedipine in a dose-dependent manner with an IC50 of 70 nM. The fraction of the nifedipine-sensitive component was 39% of the total charge movement. The potential dependence of the nifedipine-sensitive charge movement could be expressed by a Boltzmann function with values of 7.00 +/- 0.53 nC muF-1 for Qmax, 31.44 +/- 4.23 mV for V and 21.53 +/- 3.18 mV for k (n = 8). 6. The P-type calcium channel specific inhibitor, omega-Aga IVA (250 nM), had no effect on intramembrane charge movement. 7. The above results show that part of the intramembrane charge movement in Purkinje cells may be related to a conformational change of DHP receptors upon membrane depolarization.

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