FUNCTIONAL-CHARACTERIZATION OF THE EFFECT OF NIMODIPINE ON THE CALCIUM CURRENT IN RAT CEREBELLAR GRANULE CELLS

1. We investigated the effect of nimodipine on the calcium current in dissociated cerebellar granule cells from 8-day-old rats. We measured the whole cell current and the depolarization-induced internal calcium elevation in Fura2-loaded cells exposed to high-potassium solutions. 2. Nimodipine maximally depressed the peak calcium current from holding potential (V-h) = -80 mV by 25% and the inactivation resistant residual current from V-h = -50 mV by 44%. The nimodipine-sensitive current had the same amplitude under both conditions and the half-maximal inhibition concentration (IC50) was close to 50 nM. 3. In contrast to other components of the calcium current. the nimodipine-sensitive current did not inactivate significantly during 1-s depolarization and it was weakly sensitive to the holding potential and to depolarizing conditioning prepulses. The effect of nimodipine was higher on the current elicited by small depolarizations and the current at -30 mV was depressed by less than or equal to 70%. Depolarizing prepulses enhanced the effect of 1 nM (but not that of 1 mu M) nimodipine. 4. In Fura2-loaded cells, nimodipine strongly antagonized the internal calcium rise due to superfusion with 15-75 mM potassium. The effect was significantly more potent on the internal calcium rise determined by lower depolarizations, with 80% inhibition of the peak response to 25 mM KCl. The dose dependence of this depression was best approximated by a two-site curve with IC50(1) = 0.27 nM and IC50(2) = 65 nM. The time course of recovery was dependent on the duration of treatment, suggesting a close interaction between nimodipine and the lipid membrane. 5. The effect of 1 mu M nimodipine was not influenced by prede-polarizations determined by treatments with calcium-free elevated potassium solutions, but when doses as low as 1 nM were applied the fast decay of the calcium level suggested a voltage dependence of the effect. Nimodipine also depressed less than or equal to 90% of the plateau phase of the depolarization-induced internal calcium rise at different depolarizations. 6. Incubation in omega-conotoxin, fraction GVIA (5 mu M) did not cause any significant decrease in the calcium response, whereas omega-agatoxin, fraction IVA (0.5 mu M) depressed the calcium peak by 40-60% and its effect was additive with that of nimodipine. 7. We conclude that the nimodipine-sensitive current is a persistent, inactivation-resistant current that activates at a membrane potential lower than the other components and is likely to be responsible for the elevation of calcium determined by nonovershooting, prolonged depolarizations in these cells. Low doses of the drug can inhibit calcium channels only when the membrane is depolarized for a quite long time, and this effect might reflect high-affinity binding to the open state of the channel.