Inhibition of K +-evoked [ 3H]D-aspartate release and neuronal calcium influx by verapamil, diltiazem and dextromethorphan: evidence for non-L/non-N voltage-sensitive calcium channels

Abstract

The effects of inhibitors of voltage-sensitive calcium channels (VSCC) on K +-evoked [ 3H]D-aspartate release from rat hippocampal slices and the K +-evoked increase in intracellular calcium in neocortical neurons in primary culture were examined. K + caused a concentration-dependent release of [ 3H]D-aspartate that was appoximately 85% dependent on the presence of extracellular calcium. Neither the marine snail toxin, ω-conotoxin GVIA, nor the dihydropyridine VSCC antagonist, nitrendipine, had any effect on K +-evoked release of [ 3H]D-aspartate. ω-Conotoxin GVIA and nitrendipine caused a relatively small (20–30%) inhibition of K +-evoked increase in intracellular calcium in neocortical neurons in primary culture. This suggests that K +-evoked [ 3H]D-aspartate release is not dependent on L- or N-type VSCC, whereas K +-evoked neuronal calcium influx was only partially dependent on L- and N-type VSCC. Verapamil, dextromethorphan and diltiazem caused a concentration-dependent inhibition of K +-evoked release of [ 3H]D-aspartate with IC 50 values of 30, 100 and 120 μM, respectively. The K +-evoked increase in intracellular calcium was inhibited with essentially the same rank order of potency, but with slightly greater potencies (IC 50 values for verapamil, diltiazem and dextromethorphan were 20, 50 and 50 μM, respectively). At 300 μM, neither verapamil, diltiazem nor dextromethorphan inhibited [ 3H]D-aspartate release evoked by the calcium ionophore ionomycin, suggesting that these compounds are not acting intracellularly to inhibit the ability of free cytosolic calcium to evoke release. The results of the present study are consistent with the notion that K +-evoked release of [ 3H]D-aspartate from hippocampal slices is mediated by calcium influx through a channel or channels that are not N- or L-type VSCC, but are sensitive to inhibition by verapamil, diltiazem and dextromethorphan. It remains to be determined whether this represents the activity of a novel VSCC.