Bay K 8644 stimulation of calcium entry and endogenous dopamine release in rat striatal synaptosomes antagonized by nimodipine.

The calcium channel agonist Bay K 8644 (0.1-100 nM) significantly increased the fast-phase entry of calcium and release of endogenous dopamine from rat striatal synaptosomes partially depolarized with 15 mM KCl. This increase was completely blocked by 10 nM nimodipine which had no inhibitory effect on calcium influx and dopamine release in the absence of Bay K 8644. Bay K 8644's agonist effect was attenuated with higher KCl concentrations. These findings suggest that Bay K 8644, in combination with partial KCl depolarization, may expose brain synaptosomal calcium channels which are sensitive to nanomolar concentrations of dihydropyridine calcium channel blockers.