Characterization of dihydropyridine-sensitive calcium channels in rat brain synaptosomes.

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Abstract
We examined the effects of dihydropyridine Ca2+-channel agonists on synaptosomal voltage-dependent Ca2+ entry and endogenous dopamine release. The (-) isomer of Bay K 8644 and the (+) isomer of Sandoz compound 202-791 were 100-1000 times more potent than their respective opposite enantiomers in enhancing Ca2+ uptake and dopamine release from striatal synaptosomes. The active isomer of each of these compounds increased Ca2+ entry and dopamine release to the same extent at a concentration of 1 nM. Fast-phase Ca2+ entry into synaptosomes isolated from cerebellum, cortex, and hippocampus was sensitive to nanomolar concentrations of Bay K 8644. No effect of Bay K 8644 was observed in synaptosomes isolated from brainstem. Bay K 8644 increased synaptosomal Ca2+ uptake and endogenous dopamine release from striatal synaptosomes only during the initial seconds of KCl-induced depolarization. The greatest increase was observed during the first second of depolarization. No effect was observed after .gtoreq. 5 sec of depolarization. Bay K 8644 did not alter Ca2+ uptake or dopamine release under resting conditions (5 mM KCl) or in response to KCl at > 15 mM. The activity of Bay K 8644 was also attenuated by lowering concentrations of divalent cations in the incubation medium. Agonist activity was observed at Mg2+ concentrations > 500 .mu.M (Ca2+ held at 100 .mu.M) and Ca2+ concentrations > 100 .mu.M (Mg2+ held at 1000 .mu.M). These results suggest that the Ca2+ channels present in synaptosomes are sensitive to nanomolar concentrations of dihydropyridine agonists under a narrow range of experimental conditions.

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