Relationship of Dihydropyridine Binding Sites with Calcium‐Dependent Neurotransmitter Release in Synaptosomes

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Abstract
In the present work, we have studied the effect of ruthenium red (RuR), La3+, and 4-aminopyridine (4-AP) on the specific binding of (+)-[3H]PN200-110 to synaptosomes, as well as the effect of nitrendipine, nifedipine, and BAY K 8644 on .gamma.-[3H]aminobutyric acid ([3H]GABA) release induced by potassium depolarization and by 4-AP in synaptosomes. Scatchard plots indicated that neither RuR nor 4-AP modifies the KD and Bmax of [3H]PN200-110 specific binding, whereas La3+ decreased the Bmax by about 25%; when the effect of the drugs on the total binding of PN200-110 was studied, a similar inhibition by La3+ was found. The calcium antagonists, nitrendipine and nifedipine, did not affect at all the potassium-stimulated release of [3H]GABA nor its release induced by 4-AP. The calcium agonist BAY K 8644 failed to affect both the spontaneous and the potassium-stimulated GABA release. Our results suggest that the binding sites of dihydropyridines in presynaptic membranes are not related to the calcium channels involved in neurotransmitter release with which RuR, La3+, and 4-AP interact.

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