Influx of calcium, strontium, and barium in presynaptic nerve endings.

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Abstract
Depolarization-induced (K-stimulated) influx of 45Ca, 85Sr and 133Ba was measured in synaptosomes prepared from rat brain. There are 2 phases of divalent cation entry, fast and slow; each phase is mediated by channels with distinctive characteristics. The fast channels inactivate (within 1 s) and are blocked by low concentrations (< 1 .mu.M) of La. The slow channels do not inactivate (within 10 s), and are blocked by high concentrations (> 50 .mu.M) of La. Divalent cation influx through both channels saturates with increasing concentrations of permeant divalent cation; each permeant divalent cation species competitively blocks the influx of other permeant species. These results are consistent with the presence of binding sites for divalent cations in the fast and slow channels. The Ca:Sr:Ba permeability ratio, determined by measuring the influx of all 3 spp. in triple-label experiments, was 6:3:2 for the fast channel and 6:3:1 for the slow channel. A simple model for ion selectivity, based on the presence of a binding site in the channel, could account for slow and, to some extent, for fast channel selectivity data.