Some properties of potassium-stimulated calcium influx in presynaptic nerve endings.

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Abstract
K stimulated 45Ca entry into rat brain synaptosomes was measured at times ranging from 1-60 s. The K-rich solutions were used to depolarize the synaptosomes. Backflux of 45Ca from the synaptosomes was negligible during the 1st 10-20 s of incubation. An initial fast phase of K-stimulated Ca entry, lasting from 1-2 s was observed. This phase was inhibited by low concentrations of La (KI .simeq. 0.3 .mu.M). It was inactivated by incubating the synaptosomes in depolarizing solutions (containing veratridine, gramicidin or elevated [K]o) before the addition of 45Ca. An additional long lasting slow phase of K-stimulated Ca entry was detected. This slow Ca entry was much less sensitive to La (KI > 100 .mu.M), and was not affected by depolarizing the synaptosomes before the addition of 45Ca. The rate of influx during the fast phase was about 4 times the rate of Ca influx during the slow phase. Neither the fast nor slow phase of Ca entry was sensitive to tetrodotoxin (10 .mu.M), a potent blocker of Na channels, but both phases were inhibited by Ni, Mn, Mg and other agents that block Ca channels. The data are consistent with the presence of 2 distinct populations of voltage-regulated, divalent cation-selective pathways for Ca entry in presynaptic brain nerve endings.