Abstract

Human cerebral cortical synaptosomes were used to study voltage-dependent Ca²⁺ channels mediating calcium influx in human axon terminals. Synaptosomes were depolarized by elevation of the extracellular K⁺ concentration by 30 mM or by the addition of veratridine (10 μM). Increase in cytosolic concentration of calcium [Ca²⁺]_i induced by either stimulus was abolished in the absence of extracellular Ca²⁺ions. ω-Agatoxin IVA inhibited the K⁺-induced [Ca²⁺]_i increase concentration-dependently (IC₅₀: 113 nM). ω-Conotoxin GVIA (0.1 μM) inhibited K⁺-induced [Ca²⁺]_iincrease by 20%. ω-Conotoxin MVIIC (0.2 μM) caused an inhibition by 85%. Nifedipine (1 μM) had no effect on K⁺-induced [Ca²⁺]_i increase. Veratridine-induced increase in [Ca²⁺]_i was inhibited by ω-conotoxin GVIA (0.1 μM) and ω-Agatoxin IVA (0.2 μM; by about 25 and 45%, respectively). Nifedipine inhibited the veratridine-evoked [Ca²⁺]_i increase concentration-dependently (IC₅₀: 4.9 nM); Bay K 8644 (3 μM) shifted the nifedipine concentration-response curve to the right. Mibefradil (10 μM) abolished the increase in [Ca²⁺]_i evoked by K⁺ and reduced the increase evoked by veratridine by almost 90%. KB-R7943 (3 μM) an inhibitor of the Na⁺/Ca²⁺ exchanger NCX1, decreased the increase in [Ca²⁺]_i evoked by veratridine by approximately 20%. It is concluded that the increase in [Ca²⁺]_i after K⁺ depolarization caused by Ca²⁺ influx predominantly via P/Q-type Ca²⁺ channels and after veratridine depolarization via N- and P/Q-type, but also by L-type Ca²⁺ channels. The toxin- and nifedipine-resistant fraction of the veratridine response may result both from influx via R-type Ca²⁺ channels and by Ca²⁺ inward transport via Na⁺/Ca²⁺exchanger.