Calcium channels involved in K+- and veratridine-induced increase of cytosolic calcium concentration in human cerebral cortical synaptosomes

Human cerebral cortical synaptosomes were used to study voltage-dependent C a2+ 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 mu M). Increase in cytosolic conce ntration of calcium [Ca2+](i) induced by either stimulus was abolished in t he absence of extracellular Ca2+ ions. omega-Agatoxin IVA inhibited the K+- induced [Ca2+](i) increase concentration-dependently (IC50:113 nM). omega-C onotoxin GVIA (0.1 mu M) inhibited K+-induced [Ca2+](i) increase by 20%. om ega-Conotoxin MVIIC (0.2 mu M) caused an inhibition by 85%, Nifedipine (1 m u M) had no effect on K+-induced [Ca2+](i) increase. Veratridine-induced in crease in [Ca2+](i) was inhibited by omega-conotoxin GVIA (0.1 mu M) and om ega-Agatoxin IVA (0.2 mu M; by about 25 and 45%, respectively). Nifedipine inhibited the veratridine-evoked [Ca2+](i), increase concentration-dependen tly (IC50:4.9 nM); Bay K 8644 (3 mu M) shifted the nifedipine concentration -response curve to the right. Mibefradil (10 mu M) abolished the increase i n [Ca2+](i) evoked by K+ and reduced the increase evoked by veratridine by almost 90%. KB-R7943 (3 mu M) 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 v eratridine depolarization via N- and P/Q-type, but also by L-type Ca2+ chan nels. The toxin- and nifedipine-resistant fraction of the veratridine respo nse may result both from influx via R-type Ca2+ channels and by Ca2+ inward transport via Na+/Ca2+ exchanger.