Vol. 290, Issue 3, 1126-1131, September 1999

Calcium Channels Involved in K⁺- and Veratridine-Induced Increase of Cytosolic Calcium Concentration in Human Cerebral Cortical Synaptosomes¹

Wolfgang Meder, Klaus Fink, Josef Zentner and Manfred Göthert

Department of Pharmacology (W.M., K.F., M.G.) and Clinic for Neurosurgery (J.Z.), University of Bonn, Bonn, Germany

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. -Agatoxin IVA inhibited the K⁺-induced [Ca²⁺]_i increase concentration-dependently (IC₅₀: 113 nM). -Conotoxin GVIA (0.1 µM) inhibited K⁺-induced [Ca²⁺]_i increase by 20%. -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 -conotoxin GVIA (0.1 µM) and -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.