Ca2+ entry via P/Q-type Ca2+ channels and the Na+/Ca2+ exchanger in rat and human neocortical synaptosomes.
Rat or human neocortical synaptosomes were used to study the role of voltage-gated Ca(2+) channels and the Na(+)/Ca(2+) exchanger in (45)Ca(2+) influx into nerve terminals. K(+) depolarization-induced (45)Ca(2+) influx through voltage-gated Ca(2+) channels into rat or human synaptosomes was completely blocked by mibefradil 30 microM or Cd(2+) 100 microM but was not affected by tetrodotoxin 1 microM. It was reduced by omega-agatoxin IVA 0.2 microM by 68% in synaptosomes of either species, whereas omega-conotoxin GVIA 0.1 microM and nifedipine 1 microM had no effect. Veratridine-induced (45)Ca(2+) entry into rat neocortical synaptosomes was completely blocked by mibefradil 30 microM, reduced by 80% by Cd(2+) 100 microM, by 90% by tetrodotoxin 1 microM and by 53% by omega-agatoxin IVA 0.2 microM but not by omega-conotoxin GVIA 0.1 microM or nifedipine 1 microM. Na(+)/Ca(2+) exchanger-mediated (45)Ca(2+) uptake into rat neocortical synaptosomes evoked by replacement of Na(+) by choline(+) in the incubation buffer was reduced by KB-R7943 (3-50 microM), an inhibitor of the Na(+)/Ca(2+) exchanger, in a concentration-dependent manner (maximal inhibition by 46% at 50 microM; IC(23%)=7.1 microM). Mibefradil also inhibited the Na(+)/Ca(2+) exchanger-mediated Ca(2+) uptake, although at 3.7 times lower potency (IC(23%)=26 microM). It is concluded that in rat and human neocortical nerve terminals Ca(2+) entry is mediated under physiological conditions by P/Q-type, but not by N- or L-type Ca(2+) channels or the Na(+)/Ca(2+) exchanger. If the cytosolic Na(+) concentration is increased, Ca(2+) is also taken up via the Na(+)/Ca(2+) exchanger. In addition to the ability of mibefradil to block all voltage-operated Ca(2+) channels, this drug is a low potency inhibitor of the Na(+)/Ca(2+) exchanger.
Fink
Klaus
K
Institut für Pharmakologie und Toxikologie, Universitätsklinikum Bonn, Reuterstrasse 2b, 53113 Bonn, Germany. finkk@uni-bonn.de
Meder
Wolfgang P
WP
Clusmann
Hans
H
Göthert
Manfred
M
D000328:Adult; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020865:Calcium Channels, P-Type / Q000378:metabolism*; D020907:Calcium Channels, Q-Type / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D008297:Male; D020748:Mibefradil / Q000494:pharmacology; D019579:Neocortex / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism
D020865:Calcium Channels, P-Type; D020907:Calcium Channels, Q-Type; D019831:Sodium-Calcium Exchanger; D020748:Mibefradil; D002118:Calcium