Ca²⁺ entry via P/Q-type Ca²⁺ channels and the Na⁺/Ca²⁺ exchanger in rat and human neocortical synaptosomes

Abstract.

Rat or human neocortical synaptosomes were used to study the role of voltage-gated Ca²⁺ channels and the Na⁺/Ca²⁺ exchanger in ⁴⁵Ca²⁺ influx into nerve terminals. K⁺ depolarization-induced ⁴⁵Ca²⁺ influx through voltage-gated Ca²⁺ channels into rat or human synaptosomes was completely blocked by mibefradil 30 µM or Cd²⁺ 100 µM but was not affected by tetrodotoxin 1 µM. It was reduced by ω-agatoxin IVA 0.2 µM by 68% in synaptosomes of either species, whereas ω-conotoxin GVIA 0.1 µM and nifedipine 1 µM had no effect. Veratridine-induced ⁴⁵Ca²⁺ entry into rat neocortical synaptosomes was completely blocked by mibefradil 30 µM, reduced by 80% by Cd²⁺ 100 µM, by 90% by tetrodotoxin 1 µM and by 53% by ω-agatoxin IVA 0.2 µM but not by ω-conotoxin GVIA 0.1 µM or nifedipine 1 µM. Na⁺/Ca²⁺ exchanger-mediated ⁴⁵Ca²⁺ uptake into rat neocortical synaptosomes evoked by replacement of Na⁺ by choline⁺ in the incubation buffer was reduced by KB-R7943 (3–50 µM), an inhibitor of the Na⁺/Ca²⁺ exchanger, in a concentration-dependent manner (maximal inhibition by 46% at 50 µM; IC_23%=7.1 µM). Mibefradil also inhibited the Na⁺/Ca²⁺ exchanger-mediated Ca²⁺ uptake, although at 3.7 times lower potency (IC_23%=26 µM). It is concluded that in rat and human neocortical nerve terminals Ca²⁺ entry is mediated under physiological conditions by P/Q-type, but not by N- or L-type Ca²⁺ channels or the Na⁺/Ca²⁺ exchanger. If the cytosolic Na⁺ concentration is increased, Ca²⁺ is also taken up via the Na⁺/Ca²⁺ exchanger. In addition to the ability of mibefradil to block all voltage-operated Ca²⁺ channels, this drug is a low potency inhibitor of the Na⁺/Ca²⁺ exchanger.