An ion exchange chromatographic method for the determination of synaptosomal calcium uptake

A simple, rapid method for determining depolarization-induced⁴⁵Ca influx into synaptosomes is described. Synaptosomes which had been depolarized in the presence of⁴⁵Ca were applied to a small column of Chelex-100 resin to separate free Ca²⁺ from that taken up by the tissue. Approximately 70% of the synaptosomal protein applied to the column was recovered in the initial eluate. The magnitude of⁴⁵Ca uptake was dependent on the amount of Ca²⁺ in the incubation medium and on the KCl concentration. Calcium influx reached a plateau after 90 sec of incubation at 24°C. The Na⁺ channel activator veratridine also produced a substantial influx of⁴⁵Ca, and this effect was blocked by tetrodotoxin. Thus, this ion exchange procedure makes it possible to measure depolarization-induced Ca²⁺ influx in synaptosomes without subjecting them to high vacuum or centrifugation pressures or to EGTA-containing solutions.