Lack of excitatory amino acids-induced effects on calcium fluxes measured with 45Ca2+ in rat cerebral cortex synaptosomes

Ca2+ uptake was measured in purified rat cerebral cortex synaptosomes (P3 pellets) using45Ca2+ as a tracer. Ca2+ influx increased in time, and with an increase in external K+ concentration and temperature. The net (external K+-induced, depolarization-dependent) uptake follows a two-component course. The exponential term, due to the opening of voltage-operated calcium channels (VOC), has a rate constant which increases with an increase in the depolarization level (1.04 versus 0.54 nmol/s/mg protein for 50 mM-versus 15 mM [K+]-dependent net influx). The linear term, due to the Na+/Ca2+ exchange system, has a similar rate constant at all depolarization levels (0.16+/-0.05 and 0.11+/-0.02 nmol/s/mg protein). Excitatory amino acids (glutamate, kainate and n-methyl-d-aspartate-NMDA-) were tested on this preparation at doses ranging between 5×10-5 M and 5×10-3M and at multiple incubation times, under resting conditions and under two depolarizing conditions (partial depolarization: 15 mM exter...