Effect of calcium on the uptake of glutamate by synaptosomes: possible involvement of two different mechanisms.

The effects of Ca2+ on glutamate uptake by synaptosomes were investigated. Glutamate uptake was decreased in the absence of Ca2+ with 2 mM EGTA added to the medium. After a 8-min preincubation, the uptake reduced to the greatest extent (about 64% of the uptake measured in Ca2+ medium). According to the kinetic analysis, lack of Ca2+ resulted in a reduction of the Vmax for glutamate uptake. These results suggested external Ca2+ was necessary for the optimal uptake of glutamate. TTX, an inhibitor of the voltage-dependent Na+ channel, almost reversed the reduction of glutamate uptake, indicating that the uptake might be impaired via the partial activation of the voltage-dependent tetrodotoxin-sensitive Na+ channel. On the other hand, glutamate uptake was decreased by trifluoperazine, a calmodulin antagonist, and KN-62, an inhibitor of Ca2+/calmodulin-dependent protein kinase II, implying that intrasynaptosomal Ca2+ might also have a role in the uptake of glutamate, and Ca2+/calmodulin-dependent processes, such as Ca2+/calmodulin-dependent protein kinase II, might be involved in the modulation of glutamate transporter activity.