Calcium ions induce glutamate transport into rat brain membrane vesicles in the absence of sodium and chloride. Evidence for a novel uptake site?

Evidence is presented that glutamate binding to rat brain synaptic plasma membranes measured in Cl(-)- and Na+-free but Ca2+-supplemented buffers is partly due to uptake of glutamate into resealed membrane vesicles. An intravesicular volume of 7.9 microliter/mg protein was measured. Ca2+-induced glutamate binding to synaptic plasma membranes was found to be sensitive to low temperatures as well as to an increase in osmolarity and was abolished by short pulses of ultrasonication. None of several glutamate receptor agonists tested discriminated between basal and Ca2+-induced binding, but 4 out of 5 glutamate uptake inhibitors did. The Ca2+-induced increase in glutamate binding was the same irrespective of whether calcium acetate, calcium sulfate or calcium gluconate in either Tris-acetate of Tris-citrate buffer was used.