EFFECTS OF VARIATIONS IN IONIC CONCENTRATIONS ON HIGH-AFFINITY UPTAKE OF L-GLUTAMATE IN NON-GLUTAMATERGIC NEURONS AND NONNEURONAL CELLS CULTURED FROM NEONATAL RAT CORTEX

High affinity uptake of L-glutamate in the CNS is known to be strongly dependent on Na+. However, the stoichiometry and the mechanism of the relationship between Na+ and L-glutamate at the putative transport site (i.e. whether one or two Na+ are co-transported with one molecule of L-glutamate and in which order they bind to the site) have been the subject of debate because the results of quantitative studies on Na+ -dependence of L-glutamate uptake have varied from one experimental model to another. This communication discusses ionic requirements of L-glutamate uptake in two culture systems derived from neonatal rat cortex - one containing no neurons and the other including, in addition to glial cells, GABAergic neurons. The present results are consistent with a model in which one molecule of L-glutamate is co-transported with either one or two Na+ and no fixed order is required for the binding of Na+ and L-glutamate to the transport site. Also, in contrast to other studies, L-glutamate uptake was found to be significantly reduced in the absence of Ca2+ and was not affected by depolarizing concentrations of K+.