Effects of variations in ionic concentrations on high affinity uptake of l-glutamate in non-glutamatergic neurons and non-neuronal 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 Ca(2+) and was not affected by depolarizing concentrations of K(+).
Balcar
V J
VJ
Department of Anatomy, Anderson Stuart Building F13, The University of Sydney, N.S.W. 2006, Australia.