Reversible activation of glutamate transport in rat brain glia by protein kinase C and an okadaic acid-sensitive phosphoprotein phosphatase

High-affinity L-glutamate (GLU) transport is an important regulator of exci tatory amino acid (EAA) concentrations in brain extracellular fluid and may play a key role in excitatory synaptic transmission. In view of evidence t hat EAA transporters (EAAT) are heterogenous and contain consensus sites fo r phosphorylation, this investigation was undertaken to contrast the effect s of transporter phosphorylation in fractions derived from glia and neurons (synaptosomes) of the adult rat forebrain. Treatment with phorbol-12,13-di butyrate (PDBu), an activator of protein kinase C (PKC), increased the maxi mal rate of GLU transport in glial plasmalemmal vesicles by greater than 50 percent (237 +/- 18 vs. 365 +/- 27 pmol/mg protein/90s, p < 0.05) but caus ed no change in synaptosomes. The effect by PDBu was concentration and time -dependent and was inhibited completely by the PKC inhibitor calphostin C. Inhibition of serine-threonine phosphoprotein phosphatases with okadaic aci d produced similar effects which were not additive with PDBu. Together, the se results demonstrate that glial EAAT can be regulated by multiple phospho rylation processes.