RETINAL GLIAL-CELL GLUTAMATE TRANSPORTER IS COUPLED TO AN ANIONIC CONDUCTANCE

Application of L-glutamate to retinal glial (Muller) cells results in an inwardly rectifying current due to the net influx of one positive c harge per molecule of glutamate transported into the cell. However, at positive potentials an outward current can be elicited by glutamate. This outward current is eliminated by removal of external chloride ion s. Substitution of external chloride with the anions thiocyanate, perc hlorate, nitrate, and iodide, which are known to be more permeant at o ther chloride channels, results in a considerably larger glutamate-eli cited outward current at positive potentials. The large outward curren t in external nitrate has the same ionic dependence, apparent affinity for L-glutamate, and pharmacology as the glutamate transporter previo usly reported to exist in these cells. Varying the concentration of ex ternal nitrate shifts the reversal potential in a manner consistent wi th a conductance permeable to nitrate. Together, these results suggest that the glutamate transporter in retinal glial cells is associated w ith an anionic conductance. This anionic conductance may be important for preventing a reduction in the rate of transport due the depolariza tion that would otherwise occur as a result of electrogenic glutamate uptake.