COUPLED TRANSPORT OF GLUTAMATE AND SODIUM IN A CEREBELLAR NERVE CELL LINE

Abstract The cerebellar nerve cell line ε 1 has a very effective active transport system for glutamate. Glutamate uptake is dependent on extracellular Na + and furthermore, 22 Na + uptake is stimulated by glutamate, indicating that glutamate uptake and Na + uptake are coupled. Two molecules of Na + are transported for each molecule of glutamate. The K m for glutamate is found to be 5 × 10 −5 M in both the glutamate uptake assay and the 22 Na + uptake assay, providing additional evidence for glutamate‐Na + coupling. Pre‐incubation with ouabain, which inhibits the Na + ‐K + ATPase, results in a gradual inhibition of glutamate uptake due to the deterioration of the Na + gradient. Tetrodotoxin, however, has no effect on glutamate‐induced 22 Na + uptake, showing that this Na + flux does not occur via voltage‐dependent Na + channels. Studies on the specificity of the ε 1 glutamate transport system show that it is distinct from systems that transport alanine and glycine. l ‐Glutamate, d ‐aspartate, l ‐cysteate, and l ‐cysteine sulfinate are able to utilize the transport system efficiently. d ‐Glutamate, l ‐homocysteate, N ‐methyl‐ d , l ‐aspartate, and kainic acid are very poor substrates for the glutamate transport system, and in addition do not stimulate 22 Na + uptake. These data allow us to distinguish the glutamate transport system from the glutamate receptor which is known to mediate depolarization in response to all nine of the above compounds. Thus, ε 1 does not have an excitatory glutamate receptor.