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

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Abstract
The cerebellar nerve cell line ε1 has a very effective active transport system for glutamate. Glutamate uptake is dependent on extracellular Na+ and furthermore, 22Na+ 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 Km for glutamate is found to be 5 × 10−5M in both the glutamate uptake assay and the 22Na+ 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 22Na+ uptake, showing that this Na+ flux does not occur via voltage‐dependent Na+ channels. Studies on the specificity of the ε1 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 22Na+ 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, ε1 does not have an excitatory glutamate receptor.

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