Electrophysiological analysis of rat renal sugar and amino acid transport

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Abstract
We have used electrophysiological techniques to study various aspects of the transport of glutamate and aspartate in proximal tubules of the rat kidney in vivo. Single tubular cells were punctured with microelectrodes and the response of the cell membrane potential to sudden luminal or peritubular applications of these amino acids was measured. The experiments indicated that a specific transport system exists forl-glutamate andl-aspartate in the brushborder membrane, which does not transport neutral or basic amino acids. The uptake of bothl-amino acids from the lumen into the cell was found to be rheogenic, probably reflecting the cotransport of two Na+ ions together with one amino acid molecule. The transport system has a slightly greater affinity forl-glutamate, but transports the smallerl-aspartate somewhat faster. Besides thel-isomers alsod-glutamate andd-aspartate were found to depolarize the tubular cells which suggests that also thed-isomers are absorbed in the tubule, however they do not seem to use the same transport system as thel-isomers. In addition to the transport system in the brushborder, a similar Na+-dependent, rheogenic transport system forl-glutamate andl-aspartate was also found in the peritubular cell membrane, as deduced from cell depolarizations in response to these substrates applied peritubularly. The simultaneous presence of Na-driven transport systems in the apical and basal cell membrane which is not found with other amino acids, may explain the high intracellular accumulation ofl-glutamate andl-aspartate in the kidney and provides a rational basis for explaining clinically observed cases of dicarboxylic aminoacidurias.

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