Sodium gradient- and sodium plus potassium gradient-dependentl-glutamate uptake in renal basolateral membrane vesicles |
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Authors: | Bertram Sacktor Isabel L. Rosenbloom C. Tony Liang Linda Cheng |
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Affiliation: | (1) Laboratory of Molecular Aging, Gerontology Research Center, National Institute on Aging, National Institutes of Health, Baltimore City Hospitals, 21224 Baltimore, Maryland |
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Abstract: | Summary A membrane preparation enriched in the basolateral segment of the plasma membrane was isolated from the rat renal cortex by a procedure that included separation of particulates on a self-generating Percoll gradient. The uptake ofl-glutamate by the basolateral membrane vesicles was studied. A Na+ gradient ([Na+]o>[Na+]i) stimulated the uptake ofl-glutamate and provided the driving force for the uphill transport of the acidic amino acid, suggesting a Na+-l-glutamate cotransport system in the basolateral membrane. A K+ gradient ([K+]i>[K+]o) increased the uptake additionally. This effect was specific for K+ (Rb+). The action of the K+ gradient in enhancing the uptake ofl-glutamate had an absolute requirement for Na+. In the presence of Na+, but in the absence of a Na+ gradient. i.e., [Na+]o=[Na+]i, the K+ gradient also energized the concentrative uptake ofl-glutamate. This effect of the K+ gradient was not attributable to an alteration in membrane potential. The finding of a concentrative uptake system forl-glutamate energized by both Na+ ([Na+]o>[Na+]i and K+ ([K+]i>[K+]o) gradients in the basolateral membrane, combined with previous reports of an ion gradient-dependent uphill transport system for this amino acid in the brush border membrane, suggests a mechanism by whichl-glutamate is accumulated intracellularly in the renal proximal tubule to extraordinarily high concentrations. |
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