Sodium gradient- and sodium plus potassium gradient-dependentl-glutamate uptake in renal basolateral membrane vesicles

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Abstract
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.