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Sodium Uptake; Valinomycin; Phlorizin; Permeant Anion; Cotransport System
Abstract:
Brush-border membrane vesicles were isolated from the intestine and kidney of the winter flounder, Pseudopleuronectes americanus, and the transport ofd-glucose, L-alanine and sodium was examined by a rapid filtration technique. D-glucose, L-alanine, and sodium entered the same osmotically reactive space suggesting that uptake into vesicles represents transport across rather than binding to the membrane.
D-glucose and L-alanine uptake by intestinal and renal brush-border membrane vesicles was stimulated by sodium as compared to potassium or choline. In the presence of a sodium chloride gradient, overshooting uptake was observed indicating a transient intravesicular accumulation ofd-glucose and L-alanine. The sodium-dependent D-glucose uptake was inhibited by phlorizin andd-galactose while the transport of L-alanine was inhibited byl-phenylalanine. The sodium-dependent transport of D-glucose and L-alanine was affected by the electrical potential difference across the vesicle membrane; the addition of valinomycin in the presence of an inwardly directed potassium chloride gradient inhibited sodium-dependent solute uptake, whereas replacing chloride or gluconate with more permeant anions, such as SCN−, stimulated uptake. Similar results were obtained with intestinal and renal membranes; they document the presence of sodium/D-glucose and sodium/L-alanine cotransport systems in the brush-border membrane of intestine and kidney.
Sodium uptake into brush border membrane vesicles from the flounder intestine and kidney was saturable (tracer replacement) and trans-stimulated (tracer coupling), indicating transport via facilitated diffusion systems. Additionally, sodium uptake was only slightly affected by superimposing diffusion potentials demonstrating that the majority of sodium transport was by electroneutral coupled processes. In both the intestinal and kidney brush-border membrane vesicles sodium uptake was inhibited by an inwardly directed proton gradient suggesting the presence of a sodium/proton exchange mechanism. In intestinal, but not in renal membrane preparations, sodium uptake was stimulated by chloride. Chloride stimulation was abolished after preincubation with furosemide indicating the presence of an additional coupled sodium-chloride transport in the intestinal brush-border membranes.
