Mechanism of organic anion transport across the apical membrane of choroidplexus

The mechanism and membrane localization of choroid plexus (CP) organic anio n transport were determined in apical (or brush border) membrane vesicles i solated from bovine choroid plexus and in intact CP tissue from cow and rat . Brush border membrane vesicles were enriched in Na+,K+-ATPase (20-fold; a n apical marker in CP) and demonstrated specific, sodium-coupled transport of proline, glucose, and glutarate. Vesicular uptake of the anionic herbici de 2,4-dichlorophenoxyacetic acid (2,4-D) was markedly stimulated by an inw ard sodium gradient but only in the presence of glutarate, indicating the p resence of apical dicarboxylate/organic anion exchange. Consistent with thi s interpretation, an imposed outward glutarate gradient stimulated 2,4-D up take in the absence of sodium. Under both conditions, uptake was dramatical ly slowed and overshoot was abolished by probenecid. Likewise, apical accum ulation of 2,4-D by intact bovine choroid plexus tissue in vitro was stimul ated by external glutarate in the presence of sodium. Glutarate stimulation was abolished by 5 mM LiCl. Identical findings were obtained using rat CP tissue, which showed both sodium/glutarate-stimulated 2,4-D (tissue/medium (T/M) similar to 8) and p-aminohippurate (T/M = 2) transport. Finally, sinc e the renal exchanger (rROAT1) has been cloned in rat kidney, a rROAT1-gree n fluorescent protein construct was used to analyze exchanger distribution directly in transiently transfected rat CP. As predicted by the functional studies, the fluorescently tagged transporter was seen in apical but not ba solateral membranes of the CP.