Abstract
The mechanism and membrane localization of choroid plexus (CP) organic anion transport were determined in apical (or brush border) membrane vesicles isolated 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; an apical marker in CP) and demonstrated specific, sodium-coupled transport of proline, glucose, and glutarate. Vesicular uptake of the anionic herbicide 2, 4-dichlorophenoxyacetic acid (2,4-D) was markedly stimulated by an inward sodium gradient but only in the presence of glutarate, indicating the presence of apical dicarboxylate/organic anion exchange. Consistent with this interpretation, an imposed outward glutarate gradient stimulated 2,4-D uptake in the absence of sodium. Under both conditions, uptake was dramatically slowed and overshoot was abolished by probenecid. Likewise, apical accumulation of 2,4-D by intact bovine choroid plexus tissue in vitro was stimulated 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) approximately 8) and p-aminohippurate (T/M = 2) transport. Finally, since the renal exchanger (rROAT1) has been cloned in rat kidney, a rROAT1-green 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 basolateral membranes of the CP.
Highlights
From the Laboratory of Pharmacology and Chemistry, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709
The mechanism and membrane localization of choroid plexus (CP) organic anion transport were determined in apical membrane vesicles isolated from bovine choroid plexus and in intact CP tissue from cow and rat
Apical membrane vesicles from bovine choroid plexus transported the dicarboxylic acid, glutarate (10 M), very effectively in the presence of a sodium gradient, yielding a 4- to 5-fold overshoot (Fig. 3)
Summary
Vol 274, No 47, Issue of November 19, pp. 33382–33387, 1999 Printed in U.S.A. Mechanism of Organic Anion Transport across the Apical Membrane of Choroid Plexus*. The aims of the studies presented below were 2-fold: 1) to determine the mechanism responsible for removal of organic anions from the CSF, and 2) to establish whether the organic anion transport system was, like Naϩ,Kϩ-ATPase, polarized differently in choroid plexus than in renal epithelium. For these studies, we used the anionic herbicide, 2,4-D, since it is transported by the basolateral indirect sodium-coupled mechanism of the kidney [23], and it is transported more effectively by choroid plexus than the classical renal substrate, p-aminohippurate (PAH) [15]
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