Phosphate transport by capillaries of the blood-brain barrier.
Capillaries were isolated from bovine brain cortex and used for phosphate transport studies. The influx of phosphate through capillary membranes was studied by incubation with [32Pi]phosphate followed by a rapid filtration technique. Phosphate uptake by brain capillaries was mediated by a saturable high-affinity system which is independent of the sodium concentration in the incubation medium. The apparent half-saturation constant (Km) and maximal influx (Vmax) were estimated to 160 microM and 0.37 nmol/mg protein/30 s. Transport was inhibited by the phosphate analogues arsenate and phosphonoformic acid with apparent inhibition constants of 5 and 11 mM, respectively. The metabolic inhibitors cyanide and ouabain had no effect on the transport activity. Competition experiments showed that phosphate uptake was inhibited up to 41% by various anions (pyruvate, acetate, citrate, glutamate, and sulfate). In addition, phosphate uptake was significantly decreased by two selective inhibitors of anionic exchangers, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. Chloride was not a substrate of the phosphate carrier as the replacement of external chloride, by nitrate, thiocyanate, or gluconate, did not increase phosphate transport. Aminohippuric acid and N'-methylnicotinamide, two specific substrates of anionic and cationic drug exchangers, did not compete with the phosphate carrier of cerebral capillaries. However, trans-stimulation with bicarbonate increased phosphate transport by 28%, and this stimulation was inhibited by 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, suggesting that the carrier of the cerebral capillaries could exchange phosphate with bicarbonate.
Dallaire
L
L
Laboratoire de Membranologie Groupe de Recherche en Biothérapeutique Moléculaire, Université du Québec Montréal.
Béliveau
R
R
D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D000618:Aminohippuric Acids / Q000494:pharmacology; D000818:Animals; D000838:Anions; D001149:Arsenates / Q000494:pharmacology; D001639:Bicarbonates / Q000494:pharmacology; D001693:Biological Transport, Active / Q000187:drug effects; D001812:Blood-Brain Barrier; D002196:Capillaries / Q000378:metabolism*; D002417:Cattle; D002540:Cerebral Cortex / Q000378:metabolism; D007700:Kinetics; D010710:Phosphates / Q000378:metabolism*
D000618:Aminohippuric Acids; D000838:Anions; D001149:Arsenates; D001639:Bicarbonates; D010710:Phosphates; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; C025657:arsenic acid; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid