Aluminum citrate uptake by immortalized brain endothelial cells: implications for its blood-brain barrier transport

The objective was to further test the hypothesis that aluminum (Al) citrate transport across the blood-brain barrier is mediated by a monocarboxylate transporter (MCT). Speciation calculations showed that Al citrates were the predominant Al species under the conditions employed. Al citrate did not inhibit lactate uptake and was not taken up by the rat erythrocyte, suggesting it does not serve as an effective substrate for either MCT1 or the anion exchanger. Studies were conducted with b.End5 cells derived from mouse brain endothelial cells to identify the properties of the carrier(s) mediating Al citrate transport. Western blot analysis of b.End5 cells showed expression of the transferrin receptor and MCT1, but not MCT2 or MCT4. Uptake of Al citrate was approximately 70% faster than citrate. Citrate and Al citrate uptake were sodium independent. Citrate uptake increased at pH 6.9 compared to 7.4, whereas Al citrate uptake did not. Al citrate uptake was reduced by inhibitors of mitochondrial respiration and oxidative phosphorylation, suggesting ATP dependence, but not by ouabain, suggesting no role for Na/K-ATPase. Uptake was not affected by alpha-ketoglutarate or malonate, substrates for the dicarboxylate carrier. Many substrates and inhibitors of MCT1 and organic anion transporters reduced Al citrate uptake into b.End5 cells. BSP and fluorescein, organic anion transporter substrates/inhibitors, inhibited Al citrate uptake. We conclude that Al citrate transport across the blood-brain barrier is carrier-mediated, involving either an uncharacterized MCT isoform expressed in the brain such as MCT7 or MCT8 and/or one of the many members of the organic anion transporting protein family, some of which are known to be expressed at the blood-brain barrier.