CITRATE TRANSPORT IN PROXIMAL CELL-LINE

Citrate uptake into kidney proximal tubules occurs via an apical dicarboxylate transporter and a poorly characterized process in the basolateral membrane. We used OK cells, a cell line derived from opossum kidney, to study citrate transport in proximal tubule-like cells. Citrate uptake into cell monolayers was studied using [C-14]citrate with [H-3]mannitol as a volume marker. Citrate uptake into these cells was sodium dependent and saturable with increasing concentrations of citrate, In contrast to previous models, citrate transport was altered minimally by changes in pH from 6.2 to 7.0 and increased at pH 7.4 to 7.8. A variety of di- and tricarboxylates were tested for interaction with citrate transport. The dicarboxylates succinate, malate, and oxaloacetate at 1 mM concentration inhibited citrate uptake minimally (uptake at least 80% of control); one dicarboxylate, alpha-ketoglutarate, did inhibit citrate uptake significantly. In contrast, the tricarboxylates isocitrate and tricarballylate inhibited citrate uptake significantly, indicating probable competitive inhibition with the transport process. These characteristics are distinctly different from those of the apical membrane dicarboxylate transporter. 1,2,3-Benzenetricarboxylic acid, an inhibitor of the mitochondrial tricarboxylate transporter, did not alter citrate uptake. In conclusion, the OK proximal cell line exhibits a novel citrate transport process compared with the apical transport of citrate described in most proximal systems. This transport process probably involves the trivalent species of citrate in contrast to the usual predominant transport of divalent citrate. This transport process may represent a process similar to that in the basolateral membrane of the proximal tubule.