Basolateral localization of flounder Na+-dicarboxylate cotransporter (fNaDC-3) in the kidney of Pleuronectes americanus

Abstract

The purpose of this study was to provide functional and immunocytochemical evidence for the location of the winter flounder (Pleuronectes americanus) sodium-dicarboxylate cotransporter-3 (fNaDC-3) in the basolateral membrane of proximal tubule cells. fNaDC-3 was expressed in Xenopus laevis oocytes. Lowering the external pH from 7.5 to 6.5 or 5.5 modestly decreased the uptake of [C-14]succinate into fNaDC-3 expressing oocytes, but markedly increased the uptake of [C-14]citrate. As measured by the two-electrode voltage-clamp technique, the citrate concentration eliciting half-maximal current, K-0.5, decreased from 490 muM at pH 7.5 to 32 muM at pH 6.0. The maximal inwards current, DeltaI(max), increased from -27 to -72 nA, when bath pH was changed from 7.5 to 6.0. These data suggest that fNaDC-3 translocates preferably divalent citrate. cis-Aconitate, a tricarboxylate that interacts exclusively with basolateral sodium-dicarboxylate cotransport in the rat kidney, was translocated by fNaDC-3 with a K-0.5 of 300 muM. Antibodies raised against an NaDC-3-specific peptide reacted with the basal cell side of flounder renal proximal tubule segment II (PII). No other structures were stained, indicating that fNaDC-3 is located exclusively in the basolateral membrane of PII cells. We assume that fNaDC-3 provides PII cells with Krebs cycle intermediates as fuels and with alpha-ketoglutarate to drive organic anion secretion.