Polarized expression of different monocarboxylate transporters in rat medullary thick limbs of Henle.

Extracellular lactic acid is a major fuel for the mammalian medullary thick ascending limb (MTAL), whereas under anoxic conditions, this nephron segment generates a large amount of lactic acid, which needs to be excreted. We therefore evaluated, at both the functional and molecular levels, the possible presence of monocarboxylate transporters in basolateral (BLMVs) and luminal (LMVs) membrane vesicles isolated from rat MTALs. Imposing an inward H(+) gradient induced the transient uphill accumulation of L-[(14)C]lactate in both types of vesicles. However, whereas the pH gradient-stimulated uptake of L-[(14)C]lactate in BLMVs was inhibited by anion transport blockers such as alpha-cyano-4-hydroxycinnamate, 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS), and furosemide, it was unaffected by these agents in LMVs, indicating the presence of a L-lactate/H(+) cotransporter in BLMVs, but not in LMVs. Under non-pH gradient conditions, however, the uptake of L-[(14)C]lactate in LMVs was transstimulated 100% by L-lactate, but by only 30% by D-lactate. Furthermore, this L-lactate self-exchange was markedly inhibited by alpha-cyano-4-hydroxycinnamate and DIDS and almost completely by 1 mM furosemide, findings consistent with the existence of a stereospecific carrier-mediated lactate transport system in LMVs. Using immunofluorescence confocal microscopy and immunoblotting, the monocarboxylate transporter (MCT)-2 isoform was shown to be specifically expressed on the basolateral domain of the rat MTAL, whereas the MCT1 isoform could not be detected in this nephron segment. This study thus demonstrates the presence of different monocarboxylate transporters in rat MTALs; the basolateral H(+)/L-lactate cotransporter (MCT2) and the luminal H(+)-independent organic anion exchanger are adapted to play distinct roles in the transport of monocarboxylates in MTALs.