The low-affinity monocarboxylate transporter MCT4 is adapted to the exportof lactate in highly glycolytic cells

Transport of lactate and other monocarboxylates in mammalian cells is media ted by a family of transporters, designated monocarboxylate transporters (M CTs). The MCT4 member of this family has recently been identified as the ma jor isoform of white muscle cells, mediating lactate efflux out of glycolyt ically active myocytes [Wilson, Jackson, Heddle, Price, Pilegaard, Juel, Bo nen, Montgomery, Hutter and Halestrap (1998)J. Biol. Chem. 273, 15920-15926 ]. To analyse the functional properties of this transporter, rat MCT4 was e xpressed in Xenopus laevis oocytes and transport activity was monitored by flux measurements with radioactive tracers and by changes of the cytosolic pH using pH-sensitive microelectrodes. Similar to other members of this fam ily, monocarboxylate transport via MCT4 is accompanied by the transport of H+ across the plasma membrane. Uptake of lactate strongly increased with de creasing extracellular pH, which resulted from a concomitant drop in the K- m value. MCT4 could be distinguished from the other isoforms mainly in two respects. First, MCT4 is a low-affinity MCT: for L-lactate K-m values of 17 +/-3mM (pH-electrode) and 34+/-5mM (flux measurements with L-[U-C-14]lactat e) were determined. Secondly, lactate is the preferred substrate of MCT4. K -m values of other monocarboxylates were either similar to the K-m value fo r lactate (pyruvate, 2-oxoisohexanoate, 2-oxoisopentanoate, acetoacetate) o r displayed much lower affinity for the transporter (beta-hydroxybutyrate a nd short-chain fatty acids). Under physiological conditions, rat MCT will t herefore preferentially transport lactate. Monocarboxylate transport via MC T4 could be competitively inhibited by alpha-cyano-4-hydroxycinnamate, phlo retin and partly by 4,4'-di-isothiocyanostilbene- 2,2'-disulphonic acid. Si milar to MCT1, monocarboxylate transport via MCT4 was sensitive to inhibiti on by the thiol reagent p-chloromercuribenzoesulphonic acid.