Functional and Molecular Characterisation of Lactic Acid Transport in Bovine Articular Chondrocytes

Chondrocytes, which control the turnover of cartilage, undergo predominantly glycolytic metabolism due to the avascular nature of the tissue. This will result in high levels of lactic acid production, and this lactic acid must leave the cells for their normal intracellular pH to be maintained. However to date the mechanism by which lactic acid is removed from the chondrocytes has not been elucidated. In the present study lactic acid transport has been characterised using the intracellular pH-sensitive fluorimetric dye BCECF to measure intracellular pH (pH(i)). Addition of extracellular lactic acid-induced an acidification which was sensitive to alpha-cyano-4-hydroxycinnamate (alpha-CHC) and phloretin indicating the involvement of isoform(s) of the monocarboxylate transporter (MCT) family. The results studies of transport kinetics were consistent with the MCT4 isoform (K(m) 14.1mM), common to other glycolytic cells. Western blotting confirmed that MCT4 was the predominantly expressed isoform, although both MCT1 and MCT4 transcripts were present when cells were assayed by RT-PCR. Through effects on pH(i), the activity of this transporter may therefore modify cartilage turnover.