FUNCTIONAL EVIDENCE FOR A MONOCARBOXYLATE TRANSPORTER (MCT) IN STRIALMARGINAL CELLS AND MOLECULAR EVIDENCE FOR MCT1 AND MCT2 IN STRIA VASCULARIS

The transport of lactate, pyruvate and other monocarboxylates across p lasma membranes of metabolically active cells such as strial marginal cells (SMC) may be important under aerobic conditions as well as under ischemic and hypoxic conditions. This study addresses the question wh ether SMC from the gerbil contain a membrane transport mechanism for m onocarboxylates. The type of transporter was identified in functional studies by monitoring uptake of monocarboxylates into SMC through meas urement of the cytosolic pH (pH(i)) with the pH-sensitive dye s-(2-car boxyethyl)-5(6)-carboxyfluorescein(BCECF). Further, subtypes of the fu nctionally identified transporter which are present in stria vasculari s were identified as transcripts by cloning and sequencing the reverse -transcription polymerase chain reaction (RT-PCR) products. All functi onal experiments were performed under nominally HCO3--free conditions. The monocarboxylates acetate and pyruvate (each 20 mM) induced an aci dification of pH(i). In contrast, the dicarboxylate malonate (20 mM) h ad no significant effect on pH(i). alpha-Cyano-4-hydroxycinnamate (CHC ; 5 mM), a blocker of H+/ monocarboxylate(-) cotransporter (MCT), redu ced reversibly the acidification induced by 5 mM pyruvate. In contrast , 1 mu M DIDS, a blocker of band-3 protein, had no significant effect on the acidification induced by 20 mM acetate. The presence of the tra nscripts for each of the MCT subtypes, MCT1 and MCT2, was determined b y RT-PCR of stria vascularis from gerbil. RT-PCR performed with primer s for the MCT1 and MCT2 subtypes on total RNA from stria vascularis re vealed PCR products of the predicted sizes. Sequence analysis confirme d that amplified MCT1 and MCT2 cDNA fragments encoded a nucleotide seq uence of MCT1 and MCT2, respectively. These observations suggest that SMC contain a MCT and that stria vascularis contains RNA for the subty pes MCT1 and MCT2 subtypes. (C) 1997 Elsevier Science B.V.