Vestibular Dark Cells Contain an H⁺/Monocarboxylate⁻ Cotransporter in Their Apical and Basolateral Membrane

Abstract.

The transport of lactate and pyruvate across membranes of vestibular dark cells (VDC) may be important under aerobic, ischemic or hypoxic conditions. This study addresses the questions whether VDC from the gerbil contain an H⁺/monocarboxylate⁻ cotransporter (MCT) and in which membrane, apical or basolateral, MCT is located. Uptake of monocarboxylates into VDC was monitored in functional studies by measuring the cytosolic pH (pH _i ) and by measuring the pH-sensitive equivalent short circuit current (I _sc ). Subtypes of the functionally identified MCT which are present in vestibular labyrinth tissues were identified as transcripts by cloning and sequencing of reverse-transcriptase polymerase chain reaction (RT-PCR) products. Monocarboxylates but not dicarboxylates induced a transient acidification of pH _i which was inhibited by 5 mmα-cyano-4-hydroxycinnamate (CHC) but not by 1 μm DIDS or 500 μm pCMBS. The initial rate of acidification induced by monocarboxylates was dose-dependent in the range between 1 and 20 mm. K _m values were for pyruvate 1.3, acetate 3.7, l-lactate 3.8 and d-lactate 7.3 mm. Both apical and basolateral application of monocarboxylates caused a transient increase of I _sc which was sensitive to 5 mm CHC. RT-PCR revealed the presence of transcripts for the MCT subtypes MCT1 and MCT2. The identity of transcripts was confirmed by sequence analysis. These observations suggest that VDC contain an MCT in their apical and basolateral membrane and that the vestibular labyrinth contains transcripts for the subtypes MCT1 and MCT2.