CELLULAR AND SUBCELLULAR EXPRESSION OF THE MONOCARBOXYLATE TRANSPORTER MCT1 IN RAT-HEART - A HIGH-RESOLUTION IMMUNOGOLD ANALYSIS

An antibody to the C-terminus of the monocarboxylate transporter MCTI was used to study the precise cellular and subcellular distribution of this transporter in rat heart. Postembedding immunogold procedures re vealed that the labeling in the heart was restricted to cardiomyocytes and concentrated along the plasma membrane, including the transverse tubules. Gold particles occurred with highest densities in intercalate d disks, where they avoided desmosomes and gap junctions. Labeling was also associated with plasmalemmal invaginations nations having ultras tructural features typical of caveolae. Internal membrane compartments were unlabeled. Quantitative analyses following postembedding labelin g showed that the distribution of gold particles across the plasma mem brane was nearly symmetrical, indicating that the C-terminus of the tr ansporter is situated very close to the cell membrane. In preembedding immunogold experiments, the gold particles were localized at the exte rnal aspect of the plasma membrane, suggesting that the C-terminus is extracellular. From the present data, it can be concluded that even un der basal conditions the majority of the MCT1 molecules in heart is pr esent in the myocyte plasma membrane, implying that there is a constit utive functional expression of this transporter. It follows that the i ncreased trans membrane flux of lactate during exercise on pathologica l conditions such as ischemia must be a result of altered substrate gr adients rather than of translocation of MCT1 molecules to the plasma m embrane.