It is widely believed, that the heart uses blood-derived lactate as a major energy substrate. Lactate is transported into cardiomyocytes via monocarboxylate transporters (MCT) together with H⁺ at a 1:1 stoichiometry, which couples lactate uptake to changes in intra- and extracellular pH.

In this study we investigate how the interplay between different acid/base transporters and membrane-anchored, extracellular carbonic anhydrases (CA), which catalyze the reversible hydration of CO₂, can influence uptake of lactate into isolated mouse cardiomyocytes. We measured both lactate-induced acidification and lactate influx by pH imaging and a newly developed, FRET-based lactate sensor. Recordings of intracellular H⁺ concentration showed an increase in the rate of lactate-induced acidification when CA was inhibited by 6-Ethoxy-2-benzothiazolesulfonamide (EZA), while direct measurements of lactate flux demonstrated an EZA-induced decrease in MCT transport activity. The data indicate that catalytic activity of carbonic anhydrase, which could only been found on the extracellular face of the cardiomyocytes plasma membrane, does both increase lactate uptake and counteract intracellular lactate-induced acidification. Based on this, we propose a hypothetical model, in which H⁺ and HCO₃- are formed from cell-derived CO₂ at the outer surface of the myocyte plasma membrane by membrane-anchored, extracellular CA. HCO₃- is then transported into the cell to counteract intracellular acidification, while the remaining H⁺ stabilizes extracellular pH at the surface of the plasma membrane during MCT activity to enhance lactate influx into the myocyte.

This work was supported by the “Research Initiative Membrane Biology” and the “Stiftung Rheinland-Pfalz für Innovation”.