Figure 4. Determination of metabolic fluxes.

(A) Diagram of lactate metabolism. The cytosolic concentration of lactate depends on the balance between glycolytic production, mitochondrial consumption of pyruvate and lactate, and the exchange with the extracellular lactate pool via MCTs. (B) Intracellular lactate was monitored in individual HEK293 cells during transient exposures to phloretin (50 µM), in the presence of 25 mM glucose (top panel) or 1 mM lactate (bottom panel). (C) The responses of intracellular lactate to transient inhibitions of the MCT with phloretin (50 µM) or pCMBS (500 µM) were sequentially measured in the same HEK293 cell in the presence of 25 mM glucose. The straight lines represent slopes of lactate accumulation fitted by linear regression during the first minute of exposure. The bar graph summarizes data from three experiments. (D) The effect of 5 mM sodium azide was measured in an astrocyte during exposure to 50 µM phloretin.