Uptake and metabolism of glucose, alanine and lactate by red blood cells of the American eel Anguilla rostrata

The uptake and metabolism of glucose, alanine and lactate were assessed in red blood cells (RBCs) of the American eel Anguilla rostrata. l-Lactate was metabolized at the highest rates as assessed by O2 consumption and CO2 production, followed by glucose and alanine (rates were approximately half of those observed for lactate). A saturable (Km 10.36±0.60 mmol l-1, Jmax 27.42±2.16 µmol 3-OMG l-1 cell water min-1), sodium-independent but cytochalasin-B-sensitive carrier for d-glucose was observed, which was stereospecific and inhibited by other hexoses. These characteristics are in agreement with those reported for the GLUT-1 glucose carrier of human and Japanese eel erythrocytes. These cells also contained a saturable carrier for l-lactate in the concentration range 0­10 mmol l-1 (Km 6.74±0.36 mmol l-1, Jmax 2.29±0.09 mmol lactate l-1 cell water min-1) whereas, at higher concentrations (10­40 mmol l-1), transport occurred by simple diffusion. The carrier was stereospecific, sodium-independent, fully inhibited by alpha-cyano-4-hydroxycinnamate, DIDS and pyruvate, but less sensitive to SITS, IBCLA and pCMBS. We suggest that this carrier is similar to the H+/monocarboxylate carrier found in mammalian RBCs. Despite the fact that l-alanine transport did not saturate, transport was stereospecific because it was inhibited by d-alanine. These experiments do not, therefore, exclude the existence of an alanine carrier in the eel RBC. The rates of substrate uptake exceeded the ability of the RBC to metabolize the substrate (using 1 mmol l-1 extracellular concentration), with uptake rate/metabolic rate ratios being 2 for alanine, 5 for glucose and 151 for lactate. These experiments indicate that uptake does not limit the ability of the American eel RBC to utilize glucose, alanine or lactate, but that the mechanism(s) of substrate uptake is species-specific.