The contribution of glucose cycling to the maintenance of steady-state levels of lactate by hepatocytes during glycolysis and gluconeogenesis.

When hepatocytes from fasted rats were incubated with 10 mM glucose, there was a linear accumulation of lactate and pyruvate for about 80 min after which steady-state concentrations of these metabolites became established. The rate of glycolysis, determined with [6-3H]glucose, was constant over the entire incubation period and was 50% greater than that calculated from carbon balance studies. This suggests that one-third of the glycolytic products formed were recycled to glucose. To enable study of the factors associated with the generation and maintenance of the lactate steady state and to measure accurately the carbon balance, incubations were performed using supraphysiological concentrations of glucose (20-80 mM). Under these conditions the initial rate of lactate accumulation and its concentration at steady state were shown to be dependent on the concentration of extracellular glucose. Rates of glycolysis were also measured using 40 mM [6-3H]glucose and [U-14C]glucose added alone, or in combination with a steady-state lactate concentration (3 mM). There was no effect on the rate of glycolysis determine with [6-3H]glucose, even when lactate was present in the medium. The difference in rates between measurements with the two isotopes reflect the apparent degree of glucose recycling which in the absence and presence of added lactate increased from 0.26 to 0.54 mumol C3 equivalents min-1.g-1 respectively. Identical studies employing [U-14C]lactate showed that glucose and CO2 were the major products of lactate metabolism under steady-state conditions and that the formation of lactate from [U-14C]glucose exactly balanced the rate of lactate removal as a result of oxidation and gluconeogenesis. These studies provide evidence for the concomitant operation of glycolysis and gluconeogenesis, even in the presence of high glucose concentrations. They also demonstrate that lactate steady states are achieved not by the cessation of glycolysis but rather by the removal of lactate and pyruvate at a rate equal to that of their production.