Use of labeling pattern of liver glutamate to calculate rates of citric acid cycle and gluconeogenesis.

The use of the labeling pattern of hepatic glutamate during infusion of L-[3-13C]- or [3-14C]lactate to calculate rates of citric acid cycle activity and gluconeogenesis has been proposed. We tested the validity of this approach by perfusing isolated rat livers (48 h starved) with pyruvate and lactate (10% enriched with [3-13C]lactate) without (control) or with infusion of glucagon (to inhibit pyruvate kinase), mercaptopicolinate (to inhibit phosphoenolpyruvate carboxykinase), or dichloroacetate (to stimulate pyruvate dehydrogenase). Compared with control experiments, glucagon increased glucose output (P < 0.05) and decreased the calculated flux through pyruvate kinase (P < 0.05). Mercaptopicolinate almost totally suppressed glucose production and dramatically reduced the calculated gluconeogenic rate and flux through phosphoenolpyruvate carboxykinase (P < 0.001). Dichloroacetate moderately increased the calculated flux through pyruvate dehydrogenase (P < 0.05). In experiments with perfused livers from fed rats, the calculated gluconeogenic rate and flux through phosphoenolpyruvate carboxykinase were very low compared with control experiments (P < 0.001), whereas the pyruvate dehydrogenase flux was increased (P < 0.05). Therefore, the expected modifications of the citric acid cycle activity and gluconeogenic rate were clearly detected using the labeling pattern of glutamate to calculate these metabolic rates. Except for the perfusions with mercaptopicolinate, the dilution by isotopic exchange in the oxaloacetate pool calculated from the model agreed with the actual dilution of enrichment between liver pyruvate and phosphoenolpyruvate. The present results support the validity of this approach to trace liver metabolism.