AJP - Endocrinology and Metabolism, Vol 272, Issue 1 E51-E58, Copyright © 1997 by American Physiological Society
Use of labeling pattern of liver glutamate to calculate rates of citric acid cycle and gluconeogenesis
V. Large, H. Brunengraber, M. Odeon and M. Beylot
Institut National de la Sante et de la Recherche Medicale Unite 197, Lyon, France.
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.
