Calculation of the rate of gluconeogenesis from the incorporation of 14C atoms from labelled bicarbonate or acetate

Abstract

The rate of gluconeogenesis in vivo may be estimated by the incorporation of ¹⁴C atoms from a labelled precursor into plasma glucose or by introducing ¹⁴C atoms into the pathway of gluconeogenesis at known stages by metabolites which in themselves do not contribute to the net synthesis of glucose (e.g., bicarbonate or acetate). The purpose of the investigation was to examine some of the assumptions involved in the calculation of gluconeogenic flux by the second approach. [2- ¹⁴C]acetate or NaH ¹⁴CO₃ was infused to dogs, and the specific activity (SA) of glucose, bicarbonate CO₂, urea, and lactate in the plasma was followed. The incorporation of ¹⁴C atoms from [2- ¹⁴C]acetate into glucose allows the calculation of the degree of underestimation of glucose formation due to "metabolic exchange" in the hepatic oxaloacetate pool. The possible error introduced into this calculation by the incorporation of ¹⁴C atoms from ¹⁴CO₂ (a product of acetate oxidation) was found to be negligible, but the heavy labelling of plasma lactate may possibly affect the estimate of metabolic exchange. It is proposed that in the calculation of the rate of gluconeogenesis from infused NaHCO₃ the SA of hepatocellular and not of plasma bicarbonate CO₂ should be related to that of plasma glucose. This latter is expected to equal the SA of plasma urea, since the sole precursor of its C atom is hepatocellular CO₂. The rate of gluconeogenesis estimated from the SA(glucose)/SA(urea) ratio and a previously estimated correction factor for metabolic exchange was 51% of the glucose production in the postabsorptive state. The nearly identical SA(urea)/SA(CO₂) ratios, irrespective of the tracer infused, indicated that plasma CO₂ is a major precursor of urea C and that a large fraction of injected acetate is oxidized by extrahepatic tissues.