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 NaH14CO3 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.

Probing peroxisomal β-oxidation and the labelling of acetyl-CoA proxies with [1-13C]octanoate and [3-13C]octanoate in the perfused rat liver

“…Hetenyi et al [7] and Katz [8] have proposed that the labelling of the C1 + 2 fragment of BHB could be used as a proxy for the labelling of liver mitochondrial acetyl-CoA. In the present study, and a previous study [20], we were able to compare the 13 Clabelling of the C1 + 2 fragment of BHB and of the acetyl moiety of citrate.…”

“…The 13 C-labelling of the C1 + 2 fragment of BHB (another probe of mitochondrial acetyl-CoA [7,8]) was calculated as the difference between the labelling of the whole molecule and that of the C3 + 4 fragment [15]. The 13 C-labelling of liver malonyl-CoA [12] and of acetate in the effluent perfusate [16] were assayed as described previously.…”

“…We perfused rat livers with various concentrations of [1-13 C]octanoate or [3-13 C]octanoate and measured the 13 C-labelling of malonyl-CoA, the acetyl moiety of citrate, free acetate, acetylcarnitine and the C1 + 2 fragment of ketone bodies (another proxy for the labelling of mitochondrial acetyl-CoA in liver [7,8]). This protocol allowed us to test the degree of labelling homogeneity of the various acetyl-CoA proxies.…”

Fish oil after abdominal aorta aneurysm surgery

“…Glucose turnover: A primed (2 mg/kg) continuous (2% of total carbohydrate dose) infusion of 6,6 2 H 2 -glucose (Hetenyi et al, 1982;Tappy et al, 1992). Na-bicarbonate 1.4%, was run at a rate of 45 ml/h after a bolus 10 ml.…”

“…GNG was calculated according to Hetenyi et al (1982) as GNG ¼ ( 13 C plasma glucose IE Â 6)/breath 13 CO 2 IE Â glucose rate of appearance. Owing to loss of 13 C to CO 2 at the level of Krebs cycle intermediate, this method is known to yield systematically underestimated values, which however does not preclude between groups comparisons.…”