Authors’ reply

To the Editor: When labelled glucose is used to determine endogenous glucose production (EGP) by infusing [6,6-²H₂]glucose [1], the rate by which “new” glucose produced within organs (mainly the liver, but to some extent the kidney and perhaps the intestine) enters the systemic circulation can be assessed from dilution of the enrichment of glucose (or its specific activity if radioactive-labelled glucose is infused). Measuring that EGP provides answers, for example, on what contribution those glucose-producing organs make to the actual blood glucose concentration, and whether nutrients, e.g. amino acids, increase glucose production [1].

Of course, glucose-producing organs also use the glucose they produce, and to the extent that new glucose is used before entering the circulation, glucose production is greater than the amount measured by the dilution of labelled glucose in the systemic circulation. Moreover, when EGP is assessed from the arteriovenous balance technique, i.e. the difference between the quantity of glucose leaving and entering an organ, glucose produced and utilised within the organ is not measured. For clarity two terms could be used, one (“net glucose production”) for the quantity of new glucose released into the circulation, the other (“total glucose production”) for the quantity produced. The latter could also be referred to as EGP, and the former as endogenous glucose output, i.e. glucose produced and released into the circulation for use. At present these terms are generally used interchangeably and have not been employed previously in this manner to our knowledge.

If, as concluded by Burns and Cohen, glucose uptake by the liver were substantial, our estimation of gluconeogenesis (GNG) from the difference between (net) EGP and glycogenolysis measured by ¹³C nuclear magnetic resonance (NMR) spectroscopy would have been an underestimate of (total) EGP. We appreciate the extensive literature on hepatic intralobular functional heterogeneity and the effect it could have on estimates of GNG [2]. Figure 1 summarises the concept that periportal hepatocytes primarily produce whereas perivenous hepatocytes remove glucose. Consequently, if all new unlabelled glucose produced in the periportal zone was taken up in the liver lobule, it would not appear in the hepatic vein and would not enter the peripheral blood which is sampled. As a result this unlabelled glucose would not dilute the labelled glucose. On the other hand, if the new glucose does enter the systemic circulation, returns to the liver and is taken up by hepatocytes, we will measure this glucose by the dilution of the labelled glucose. While the portal vein is usually not accessible for studies in humans, rates of glucose uptake in the splanchnic bed and in the kidney have been estimated by combining arteriovenous balance and labelled glucose techniques. Even if all glucose uptake by the splanchnic bed is attributed to the liver, whether from its perivenous zone or not, the amount taken up in normal subjects [3], as well as in Type 2 diabetic patients [4], is small compared to the overall amount of glucose produced. Moreover, there does not seem to be a major difference between the periportal and perivenous zones in the contribution of the gluconeogenic (indirect) and direct pathways to glycogen synthesis, at least in rats following a glucose load [5].

Fig. 1

The fate of labelled glucose (*Glucose) and new unlabelled glucose (°Glucose) within the liver lobule. Glucose production is ultimately regulated by the balance of fluxes through glucokinase (GK) and glucose-6-phosphatase (G6Pase). The intracellular fate of glucose metabolism depends on fluxes through the gluconeogenic pathway (GNG), glycogen synthase (GS) and glycogen phosphorylase (GP)

We also directly measured the contribution of GNG to glucose production from the ratio of ²H enrichments from ²H₂O in the hydrogens at carbons 5 and 2 of glucose [1]. That ratio provides the measure of the contribution of GNG to the production of the new glucose in the circulation. Rates of GNG were not different when calculated either by ¹³C NMR spectroscopy or the ²H₂O method (see Table 2 of [1]), providing further evidence that the estimate obtained by NMR spectroscopy was correct.