Sources of carbon for hepatic glycogen synthesis in the conscious dog

To identify the source(s) of carbon for the indirect pathway of hepatic glycogen synthesis, we studied nine 42-h fasted conscious dogs given a continuous intraduodenal infusion of glucose, labeled with [1-13C]glucose and [3-3H]glucose, at 8 mg.kg-1.min-1 for 240 min. Glycogen formation by the direct pathway was measured by 13C-NMR. Net hepatic balances of glucose, gluconeogenic amino acids, lactate, and glycerol were determined using the arteriovenous difference technique. During the steady-state period (the final hour of the infusion), 81% of the glucose infused was absorbed as glucose. Net gut output of lactate and alanine accounted for 5% and 3% of the glucose infused, respectively. The cumulative net hepatic uptakes were: glucose, 15.5 +/- 3.8 g; gluconeogenic amino acids, 32.2 +/- 2.2 mmol (2.9 +/- 0.2 g of glucose equivalents); and glycerol, 6.1 +/- 0.9 mmol (0.6 +/- 0.1 g of glucose equivalents). The liver produced a net of 29.2 +/- 9.6 mmol of lactate (2.6 +/- 0.8 g of glucose equivalents). Net hepatic glycogen synthesis totaled 9.3 +/- 2.5 g (1.8 +/- 0.4 g/100 g liver), with the direct pathway being responsible for 57 +/- 10%. Thus, net hepatic glucose uptake was sufficient to account for all glycogen formed by both the direct and indirect pathways. Total net hepatic uptake of gluconeogenic precursors (gluconeogenic amino acids, glycerol, and lactate) was able to account for only 20% of net glycogen synthesis by the indirect pathway. In a net sense, our data are consistent with an intrahepatic origin for most of the three-carbon precursors used for indirect glycogen synthesis.