Plasma glucose concentration determines direct versus indirect liver glycogen synthesis

In the present study hepatic glycogenesis by the direct versus indirect pathway was determined as a function of the glucose infusion rate. Glycogen synthesis was examined in catheterized conscious rats that had been fasted 48 h before receiving a 3-h infusion (iv) of glucose. Glucose, containing tracer quantities of [U-¹⁴C]- and [6-³H]glucose, was infused at rates ranging from 0 to 230 μmol · min^-1 · kg^-1. Plasma concentrations of glucose, lactate, and insulin were positively correlated with the glucose infusion rate. Despite large changes in plasma glucose, lactate, and insulin concentrations, the rate of hepatic glycogen deposition (0.46 ± 0.03 μmol · min^-1 · g^-1) did not vary significantly between glucose infusion rates of 20 and 230 μmol · min^-1 · kg^-1. However, the percent contribution of the direct pathway to glycogen repletion gradually increased from 13 ± 2 to 74 ± 4% in the lowest to the highest glucose infusion rates, with prevailing plasma glucose concentrations from 9.4 ± 0.5 to 21.5 ± 2.1 mM. Endogenous glucose production was depressed (by up to 40%), but not abolished by the glucose infusions. Only a small fraction (7-14%) of the infused glucose load was incorporated into liver glycogen via the direct pathway irrespective of the glucose infusion rate. Our data indicate that the relative contribution of the direct and indirect pathways of hepatic glycogen synthesis are dependent on the glucose load or plasma glucose concentration and emphasize the predominance of the indirect pathway of glycogenesis at plasma glucose concentrations normally observed after feeding.