Hepatic uptake and metabolism of oral galactose in adult fasted rats

Galactose is incorporated into glycogen by a different metabolic route than glucose and fructose, the other major dietary monosaccharides. Oral galactose (4 g/kg) was given to 24-h-fasted adult rats to 1) compare quantitatively the disposition of galactose with that of glucose and fructose; 2) examine the effects of galactose on hepatic utilization of other metabolic fuels; and 3) examine circulating and liver galactose concentrations to determine whether net hepatic uptake of galactose, like glucose, occurs against a concentration gradient. Galactose absorption, hepatic blood flow, portal venous, arterial, hepatic venous, and liver concentrations of galactose, glucose, lactate, and alanine, and hepatic glycogen concentrations were measured at intervals up to 240 min. Concentrations entering and exiting the liver, hepatic intracellular concentrations, and net hepatic uptake/output were calculated. Galactose concentration entering the liver increased to a peak of 18.8 ± 0.8 μmol/ml plasma water at 60 min and then decreased but remained above the control value. Liver galactose concentration increased dramatically from 0.28 ± 0.04 to 21.2 ± 1.1 μmol/ml liver water and exceeded plasma concentrations, even during the 1st 120 min when concentration gradients across the liver indicated net galactose extraction. Whole blood galactose concentrations initially were lower and then exceeded plasma concentrations, indicating that erythrocytes maintained galactose concentrations exceeding those in plasma. The data suggest that the hepatic and erythrocyte transport systems for galactose represent active mechanisms. Fifty-one percent of absorbed galactose was lost in urine; 18% of the remaining galactose load could be accounted for by net glycogen accumulation. Net increases in galactose, lactate, and alanine uptake could account for the glycogen synthesized but not for the net hepatic glucose output, which changed very little (6% increase).