Sugar absorption by the biliary ductular epithelium under steady-state conditions was examined using isolated perfused rat liver. The test
sugar and
mannitol (as a putative marker of paracellular entry) were added to the
glucose-free recirculating perfusate each at a concentration of 5 mmol/L, and apparent active biliary ductular absorption equated with the change in concentration of the test
sugar relative to that of
mannitol. A metabolizable
hexose (
D-glucose),
pentose (
D-xylose), and three nonmetabolizable
hexoses (alpha-methyl-glucoside, 3-o-methyl-glucose, and L-glucose) were used. All five
monosaccharides were well absorbed at constant rates for 2 hours with apparent rates of absorption (mumol.kg
body weight-1.min-1, mean +/- SE) of
D-glucose, 0.24 +/- 0.01;
L-glucose, 0.20 +/- 0.02; 3-o-methyl-glucose, 0.19 +/- 0.02; alpha-methyl-
glucoside, 0.16 +/- 0.03; and
D-xylose, 0.10 +/- 0.04. The addition of
phloridzin to the perfusate inhibited
D-glucose absorption in part but did not inhibit
L-glucose absorption. When perfusate Na+ was replaced by N-
methylglucamine, the bile-plasma ratio of
mannitol remained unchanged, as did the apparent absorption rate of
D-glucose and 3-o-methyl-glucose. In contrast, absorption of
L-glucose and
alpha-methyl-D-glucoside gradually ceased. The addition of 15 mmol/
L glucose to the perfusate caused decreased bile flow and increased
taurocholate concentration in bile, suggesting that
glucose absorption by the biliary ductules induced water reabsorption. It is concluded that
sugars are absorbed by the biliary ductular system by Na(+)-dependent and Na(+)-independent transport systems, the substrate affinities of which differ from those reported for apical membrane
hexose transport systems in renal tubular and intestinal epithelia. Ductular absorption of solutes such as
glucose that enter bile passively may have
biological use, because ductular absorption decreases the concentration of substrates for bacterial growth in gallbladder bile. On the other hand, ductular absorption of solutes induces reabsorption of biliary water, resulting in decreased bile flow; this might contribute to
cholestasis during prolonged hyperalimentation with solutions containing
glucose.