MEDIATION OF HEXOSE TRANSPORT BY THE LIVER CELL MEMBRANE

MEDIATION OF HEXOSE TRANSPORT BY THE LIVER CELL MEMBRANE

Abstract:

1. Transport of D-glucose, D-galactose and D-fructose across the liver cell membrane was studied using an in situ rat isolated liver perfusion technique. 2. A bench-top liver perfusion apparatus which maintains the liver temperature constant for several hours of perfusion is described. This apparatus has the advantages that it is simple, cheap, and effective. 3. The perfusion medium used was Krebs bicarbonate buffer to which were added washed bovine erythrocytes and bovine albumin. This medium was found to meet the requirements for a viable liver perfusion. 4. Stringent viability criteria were adopted and livers which failed these criteria were rejected. Electron microscopic studies of perfused and unperfused livers were undertaken as adjunct to viability assessment of these livers. 5. The suitability of radioactively labelled sucrose as an extracellular reference material in a double indicator dilution technique in the perfused rat liver was investigated. 6. The hepatocellular transport of each of D-glucose, D-galactose and D-fructose was saturable. All the three hexoses showed mutual competition. The single pass extraction values for the hexoses were consistently different such that D-glucose>>D-galactose»D-fructose»»L-glucose. 7. D-glucose transport is by far greater than L-glucose transport suggesting stereospecificity of glucose transport mechanism, confirming the finding of the only previous study (WILLIAMS et al, 1968). 8. Both phlorizin and phloretin inhibited uptake of these hexoses. Of these two phloretin was by far the better inhibitor, a finding suggesting that the hepatic uptake of these sugars is analogous to known passive (e.g. human erythrocyte) rather than active (e.g. e.nterocyte) types of hexose transport. 9. Some dissaccharides were found to inhibit the uptake of these hexoses. Lactose inhibited the uptake of all three hexoses. Maltose inhibited uptake on D-glucose and D-fructose only (it had no effect on D-galactose uptake). Sucrose had no effect on any of the three hexoses. 10. Galactose transport was found to be temperature dependent. On an Arrhenius plot there was a discontinuity of this temperature dependence suggesting a transition temperature of about 31 C. A relatively high activation energy (67 Kj mole at 37°C) was calculated from these findings. 11. It was concluded, on the basis of the findings of saturability, competition, stereospecificity and greater chemical inhibition of hexose transport by phloretin than phlorizin, that these hexoses are transported across the liver cell membrane by a facilitated diffusion (carrier mediated) transport mechanism similar to that existing in human erythrocytes. On the basis of dissacharide effects, it was inferred that the D-galactose carrier mechanism is distinct at the external surface of the membrane from the D-glucose/fructose carrier mechanism. This implies an asymmetry between the external and internal carrier sites, the latter probably being common to all three hexoses whereas the former is distinct. An architecture for these carrier sites in the liver cell membrane is proposed

MEDIATION OF HEXOSE TRANSPORT BY THE LIVER CELL MEMBRANE