Influence of food deprivation on the transport of 3-O-methyl-alpha-D-glucose across the isolated ruminal epithelium of sheep.

Recent studies provided evidence that the ruminal epithelium is able to absorb D-glucose even at physiologically low intraruminal concentrations. To elucidate whether ruminal D-glucose transport shows adaptive responses during food deprivation, transport of 3-0-methyl-alpha-D-glucose (3-OMG), a hardly metabolizable D-glucose analogue, was measured in isolated ruminal epithelia obtained from hay-fed or food-deprived adult sheep. In both groups, a significant net absorption of 3-OMG to the serosal side (in vivo: blood side oriented) could be detected at 3-OMG concentrations between 0.25 mM and 5 mM. Net absorption of 3-OMG was abolished by mucosal (in vivo: lumen side oriented) addition of phlorizin, an inhibitor of the sodium glucose-linked transporter 1 (SGLT-1). Net absorption of 3-OMG followed Michaelis-Menten kinetics, but apparent affinity and maximal transport capacity were lower in epithelia obtained from food-deprived sheep. In contrast to the decrease of the (secondary) active 3-OMG transport, serosal-to-mucosal permeation of 3-OMG increased after food deprivation, suggesting an elevated passive 3-OMG transfer. It is concluded that the altered transport characteristics are either part of a global energy-sparing process during food deprivation (i.e., a lowered activity of the Na+/K+-ATPase) or result from specific down-regulation of SGLT-1.