  Intestinal hexose absorption and gluconeogenesis have been studied in
relation to refeeding after two different fasting phases: a long period of
protein sparing during which energy expenditure is derived from lipid oxidation
(phase II), and a later phase characterized by a rise in plasma corticosterone
triggering protein catabolism (phase III). Such a switch in body fuel uses,
leading to changes in body reserves and gluconeogenic precursors, could
modulate intestinal gluconeogenesis and glucose transport. The gene and protein
levels, and the cellular localization of the sodium-glucose cotransporter
SGLT1, and of GLUT5 and GLUT2, as well as that of the key gluconeogenic enzymes
phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (Glc6Pase)
were measured. PEPCK and Glc6Pase activities were also determined. In phase III
fasted rats, SGLT1 was up-regulated and intestinal glucose uptake rates were
higher than in phase II fasted and fed rats. PEPCK and Glc6Pase mRNA, protein
levels and activities also increased in phase III. GLUT5 and GLUT2 were
down-regulated throughout the fast, but increased after refeeding, with GLUT2
recruited to the apical membrane. The increase in SGLT1 expression during phase
III may allow glucose absorption at low concentrations as soon as food is
available. Furthermore, an increased epithelial permeability due to fasting may
induce a paracellular movement of glucose. In the absence of intestinal GLUT2
during fasting, Glc6Pase could be involved in glucose release to the
bloodstream via membrane trafficking. Finally, refeeding triggered GLUT2 and
GLUT5 synthesis and apical recruitment of GLUT2, to absorb larger amounts of
hexoses.
