Energetics of sugar transport by isolated intestinal epithelial cells: effects of cytochalasin B.

The capability of isolated intestinal epithelial cells to establish concentration gradients of 3-O-methylglucose (3-OMG) by a Na+-dependent transport system is limited by concomitant function of a Na+-independent, facilitated diffusion transport system. Monosaccharides accumulated by the active system are continuously lost via the passive system, which acts to lower steady-state sugar gradients maintained by the cell. Cytochalasin B is a potent inhibitor of the passive system and allows the cells to establish a sugar gradient that is much higher than normal. When extracellular [3-OMG] is 1 mM, cytochalasin induces sugar accumulation ratios of 30-fold (+/- phlorizin) in contrast to control ratios of approximately 10-fold. When [3-OMG] is 0.1 mM, cytochalasin (0.1 mM) induces 40-fold accumulation ratios. When changes in extracellular sugar concentration are considered, steady-state concentration gradients observed are 70-fold. For a Na:sugar coupling stoichiometry of 1:1, gradients of this magnitude represent the approximate theoretical maximum for a transport system driven exclusively by the transmembrane electrochemical potential for Na+.