Na+-dependent sugar transport in a cultured epithelial cell line from pig kidney

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Abstract
Summary A Na+-dependent hexose transport system with similar characteristics to that observed in the kidney is retained in a cultured epithelial cell line from pig kidney (LLC-PK1). The active transport of α methyl-d-glucoside (α MGP), a nonmetabolizable sugar, which shares the glucose-galactose transport system in kidney cells is mediated through a Na+-dependent, substrate-saturable process. The kinetic analysis of the effect of Na+ on the uptake of αMGP indicated that the Na+-sugar cotransport system is an affinity type system in which the binding of either sugar or Na+ to carrier increases the affinity for the other ligand without affecting theV max. The sequence of selectivity for different sugars studied by the inhibition produced in the uptake of αMGP is very similar to that reported in rat kidney, rabbit kidney cortex slices, and rabbit renal brush border membrane vesicles. Phlorizin, even at very low concentration, almost completely inhibits αMGP uptake. Conversely, phloretin at the same low concentration stimulated the sugar accumulation by inhibition of efflux, probably at the level of the basolateral membrane. Sulfhydryl group inhibitors also blocked the αMGP uptake, suggesting that these groups were required for normal functioning of the sugar carrier system. This sugar transport system is an important functional marker to study the molecular events associated with the development of polarization in epithelial cells.