Regulation of sodium-coupled glucose transport by glucose in a cultured epithelium.

Cultured porcine kidney cells (LLC-PK1) form polarized epithelia that transport glucose from apical to basal surface as in the renal proximal tube. The ability of these cells to transport glucose is known to increase as the epithelium forms and matures in culture. We find that epithelia grown in medium containing 25 mM glucose have reduced hexose transport compared to epithelia grown in 5 mM glucose. This difference is not the result of differences in seeding efficiency and can be reversed by changing the concentration of glucose in the growth medium. Increased transport in epithelia grown in 5 mM glucose is the result of increased influx on the sodium-coupled apical membrane transporter rather than changes in efflux. This difference is apparently the result of more apical membrane transporters in epithelia grown in 5 mM glucose. The number of high affinity phlorizin-binding sites is greater in epithelia grown in 5 mM glucose (about 0.8 pmol/10(6) cells) than in 25 mM glucose (about 0.25 pmol/10(6) cells). The increase in the number of glucose transporters induced by the low glucose medium is specific in that there is not a comparable change in activity of marker enzymes (alkaline phosphatase, acid phosphatase, or glucose 6-phosphatase). The nature of the intracellular signal elicited by extracellular glucose remains to be determined.