Absence of a transcellular oxalate transport mechanism in LLC-PK1 and MDCK cells cultured on porous supports.

Transepithelial oxalate transport across polarized monolayers of LLC-PK1 cells, grown on collagen-coated microporous membranes in Transwell culture chambers, was studied in double-label experiments using [14C]-oxalate together with [3H]-D-mannitol as an extracellular marker. The [14C]-labeled glucose analog alpha-methyl-glucoside (alpha-MG) was used as functional marker for active proximal tubular sugar transport. Cellular uptake of oxalate and alpha-MG at both the apical and basolateral plasma membrane was determined. When added to the upper compartment, alpha-MG was actively taken up at the apical membrane, directed through the cells to the basolateral membrane and transported to the lower compartment, indicating functional epithelial sugar transport by LLC-PK1 cells. In LLC-PK1 cells, the uptake of alpha-MG at the apical membrane was approximately 50 times higher than that at the basolateral membrane. In contrast to this active transport of sugar, LLC-PK1 cells did not demonstrate oxalate uptake either at the apical or basolateral plasma membrane. The apical-to-basolateral (A- > B) flux of oxalate in LLC-PK1 cells was identical to the basolateral-to-apical (B- > A) oxalate flux in these cells. Moreover these flux characteristics were similar to those found for D-mannitol, indicating paracellular movement for both compounds. From these data, it is concluded that, under the experimental conditions used, LLC-PK1 cells do not exhibit a specific transcellular transport system for oxalate.