Transmembrane Transport of Water and Urea in Rat Corneal Endothelial Cells

This study investigated permeability of the apical and basolateral membranes of rat corneal endothelial cells to water and urea. We demonstrated that the apparent water permeability of the basolateral membrane of endothelial cells (4.43E-05 ± 7.57E-07 cm/s) is more than three times higher than that of the apical membrane (1.21E-05 ± 1.03E-07 cm/s). Permeability of the basolateral membrane to urea (1.23E-04 ± 1.56E-06 cm/s) was statistically significantly higher than that of the apical membrane (9.52E-05 ± 1.02E-06 cm/s) by approximately 30%. We examined contribution of the phloretin-inhibited urea transport across the apical and basolateral membranes in these cells. Phloretin at concentration of 0.1 mM significantly reduced urea permeability by more than 20% through both the apical and basolateral membranes. The results suggest that the compositions of transporters involved in water transport in the apical and basolateral membranes differ significantly. It is hypothesized that high apparent water permeability of the basolateral membrane of endothelial cells is due to contribution of the concomitant water transport with ions involved in active transport processes. Presence of the phloretin-sensitive urea transporters in the plasma membrane of endothelial cells, likely involved in its transcellular transport, has been demonstrated. The results indicate potential significance of urea for corneal function.