Water and urea diffusive permeabilities in isolated proximal tubule cells

We measured the water (3H2O) and urea ([14C]urea) diffusive permeabilities (Pd) in intact proximal tubule cells (PTC). Isolated rabbit PTC were packed in polyethylene tubes to measure the diffusion coefficients for 3H2O and [14C]urea at 14 degrees C. Pd values for water and urea were estimated from the diffusion coefficients in intact cells, in extracellular and in intracellular media using a model of parallel and series diffusion in the packed PTC. Pd was 65 +/- 8 and 18 +/- 3 microns/s for water and urea, respectively. We examined the effect of different pharmacological agents on the Pd values for water and for urea. p-Chloromercuribenzenesulfonate at 2 mM markedly inhibited the Pd of water and urea. Phloretin (1 mM) inhibited the Pd for urea, whereas it increased Pd for water. KMnO4 (30 microM) markedly inhibited urea Pd but did not alter water Pd. The values for energy of activation of Pd for water and for urea were 2.9 +/- 2 and 7.3 +/- 4 kcal/mol, respectively. These results show that water and urea do not share the same pathway across the PTC membrane.