Osmotic gradient dependence of osmotic water permeability in rabbit proximal convoluted tubule

Summary To assess steady-state transepithelial osmotic water permeability (P _f ), rabbit proximal convoluted tubules were perfused in vitro with the impermeant salt, sodium isethionate at 26°C. Osmotic gradients () were established by varying the bath concentration of the impermeant solute, raffinose. When lumen osmolality was 300 mOsm and bath osmolality was 320, 360 and 400 mOsm, apparentP _f decreased from 0.5 to 0.10 to 0.08 cm/sec, respectively. Similar data were obtained when lumen osmolality was 400 mOsm. Five possible causes of the dependence of apparentP _f were considered experimentally and/or theoretically: (1) external unstirred layer (USL); (2) cytoplasmic USL; (3) change in surface area; (4) saturation of water transport; (5) down-regulation ofP _f . ApparentP _f was inhibited 83% byp-chloromercuribenzene sulfonate (pCMBS) at 20 mOsm, but not at 60 mOsm , suggesting presence of a serial barrier resistance to water transport. Increases in perfusate or bath solution flow rate and viscosity did not alter apparentP _f , ruling out an external USL. A simple cytoplasmic USL, described by a constant USL thickness and solute diffusion coefficient, could not account for the dependence of apparentP _f according to a mathematical model. The activation energy (E _a ) for apparentP _f increased from 7.0 to 12.5 kcal/mol when was increased from 20 to 60 mOsm, not consistent with a simple USL or a change in membrane surface area with transepithelial water flow. These findings are most consistent with a complex cytoplasmic USL, where the average solute diffusion coefficient and/or the area available for osmosis decrease with increasing . These results (1) indicate that trueP _f (at physiologically low ) is very high (>0.5 cm/sec) in the rabbit proximal tubule; (2) provide an explanation for the wide variation inP _f values reported in the literature using different , and (3) suggest the presence of a flow-dependent cytoplasmic barrier to water flow.