Evidence for proteic water pathways in the luminal membrane of kidney proximal tubule.

The osmotic permeability of the apical membrane of proximal tubule cells was studied on rat brush-border membrane vesicles by following their rate of shrinkage with a stopped-flow device coupled to light transmission recording. The mercuric sulfhydryl reagent para-chloromercuribenzenesulfonic acid (PCMBS) reduced the water permeability of the membrane, in a time- and dose-dependent manner, to 35% of the control value. Mercuric chloride was a more potent inhibitor and decreased the osmotic water permeability of the brush-border membrane to 15% of the control. This inhibition was reversed by an excess of cysteine, while cysteine per se did not modify the rate of vesicle shrinkage. These results suggest that most of the osmotic water movements across kidney brush-border membranes are through polar pathways which involve the integrity of the membrane proteins.