Comparison of transcellular and transepithelial water osmotic permeabilities (Pos) in the isolated proximal straight tubule (PST) of the rabbit kidney.

Measurements of the water osmotic permeabilities of apical and basolateral membranes of PST cells and of the transepithelial permeability have been carried out using a very fast method with high temporal and spatial resolution. At 25 degrees C the values obtained are: 80.8 +/- 11.9 x 10(-4) cm3/s osmol cm2 of apical (luminal) surface area and 90.1 +/- 13.0 x 10(-4) cm3/s osmol cm2 of basement membrane area (no membrane invaginations taken in account). These values are higher than previously published values due to the use of a faster and more accurate volume measuring and recording system. The transepithelial water osmotic permeability at 25 degrees C is 77 +/- 11 in units of 10(-4) cm3/s osmol cm2 basement membrane area. The transcellular water osmotic permeability is 32 +/- 7 (same units), leaving a paracellular contribution of 45 +/- 10 (same units). In the presence of 2.5 mM parachloromercuribenzenesulfonate (pCMBS) the apical permeability is reduced with an incubation of 10-15 min to 23% of its control value and the basolateral permeability to 8% of its control value (after 25 min) but the transepithelial permeability is only reduced to about 1/2 of the control value. This leaves a transcellular permeability of 6 x 10(-4) cm3/s osmol cm2 of basement membrane area and a paracellular contribution of 33 +/- 6 (same units). These results indicate a significant contribution of the paracellular pathway to the transepithelial water osmotic permeabilities in PST.