Cell osmotic water permeability of isolated rabbit proximal convoluted tubules

Cell osmotic water permeability, Pcos, of the peritubular aspect of the proximal convoluted tubule (PCT) was measured from the time course of cell volume changes subsequent to the sudden imposition of an osmotic gradient, delta Cio, across the cell membrane of PCT that had been dissected and mounted in a chamber. The possibilities of artifact were minimized. The bath was vigorously stirred, the solutions could be 95% changed within 0.1 s, and small osmotic gradients (10-20 mosM) were used. Thus, the osmotically induced water flow was a linear function of delta Cio and the effect of the 70-microns-thick unstirred layers was negligible. In addition, data were extrapolated to delta Cio = 0. Pcos for PCT was 41.6 (+/- 3.5) X 10(-4) cm3 X s-1 X osM-1 per cm2 of peritubular basal area. The standing gradient osmotic theory for transcellular osmosis is incompatible with this value. Published values for Pcos of PST are 25.1 X 10(-4), and for the transepithelial permeability Peos values are 64 X 10(-4) for PCT and 94 X 10(-4) for PST, in the same units. These results indicate that there is room for paracellular water flow in both nephron segments and that the magnitude of the transcellular and paracellular water flows may vary from one segment of the proximal tubule to another.

Solvent drag of large solutes indicates paracellular water flow in leaky epithelia

Net fluxes ( J s n ) of sucrose, inulin, dextran (relative molecular mass 15000–17000), albumin and haemoglobin were measured across guineapig gall bladder unilateral preparations in which the absorptive flow ( J v ) was varied over a wide range (without transepithelial osmotic gradients) by perfusing their lumina either with isosmotic solutions or with solutions of reduced osmolalities (a procedure that increased J v ). The absorptive flow J v was inhibited by 10 –4 M ouabain. The net flux J s n was a linear function of J v for sucrose, inulin and dextran, the slopes being inversely related to the size of the solute. Albumin and haemoglobin barely crossed the preparation. The large increase in J s n cannot be accounted for by increased leakiness of the preparation or by unstirred layer effects. The logarithms of the unidirectional flux ratios for sucrose, inulin and dextran in bilateral preparation were also linear functions of J v . It is concluded that solvent drags these solutes via the paracellular pathway. More than 50% of the water flows paracellularly also under normal physiological conditions, since J s n for sucrose was also a linear function of J v in experiments performed only at the physiological osmolality (0.3osm). This value is calculated from the slope of the line relating J s n to J v .