Isosmotic volume reabsorption in rat proximal tubule.

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Abstract
A theoretical model incorporating active and passive forces was developed for fluid reabsorption from split oil droplets in rat intermediate and late proximal tubules. Simplifying assumptions were introduced; it was assumed that the epithelium can be treated as a single membrane and that the membrane effective HCO3 permeability is near zero. Based on this model with its underlying assumptions, the following conclusions are drawn. Regardless of the presence or absence of active NaCl transport, fluid reabsorption from the split oil droplet is isosmotic. The reabsorbate osmolarity can be affected by changes in tubular permeability parameters and applied forces but is not readily altered from an osmolarity essentially equal to that of plasma. In a split droplet, isosmotic flow need not be a special consequence of active Na transport, is not the result of a particular set of permeability properties and is not merely a trivial consequence of a very high hydraulic conductivity. Isosmotic flow can be obtained with hydraulic conductivity nearly an order of magnitude lower than that previously measured in the rat proximal convoluted tubule. Isosmotic reabsorption is, in part, the result of the interdependence of salt and water flows, their changing in parallel and their ratio, the reabsorbate concentration being relatively invariant. Active NaCl transport can cause osmotic water flow by reducing the luminal fluid osmolarity. In the presence of passive forces the luminal fluid can be hypertonic to plasma, and active NaCl transport can still exert its osmotic effect on volume flow. There are 2 passive forces for volume flow: the Cl gradient and the difference in effective osmotic pressure; they have an approximately equivalent effect on volume flow. Experimentally, the volume changes were measured in a droplet made hyperosmotic by the addition of 50 mM NaCl; the experimental results are predicted reasonably well by the theoretical model.