Flow dependence of fluid transport in the isolated superficial pars recta: evidence that osmotic disequilibrium between external solutions drives isotonic fluid absorption.

The present studies tested the hypothesis that osmotic disequilibrium between luminal and peritubular fluids is the driving force for net volume absorption in the isolated proximal straight tubule. Isolated tubule segments from superficial rabbit renal cortex were perfused at varying rates with a high chloride and bicarbonate-free solution as they were bathed with a normal bicarbonate-Krebs-Ringer buffer solution at 38 degrees C. Increasing the perfusion rate from congruent to 4 to congruent to 30 nl/min produced a monotonic increase in net volume absorption (Jv) from 0.18 +/- (sem) 0.03 to 0.62 +/- 0.08 nl . min-1. The chloride concentration in collected fluid samples rose from congruent to 137 to congruent to 147 mEq/liter over the same perfusion rate range. Ouabain (10(-4) m) added to the bathing solution inhibited Jv by a rate which varied from 0.20 to 0.28 nl . min-1 . min-1, depending on the perfusion rate. A mathematical model of the axial flows and transepithelial transport processes was developed. This model, and the experimental data, is consistent with the view that the driving force for isotonic fluid absorption in these tubules depends on the axial maintenance of osmotic disequilibrium between the perfusate and the bathing solution. Increasing the perfusion rate opposes osmotic equilibration by minimizing the extent to which dissipative fluxes of chloride and bicarbonate ions change the transepithelial chloride and bicarbonate concentration gradients, and by minimizing the tendency of the luminal cryoscopic osmolality to increase as volume absorption occurs.