title: Volume absorption in the pars recta. III. Luminal hypotonicity as a driving force for isotonic volume absorption.
abstract: This paper examines the possibility that osmotic disequilibrium between luminal and bathing solutions may account for isotonic fluid absorption coupled to active Na+ absorption observed when superficial proximal straight tubules isolated from rabbit kidney are perfused and bathed with NaCl solutions in the absence of CO2, HCO3-, and luminal organic solutes. If luminal hypotonicity provides a driving force for isotonic fluid absorption under these conditions, the luminal fluid must be nearly isotonic; and steady-state luminal hypotonicity should develop sufficiently rapidly that the absolute rate of volume absorption ('JV, nl min-1) coupled to active Na+ transport is relatively independent of perfusion rate, so that the normalized rate of fluid absorption (JV, nl min-1 mm-1) is approximately constant. Our theoretical calculations indicate that these expectations are fulfilled. A 0.42-0.56 mM reduction in luminal NaCl concentration adequately accounts for the JV observed under such conditions, because of the high hydraulic conductivity of these tubules; and within the range of tubule lengths normally employed with isolated proximal straight tubules, JV is relatively indepedent of perfusion rate within the generally observed range of experimental error.
mesh_terms: D000042:Absorption; D000818:Animals; D001692:Biological Transport; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D008433:Mathematics; D008954:Models, Biological; D009997:Osmotic Pressure; D011817:Rabbits; D012964:Sodium / Q000378:metabolism*; D014867:Water / Q000378:metabolism*
chemical_list: D014867:Water; D012964:Sodium