title: Luminal hypotonicity: a driving force for fluid absorption from the proximal tubule.
abstract: The ability of rat proximal tubules to generate a hypotonic luminal fluid was investigated. Simultaneous perfusion of tubules and peritubular capillaries was performed with simple solutions. When tubules were perfused at 10 nl X min-1 and NaCl was the perfusate for tubules and capillaries, solute and fluid (0.41 nl X min-1 X mm-1) were transported and the luminal fluid became hypotonic (delta osmol = -1.7 mosmol X kg-1). When the same solutions were used but the tubule was perfused at 45 nl X min-1, more fluid (0.89 nl X min-1 X mm-1) was reabsorbed and the fluid became more hypotonic (delta osmol = -3.9 mosmol X kg-1). Bicarbonate in the peritubular capillaries increased the fluid reabsorption (1.21 nl X min-1 X mm-1) but did not generate cryoscopically hypotonic fluid. Cyanide abolished all net movement of fluid and solute. It is concluded that the tubule can generate a hypotonic fluid, that the hydraulic conductivity for proximal tubular epithelium is 3,200-3,400 microns X s-1, and that the reflection coefficient for NaHCO3 is slightly higher than for NaCl.
mesh_terms: D000042:Absorption; D000818:Animals; D001696:Biomechanical Phenomena; D002712:Chlorides / Q000378:metabolism; D007038:Hypotonic Solutions; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D008297:Male; D009994:Osmolar Concentration; D010477:Perfusion; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D002712:Chlorides; D007038:Hypotonic Solutions