In vitro permeability of medullary collecting ducts to water and urea

Abstract

Collecting ducts of an isolated rat papilla were perfused by free-flow and stop-flow methods. The permeability coefficient for water was 53[plus or minus]4.3 (SE) cm sec-1 x 10-5 and increased to 95[plus or minus]8.5 when [anti-diuretic hormone] ADH was added. The corresponding values for urea were 13.8[plus or minus]0.79 and 20.2[plus or minus]1.3, respectively. When the osmolality of the medium was higher than that of the perfusion solution, there was net abstraction of water and ADH increased the rate of abstraction from 4.1[plus or minus]0.9 nlcm~2 min-1 milliosmol-1 to 21.0[plus or minus]2.3. Urea and NaCl did not remove as much water as an equal osmolal concentration of mannitol. Urea removed the least water reflecting the membranes'' high permeability to urea. The reflection coefficients to NaCl and urea were 0.9 and 0.4, respectively.