Urea transport in isolated thick ascending limbs and collecting ducts from rats

The use of pathogen-free rats allows the dissection and in vitro perfusion of several rat nephron segments not previously studied. In the present experiments, net urea fluxes were measured in isolated perfused cortical and medullary thick ascending limbs and cortical collecting ducts from pathogen-free Sprague-Dawley rats. No evidence for active transport of urea was found in either cortical or medullary thick ascending limbs. Permeabilities were calculated from urea fluxes measured with 5 mM urea either in the bath or perfusate and with no urea on the opposite side of the epithelium. Permeability coefficients (cm/s X 10(-5) +/- SE) in different portions of the thick ascending limb were: inner stripe, short-looped nephrons, 0.9 +/- 0.2; inner stripe, long-looped nephrons, 0.6 +/- 0.2 (not significantly different vs. short loops); outer stripe, 1.4 +/- 0.3 (P less than 0.05 vs. inner stripe); and cortical, 1.5 +/- 0.3 (P less than 0.05 vs. inner stripe). The relatively high urea permeability of thick ascending limbs in the outer stripe of the outer medulla and medullary rays is likely to permit substantial passive absorption of urea from these segments in vivo. This will contribute to dilution of the tubule fluid in thick ascending limbs and thus indirectly enhance urinary concentrating ability. In cortical collecting ducts, the urea permeability was relatively low both in the presence of 100 microU/ml arginine vasopressin in the bath (0.5 +/- 0.1 X 10(-5) cm/s) and in its absence (0.4 +/- 0.1). These permeability values are similar to values previously measured in rabbit cortical collecting ducts.