Urea transport in the proximal tubule and the descending limb of Henle

Urea transport in proximal convoluted tubule (PCT) and descending limb of Henle (DLH) was studied in perfused segments of rabbit nephrons in vitro.Active transport of urea was ruled out in a series of experiments in which net transport of fluid was zero. Under these conditions the collected urea concentration neither increased nor decreased when compared to the mean urea concentration in the perfusion fluid and the bath.Permeability coefficient for urea (P(urea)) was calculated from the disappearance of urea-(14)C added to perfusion fluid. Measurements were obtained under conditions of zero net fluid movement: DLH was perfused with isosmolal ultrafiltrate (UF) of the same rabbit serum as the bath, while PCT was perfused with equilibrium solution (UF diluted with raffinose solution for fluid [Na] = 127 mEq/liter). Under these conditions P(urea) per unit length was 3.3+/-0.4 x 10(-7) cm(2)/sec (5.3+/-0.6 x 10(-5) cm/sec assuming I.D. = 20mu) in PCT and 0.93+/-0.4 x 10(-7) cm(2)/sec (1.5+/-0.5 x 10(-5) cm/sec) in DLH. When compared to previously published results, these values show that the PCT is 2.5 times less permeable to urea than to Na, while the DLH is as impermeable to urea as to Na. These results further indicate that the DLH is less permeable to both Na and urea than the PCT.The reflection coefficient for urea, sigma(urea), was calculated as the ratio of induced solution efflux when 95 mOsm/liter of urea was added to the bath, as compared to net fluid movement induced by addition to the bath of equivalent amount of raffinose, sigma(urea) in DLH is 0.95+/-0.4 as compared to 0.91+/-0.05 in PCT. sigma(urea) in DLH is approximately equal to sigma(Na); however, sigma(urea) in PCT is higher than sigma(Na) (0.68).Several types of studies were conducted to examine the role of urea and urea plus sodium chloride in concentrating the fluid in the DLH. From the obtained results it was concluded that the intraluminal fluid of DLH is primarily concentrated by abstraction of water without significant net entry of solute. These results are discussed with respect to possible significance in the overall operation of the countercurrent system.