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

Abstract

A B S T R A C T 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 (Purea) was calculated from the disappearance of urea-'4C 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 Pu,.. per unit length was 3.3±0.4 X 10 cm'/sec (5.3±(0.6 X 10 cm/sec assuming I.D. = 20) in PCT and 0.93±0.4 X 10-7 cm'/sec (1.5±0.5 X 10' 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,°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.°urea in DLH is 0.95±0.4 as compared to 0.91±0.05 in PCT. ourea in DLH is approximately equal to OuNs; however, curea in PCT is higher than ffNa (0.68).

Sodium chloride, urea, and water transport in the thin ascending limb of Henle. Generation of osmotic gradients by passive diffusion of solutes.

“…According to the model as proposed by Kokko and Rector (6), the addition of salt in excess of water to the interstitial fluid and the subsequent formation of dilute intraluminal fluid by the tALH requires that this segment of the loop have the following unique permeability properties: impermeability to water; high permeability to NaCI; and moderate permeability to urea. This is in marked contrast to the permeability characteristics observed by Kokko (8,9) for the DLH, which was found to be highly permeable to water and impermeable to salt (8) and urea (9).…”

“…Segments of tALH were perfused in vitro by the same technique described previously for the thin DLH (8,9).…”

“…In the above studies the perfusate was made hypertonic by addition of NaCl because in our earlier studies we found that in vitro perfused DLH of rabbit osmotically equilibrated principally by water abstraction rather than urea entry, and it was thus predicted that in vivo the fluid leaving the DLH and entering the tALH would have a high NaCl concentration (9). However, such large NaCl concentration gradients between the tALH and other medullary structures have not been observed in vivo micropuncture studies (16,23).…”

The Diuretic Effect of Sairei-to Is Mediated by Nitric Oxide Production in Pentobarbital-Anesthetized Rats

“…On the other hand, TJ-114 increased urinary urea nitrogen levels and the effect was inhibited by pre-intraperitoneal injection of L-NAME. It has been reported that the urea filtered from glomeruli was reabsorbed passively in renal proximal tubules during the reabsorption of water or sodium (13,14). Thus, it is considered that the increase in urinary urea nitrogen with TJ-114 is associated with the secretion of water and sodium in proximal tubules.…”

Simulation of the Profile of Water, NaCl, and Urea Transport in the Countercurrent Multiplication System between Thin Ascending Limb and Inner Medullary Collecting Duct.

“…In addition, characteristics of solute and water transport across the thin descending and ascending limb of Henle's loop do not fit well with the active NaCI transport model (Gottschalk et al 1963 ;Jamison 1968 ; Morgan and Berliner 1968 ;Marsh 1970 ; Kokko 1970Kokko , 1972Pennel et al 1974 ; Imai andKokko 1974, 1976 ;Elalouf et al 1985Elalouf et al , 1986a. Stephenson (1972) and Kokko and Rector (1972) independently proposed models which incorporated passive movement of NaCl and urea into a "single effect" operating in thin ascending limb of Henle's loop (TAL).…”