Effect of urea concentration of pelvic fluid on renal concentrating ability

Solute and water transport along an inner medullary collecting duct undergoing peristaltic contractions

The mechanism by which solutes accumulate in the inner medulla of the mammalian kidney has remained incompletely understood. That persistent mystery has led to hypotheses based on the peristaltic contractions of the pelvic wall smooth muscles. It has been demonstrated the peristaltic contractions propel fluid down the collecting duct in boluses. In antidiuresis, boluses are sufficiently short that collecting ducts may be collapsed most of the time. In this study, we investigated the mechanism by which about half of the bolus volume is reabsorbed into the collecting duct cells despite the short contact time. To accomplish this, we developed a dynamic mathematical model of solute and water transport along a collecting duct of a rat papilla undergoing peristaltic contractions. The model predicts that, given preexisting axial concentration gradients along the loops of Henle, ∼40% of the bolus volume is reabsorbed as the bolus flows down the inner medullary collecting duct. Additionally, simulation results suggest that while the contraction-induced luminal hydrostatic pressure facilitates water extraction from the bolus, that pressure is not necessary to concentrate the bolus. Also, neither the negative interstitial pressure generated during the relaxation phase nor the concentrating effect of hyaluronic acid has a significant effect on bolus concentration. Taken together, these findings indicate that the high collecting duct apical water permeability allows a substantial amount of water to be extracted from the bolus, despite its short transit time. However, the potential role of the peristaltic waves in the urine-concentrating mechanism remains to be revealed.

Two fluid compartments in the renal inner medulla: a view through the keyhole of the concentrating process

Approximately four decades ago, the countercurrent theory became influential in studies on the concentrating process in the mammalian kidney. The theory successfully represented the concentrating process in the outer medulla, but the problem of the concentrating mechanism in the inner medulla, as defined by Homer Smith has remained essentially intractable. In a recent comprehensive review by Knepper and coworkers of various theories and models, attention was refocused on the possible role of hyaluronate (HA) in the inner medullary concentrating process. The authors proposed a hypothesis that HA can convert hydrostatic pressure to concentrating work.Here, we briefly survey the earlier ideas on the role imputed to HA and present a new hypothesis which is different from that of Knepper and coworkers. We estimate that the hydrostatic pressures available in the inner medulla can account only for a very small fraction of the concentrating work. We hypothesize that the role of HA is tied up with extravasated plasma albumin and suggest that owing to the property of HA solutions to exclude other macromolecules, extravasated plasma albumin and HA constitute two fluid compartments in the interstitium in the inner medulla. In this proposed two-compartment model, the Gibbs-Donnan distribution influences the movement of ions and water between the HA and the extravasated albumin compartment. To relate the hypothetical role of HA to the concentrating process, we briefly describe new results obtained by other investigators on the accumulation of urea in the inner medulla. This subject has been critically reviewed recently by Yang & Bankir.Many processes have been identified as contributing to the concentrating process in the mammalian inner medulla. We speculate that among these many processes, the primary responsibility for the final concentration of the excreted urine may be portioned out differently in different mammalian species.

Countercurrent exchange in the renal medulla

The microcirculation of the renal medulla traps NaCl and urea deposited to the interstitium by the loops of Henle and collecting ducts. Theories have predicted that countercurrent exchanger efficiency is favored by high permeability to solute. In contrast to the conceptualization of vasa recta as simple "U-tube" diffusive exchangers, many findings have revealed surprising complexity. Tubular-vascular relationships in the outer and inner medulla differ markedly. The wall structure and transport properties of descending vasa recta (DVR) and ascending vasa recta (AVR) are very different. The recent discoveries of aquaporin-1 (AQP1) water channels and the facilitated urea carrier UTB in DVR endothelia show that transcellular as well as paracellular pathways are involved in equilibration of DVR plasma with the interstitium. Efflux of water across AQP1 excludes NaCl and urea, leading to the conclusion that both water abstraction and diffusion contribute to transmural equilibration. Recent theory predicts that loss of water from DVR to the interstitium favors optimization of urinary concentration by shunting water to AVR, secondarily lowering blood flow to the inner medulla. Finally, DVR are vasoactive, arteriolar microvessels that are anatomically positioned to regulate total and regional blood flow to the outer and inner medulla. In this review, we provide historical perspective, describe the current state of knowledge, and suggest areas that are in need of further exploration.

Cloning and characterization of the urea transporter UT3: localization in rat kidney and testis

Urea transport in the kidney plays an important role in urinary concentration and nitrogen balance. Recently, three types of urea transporters have been cloned, UT1 and UT2 from rat and rabbit kidney and HUT11 from human bone marrow. To elucidate the physiological role of the latter urea transporter, we have isolated the rat homologue (UT3) of HUT11 and studied its distribution of expression and functional characteristics. UT3 cDNA encodes a 384 amino acid residue protein, which has 80% identity to the human HUT11 and 62% identity to rat UT2. Functional expression in Xenopus oocytes induced a large (approximately 50-fold) increase in the uptake of urea compared with water-injected oocytes. The uptake was inhibited by phloretin (0.75 mM) and pCMBS (0.5 mM) (55 and 32% inhibition, respectively). Northern analysis gave a single band of 3.8 kb in kidney inner and outer medulla, testis, brain, bone marrow, spleen, thymus, and lung. In situ hybridization of rat kidney revealed that UT3 mRNA is expressed in the inner stripe of the outer medulla, inner medulla, the papillary surface epithelium, and the transitional urinary epithelium of urinary tracts. Co-staining experiments using antibody against von Willebrand factor showed that UT3 mRNA in the inner stripe of the outer medulla is expressed in descending vasa recta. These data suggest that UT3 in kidney is involved in counter current exchange between ascending and descending vasa recta, to enhance the cortico-papillary osmolality gradient. In situ hybridization of testis revealed that UT3 is located in Sertoli cells of seminiferous tubules. The signal was only detected in Sertoli cells associated with the early stages of spermatocyte development, suggesting that urea may play a role in spermatogenesis.

Urea permeability of mammalian inner medullary collecting duct system and papillary surface epithelium

To compare passive urea transport across the inner medullary collecting ducts (IMCDs) and the papillary surface epithelium (PSE) of the kidney, two determinants of passive transport were measured, namely permeability coefficient and surface area. Urea permeability was measured in isolated perfused IMCDs dissected from carefully localized sites along the inner medullas of rats and rabbits. Mean permeability coefficients (X 10(-5) cm/s) in rat IMCDs were: outer third of inner medulla (IMCD1), 1.6 +/- 0.5; middle third (IMCD2), 46.6 +/- 10.5; and inner third (IMCD3), 39.1 +/- 3.6. Mean permeability coefficients in rabbit IMCDs were: IMCD1, 1.2 +/- 0.1; IMCD2, 11.6 +/- 2.8; and IMCD3, 13.1 +/- 1.8. The rabbit PSE was dissected free from the underlying renal inner medulla and was mounted in a specially designed chamber to measure its permeability to urea. The mean value was 1 X 10(-5) cm/s both in the absence and presence of vasopressin (10 nM). Morphometry of renal papillary cross sections revealed that the total surface area of IMCDs exceeds the total area of the PSE by 10-fold in the rat and threefold in the rabbit. We conclude: the IMCD displays axial heterogeneity with respect to urea permeability, with a high permeability only in its distal two-thirds; and because the urea permeability and surface area of the PSE are relatively small, passive transport across it is unlikely to be a major source of urea to the inner medullary interstitium.

Examination of transepithelial exchange of water and solute in the rat renal pelvis

Severance of the ureter beyond the renal papilla causes a fall in urinary osmolality, which suggests that exchange of water or solute between urine and renal parenchyma normally occurs in the intact renal pelvis. We examined water and solute flux in the renal pelvis with micropuncture and microcatheterization techniques. Four groups of antidiuretic rats were studied. Group I (n = 17) underwent micropuncture through the intact contracting ureter. Urine samples were obtained at the papillary tip, and in the pelvis beside the base of the extrarenal papilla. Urinary osmolality at the base, 880 +/- 97 mosmol/kg H2O (mean +/- SE), was less than that at the tip, 1,425 +/- 104 mosmol/kg H2O (P less than 0.005). In group II (n = 24), samples were analyzed for inulin and osmolality. In 15 rats (group IIA), comparison was made between base and tip samples. In the other nine animals (group IIB), comparisons were made among base, tip, and bladder samples and urea was also measured. In group II (A and B combined) urine-to-plasma (U/P) osmolality was lower at the base, 4.31 +/- 0.27, than at the tip, 6.08 +/- 0.23 (P less than 0.001), and U/P inulin was lower at the base, 192 +/- 25, than at the tip, 306 +/- 16 (P less than 0.001). In group IIB, the bladder urine had a lower U/P osmolality, 5.27 +/- 0.25, than the tip, 6.01 +/- 0.31 (P less than 0.02). The U/P urea was 59 +/- 10.6 (base), 98 +/- 9.4 (tip) (base vs. tip, P less than 0.05), and 81 +/- 6.5 (bladder, P less than 0.005, compared with tip). In group III (n = 8), samples were obtained by microcatheter from the fornices, the deepest intrarenal extensions of the pelvis, and compared with samples at the tip. Urinary osmolality was lower in the fornix, 646 +/- 106 mosmol/kg H2O, than at the tip, 1,296 +/- 99 mosmol/kg H2O (P less than 0.001). Similarly, U/P inulin was lower in the fornix, 48 +/- 14, than at the tip, 128 +/- 12 (P less than 0.001). The lower U/P inulin in the pelvic urine is the result of either the addition of fluid to the pelvis, or the backleak of inulin across the epithelium lining the pelvis. To verify that the pelvic epithelium was impermeable to inulin, in group IVA (n = 4) the left renal pelvis was superfused with a solution of chemical inulin. Cumulative absorption of inulin from the left kidney was 0.15 +/- 0.08% of that superfused. Using [14C]inulin in group IVB (n= 3), similar results were obtained (0.05 +/- 0.02%). These findings indicate that in the renal pelvis, fluid is added to urine after it emerges from the collecting ducts. We suggest that reflux of hyperosmotic urine over the renal papilla creates a transepithelial gradient for the flux of water into the pelvis. A model that incorporates diffusive and convective forces for water and solute transport is proposed to account for these findings.

Urinary concentration in the papillary collecting duct of the rat. Role of the ureter

Urine was observed to flow intermittently in the collecting ducts of the extrarenal papilla of antidiuretic rats. The purpose of this investigation was to test Reinking and Schmidt-Nielsen's hypothesis that intermittent flow plays an important role in the production of maximally concentrated urine. Samples of collecting duct fluid were obtained from the base and tip of the papilla by micropuncture through the intact ureter. Fluid osmolality rose sharply from base, 894+/-120 mosmol/kg H(2)O(-1) (mean+/-SE), to tip, 1,667+/-114 (P<0.001), a distance of only 2 mm, and was due exclusively to reabsorption of water. After excision of the ureter, which abolished intermittent flow, osmolality fell modestly at the base to 723+/-82 mosmol/kg H(2)O(-1) (P < 0.02), but strikingly at the tip to 1,012+/-103 (P < 0.001). The pelvic ureter was paralyzed by topical verapamil and dimethylsulfoxide, which abolished intermittent flow. Osmolality of urine at the tip was not changed (1,959+/-184 mosmol/kg H(2)O(-1) before, vs. 1,957+/-126 after paralysis). The ureter was severed just beyond the papillary tip, a maneuver which preserved intermittent flow but abolished urinary reflux over the papilla. Urinary osmolality fell from 1,876+/-134 mosmol/kg H(2)O(-1) to 1,284+/-115 (P < 0.005). These findings demonstrate that when the ureter is intact, over half of the increase in urinary osmolality above isotonicity occurs in the terminal one-fourth of the medullary collecting duct and is due exclusively to water reabsorption (no net solute addition). It is the continuity of the ureter, rather than intermittent flow due to ureteral peristalsis, which is essential for the formation of a maximally concentrated urine.