Water extraction from the inner medullary collecting tubule system: a role for urea

Concentration of solutes in the renal inner medulla: interstitial hyaluronan as a mechano-osmotic transducer

Although the concentrating process in the renal outer medulla is well understood, the concentrating mechanism in the renal inner medulla remains an enigma. The purposes of this review are fourfold. 1) We summarize a theoretical basis for classifying all possible steady-state inner medullary countercurrent concentrating mechanisms based on mass balance principles. 2) We review the major hypotheses that have been proposed to explain the axial osmolality gradient in the interstitium of the renal inner medulla. 3) We summarize and expand on the Schmidt-Nielsen hypothesis that the contractions of the renal pelvocalyceal wall may provide an important energy source for concentration in the inner medulla. 4) We discuss the special properties of hyaluronan, a glycosaminoglycan that is the chief component of a gel-like renal inner medullary interstitial matrix, which may allow it to function as a mechano-osmotic transducer, converting energy from the contractions of the pelvic wall to an axial osmolality gradient in the medulla. These considerations set the stage for renewed experimental investigation of the urinary concentrating process and a new generation of mathematical models of the renal concentrating mechanism, which treat the inner medullary interstitium as a viscoelastic system rather than a purely hydraulic system.

Regulation of renal urea transporters

Urea is important for the conservation of body water due to its role in the production of concentrated urine in the renal inner medulla. Physiologic data demonstrate that urea is transported by facilitated and by active urea transporter proteins. The facilitated urea transporter (UT-A) in the terminal inner medullary collecting duct (IMCD) permits very high rates of transepithelial urea transport and results in the delivery of large amounts of urea into the deepest portions of the inner medulla where it is needed to maintain a high interstitial osmolality for concentrating the urine maximally. Four isoforms of the UT-A urea transporter family have been cloned to date. The facilitated urea transporter (UT-B) in erythrocytes permits these cells to lose urea rapidly as they traverse the ascending vasa recta, thereby preventing loss of urea from the medulla and decreasing urine-concentrating ability by decreasing the efficiency of countercurrent exchange, as occurs in Jk null individuals (who lack Kidd antigen). In addition to these facilitated urea transporters, three sodium-dependent, secondary active urea transport mechanisms have been characterized functionally in IMCD subsegments: (1) active urea reabsorption in the apical membrane of initial IMCD from low-protein fed or hypercalcemic rats; (2) active urea reabsorption in the basolateral membrane of initial IMCD from furosemide-treated rats; and (3) active urea secretion in the apical membrane of terminal IMCD from untreated rats. This review focuses on the physiologic, biophysical, and molecular evidence for facilitated and active urea transporters, and integrative studies of their acute and long-term regulation in rats with reduced urine-concentrating ability.

Influence of a low protein diet on radiation nephropathy in the pig

Nine mature, approximately 45-week-old pigs, were fed a standard diet (SD) containing 16% protein; 10 pigs were fed an isocaloric low protein (LP) pig feed containing 4% protein 2 weeks prior to and 16 weeks after renal or sham-irradiation. The pigs then received the SD for a further 4 weeks. Both kidneys of seven pigs fed the LP diet, and six pigs fed the SD, were irradiated with a single dose of 9.8 Gy of 60Co gamma-rays. The remaining pigs received sham-irradiation. The individual kidney glomerular filtration rate (GFR), effective renal plasma flow (ERPF), and the haematocrit were serially measured before and up to 20 weeks after irradiation. The radiation-induced reduction in mean individual kidney GFR, expressed as either absolute values or as a percentage of the respective sham-irradiated controls, was significantly greater in pigs fed the SD compared with pigs fed the LP diet (p < 0.05). In contrast, the radiation-induced reduction in mean individual kidney ERPF, expressed as absolute values, observed in pigs fed the SD was not significantly different from that seen in pigs fed the LP diet. However, if these data were expressed as a percentage of the respective sham-irradiated controls, then a significantly greater reduction in ERPF was seen in pigs fed the SD compared with pigs fed the LP diet (p < 0.001). Morphological analysis of renal tissue removed 20 weeks after irradiation showed that the extent and severity of the glomerular lesions seen in pigs fed the SD were significantly reduced in pigs fed the LP diet. Thus dietary protein restriction appears to reduce some of the signs of radiation nephropathy in the pig.

Countercurrent system

Urinary concentration is achieved by countercurrent multiplication in the inner medulla. The single effect in the outer medulla is active NaCl absorption from the thick ascending limb. While the single effect in the inner medulla is not definitively established, the majority of experimental data favors passive NaCl absorption from the thin ascending limb. Continued experimental studies in inner medullary nephron segments will be needed to elucidate fully the process of urinary concentration.

The role of the collecting duct in urinary concentration

In summary, the three major segments of the collecting duct subserve three different functions in the urinary concentrating mechanism. The main function of the cortical collecting tubule is to raise the fractional solute contribution and absolute concentration of urea in fluid that it delivers to the outer medullary collecting duct. The function of the outer medullary collecting duct is to raise further the absolute intraluminal urea concentration. Finally, the inner medullary collecting duct has two major functions in urinary concentration: first, it adds net urea to the papillary interstitium, and second, it allows the generation of maximally concentrated urine due to osmotic water equilibration. Indeed, the urine osmolality can rise to levels higher than the papillary interstitial osmolality as a consequence of inequalities of the reflection coefficients of urea and sodium chloride.