Effect of adrenalectomy on transport in the rat medullary thick ascending limb.

Previous studies in adrenalectomized (Adx) rats suggest that aldosterone may regulate ion transport in the ascending portion of Helen's loop. In order to examine directly the effect of adrenalectomy on transport, medullary thick ascending limb (Mtal) segments were isolated from Adx, Adx replaced with aldosterone (Adx + Ald, 0.5 micrograms X 100 g X body wt X d), and control Sprague-Dawley rats. Both net sodium and net chloride fluxes were significantly less in the Mtal segments from Adx rats compared with those in the control or Adx + Ald group. Physiologic levels of exogenous aldosterone increased net sodium chloride flux toward control values in the Adx + Ald group. Net potassium flux was not different among the three groups. We conclude that adrenalectomy impairs reabsorptive NaCl but not K transport in the Mtal, and that aldosterone restores this process. This reabsorptive defect may contribute to the urinary concentrating and diluting abnormality associated with adrenal insufficiency.

A micropuncture study of collecting tubule function in rats with hereditary diabetes insipidus.

The reabsorption of water and solute by the papillary collecting duct was studied during water diuresis and vasopressin-induced antidiuresis in young rats with hereditary hypothalamic diabetes insipidus. The tip of the left renal papilla was exposed and fluid was obtained by micropuncture from loops of Henle and from collecting ducts at the papillary tip, and at an average of 1 mm proximal to the tip. In water diuresis the ratio of tubule fluid to plasma (TF/P) osmolality (osm) of loop fluid was 1.73 +/-0.058 (SE); of fluid from the proximal collecting duct, 0.63 +/-0.027; and from the tip, 0.55 +/-0.024; indicating a substantial osmotic pressure difference across the collecting duct epithelium. The fraction of filtered water reabsorbed (x 100) by the terminal collecting duct was 1.58% +/-0.32. In antidiuresis the TF/P osm of loop fluid was 2.65 +/-0.109; of fluid from the proximal collecting duct, 2.20 +/-0.093; and from the tip, 2.71 +/-0.111; indicating a marked decrease in the driving force for water reabsorption. The fraction of filtered water reabsorbed (x 100) by the terminal collecting duct was reduced to 0.58% +/-0.08, while the delivery of solute to the same segment was unchanged from that in water diuresis. The glomerular filtration rate (GFR) of the right kidney declined from 327 +/-24.4 mul/min in water diuresis to 274 +/-24.4 mul/min in antidiuresis (P < 0.005); similar results were obtained in a study comparing right and left GFRs in five additional rats. Thus, fractional reabsorption (and very likely the absolute volume) of water reabsorbed by the terminal collecting duct was less in antidiuresis than in water diuresis (mean difference, 1.01% +/-0.29, P < 0.005).

Effect of arginine vasopressin on renal medullary blood flow. A videomicroscopic study in the rat.

The role of arginine vasopressin (AVP) in the regulation of renal medullary blood flow is uncertain. To determine if AVP has a direct vasoconstrictive action on vasa recta, the effect of AVP on erythrocyte velocity (VRBC), diameter, and blood flow (QVR) in descending vasa recta (DVR) and ascending vasa recta (AVR) was studied in the exposed renal papilla of four groups of chronically water diuretic rats using fluorescence videomicroscopy. There were three periods: control (period 1), experimental (period 2), and recovery (period 3). In periods 1 and 3, all groups received hypotonic saline. In period 2, group I rats (AVP) received AVP (45 ng/h per kg body wt); group II (time) received hypotonic saline alone; group III (AVP plus V1-inhibitor) received AVP plus its vascular antagonist, d(CH2)5Tyr(Me)AVP; and group IV (V1-inhibitor) received the vascular antagonist alone. Another group of rats (group V) was employed to demonstrate that the rise in blood pressure induced by a 3- or 10-ng/kg injection of AVP was virtually abolished by the prior infusion of the V1-inhibitor. The urine of group III as well as group I rats was concentrated (Uosm = 721 +/- 62 H2O vs. 670 +/- 39 mosM/kg), while urine remained dilute in groups II and IV. In period 2, VRBC and QVR in DVR and AVR decreased in group I, did not decrease in group III, and increased in groups II and IV. The vascular antagonist thus completely abolished the AVP-induced decrease in QVR in group III. These findings unequivocally establish that AVP in physiological amounts reduces medullary blood flow, at least in part, by a direct vasoconstrictive action on the medullary microcirculation. They also show that an effect of AVP on medullary blood flow is not necessary for its antidiuretic effect.

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.

Transport of sodium and urea in outer medullary descending vasa recta.

We dissected and perfused outer medullary vasa recta (OMVR) from vascular bundles in the rat. Permeabilities of sodium (PNa) and urea (Pu) were simultaneously determined from the lumen-to-bath efflux of 22Na and [14C]urea. PNa and Pu were also measured by in vivo microperfusion of descending (DVR) and ascending vasa recta (AVR) at the papillary tip of Munich-Wistar rats. In some OMVR PNa was indistinguishable from zero. The mean +/- SE of PNa (x 10(-5), cm/s) in OMVR was 76 +/- 9. Pu in OMVR was always very high (x 10(-5), cm/s), 360 +/- 14. There was no correlation between OMVR PNa and Pu. Inner medullary AVR and DVR had PNa of 115 +/- 10 and 75 +/- 10, respectively, and Pu of 121 +/- 10 and 76 +/- 11, respectively. PNa and Pu in papillary vasa recta were always nearly identical and highly correlated. Transport of [14C] urea in OMVR was reversibly inhibited by addition of unlabeled urea or phloretin to the bath and lumen, providing evidence for carrier-mediated transport. These data suggest that sodium and urea might traverse the wall of inner medullary vasa recta by a paracellular pathway while urea also crosses by a transcellular route in OMVR. Electron microscopic examination of seven in vitro perfused OMVR revealed no fenestrations and exposure of these vessels to 10 microM calcium ionophore A23187 or 1 nM angiotensin II resulted in reversible contraction, suggesting that in vitro perfused OMVR are DVR only.

Evidence for a concentration gradient favoring outward movement of sodium from the thin loop of Henle.

Recent models of the urinary concentrating mechanism have postulated that urea in the medullary interstitium creates a transtubular concentration gradient for sodium between fluid at the end of the descending limb of Henle's loop and the medullary interstitium, favoring the passive outward movement of sodium from Henle's thin ascending limb. These experiments were designed to determine whether such a gradient normally exists. Young nondiuretic Munich-Wistar rats were prepared for micropuncture of the exposed left renal papilla. Samples of loop of Henle fluid and vasa recta plasma (assumed to reflect the composition of interstitial fluid) were obtained from adjacent sites. Loop fluid values in 21 comparisons from 18 rats (mean +/- SE) were: sodium 344 +/- 12 meq/liter; potassium, 26 +/- 2 meq/liter; osmolality, 938 +/- 37 mosmol/kg H23. Vasa recta plasma values (in corresponding units of measurement) were: sodium, 284 +/- 11; potassium, 34 +/- 2; osmolality, 935 +/- 34. Mean values of paired differences (loop fluid minus vasa recta plasma) were: delta sodium, 60 +/- 11.1 (P less than 0.001); delta potassium, -8.0 +/- 2.1 (P less than 0.001); delta osmolality, 4 +/- 16 (NS). Corrected for plasma water, the loop fluid minus vasa recta differences (in milliequivalents per kilogram H2O) were: delta sodium, 40 +/- 11.4 (P less than 0.005); delta potassium, -9.7 +/- 1.9 (P less than 0.001). We interpret these findings to indicate that in the papilla of nondiuretic rats, a significant difference in sodium concentration exists across the thin loop of Henle favoring outward movement of sodium, which confirms a key requirement of the passive models. A concentration difference for potassium in the reverse direction was also observed.

The effect of urea infusion on the urinary concentrating mechanism in protein-depleted rats.

To explore the role of urea in the urinary concentrating mechanism, the contents of vasa recta, Henle's descending limbs and collecting ducts were sampled by micropuncture of the renal papilla before and after infusion of urea in 10 protein-depleted rats. Eight protein-depleted rats not given urea were similarly studied as a control group. After urea administration, osmolality and the concentrations of urea and nonurea solute of urine from both exposed and contralateral kideny increased significantly. The osmolality and urea concentration of fluid from the end of Henle's descending limb and vasa recta plasma and the tubule fluid-to-plasma inulin ratio in the end-descending limb all increased significantly after urea infusion. We interpret these observations to indicate that urea enhances urinary concentration by increasing the abstraction of water from the juxtamedullary nephron (presumably the descending limb), in agreement with the prediction of recent passive models of the urinary concentrating mechanism. However, the concentration of urea in fluid from the descending limb after urea infusion was high (261 plus or minus 31 mM) and the difference in solium concentration between descending limb fluid and vasa recta was small and statistically insignificant.

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.

Effect of chronic potassium loading on potassium secretion by the pars recta or descending limb of the juxtamedullary nephron in the rat.

Recently we demonstrated potassium secretion by the pars recta or by the descending limb of the juxtamedullary nephron. The purpose of this present investigation is to study the effect of a chronic high-potassium intake on this phenomenon. Fractional reabsorption of water and sodium by the juxtamedullary proximal nephron was decreased when compared to that in normal hydropenic rats. There was a striking increase in the fraction of filtered potassium at the end of the juxtamedullary descending limb from 94+/11% to 180+/18%, which was principally a result of enhanced potassium secretion. When the concentration of potassium in the collecting tubule fluid of potassium-loaded rats was reduced after the administration of amiloride, a sharp fall was observed in the amount of potassium which reached the end of the descending limb (64+/8%). A direct correlation was observed between the fraction of filtered potassium at the descending limb and the potassium concentration in the final urine (P less than 0.001). The findings suggest that potassium, like urea, normally undergoes medullary recycling, which is enhanced by chronic potassium loading.

Identification of "B" receptor for natriuretic peptide in human kidney.

Three distinct receptor types for natriuretic peptides (NP) have been identified in human tissue. "A" and "B" receptors initiate biological actions, whereas the "C" receptor has a clearance function. It has been proposed that the natural ligand for the B receptor is c-type natriuretic peptide (CNP), rather than atrial natriuretic peptide (ANP), and that the B receptor is only found in the central nervous system (CNS) and is responsible for all NP-mediated effects in the CNS. Contrary to this hypothesis, we have identified, by means of the polymerase chain reaction (PCR), the B receptor in human kidney tissue. To detect A and C receptors, the PCR reaction was performed with primers which yielded predicted 600 and 378 base pair (bp) products, respectively. For the B receptor, 3 different primer sets were used, resulting in the expected 785, 453 and 228 bp fragments. Restriction mapping of the latter two products with Rsa I yielded the expected fragment numbers and sizes, indicating the PCR products were from B receptor mRNA. These results indicate that the human kidney has B as well as A and C receptors. Thus CNP may have a renal as well as a CNS site of action.

Disorders of urinary concentration and dilution.

A new approach to the classification of disorders of urinary concentration and dilution is recommended based on recent studies of how the kidney elaborates a urine of widely varying osmolality. The capacity to concentrate urine depends on ft, the fractional reabsorption of solute delivered to the loop of Henle; fu, the excretion of solute relative to the sum of solute excretion and solute delivery to Henle's loop; fw, the fraction of solute loss by vascular outflow from the medulla relative to that reabsorbed by the loop; and finally, collecting duct response to antidiuretic hormone (ADH). A decrease in ft or in increased fu or fw will diminish urinary concentrating ability, as will resistance of the tubule to ADH. Conversely, urinary dilution depends on the delivery of sodium and water to the ascending limb; NaCl reabsorption by the ascending limb; and the absence of ADH. A decrease in sodium and water delivery to the ascending limb or in NaCl reabsorption by the ascending limb will impair urinary diluting ability, as will the presence of ADH. The consequences of disorders in urinary concentrating and diluting ability vary widely. In an alert patient with an intact thirst center, there may be no consequence; in a patient unable to communicate thirst or whose thirst center is deranged, the results may be catastrophic. Keeping in mind the kidney's few basic requirements for formation of concentrated or dilute urine may help the physician avoid these potentially serious dislocations of water balance.

Glomerular and tubular function in non-oliguric acute renal failure.

Glomerular and tubular function were evaluated in 30 non-oliguric patients with increasing azotemia following open heart surgery. Fractional clearances (theta) of test solutes relative to that of inulin were determined. In 16 patients, theta dextran (radius 22 to 30 A) exceeded unity, a finding attributed to inulin backleak through necrotic tubules. These patients were classified as having acute renal failure; 14 subsequently required dialysis. In the remaining patients (N = 14), theta dextran was normal. These patients were considered to have prerenal failure; all recovered spontaneously. clearance of inulin (Cin) was lower in acute renal failure than in prerenal failure (12 +/- 2 versus 18 +/- 2 ml/min/1.73 m2; p less than 0.025). The apparent difference in glomerular filtration rate when Cin is used as an index was abolished, however, when Cin in acute renal failure was corrected for tubule backleak of inulin. In acute renal failure, fractional clearance of p-aminohippurate (theta PAH) was 7.1 +/- 1.0, and fractional excretion of potassium (FEk) was 160 +/- 18 percent. These findings strongly suggest that secretory ability in both proximal and terminal tubule augments, respectively, is preserved in acute renal failure. Compared with prerenal failure, the urine-to-plasma inulin ratio was lower (U/Pin = 10 +/- 1 versus 25 +/- 4; p less than 0.005) and FENa was higher (FENa = 5.1 +/- 1.5 versus 0.5 +/- 1.0 percent; p less than 0.01) in acute renal failure.

Surreptitious diuretic ingestion and pseudo-Bartter's syndrome.

A patient with profound hypokalemia satisfied the criteria for Bartter's syndrome, including hyperreninemia, aldosteronism, normal blood pressure, and hyperplasia of the juxtaglomerular apparatus. Two screening tests of urine and one of plasma for diuretic agents gave negative results. A third urinary sample gave negative results for thiazide but positive for furosemide; the fourth and fifth samples gave negative results for furosemide but positive for thiazide. Urinary prostaglandin excretion was normal. We conclude that this apparent case of Bartter's syndrome was caused by long term surreptitious diuretic ingestion and suggest this may occur more frequently than is generally appreciated.

Acute renal failure following cardiac surgery.

In a prospective 6 month study of 204 patients requiring cardiac operations, five (2.5 percent) developed acute renal failure (ARF) and five (2.5 percent) had documented renal dysfunction (RD). Preoperative left ventricular dysfunction and prolonged cardiopulmonary bypass (CPB) were important predictors of subsequent RD/ARF; CPB pressure per se was not. Physiological and clinical studies in 51 selected patients studied over an 18 month period documented the effectiveness of low flow, low pressure CPB in preserving postoperative renal function. Twenty-two patients with nonazotemic postoperative courses demonstrated moderate depression of cardiac function while the glomerular filtration rate (GFR) was normal (98 +/- 30 ml./min/1.73 M.2) within 24 hours of operation. Seventeen high risk patients developed AFF (65 percent mortality rate) and 12 experienced severe RD without ARF (17 percent mortality). ARF (65 percent mortality rate) and 12 experienced severe RD without ARF (17 percent mortality). Eleven patients with ARF and 11 with RD were studied in the early postoperative period; at this time, all 22 patients demonstrated RD with equivalent severe depression of cardiac and renal function. Superposition of further hemodynamic or toxic insults upon ischemic kidneys was usually necessary for ARF to occur.

Hepatic resection for metastatic colorectal cancer results in cure for some patients.

OBJECTIVES: To determine the long-term disease-free and overall survivals for patients undergoing hepatic resection for colorectal cancer metastases and to define significant predictors of improved patients survival. DESIGN: Retrospective review. SETTING: Single tertiary care center. PATIENTS: Two hundred eighty consecutive patients underwent hepatic resection for colorectal cancer metastases at the Mayo Clinic from 1960 to 1987. Fifty patients alive at the completion of the study had a mean follow-up of 11.3 years (median, 121 months). MAIN OUTCOME MEASURES: Disease-free interval following initial hepatic resection and death. RESULTS: The overall 5-year survival of the 280 patients was 27%. Twenty-eight patients were alive at 10 years from the time of hepatic resection, and the 10-year actuarial survival was 20%. Only 2 patients alive and free of disease at 5 years had recurrent disease. For all other patients who were free of disease more than 5 years after hepatic resection and died, the cause of death was not cancer related. No patients characteristics or features of the primary tumor affected survival. Clinical presentation of metastatic disease, configuration of hepatic lesions, the presence of extrahepatic lymph node involvement, and the existence of resectable extrahepatic disease significantly affected long-term patient survival. Need for perioperative blood product transfusion was associated with a lower probability of long-term survival. CONCLUSIONS: Disease-free patient survival beyond 5 years from surgical resection of colorectal cancer metastases to the liver represents patient cure in nearly all instances.

Long-term consequences of intraoperative spillage of bile and gallstones during laparoscopic cholecystectomy.

Laparoscopic cholecystectomy is associated with a higher incidence of iatrogenic perforation of the gallbladder than open cholecystectomy. The long-term consequences of spilled bile and gallstones are unknown. Data were collected prospectively from 1059 consecutive patients undergoing laparoscopic cholecystectomy over a 3-year period. Details of the operative procedures and postoperative course of patients in whom gallbladder perforation occurred were reviewed. Long-term follow-up (range 24 to 59 months) was available for 92% of patients. Intraoperative perforation of the gallbladder occurred in 306 patients (29%); it was more common in men and was associated with increasing age, body weight, and the presence of omental adhesions (each P < 0.001). There was no increased risk in patients with acute cholecystitis (P = 0.13). Postoperatively pyrexia was more common in patients with spillage of gallbladder contents (18% vs. 9%; P < 0.001). Of the patients with long-term follow-up, intra- abdominal abscess developed in 1 (0.6%) of 177 with spillage of only bile, and in 3 (2.9%) of 103 patients with spillage of both bile and gallstones, whereas no intra- abdominal abscesses occurred in the 697 patients in whom the gallbladder was removed intact ( P < 0.001). Intraperitoneal spillage of gallbladder contents during laparoscopic cholecystectomy is associated with an increased risk of intra-abdominal abscess. Attempts should be made to irrigate the operative field to evacuate spilled bile and to retrieve all gallstones spilled during the operative procedure.

Diabetes insipidus: a physiologic approach to diagnosis.

Diabetes insipidus can be delineated in the context of the normal physiology of water metabolism. This approach highlights the common pathway taken by the variety of diseases that can progress to an insufficiency of antidiuretic hormone (ADH) and to diabetes insipidus. A simple diagnostic approach uses homeostatic pathways to separate diabetes insipidus from the other polyuric states. New developments in the biochemical alteration of ADH have improved the ability to individualize hormonal replacement and promise better therapy in the near future.

Short and long loop nephrons.

The explanation for the necessity to have both short and long loop nephrons for urinary concentration is unknown but may represent nature's resolution of conflicting ideal conditions for maximum urinary concentration. Ideally, one would like the thick ascending limb to extend throughout the entire medulla to the papillary tip and be supplied by a blood flow vigorous enough to provide oxygen and remove waste products as rapidly as needed. One would also like to have a progressively smaller volume of tissue to be concentrated toward the papillary tip to lessen the osmotic work required and a highly efficient vascular exchange system to sequester the medullary interstitial solute effectively. But the same efficiency of countercurrent exchange of oxygen causes the inner medulla to have a relatively low oxygen content. The presence of the thin loops of Henle in the inner medulla may represent a compromise between these conflicting ideals. The papilla tapers to a low mass, which allows a mechanism requiring only a modest energy supply to increase the tonicity of the interstitium enormously. The reduced work requirement obivates the need for thick ascending limbs to extend into the papilla where they would be highly vulnerable to anoxia. The outer medulla with its larger mass and thick ascending limbs supplied by a high blood flow can initiate the operation to reduce the volume of fluid and solute to be concentrated, and at the same time carry out other functions required of the filtration-reabsorption kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

The renal concentrating mechanism: micropuncture studies of the renal medulla.

Micropuncture of the rat renal papilla has disclosed an outward transepithelial gradient for NaCl at the bend of Henle's thin loop and an electrical potential difference, lumen positive, in the ascending thin limb. Substantial water extraction and urea secretion occur somewhere proximal to the bend, but direct evidence for transepithelial NaCl movement across the rat descending thin limb is lacking. In the hamster, water is extracted and urea secreted, but no NaCl gradient has been found, and in Psammomys there is indirect evidence for transepithelial entry of NaCl into the descending limb. Fluid is diluted in the ascending thin limb by reabsorption of NaCl. The lack of unequivocal evidence for active NaCl reabsorption has stimulated a search for alternative mechanisms of osmotic work in the inner medulla. The collecting duct plays a crucial role by its differential reabsorption of water (primarily in the cortex) and urea (exclusively in the inner medulla) but has not yet been shown to supply useful energy to the concentrating mechanism by active sodium reabsorption. Exposure of the papillary tip by ureteral excision impairs urinary osmolality. Ureteral peristalsis normally causes intermittent flow of fluid in the collecting duct, but abolition of intermittent flow by paralysis of the ureter does not decrease urinary osmolality. Superperfusion of the exposed papilla by a urea solution prevents the decline in osmolality but the amount of urea used greatly exceeds that available from the urine. Nevertheless, it is the intactness of the ureter that is somehow essential to maximum urinary concentration, perhaps by preventing loss of solute from the papilla rather than by supplying energy.