Acquired causes of obstruction may be intrinsic to the urinary tract (i.e., resulting from intraluminal or intramural processes) or may arise from extrinsic causes ( Box 39.2 ).

Intrinsic intraluminal causes of obstruction may be intrarenal or extrarenal. Intrarenal causes arise from the formation of casts or crystals within the renal tubules. These include uric acid nephropathy, which usually results from the large uric acid load released when chemotherapeutic agents abruptly destroy large numbers of tumor cells in the treatment of patients with malignant hematopoietic neoplasms. Uric acid nephropathy may also occur in the setting of disseminated adenomatous carcinoma of the gastrointestinal tract. Drug-induced precipitations have been known to occur with the use of sulfonamides, acyclovir, indinavir, and ciprofloxacin. Sulfadiazine, a relatively lipophilic sulfonamide, penetrates into the brain and has proven to be an excellent treatment for toxoplasmosis. However, the same lipophilicity makes the drug prone to formation of intrarenal crystals, which can lead to acute kidney injury when the drug is given in large doses. ^, In multiple myeloma, a common renal complication is “myeloma kidney,” a condition also known as “myeloma cast nephropathy.” The renal lesions (casts) are directly related to the production of monoclonal immunoglobulin free light chains (FLCs), which coprecipitate with Tamm-Horsfall glycoprotein (THP) in the lumen of the distal nephron, obstructing tubular fluid flow. ^, Promising experiments have identified the determinants of the molecular interaction between FLCs and THP, which permitted development of a peptide that demonstrated strong inhibitory capability in the binding of FLCs to THP in vitro.

Several intrinsic intraluminal, extrarenal, or intraureteral processes may also cause obstruction. Nephrolithiasis represents the most common cause of ureteral obstruction in younger men. In the U.S. population, the prevalence of kidney stones in adults older than 20 years rose from 3.2% in 1980 to 10.1% in 2016. The significance of this is also reflected by the large number of hospital admissions due to calculus of the kidney and ureters, amounting to 166,000 hospital stays in 2006. Calcium oxalate stones occur most commonly. Obstruction caused by such stones occurs sporadically and tends to be acute and unilateral, usually without long-term decrements in kidney function. Of course, when a stone obstructs a solitary kidney, the result can be anuric or oliguric acute kidney injury. Less common types of stones, such as struvite (ammonium-magnesium-sulfate) and cysteine stones, more frequently cause significant kidney damage because these substances accumulate over time and often form staghorn calculi. Stones tend to lodge and obstruct urine flow at narrowings along the ureter including the UPJ, pelvic brim, and ureterovesical junction.

Other processes that cause ureteral obstruction include papillary necrosis, blood clots, and cystic inflammation. Papillary necrosis may result from sickle cell disease or trait, amyloidosis, acute pyelonephritis, exposure to nonsteroidal antiinflammatory drugs, or diabetes mellitus. Acute obstruction may even require surgical intervention. Blood clots secondary to a benign or malignant lesion of the urinary tract or cystic inflammation of the ureter (ureteritis cystica) can also lead to obstruction and hydronephrosis.

Intrinsic intramural processes that cause obstruction include dysfunctional voiding and acute or chronic urinary retention. Bladder storage of urine and micturition require complex interplay of spinal reflexes, midbrain, and cortical function. Neurologic dysfunction occurring in diabetes mellitus, multiple sclerosis, spinal cord injury, cerebrovascular disease, and Parkinson disease can result from upper motor neuron damage. Such lesions may disturb the intricate balance governing storage and micturition, leading to detrusor overactivity, a low compliance, high-pressure bladder, and voiding dysfunction as a result of detrusor sphincter dyscoordination leading to bladder emptying issues. The ensuing high bladder pressures are thus transmitted to the upper tracts by vesicoureteral reflux or obstruction results from the increased hydrostatic pressures needed to stretch the hypertrophic detrusor muscle. ^,

Various drugs may cause intrinsic intramural obstruction by disrupting the normal function of the smooth muscle of the urinary tract. Anticholinergic agents may interfere with bladder contraction, whereas levodopa may mediate an α-adrenergic increase in urethral sphincter tone, resulting in increased bladder outlet resistance. In all circumstances when the bladder does not void normally, kidney damage may develop because of recurrent urinary tract infections and increased detrusor pressures.

Acquired anatomic abnormalities of the wall of the urinary tract include ureteral strictures and benign, as well as malignant, tumors of the urethra, bladder, ureter, or renal pelvis. Ureteral strictures may develop due to stone impaction and as a complication of ureteral instrumentation or surgery. ^, Furthermore, radiation therapy in children and in adults treated for pelvic or lower abdominal cancers, such as cervical cancer, can result in ureteral fibrosis, strictures, and subsequent complications.

Infectious organisms may also produce intrinsic obstruction of the urinary tract. Schistosoma haematobium afflicts nearly 100 million people worldwide. Although active infection can be treated and obstructive uropathy may resolve, chronic schistosomiasis (bilharziasis) may develop in untreated cases, leading to irreversible ureteral or bladder fibrosis and obstruction. Of other infections, the incidence of genitourinary tuberculosis has remained constant over the 30 years, amounting to 3% to 5% of patients with tuberculosis. Mycoses caused by Candida albicans or Candida tropicalis may also result in obstruction due to intraluminal obstruction (fungus ball) or invasion of the ureteral wall.

Acquired extrinsic urinary tract obstruction occurs in a wide variety of settings. The relatively high frequency of obstructive uropathy from processes in the female reproductive tract, such as pregnancy and pelvic neoplasms, results in higher rates of urinary tract obstruction in younger women than in younger men. ^, The advent of routine abdominal and fetal ultrasonography in pregnant women has revealed that 50% to 90% of women entering their third trimester demonstrate some degree of dilatation of the collecting system. The obstruction that is usually mild and subsides several weeks after delivery occurs at the level where the ureters cross the pelvic brim. Mechanical and hormonal factors affect these changes, in that hypertrophy of the Waldeyer sheath (the smooth muscle layer encompassing the distal ureter) and compression from the gravid uterus can impinge on the ureters. Additionally, high progesterone levels reduce ureteral tone and decrease peristalsis. As the uterus is normally dextro-rotated and the right ovarian vein crosses the ureter, dilatation is more common on the right side. ^, Clinically significant obstructive uropathy in pregnancy almost always presents with flank pain. In these cases, ultrasonography serves as a useful initial screening test and magnetic resonance imaging (MRI) can be used if ultrasound is not conclusive. In cases of symptomatic or significant hydronephrosis, relief of obstruction can be attained by placing an internal stent or percutaneous nephrostomy catheter. Clinically significant ureteral obstruction is rare in pregnancy, and bilateral obstruction leading to acute kidney injury is exceptionally rare. Conditions in pregnancy that may predispose to obstructive uropathy and acute kidney injury include multiple fetuses, polyhydramnios, an incarcerated gravid uterus, or a solitary kidney.

Pelvic malignancies, especially cervical adenocarcinomas, represent the second most common cause of extrinsic obstructive uropathy in women. In older women, pelvic organ prolapse leads mainly to lower urinary tract obstruction; however, approximately 7.7% of women scheduled for prolapse surgery had hydronephrosis on preoperative ultrasound. ^, Other benign pelvic abnormalities may cause ureter obstruction including uterine fibroids and ovarian cysts. ^, Pelvic lipomatosis, a disease with an unclear etiology seen more often in men, is another less common reason for compressive urinary tract obstruction.

Endometriosis affects the urinary tract in up to 5% of women afflicted, especially in cases of deep infiltrating endometriosis. The ureters are involved in 9% to 23% of cases, usually unilaterally and with a predilection for the left side. Obstruction may arise due to the inflammation and adhesions caused by endometrial peritoneal deposits, leading to extrinsic compression of the ureters. An alternative theory contends that hematogenic spread may explain intrinsic ureteral involvement. The onset of obstruction may be insidious, and the process is usually confined to the pelvic portion of the ureter. It is thus imperative to screen for obstruction in cases of advanced endometriosis. Ultrasound should be the first-line imaging modality; computed tomography (CT) scanning or MRI can help to determine more precisely the type of ureteral involvement. It is of note that approximately 50% of inadvertent ligations of the ureter in abdominal and retroperitoneal operations occur during gynecologic and obstetric procedures.

After the age of 60 years, obstructive uropathy occurs more commonly in men. Benign prostatic hyperplasia, which is by far the most common cause of urinary tract obstruction in men, produces symptoms of bladder outlet obstruction in approximately 75% of men aged 50 years and older. ^, Upper urinary tract obstruction may also ensue in severe cases. Presenting symptoms of bladder outlet obstruction include difficulty initiating micturition, weak urinary stream, dribbling at the end of micturition, incomplete bladder emptying, and nocturia. The diagnosis may be established by history and urodynamic studies, as well as imaging in some cases. ^{, ,}

Urologic malignancies have, in light of their anatomic origin, a particular propensity to cause both upper and lower urinary tract obstruction. Urothelial carcinoma, which accounts for 3% of global cancer diagnoses, ^, can manifest anywhere along the lining of the urinary tract and may lead to obstruction at the ureterovesical junction in cases of bladder cancer or anywhere along the ureter. Presence of hydronephrosis in bladder cancer and upper urinary tract urothelial carcinoma has been found to adversely affect overall prognosis. ^, Despite stage migration to more organ-confined disease in the era of prostate-specific antigen, obstruction due to prostate cancer compressing the bladder neck and invading the ureteral orifices is still relatively common. ^, Urinary tract obstruction in advanced and metastatic prostate cancer can have a varied presentation because it may occur in multiple anatomic locations including the ureter and pelvic lymph nodes.

Gastrointestinal processes may also cause obstructive uropathy. Inflammatory bowel disease (IBD) may extend into the retroperitoneum, leading to obstruction of the ureters. Additionally, the malabsorption associated with IBD and bariatric surgery can lead to nephrolithiasis due to metabolic abnormalities including hyperoxaluria and hypocitraturia. ^, Disorders of the colon including diverticulitis and fecal impaction may also on occasion lead to ureteral obstruction. ^, Acute and chronic pancreatitis are also rare causes of mainly right-sided hydronephrosis.

Vascular disease also constitutes a rare cause of obstruction. Aneurysms of the abdominal aorta and iliac arteries may lead to ureteral obstruction either by direct pressure of the aneurysm or as a result of the associated retroperitoneal fibrosis. ^, In addition, and also rarely, vasculitis caused by systemic lupus erythematosus, polyarteritis nodosa, granulomatosis with polyangiitis, and Henoch–Schönlein purpura ^, has been reported to cause obstruction.

Retroperitoneal neoplastic processes can obstruct the ureters at multiple levels by direct extension, encasement, or invasion as seen with periaortic nodal masses resulting from primary nodal disease, such as lymphoma or retroperitoneal metastases from urologic, gastrointestinal, or germ cell tumors. Relief of obstruction can be achieved by stenting or nephrostomy tube placement. Idiopathic, retroperitoneal fibrosis, on the other hand, ^, usually involves the middle third of the ureter and affects men and women equally, predominantly those in the fifth and sixth decades of life. Retroperitoneal fibrosis may also be drug induced (e.g., methysergide), or it may occur as a consequence of scarring from multiple abdominal surgical procedures. It may also be associated with conditions as varied as gonorrhea, sarcoidosis, chronic urinary tract infections, Henoch-Schönlein purpura, tuberculosis, biliary tract disease, and inflammatory processes of the lower extremities with ascending lymphangitis.

Rare childhood tumors such as pelvic neurofibromas can induce upper urinary tract obstruction in up to 60% of patients. Wilms tumor may obstruct via local compression of the renal pelvis, and neuroblastoma may cause both upper and lower urinary tract obstruction. Miscellaneous inflammatory processes can also result in obstruction. These include granulomatous causes such as sarcoidosis and chronic granulomatous disease of childhood. Amyloid deposits may produce isolated involvement of the ureter. Furthermore, a pelvic mass or inflammatory process associated with actinomycosis may cause external ureteral compression. ^, Retrovesical echinococcal cysts can also impede urine flow. Retroperitoneal malacoplakia can also be a rare cause of urinary obstruction.

The laboratory evaluation includes urinalysis and examination of the sediment on a fresh specimen by an experienced observer. Unexplained renal failure with benign urinary sediment should suggest urinary tract obstruction. Microscopic hematuria without proteinuria may suggest calculus or tumor. Pyuria and bacteriuria may indicate pyelonephritis; bacteriuria alone may suggest stasis. Crystals in a freshly voided specimen should lead to consideration of nephrolithiasis or intrarenal crystal deposition.

Hematologic evaluation includes the hemoglobin/hematocrit and mean corpuscular volume (to identify anemia of chronic renal disease) and white blood cell count (to identify possible hematopoietic system neoplasm or infection). Serum electrolytes (Na ⁺ , Cl ^– , K ⁺ , and HCO ₃ ^– ), blood urea nitrogen concentration, creatinine, Ca2 ⁺ , phosphorus, Mg2 ⁺ , uric acid, and albumin levels should be measured. These will help identify disorders of distal nephron function (impaired acid excretion or osmoregulation) and uremia. Urinary chemistries may also suggest distal tubular dysfunction (high urine pH, isosthenuric urine) and inability to reabsorb sodium normally (urinary Na ⁺ >20 mEq/L, fractional excretion of Na ⁺ [FENa] >1%, and osmolality <350 mOsm/L). Alternatively, in acute obstruction, urinary chemistry values may be consistent with prerenal azotemia (urinary Na ⁺ <20 mEq/L, FENa <1%, and osmolality >500 mOsm/L).

Novel biomarkers relevant for functional and cellular and molecular changes are being developed as an index of kidney injury and to predict functional reserve or recovery after reconstruction. Simple tests examining the value of risk factor proteins have suggested that elevated levels of neutrophil gelatinase-associated lipocalin and β2-microglobulin are present in the urine from obstructed kidneys. Attempts to predict the clinical outcome of congenital unilateral UPJ obstruction in newborns by urine proteome analysis reveal an example of this powerful new technology. Polypeptides in the urine were identified and enabled diagnosis of the severity of obstruction, and using this technique the clinical evolution was predicted with 94% precision in neonates, whereas the precision was only 20% in older children with UPJ obstruction. Thus far, application of large-scale urinary proteomic analysis holds promise for better classification of individuals with hydronephrosis for early selection of surgical candidates, but long-term follow-up studies are warranted to determine the true clinical value of this diagnostic approach. Use of a proteomic analysis of fetal urine using a 12-peptide signature expressed in fetuses who go on to develop end-stage renal disease by the age of 2 years has recently shown promise in determining prognosis of posterior urethral valves.

In an attempt toward improved understanding of obstructive nephropathy and improved translatability of the results to clinical practice, a systems biology approach combining omics data of both human and mouse obstructive nephropathy was developed. Using the urinary miRNome of infants with UPJ obstruction and the kidney tissue miRNome and transcriptome of the corresponding neonatal partial unilateral ureteral obstruction (PUUO) mouse model revealed that let-7a and miR-29b are potentially involved in the development of fibrosis in UPJ obstruction via the control of DTX4 in both humans and mice.

The history, physical examination, and initial laboratory studies should guide the medical imaging evaluation. Pain, degree of impaired kidney function, and the presence of infection dictate the speed and nature of the evaluation. Numerous imaging techniques are available; each has its own advantages and disadvantages, including the ability to identify the site and cause of the obstruction and to separate functional obstruction from mere dilatation of the urinary tract. Patient-specific factors, such as the risks of radiocontrast in the setting of impaired kidney function or exposure to radiation in pregnant women, must also be weighed.

Ultrasonography

Ultrasonography is usually the initial investigative modality employed when urinary tract obstruction is suspected. Ultrasound provides excellent anatomic information about obstruction, primarily a dilated collecting system and the degree hereof. It can also provide information regarding the level and cause of obstruction. Ultrasound is safe and readily available and avoids ionizing radiation, thus making it ideal for pregnant and pediatric patients. Moreover, as no radiographic contrast is required, it is well suited in the assessment of suspected obstruction in the setting of impaired kidney function, where iodinated contrast administration should be used cautiously and in patients allergic to contrast material. It may also determine the size and shape of the kidney and may demonstrate thinned cortex in case of severe long-standing hydronephrosis ( Fig. 39.1 ). Additionally, ultrasound may detect perinephric fluid collections, which may indicate urine leakage from an obstructed system or signs of perinephric abscess.

Renal ultrasound.

(A) Normal kidney. (B) Hydronephrotic kidney: dilated calices and pelvis (arrows).

Although ultrasound has a sensitivity of 98% for detecting hydronephrosis, it does not provide physiologic or functional data with respect to the dilatation observed, nor does ultrasound provide information on kidney function. The specificity of ultrasound has been criticized, as there are multiple conditions other than obstruction that can cause dilatation, such as vesicoureteral reflux, a dilated extrarenal pelvis, and residual dilatation after previous obstruction, which can lead to false-positive interpretations. False-negative results are also known to occur, especially during the first 48 hours of obstruction, ^, in cases of dehydration, staghorn calculi, nephrocalcinosis, and retroperitoneal fibrosis, as well as with tumor encasement of the collecting system. A dilated collecting system without obstruction may also be observed in up to 50% of patients with urinary diversion through ileal conduits, especially on the left side. In suspected stone disease, ultrasound is recommended as the initial investigative imaging modality in young patients and pregnant women; however, for adults with renal colic and especially those who are obese, most guidelines recommend the use of CT because its accuracy for diagnosing and characterizing stones, location, and burden exceeds that of ultrasound.

Intravenous Urography

Intravenous urography (also known as “intravenous pyelography”) was for many years the imaging modality of choice for patients suspected of having urinary tract obstruction ( Fig. 39.2 ). Intravenous urography requires administration of iodinated contrast, is time consuming to perform, and for all purposes, it is obsolete in institutions where CT is available.

Intravenous pyelography.

Normal right kidney and dilated collecting system on the left. The obstruction was relieved with a stent.

Computed Tomography

CT as an imaging modality has become ubiquitous in urologic diagnostics, especially in urothelial disease, renal masses, work-up of hematuria, and obstructions. CT has a particular advantage because it can visualize a dilated collecting system without the requirement for contrast enhancement ( Fig. 39.3 ). Because of its exquisite sensitivity to density, CT can identify even radiolucent stones because uric acid stone density is at least 100 Hounsfield units (HU), which is higher than that of soft tissue (usually 10–70 HU).

Computed tomography, noncontrast study.

(A) Left hydronephrosis: dilated renal pelvis (arrows), with normal kidney on the right. (B) Reason for obstruction: left midureteral stone (arrow).

CT is especially effective in identifying extrinsic causes of obstruction (e.g., retroperitoneal fibrosis, lymphadenopathy, and hematoma) and when contrast-enhanced CT is used, obstructing urothelial lesions in the ureters or bladder along with prostate pathology ( Fig. 39.4 ) can easily be visualized. Helical multiphasic contrast-enhanced CT has also proven to be an accurate and noninvasive method of demonstrating crossing vessels in UPJ obstruction.

Computed tomography of the pelvis.

(A) Large postvoiding residual urine in the bladder. (B) Enlarged prostate (arrows), leading to urinary retention.

Furthermore, CT can detect extra-urinary pathology and can establish non-urogenital causes of pain. All of these advantages establish non-contrast-enhanced helical CT as the diagnostic study of choice for the evaluation of the patient with acute flank pain. Ultrasound may be the first method of diagnosis in this setting ( Fig. 39.5 ), but CT resolution and depiction of details are usually superior to those of ultrasound.

Pelvic ultrasound.

(A) Distended bladder (arrowheads). (B) Enlarged prostate (arrows), causing infravesical urinary obstruction.

In this regard, it must be emphasized that there is a minimal cancer risk from radiation exposure in connection with CT scans. This incurred theoretical risk must be considered in the context of the clinical benefit derived from a medically indicated CT and the likelihood of cancer occurrence in the general population. The optimal method of mitigating this risk is avoidance of unnecessary scans and optimizing the technical aspects of medically justified ones. This issue is especially pertinent in children who may be at higher risk of developing leukemias and brain tumors.

Magnetic Resonance Imaging

MRI generates images by transmitting and receiving radiowave frequencies while altering the magnetic field strength to determine the spatial orientation and contrast of images. The energy used is nonionizing and therefore, as discussed earlier, the modality can safely be used in children and during pregnancy. However, MRI studies are relatively expensive, and acquisition times are relatively long compared with other modalities. A full MRI protocol can take up to 15 to 30 minutes, with any motion from the patient potentially degrading image quality. Sedation may therefore be necessary for children.

MRI sequences fall into T1-weighted or T2-weighted categories, with T2-weighted sequences being sensitive for fluids, which appear bright on the image, and hence protocols with multiple specialized T2-weighted sequences can be constructed to simulate a CT urogram ( Fig. 39.6 ). As in CT, MRI sequences can be enhanced by administering an intravenous contrast agent. Enhanced functional MRI studies can combine anatomic visualization of the kidneys and urinary tract with functional information acquired from the transit of the contrast from the cortex and medullary pyramids into the collecting system. This enables measurement of glomerular filtration rate; assessment of potential prognostic factors (hypoxia, inflammation, cell viability, degree of tubular function, and interstitial fibrosis); and monitoring of new therapies. Functional MRI can also provide valuable information regarding tissue energy consumption from so-called blood oxygenation level dependent (BOLD) imaging; these data may be helpful in the future in predicting the recovery of kidney function following obstruction. ^,

Magnetic resonance (MR) imaging urography of left-sided hydronephrosis with parenchymal thinning.

MR urographic image shows large dilatation of the left pelvicalyceal system and narrowing of the left ureteropelvic junction segment.

Studies in children and adults have shown high sensitivity and specificity for magnetic resonance urography in diagnosing obstruction; however, identifying cause of obstruction was only possible in approximately 50%. This is especially true for stone disease because MRI cannot directly detect calcifications or stone material.

Concerns related to the use of MRI in patients with kidney disease were highlighted by the risk of developing nephrogenic systemic fibrosis (NSF) induced by the toxicity of several gadolinium-based contrast agents in patients with severely impaired kidney function, although newer formulations of gadolinium do not appear to contribute to NSF.

Retrograde and Antegrade Pyelography

When other tests do not provide adequate anatomic detail or when obstruction must be relieved (e.g., obstruction of a solitary kidney, bilateral obstruction, and symptomatic infection in the obstructed system), more invasive investigation, with a combination of treatments, may be necessary. When retrograde pyelography is performed, this takes place during cystoscopy, by cannulating the ureteral orifice and injecting contrast. In some cases of complete obstruction, contrast may not reach the kidney, but the procedure will define the lower level of the obstruction. Retrograde pyelography can be combined with placement of a ureteral stent to relieve an obstruction or with possible stone extraction. Because the catheter passes through the bladder to reach the upper urinary tract, the risk of introducing infection must be kept in mind. Antegrade pyelography is performed by percutaneous cannulation of the renal pelvis and injection of the contrast material into the kidney and ureter. ^, This procedure should establish the proximal level of obstruction and may also serve as a first step in relieving obstruction by means of percutaneous nephrostomy ( Fig. 39.7 ).

Antegrade pyelography.

(A) Dilated renal pelvis and calices on left. (B) Stones (arrowheads) as filling defects in the distal ureter (not seen on plain film). Intravenous pyelography was unsuccessful owing to the obstructed and malfunctioning kidney.

In the immediate 2 to 3 hours after the onset of ureteral obstruction, blockade of antegrade urine flow markedly increased proximal tubule hydraulic pressure. This increase in pressure in the Bowman space would be expected to halt GFR immediately. However, during this early phase of obstruction, the afferent arterioles dilate, decreasing afferent arteriolar resistance, increasing glomerular capillary pressure, and counteracting the increase in proximal tubule hydraulic pressure. ^, Because this vasodilator or “hyperemic response” occurs in denervated kidneys in situ and in isolated perfused kidneys, ^, it must result from intrarenal mechanisms. In fact, glomeruli of individual nephrons exhibit the same response in in vivo micropuncture experiments when antegrade urine flow is blocked by placement of a wax block in the tubule of the nephron.

Many mechanisms may mediate this afferent vasodilatation, including increases in vasodilatory hormones such as prostaglandins, reduced NaCl concentration at the macula densa, and a direct myogenic reflex. This hyperemic response is not attenuated by renal nerve stimulation or infusion of catecholamines and may be linked to changes in interstitial pressure.

In the tubuloglomerular feedback response, reduced tubular flow past the macula densa induces reductions in afferent arteriolar resistance and increases in glomerular capillary pressure, so single-nephron GFR rises. Similarly, because obstruction reduces urine flow past the macula densa, it would be predicted to induce afferent vasodilatation. However, elegant micropuncture studies separated the stoppage in flow from increases in the proximal tubule by placing an additional puncture in the tubule that was proximal to the blockage of flow to the macula densa. In this setting, flow past the macula densa was halted but proximal tubule remained normal because accumulating tubular fluid was permitted to leak out. In such nephrons the increase in glomerular capillary pressure observed in obstructed tubules did not occur, indicating that the obstruction itself and not the macula densa stimulates afferent vasodilatation.

Renal prostaglandins and renal nerves play important roles in the hyperemic response. Indomethacin blocks the hyperemic response, indicating that vasodilator prostaglandins are critical to afferent vasodilatation. ^, A renorenal reflex mechanism in the hemodynamic response to obstruction can be discerned from studies in bilateral obstruction, where the afferent vasodilatation response is absent or markedly attenuated. ^, Obstruction of the left kidney augments afferent renal nerve activity from the left kidney and efferent nerve activity to the right kidney. Increased efferent nerve activity to the right kidney was accompanied by reduced blood flow to that kidney. This vasoconstrictor response was ablated by denervation of either the left or right kidney before induction of left ureteral obstruction, suggesting that increased afferent renal nerve traffic triggers vasoconstrictive renorenal reflex activity that counteracts the early intrinsic renal vasodilator effects of obstruction in bilateral ureteral obstruction.

Because obstruction results in cessation of glomerular filtration, efforts to study the regulation of single-nephron GFR later in obstruction have measured determinants of GFR immediately after release of obstruction. ^, Using this approach, investigators have shown that renal blood flow declines progressively after 3 hours of unilateral obstruction and through 12 to 24 hours of obstruction. ^, Interestingly, although tubular pressures rise initially after obstruction, they then decline, so by 24 hours renal plasma flow, GFR, and intratubular pressures have all dropped below normal values. ^{, , ,} At 24 hours into the obstruction, examination of regional blood flow in the kidney by injections of silicone rubber reveals large areas of the cortical vascular bed that are either underperfused or not perfused at all. ^{, ,} Depending on the species, the different vascular beds in the outer and juxtamedullary cortex receive differing proportions of the renal blood flow under basal conditions and following obstruction. However, at 24 hours of obstruction, reduced whole-kidney GFR is clearly due, in large part, to nonperfusion of many glomeruli.

Beyond 24 hours of obstruction, the single-nephron GFR of glomeruli that remain perfused is markedly decreased because of reduced blood flow to the afferent arteriole and afferent vasoconstriction, which, in turn, reduces glomerular capillary pressure. ^, Because glomerular capillary pressure responds in the same manner when the individual nephron is blocked with oil for 24 hours before micropuncture measurements are performed, it is clear that afferent arteriolar vasoconstriction plays an important role in attenuating single-nephron GFR during the established phase of obstruction. These results indicate that, like the early hyperemic response, intrarenal mechanisms play the major role in the late vasoconstrictive response to ureteral obstruction. In bilateral obstruction, renal blood flow is reduced to levels 30% to 60% below normal ( Table 39.1 ). ^{, ,} In both unilateral and bilateral obstruction, single-nephron GFR falls to a similar degree. However, the mechanisms involved are different in the two conditions. In unilateral obstruction, reduced glomerular capillary pressure lowers the driving pressure for filtration when set against a nearly normal proximal tubular hydraulic pressure. By contrast, in bilateral obstruction, glomerular capillary pressure remains normal and GFR is halted by a highly elevated proximal tubular hydraulic pressure. These results suggest that systemic factors, such as accumulation of extracellular fluid volume and urea, an increase in natriuretic substances, and alterations in renal nerve activity, modulate the vasoconstrictive effect of obstruction on the affected kidney.

The reduction in renal blood flow and GFR after release of obstruction varies with the species studied and duration of obstruction. Following release of a 24-hour complete unilateral obstruction, GFR remains below 50% of normal in dogs and 25% of normal in rats; renal blood flow remains markedly reduced in both species. After release of bilateral ureteral obstruction, renal blood flow increases to levels higher than those observed after unilateral obstruction. This increase might be due to systemic natriuretic influences such as volume accumulation, reduced sympathetic tone, or increased circulating atrial natriuretic peptide. However, despite an increase in renal blood flow, GFR remains significantly impaired. The persistent reduction in GFR is due in part to nonperfusion or underperfusion of many glomeruli as shown in silicone rubber injection experiments. ^, Where glomeruli remain perfused, strong afferent vasoconstriction reduces glomerular capillary pressure. This, combined with the decline in proximal tubule hydraulic pressure after release of obstruction, leads to a low driving force for glomerular filtration. ^, In addition, a sharp reduction in Kf further contributes to the decline in GFR upon release of both unilateral and bilateral obstruction. ^,

Several mechanisms contribute to afferent vasoconstriction and a reduced Kf. First, release of obstruction strikingly augments the flow of tubular fluid past the macula densa. Although the absolute rate of flow is still far below normal, the macula densa likely senses the change in flow, which may lead to intense vasoconstriction. In favor of this view, the sensitivity of the tubuloglomerular feedback mechanism is enhanced in unilateral, as compared with bilateral, obstruction, suggesting that the ability of the mechanism to regulate afferent arteriolar tone is modulated by the extrarenal hormonal milieu.

There is substantial evidence that increased intrarenal secretion of angiotensin II participates actively in afferent vasoconstriction and reduced Kf following release of ureteral obstruction. Ureteral obstruction rapidly increases renal vein renin levels at a time when renal blood flow is normal or elevated, but at later time points, renal vein renin levels return to normal. In addition, infusion of captopril, an angiotensin-converting enzyme (ACE) inhibitor, attenuated the declines in renal blood flow and GFR observed in both unilateral and bilateral obstruction. ^, The effect of captopril persisted regardless of whether kinin activity was eliminated through infusions of either carboxypeptidase B, which destroys kinins, or aprotinin, which blocks kinin generation. This suggests that captopril reduced afferent arteriolar resistance primarily by blocking generation of angiotensin II rather than affecting kinins. The significance of the renin–angiotensin system as an important contributor to vasoconstriction has recently been highlighted in studies where AT1 receptor blocker treatment attenuated the reduction in GFR in the postobstructive period in both adult rats and rats with neonatally induced unilateral partial obstruction in response to long-term AT1 receptor antagonist treatment.

Thromboxane A2 (TXA2) plays a role in obstruction-induced vasoconstriction. ^, Chronically, hydronephrotic kidneys exhibit increased TXA2 accumulation, as measured by accumulation of its more stable metabolite TXB2. Furthermore, whole-kidney GFR and renal blood flow were increased in response to thromboxane synthase inhibitor treatment, ^, likely by reducing afferent arteriolar resistance and thereby increasing Kf. From these results, TXA2 appears to be generated in the kidney after release of obstruction and mediates afferent vasoconstriction and reductions in Kf.

Although the source of TXA2 generation remains unclear in some cases, but not all, glomeruli isolated from obstructed kidneys have shown increased ability to synthesize TXA2 and other studies have suggested inflammatory cells as the source of TXA2. This is consistent with the observations that suppressor T cells and macrophages migrate to the renal cortex and medulla during the first 24 hours of obstruction, reaching levels fifteen fold higher than those observed in normal kidneys and a parallel rise in TXA2 release and the fall in GFR. These changes can be attenuated by renal irradiation, suggesting that obstruction stimulates migration of inflammatory leukocytes, which, in turn, generate vasoconstrictors such as TXA2. ^, The role of angiotensin II in this process is highlighted because glomeruli isolated from obstructed kidneys showed increased eicosanoid synthesis after angiotensin II stimulation. In addition, treatment of animals with urinary tract obstruction with ACE inhibitors enhanced GFR and reduced TXA2 generation by glomeruli isolated from these animals. Thus these vasoconstrictors may contribute to regulate afferent arteriolar resistance and GFR after release of obstruction.

Less severe vasoconstriction in animals with bilateral ureteral obstruction, as noted earlier, suggests that extrarenal factors play a major role in modulating the hemodynamic response of the kidney to obstruction and release of obstruction. In addition to renorenal reflexes, factors including volume accumulation, urea, atrial natriuretic peptide, and other natriuretic substances appear to mitigate vasoconstriction in response to bilateral obstruction. ^, After 24 hours of obstruction, GFR is preserved to some degree if the contralateral kidney is also obstructed or removed. In addition, reinfusion urea, salt, and water content of the urine from the contralateral kidney markedly increase GFR in animals after release of 24 hours of unilateral obstruction. ^, Taken together, this suggests that atrial natriuretic peptide, urea, and other excreted urine solutes have a protective effect and can ameliorate vasoconstriction post release of ureteral obstruction by direct vasodilatation of afferent arterioles, constriction of efferent arterioles, and an increase in Kf.

Additional studies in dogs and rats have implicated endothelins as contributors to reduced GFR in obstruction, while prostaglandin E2 (PGE2) and nitric oxide (NO) help alleviate glomerular vasoconstriction in the chronic obstructed kidneys. ^, Renal PGE2 levels increase markedly in obstruction (see later) and in states of extracellular volume expansion, as occurs in bilateral ureteral obstruction. Given the vasodilator effects of PGE2, it appears likely that increased levels could ameliorate falls in GFR in obstruction. Bilateral obstruction may reduce generation of NO, leading to a net vasoconstrictive effect.

PUUO is associated with increased renin-angiotensin-aldosterone system activity, elevated oxidative stress, reduced NO bioavailability, and sensitized afferent arteriolar reactivity and renal autoregulation, leading to blood pressure elevation in both mice and rats. Renal denervation in PUUO rats attenuated both hypertension and salt sensitivity and normalized the renal excretion pattern, whereas the degree of renal fibrosis and inflammation was not changed. This suggests a link among renal nerves, increased blood pressure, and modulation of nicotinamide adenine dinucleotide phosphate oxidase function.

In summary, both intrarenal and extrarenal factors combine to profoundly decrease GFR during and immediately after release of obstruction. The decrease in GFR is caused by a sharp reduction in the number of perfused glomeruli and by a reduction in the single-nephron GFR of functioning nephrons. Decreased Kf and increased afferent arteriolar resistance reduce single-nephron GFR. Increases in various vasoconstrictors, such as angiotensin II and TXA2, as well as other vasoconstrictors, some coming from inflammatory cells, augment these hemodynamic effects. In the setting of bilateral obstruction, retention of urea and other solutes, as well as volume expansion and increases in circulating levels of vasodilators such as atrial natriuretic peptide, helps offset these vasoconstrictive effects, but only partially.

The extent of recovery of glomerular filtration after release of obstruction depends on several factors including the duration and extent of obstruction, presence or absence of a functioning contralateral kidney, presence or absence of associated infection, and level of preobstruction renal blood flow. ^, In a classic experiment in dogs subjected to a 1-week period of complete UUO, GFR fell to 25% of normal on release of the obstruction and recovered gradually to 50% of normal levels 2 years later, indicating persisting irreversible changes. In rats, release of UUO of 7- and 14-day duration left residual GFR at 17% and 9% of control levels, respectively, when the contralateral kidney was left in place, and at 31% and 14% when the animals underwent contralateral nephrectomy at the time of release of the obstruction. A similar beneficial effect on the obstructed kidney of contralateral nephrectomy was observed in rats subjected to chronic partial obstruction. As discussed earlier, this beneficial effect likely results from the accumulation of urea and other solutes and increased levels of ANP when the functioning contralateral kidney is absent.

The partial recovery of total GFR after release of obstruction masks an uneven distribution of blood flow and nephron function. In micropuncture studies, some nephrons never regain filtration function, whereas others reveal striking hyperfiltration. It appeared in some studies that surface nephrons exhibited normal single-nephron GFR, whereas the whole-kidney GFR was reduced to 18% of normal. These results suggest that chronic partial obstruction causes selective damage to juxtamedullary and deep cortical nephrons. ^{, ,} Similarly, studies of the long-term outcome of complete 24-hour ureteral obstruction revealed that total GFR recovered to normal levels by 14 and 60 days after release of obstruction. However, 15% of the glomeruli were not filtering in recovered kidneys, and other nephrons were hyperfiltering. In this model of complete obstruction, there appeared to be no selective advantage for surface glomeruli over deep cortical and juxtamedullary glomeruli.

Similarly, in the developing kidney, the duration of obstruction and timing of release significantly impact long-term renal function. Releasing obstruction after 1 week prevented hydronephrosis and reduced renal blood flow and GFR impairment in rats with PUUO from birth. In contrast, releasing obstruction after 4 weeks resulted in minimal or no kidney function in the neonatal obstructed kidney, highlighting the importance of early release of obstruction for optimal protection of kidney function after the maturation process is completed. Similarly, neonatal unilateral partial obstruction in pigs demonstrated that impaired nephrogenesis resulted in a reduced number of glomeruli in the obstructed kidney. Preserved whole-kidney function suggests some degree of glomerular hyperfiltration. In line with the hypothesis that hyperfiltration is associated with an increased risk of systemic hypertension, studies in pigs and rats have demonstrated that renal expressions of neuronal nitric oxide synthase (NOS) and endothelial NOS proteins were lower in animals with hydronephrosis. ^, These findings suggest that the reduced NO response in the obstructed hydronephrotic kidney and subsequent resetting of the tubuloglomerular feedback mechanism play an important role in the development of hypertension in hydronephrosis.

Obstruction severely impairs the ability of renal tubules to transport Na ⁺ , K ⁺ , and H ⁺ and reduces their ability to concentrate and dilute the urine ( Table 39.2 ). ^, The resulting inability to reabsorb water and solutes facilitates postobstructive diuresis and natriuresis. As in the case with glomerular filtration, the extent of disruption of tubular transport depends directly on the duration and severity of the obstruction. Pathologically, prolonged obstruction leads to profound tubular atrophy and chronic interstitial inflammation and fibrosis (see the following section), whereas at early time points after the onset of obstruction, such as at 24 hours, there are only slight structural and ultrastructural changes including mitochondrial swelling, modest blunting of basolateral interdigitations in the thick ascending limb and proximal tubule epithelial cells, flattening of the epithelium, and some widening of the intercellular spaces in the collecting ducts. ^{, ,} The only cell death at early time points is observed at the tip of the papilla, where focal necrosis may be observed. Because there is so little cell damage and the model is simple, most investigators have examined the effect of 24 hours of complete ureteral obstruction on tubular function. As discussed later, regulation of tubular transport is complex and due to both direct damage of epithelial cells and the action of extra tubular mediators, arising from the kidney and extrarenal sources.

Following release of 24 hours of UUO, volume excretion from the postobstructed kidney is normal or slightly increased ^{, , ,} (see Table 39.2 ). However, as discussed earlier, normal volume excretion occurs in the setting of a markedly reduced (20% of normal) GFR. This leads to a higher fractional excretion of sodium (FENa). After release of bilateral obstruction, salt and water excretion rises from fivefold to ninefold higher than normal. ^{, , ,} Given the decreased GFR in this setting, FENa can even be 20-fold higher than normal.

The micropuncture studies summarized in Table 39.2 demonstrate that the reabsorption defect after release of obstruction is localized similarly in both unilateral and bilateral ureteral obstruction. Obstruction reduced net salt and water reabsorption in the medullary thick ascending limb (MTAL), distal convoluted tubule, and entire length of the collecting duct including its cortical, outer medullary, and inner medullary segments.

These studies in whole animals were confirmed and extended by a series of studies from multiple laboratories using isolated perfused tubule and cell suspension preparations ( Table 39.3 ). As shown, the segments including proximal straight tubule, MTAL, and cortical collecting duct isolated from unilaterally or bilaterally obstructed animals exhibited profound impairment of sodium reabsorptive capacity. ^, This finding was confirmed in studies of freshly prepared suspensions of MTAL cells from obstructed kidneys, in which transport-dependent oxygen consumption, a measure of sodium reabsorptive capacity, was markedly reduced. Given the regulating role of mineralocorticoids in the collecting duct, reductions in collecting duct sodium reabsorption capacity are observed in tubules taken from obstructed kidneys, regardless of prior mineralocorticoid treatment. ^{, ,} As the inner medullary collecting duct is challenging to study directly, investigations using cell suspensions show a marked reduction in transport-dependent oxygen consumption in inner medullary collecting duct cells isolated from animals with bilateral obstruction.

Taken together, the data derived from micropuncture, tubule perfusion, and cell suspension studies reveal a striking impairment of sodium reabsorption in the proximal straight tubule, MTAL, and entire collecting duct. Because these functional derangements occur in the absence of clear-cut ultrastructural damage to the epithelial cells, obstruction likely induces a selective impairment in the regulation of active cellular transport mechanisms. Unlike the situation with glomerular filtration, the functional impairment of tubules appears similar in both unilateral and bilateral obstruction. ^{, ,} Thus it appears that a major component of impaired active transport is likely due to direct tubular cell injury rather than continuous action of natriuretic substances. Added to this intrinsic injury, natriuretic substances may be responsible for the apparent secretion of sodium and water in the inner medullary collecting duct of animals after release of bilateral obstruction (see Table 39.2 ).

A combination of studies of cell suspensions and antibody-based targeted proteomics where long-term regulation of renal transporters and channels can be examined in intact animals to understand the integrated response to obstruction has improved the molecular understanding of mechanisms by which tubular epithelial cell sodium reabsorption is impaired in the setting of obstruction. Active tubular Na ⁺ transport requires an apical entry step (e.g., NKCC2 cotransporter in MTAL or epithelial Na ⁺ channels [ENaCs] in the collecting duct) coupled to the basolateral Na ⁺ , K ⁺ -ATPase. In addition, the cell must generate sufficient adenosine triphosphate (ATP) to fuel active transport by the ATPase. Suspensions of MTAL cells from obstructed kidneys exhibited markedly reduced furosemide-sensitive oxygen consumption, indicating striking decreases in apical NKCC2 cotransporter activity in these cells. Isotopic bumetanide binding revealed a marked reduction in the number of cotransporter protein molecules available for binding on the membrane, with no change in affinity of binding, indicating that obstruction downregulates the expression of the cotransporter protein on the membrane surface. More recent studies using antibody-based targeted approaches clearly showed that obstruction diminishes expression of the cotransporter protein on the MTAL cell apical membrane. Similar approaches demonstrated downregulation of Na ⁺ , K ⁺ -ATPase of both α- and β-subunits at the transcriptional and posttranscriptional levels.

In the inner medullary collecting duct, similar studies demonstrated downregulation of ENaC. Consistent with this, suspensions from obstructed kidneys showed marked decreases in amiloride-sensitive oxygen consumption, as well as amiloride-sensitive isotopic sodium entry into hyperpolarized cells.

Obstruction reduces oxygen consumption and ATPase activity in inner medullary collecting duct cells, similar to MTAL cells. In addition, the levels of both Na ⁺ , K ⁺ -ATPase subunits decrease in these cell preparations. Patterns of messenger RNA (mRNA) expression resemble MTAL, indicating that the pump subunits can be downregulated at both transcriptional and posttranscriptional levels. Using the targeted antibody-based approach, it has been demonstrated that in both unilateral and bilateral ureteral obstruction, the expressions of Na ⁺ /H ⁺ exchanger (NHE3) and the Na ⁺ /PO43 ^– exchanger (NaPi-2) were strikingly decreased in the proximal tubule. ^, These changes occurred in both the proximal convoluted and proximal straight tubule, even though the micropuncture and tubule perfusion studies cited earlier demonstrated preserved proximal convoluted tubule salt reabsorption and inhibition of proximal straight tubule reabsorption. ^, Transporter protein and apical membrane expression of the distal convoluted tubule Na ⁺ /Cl ^– cotransporter are also reduced, implying similar obstruction effects as seen in the MTAL and collecting duct. ^,

Collectively, these findings reveal that obstruction leads to decreased membrane expression of proteins involved in sodium transport. In addition, metabolic studies suggest a decrease in oxidative and glycolytic enzyme activities, implying reduced energy production in these cells. This might be intensified by decreased basolateral folding and mitochondrial density in obstructed kidney tubules. Notably, in MTAL and collecting duct suspensions, obstruction reduces oxygen consumption linked to transport, implying ATP generation is not the limiting factor for active transport. Thus the decline in epithelial sodium transport due to obstruction seems to be a controlled response tied to lowered metabolic demands during obstruction.

The mechanisms and pathways responsible for downregulation of transport proteins in tubular epithelial cells by obstruction remain largely incomplete. Possible signals include the halting of urine flow, increased hydrostatic pressure on tubular epithelial cells, changes in blood flow to the tubules or in interstitial pressure, and generation of natriuretic substances in the kidney that result in long-term inhibition of transporter function. Powerful mass spectrometry analysis of tissue from obstructed rat kidneys and mpkCCD cells has led to proteomic identification of significant changes in more than 100 proteins including those belonging to the cytoskeleton. These findings suggest that obstruction induces acute molecular changes in the renal cytoskeleton, in part, mediated by increased stretch of the renal tubular cells during obstruction.

Obstruction disrupts glomerular filtration, causing a dramatic reduction (or even cessation) in urine production. This lowers sodium delivery to tubular segments, resulting in sluggish apical membrane Na ⁺ entry due to unfavorable electrochemical gradients. Reduced Na ⁺ entry might then directly stimulate downregulation of transporter activity and expression. In MTAL and inner medullary collecting duct cells, blocking Na ⁺ entry by furosemide or amiloride, respectively, promptly reduced ouabain-sensitive oxygen consumption, ^, indicating acute downregulation of Na ⁺ , K ⁺ -ATPase. In addition, in mineralocorticoid-clamped animals, chronic Na ⁺ entry blockade at the MTAL or cortical collecting duct further reduced ouabain-sensitive ATPase in microdissected tubule segments. ^,

This suggests that halted urine flow could be a primary signal for obstruction-induced Na ⁺ transport downregulation. Testing this hypothesis, inhibiting apical Na ⁺ entry for 24 hours in A6 cells (mimicking cortical collecting duct cells) led to reduced apical sodium entry even after blockade removal. This downregulation coincides with selective β-subunit decrease in ENaC expression on apical membranes of the A6 cells. In whole cells, α- and γ-subunits remain unaffected. In rats with obstruction, α-, β-, and γ-subunits of ENaC are downregulated, suggesting all three subunits contribute to impaired sodium reabsorption. In contrast to results in cell suspensions or whole kidney, ^{, , ,} inhibition of apical sodium entry had no effect on expression of either subunit of Na ⁺ , K ⁺ -ATPase. This directly proves that reduced Na ⁺ entry rate (likely during blocked urine flow) directly downregulates renal epithelial cell Na ⁺ transport.

In addition to the direct effects of halting urine flow, changes in intrarenal mediators and subcellular pathways likely play a critical role in the reduction of salt transport observed with obstruction. Obstruction markedly accelerates the already rapid generation of PGE2 in the renal medulla. ^{, , ,} The molecular basis for this is a dramatic medullary cyclooxygenase-2 (COX-2) induction. ^, Consistent with the known effect of PGE2 to markedly inhibit Na ⁺ reabsorption in the MTAL, as well as in the cortical and inner medullary collecting ducts, COX-2 inhibition in rats with obstruction and release of obstruction attenuated the downregulation of NHE2, NKCC2, and Na ⁺ , K ⁺ -ATPase. ^, From these results, obstruction likely reduces apically localized sodium cotransport proteins in the tubule epithelium and sodium pump activity in tubular epithelia in part by increasing renal levels of PGE2.

As discussed earlier, obstruction leads to a monocellular infiltrate in the kidney, mainly around the peritubular distribution. When obstructed kidneys were exposed to radiation, medullary inflammation diminished and there was a modest reduction in fractional excretion of sodium. In addition, obstruction increases renal angiotensin II generation, which might impact the regulation of renal sodium handling. Inhibiting the AT1 significantly lessened the downregulation of NHE3 and NKCC2, leading to a decrease in renal sodium loss.

In summary, obstruction reduces net reabsorption of sodium in several nephron segments including the proximal straight tubule, MTAL, and cortical and inner medullary collecting ducts by downregulating the expression and activities of specific transporter proteins. Several signals mediate this downregulation, such as the cessation of urine flow with its attendant reduction of the rate of Na ⁺ entry across the apical membrane, increased levels of natriuretic substances such as PGE2, and infiltration of the obstructed kidney by mononuclear cells.

When both ureters are obstructed, extrarenal factors markedly enhance the sodium-wasting tendency already present in the obstructed kidney. One mechanism involves the volume expansion that occurs when bilateral obstruction ablates all renal function. Volume expansion impairs activity in the sympathetic nervous system, reduces circulating levels of aldosterone along with reduced renal clearance, and increases levels of atrial natriuretic peptide. Reduced sympathetic tone and aldosterone, coupled with increased atrial natriuretic peptide, markedly stimulate sodium excretion. Atrial natriuretic peptide likely represents a particularly important mediator of salt wasting in bilateral obstruction. Levels of atrial natriuretic peptide are markedly elevated in bilateral but not unilateral obstruction. Atrial natriuretic peptide enhances salt wasting at several nephron segments. By blocking renin release in the macula densa and angiotensin action in the proximal tubule, atrial natriuretic peptide reduces proximal tubule sodium reabsorption. ^{, ,} Atrial natriuretic peptide also reduces aldosterone release and directly inhibits sodium reabsorption in the collecting ducts. ^{, ,} In agreement with this mechanism, infusion of atrial natriuretic peptide into animals in which obstruction has just been released leads to marked increases in sodium and water excretion. Moreover, efforts to reduce circulating atrial natriuretic peptide levels following bilateral obstruction attenuated sodium excretion somewhat.

Because obstruction reduces the ability of the renal tubules to concentrate and dilute the urine, urine osmolality following release of obstruction in humans and experimental animals approaches that of plasma. ^{, ,} Dilution of the urine requires that the thick ascending limb reabsorbs sodium without water and that the collecting duct maintains the dilute urine by not reabsorbing water along its length, despite the presence of a concentrated medullary interstitium. Concentration of the urine requires active sodium reabsorption in the thick limb and the action of the countercurrent multiplier to generate a concentrated medullary interstitium, as well as the ability of the collecting duct to insert the vasopressin-regulated water channel aquaporin-2 (AQP2) into the apical membrane. ^,

Obstructive nephropathy disrupts several of these mechanisms. ^{, , ,} As noted earlier, obstruction also markedly reduces MTAL sodium reabsorption, limiting this segment’s ability to dilute the urine and generate a high medullary interstitial osmolality. Indeed, interstitial osmolality has been shown to be reduced in obstructed kidneys. In addition, collecting ducts isolated from obstructed kidneys reveal normal basal water permeabilities but a marked reduction in their ability to increase water permeability in response to antidiuretic hormone or other stimulants of cyclic adenosine monophosphate (cAMP) accumulation in the cells. As was the case with sodium transport, the effects were similar in unilateral and bilateral obstruction. ^, Detailed mechanistic studies show that obstruction markedly reduces transcription of mRNA encoding AQP2, as well as synthesis of AQP2 protein, and that collecting duct cells in obstructed kidneys do not traffic AQP2-containing vesicles effectively to the apical surface in response to vasopressin or increased cAMP. ^, Part of this failure in trafficking results from a decrease in phosphorylation of AQP2 in obstructed kidneys and likely also due to the fact that V2 receptor protein expression is downregulated. Redistribution of AQP2 and AQP2 phosphorylated at ser261 to more intracellular localizations after bilateral obstruction and colocalization with the early endosomal marker EEA1 and the lysosomal marker cathepsin D suggest that early downregulation of AQP2 could in part be caused by degradation of AQP2 through a lysosomal degradation pathway. In addition, UUO markedly decreases synthesis and deployment to the basolateral membrane of aquaporin-3 and aquaporin-4; when AQP2 is in the apical membrane, these aquaporins mediate the water flux across the basolateral membrane. Enhancing the causal relationship of the changes in aquaporin activity and ability to concentrate the urine, expression of AQP2 remains suppressed for 7 days following relief of the obstruction, and the rise in urinary concentration parallels the recovery in AQP2 expression. ^{, , ,}

The fact that collecting ducts from obstructed kidneys do not respond to cAMP indicates that the lesion also involves sites other than the receptor for antidiuretic hormone. Consistent with the hypothesis that PGE2-mediated inhibition of collecting duct water permeability does not directly affect cAMP levels but may have post-cAMP effects rather than actions via cAMP regulation, previous studies have shown that cyclooxygenase-2 (COX-2) inhibition prevented dysregulation of AQP2 in obstructed kidneys where COX-2 protein expression was markedly increased ( Fig. 39.8 ). ^, Consistent with the COX-2 induction observed in the kidney inner medulla in bilateral ureteral obstruction (BUO), COX-2 mRNA and protein levels are also increased in response to UUO. ^, Reactive oxygen species are shown to trigger this increase in rats with 3-day UUO, but antioxidants like nicotinamide adenine dinucleotide phosphate oxidase inhibitor and rotenone can counteract this effect. Treating with superoxide dismutase 2 (SOD2)-mimic manganese (III) tetrakis ([4-benzoic acid] porphyrin chloride [MnTBAP]) reduces AQP2 downregulation after 7 days of UUO and also suppresses COX-2 induction. This finding suggests that mitochondrial oxidative stress partly mediates AQP2 downregulation in obstructive nephropathy through the COX-2 pathway.

On the basis of these results, it can be concluded that the defect in urinary dilution in obstruction is due to reduced ability of the thick ascending limb to dilute the urine by transporting salt from the lumen of the tubule to its basolateral side. The collecting duct in obstructed kidneys maintains its low water permeability in the absence of antidiuretic hormone, so the failure to dilute the urine is not due to collapse of osmotic gradients in the collecting duct. The inability to concentrate the urine results from the failure of the thick limb to generate a concentrated interstitium, as well as the inability of the collecting duct to synthesize and traffic AQP2 and other water channels in response to antidiuretic hormone.

Obstruction dramatically reduces urinary acidification capacity in experimental animals and humans. In humans, release of obstruction does not lead to bicarbonate wasting, indicating that proximal tubular bicarbonate reclamation is maintained. By contrast, in both experimental animals and patients following release of obstruction, the urine pH does not decrease in response to an acid load, indicating that obstruction impairs the ability of the distal nephron to acidify the urine. This defect likely involves proton transport proteins in the collecting duct ^, and proximal tubule and thick ascending limb of Henle. ^,

Reduced collecting duct acid secretion could result from defects in H ⁺ (H ⁺ -ATPase or H ⁺ /K ⁺ -ATPase) or HCO ₃ ^– (e.g., Cl ^– /HCO ₃ ^– exchange) transport pathways, back leak of protons down their electrochemical gradient from the lumen to the basolateral side of the tubule, or, in the cortical collecting duct, the failure to generate a sufficiently lumen-negative transepithelial voltage. ^{, ,} As described in detail earlier, obstruction reduces the activity of apical ENaC in the cortical collecting duct; the resulting loss of luminal negativity may attenuate acid secretion in these segments. ^,

In the rat inner medullary collecting duct (studied by micropuncture) and in isolated perfused rat and rabbit outer medullary collecting duct, obstruction markedly reduces luminal acidification rates. Because Na ⁺ transport does not play a major role in acidification in these segments, the defect must be due to direct inhibition of acid or HCO ₃ ^– transport pathways, or back leak of protons from lumen to interstitium.

Antibody-based targeted studies examining the Cl ^– /HCO ₃ ^– exchanger and subunits of the H ⁺ -ATPase revealed reduced expression of these transporters in collecting ducts of unilaterally obstructed compared with contralateral, and control kidneys. ^, Two possible mechanisms of reduced acid secretion were explored. One was that the intercalated cells in obstructed kidneys would exhibit a high proportion of “reverse” orientation, with the proton pump in the basolateral membrane and the Cl ^– /HCO ₃ ^– exchanger in the apical membrane. The other possibility was that the orientation of intercalated cells would not change, but there would be reduced expression of the H ⁺ or HCO ₃ ^– transporter. The orientation of the intercalated cells was not altered by obstruction. However, obstruction did reduce the appearance of H ⁺ -ATPase along the apical membranes of intercalated cells, without altering the total content of H ⁺ -ATPase in extracts of renal cortex or medulla, in unilaterally obstructed as compared with contralateral kidneys.

In addition to defective collecting duct, H ⁺ transport–reduced generation of the main buffer that carries acid equivalents in the urine, ammonia has also been observed in kidneys released from obstruction. Cortical slices of obstructed kidneys exhibit reduced glutamine uptake and oxidation, reduced gluconeogenesis, and reduced total oxygen consumption, resulting in a reduced ability to generate ammonia from glutamine. ^,