The role of the medullary collecting ducts in postobstructive diuresis

Presentation Variables Associated With the Development of Severe Post-obstructive Diuresis in Male Cats Following Relief of Urethral Obstruction

Background

Diuresis following relief of urethral obstruction is a potentially life-threatening complication of feline urethral obstruction. Evidence regarding the incidence of post-obstructive diuresis (POD) in cats is scarce. Establishing historical, physical examination, and initial clinicopathologic variables associated with risk for developing POD may better enable clinicians to direct treatment for this common feline emergency and to educate clients regarding financial expectations.

Objectives

To report the incidence of POD in a large group of cats with urethral obstruction and determine whether select presenting physical examination or initial clinicopathologic variables may predict the onset or severity of POD.

Methods

The records of 260 cats that were admitted to the University of California, Davis, Veterinary Medical Teaching Hospital for urethral obstruction were reviewed. Urine output after urethral catherization was categorized into no POD (urine output ≤ 2 mL/kg/h), mild-moderate POD (urine output > 2 but <5 mL/kg/h) and severe POD (urine output ≥ 5 mL/kg/h). Select presentation physical examination, venous acid-base, electrolyte, serum biochemistry, and urinalysis results were compared among the groups.

Results

67.7% of cats experienced POD, and in 35% of cats it was categorized as severe. Evaluated historical and physical examination variables correlated with development of POD were lower body weight and, for severe POD, hypovolemia. Clinicopathologic variables associated with development of POD included acidemia, azotemia, hyperphosphatemia, hyperkalemia, hyponatremia, hypochloremia, hypocalcemia, hypermagnesemia, and hypoalbuminemia. Cats with severe POD were hospitalized a median of 1 day longer than those without POD.

Conclusions

Results of the present study indicate that there are presentation variables associated with onset and severity of POD following relief of feline urethral obstruction.

Postobstructive diuresis: pay close attention to urinary retention

OBJECTIVE

To educate primary health care professionals about the diagnosis and treatment of postobstructive diuresis (POD), a rare but potentially lethal complication associated with the relief of urinary obstructions.

SOURCES OF INFORMATION

The main concepts and clinical evidence reviewed in this article were derived from a literature search of PubMed and Google Scholar. Expert opinion was used to supplement recommendations in areas with little evidence.

MAIN MESSAGE

Urinary retention is a frequently encountered presentation seen by all physicians. Most family physicians are comfortable treating these patients, initiating investigations, and organizing appropriate follow-up. This article reviews a rare but potentially lethal complication known as POD. Postobstructive diuresis is a polyuric response initiated by the kidneys after the relief of a substantial bladder outlet obstruction. In severe cases this condition can become pathologic, resulting in dehydration, electrolyte imbalances, and death if not adequately treated. Primary care physicians should be familiar with this potential clinical entity, especially as they are generally the first to encounter and treat these patients.

CONCLUSION

Physicians aware of POD will be able to identify patients at risk and arrange the appropriate monitoring after relieving a urinary obstruction. Early diagnosis and treatment of pathologic POD will prevent mortality.

Potential involvement of P2Y2 receptor in diuresis of postobstructive uropathy in rats

AVP resistance of the medullary collecting duct (mCD) in postobstructive uropathy (POU) has been attributed to increased production of PGE2. P2Y2 receptor activation causes production of PGE2 by the mCD. We hypothesize that increased P2Y2 receptor expression and/or activity may contribute to the diuresis of POU. Sprague-Dawley rats were subjected to bilateral ureteral obstruction for 24 h followed by release (BUO/R, n = 17) or sham operation (SHM/O, n = 15) and euthanized after 1 wk or 12 days. BUO/R rats developed significant polydipsia, polyuria, urinary concentration defect, and increased urinary PGE2 and decreased aquaporin-2 protein abundance in the inner medulla compared with SHM/O rats. After BUO/R, the relative mRNA expression of P2Y2 and P2Y6 receptors was increased by 2.7- and 4.9-fold, respectively, without significant changes in mRNA expression of P2Y1 or P2Y4 receptor. This was associated with a significant 3.5-fold higher protein abundance of the P2Y2 receptor in BUO/R than SHM/O rats. When freshly isolated mCD fractions were challenged with different types of nucleotides (ATPgammaS, ADP, UTP, or UDP), BUO/R and SHM/O rats responded to only ATPgammaS and UTP and released PGE2, consistent with involvement of the P2Y2, but not P2Y6, receptor. ATPgammaS- or UTP-stimulated increases in PGE2 were much higher in BUO/R (3.20- and 2.28-fold, respectively, vs. vehicle controls) than SHM/O (1.68- and 1.30-fold, respectively, vs. vehicle controls) rats. In addition, there were significant 2.4- and 2.1-fold increases in relative mRNA expression of prostanoid EP1 and EP3 receptors, respectively, in the inner medulla of BUO/R vs. SHM/O rats. Taken together, these data suggest that increased production of PGE2 by the mCD in POU may be due to increased expression and activity of the P2Y2 receptor. Increased mRNA expression of EP1 and EP3 receptors in POU may also help accentuate PGE2-induced signaling in the mCD.

Angiotensin II regulates V2 receptor and pAQP2 during ureteral obstruction

Release of bilateral ureteral obstruction (BUO) is associated with nephrogenic diabetes insipidus (NDI) and a reduced abundance of the vasopressin-regulated aquaporins. To evaluate the role of the vasopressin type 2 receptor (V2R), we determined V2R abundance in kidneys from rats subjected to 24-h BUO or 24-h unilateral ureteral obstruction (UUO) followed by 48-h release. Because angiotensin II type 1 (AT1) receptor blockade attenuates postobstructive polyuria and aquaporin-2 (AQP2) downregulation, we examined the effect of AT1 receptor blockade on AQP2 phosphorylated at serine 256 (pS256-AQP2) and V2 receptor complex abundance in kidney inner medulla (IM). Furthermore, cAMP generation in sodium fluoride- and forskolin-stimulated inner medullary membrane fractions was studied after release of BUO. V2R was significantly reduced to 12% of sham levels in IM and to 52% of sham levels in cortex and outer stripe of outer medulla (OSOM) from BUO rats. In UUO rats, V2R abundance in the obstructed kidney IM decreased to 35% of sham levels, whereas it was comparable to sham levels in the nonobstructed kidney IM. No significant change was observed in cortex and OSOM. AT1 receptor blockade attenuated V2R, pS256-AQP2, and G(s)alpha protein downregulation in IM and partially reversed the obstruction-induced inhibition of sodium fluoride- and forskolin-stimulated cAMP generation in inner medullary membrane fractions from BUO rats. In conclusion, V2R downregulation plays a pivotal role in development of NDI after release of BUO. In addition, we have shown that angiotensin II regulates the V2 receptor complex and pS256-AQP2 in postobstructive kidney IM, probably by stimulating cAMP generation.

COX-2 activity transiently contributes to increased water and NaCl excretion in the polyuric phase after release of ureteral obstruction

Release of bilateral ureteral obstruction (BUO) is associated with reduced expression of renal aquaporins (AQPs), polyuria, and impairment of urine-concentrating capacity. Recently, we demonstrated that 24 h of BUO is associated with increased cyclooxygenase (COX)-2 expression in the inner medulla (IM) and that selective COX-2 inhibition prevents downregulation of AQP2. In the present study, we tested the hypothesis that COX-2 activity increases in the postobstructive phase and that this increase in COX-2 activity contributes to polyuria and impaired urine-concentrating capacity. We examined the effect of the selective COX-2 inhibitor parecoxib (5 mg.kg(-1).day(-1) via osmotic minipumps) on renal functions and protein abundance of AQP2, AQP3, Na-K-2Cl cotransporter type 2 (NKCC2), and Na-K-ATPase 3 days after release of BUO. At 3 days after release of BUO, rats exhibited polyuria, dehydration and urine and IM tissue osmolality were decreased. There were inverse changes of COX-1 and COX-2 in the IM: COX-2 mRNA, protein, and activity increased, while COX-1 mRNA and protein decreased. Parecoxib reduced urine output 1 day after release of BUO, but sodium excretion and glomerular filtration rate were unchanged. Parecoxib normalized urinary PGE(2) and PGI(2) excretion and attenuated downregulation of AQP2 and AQP3, while phosphorylated AQP2 and NKCC2 remained suppressed. Parecoxib did not improve urine-concentrating capacity in response to 24 h of water deprivation. We conclude that decreased NKCC2 and collapse of the IM osmotic gradient, together with suppressed phosphorylated AQP2, are likely causes for the impaired urine-concentrating capacity and that COX-2 activity is not likely to mediate these changes in the chronic postobstructive phase after ureteral obstruction.

alpha-MSH prevents impairment in renal function and dysregulation of AQPs and Na-K-ATPase in rats with bilateral ureteral obstruction

The purpose of this study was to evaluate the effects of the anti-inflammatory hormone alpha-melanocyte-stimulating hormone (alpha-MSH) treatment on renal function and expression of aquaporins (AQPs) and Na-K-ATPase in the kidney in response to 24 h of bilateral ureteral obstruction (BUO) or release of BUO (BUO-R). In rats with 24-h BUO, immunoblotting revealed that downregulation of AQP2 and AQP3 was attenuated (AQP2: 38 +/- 5 vs. 13 +/- 4%; AQP3: 44 +/- 3 vs. 19 +/- 4% of sham levels; P < 0.05), whereas downregulation of Na-K-ATPase was prevented by alpha-MSH treatment (Na-K-ATPase: 94 +/- 7 vs. 35 +/- 5% of sham levels; P < 0.05). Immunocytochemistry confirmed the changes in AQP1 and Na-K-ATPase expression. Renal tubular cell apoptosis was confirmed in BUO kidneys, and alpha-MSH treatment virtually completely abolished apoptosis. Furthermore, we measured glomerular filtration rate (GFR) and effective renal plasma flow (ERPF), respectively. Forty-eight hours after BUO-R demonstrated that alpha-MSH treatment almost completely prevented the decrease in GFR (nontreated: 271 +/- 50; alpha-MSH: 706 +/- 85; sham: 841 +/- 105 microl x min(-1).100 g body wt(-1), P < 0.05) and ERPF (nontreated: 1,139 +/- 217; alpha-MSH: 2,598 +/- 129; sham: 2,633 +/- 457 microl x min(-1).100 g body wt(-1), P < 0.05). alpha-MSH treatment also partly prevented the downregulation of AQP1 and Na-K-ATPase expression in rats after BUO-R for 48 h. In conclusion, alpha-MSH treatment significantly prevents impairment in renal function and also prevents downregulation of AQP2, AQP3, and Na-K-ATPase during BUO or AQP1 and Na-K-ATPase after BUO-R, demonstrating a marked renoprotective effect of alpha-MSH treatment in conditions with urinary tract obstruction.

Diminished expression of aquaporin water channels in ureteral-obstructed kidney in rats

OBJECTIVE

We investigated whether regulation of aquaporin (AQP) water channels is altered in the ureteral-obstructed kidney.

MATERIAL AND METHODS

Male Sprague-Dawley rats were unilaterally obstructed of their left proximal ureters for 48 h. The protein expression of AQP1-3 channels was determined in the kidney by Western blot analysis. The expression of AQP2 was also determined by immunohistochemistry. In order to specifically determine primary impairment of the pathway leading to an altered regulation of AQP channels stimulated by the arginine vasopressin (AVP)/cyclic adenosine monophosphate (cAMP) pathway, the catalytic activity and protein expression of different parts of the adenylyl cyclase complex were separately determined.

RESULTS

In the previously obstructed kidney, urinary osmolality and free water reabsorption were greatly decreased. The expression of AQP2 proteins was decreased in the cortex, outer medulla and inner medulla. Immunohistochemistry also showed a marked decrease in AQP2 expression. The expression of AQP1 and AQP3 was decreased in the outer medulla and inner medulla. cAMP generation in response to AVP, sodium fluoride or forskolin was greatly decreased. The expression of Gsalpha and adenylyl cyclase VI proteins was decreased. The contralateral kidney showed minimal or no changes in these parameters.

CONCLUSIONS

The reduced abundance of AQP water channels may at least partly account for the urinary concentration defect in the ureteral-obstructed kidney. The primary point of impairment of AQP channels regulated by the AVP/cAMP pathway may lie at the level of the catalytic unit of adenylyl cyclase.

Altered expression of major renal Na transporters in rats with bilateral ureteral obstruction and release of obstruction

Urinary tract obstruction impairs urinary concentrating capacity and reabsorption of sodium. To clarify the molecular mechanisms of these defects, expression levels of renal sodium transporters were examined in rats with 24-h bilateral ureteral obstruction (BUO) or at day 3 or 14 after release of BUO (BUO-R). BUO resulted in downregulation of type 3 Na+/H+ exchanger (NHE3) to 41 +/- 14%, type 2 Na-Pi cotransporter (NaPi-2) to 26 +/- 6%, Na-K-ATPase to 67 +/- 8%, type 1 bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1) to 20 +/- 7%, and thiazide-sensitive cotransporter (TSC) to 37 +/- 9%. Immunocytochemistry confirmed downregulation of NHE3, NaPi-2, Na-K-ATPase, BSC-1, and TSC. Consistent with this downregulation, BUO-R was associated with polyuria, reduced urinary osmolality, and increased urinary sodium and phosphate excretion. BUO-R for 3 days caused a persistant downregulation of NHE3 to 53 +/- 10%, NaPi-2 to 57 +/- 9%, Na-K-ATPase to 62 +/- 8%, BSC-1 to 50 +/- 12%, and TSC to 56 +/- 16%, which was associated with a marked reduction in the net renal reabsorption of sodium (616 +/- 54 vs. 944 +/- 24 micromol x min-1 x kg-1; P < 0.05) and phosphate (6.3 +/- 0.9 vs. 13.1 +/- 0.4 micromol x min-1. kg-1; P < 0.05) demonstrating a defect in renal sodium and phosphate reabsorption capacity. Moreover, downregulation of Na-K-ATPase and TSC persisted in BUO-R for 14 days, whereas NHE3, NaPi-2, and BSC-1 were normalized to control levels. In conclusion, downregulation of renal Na transporters in rats with BUO and release of BUO are likely to contribute to the associated urinary concentrating defect, increased urinary sodium excretion, and postobstructive polyuria.

Altered expression of immune modulator and structural genes in neonatal unilateral ureteral obstruction

BACKGROUND

Congenital obstructive nephropathy is a condition characterized by hydronephrosis, tubular dilatation, apoptosis, and atrophy, as well as interstitial cellular infiltration and progressive interstitial fibrosis. The renal consequences of chronic unilateral ureteral obstruction (UUO) in the neonatal rat are similar to those of clinical congenital obstructive nephropathy.

METHODS

To define alterations in renal gene expression induced by chronic neonatal UUO, Sprague-Dawley rats were subjected to UUO or sham operation within the first 2 days of life, and kidneys were harvested after 12 days.

RESULTS

Microarray analysis revealed that the mRNA expression of multiple immune modulators, including krox24, interferon-gamma regulating factor-1 (IRF-1), monocyte chemoattractant protein-1 (MCP-1), interleukin-1beta (IL-1beta), CCAAT/enhancer binding protein (C/EBP), p21, c-fos, c-jun, and pJunB, was significantly increased in obstructed compared to sham-operated kidneys (all P < 0.05). Western blot analysis revealed significant changes in immune modulator protein abundance in the obstructed versus sham-operated kidney for krox24 (P = 0.0004), IRF-1 (P = 0.005), MCP-1 (P = 0.01), and JunD (P = 0.0008). Alternatively, the abundance of all of the immune modulator proteins was similar in sham-operated and obstructed kidneys in rats subjected to acute (4 days) neonatal UUO. Microarray analysis studies also reveal that structural genes that comprise the cytoskeleton and cell matrix are significantly up-regulated by chronic neonatal UUO, including calponin, desmin, dynamin, and lumican (all P < 0.05).

CONCLUSION

Multiple genes are aberrantly expressed in the kidney of rats subjected to chronic neonatal UUO. Elucidation of these genes involved in neonatal UUO may lead to new insight about congenital obstructive nephropathy.

Altered expression of major renal Na transporters in rats with unilateral ureteral obstruction

It has been demonstrated previously that ureteral obstruction was associated with downregulation of renal AQP2 expression and an impaired urinary concentrating capacity (Li C, Wang W, Kwon TH, Isikay L, Wen JG, Marples D, Djurhuus JC, Stockwell A, Knepper MA, Nielsen S, and Frøkiaer J. Am J Physiol Renal Physiol 281: F163-F171, 2001). In the present study, changes in the expression of major renal Na transporters were examined in a rat model with 24 h of unilateral ureteral obstruction (UUO) to clarify the molecular mechanisms of the marked natriuresis seen after release of UUO. Urine collection for 2 h after release of UUO revealed a significant reduction in urinary osmolality, solute-free water reabsorption, and a marked natriuresis (0.29 +/- 0.03 vs. 0.17 +/- 0.03 micromol/min, P < 0.05). Consistent with this, immunoblotting revealed significant reductions in the abundance of major renal Na transporters: type 3 Na(+)/H(+) exchanger (NHE3; 24 +/- 4% of sham-operated control levels), type 2 Na-P(i) cotransporter (NaPi-2; 21 +/- 4%), Na-K-ATPase (37 +/- 4%), type 1 bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1; 15 +/- 3%), and thiazide-sensitive Na-Cl cotransporter (TSC; 15 +/- 4%). Immunocytochemistry confirmed the downregulation of NHE3, BSC-1, and TSC in response to obstruction. In nonobstructed contralateral kidneys, a significant reduction in the abundance of inner medullary Na-K-ATPase and cortical NaPi-2 was found. This may contribute to the compensatory increase in urinary production (23 +/- 2 vs. 13 +/- 1 microl x min(-1). kg(-1)) and increased fractional excretion of urinary Na (0.62 +/- 0.03 vs. 0.44 +/- 0.03%, P < 0.05). In conclusion, downregulation of major renal Na transporters in rats with UUO may contribute to the impairment in urinary concentrating capacity and natriuresis after release of obstruction, and reduced levels of Na-K-ATPase and NaPi-2 in the contralateral nonobstructed kidney may contribute to the compensatory increase in water and Na excretion from that kidney during UUO and after release of obstruction.

Downregulation of AQP1, -2, and -3 after ureteral obstruction is associated with a long-term urine-concentrating defect

Previously, we demonstrated that 24 h of bilateral ureteral obstruction (BUO) and short-term release of BUO was associated with a decrease in the expression of aquaporin-2 (AQP2), polyuria, and a reduced urinary concentrating capacity (10). The purposes of the present study were to examine whether BUO and the long-term release of BUO (BUO-R) for 3, 14, and 30 days were associated with changes in the expression of renal AQP1, AQP2, and AQP3 and whether such changes were associated with parallel changes in urinary output and urinary concentrating capacity. Rats (n = 4-7 in each group) were kept in metabolic cages for measurements of urinary output. Kidneys were removed to determine the expression levels of AQP1, AQP2, and AQP3 by semiquantitative immunoblotting. AQP2 was downregulated after 24 h of BUO (42 +/- 3%). Downregulation of AQP2 persisted 3 (43 +/- 14%; P < 0.01) and 15 days after BUO-R (48 +/- 11%; P < 0.01) but was normalized 30 days after BUO-R. AQP3 showed a similar pattern. Moreover, AQP1 was downregulated in response to BUO (65 +/- 7%) and remained downregulated 3 days after BUO-R (41 +/- 5%), 14 days after BUO-R (57 +/- 8%), and 30 days after BUO-R (59 +/- 5%). BUO-R resulted in a significant polyuria that gradually decreased, although it remained significant at day 30. Urinary concentrating capacity remained significantly impaired when determined 3, 14, and 30 days after BUO-R in response to a 24-h period of thirst (1,712 +/- 270 vs. 2,880 +/- 91 mosmol/kgH2O at day 30, P < 0.05). In conclusion, the expression of AQP1, AQP2, and AQP3 were long-term downregulated after BUO-R, suggesting that dysregulation of aquaporins located at the proximal tubule, thin descending limb of the loop of Henle, and the collecting duct may contribute to the long-term polyuria and impairment of urinary concentrating capacity associated with obstructive nephropathy.

Increased expression of atrial natriuretic peptide in ureteral obstructed kidneys in rats

OBJECTIVES

Whether the natriuresis in the previously ureteral obstructed kidney may be related to an altered regulation of local atrial natriuretic peptide (ANP) was investigated.

MATERIALS AND METHODS

Male Sprague-Dawley rats were unilaterally obstructed of the left ureters for 48 hours. The left and right ureters were separately cannulated to collect urine samples. In some rats, both kidneys were removed without releasing the obstruction. Control rats were treated the same except that no ureteral obstruction was made. The mRNA expression of ANP and natriuretic peptide receptor (NPR)-A was determined in the kidney by reverse transcription-polymerase chain reaction. The guanylyl cyclase activity was measured by the amount of cGMP generated in response to ANP. Plasma levels of ANP were measured by radioimmunoassay.

RESULTS

The ureteral obstruction did not significantly affect the plasma ANP levels. In the obstructed kidney, the creatinine clearance was decreased, while the fractional excretion of sodium increased. The expression of ANP mRNA was increased in the obstructed kidney. The NPR-A mRNA expression was not altered in the glomerulus, but was decreased in the papilla of the obstructed kidney. Nor was the guanylyl cyclase activity in the glomerulus altered. Although the guanylyl cyclase activity in the papilla was significantly decreased in the obstructed kidney, it was rapidly resumed upon releasing the obstruction.

CONCLUSIONS

An increased local expression of ANP may, at least in part, account for the natriuresis in the previously ureteral obstructed kidney.

Transport defects of rabbit medullary thick ascending limb cells in obstructive nephropathy

To characterize the sodium transport defect responsible for salt wasting in obstructive nephropathy, the major sodium transporters in the medullary thick ascending limb (mTAL), the apical Na-K-2Cl cotransporter and the basolateral Na-K-ATPase, were studied in fresh suspensions of mTAL cells and outer medulla plasma membranes prepared from obstructed and untreated kidneys. Oxygen consumption (QO2) studies in intact cells revealed marked reductions in the inhibitory effects of both furosemide and ouabain on QO2 in cells from obstructed, as compared with control animals, indicating a reduction in activities of both the Na-K-2Cl cotransporter and the Na-K-ATPase. Saturable [3H]bumetanide binding was reduced in membranes isolated from obstructed kidneys, but the Kd for [3H]bumetanide was unchanged, indicating a decrease in the number of functional luminal Na-K-2Cl cotransporters in obstructed mTAL. Ouabain sensitive Na-K-ATPase activity in plasma membranes was also reduced, and immunoblots using specific monoclonal antibodies directed against the alpha and beta subunits of rabbit Na-K-ATPase showed decreased amounts of both subunits in outer medullas of obstructed kidney. A significant decrease in [3H]bumetanide binding was detected after 4 h of ureteral obstruction, whereas Na-K-ATPase activity at this time was still not different from control. We conclude that ureteral obstruction reduces the amounts of both luminal Na-K-2Cl cotransporter and basolateral Na-K-ATPase in mTAL of obstructed kidney and that these reductions contribute to the salt wasting observed after release of obstruction.

Nephrogenic diabetes insipidus. An unusual presentation of recurrent rectal cancer

The occurrence of diabetes insipidus (DI) in patients with systemic cancer is caused usually by tumor metastasis to the hypothalamus or posterior pituitary. A 43-year-old man with DI 8 months after radical surgery for a poorly differentiated adenocarcinoma of the distal rectum is reported. A therapeutic trial of intranasal desmopressin acetate did not correct the hyposthenuria, thus localizing the defect to the kidneys. A large tumor recurrence in the pelvis caused bilateral hydroureteronephrosis, resulting in nephrogenic DI. This report shows that not all cases of DI in cancer patients are of central origin.

Effects of amiloride in the medullary collecting duct of rat kidney

The in vivo microcatheterization technique was used to study amiloride-induced transport alterations in the inner medullary collecting duct. Amiloride treated rats (0.1 mg/hr) had significant diuresis and natriuresis, as well as antikaliuresis, compared to untreated controls. The relative decrease in potassium excretion was associated with a significant rise in plasma potassium concentration. Net sodium transport in the duct was decreased from 83 + 3 to 46 + 6 per cent of delivered load, as a result of amiloride treatment. Smaller, but statistically significant, reductions (P less than 0.01) were seen for fluid and chloride reabsorptions (from 66 + 3 to 51 + 4%, and from 72 + 4 to 52 + 5%, respectively). Potassium reabsorption increased from 15 + 8 to 61 + 6% of delivered load. The data indicated that amiloride natriuresis is determined primarily by inhibition of sodium reabsorption in the medullary collecting duct, probably due to blockade of a specific Na channel. The antikaliuresis, on the other hand, appears to be due to inhibition of secretion both in upstream nephron segments and in the duct itself.

Acidification in the medullary collecting duct following ureteral obstruction

A defect in urine acidification has been described in obstructive uropathy. Since the collecting tubule from the inner stripe of the outer medulla (OMCTi) is the major site for distal acidification, isolated OMCTi nephron segments from control rabbits and from rabbits after 24 hr of unilateral (UUO) or bilateral (BUO) ureteral obstruction were studied. Tubules were perfused (4 nliter/min) and bathed with an artificial solution resembling rabbit serum ultrafiltrate, and 3H inulin was incorporated in the perfusate as a volume marker. Water absorption (Jv) was -0.03 +/- 0.03 nliter X mm-1 X min-1 in control tubules, and was significantly (P less than 0.05) negative in UUO (-0.48 +/- 0.12 nliter X mm-1 X min-1) and BUO (-0.29 +/- 0.07 nliter/mm-1 X min-1) tubules, as a result of an inulin leak. Bicarbonate absorption (JHCO3) in control tubules was 11.61 +/- 1.21 pmole X mm-1 X min-1 and was significantly lower in UUO tubules (7.59 +/- 1.09 pmole X mm-1 X min-1, P less than 0.05). JHCO3 in BUO tubules although lower than control (7.96 +/- 2.75 pmole X mm-1 X min-1) did not achieve statistical significance because of a high degree of heterogeneity among tubules. To determine whether the acidification disorder was due to a gradient or capacity defect, the ability of the tubules to lower HCO3- concentration (delta HCO3) at low rate of perfusion (1 nliter/min) was examined. No difference in delta HCO3 was found among the three groups being 8.98 +/- 0.54, 9.95 +/- 1.76, and 8.93 +/- 2.19 mmole in control, UUO and BUO tubules respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term effects of 24-hr unilateral ureteral obstruction on renal function in the rat

To characterize the pattern of recovery following release of unilateral ureteral obstruction of 24-hr duration, rats were studied with whole kidney clearance techniques, 3 hrs, 8, 14, and 60 days after release. The single nephron glomerular filtration rate (SNGFR) of superficial and juxtamedullary nephrons was estimated with a modification of Hanssen's technique in rats studied 8 and 60 days after ureteral release. The whole kidney glomerular filtration rates (GFR) were decreased markedly 3 hrs after relief of obstruction, but gradually increased and by 14 days, the GFR of the postobstructed kidney (POK) and the contralateral kidney (CK) were comparable. This recovery of GFR was not a consequence of a homogeneous improvement in SNGFR. At 8 days, more than 15% of superficial and juxtamedullary nephrons were not filtering in the POK. This decrease in the percent of filtering nephrons persisted to 60 days post release, indicating a permanent loss of nephron units. The SNGFR of the residual nephrons of the POK was significantly greater than that of the CK at 8 and 60 days following ureteral release. Thus, acute unilateral ureteral obstruction results in a permanent loss of filtering nephrons, which is offset by hyperfiltration of those remaining. Abnormalities in renal tubule function persisted beyond the time (14 days) when whole kidney GFR had returned to normal. These abnormalities were in distal tubule function. Urine osmolality was consistently lower at all time intervals post release, as was net acid excretion. The results of the present study suggest that these abnormalities are a consequence of the reduction in the number of filtering juxtamedullary nephrons and/or to abnormalities in collecting duct function.

Experimental obstructive hydronephrosis in newborn rats. V. Long-term effects on renal tissue solute content

Partial obstruction of 1 ureter was created in newborn rats and its effects were studied in the adult rat. The obstructed renal pelvis was found to be about 6 times enlarged and the weight of the kidney was 85 per cent of the contralateral intact one. Despite considerable distortion of the inner medulla on the obstructed side, no loss of weight in this region was observed. The only changes observed with respect to tissue concentrations--which were significantly due to the obstruction--were increases in urea in the cortex (110 per cent) and in potassium in the inner medulla (21 per cent); thus, the changes were few and, in part, moderate. The findings are compared with previous observations of solute excretions and the pathophysiological implications are discussed. The conclusion is that although the inner medulla was considerably distorted, the solute content was far from being affected to a corresponding degree--at least not in this experimental preparation.

Tubuloglomerular feedback and interstitial pressure in obstructive nephropathy

The possible role of the tubuloglomerular feedback (TGF) mechanism in the altered glomerular hemodynamics and tubular reabsorption which occur with prolonged (24-hr) ureteral obstruction and the changes in renal interstitial hydrostatic and oncotic pressure which may modulate TGF sensitivity were examined. The proximal tubule stop-flow pressure (PSF) response to increased distal tubular flow rates (TGF activity) was determined in rats with sham operation, 24-hr unilateral ureteral obstruction (UUO), or 24-hr bilateral ureteral obstruction (BUO), both before and for 2 hr after relief of obstruction. Subcapsular hydrostatic pressure, lymph flow and oncotic pressure, clearance and excretory data were measured in the second series of animals. During and after release of UUO, TGF sensitivity was increased, as indicated by the marked decrease in the loop perfusion rate at which 50% of the maximum decrease in PSF occurred (the turning point of TGF activation). Interstitial oncotic pressure but not hydrostatic pressure was significantly increased in UUO kidneys. In BUO rats, the turning point for TGF activation was slightly higher than the controls and the change in PSF with maximum loop perfusion rates was reduced, indicating a blunting of the TGF response before and particularly during postobstructive diuresis after release of BUO. Interstitial hydrostatic and oncotic pressures were both slightly increased resulting in no changes in net interstitial Starling forces. We conclude that enhanced TGF sensitivity after release of prolonged UUO, associated with increased interstitial oncotic pressure, may play a role in preventing postobstructive diuresis, while the blunting of TGF sensitivity after BUO may contribute to this phenomenon.

Thiazide diuretic effect on medullary collecting duct function in the rat

The distal convoluted tubule is thought to be the principal site of action of thiazide diuretics, but, to our knowledge, there are no studies of their possible effects on collecting duct transport. Microcatheterization of the inner medullary collecting duct (IMCD) was carried out in rats undergoing a modest diuresis, natriuresis, and chloriuresis from hydro-chlorothiazide (2 mg/kg/hr) and in normal controls. Delivery of fluid, sodium, and chloride to the beginning of the IMCD was increased, but not significantly, while the load remaining at the papillary tip (end) of the duct was increased markedly by hydrochlorothiazide. Chloride reabsorption in the IMCD was affected most markedly; the chloride reabsorption between the beginning and end of the duct, as a fraction of the delivered load, was reduced from 70.4 +/- 5.4% in controls to insignificant amounts with hydrochlorothiazide (8.2 +/- 11.5%, P less than 0.001). The fraction of delivered sodium reabsorbed along the collecting duct was decreased from 78.5 +/- 4.9% in controls to 37.2 +/- 12.4% (P less than 0.005) in thiazide-treated rats and fluid reabsorption was decreased from 59.4 +/- 4.0% in controls to 31.9 +/- 5.1% (P less than 0.005). Small but significant potassium secretion into the IMCD occurred with hydrochlorothiazide, probably secondary to the marked increase in potassium delivery to the duct. Increased potassium excretion could account for a maximum of 50% of chloriuresis with hydrochlorothiazide. The observation that thiazide diuretics decrease chloride, sodium, and fluid reabsorption in the medullary collecting duct, like the recently demonstrated inhibitory effect of furosemide on this nephron segment, has significant implications for the rationale for diuretic use.

Kidney pressures after the release of 24 hours of bilateral ureteral ligation in the rat

Kidney function was studied in rats after the release of 24 hours of bilateral ureteral ligation (BUL). After deligation, natriuresis was observed for 1 day while diuresis persisted up to 4 days. The possible defective collecting tubule to reabsorb sodium may improve within 1 day, since the urinary sodium concentration was well below the sham value. During the early phase of deligation, the glomerular filtration rate (GFR) averaged only 15 per cent of the sham value. Heterogeneity of the surface nephrons was apparent. About 17 per cent of the observed surface nephrons had no tubular fluid flow, suggesting possibly that these nephrons may not contribute to the kidney GFR. In the other 83 per cent of the nephrons, the stop-flow pressure, 29.0 mm. Hg, was lower than the sham value, 33.6 mm. Hg, whereas the proximal tubular pressure of 15.1 mm. Hg was significantly above the sham value of 13.2 mm. Hg. Both these factors may have induced a reduction in the effective filtration pressure, from 20.1 to 13.9 mm. Hg, and may have contributed to the reduction in GFR in those functioning nephrons. Four days later, a similar proportion of the surface nephrons still showed a marked reduction in function, whereas others had improved significantly. The proximal tubular pressure, the stop-flow and the effective filtration pressures approached the sham values. These improvements may be associated with an increase in GFR to 42 per cent of the sham value. It is suggested that the prolong suppression of the kidney GFR may not be due to the drastic change in the glomerular capillary pressure of the surface nephrons, at this latter period.

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.

Isolated nephron segments from rabbit models of obstructive nephropathy

Micropuncture and microcatheterization studies have been used extensively to investigate the pathophysiologic alterations in renal function induced by urinary tract obstruction. The present isolated tubule microperfusion studies were designed to examine the intrinsic alterations in segmental nephron function induced by 24 h of bilateral (BUO) and unilateral (UUO) urinary tract obstruction. Following UUO superficial proximal convoluted tubule reabsorption rate (J(v)) was not different from contralateral control (0.75+/-0.08 vs. 0.73+/-0.11 nl/mm per min, NS). Following UUO J(v) in juxtamedullary proximal convoluted tubules (JMPCT) was reduced 32% (0.69+/-0.06 vs. 0.47+/-0.04 nl/mm per min, P < 0.02). Following UUO J(v) in proximal straight tubules (PST) was reduced 52% (0.25+/-0.02 vs. 0.12+/-0.03, P < 0.01). Thick ascending limb (T-ALH) function was assessed by measurement of ability to lower perfusate chloride ion concentration (DeltaCl). Following UUO DeltaCl was reduced 76% (-39+/-9 vs. -9+/-1 meq/liter, P < 0.001). Cortical collecting tubule (CCT) function was assessed by measurement of antiduiretic hormone (ADH)-dependent osmotic water flow. Following UUO osmotic water flow was reduced 76% (0.90+/-0.08 vs. 0.22+/-0.04 nl/mm per min, P < 0.01) and this ADH resistance could not be overcome by cAMP. Nephron segments were then examined following relief of BUO. There were no differences in intrinsic function following relief of BUO when compared with UUO. We conclude that in UUO and BUO (a) the intrinsic tubular defects are identical, (b) the natriuresis noted is due, in part, to disordered JMPCT, PST, and T-ALH NaCl reabsorption, (c) the impaired concentrating ability is due, in part, to depressed function in T-ALH and ADH resistance of the CCT, and (d) the ADH resistance occurs at a site distal to the intracellular generation of cAMP.

Urea handling by the medullary collecting duct of the rat kidney during hydropenia and urea infusion

Previous micropuncture studies of distal tubule fluid and ureteral urine have indicated a varying degree of urea reabsorption in the collecting duct. In the present experiments the microcatheterization technique was used to directly determine urea, Na, K, total solute and fluid reabsorption along the length of the medullary collecting duct in anesthetized hydropenic rats and in rats given low dose urea infusion (Purea 18.9 mM/l). In hydropenic rats, the remaining fraction of filtered urea did not change significantly along the collecting duct, as indicated both by regression analysis of all samples and by comparison of paired samples from the corticomedullary junction and papillary tip. During low dose urea infusion, urine osmolality increased in proportion to the increase in urea concentration and again there was no net urea reabsorption between the beginning and end of the duct. However, during urea infusion, analysis of samples from the beginning, mid-zone, and end of the collecting duct indicated that urea entry occurred in the proximal portion of the duct (beginning to mid-zone, P less than 0.01) and that urea reabsorption occurred in the distal portion (mid-zone to end, P less than 0.01). The lack of significant net urea reabsorption along the duct despite the excretion of moderately concentrated urine, has despite the excretion of moderately concentrated urine, has implications for the concept of medullary urea recycling and for models of the urinary concentrating mechanism. The finding of functional heterogeneity with respect to urea handling in the collecting duct in vivo, with both reabsorption are secretion being demonstrated, raises the possibility that internal recycling of urea in the medullary collecting duct itself may contribute to maintenance of a high papillary interstitial urea concentration.

Medullary collecting duct function in the remnant kidney before and after volume expansion

The role of the medullary collecting ducts (CD) in the regulation of water and electrolyte excretion by the remnant kidney has not been determined. Medullary CD function was therefore studied by the microcatheterization technique during hydropenia and after volume expansion with an isotonic saline load in rats with sham-operated normal kidney (stage I), remnant kidney (stage II), remnant kidney after contralateral nephrectomy (stage III), and sham-operated normal kidney after contralateral nephrectomy (stage-III control). Sodium, potassium, water, and solute reabsorption along the medullary CD were not altered in stage II during hydropenia when compared to normal control (stage I), but water reabsorption proximal to the medullary CD was decreased. In stage III, where the uremia was mild (BUN, 41 mg/dl), the fractions of the filtered load of sodium (72%) and water (62%) which were reabsorbed along the medullary CD were reduced in comparison to stage II (94% and 83%, respectively), stage I, or stage-III control. The fraction of filtered potassium entering the medullary CD was increased to 53% in stage III, compared to 16% in stage II, and 10 or 11% in stage I or stage-III control. No change in the fraction of filtered potassium remaining along the collecting ducts was observed. After volume expansion, there was no significant change in the fraction of filtered sodium and water remaining along the medullary CD in stage I, II, or III. The greater fractional excretion in stage III resulted from decreased reabsorption in more proximal nephron segments. The results indicate that (a) during hydropenia, fractional reabsorption of sodium and water is decreased along the medullary CD of the stage-III remnant kidney in the presence of mild uremia, (b) the increased fractional excretion of potassium in the remnant kidney with uremia is not determined by altered potassium handling in the medullary CD but occurs proximal to this nephron segment, and (c) extracellular fluid volume expansion with isotonic saline results in similar inhibition of fractional sodium and water reabsorption in the collecting ducts of both remnant and normal kidneys.

Effects of acute bilateral ureteral obstruction on deep nephron and terminal collecting duct function in the young rat

The effects of acute bilateral ureteral obstruction (BUO) of 18-h duration on deep nephron and collecting duct function were studied by micropuncture in 11 weanling rats. After release of BUO glomerular filtration rate was reduced (178+/-15 vs. 1,343+/-119 mul/min per g kidney weight in shams), while urine flow was increased averaging 17.5+/-1.3 vs. 6.8+/-0.72 mul/min per g kidney weight in controls. There was a marked increase in the absolute and fractional excretion of Na. Single nephron glomerular filtration rate of deep nephrons was reduced in the BUO group, mean 19.4+/-3.5 vs. 77.0+/-7.7 nl/min per g kidney weight in shams. Single nephron glomerular filtration rate of superficial nephrons fell to the same extent after relief of BUO. Mean tubular fluid to plasma inulin ratio of fluid from Henle's loop was 2.46+/-0.20 after relief of BUO vs. 8.23+/-0.85 in shams. This suggested a reduction in the reabsorption of Na and water before the bend of the loop of Henle, most likely in both the proximal tubule and descending limb. Fluid osmolality was depressed due to a decline in both Na and nonelectrolyte solute content. After release of BUO the percentage of filtered water remaining in the collecting duct (CD) at the base of the papilla was greater than in controls (13.3+/-2.0 and 1.72+/-0.01%, respectively) but fell significantly by the tip of the papilla to 7.92+/-1.12 vs. 1.17+/-0.02% in controls. These results indicate that water was reabsorbed along the terminal CD after relief of ureteral obstruction. In fact, a greater fraction was reabsorbed in this segment after release of BUO (5.37+/-1.58%) than after sham operation (0.55+/-0.15%). Similar changes were seen in Na excretion. Thus alterations in deep nephron function appear to contribute to the natriuresis and diuresis which follow release of BUO while terminal CD function in this model appears intact.

Renal cortical blood flow distribution in obstructive nephropathy in rats

To examine the role of intrarenal hemodynamics in in obstructive nephropathy, we determined cortical blood flow distribution (CBFD) in rats with bilateral ureteral occlusion (BUO) and unilateral ureteral occlusion (UUO) during and after release of obstruction. Prior to release of obstruction of 24 hours' duration, we found that outer cortical perfusion decreased by 20+/-5% in both BUO and UUO rats. Furthermore, one hour after release of BUO, there was rapid normalization of CBFD associated with a modest return of glomerular filtration rate (GFR), an almost complete return of renal blood flow (RBF), and a marked postobstructive diuresis. In contrast, after release of UUO, we observed that outer cortical perfusion remained decreased by 21+/-31%, both GFR and RBF remained markedly depressed, and no diuresis occurred. These data demonstrate (1) marked ischemia of the outer cortex in both BUO and UUO during obstruction, (2) a rapid return of CBFD to a normal pattern after release of BUO, but (3) persistent outer cortical ischemia following release of UUO.

Effect of adrenalectomy on medullary collecting-duct function in rats before and during blood volume expansion

Adrenalectomized rats, kept on tap water for 3 days and infused with the glucocorticoid dexamethasone during the experiment, were compared to similarly treated sham-operated rats. Using the microcatheterization technique, reabsorption of fluid, sodium and potassium in the medullary collecting duct was studied before and after infusion of donor blood (2.3% of body weight). Before intravascular volume expansion sodium excretion in adrenalectomized rats was greater than in sham-operated ones. Extensive overlap between the two groups made this difference not statistically significant. However, the fraction of filtered sodium excreted was significantly greater after adrenalectomy, indicating the expected tubular transport defect. Fluid reabsorption from the medullary collecting-duct system was comparable in both series. Adrenalectomy did not inhibit net sodium reabsorption from the inner medullary duct, although reduction of Na transport in the outer medullary collecting system could be inferred. Renal excretion of potassium was not associated with net K secretion in the collecting duct in either group. During hypervolemia induced by intravenous infusion of donor blood, marked diuresis, natriuresis and kaliuresis were observed in all animals, associated with inhibition of net fluid and sodium reabsorption along the collecting system in both inner and outer medulla. Small, but statistically significant secretion of potassium became evident. The relatively reduced renal response in adrenalectomized animals could be attributed in part to a decreased filtered load compared to sham-operated rats. It is concluded: (1) that lack of mineralocorticoid does not prevent the normal fluid and sodium reabsorption from the lumen of the inner medullary collecting system, and (2) that the inhibition of this reabsorption consequent to hypervolemia is independent of changes in plasma aldosterone levels.