Tubuloglomerular feedback in animals with unilateral, partial ureteral occlusion

How should we treat acute kidney injury caused by renal congestion?

Decreased kidney function is associated with increased risk of cardiovascular events and mortality, and heart failure (HF) is a wellknown risk factor for renal dysfunction. Acute kidney injury (AKI) in patients with HF often is attributed to prerenal factors, such as renal hypoperfusion and ischemia as a result of decreased cardiac output. Another such factor is reduction of absolute or relative circulating blood volume, with the decrease in renal blood flow leading to renal hypoxia followed by a decrease in the glomerular filtration rate. However, renal congestion is increasingly being recognized as a potential cause of AKI in patients with HF. Increased central venous pressure and renal venous pressure lead to increased renal interstitial hydrostatic pressure and a reduction of the glomerular filtration rate. Both decreased kidney function and renal congestion have been shown to be important prognostic factors of HF, and adequate control of congestion is important for improving kidney function. Loop and thiazide diuretics are recommended as standard therapies to reduce volume overload. However, these agents are associated with worsening renal function even though they are effective for improving congestive symptoms. There is growing interest in tolvaptan, which can improve renal congestion by increasing excretion of free water and decreasing the required dose of loop diuretic, thereby improving kidney function. This review summarizes renal hemodynamics, the pathogenesis of AKI due to renal ischemia and renal congestion, and diagnosis and treatment options for renal congestion.

Hydronephrosis and risk of later development of hypertension

AIM

Congenital ureteral obstruction is a fairly common condition in infants, and its clinical management has been long debated during the last decade. The long-term physiological consequences of today's conservative non-surgical management in many asymptomatic hydronephrotic children are unclear.

METHODS

Experimental studies in rats and mice, retrospective studies in children and adults, as well as prospective studies in children are included in this mini review.

RESULTS

Experimental models of hydronephrosis in rats and mice have demonstrated that partial ureteropelvic junction obstruction (UPJO) is casually linked with development of hypertension and renal injuries in later life. The mechanisms are multifactorial and involve increased activity of the renin-angiotensin-aldosterone system and renal sympathetic nerve activity. Furthermore, oxidative stress and nitric oxide deficiency in the affected kidney appear to play important roles in the development and maintenance of hypertension. Clinical case reports in adults and recent prospective studies in children have associated hydronephrosis with elevated blood pressure, which could be reduced by surgical management of the obstruction.

CONCLUSION

Based on current experimental and clinical knowledge regarding the link between partial UPJO and changes in blood pressure, it is proposed that today's non-operative management of hydronephrosis should be reconsidered to reduce the risk of developing elevated blood pressure or hypertension in later life.

Pathophysiological and molecular mechanisms involved in renal congestion in a novel rat model

Increased central venous pressure in congestive heart failure causes renal dysfunction; however, the underlying mechanisms are unclear. We created a rat renal congestion model and investigated the effect of renal congestion on hemodynamics and molecular mechanisms. The inferior vena cava (IVC) between the renal veins was ligated by suture in male Sprague-Dawley rats to increase upstream IVC pressure and induce congestion in the left kidney only. Left kidney congestion reduced renal blood flow, glomerular filtration rate, and increased renal interstitial hydrostatic pressure. Tubulointerstitial and glomerular injury and medullary thick ascending limb hypoxia were observed only in the congestive kidneys. Molecules related to extracellular matrix expansion, tubular injury, and focal adhesion were upregulated in microarray analysis. Renal decapsulation ameliorated the tubulointerstitial injury. Electron microscopy captured pericyte detachment in the congestive kidneys. Transgelin and platelet-derived growth factor receptors, as indicators of pericyte-myofibroblast transition, were upregulated in the pericytes and the adjacent interstitium. With the compression of the peritubular capillaries and tubules, hypoxia and physical stress induce pericyte detachment, which could result in extracellular matrix expansion and tubular injury in renal congestion.

Hypertension and hydronephrosis: rapid resolution of high blood pressure following relief of bilateral ureteric obstruction

Hypertension secondary to hydronephrosis is not commonly reported in the medical literature. Tubuloglomerular feedback and the renin-angiotensin-aldosterone axis are thought to mediate this process. We describe a patient presenting with acute kidney injury and bilateral hydronephrosis secondary to pelvic malignancy in which peripheral venous renin and aldosterone were elevated. Her blood pressure improved rapidly following insertion of bilateral nephrostomies. The speed of resolution of hypertension following relief of obstruction suggests that humorally mediated vasoconstriction can play an important role in the mechanism by which hydronephrosis causes hypertension. We also discuss other causes of renal parenchymal compression that may lead to the development of hypertension.

Nitric Oxide Deficiency and Increased Adenosine Response of Afferent Arterioles in Hydronephrotic Mice With Hypertension

Afferent arterioles were used to investigate the role of adenosine, angiotensin II, NO, and reactive oxygen species in the pathogenesis of increased tubuloglomerular feedback response in hydronephrosis. Hydronephrosis was induced in wild-type mice, superoxide dismutase-1 overexpressed mice (superoxide-dismutase-1 transgenic), and deficient mice (superoxide dismutase-1 knockout). Isotonic contractions in isolated perfused arterioles and mRNA expression of NO synthase isoforms, adenosine, and angiotensin II receptors were measured. In wild-type mice, N G -nitro- l -arginine methyl ester ( l -NAME) did not change the basal arteriolar diameter of hydronephrotic kidneys (−6%) but reduced it in control (−12%) and contralateral arterioles (−43%). Angiotensin II mediated a weaker maximum contraction of hydronephrotic arterioles (−18%) than in control (−42%) and contralateral arterioles (−49%). The maximum adenosine-induced constriction was stronger in hydronephrotic (−19%) compared with control (−8%) and contralateral kidneys (±0%). The response to angiotensin II became stronger in the presence of adenosine in hydronephrotic kidneys and attenuated in contralateral arterioles. l -NAME increased angiotensin II responses of all of the groups but less in hydronephrotic kidneys. The mRNA expression of endothelial NO synthase and inducible NO synthase was upregulated in the hydronephrotic arterioles. No differences were found for adenosine or angiotensin II receptors. In superoxide dismutase-1 transgenic mice, strong but similar l -NAME response (−40%) was observed for all of the groups. This response was totally abolished in arterioles of hydronephrotic superoxide dismutase-1 knockout mice. In conclusion, hydronephrosis is associated with changes in the arteriolar reactivity of both hydronephrotic and contralateral kidneys. Increased oxidative stress, reduced NO availability, and stronger reactivity to adenosine of the hydronephrotic kidney may contribute to the enhanced tubuloglomerular feedback responsiveness in hydronephrosis and be involved in the development of hypertension.

Role of nitric oxide deficiency in the development of hypertension in hydronephrotic animals

Hydronephrotic animals develop renal injury and hypertension, which is associated with an abnormal tubuloglomerular feedback (TGF). The TGF sensitivity is coupled to nitric oxide (NO) in the macula densa. The involvement of reduced NO availability in the development of hypertension in hydronephrosis was investigated. Hydronephrosis was induced by ureteral obstruction in young rats. Blood pressure and renal excretion were measured in adulthood, under different sodium conditions, and before and after chronic administration of either NG-nitro-l-arginine methyl ester (l-NAME) or l-arginine. Blood samples for ADMA, SDMA, and l-arginine analysis were taken and the renal tissue was used for histology and determination of NO synthase (NOS) proteins. TGF characteristics were determined by stop-flow pressure technique before and after administration of 7-nitroindazole (7-NI) or l-arginine. Hydronephrotic animals developed salt-sensitive hypertension, which was associated with pressure natriuresis and diuresis. The blood pressure response to l-NAME was attenuated and l-arginine supplementation decreased blood pressure in hydronephrotic animals, but not in the controls. Under control conditions, reactivity and sensitivity of the TGF response were greater in the hydronephrotic group. 7-NI administration increased TGF reactivity and sensitivity in control animals, whereas, in hydronephrotic animals, neuronal NOS (nNOS) inhibition had no effect. l-Arginine attenuated TGF response more in hydronephrotic kidneys than in controls. The hydronephrotic animals displayed various degrees of histopathological changes. ADMA and SDMA levels were higher and the renal expressions of nNOS and endothelial NOS proteins were lower in animals with hydronephrosis. Reduced NO availability in the diseased kidney in hydronephrosis, and subsequent resetting of the TGF mechanism, plays an important role in the development of hypertension.

Hydronephrosis causes salt-sensitive hypertension and impaired renal concentrating ability in mice

AIM

Hypertension is a common disease in the industrialized world and approximately 5% of all cases are secondary to kidney malfunction. We have recently shown that hydronephrosis due to partial unilateral ureteral obstruction (PUUO) causes salt-sensitive hypertension in rats. The mechanisms are still unclear, but appear to be intrarenal and primarily located to the diseased kidney. In the present study, we have developed a model for PUUO to study if hydronephrotic mice develop salt-sensitive hypertension.

METHODS

PUUO was created in 3-week-old mice (C57bl/6J). Blood pressure and heart rate were measured telemetrically in adult animals on normal and high salt diets. Metabolism cages were used to study the renal excretion of electrolytes and water. Plasma samples for renin analysis were collected and renal histological changes were evaluated.

RESULTS

All hydronephrotic animals developed salt-sensitive hypertension that correlated to the degree of hydronephrosis. In hydronephrotic animals, blood pressure increased from 114 +/- 1 mmHg on normal salt diet to 120 +/- 2 mmHg on high salt diet, compared with 103 +/- 1 to 104 +/- 1 in controls. Hydronephrotic animals showed increased diuresis and reduced ability to regulate electrolyte concentration. No differences in plasma renin concentration were found between the groups. The parenchymal weight and glomerular area of contralateral kidneys were significantly increased in the hydronephrotic animals. Histopathology of the hydronephrotic kidneys displayed areas with fibrosis, inflammation and glomerular changes.

CONCLUSION

This study provides a model for PUUO in mice and demonstrates the presence of salt-sensitive hypertension and an impaired renal concentrating ability in mice which has not been described before.

Relief of chronic partial ureteral obstruction attenuates salt-sensitive hypertension in rats

AIM

The incidence of hydronephrosis due to ureteropelvic junction obstruction is approx. 0.5%. During the last decade, the management of non-symptomatic hydronephrosis has become much more conservative, but the long-term physiological consequences of this policy are not clear. Previously, we have shown that animals with chronic partial unilateral ureteral obstruction develop salt-sensitive hypertension. In this study, the effects of ipsilateral and contralateral nephrectomy and ureterovesicostomy on blood pressure were studied in hydronephrotic animals.

METHODS

Partial unilateral ureteral obstruction was created in 3-week-old male Sprague-Dawley rats and blood pressure was measured telemetrically 4-6 weeks later during a normal and high salt diet before and after uninephrectomy or ureterovesicostomy. Plasma samples for renin assay were collected during both diets before and after ipsilateral nephrectomy.

RESULTS

All hydronephrotic animals developed salt-sensitive hypertension, of different degrees. Before nephrectomy the plasma renin concentration was significantly higher in the hydronephrotic animals than in controls (160 +/- 15 microGU mL(-1) vs. 96 +/- 12 microGU mL(-1), respectively), but after the ipsilateral nephrectomy no differences were found between the groups. In the hydronephrotic animals both ipsilateral nephrectomy and ureterovesicostomy reduced the blood pressure and salt-sensitivity but the former still differed significantly from the controls. In contralaterally, nephrectomized hydronephrotic animals the salt-sensitive hypertension became more pronounced.

CONCLUSION

Hydronephrosis in rats causes salt-sensitive hypertension that can be markedly reduced by removing the hydronephrotic kidney or relieving the obstruction by ureterovesicostomy. The mechanisms appear to be intrarenal and primarily located in the diseased kidney, but a secondary mechanism is also present.

Hydronephrosis causes salt-sensitive hypertension in rats

BACKGROUND

Hypertension is a common disease in the Western world and approximately 5% of all cases are secondary to kidney malfunction. It is not clear whether unilateral hydronephrosis due to partial obstruction affects blood pressure.

AIM

The aim of this study was to determine whether hypertension develops and to investigate the effects of different salt diets on the blood pressure in hydronephrotic animals.

METHODS

Unilateral partial ureteral obstruction was created in 3-week-old Sprague-Dawley rats. A telemetric device was implanted 4-6 weeks later and blood pressure was measured on normal, low- and high-salt diets. Plasma samples were collected on all diets for renin analysis.

RESULTS

All hydronephrotic animals developed hypertension that correlated to the degree of hydronephrosis. The blood pressure increased slowly with time and was salt sensitive. In severe hydronephrosis, blood pressure increased from 118 +/- 5 mmHg on low salt to 140 +/- 6 mmHg on high salt intake, compared to control levels of 82 +/- 2 and 84 +/- 2 mmHg, respectively. Plasma renin concentration was increased in the hydronephrotic group of animals compared to controls on all diets, but the difference was only significant on a normal salt diet, 165 +/- 15 versus 86 +/- 12 microGU/ml respectively. In animals with severe hydronephrosis the plasma renin levels were lower, and the changes less, than in those with mild and moderate hydronephrosis.

CONCLUSION

This study demonstrates the presence of a salt-sensitive hypertension in hydronephrosis. A systemic effect of the renin-angiotensin system alone cannot be responsible for the hypertension.

Early release of neonatal ureteral obstruction preserves renal function

The incidence of congenital hydronephrosis is approximately 1% and is often associated with renal insufficiency. It is unknown whether early release is essential to prevent deterioration of renal function. Rats were subjected to partial unilateral ureteral obstruction (PUUO) on postnatal day 2. The obstruction was left in place or released after 1 or 4 wk. Renal blood flow (RBF) and kidney size were measured sequentially over 24 wk using MRI. In rats in which the obstruction was left in place, RBF of the obstructed kidney was progressively reduced to 0.92 +/- 0.17 vs. 1.79 +/- 0.12 ml.min(-1).100 g body wt(-1) (P < 0.05) after 24 wk. Similarly, glomerular filtration rate of the obstructed kidney was severely reduced at 24 wk: 172 +/- 36 vs. 306 +/- 42 microl.min(-1).100 g body wt(-1) (P < 0.05). These changes were preceded by development of severe hydronephrosis and obstructive nephropathy with a reduction in total protein content: 45 +/- 3 vs. 58 +/- 4 mg/kidney. Moreover, nonreleased PUUO caused a marked natriuresis (0.32 +/- 0.07 vs. 0.11 +/- 0.02 micromol.min(-1).100 g body wt(-1), P < 0.05) and impaired solute free water reabsorption (0.47 +/- 0.16 vs. 2.71 +/- 0.67 microl.min(-1).100 g body wt(-1), P < 0.05), consistent with a significant downregulation of Na-K-ATPase to 62 +/- 7%, aquaporin-1 to 53 +/- 3%, and aquaporin-3 to 53 +/- 7% of sham levels. Release after 1 wk completely prevented development of hydronephrosis, reduction in RBF and glomerular filtration rate, and downregulation of renal transport proteins, whereas release after 4 wk had no effect. These results suggest that early release of neonatal obstruction provides dramatically better protection of renal function than release of obstruction after the maturation process is completed.

Mild hyperuricemia induces glomerular hypertension in normal rats

Mildly hyperuricemic rats develop renin-dependent hypertension and interstitial renal disease. Hyperuricemia might also induce changes in glomerular hemodynamics. Micropuncture experiments under deep anesthesia were performed in Sprague-Dawley rats fed a low-salt diet (LS group), fed a low-salt diet and treated with oxonic acid (OA/LS group), and fed a low-salt diet and treated with oxonic acid + allopurinol (OA/LS/AP group) for 5 wk. The OA/LS group developed hyperuricemia and hypertension compared with the LS group: 3.1 +/- 0.2 vs. 1.1 +/- 0.2 mg/dl (P < 0.01) and 143 +/- 4 vs. 126 +/- 2 mmHg (P < 0.01). Hyperuricemic rats developed increased glomerular capillary pressure compared with the LS rats: 56.7 +/- 1.2 vs. 51.9 +/- 1.4 mmHg (P < 0.05). Pre- and postglomerular resistances were not increased. Histology showed afferent arteriolar thickening with increased alpha-smooth muscle actin staining of the media. Allopurinol prevented hyperuricemia (1.14 +/- 0.2 mg/dl), systemic (121.8 +/- 2.8 mmHg) and glomerular hypertension (50.1 +/- 0.8 mmHg), and arteriolopathy in oxonic acid-treated rats. Linear regression analysis showed that glomerular capillary pressure and arteriolar thickening correlated positively with serum uric acid and systolic blood pressure. Glomerular hypertension may be partially mediated by an abnormal vascular response to systemic hypertension due to arteriolopathy of the afferent arteriole.

Recovery from release of ureteral obstruction in the rat: relationship to nephrogenesis

BACKGROUND

Obstructive nephropathy is a major cause of renal insufficiency in infants and children. Despite release of unilateral ureteral obstruction (UUO) in the first five days of life in the rat, renal growth is impaired, while glomerular filtration rate (GFR) is preserved at one month, but decreases markedly by one year. To test the hypothesis that renal recovery from UUO depends on the stage of nephrogenesis at the time of relief of obstruction, renal recovery from relief of five days UUO following completion of nephrogenesis (days 14 to 19) was compared with UUO during nephrogenesis (days 1 to 5).

METHODS

Rats underwent UUO or sham operation at one day of age, with relief five days later. In additional groups of neonatal rats, the operation was at 14 days, with relief at 19 days. Three months later, blood pressure, GFR, urine flow, sodium and potassium excretion, and kidney weight were measured. In addition, the number of glomeruli, glomerular maturation, glomerular diameter, tubular atrophy, and interstitial fibrosis were determined in each kidney. The effects of five-day UUO on number of glomeruli was determined also in adult rats one month following relief of obstruction.

RESULTS

Three months following relief of UUO during days 14 to 19, renal growth was decreased by 50%, compared to a 30% reduction following relief of UUO during days 1 to 5 (P < 0.05). The number of glomeruli was reduced by approximately 50% regardless of the timing of UUO, but glomerular size was reduced only in rats with UUO from days 14 to 19. Blood pressure and tubular atrophy were increased, and GFR, urine flow, sodium and potassium excretion were decreased in the postobstructed kidney of both neonatal groups. In the adult rat, the five-day UUO did not result in a decrease in the number of glomeruli.

CONCLUSIONS

In the period immediately following nephrogenesis, the kidney is particularly susceptible to long-term injury from temporary UUO. This suggests that a delay in relief of significant ureteral obstruction should be avoided if diagnosed in the perinatal or neonatal period.

Pressure flow measurement of hydronephrosis in children: a new approach to definition and quantification of obstruction

PURPOSE

Hydronephrosis due to obstruction of the ureteropelvic junction is not uncommon in children. The critical level of outflow resistance characterizing obstruction remains to be established. It was previously found in our laboratory that the outflow resistance could be calculated in an animal model by servoregulating the infused flow to preset pressure levels. We determine whether this procedure is also feasible in patients with hydronephrosis.

MATERIALS AND METHODS

A total of 46 patients with 48 hydronephrotic kidneys were examined. There were 32 boys and 14 girls with a diagnosis of hydronephrosis and suspected ureteropelvic junction obstruction who underwent a pressure flow study under general anesthesia for evaluation before possible surgery. The kidney was punctured percutaneously under ultrasonic guidance, and the study was performed under fluoroscopy. Flow levels were measured when steady state flow was achieved at pressures 5, 10, 15, 20, 25 and 30 mm. Hg greater than the intra-abdominal pressure. A conventional Whitaker test at an infusion rate of 10 ml. per minute was performed for comparison. The patient then underwent pyeloplasty if the kidney was considered obstructed.

RESULTS

There were 2 patients excluded from evaluation because of significant leakage of contrast medium. Stable and repeatable recordings were obtained in all pressure flow studies but only in 7 of 46 Whitaker tests. A total of 41 kidneys were considered obstructed at pressure flow studies and operated on. The resistance was pressure dependent. Thus, mean resistance in the previous range 5 to 15 mm. Hg was considered a good overall measure of outflow resistance. Mean resistance less than 0.75 was normal and greater than 1.25 obstruction. There were 7 patients who had crossing vessels and a high mean resistance (3 or greater).

CONCLUSIONS

The Whitaker test proved to be of no value for calculating the outflow resistance in ureteropelvic junction obstruction. The pressure flow study is superior to the Whitaker test for determining the outflow resistance and also allowing categorization of ureteropelvic junction obstruction.

Chronic regulation of transepithelial Na+ transport by the rate of apical Na+ entry

In several settings in vivo, prolonged inhibition of apical Na+ entry reduces and prolonged stimulation of apical entry enhances the ability of renal epithelial cells to reabsorb Na+, an important feature of the load-dependent regulation of renal tubular Na+ transport. To model this load dependency, apical Na+ entry was inhibited or stimulated for 18 h in A6 cells and vectorial transport was measured as short-circuit current (Isc) across monolayers on filter-bottom structures. Basal amiloride-sensitive Isc represents the activity of apical Na+ channels, whereas Isc after permeabilization of the apical membrane to cations with nystatin represents maximal activity of the basolateral Na(+)-K(+)-ATPase. Chronic inhibition of apical Na+ entry by 18-h apical exposure to amiloride or replacement of apical Na+ with tetramethylammonium (TMA+), followed by washing and restoration of normal apical medium, revealed a persistent decrease in Isc that remained despite exposure to nystatin. Both basal and nystatin-stimulated Isc recovered progressively after restoration of normal apical medium. In contrast, chronic stimulation of apical Na+ entry by short circuiting the epithelium increased Isc in the absence and presence of nystatin, indicating upregulation of both apical Na+ channels and basolateral Na(+)-K(+)-ATPase. Basolateral equilibrium [3H]ouabain binding was reduced to 67 +/- 5% in TMA+ vs. control cells, whereas values in 18-h short-circuited cells increased by 42 +/- 19%. The results demonstrate that load dependency of tubular Na+ transport can be modeled in vitro and indicate that the regulation of Na(+)-K(+)-ATPase observed in these studies occurs in part by changes in the density of functional transporter proteins within the basolateral membrane.

A new method for exchange of perfusion fluid during in vivo micropuncture

The first step in the tubuloglomerular feedback (TGF) mechanism is postulated to be the sensing of changes in tubular NaCl concentration by the macula densa (MD) cells. Despite this, few in vivo studies using different tubular NaCl concentrations administered to the MD site have been completed. Methodological problems associated with retrograde perfusion might possibly explain this. In the present study we present a modification of a method used in in vitro tubular perfusion experiments which makes it possible to determine the TGF response using retrograde perfusion with a single perfusion pipette. The system is based on the use of a fluid exchange pipette introduced through a chamber in the holder of the perfusion pipette all the way to the tip of this pipette. The perfusion flow was regulated by a pressure head, and by regulating the outflow from the chamber. The flow was determined for different tip diameters and for different perfusion pressures. Using a tip diameter of 4 microns, the flow through the tip was in the range 18-25 nL min-1 at a pressure of 60-70 mmHg. The fluid-exchange pipette was tested in vivo by measuring proximal tubular stop-flow pressure while perfusing the macula densa region with different NaCl concentration.

Renal tubular apoptosis after partial ureteral obstruction

Partial ureteral obstruction in the weanling rat leads to hydronephrosis of the ipsilateral kidney and renal cell deletion through the process of programmed cell death known as apoptosis. The apoptotic response following partial ureteral obstruction in weanling Sprague-Dawley rats was studied using the traditional markers of apoptosis, including deoxyribonucleic acid (DNA) laddering pattern on agarose gel electrophoresis, in situ gap labeling of fragmented DNA for quantitative apoptotic body determination, polyadenylated messenger ribonucleic acid (mRNA) expression of sulfated glycoprotein-2, and polyadenylated mRNA expression of epidermal growth factor and transforming growth factor-beta. Partial ureteral obstruction resulted in a progressive increase in the intensity of DNA fragmentation associated with apoptosis during the initial 3 weeks. Quantitative apoptotic body counting revealed a 3-fold increase by week 3 of partial obstruction. This increase represented a level of apoptosis, which is 65% of that observed in complete ureteral obstruction. By week 2 of partial obstruction there was a 13-fold increase in the expression of sulfated glycoprotein-2 mRNA, as well as changes in the growth factor environment characterized by a decline in the constitutive expression of epidermal growth factor mRNA and an increase in the expression of transforming growth factor-beta mRNA. These altered levels represent changes in expression comparable to those observed during the apoptotic response following complete ureteral obstruction, although the time course is delayed by 2 to 3 weeks.

Alterations in glomerular dynamics in congenital, unilateral hydronephrosis

We have previously shown that rats with congenital, unilateral hydronephrosis exhibit a reduction in GFR that returns to normal when either the renin angiotensin system or thromboxane A2 (TxA2) is blocked. The current study defines the single nephron defect in congenital, unilateral hydronephrosis and evaluates the roles of angiotensin II (Ang II) and TxA2 in this renal derangement. Renal micropuncture experiments were performed on the right kidney of rats from an inbred colony with unilateral right-sided hydronephrosis (HYDRO), or non-affected litter mates (CONTROL). In addition, four separate groups of hydronephrotic animals were treated with either the TxA2 receptor antagonist SQ-29548 (SQ), one of two Ang II receptor antagonists [saralasin (SAR) or DuP-753 (DUP)]; or combined treatment with DuP-753 and SQ-29,548 (S&D). SNGFR was significantly reduced (P < 0.05) in HYDRO compared to CONTROL (17.6 +/- 2.0 vs. 35.9 +/- 3.7 nl/min, respectively). Treatment with SQ-29,548 normalized SNGFR (29.0 +/- 3.0 nl/min), while saralasin and DuP-753 resulted in only a partial recovery of function (25.6 +/- 1.6 and 27.8 +/- 1.4 nl/min, respectively). Combined SQ-29,548 and DuP-753 treatment resulted in full recovery of SNGFR to 32.9 +/- 4.4 nl/min. The glomerular ultrafiltration coefficient (Kf) was reduced (P < 0.05) approximately 45% in HYDRO compared to CONTROL (1.64 +/- .08 vs. 2.84 +/- .22 nl/min/mm Hg, respectively). Kf returned to control levels in SAR, DUP and SQ, and increased above control in S&D (5.58 +/- 1.6 nl/min/mm Hg). There were no differences (P > 0.05) in hydrostatic or oncotic pressures across the glomerular capillary between any of the groups studied. The observation that Kf increases above CONTROL with combined blockade of TxA2 and Ang II suggests that these regulatory hormones decrease Kf via independent mechanisms. These data indicate that the reduction in SNGFR in congenital, unilateral hydronephrosis is a result of a marked fall in Kf that is mediated by both Ang II and TxA2.

Resetting of tubuloglomerular feedback by interrupting early distal flow

Cell-cell contact between the macula densa and the glomerular arterioles is is thought to provide the information pathway for the tubuloglomerular feedback (TGF) mechanism. When concentrations of sodium and chloride in the macula densa segment are increased, a signal is transmitted through the extraglomerular mesangium to contract the afferent arteriole. In addition, some observers have described a second region of contact between a later part of the distal tubule and the afferent arteriole of the same nephron. In this region the connecting tubule (CNT), and sometimes nerves that make contact with the cells of this CNT, were found. This arrangement gives another potential tubular segment, besides the macula densa plaque, in which the composition of tubular fluid may regulate glomerular dynamics. The present study was designed to investigate whether interrupting flow in the distal tubule downstream from the macula densa would influence the TGF mechanism. TGF was examined in rats by orthograde microperfusion, before and after blockade of the distal nephron with castor oil. Two variables were measured: maximum decrease in stop-flow pressure (delta Psf), and perfusion rate which elicits half-maximal decrease in delta Psf (V1/2). The fluid arriving at the blocking point was collected into a micro-pipette. The results show a significant increase in V1/2 from 19 to 25 nl min-1 after 30 min of blockade. In conclusion the results support a role of the distal nephron in the control of the TGF mechanism.

Renal interstitial pressure and tubuloglomerular feedback control in rats during infusion of atrial natriuretic peptide (ANP)

Atrial natriuretic peptide (ANP), injected at physiological concentrations, is known to induce both natriuresis and diuresis. It has been suggested by some investigators that these changes result from an increasing glomerular filtration rate (GFR), but others have been unable to demonstrate an increased GFR. The tubuloglomerular feedback (TGF) mechanism is an important regulator of GFR, and the sensitivity of TGF is decreased during ANP administration. Furthermore, resetting of TGF is, in most instances, related to changes in renal interstitial hydrostatic and oncotic pressures. It is also known that ANP may increase capillary permeability which may change renal interstitial pressure. The present study was performed to examine renal interstitial pressures and the TGF mechanism during ANP infusion. In accordance with previous studies, TGF sensitivity was found to be decreased. The tubular flow rate which elicited half the maximal drop in stop-flow pressure (Psf) was increased from 18.5 to 25.7 nl min-1. In contrast, ANP infusion resulted in a decreased interstitial hydrostatic pressure and an increased interstitial oncotic pressure. From previous experiments, such changes in interstitial pressures would be expected to increase TGF sensitivity. The changes in interstitial pressure cannot, therefore, directly explain the resetting of the feedback mechanism. In conclusion, the present paper shows a decreased renal net interstitial pressure after intravenous administration of ANP.

Tubular sodium handling and tubuloglomerular feedback in compensatory renal hypertrophy

Tubular sodium handling and tubuloglomerular feedback (TGF) activity were assessed in established compensatory renal hypertrophy in Sprague Dawley rats. Hyperfiltration at the level of the single nephron was confirmed 4-6 weeks following a reduction in renal mass. TGF activity, determined as the difference between late proximal and early distal measurements of single-nephron glomerular filtration rate (SNGFR), was significantly increased in compensatory renal hypertrophy, being 7.8 +/- 1.0 vs 23.3 +/- 1.9 vs 25.5 +/- 2.6 nl/min (P for analysis of variance less than 0.05) following sham operation, unilateral nephrectomy, and 1 1/3 nephrectomy, respectively. Enhanced net tubular Na transport was also observed, with total Na reabsorption up to the late proximal site being 1.8 +/- 0.2 vs 2.7 +/- 0.1 vs 3.1 +/- 0.3 nmol/min (P less than 0.05), and to the early distal site being 3.4 +/- 0.5 vs 5.8 +/- 0.6 vs 7.9 +/- 0.8 nmol/min (P less than 0.05) in the three animal groups respectively. Comparison of proximal tubular length demonstrated a 71.9 +/- 8.1% increase in uninephrectomised vs sham-operated animals. This increase was proportionately greater than the increase in proximal Na reabsorption (50.0 +/- 4.0%) observed in the corresponding animal groups. Concurrent electron microprobe experiments in uninephrectomised and sham-operated animals demonstrated that the proximal tubular intracellular Na concentration was significantly lower following uninephrectomy (16.8 +/- 0.6 vs 18.9 +/- 0.5 mmol/kg wet weight, P less than 0.01), in association with evidence of reduced basolateral Na/K-ATPase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Urinary excretion of brush border antigens and other proteins in children with vesico-ureteric reflux

This study was designed to evaluate the occurrence and the type of proteinuria in 82 children with vesico-ureteric reflux (VUR) with or without renal scars. The urinary excretion of the high molecular weight protein albumin was taken as an index of glomerular alterations and the excretion of retinol-binding protein (RBP), beta 2-microglobulin and brush border antigens (BBA) (measured by monoclonal antibody-based enzyme-linked immunosorbent assay) was taken as an index of tubular alterations. All such markers were increased in children with VUR and were related to the degree of renal function. Patients showing reduced creatinine clearance had very high levels of albuminuria, microproteinuria and BBA, with all these variables reciprocally correlated. In children with normal renal function however, only microproteins (not albumin or BBA) were slightly increased, thus indicating an isolated tubular defect without involvement of the proximal segment of the tubule. However, microprotein excretion did not correlate with the grade of scarring (99mtechnetium-dimercaptosuccinic acid scan), both RBP and beta 2-microglobulin excretion being normal in 75% of children with radioisotopic signs of renal lesions but increased in 17% of children without scars. Therefore, tubular proteinuria identifies different groups of children with VUR but is not related to renal scarring. Prospective studies will define the usefulness of proteinuria as a reliable indicator of renal outcome.

Long-term partial ureteral obstruction and its effects on kidney function

Previously it has been shown that partial ureteral obstruction present in young rats for 12 weeks results in small morphological changes in the kidney as well as slightly decreased kidney function. In the present study the aim was to examine whether rats obstructed for one year had more advanced changes in morphology and kidney function. The first group of animals examined after three weeks of obstruction showed only modest changes in kidney function with a reduced potassium concentration in the urine but no reduction in the glomerular filtration rate. After one year there was a reduction in urine flow as well as in the excretion of both potassium and sodium. Urine osmolality was also reduced. Glomerular filtration rate measured in this group of animals was reduced in the obstructed kidney by about 60% compared to the contralateral one. There were only small changes in the morphology with no loss in parenchymal weight or compensatory hypertrophy, but there was a significant deformation of the papilla and an increase in inflammatory cells in the parenchyma. In conclusion hydronephrosis during a shorter period is not harmful to kidney function but if sustained for an extended time period kidney function will deteriorate.

Effect of thromboxane inhibition on tubuloglomerular feedback in hydronephrotic kidneys

The tubuloglomerular feedback (TGF) system is less sensitive after extracellular volume expansion (VE). In rats with partial ureteral obstruction, however, we have previously found increased TGF sensitivity during such expansion. Thromboxane A2 (TXA2) has been reported to be increased in the hydronephrotic kidney, and the present study was undertaken to investigate whether TXA2 might be responsible for these findings. The TGF characteristics were studied, by means of the stop-flow technique, in control and hydronephrotic rats before and after intravenous injection of either a thromboxane synthetase inhibitor or a thromboxane receptor antagonist during hydropenia. After VE, TGF was studied again. Proximal tubular stop-flow pressure (Psf) was measured during perfusion of the loop of Henle with a modified Ringer's solution, the maximal response (delta Psf) to an increased tubular flow rate was determined and the tubular perfusion rate which elicited a half-maximal decrease in Psf, designated the turning point, was recorded. In hydropenic control or hydronephrotic animals, TXA2 did not permanently change the TGF characteristics. During VE of the TXA2-blocked hydronephrotic animals, blocked both with synthetase inhibitor or receptor-antagonist, TGF was reset to a lower sensitivity like VE controls, as indicated by a high turning point and a low delta Psf. It was therefore concluded that thromboxane A2 inhibition does not influence the TGF system in hydropenia, but that the production of thromboxane A2 is responsible for the resetting of the TGF system during VE in hydronephrotic animals.