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RENAL BLOOD FLOW INCREASE DURING VOLUME EXPANSION IN HYDRONEPHROTIC RATS
0022-5347/01/1655-1696/0 THE JOURNAL OF UROLOGY® Copyright © 2001 by AMERICAN UROLOGICAL ASSOCIATION, INC.®
Vol. 165, 1696 –1699, May 2001 Printed in U.S.A.
RENAL BLOOD FLOW INCREASE DURING VOLUME EXPANSION IN HYDRONEPHROTIC RATS NILS WÅHLIN,* ARNE STENBERG
AND
A. ERIK G. PERSSON
From the Department of Pediatric Surgery, University Childrens Hospital and Department of Physiology, University of Uppsala, Uppsala, Sweden
ABSTRACT
Purpose: We assessed the renal blood flow pattern in experimental hydronephrosis during normal hydration and extracellular volume expansion. Materials and Methods: Partial obstruction of the left ureter was created in 3-week-old SpragueDawley rats by embedding the ureter in a psoas muscle groove. Moderate hydronephrosis without kidney weight reduction developed in all cases. The effects on renal hemodynamics were studied with real-time ultrasound flowmetry 3 weeks later during normal hydration and then during volume expansion. The degree of hydronephrosis was classified as mild, moderate or severe. Results: Under baseline conditions renal blood flow was normal in mild and moderate hydronephrosis but low in severe hydronephrosis. During volume expansion renal blood flow increased significantly in all experimental animals (mean 14%) compared to that in controls, which remained unaffected or decreased (mean ⫺3%). The flow increase was related to the degree of dilatation, which was 2% in mild, 13% in moderate and 44% in severe hydronephrosis when the groups were considered separately. Conclusions: A significant increase in renal blood flow proportional to the degree of hydronephrosis occurred as a result of volume expansion. This finding may be explained by a state of vasodilatation combined with a reduction in the filtration coefficient. KEY WORDS: kidney; renal circulation; hydronephrosis; ureteral obstruction; rats, Sprague-Dawley
The regulation of renal blood flow is intimately coupled to the glomerular filtration rate. The glomerular filtration rate depends on the surface area available for filtration, its permeability, the blood flow over it and the actual pressure force, net filtration pressure. Normally renal blood flow is automatically regulated at a constant level even under different circumstances.1 The renal blood flow pattern in the acute phase of and after complete urinary tract obstruction is well known.2, 3 The mechanisms involved in partial obstruction have been thought to be mainly of the same character but more subtle and slower in onset. The severity and duration of obstruction as well as the age of the individual at onset appear to be important to the outcome.4, 5 In previous experimental studies at our own laboratory rats underwent partial ureteral obstruction at age 3 weeks, leading to considerable hydronephrosis. Histologically moderate changes with a 35% reduction in the number of glomeruli were noted but there was no kidney weight loss.6 Physiologically hydronephrotic kidneys showed normal single nephron glomerular filtration rate and excretory behavior under baseline conditions. However, micropuncture studies during volume expansion showed major changes in filtration regulation and tubuloglomerular feedback mechanism characteristics.7 The tubuloglomerular feedback mechanism acts as a negative feedback control whereby the macula densa cells respond to the salt concentration of fluid in the distal tubule, normally by regulating the tone of the afferent arteriole and, thereby, filtration pressure of the glomerulus. The sensitivity and activity of the tubulo-glomerular feedback mechanism are modulated by several factors, such as extracellular volume conditions and, thus, interstitial pressure in the kidney as well as by various vasoactive substances, such as angiotensin II, thromboxane A2 and nitric oxide.8, 9
During volume expansion in the nonhydronephrotic control state great diuresis occurs. Afferent vasodilatation leads to elevated filtration pressure and, thereby, an increase in the glomerular filtration rate. In addition, the sensitivity of the tubuloglomerular feedback mechanism is reset to a minimum level that abolishes any vascular effect as a response to tubular signals and high filtration is maintained. On the other hand, in hydronephrotic kidneys tubuloglomerular feedback mechanism sensitivity is reset to a higher level and is strongly active during volume expansion. As a result, filtration pressure remains unchanged, and the single nephron glomerular filtration rate and urinary output are constant or only slightly increased. Thus, most of the fluid load is excreted by the contralateral intact kidney. These effects may be reversed by blocking thromboxane synthesis or receptors.7, 10 We determined the pattern of total renal blood flow in rats with partial ureteral obstruction and moderate hydronephrosis under baseline conditions, and during and after extracellular volume expansion.
MATERIALS AND METHODS
Creation of hydronephrosis. Unilateral partial ureteral obstruction was created in 3-week-old male Sprague-Dawley rats (Mo¨llegaard, Copenhagen, Denmark) using the method originally described by Ulm and Miller in dogs.11 The animals were anesthetized with 30 mg./kg. body weight methohexital administered intraperitoneally. The abdomen was opened through a midline incision and the left ureter was dissected free under microscopy. The underlying psoas muscle was split longitudinally to form an approximate 15 mm. groove and the ureter was placed into it. The muscle edges were adapted above the ureter with 2, 6-zero silk sutures and, thus, the ureter was embedded in a tunnel. The abdomen was then closed with interrupted 5-zero polyamid sutures. Sham operations in controls were performed in the same manner but without dissecting the ureter. After the
Accepted for publication November 7, 2000. Supported by grants from Her Royal Highness the Crown Princess Lovisas Fund for Scientific Research, Åke Wiberg Foundation, Gillbergska Foundation and Grant B98-14x-63522-27D. * Requests for reprints: Department of Pediatric Surgery, Section of Urology, University Children’s Hospital, S-751 85 Uppsala, Sweden. 1696
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RENAL BLOOD FLOW INCREASE DURING VOLUME EXPANSION IN HYDRONEPHROSIS
operation the animals were allowed to awaken beneath a heating lamp and were not returned to the cages until completely awake. They were then left to grow with free access to R2 food (Ewos, So¨derta¨lje, Sweden) and water. Experimental preparation. Experiments were done about 3 weeks later at a mean rat body weight of 286 gm. (range 235 to 350). The rats were anesthetized with 120 mg./kg. body weight Inactin (Byk Gulden, Konstanz, Germany) administered intraperitoneally and placed on a servoregulated heating pad to maintain body temperature at 37.5C. Tracheotomy was performed, the right femoral artery and vein were catheterized for blood pressure monitoring and infusion, and the rats were then placed on the right side. Baseline infusion was started of 0.5 ml. 0.9% NaCl/100 gm. body weight per hour. The left kidney was exposed through a subcostal flank incision and the renal vein was dissected free from the stalk with care taken to avoid nerve damage. The kidney was placed in a polymerized methyl methacrylate cup and gently fixed with cotton fiber and water soluble gel, preventing stalk tension. A real-time ultrasound recorder (Transonic Systems, Ithaca, New York) was used for blood flow measurements. A 1 mm. probe was attached to the renal vein and retained in position with a clamp holder. The space around the vessel and probe was carefully filled with inert Derma-Jel gel (Physio-Control Corporation, Redmond, Oregon) to establish optimal measurement conditions. The recorder was then connected to a Goertz Servogor 400 printer (ABB Goertz AG, Vienna, Austria), which continuously displayed blood pressure and renal blood flow values. After a 30-minute stabilizing period saline volume expansion at 5% ⫻ body weight per hour was begun and continued for 30 minutes. Thereafter the infusion was returned to the original baseline rate of 0.5 ml. per 100 gm. body weight per hour and recording was continued for another 30 minutes. The experiment was then completed by sacrificing the animal and the zero flow versus previous in vitro calibration was assessed with the probe in situ. Quantification of hydronephrosis. Pelvic volume was then determined by placing a ligature around the ureter, cutting the vessels and removing the kidney by blunt dissection. The kidney was first weighed and then cut into slices, thereby, emptying the renal pelvis completely, after which it was weighed again. Pelvic volume was calculated by subtraction and hydronephrosis was classified as mild, moderate or severe. To quantify these categories and compensate for differences in body weight we used the term hydronephrotic ratio, defined as pelvic volume as measured by weight divided by renal parenchymal weight. Normal kidneys have a hydronephrotic ratio of less than 0.1.12, 13 Kidneys with a hydronephrotic ratio of 0.1 to 0.3 were classified as having mild hydronephrosis (group 1), those with a ratio of between 0.3 and 1.0 were classified as moderate (group 2) and those with a ratio of above 1 were classified as severe (group 3). Controls were examined in exactly the same manner for comparison, including weight measurement of the kidneys. Calculations and statistics. The experimental and control groups were compared concerning body weight, kidney wet weight, degree of hydronephrosis and renal blood flow values before, during and after volume expansion. Renal blood flow changes during volume expansion were calculated as the
percent difference in the renal blood flow value in the middle of the expansion period and the pre-expansion value, which proved to be the most consistent pattern. All values are expressed as the mean plus or minus standard error of mean. The groups were compared using Student’s t test with p ⬍0.05 considered statistically significant. RESULTS
The experimental animals were in good condition and there were no differences in general characteristics among the 3 experimental groups (table 1). All animals in groups 1 and 2 were normotensive but those in group 3 were slightly hypertensive with a mean arterial blood pressure of 129 ⫾ 11 mm. Hg. Parenchymal weight correlated with body weight was normal in all groups. Renal blood flow during normovolemic conditions was similar in the group of experimental animals overall and in controls, and was within the normal range with a mean value of 6.4 ⫾ 0.7 and 6.7 ⫾ 0.6 ml. per minute, respectively. Regarding the separate experimental groups, the 10 animals in group 2 with moderate hydronephrosis had a higher blood flow level than the 7 in group 1 with mild hydronephrosis or controls, although the differences were not significant. On the other hand, the 3 group 3 animals with severe hydronephrosis had significantly lower renal blood flow than those in the other 2 groups or controls (table 2). During volume expansion there was a clear discrepancy in the appearance of the renal blood flow pattern in hydronephrotic and control kidneys. In the experimental group blood flow increased significantly in response to volume expansion. The blood flow rise generally began immediately after the start of volume expansion with a clearly visible onset and continued to rise for 10 to 15 minutes to reach a steady state with a maximum level in the middle of the volume expansion period. The controls had no reaction with a renal blood flow curve visibly identical to a continued baseline recording or slowly falling renal blood flow. Moreover, the renal blood flow increase was proportional to the degree of obstruction, insofar as the more severe the degree of obstruction, the greater the blood flow increase (see figure). In addition, the greater the degree of obstruction, the more sustained the renal blood flow reaction and the higher the blood flow level at the end of the experiment. Thus, animals with mild, moderate and severe hydronephrosis had a mean end renal blood flow that was 3%, 11% and 59% above the preexpansion value, respectively, whereas controls had no increase (table 2). DISCUSSION
In our study there was no reduction in the renal blood flow level compared to controls except in the most hydronephrotic kidneys. However, as a response to volume expansion, blood flow rose markedly in hydronephrotic kidneys but remained unchanged or decreased in controls. Furthermore, the more pronounced the degree of hydronephrosis, the greater the increase in blood flow. The experimental model used has been shown to produce consistent partial obstruction with moderate reduction in
TABLE 1. Body and kidney weight data No. Subjects Experimental groups overall 20 Controls 10 Experimental group: 1 7 2 10 3 3 No significant difference in mean body weight, left kidney * Versus controls and other experimental groups p ⬍0.05.
Mean Body Wt. ⫾ SEM
Mean Lt. Kidney Wet Wt. ⫾ SEM
Lt. Kidney Wet Wt./Body Wt.
Mean Hydronephrotic Ratio ⫾ SEM
274 ⫾ 11.5 300 ⫾ 17.3
1.51 ⫾ 0.08 1.48 ⫾ 0.08
0.0055 0.0049
0.59 ⫾ 0.12* 0.08 ⫾ 0.01
1.43 ⫾ 0.15 1.56 ⫾ 0.08 1.47 ⫾ 0.28 wet weight/body weight.
0.0057 0.0055 0.0057
0.19 ⫾ 0.02* 0.54 ⫾ 0.08* 1.69 ⫾ 0.28*
263 ⫾ 22.7 286 ⫾ 16.1 258 ⫾ 19.7 wet weight or left kidney
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RENAL BLOOD FLOW INCREASE DURING VOLUME EXPANSION IN HYDRONEPHROSIS TABLE 2. Renal blood flow data in hydronephrotic and control animals Mean Renal Blood Flow ⫾ SEM No. Subjects
Experimental group overall Controls Experimental group: 1 2 3 * Versus controls p ⬍0.05. † Versus controls and other experimental
Ml./Min. Pre-Expansion
Ml./Min. During Vol. Expansion
% Increase During Vol. Expansion
Ml./Min. End Level
20 10
6.4 ⫾ 0.6 6.7 ⫾ 0.5
7.1 ⫾ 0.6 6.7 ⫾ 0.6
13.9 ⫾ 3.9* ⫺3.4 ⫾ 1.3
7.3 ⫾ 0.7 6.6 ⫾ 0.6
7 10 3
6.1 ⫾ 0.5 8.1 ⫾ 0.8 2.1 ⫾ 0.1†
6.1 ⫾ 0.5 9.1 ⫾ 0.7† 3.7 ⫾ 0.4†
2.3 ⫾ 1.5† 12.9 ⫾ 2.8† 44.3 ⫾ 14.3†
6.3 ⫾ 0.5 8.9 ⫾ 0.9† 3.3 ⫾ 0.3†
groups p ⬍0.05.
Renal blood flow increase related to degree of hydronephrosis. ⌬RBF(VE)%, percent of renal blood flow increase during volume expansion. HNR, hydronephrotic ratio. Open circles represent 10 controls. Black circles represent 20 experimental animals.
kidney function.6, 7, 10 The mean degree of hydronephrosis in this study was the same as earlier. However, the degree of obstruction in group 3 was higher than generally noted previously. Therefore, it is not surprising that renal blood flow in group 3 kidneys was lower. This finding has been noted in several studies in which the degree of obstruction was more pronounced and the reduction in renal blood flow correlated well with the severity and duration of obstruction.3–5, 8, 14 On the other hand, the finding of normal or even above normal renal blood flow levels in the remainder of the animals was surprising. In earlier investigations at our laboratory we found that hydronephrotic kidneys showed a 35% reduction in the number of glomeruli after 3 weeks of partial obstruction.6 The single nephron glomerular filtration rate during baseline conditions was about 10% lower in hydronephrotic kidneys than in controls, in which filtration pressure levels were identical.7 Thus, the single nephron blood flow must be higher in hydronephrotic kidneys since the number of glomeruli is reduced by as much as 35%. Nevertheless, the single nephron glomerular filtration rate was lower in hydronephrotic kidneys. A reduction in the filtration coefficient must be assumed to be responsible for that result. Several groups have found that a reduction in the filtration coefficient is part of the adaptive mechanism to obstructive injury.14 –17 Others have used a strain of inbred Wistar rats with congenital unilateral hydronephrosis that is apparently obstructive with functional retardation on the affected side.16 –20 Kidney parenchymal weights were identical to those in controls but the glomerular filtration rate was lower by about 25%. Glomerular filtration rate values were restored to normal by the blockade of angiotensin II or thromboxane, or to even su-
pernormal values by a combination of the 2 substances. Detailed studies of the determinants of the single nephron glomerular filtration rate showed that the functional reduction in these kidneys was due to a decreased filtration coefficient. Chevalier et al studied the effects on neonatal obstruction combined with contralateral nephrectomy.21 Unlike animals with the other side intact, in which obstruction resulted in increased renal vascular resistance and reduced renal blood flow, uninephrectomy resulted in little or no decrease in renal blood flow, whereas the glomerular filtration rate was reduced by as much as 50%. A reduction in the filtration coefficient was supposed to be responsible for that effect. In our series a significant renal blood flow increase occurred during volume expansion at the same time during the experiment as the aforementioned changes in tubuloglomerular feedback mechanism characteristics were noted.7 In contrast to that in controls, the tubuloglomerular feedback mechanism in hydronephrotic animals was reset toward higher sensitivity and reactivity, and glomerular capillary pressure remained unchanged during volume expansion. As a result, the single nephron glomerular filtration rate in control kidneys was increased by 18% to 20% but only by 8% to 10% during volume expansion in hydronephrotic kidneys. Thromboxane blockade increased glomerular capillary pressure and the single nephron glomerular filtration rate during volume expansion to control values.10 Thus, the tubuloglomerular feedback mechanism response to volume expansion in hydronephrotic kidneys appears to occur by vasodilatation in contrast to that in normal kidneys. Interestingly the more severe the hydronephrosis, the more pronounced the relative blood flow increase. This finding may possibly be a diuretic force of greater magnitude than the lack of an increase in glomerular capillary pressure. Nevertheless, the single nephron glomerular filtration rate increased less in hydronephrotic kidneys during volume expansion in the aforementioned study,7 which indicates that the filtration coefficient was maintained at a low level or even further reduced during volume expansion. This hypothesis is further supported by the results of our study of partial bilateral ureteral obstruction.22 Kidneys were classified as least or most obstructed due to the degree of hydronephrosis and the tubuloglomerular feedback mechanism characteristics were examined by micropuncture experiments during normovolemia and volume expansion. During normovolemic conditions tubular free flow pressure was equal in each kidney. During volume expansion tubular free flow pressure increased significantly less in the most obstructed kidney. Few investigators have studied renal blood flow in partial obstruction during volume expansion. However, Josephsson et al submitted animals obstructed at weanling age to 3% volume expansion at ages 7 to 9 weeks using the same technique as described.23 These animals were hypertensive and possibly the degree of obstruction in that series was higher. Kidney weight was not measured. Renal blood flow and the glomerular filtration rate were markedly reduced during normovolemia but rose significantly to near normal values after volume expansion. This finding is in accordance with our results.
RENAL BLOOD FLOW INCREASE DURING VOLUME EXPANSION IN HYDRONEPHROSIS
However, others have addressed renal blood flow regulation in experimental partial obstruction. Chevalier and Peach found an angiotensin II dependent reduction in the number of filtering nephrons and consequently in renal blood flow in hydronephrotic kidneys.8 They submitted neonatal guinea pigs to partial obstruction for 3 weeks. Obstruction led to a significant reduction in kidney weight, renal blood flow and the number of perfused glomeruli. After the administration of enalapril to the animals during the obstruction period before the experiments renal blood flow increased from 50% to 80% of control values but the number of perfused glomeruli increased by only 26%, indicating that single nephron blood flow was also increased. Others have found that an angiotensin II dependent reduction of the single nephron blood flow was responsible with a reduced filtration coefficient for the reduced whole kidney glomerular filtration rate.15–19 Angiotensin II blockade led to an immediate increase in single nephron blood flow. An increased release of atrial natriuretic peptide occurs during volume expansion in rats.24 Atrial natriuretic peptide increases renal blood flow. It also increases glomerular capillary pressure due to efferent vasoconstriction and increases the filtration coefficient in the normal kidney.25, 26 As a result, diuresis is elevated. Similar effects of atrial natriuretic peptide in the split hydronephrotic kidney preparation were found by Endlich et al.27 Atrial natriuretic peptide induced pre-glomerular vasodilatation and post-glomerular vasoconstriction as well as an increase in glomerular blood flow in superficial and deep nephrons. Together these results imply that increased atrial natriuretic peptide activity is not responsible for the renal blood flow increase during volume expansion in our series since the increase would then have occurred similarly in the hydronephrotic and control kidneys. Furthermore, earlier a clear discrepancy was observed in the single nephron glomerular filtration rate pattern during volume expansion in hydronephrotic and control kidneys.7 The single nephron glomerular filtration rate was elevated mainly in the normal kidney during volume expansion. The current experiments may be interpreted. The capability of the partially obstructed kidney to respond to various stimuli with a blood flow increase is clear. The question of whether general vasodilatation occurs in response to volume expansion and/or whether there is redistribution of the renal blood flow was not decided according to our data. However, single nephron glomerular filtration rate characteristics during volume expansion in untreated hydronephrotic and thromboxane blockade animals strongly support vascular changes within each measured nephron. Thus, the reperfusion of resting nephrons alone does not seem to be responsible for the renal blood flow increase. A reduction in the filtration coefficient in hydronephrotic animals also seems likely. The influence of vasoactive substances on these phenomena was not examined in this study and remains to be established. Hypothetically there may be a general afferent vasodilatory response to volume expansion in normal and hydronephrotic kidneys that is offset by simultaneous efferent vasodilatation and reduction in the filtration coefficient in hydronephrotic kidney. Thus, the net result would be a low filtered load to protect the hydronephrotic kidney from potentially damaging high hydrostatic pressures. CONCLUSIONS
The severity and duration of ureteral obstruction and the age of the individual on its occurrence seem to be important for the outcome regarding kidney function. In this series partial obstruction was created in weanling rats and kidney function was studied in adulthood. Obstruction appears to have been significant and persistent at the time of the experiments. Baseline renal blood flow and kidney weight were normal compared to those in controls. A significant blood flow increase that was proportional to the degree of obstruction occurred as a result of volume expansion. This finding may be explained by a state of afferent and efferent vasodilatation combined with a reduction of the filtration coefficient.
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REFERENCES
1. Dworkin, L. D. and Brenner, B. M.: The renal circulations. In: The Kidney, 5th ed. Edited by B. M. Brenner and F. C. Rector. Philadelphia: W. B. Saunders, vol. 1, chapt. 6, p. 273, 1996 2. Wright, F. S.: Effects of urinary tract obstruction on glomerular filtration rate and renal blood flow. Semin Nephrol, 2: 5, 1982 3. Vaughan, E. D., Sweet, R. C. and Gillenwater, J. Y.: The renal haemodynamic response to chronic complete ureteral occlusion. Invest Urol, 8: 78, 1970 4. Leahy, A. L., Ryan, P. C., McEntee, G. M. et al: Renal injury and recovery in partial ureteric obstruction. J Urol, 142: 199, 1970 5. Taki, M., Goldsmith, D. I. and Spitzer, A.: Impact of age on effects of ureteral obstruction on renal function. Kidney Int, 24: 602, 1983 6. Stenberg, A., Olsen, L., Josephsson, S. et al: Partial ureteric obstruction in weanling rats. 1. Long-term effects on renal morphology. Scand J Urol Nephrol, 19: 139, 1985 7. Morsing, P., Stenberg, A., Muller-Suur, C. et al: Tubuloglomerular feedback in animals with unilateral partial ureteral occlusion. Kidney Int, 32: 212, 1987 8. Chevalier, R. L. and Peach, M. J.: Hemodynamic effects of enalapril on neonatal chronic partial obstruction. Kidney Int, 28: 891, 1985 9. Navar, L. G.: Integrating multiple paracrine regulators of renal microvascular dynamics. Am J Physiol, 274: 433, 1998 10. Morsing, P., Stenberg, A. and Persson, A. E. G.: Effect of thromboxane inhibition on tubuloglomerular feedback in hydronephrotic kidneys. Kidney Int, 36: 447, 1989 11. Ulm, A. H. and Miller, F.: An operation to produce experimental reversible hydronephrosis in dogs. J Urol, 88: 337, 1962 12. Morsing, P. and Persson, A. E. G.: Pelvic pressure and tubuloglomerular feedback in hydronephrosis. Renal Physiol Biochem, 13: 181, 1990 13. Stenberg, A., Olsen, L., Engstrand, U. et al: Pressure and flow measurements in the partially obstructed ureter of the rat. Scand J Urol Nephrol, 22: 279, 1988 14. Ichikawa, I. and Brenner, B. M.: Local intrarenal vasoconstrictorvasodilator interactions in mild partial ureteral obstruction. Am J Physiol, 236: F 131, 1979 15. Sheehan, S. J., Moran, K. T., Dowsett, D. J. et al: Renal haemodynamics and prostaglandin synthesis in partial unilateral ureteric obstruction. Urol Res, 22: 279, 1994 16. Vari, R. C., Boineau, F. G. and Lewy, J. E.: Angiotensin or thromboxane receptor antagonism in rats with congenital hydronephrosis. J Am Soc Nephrol, 3: 1522, 1992 17. Hanss, B. G., Lewy, J. E. and Vari, R. C.: Alterations in glomerular dynamics in congenital, unilateral hydronephrosis. Kidney Int, 46: 48, 1994 18. Boineau, F. G., Vari, R. C. and Lewy, J. E.: Reversible vasoconstriction in rats with congenital unilateral hydronephrosis. Pediatr Nephrol, 1: 498, 1987 19. Friedman, J., Hoyer, J. R., McCormick, B. et al: Congenital unilateral hydronephrosis in the rat. Kidney Int, 15: 567, 1979 20. Fichtner, J., Boineau, F. G., Lewy, J. E. et al: Congenital unilateral hydronephrosis in a rat model; continuous renal pelvic and bladder pressures. J Urol, 152: 652, 1994 21. Chevalier, R. L., Sturgill, C., Jones, C. E. et al: Morphologic correlates of renal growth arrest in neonatal partial ureteral obstruction. Pediatr Res, 21: 338, 1986 22. Morsing, P., Stenberg, A., Wåhlin, N. et al: Tubuloglomerular feedback in rats with chronic partial bilateral ureteral obstruction. Renal Physiol Biochem, 18: 27, 1995 23. Josephsson, S., Aperia, A., La¨nnergen, K. et al: Partial ureteric obstruction in the pubescent rat I. Long term effects on renal function. J Urol, 138: 414, 1987 24. Pettersson, A., Ricksten, S.-E., Towle, A. C. et al: Effect of blood volume expansion and sympathetic denervation on plasma levels of atrial natriuretic factor (ANF) in the rat. Acta Physiol Scand, 124: 309, 1985 25. Fried, T. A., Mc Coy, R. N., Osgood, R. W. et al: Effect of atriopeptin II on determinants of glomerular filtration rate in the in vitro perfused dog glomerulus. Am J Physiol, 250: 1119, 1986 26. Dunn, B. R., Ichikawa, I., Pfeffer, J. M. et al: Renal and systemic hemodynamic effects of synthetic atrial natriuretic peptide in the anesthetized rat. Circ Res, 59: 237, 1986 27. Endlich, K., Forssmann, W.-G. and Steinhausen, M.: Effects of urodilatin in the rat kidney: comparison with ANF and interaction with vasoactive substances. Kidney Int, 47: 1558, 1995
Vol. 165, 1696 –1699, May 2001 Printed in U.S.A.
RENAL BLOOD FLOW INCREASE DURING VOLUME EXPANSION IN HYDRONEPHROTIC RATS NILS WÅHLIN,* ARNE STENBERG
AND
A. ERIK G. PERSSON
From the Department of Pediatric Surgery, University Childrens Hospital and Department of Physiology, University of Uppsala, Uppsala, Sweden
ABSTRACT
Purpose: We assessed the renal blood flow pattern in experimental hydronephrosis during normal hydration and extracellular volume expansion. Materials and Methods: Partial obstruction of the left ureter was created in 3-week-old SpragueDawley rats by embedding the ureter in a psoas muscle groove. Moderate hydronephrosis without kidney weight reduction developed in all cases. The effects on renal hemodynamics were studied with real-time ultrasound flowmetry 3 weeks later during normal hydration and then during volume expansion. The degree of hydronephrosis was classified as mild, moderate or severe. Results: Under baseline conditions renal blood flow was normal in mild and moderate hydronephrosis but low in severe hydronephrosis. During volume expansion renal blood flow increased significantly in all experimental animals (mean 14%) compared to that in controls, which remained unaffected or decreased (mean ⫺3%). The flow increase was related to the degree of dilatation, which was 2% in mild, 13% in moderate and 44% in severe hydronephrosis when the groups were considered separately. Conclusions: A significant increase in renal blood flow proportional to the degree of hydronephrosis occurred as a result of volume expansion. This finding may be explained by a state of vasodilatation combined with a reduction in the filtration coefficient. KEY WORDS: kidney; renal circulation; hydronephrosis; ureteral obstruction; rats, Sprague-Dawley
The regulation of renal blood flow is intimately coupled to the glomerular filtration rate. The glomerular filtration rate depends on the surface area available for filtration, its permeability, the blood flow over it and the actual pressure force, net filtration pressure. Normally renal blood flow is automatically regulated at a constant level even under different circumstances.1 The renal blood flow pattern in the acute phase of and after complete urinary tract obstruction is well known.2, 3 The mechanisms involved in partial obstruction have been thought to be mainly of the same character but more subtle and slower in onset. The severity and duration of obstruction as well as the age of the individual at onset appear to be important to the outcome.4, 5 In previous experimental studies at our own laboratory rats underwent partial ureteral obstruction at age 3 weeks, leading to considerable hydronephrosis. Histologically moderate changes with a 35% reduction in the number of glomeruli were noted but there was no kidney weight loss.6 Physiologically hydronephrotic kidneys showed normal single nephron glomerular filtration rate and excretory behavior under baseline conditions. However, micropuncture studies during volume expansion showed major changes in filtration regulation and tubuloglomerular feedback mechanism characteristics.7 The tubuloglomerular feedback mechanism acts as a negative feedback control whereby the macula densa cells respond to the salt concentration of fluid in the distal tubule, normally by regulating the tone of the afferent arteriole and, thereby, filtration pressure of the glomerulus. The sensitivity and activity of the tubulo-glomerular feedback mechanism are modulated by several factors, such as extracellular volume conditions and, thus, interstitial pressure in the kidney as well as by various vasoactive substances, such as angiotensin II, thromboxane A2 and nitric oxide.8, 9
During volume expansion in the nonhydronephrotic control state great diuresis occurs. Afferent vasodilatation leads to elevated filtration pressure and, thereby, an increase in the glomerular filtration rate. In addition, the sensitivity of the tubuloglomerular feedback mechanism is reset to a minimum level that abolishes any vascular effect as a response to tubular signals and high filtration is maintained. On the other hand, in hydronephrotic kidneys tubuloglomerular feedback mechanism sensitivity is reset to a higher level and is strongly active during volume expansion. As a result, filtration pressure remains unchanged, and the single nephron glomerular filtration rate and urinary output are constant or only slightly increased. Thus, most of the fluid load is excreted by the contralateral intact kidney. These effects may be reversed by blocking thromboxane synthesis or receptors.7, 10 We determined the pattern of total renal blood flow in rats with partial ureteral obstruction and moderate hydronephrosis under baseline conditions, and during and after extracellular volume expansion.
MATERIALS AND METHODS
Creation of hydronephrosis. Unilateral partial ureteral obstruction was created in 3-week-old male Sprague-Dawley rats (Mo¨llegaard, Copenhagen, Denmark) using the method originally described by Ulm and Miller in dogs.11 The animals were anesthetized with 30 mg./kg. body weight methohexital administered intraperitoneally. The abdomen was opened through a midline incision and the left ureter was dissected free under microscopy. The underlying psoas muscle was split longitudinally to form an approximate 15 mm. groove and the ureter was placed into it. The muscle edges were adapted above the ureter with 2, 6-zero silk sutures and, thus, the ureter was embedded in a tunnel. The abdomen was then closed with interrupted 5-zero polyamid sutures. Sham operations in controls were performed in the same manner but without dissecting the ureter. After the
Accepted for publication November 7, 2000. Supported by grants from Her Royal Highness the Crown Princess Lovisas Fund for Scientific Research, Åke Wiberg Foundation, Gillbergska Foundation and Grant B98-14x-63522-27D. * Requests for reprints: Department of Pediatric Surgery, Section of Urology, University Children’s Hospital, S-751 85 Uppsala, Sweden. 1696
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RENAL BLOOD FLOW INCREASE DURING VOLUME EXPANSION IN HYDRONEPHROSIS
operation the animals were allowed to awaken beneath a heating lamp and were not returned to the cages until completely awake. They were then left to grow with free access to R2 food (Ewos, So¨derta¨lje, Sweden) and water. Experimental preparation. Experiments were done about 3 weeks later at a mean rat body weight of 286 gm. (range 235 to 350). The rats were anesthetized with 120 mg./kg. body weight Inactin (Byk Gulden, Konstanz, Germany) administered intraperitoneally and placed on a servoregulated heating pad to maintain body temperature at 37.5C. Tracheotomy was performed, the right femoral artery and vein were catheterized for blood pressure monitoring and infusion, and the rats were then placed on the right side. Baseline infusion was started of 0.5 ml. 0.9% NaCl/100 gm. body weight per hour. The left kidney was exposed through a subcostal flank incision and the renal vein was dissected free from the stalk with care taken to avoid nerve damage. The kidney was placed in a polymerized methyl methacrylate cup and gently fixed with cotton fiber and water soluble gel, preventing stalk tension. A real-time ultrasound recorder (Transonic Systems, Ithaca, New York) was used for blood flow measurements. A 1 mm. probe was attached to the renal vein and retained in position with a clamp holder. The space around the vessel and probe was carefully filled with inert Derma-Jel gel (Physio-Control Corporation, Redmond, Oregon) to establish optimal measurement conditions. The recorder was then connected to a Goertz Servogor 400 printer (ABB Goertz AG, Vienna, Austria), which continuously displayed blood pressure and renal blood flow values. After a 30-minute stabilizing period saline volume expansion at 5% ⫻ body weight per hour was begun and continued for 30 minutes. Thereafter the infusion was returned to the original baseline rate of 0.5 ml. per 100 gm. body weight per hour and recording was continued for another 30 minutes. The experiment was then completed by sacrificing the animal and the zero flow versus previous in vitro calibration was assessed with the probe in situ. Quantification of hydronephrosis. Pelvic volume was then determined by placing a ligature around the ureter, cutting the vessels and removing the kidney by blunt dissection. The kidney was first weighed and then cut into slices, thereby, emptying the renal pelvis completely, after which it was weighed again. Pelvic volume was calculated by subtraction and hydronephrosis was classified as mild, moderate or severe. To quantify these categories and compensate for differences in body weight we used the term hydronephrotic ratio, defined as pelvic volume as measured by weight divided by renal parenchymal weight. Normal kidneys have a hydronephrotic ratio of less than 0.1.12, 13 Kidneys with a hydronephrotic ratio of 0.1 to 0.3 were classified as having mild hydronephrosis (group 1), those with a ratio of between 0.3 and 1.0 were classified as moderate (group 2) and those with a ratio of above 1 were classified as severe (group 3). Controls were examined in exactly the same manner for comparison, including weight measurement of the kidneys. Calculations and statistics. The experimental and control groups were compared concerning body weight, kidney wet weight, degree of hydronephrosis and renal blood flow values before, during and after volume expansion. Renal blood flow changes during volume expansion were calculated as the
percent difference in the renal blood flow value in the middle of the expansion period and the pre-expansion value, which proved to be the most consistent pattern. All values are expressed as the mean plus or minus standard error of mean. The groups were compared using Student’s t test with p ⬍0.05 considered statistically significant. RESULTS
The experimental animals were in good condition and there were no differences in general characteristics among the 3 experimental groups (table 1). All animals in groups 1 and 2 were normotensive but those in group 3 were slightly hypertensive with a mean arterial blood pressure of 129 ⫾ 11 mm. Hg. Parenchymal weight correlated with body weight was normal in all groups. Renal blood flow during normovolemic conditions was similar in the group of experimental animals overall and in controls, and was within the normal range with a mean value of 6.4 ⫾ 0.7 and 6.7 ⫾ 0.6 ml. per minute, respectively. Regarding the separate experimental groups, the 10 animals in group 2 with moderate hydronephrosis had a higher blood flow level than the 7 in group 1 with mild hydronephrosis or controls, although the differences were not significant. On the other hand, the 3 group 3 animals with severe hydronephrosis had significantly lower renal blood flow than those in the other 2 groups or controls (table 2). During volume expansion there was a clear discrepancy in the appearance of the renal blood flow pattern in hydronephrotic and control kidneys. In the experimental group blood flow increased significantly in response to volume expansion. The blood flow rise generally began immediately after the start of volume expansion with a clearly visible onset and continued to rise for 10 to 15 minutes to reach a steady state with a maximum level in the middle of the volume expansion period. The controls had no reaction with a renal blood flow curve visibly identical to a continued baseline recording or slowly falling renal blood flow. Moreover, the renal blood flow increase was proportional to the degree of obstruction, insofar as the more severe the degree of obstruction, the greater the blood flow increase (see figure). In addition, the greater the degree of obstruction, the more sustained the renal blood flow reaction and the higher the blood flow level at the end of the experiment. Thus, animals with mild, moderate and severe hydronephrosis had a mean end renal blood flow that was 3%, 11% and 59% above the preexpansion value, respectively, whereas controls had no increase (table 2). DISCUSSION
In our study there was no reduction in the renal blood flow level compared to controls except in the most hydronephrotic kidneys. However, as a response to volume expansion, blood flow rose markedly in hydronephrotic kidneys but remained unchanged or decreased in controls. Furthermore, the more pronounced the degree of hydronephrosis, the greater the increase in blood flow. The experimental model used has been shown to produce consistent partial obstruction with moderate reduction in
TABLE 1. Body and kidney weight data No. Subjects Experimental groups overall 20 Controls 10 Experimental group: 1 7 2 10 3 3 No significant difference in mean body weight, left kidney * Versus controls and other experimental groups p ⬍0.05.
Mean Body Wt. ⫾ SEM
Mean Lt. Kidney Wet Wt. ⫾ SEM
Lt. Kidney Wet Wt./Body Wt.
Mean Hydronephrotic Ratio ⫾ SEM
274 ⫾ 11.5 300 ⫾ 17.3
1.51 ⫾ 0.08 1.48 ⫾ 0.08
0.0055 0.0049
0.59 ⫾ 0.12* 0.08 ⫾ 0.01
1.43 ⫾ 0.15 1.56 ⫾ 0.08 1.47 ⫾ 0.28 wet weight/body weight.
0.0057 0.0055 0.0057
0.19 ⫾ 0.02* 0.54 ⫾ 0.08* 1.69 ⫾ 0.28*
263 ⫾ 22.7 286 ⫾ 16.1 258 ⫾ 19.7 wet weight or left kidney
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RENAL BLOOD FLOW INCREASE DURING VOLUME EXPANSION IN HYDRONEPHROSIS TABLE 2. Renal blood flow data in hydronephrotic and control animals Mean Renal Blood Flow ⫾ SEM No. Subjects
Experimental group overall Controls Experimental group: 1 2 3 * Versus controls p ⬍0.05. † Versus controls and other experimental
Ml./Min. Pre-Expansion
Ml./Min. During Vol. Expansion
% Increase During Vol. Expansion
Ml./Min. End Level
20 10
6.4 ⫾ 0.6 6.7 ⫾ 0.5
7.1 ⫾ 0.6 6.7 ⫾ 0.6
13.9 ⫾ 3.9* ⫺3.4 ⫾ 1.3
7.3 ⫾ 0.7 6.6 ⫾ 0.6
7 10 3
6.1 ⫾ 0.5 8.1 ⫾ 0.8 2.1 ⫾ 0.1†
6.1 ⫾ 0.5 9.1 ⫾ 0.7† 3.7 ⫾ 0.4†
2.3 ⫾ 1.5† 12.9 ⫾ 2.8† 44.3 ⫾ 14.3†
6.3 ⫾ 0.5 8.9 ⫾ 0.9† 3.3 ⫾ 0.3†
groups p ⬍0.05.
Renal blood flow increase related to degree of hydronephrosis. ⌬RBF(VE)%, percent of renal blood flow increase during volume expansion. HNR, hydronephrotic ratio. Open circles represent 10 controls. Black circles represent 20 experimental animals.
kidney function.6, 7, 10 The mean degree of hydronephrosis in this study was the same as earlier. However, the degree of obstruction in group 3 was higher than generally noted previously. Therefore, it is not surprising that renal blood flow in group 3 kidneys was lower. This finding has been noted in several studies in which the degree of obstruction was more pronounced and the reduction in renal blood flow correlated well with the severity and duration of obstruction.3–5, 8, 14 On the other hand, the finding of normal or even above normal renal blood flow levels in the remainder of the animals was surprising. In earlier investigations at our laboratory we found that hydronephrotic kidneys showed a 35% reduction in the number of glomeruli after 3 weeks of partial obstruction.6 The single nephron glomerular filtration rate during baseline conditions was about 10% lower in hydronephrotic kidneys than in controls, in which filtration pressure levels were identical.7 Thus, the single nephron blood flow must be higher in hydronephrotic kidneys since the number of glomeruli is reduced by as much as 35%. Nevertheless, the single nephron glomerular filtration rate was lower in hydronephrotic kidneys. A reduction in the filtration coefficient must be assumed to be responsible for that result. Several groups have found that a reduction in the filtration coefficient is part of the adaptive mechanism to obstructive injury.14 –17 Others have used a strain of inbred Wistar rats with congenital unilateral hydronephrosis that is apparently obstructive with functional retardation on the affected side.16 –20 Kidney parenchymal weights were identical to those in controls but the glomerular filtration rate was lower by about 25%. Glomerular filtration rate values were restored to normal by the blockade of angiotensin II or thromboxane, or to even su-
pernormal values by a combination of the 2 substances. Detailed studies of the determinants of the single nephron glomerular filtration rate showed that the functional reduction in these kidneys was due to a decreased filtration coefficient. Chevalier et al studied the effects on neonatal obstruction combined with contralateral nephrectomy.21 Unlike animals with the other side intact, in which obstruction resulted in increased renal vascular resistance and reduced renal blood flow, uninephrectomy resulted in little or no decrease in renal blood flow, whereas the glomerular filtration rate was reduced by as much as 50%. A reduction in the filtration coefficient was supposed to be responsible for that effect. In our series a significant renal blood flow increase occurred during volume expansion at the same time during the experiment as the aforementioned changes in tubuloglomerular feedback mechanism characteristics were noted.7 In contrast to that in controls, the tubuloglomerular feedback mechanism in hydronephrotic animals was reset toward higher sensitivity and reactivity, and glomerular capillary pressure remained unchanged during volume expansion. As a result, the single nephron glomerular filtration rate in control kidneys was increased by 18% to 20% but only by 8% to 10% during volume expansion in hydronephrotic kidneys. Thromboxane blockade increased glomerular capillary pressure and the single nephron glomerular filtration rate during volume expansion to control values.10 Thus, the tubuloglomerular feedback mechanism response to volume expansion in hydronephrotic kidneys appears to occur by vasodilatation in contrast to that in normal kidneys. Interestingly the more severe the hydronephrosis, the more pronounced the relative blood flow increase. This finding may possibly be a diuretic force of greater magnitude than the lack of an increase in glomerular capillary pressure. Nevertheless, the single nephron glomerular filtration rate increased less in hydronephrotic kidneys during volume expansion in the aforementioned study,7 which indicates that the filtration coefficient was maintained at a low level or even further reduced during volume expansion. This hypothesis is further supported by the results of our study of partial bilateral ureteral obstruction.22 Kidneys were classified as least or most obstructed due to the degree of hydronephrosis and the tubuloglomerular feedback mechanism characteristics were examined by micropuncture experiments during normovolemia and volume expansion. During normovolemic conditions tubular free flow pressure was equal in each kidney. During volume expansion tubular free flow pressure increased significantly less in the most obstructed kidney. Few investigators have studied renal blood flow in partial obstruction during volume expansion. However, Josephsson et al submitted animals obstructed at weanling age to 3% volume expansion at ages 7 to 9 weeks using the same technique as described.23 These animals were hypertensive and possibly the degree of obstruction in that series was higher. Kidney weight was not measured. Renal blood flow and the glomerular filtration rate were markedly reduced during normovolemia but rose significantly to near normal values after volume expansion. This finding is in accordance with our results.
RENAL BLOOD FLOW INCREASE DURING VOLUME EXPANSION IN HYDRONEPHROSIS
However, others have addressed renal blood flow regulation in experimental partial obstruction. Chevalier and Peach found an angiotensin II dependent reduction in the number of filtering nephrons and consequently in renal blood flow in hydronephrotic kidneys.8 They submitted neonatal guinea pigs to partial obstruction for 3 weeks. Obstruction led to a significant reduction in kidney weight, renal blood flow and the number of perfused glomeruli. After the administration of enalapril to the animals during the obstruction period before the experiments renal blood flow increased from 50% to 80% of control values but the number of perfused glomeruli increased by only 26%, indicating that single nephron blood flow was also increased. Others have found that an angiotensin II dependent reduction of the single nephron blood flow was responsible with a reduced filtration coefficient for the reduced whole kidney glomerular filtration rate.15–19 Angiotensin II blockade led to an immediate increase in single nephron blood flow. An increased release of atrial natriuretic peptide occurs during volume expansion in rats.24 Atrial natriuretic peptide increases renal blood flow. It also increases glomerular capillary pressure due to efferent vasoconstriction and increases the filtration coefficient in the normal kidney.25, 26 As a result, diuresis is elevated. Similar effects of atrial natriuretic peptide in the split hydronephrotic kidney preparation were found by Endlich et al.27 Atrial natriuretic peptide induced pre-glomerular vasodilatation and post-glomerular vasoconstriction as well as an increase in glomerular blood flow in superficial and deep nephrons. Together these results imply that increased atrial natriuretic peptide activity is not responsible for the renal blood flow increase during volume expansion in our series since the increase would then have occurred similarly in the hydronephrotic and control kidneys. Furthermore, earlier a clear discrepancy was observed in the single nephron glomerular filtration rate pattern during volume expansion in hydronephrotic and control kidneys.7 The single nephron glomerular filtration rate was elevated mainly in the normal kidney during volume expansion. The current experiments may be interpreted. The capability of the partially obstructed kidney to respond to various stimuli with a blood flow increase is clear. The question of whether general vasodilatation occurs in response to volume expansion and/or whether there is redistribution of the renal blood flow was not decided according to our data. However, single nephron glomerular filtration rate characteristics during volume expansion in untreated hydronephrotic and thromboxane blockade animals strongly support vascular changes within each measured nephron. Thus, the reperfusion of resting nephrons alone does not seem to be responsible for the renal blood flow increase. A reduction in the filtration coefficient in hydronephrotic animals also seems likely. The influence of vasoactive substances on these phenomena was not examined in this study and remains to be established. Hypothetically there may be a general afferent vasodilatory response to volume expansion in normal and hydronephrotic kidneys that is offset by simultaneous efferent vasodilatation and reduction in the filtration coefficient in hydronephrotic kidney. Thus, the net result would be a low filtered load to protect the hydronephrotic kidney from potentially damaging high hydrostatic pressures. CONCLUSIONS
The severity and duration of ureteral obstruction and the age of the individual on its occurrence seem to be important for the outcome regarding kidney function. In this series partial obstruction was created in weanling rats and kidney function was studied in adulthood. Obstruction appears to have been significant and persistent at the time of the experiments. Baseline renal blood flow and kidney weight were normal compared to those in controls. A significant blood flow increase that was proportional to the degree of obstruction occurred as a result of volume expansion. This finding may be explained by a state of afferent and efferent vasodilatation combined with a reduction of the filtration coefficient.
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REFERENCES
1. Dworkin, L. D. and Brenner, B. M.: The renal circulations. In: The Kidney, 5th ed. Edited by B. M. Brenner and F. C. Rector. Philadelphia: W. B. Saunders, vol. 1, chapt. 6, p. 273, 1996 2. Wright, F. S.: Effects of urinary tract obstruction on glomerular filtration rate and renal blood flow. Semin Nephrol, 2: 5, 1982 3. Vaughan, E. D., Sweet, R. C. and Gillenwater, J. Y.: The renal haemodynamic response to chronic complete ureteral occlusion. Invest Urol, 8: 78, 1970 4. Leahy, A. L., Ryan, P. C., McEntee, G. M. et al: Renal injury and recovery in partial ureteric obstruction. J Urol, 142: 199, 1970 5. Taki, M., Goldsmith, D. I. and Spitzer, A.: Impact of age on effects of ureteral obstruction on renal function. Kidney Int, 24: 602, 1983 6. Stenberg, A., Olsen, L., Josephsson, S. et al: Partial ureteric obstruction in weanling rats. 1. Long-term effects on renal morphology. Scand J Urol Nephrol, 19: 139, 1985 7. Morsing, P., Stenberg, A., Muller-Suur, C. et al: Tubuloglomerular feedback in animals with unilateral partial ureteral occlusion. Kidney Int, 32: 212, 1987 8. Chevalier, R. L. and Peach, M. J.: Hemodynamic effects of enalapril on neonatal chronic partial obstruction. Kidney Int, 28: 891, 1985 9. Navar, L. G.: Integrating multiple paracrine regulators of renal microvascular dynamics. Am J Physiol, 274: 433, 1998 10. Morsing, P., Stenberg, A. and Persson, A. E. G.: Effect of thromboxane inhibition on tubuloglomerular feedback in hydronephrotic kidneys. Kidney Int, 36: 447, 1989 11. Ulm, A. H. and Miller, F.: An operation to produce experimental reversible hydronephrosis in dogs. J Urol, 88: 337, 1962 12. Morsing, P. and Persson, A. E. G.: Pelvic pressure and tubuloglomerular feedback in hydronephrosis. Renal Physiol Biochem, 13: 181, 1990 13. Stenberg, A., Olsen, L., Engstrand, U. et al: Pressure and flow measurements in the partially obstructed ureter of the rat. Scand J Urol Nephrol, 22: 279, 1988 14. Ichikawa, I. and Brenner, B. M.: Local intrarenal vasoconstrictorvasodilator interactions in mild partial ureteral obstruction. Am J Physiol, 236: F 131, 1979 15. Sheehan, S. J., Moran, K. T., Dowsett, D. J. et al: Renal haemodynamics and prostaglandin synthesis in partial unilateral ureteric obstruction. Urol Res, 22: 279, 1994 16. Vari, R. C., Boineau, F. G. and Lewy, J. E.: Angiotensin or thromboxane receptor antagonism in rats with congenital hydronephrosis. J Am Soc Nephrol, 3: 1522, 1992 17. Hanss, B. G., Lewy, J. E. and Vari, R. C.: Alterations in glomerular dynamics in congenital, unilateral hydronephrosis. Kidney Int, 46: 48, 1994 18. Boineau, F. G., Vari, R. C. and Lewy, J. E.: Reversible vasoconstriction in rats with congenital unilateral hydronephrosis. Pediatr Nephrol, 1: 498, 1987 19. Friedman, J., Hoyer, J. R., McCormick, B. et al: Congenital unilateral hydronephrosis in the rat. Kidney Int, 15: 567, 1979 20. Fichtner, J., Boineau, F. G., Lewy, J. E. et al: Congenital unilateral hydronephrosis in a rat model; continuous renal pelvic and bladder pressures. J Urol, 152: 652, 1994 21. Chevalier, R. L., Sturgill, C., Jones, C. E. et al: Morphologic correlates of renal growth arrest in neonatal partial ureteral obstruction. Pediatr Res, 21: 338, 1986 22. Morsing, P., Stenberg, A., Wåhlin, N. et al: Tubuloglomerular feedback in rats with chronic partial bilateral ureteral obstruction. Renal Physiol Biochem, 18: 27, 1995 23. Josephsson, S., Aperia, A., La¨nnergen, K. et al: Partial ureteric obstruction in the pubescent rat I. Long term effects on renal function. J Urol, 138: 414, 1987 24. Pettersson, A., Ricksten, S.-E., Towle, A. C. et al: Effect of blood volume expansion and sympathetic denervation on plasma levels of atrial natriuretic factor (ANF) in the rat. Acta Physiol Scand, 124: 309, 1985 25. Fried, T. A., Mc Coy, R. N., Osgood, R. W. et al: Effect of atriopeptin II on determinants of glomerular filtration rate in the in vitro perfused dog glomerulus. Am J Physiol, 250: 1119, 1986 26. Dunn, B. R., Ichikawa, I., Pfeffer, J. M. et al: Renal and systemic hemodynamic effects of synthetic atrial natriuretic peptide in the anesthetized rat. Circ Res, 59: 237, 1986 27. Endlich, K., Forssmann, W.-G. and Steinhausen, M.: Effects of urodilatin in the rat kidney: comparison with ANF and interaction with vasoactive substances. Kidney Int, 47: 1558, 1995