Full text of DOI:10.1016/S0022-5347(05)67594-5

0022-5347/00/1636-1904/0
®
THE JOURNAL OF UROLOGY
Vol. 163, 1904–1907, June 2000
Copyright © 2000 by AMERICAN UROLOGICAL ASSOCIATION, INC.®
Printed in U.S.A.
HIGH URINARY FLOW ACCELERATES RENAL INJURY IN YOUNG RATS
WITH PARTIAL UNILATERAL URETERAL OBSTRUCTION
HIEP-THIEU NGUYEN, HSI-YANG WU, LAURENCE S. BASKIN AND BARRY A. KOGAN
From the Department of Urology, University of California, San Francisco, San Francisco, California, and Division of Urology, Albany
Medical College, Albany, New York
ABSTRACT
Purpose: We studied the effect of dietary manipulation and high urine flow on neonatal partial
ureteral obstruction in a weanling rat preparation.
Materials and Methods: A total of 40, 3-week old Sprague-Dawley rats underwent unilateral
ureteral obstruction by burial of the right ureter in the psoas muscle and 13 underwent sham
operation. Low, high and normal salt, and high sucrose diets were administered for 2 months.
The glomerular filtration rate of each kidney was measured by iothalamate clearance. Intrapelvic
pressure and renal blood flow were measured before and after acute volume loading.
Results: Fluid intake and urine output were 8 to 10-fold greater in animals on high salt and
high sucrose diets compared to those in rats on normal and low salt diets. Hydronephrosis was
observed only in rats with partial obstruction and high urine flow. No difference in renal weight
was noted. Relative glomerular filtration rate of the partially obstructed kidney was maintained
when urine flow was normal but decreased significantly with high urine flow. Total glomerular
filtration rate also decreased with high urine flow. Intrapelvic pressure was elevated signifi-
cantly at baseline in partially obstructed kidneys with high urine flow. All kidneys with partial
obstruction had significantly increased intrapelvic pressure with volume loading. Renal blood
flow was not significantly decreased in rats with high urine flow.
Conclusions: Chronic high urine flow causes loss of renal function in partially obstructed
weanling rat kidneys. Research should be done to determine whether human infants with
hydronephrosis and partial ureteral obstruction would benefit from the prevention of increased
fluid and salt intake.
KEY WORDS: ureter; salts; rats, Sprague-Dawley; hydronephrosis; kidney
The proper management of hydronephrosis detected pre- salt diet accelerates renal injury in partially obstructed kid-
natally is controversial because to our knowledge it is not neys and a low salt diet protects against renal injury. Results
possible currently to predict whether hydronephrosis would would be useful clinically for advising parents on the proper
stabilize, resolve spontaneously or progress to require surgi- diet for their child with hydronephrosis who is under obser-
cal intervention. A critical issue is whether hydronephrosis vation, and would also explain why the condition of some
indicates clinically significant obstruction.1 In the majority of patients with hydronephrosis improves and that of others
cases hydronephrosis resolves or stabilizes without signifi- deteriorates.
cant complications.2 Although Peters took the approach that
all patients with hydronephrosis should be offered early re-
MATERIALS AND METHODS
pair to prevent acidification or concentrating defects that are
not readily measured by renal scanning,3 Koff and Campbell
We used 3-week-old, newly weaned Sprague-Dawley rats
pointed out that relatively few with unilateral partial ob- for this experiment. Housing, diet and surgical procedures
struction require surgery when they are properly observed.4 were approved by the Committee on Animal Research at the
Koff et al proposed 2 types of partial ureteropelvic junction University of California, San Francisco. A total of 40 rats
obstruction, namely intrinsic and extrinsic.5 In the intrinsic underwent exposure of the right ureter, followed by burial of
type obstruction is fixed and as urine flow increases, renal the ureter in the psoas muscle according to the technique of
pelvic pressure increases in a linear fashion. In the extrinsic Ulm and Miller.6 This maneuver created partial unilateral
type the degree of obstruction increases with elevated urine ureteral obstruction. Another 13 rats underwent sham oper-
flow. Therefore, flow decreases as renal pelvic pressure in- ation with exposure of the right ureter without burial in the
creases. In either case the prevention of high urine flow psoas muscle. Three salt diets were administered, including
would prevent the onset of elevated intrapelvic pressure and low with 0.07, high with 0.9 and normal with 0.4 gm. sodium
progressive hydronephrosis.
chloride per 100 cc.7 The latter group served as a control.
Unfortunately to our knowledge no studies have evaluated Chronic high urine output that was not due to salt intake was
the effect of diet on urine flow and hydronephrosis. Since modeled by feeding rats a high sucrose solution of 1 gm./100
most patients with asymptomatic, prenatally detected hydro- cc, resulting in a 7-fold increase in urine output.
nephrosis initially undergo observation, we determined
Rats were placed into metabolic cages every 2 weeks to
whether high urine flow has a role in renal damage due to monitor 24-hour intake, urine output and weight. They were
partial ureteral obstruction. We hypothesized that a chronic anesthetized with intraperitoneal thiopental 2 months after
increase in urine flow increases hydronephrotic injury, a high surgery. The glomerular filtration rate, renal blood flow and
intrapelvic pressure were measured before sacrifice.
The glomerular filtration rate of each kidney was meas-
ured by iothalamate clearance. The jugular vein was cannu-
Accepted for publication January 28, 2000.
Supported by grants from the National Kidney Foundation of
Northern California.
1904

EFFECT OF DIET ON PARTIAL URETERAL OBSTRUCTION
1905
lated with polyethylene tubing and iothalamate was infused
at a rate of 12 ng. per minute for 1 hour. The corresponding
femoral vein and ureter were then cannulated to obtain se-
rum and urine samples. When the kidney was experimen-
tally obstructed, the ureter was cannulated below the level of
obstruction. Renal blood flow was measured using an ultra-
sonic Doppler flow transducer. Intrapelvic pressure at base-
line was determined by cannulating the ureter with polyeth-
ylene tubing. Saline (5% of body weight) was infused for
acute volume expansion. After volume loading intrapelvic
pressure was measured. In addition, the kidneys were har-
vested and wet weight was determined.
The glomerular filtration rate was calculated after high
pressure liquid chromatography assay of serum and urine
iothalamate.8 Data are presented as the mean plus or minus
standard deviation. Statistical analysis was done using se-
quential Student’s t tests for each pair.
FIG. 2. Total glomerular filtration rate (GFR) in sham operated
versus partially obstructed groups. In high urine flow diets partially
obstructed rats had significantly lower total glomerular filtration
rate than sham operated rats. Asterisk indicates p Ͻ0.05 versus
sham operated group.
RESULTS
High salt and high sucrose intake individually and to-
gether induced an 8 to 10-fold increase in urine flow. There
was no difference in urine output in the groups fed high salt,
high sucrose or high salt and high sucrose diets. High urine
flow resulted in marked dilatation of the renal pelvis of the
partially obstructed kidney, compared to only mild dilatation
of the renal pelvis when normal urine flow was maintained.
No difference in sham operated or obstructed renal weights
was noted in the normal and high urine flow groups.
The obstructed kidney contributed significantly less to glo-
merular filtration rate in all groups with high urine flow (fig.
1). In addition, the total rate was significantly lower in rats
with unilateral obstruction and high urine flow than in those
on a normal or low salt diet (fig. 2). Baseline intrapelvic
pressure was significantly higher in the partially obstructed
kidneys of animals with high urine flow than in sham oper-
ated and partially obstructed kidneys in rats on a low or
normal salt diet (fig. 3). The change in intrapelvic pressure
after acute hydration was significantly increased in the ob-
structed kidneys in all groups but rats on a high salt intake
continued to have greater intrapelvic pressure (fig. 4). Al-
though there was a trend toward higher renal blood flow
after acute volume loading in partially obstructed kidneys,
this difference was not statistically significant compared to
that in sham operated kidneys. There were no statistically
significant changes in absolute renal blood flow in any group.
FIG. 3. Baseline renal pelvic pressure. Partial obstruction was
created in all but sham operated rats yet baseline renal pelvic pres-
sure significantly increased only when urine flow was increased by
dietary manipulation. Asterisk indicates p Ͻ0.05 versus sham oper-
ated group.
FIG. 4. Renal pelvic pressure significantly increased in all par-
tially obstructed kidneys regardless of diet, confirming partial ob-
struction. Asterisk indicates p Ͻ0.05 versus sham operated group.
DISCUSSION
Although many studies have been performed on the effect
FIG. 1. Relative glomerular filtration rate (GFR) in sham oper- of high versus low salt diet for managing hypertension or
ated, partially obstructed and contralateral kidneys. In high urine
proteinuria, to our knowledge none has evaluated the effect
flow diets of high salt, high sucrose, high salt and high sucrose
of a high versus a low salt diet in the setting of obstruction or
partially obstructed kidney contributed significantly less and con-
high urine flow only without salt loading. We report that in
young rats with partial unilateral ureteral obstruction
tralateral kidney contributed significantly more than sham operated
kidney. Asterisk indicates p Ͻ0.05 versus sham operated group.

1906
EFFECT OF DIET ON PARTIAL URETERAL OBSTRUCTION
chronic high urine flow secondary to dietary manipulation ble for elevated pressure. One would guess that chronic high
may cause renal injury. In contrast, in our preparation of 2 flow prevents the equilibration in renal pelvic size and com-
months of partial obstruction the relative and total glomer- pliance that may otherwise occur. All partially obstructed
ular filtration rates were maintained, provided that urine kidneys had a significant increase in intrapelvic pressure
flow was normal. Similarly although baseline renal pelvic after acute hydration regardless of diet, confirming that true
pressure equilibrated by 2 months after unilateral obstruc- obstruction was created.
tion under normal circumstances, baseline renal pelvic pres-
Renal blood flow was maintained in all dietary groups and
sure in the animals with high urine flow remained elevated. was similar in the sham operated, obstructed and contralat-
High salt and high sucrose diets increased fluid intake and eral kidneys. Renal blood flow was increased in obstructed
urine output 8 to 10-fold. It appears that the increased salt high flow kidneys, although this change was not statistically
and sucrose diets caused an obligatory increase in fluid in- significant. As we showed previously in fetal lambs, partial
take, as shown by Walker and Olson.7 However, rats on a low ureteral obstruction does not necessarily lead to a decrease in
salt diet did not have decreased fluid intake or urine output renal blood flow, although that was the classic finding
to below normal levels and they also maintained similar in complete ureteral obstruction models.15 The effect of par-
relative renal weight. These data raise the question of tial unilateral ureteral obstruction on renal blood flow de-
whether the harmful effect of a high salt diet is due to salt or pends on the animal age, species, severity of partial obstruc-
urine volume. Is there truly a salt sensitive effect due to tion and normal renal innervation. Josephson noted a 12%
having fewer nephrons, as proposed by Brenner et al?9 Our decrease in renal blood flow in neonatal rats.10 Chevalier and
high sucrose group allowed us to discriminate the effect Kaiser observed elevated renal blood flow in neonatal guinea
of high urine flow only from that of high salt intake. No pigs with mild partial unilateral ureteral obstruction and
difference was noted in the high salt and high sucrose groups, decreased renal blood flow only in severe partial unilateral
implying that it is primarily increased flow that damaged the ureteral obstruction.16 The effect on renal blood flow appears
kidney in our preparation.
to be more severe when obstruction occurs earlier in life.17 An
The relative glomerular filtration rate in obstructed kid- explanation is based on the various activities of the renin-
neys did not decrease in the low salt group but decreased 20% angiotensin system in young versus adult rats.18 Chevalier
in the normal salt group, which was not statistically different and Thornhill also showed that de-innervation prevented
from that in the sham operated group. In the high urine flow contralateral vasodilatation after ipsilateral obstruction.19
groups a 60% to 80% loss of absolute glomerular filtration We do not believe that renal de-innervation is a confounding
rate was noted in obstructed kidneys with a compensatory factor in our preparation since the ureter was buried away
increase of 20% to 30% in the absolute rate in contralateral from the hilar region. Irrespective of the cause we believe
kidneys, resulting in an overall 20% loss of the total glomer- that our data show that ischemia is not the cause of de-
ular filtration rate. Josephson et al reported a 10% decrease creased renal function.
in the relative glomerular filtration rate in neonatal rats10
Studies of the effect of high versus low salt diets have
and up to a 30% decrease in the relative rate in 6-week-old primarily used a preparation of renal hypertrophy driven by
rats with partial unilateral ureteral obstruction.11 However, contralateral or five-sixths nephrectomy consisting of con-
Claesson et al believed that parenchymal atrophy did not tralateral nephrectomy and ischemia of two-thirds of the
progress after the initial 3 weeks.12 Stenberg et al used a remnant kidney. Daniels and Hostetter showed in a five-
model of partial unilateral ureteral obstruction in weanling sixths nephrectomy model that serum creatinine was the
rats and observed a 50% decrease in the relative glomerular same in rats fed a low or normal salt diet but those on a low
filtration rate after 1 year of partial obstruction with a con- salt diet had less proteinuria despite a high renin level.20
tralateral 16% increase.13 The marked difference in relative Benstein et al observed in spontaneously hypertensive rats
glomerular filtration rates was explained by postulating that that underwent nephrectomy that a low salt diet resulted in
weanling rats are more susceptible to damage from obstruc- less proteinuria and glomerulosclerosis, presumably by pre-
tion compared to neonatal or 6-week-old rats. Our findings venting hypertrophy.21 This effect was believed to be inde-
show more loss of the glomerular filtration rate in obstructed pendent of the renin level because micropuncture studies
kidneys than in any other study. This difference is likely due showed similarly elevated glomerular pressure despite a nor-
to additional damage caused by high urine flow and high mal or low salt diet, or hydrochlorothiazide administration.
renal pelvic pressure. A low salt diet may be an advantage for Also, Lax et al reported that rats with five-sixths nephrec-
preserving the relative glomerular filtration rate, whereas a tomy had stable glomerular size when maintained on a low
normal salt diet resulted in a slight decrease in the relative salt diet, whereas glomerular size increased markedly when
rate that was not statistically significant, as in other studies. they were placed on a high salt diet.22
The overall glomerular filtration rate was maintained in
These 3 studies suggest that salt restriction prevents pro-
rats with normal but decreased by 20% in those with high teinuria in the setting of renal hypertrophy by limiting the
urine flow. Josephson et al reported no overall rate decrease degree of compensatory growth, and the renin level and glo-
in newborn and 6-week-old rats that underwent partial uni- merular pressure are not affected by dietary salt. To make
lateral ureteral obstruction using the same technique.10, 11 these data applicable to the partial obstruction model one
Stenberg et al also noted no overall decrease in the glomer- must ask whether limiting salt intake has the same effect
ular filtration rate in weanling rats that underwent burial of when there is no stimulus for hypertrophy or when patholog-
the ureter in the psoas muscle.13 Similarly Chevalier ob- ical changes are due to apoptosis.23 In fact, Chevalier et al
served no decrease in the glomerular filtration rate after mild suggested that limiting salt intake may worsen apoptotic loss
partial unilateral ureteral obstruction was created in neona- in adult animals with complete ureteral obstruction.24 Be-
tal guinea pigs.14 Our results confirm those of previously cause infants require a positive sodium balance for somatic
published studies, illustrating that the overall glomerular growth, it is possible that a low salt diet may limit renal
filtration rate is maintained in partial unilateral ureteral growth.25 It is difficult to draw conclusions on glomerular
obstruction when urine flow is normal. Loss of the overall pressure in the partial obstruction model. Therefore, based
glomerular filtration rate in rats with high urine flow is on previous studies it is not clear whether limiting salt in-
likely due in some way to elevated intrapelvic pressure.
take in the setting of partial obstruction would be beneficial
The fact that rats with normal urine flow maintain normal by maintaining low urine flow and decreasing renal pelvic
baseline intrapelvic pressure while those with elevated urine pressure, or detrimental by preventing appropriate renal
flow have elevated pressure implies that elevated flow, irre- growth or promoting apoptosis in an obstructed kidney.
spective of whether it is salt or sucrose induced, is responsi-
Since we demonstrated the benefits of maintaining normal

EFFECT OF DIET ON PARTIAL URETERAL OBSTRUCTION
1907
urine flow in young rats with partial unilateral ureteral
obstruction, we wondered about the clinical correlates. In-
fants generally ingest fluid in excess of their nutritional
requirement but their urine concentrates only to 900 to 1,100
mOsm./l. The renal solute load is calculated based on elec-
trolytes and protein breakdown products in a given diet,
which is a more accurate way of estimating what the kidney
processes rather than osmolality only.26 Human breast milk
has a lower solute load than milk or soy based formulas and
it is much more dilute than whole milk. Therefore, the min-
imum volume that an infant must consume to excrete the
solute load would likely be the least when human breast milk
is given. Therefore, it is theoretically possible that breast fed
patients with prenatal hydronephrosis have less postnatal
hydronephrosis than those given formula. Between ages 1
and 4 months breast fed infants gain 2.5 gm./kg. daily com-
pared to those fed formula, who gain 3.5 gm./kg. daily. This
difference is not maintained indefinitely and as the child
grows it may reflect the importance of other factors, such as
the introduction of solid foods.27 Whole and evaporated milk
may cause hypernatremia in infants due to the high solute
load and they should be avoided even when a child has
normal kidneys. Muchant et al suggested that blunted natri-
uresis and diuresis with a more activated renin-angiotensin
system in neonates may be a developmental adaptation to
breast milk, which is more dilute.25
5. Koff, S. A., Hayden, L. J., Cirulli, C. et al: Pathophysiology of
ureteropelvic junction obstruction: experimental and clinical
observations. J Urol, part 2, 136: 336, 1986
6. Ulm, A. H. and Miller, F.: An operation to produce experimental
reversible hydronephrosis in dogs. J Urol, 88: 337, 1962
7. Walker, R. L. and Olson, M. E.: Renal function in the laboratory
rat: a student exercise. Am J Physiol, 268: S49, 1995
8. Guesry, P., Kaufman, L., Orloff, S. et al: Measurement of glo-
merular filtration rate by fluorescent excitation of non-
radioactive meglumine iothalamate. Clin Nephrol, 3: 134,
1975
9. Brenner, B. M., Garcia, D. L. and Anderson, S.: Glomeruli and
blood pressure. Less of one, more the other? Am J Hypertens,
1: 335, 1988
10. Josephson, S.: Experimental obstructive hydronephrosis in new-
born rats. III. Long-term effects on renal function. J Urol, 129:
396, 1983
11. Josephson, S., Aperia, A., La¨nnergren, K. et al: Partial ureteric
obstruction in the pubescent rat. I. Long-term effects on renal
function. J Urol, 138: 414, 1987
12. Claesson, G., Josephson, S. and Robertson, B.: Experimental
partial ureteric obstruction in newborn rats. IV. Do the mor-
phological effects progress continuously? J Urol, 130: 1217,
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13. Stenberg, A., Jacobsson, E., Larsson, E. et al: Long-term partial
ureteral obstruction and its effects on kidney function. Scand
J Urol Nephrol, 26: 35, 1992
14. Chevalier, R. L.: Chronic partial ureteral obstruction in the
neonatal guinea pig. II. Pressure gradients affecting glomeru-
lar filtration rate. Pediatr Res, 18: 1271, 1984
The response of the partially obstructed kidney to chal-
lenges of fluid and solute loads has been shown to be normal
in weanling rats with short-term obstruction28 and markedly
worse in weanling rats after 1 year of obstruction (40% de-
crease in concentrating ability).13 In human infants with
unobstructed kidneys the fractional excretion of sodium de-
creases from 5% prenatally to less than 0.2% postnatally.29
Since the glomerular filtration rate also doubles in week 1 of
life and concentrating ability increases,18 one may postulate
that urine flow decreases as urine becomes more concen-
trated postnatally. This hypothesis may be another explana-
tion for the fact that most cases of prenatally detected hydro-
nephrosis resolve spontaneously. On the other hand, if an
infant starts with high urine flow due to tubular dysfunction
from obstruction or reflux, that condition not only may
worsen renal pelvic pressure, but also interfere further with
normal maturation of kidney concentrating ability, leading to
persistent high urine flow and worsening hydronephrosis.
Moreover, although these findings are true in intrinsic ob-
struction, they may have a significantly greater effect in
extrinsic obstruction. The effect of diet on the natural history
of hydronephrosis may partially explain why the condition of
some patients improves while that of others deteriorates.
15. Nguyen, H. T. and Kogan, B. A.: Renal hemodynamic changes
after complete and partial unilateral ureteral obstruction in
the fetal lamb. J Urol, part 2, 160: 1063, 1998
16. Chevalier, R. L. and Kaiser, D. L.: Chronic partial ureteral
obstruction in the neonatal guinea pig. I. Influence of unine-
phrectomy on growth and hemodynamics. Pediatr Res, 18:
1266, 1984
17. Chevalier, R. L.: Effects of ureteral obstruction on renal growth.
Semin Nephrol, 15: 353, 1995
18. Chevalier, R. L. and Howards, S. S.: Renal function in the fetus,
neonate, and child. In: Campbell’s Urology, 7th ed. Edited by
P. C. Walsh, A. B. Retik, E. D. Vaughan, Jr. et al. Philadelphia:
W. B. Saunders, vol. 2, chapt. 55, pp. 1655–1668, 1998
19. Chevalier, R. L. and Thornhill, B. A.: Ureteral obstruction in the
neonatal rat: renal nerves modulate hemodynamic effects. Pe-
diatr Nephrol, 9: 447, 1995
20. Daniels, B. and Hostetter, T.: Influence of dietary salt on chronic
renal injury. Kidney Int, 29: 244, 1986
21. Benstein, J. A., Feiner, H. D., Parker, M. et al: Superiority of salt
restriction over diuretics in reducing renal hypertrophy and
injury in uninephrectomized SHR. Am J Physiol, 258: F1675,
1990
22. Lax, D. S., Benstein, J. A., Tolbert, E. et al: Effects of salt
restriction on renal growth and glomerular injury in rats with
remnant kidneys. Kidney Int, 41: 1527, 1992
23. Kennedy, W. A., II, Stenberg, A., Lackgren, G. et al: Renal
tubular apoptosis after partial ureteral obstruction. J Urol,
152: 658, 1994
24. Chevalier, R., Smith, C., Thornhill, B. et al: Sodium deprivation
increases apoptosis resulting from unilateral ureteral obstruc-
tion. Pediatr Res, 359A, 1996
25. Muchant, D. G., Thornhill, B. A., Belmonte, D. C. et al: Chronic
sodium loading augments natriuretic response to acute vol-
ume expansion in the preweaned rat. Am J Physiol, 269: R15,
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26. Fomon, S. and Ziegler, E.: Water and renal solute load. In:
Nutrition of Normal Infants. Edited by S. Fomon. St. Louis:
Mosby, chapt. 6, p. 91, 1993
CONCLUSIONS
Chronic high urine flow caused loss of renal function in
partially obstructed weanling rat kidneys. Research should
be done to determine whether human infants with hydrone-
phrosis and partial ureteral obstruction would benefit from
the prevention of excessive fluid and salt intake.
Dr. Frank Hinman, Jr. encouraged us to study the effect of
diet on hydronephrosis.
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