AJP - Regu AJP: Regulatory, Integrative and Comparative Physiology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Regul Integr Comp Physiol 274: R1718-R1724, 1998;
0363-6119/98 $5.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Guron, G.
Right arrow Articles by Friberg, P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Guron, G.
Right arrow Articles by Friberg, P.
Vol. 274, Issue 6, R1718-R1724, June 1998

Urinary acidification and net acid excretion in adult rats treated neonatally with enalapril

Gregor Guron1, Niels Marcussen2, and Peter Friberg1

1 Department of Physiology, Institute of Physiology and Pharmacology, Göteborg University, S413-90 Göteborg, Sweden; and 2 Department of Pathology, Århus Kommune Hospital, Århus University, DK-8000 Århus C, Denmark

    ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References

Neonatal blockade of the renin-angiotensin system in rats induces irreversible renal histological abnormalities, including papillary atrophy and an impaired urinary concentrating ability. The aim was to investigate urinary acidification and net acid excretion in adult Wistar rats treated neonatally with enalapril (10 mg · kg-1 · day-1) or vehicle from 5 to 24 days of age. Analyses were performed in both metabolic balance studies and renal clearance experiments performed under pentobarbital sodium anesthesia. There were no differences between groups in urine pH or urinary excretion rates of bicarbonate, titratable acid, or ammonium, neither during control conditions nor after chronic NH4Cl loading (assessed before and after Na2SO4 infusion). Glomerular filtration rate, maximal tubular bicarbonate reabsorption, and the urine-to-blood PCO2 gradient in alkaline urine during NaHCO3 infusion did not differ between groups. Neonatally enalapril-treated rats showed a urine concentration defect and papillary damage. In conclusion, neonatal enalapril treatment produces a differentiated abnormality in tubular function in which urine concentration is impaired but urinary acidification and net acid excretion are intact.

renin-angiotensin system; angiotensin-converting enzyme inhibitor; renal development; renal medulla

    INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References

NEONATAL angiotensin-converting enzyme (ACE) inhibition or ANG II type 1 (AT1) receptor blockade in the rat inhibits renal growth (25, 31) and induces renal histological abnormalities (10, 13-15, 24, 25, 31), indicating a crucial role for ANG II in renal morphogenesis. In support of this notion, all components of the renin-angiotensin system (RAS) are expressed in the immature rat kidney and show an upregulated gene expression perinatally compared with the adult, leading to high intrarenal ANG II levels (11, 34). Furthermore, the spatiotemporal pattern of the expression of renal ANG II receptors (1, 28), together with in vitro evidence showing trophic effects of ANG II on different renal cell types (33), agrees with an involvement of ANG II in renal differentiation during development. In recent gene-targeting studies, evidence have been provided confirming the essential role for the RAS in the maintenance and development of normal renal morphology. Mice deficient in ACE (8, 20) or angiotensinogen (19, 27) developed marked alterations in renal histology that were similar to those observed after neonatal pharmacological blockade of the RAS in rats (10, 13-15, 24, 25, 31).

We have previously shown that neonatal ACE inhibition or AT1 receptor antagonism during the first 3 wk of life in rats induces irreversible renal histopathological changes, mainly characterized by papillary atrophy, vascular alterations, and chronic interstitial inflammation and fibrosis (10, 13-15). These histological abnormalities were associated with an impairment in urinary concentrating ability of renal origin (13, 15), sodium retention during dietary sodium loading (14), and a modest renal potassium wastage during dietary potassium restriction (14), defects that may be attributed to the papillary atrophy. In addition to its role in the final regulation of sodium, potassium, and water excretion, the inner medullary collecting duct is an important site of urinary acidification and net acid secretion (12). In vivo experiments using micropuncture (7) and microcatheterization (4, 12) techniques, as well as in vitro microperfusion studies (32), have uniformly demonstrated significant urinary acidification and/or net acid secretion along the inner medullary collecting duct in rats.

Accordingly, the aim of the present study was to assess urinary acidification and renal net acid excretion in neonatally enalapril-treated rats with papillary damage. Analyses were performed in both metabolic balance studies and renal clearance experiments under pentobarbital sodium anesthesia.

    METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References

General Procedures

Thirty-one male Wistar rats (Möllegaard Breeding Center, Ejby, Denmark) were used. Pregnant rats were carefully observed near end gestation for determination of the exact date of birth. Sex was determined in 4-day-old pups, and litters containing only males were transported to our facilities. Weight-matched male pups were divided into groups receiving daily intraperitoneal injections from 5 to 24 days of age with either enalapril maleate (10 mg/kg; Merck Sharp & Dohme, Sollentuna, Sweden) (n = 17) or isotonic saline vehicle (n = 14) in equivalent volumes of 10 ml/kg. After the neonatal treatment period, rats were left untreated until 9 wk of age, at which time metabolic balance experiments were begun. Rats had free access to normal rat chow and tap water (when they were not subjected to any experimental dietary regimen) and kept in rooms with a controlled temperature of 24°C and a 12:12-h dark-light cycle (6 PM-6 AM) throughout the study. All experiments were approved by the regional ethics committee in Göteborg.

Protocol

Group A. The experimental protocol for group A (enalapril, n = 9; vehicle, n = 7) consisted of the following: 1) an assessment of baseline fluid handling and urinary concentrating ability when rats were 9 wk of age, 2) a metabolic balance study analyzing renal acid excretion during chronic NH4Cl loading when rats were 10 wk of age, and 3) renal clearance experiments in anesthetized, chronically NH4Cl-loaded rats for assessments of renal acid excretion before and after Na2SO4 infusion at 12-13 wk of age.

Group B. The experimental protocol for group B (enalapril, n = 8; vehicle, n = 7) consisted of the following: 1) renal clearance experiments in anesthetized rats for assessments of renal function and acid excretion during baseline conditions, 2) an assessment of tubular bicarbonate reabsorption during graded NaHCO3 infusion, and 3) an analysis of the urine-to-blood PCO2 gradient (U-B PCO2) in alkaline urine during NaHCO3 administration. Clearance experiments were carried out at 14-15 wk of age.

Metabolic Balance Studies

General procedures. Rats were kept individually in metabolic cages with free access to powdered rat chow (Na+, 120 mmol/kg; K+, 153 mmol/kg) and drinking fluid throughout experiments. Food and water intake, urine volume, and body weight were measured daily. Urine was collected in preweighed vials under mineral oil. Water intake and urine volume were determined by weighing (1 ml = 1 g).

Fluid handling and urinary concentrating ability. After 2 days of acclimatization in metabolic cages, baseline measurements were performed during 24 h. Subsequently, rats were deprived of food and water for 24 h, followed by a 6-h period of urine collection (6 PM-12 PM). Urine osmolality (Uosm) after 24-30 h of water deprivation was considered as maximal urine osmolality (Uosmmax).

Chronic NH4Cl loading. After 2 days of acclimatization in metabolic cages, baseline measurements were carried out for 2 days on rats consuming standard rat chow and tap water. Thereafter NH4Cl loading was performed for the following 5 days. All rats were offered rat chow supplemented with NH4Cl in a concentration of 1% (187 mmol/kg). In addition, vehicle-treated rats drank 1% (187 mmol/l) and neonatally enalapril-treated rats 0.75% (140 mmol/l) NH4Cl in tap water. The reduced NH4Cl concentration in the drinking fluid of enalapril-treated rats had been determined in prior pilot studies and was to compensate for the increased fluid intake in these rats, thereby matching the total NH4Cl intake in the two groups. After measurement of urine volumes, urine was kept under mineral oil and promptly analyzed for pH and titratable acid (TA). In addition, urine samples were stored at -20°C and analyzed for osmolality and sodium, potassium, and ammonium concentrations within 2 wk time. Metabolic cages and vials used for the collection of urine were carefully cleaned and disinfected daily.

Renal Clearance Experiments in Anesthetized Rats

General procedures. Glomerular filtration rate (GFR) was measured by the urinary clearance of 51Cr-labeled EDTA (Amersham Laboratories, Buckinghamshire, UK). Rats were anesthetized with pentobarbital sodium (60 mg/kg ip) and tracheotomized with a polyethylene catheter (PE-240), and body temperature was maintained at 38°C throughout the experiment. The left jugular vein and carotid artery were catheterized with PE-50 tubing. The urinary bladder was catheterized through a midline abdominal incision with a PE-160 catheter. Throughout the experiment, rats were infused with 51Cr-EDTA (20 µCi · kg-1 · h-1 iv) and pentobarbital sodium (12 mg · kg-1 · h-1 intra-arterially) dissolved in isotonic saline, yielding a total infusion rate of 7 ml · kg-1 · h-1. A 45-min equilibration period was allowed before the start of clearance measurements. Urine was collected in preweighed vials under mineral oil, and arterial blood was sampled anaerobically (0.3 ml) at the midpoint of each collection period. Urine was kept under mineral oil, handled anaerobically, and promptly analyzed for pH, TA, and PCO2. Urine was also stored at -20°C and analyzed within 2 wk for osmolality and the concentration of sodium, potassium, and ammonium. Mean arterial blood pressure (MAP) and heart rate were recorded continuously with Statham pressure transducers connected to a Grass polygraph.

Chronic NH4Cl loading and Na2SO4 infusion. Rats were NH4Cl loaded, identically to the procedure during the metabolic balance study, for 5 days before experimentation. After two baseline 40-min clearance measurements, an infusion of 4% Na2SO4 (12 ml · kg-1 · h-1 iv) was initiated, as previously described (2, 29). After 40 min of equilibration, two consecutive 15-min clearance periods were carried out during the Na2SO4 infusion. Results are presented as the average for clearance measurements before and after infusing Na2SO4.

Baseline renal function, tubular bicarbonate reabsorption, and U-B PCO2 in alkaline urine. Rats consumed ordinary chow and tap water before experimentation. After two baseline 40-min clearance measurements, an infusion of 0.9 M NaHCO3 was initiated and the infusion rate elevated in a stepwise fashion from 4 to 26 ml · kg-1 · h-1, producing plasma bicarbonate concentrations from ~20 to 65 mmol/l. After each increase in infusion rate, a 15-min equilibration period was performed before clearance measurements began. On each rat, 7-10 clearance periods were performed. At least three clearance periods were carried out per rat when urine pH exceeded 7.8 for the analyses of U-B PCO2 in highly alkaline urine (3).

Kidney Weight and Histology

After renal clearance experiments, kidneys were rapidly excised, decapsulated, and weighed. Left kidneys were dried for 24 h at 100°C and reweighed for dry weight. After weighing, right kidneys were immediately immersion fixed in 4% formaldehyde and processed for semiquantitative histological analysis by light microscopy using an arbitrary scale where 0 is normal, 1 is mild, 2 is moderate, and 3 is severe change, similar to what has been described previously (13). Assessments were made by an investigator blind to the treatment group.

Analytic Methods

Osmolality, sodium and potassium concentrations, and radioactivity were measured as previously described (13). Urine ammonium was analyzed enzymatically using glutamate dehydrogenase (18) (Sigma Chemical, St. Louis, MO). TA was assessed by the amount of 0.01 M NaOH used to titrate 1 ml of urine to the arterial pH (during metabolic balance studies no blood was sampled and urine was titrated to a pH of 7.40). Arterial pH, PCO2, and urine PCO2 were analyzed with an ABL 510 blood-gas analyzer (Radiometer, Copenhagen, Denmark). Urine pH was analyzed with a 691 pH meter (Metrohm, Herisau, Switzerland).

Calculations

Standard equations for clearance calculations were used. Fractional excretion rates of sodium (FENa, %), potassium (FEK, %), and water (FEH2O, %) were estimated as the ratio of their respective clearances to that of 51Cr-EDTA, taken as GFR, × 100. The bicarbonate concentration in plasma and urine was calculated from the Henderson-Hasselbalch equation as previously described (21). A pK value of 6.1 was used for blood. The pK used for urine was calculated as 6.33 - 0.5(UNa + UK)1/2, where UNa and UK are the urine concentrations of sodium and potassium, expressed in moles per liter. The solubility constants for CO2 used in calculations were 0.0301 and 0.0309 in blood and urine, respectively. The maximal rate of tubular bicarbonate reabsorption was calculated using a nonlinear regression model. Net acid excretion was calculated as the sum of urinary TA and ammonium excretion minus urinary bicarbonate excretion.

Statistics

Data in text and tables are presented as means ± SE. Results from metabolic balance studies were analyzed by analysis of variance. Cross-sectional data were analyzed using unpaired and paired t-tests for statistical analysis of data between and within groups, respectively. The Mann-Whitney nonparametric test was used on renal histopathological parameters. A P < 0.05 was considered statistically significant. Analyses were performed using software Statview 4.1 (Abacus Concepts) and Systat 5.2 (Systat) for Macintosh.

    RESULTS
Top
Abstract
Introduction
Methods
Results
Discussion
References

Fluid Handling and Urinary Concentrating Ability

Water intake and urine flow rate (V) were elevated (water intake: 129 ± 4 vs. 115 ± 5 ml · kg-1 · 24 h-1, P < 0.05; V: 51 ± 3 vs. 28 ± 2 ml · kg-1 · 24 h-1, P < 0.05) and Uosm reduced (1,020 ± 34 vs. 1,639 ± 124 mosmol/kg, P < 0.05) in neonatally enalapril-treated rats with free access to tap water. Although Uosm rose in both groups after 24 h of water deprivation, Uosmmax was markedly reduced in enalapril- compared with vehicle-treated rats (2,112 ± 70 vs. 2,944 ± 90 mosmol/kg, P < 0.05).

Metabolic Balance Study During Chronic NH4Cl Loading

There were no differences between neonatally enalapril- and vehicle-treated rats in urinary excretion rates of TA, ammonium, or total acid (sum of TA and ammonium) throughout the study period (Fig. 1). Urine pH in neonatally enalapril-treated rats was transiently greater than that of vehicle-treated rats on days 2, 3, and 4 of NH4Cl loading, but did not differ from vehicle on the last day of experimentation (5.63 ± 0.03 vs. 5.55 ± 0.03, in neonatally enalapril- and vehicle-treated rats, respectively). The dietary intake of NH4Cl was similar in the groups during NH4Cl loading (37 ± 1 vs. 36 ± 1 mmol · kg-1 · 24 h-1, in neonatally enalapril- and vehicle-treated rats, respectively). Water intake and V were elevated and Uosm reduced in neonatally enalapril- compared with vehicle-treated rats throughout the study period (data not shown). Although arterial acid-base parameters were not monitored during the metabolic balance study, a similar degree of metabolic acidosis was demonstrated in the two groups on the 5th day of NH4Cl loading (Table 1).


View larger version (14K):
[in this window]
[in a new window]
  		Fig. 1.   Results from metabolic balance study during NH4Cl loading in 10-wk-old rats treated neonatally from 5 to 24 days of age with enalapril (10 mg · kg-1 · day-1, n = 9) or isotonic saline vehicle (n = 7). NH4Cl loading was performed for 5 days (see METHODS). UNH4V, urinary ammonium excretion; UTAV, urinary titratable acid excretion; U-pH, urine pH. Values are means ± SE. * P < 0.05.

                              
View this table:
[in this window]
[in a new window]
  		Table 1.   Arterial acid-base status after NH4Cl loading

Renal Function and Acid Excretion in NH4Cl-Loaded Rats

Arterial pH and plasma bicarbonate concentrations were similar in neonatally enalapril- and vehicle-treated rats throughout clearance experiments (Table 2). There were no significant differences between groups in urine pH or urinary excretion rates of TA, ammonium, bicarbonate, or net acid, neither before nor after the infusion of Na2SO4 (Table 2). In addition, both groups showed an increase in urinary excretion rates of TA, ammonium, and net acid in response to Na2SO4 administration (Table 2). There was no difference between neonatally enalapril- and vehicle-treated rats in GFR, FENa, or FEK, neither during baseline nor after infusing Na2SO4 (Table 2). Administration of Na2SO4 produced marked elevations in V, FENa, and FEK, and a reduction in the plasma potassium concentration in both groups (Table 2).

                              
View this table:
[in this window]
[in a new window]
  		Table 2.   Renal function and net acid excretion in chronically NH4Cl-loaded rats

Baseline Renal Function and Acid Excretion

V and FENa were elevated and Uosm was reduced in neonatally enalapril-treated rats (Table 3). GFR, MAP, FEK, and plasma sodium and potassium concentrations did not differ between groups (Table 3). There was no difference between groups in arterial acid-base parameters, urine pH, or urinary excretion rates of TA, ammonium, bicarbonate, or net acid, during baseline conditions (Table 4).

                              
View this table:
[in this window]
[in a new window]
  		Table 3.   Baseline renal function and hemodynamics in anesthetized rats

                              
View this table:
[in this window]
[in a new window]
  		Table 4.   Baseline renal net acid excretion in anesthetized rats

Tubular Bicarbonate Reabsorption

There was no difference between neonatally enalapril- and vehicle-treated rats in tubular bicarbonate reabsorption at any level of plasma bicarbonate concentration (Fig. 2). The maximal rate of tubular bicarbonate reabsorption was 35.1 ± 1.9 vs. 33.4 ± 1.7 mmol/l GFR in neonatally enalapril- and vehicle-treated rats, respectively.


View larger version (17K):
[in this window]
[in a new window]
  		Fig. 2.   Tubular bicarbonate reabsorption (THCO3) expressed per liter glomerular filtration rate (GFR) versus plasma bicarbonate concentration in pentobarbital sodium-anesthetized rats during graded NaHCO3 infusion. Experiments were performed in 14- to 15-wk-old rats, treated neonatally from 5 to 24 days of age with enalapril (10 mg · kg-1 · day-1, n = 8) or isotonic saline vehicle (n = 7). There was no significant difference between groups in the rate of THCO3/GFR throughout the investigated range of plasma bicarbonate concentrations.

U-B PCO2 in Alkaline Urine

The maximal U-B PCO2 gradient in highly alkaline urine during NaHCO3 infusion and at comparable urine bicarbonate concentrations did not differ between groups (32 ± 2 vs. 33 ± 3 mmHg, in enalapril- and vehicle-treated rats, respectively; Fig. 3).


View larger version (12K):
[in this window]
[in a new window]
  		Fig. 3.   Bar graphs show urine pH, urine bicarbonate concentration, and the urine-to-blood PCO2 gradient (U-B PCO2) in highly alkaline urine during NaHCO3 infusion in 14- to 15-wk-old neonatally enalapril (10 mg · kg-1 · day-1, n = 8)- or isotonic saline vehicle (n = 7)-treated rats. There was no significant difference between groups in any of the presented parameters. Values represent the average of 3 urine collection periods when urine pH exceeded 7.8.

Kidney Weight and Histology

Renal histological abnormalities in neonatally enalapril-treated rats were qualitatively similar to those described in more detail previously (13, 14) and mainly consisted of significant degrees of papillary atrophy (1.1 ± 0.2 vs. 0.0 ± 0.0 arbitrary units, P < 0.05), interstitial inflammation and fibrosis, tubular atrophy, and concentric wall thickening comprising both the intima and media of interlobular arteries (data not shown). In addition to a reduction in the size of the papilla, papillae of neonatally enalapril-treated rats showed a markedly distorted structural architecture, an increase in the proportion of interstitium associated with inflammatory and fibrotic changes, and dilated collecting ducts. There were no differences between groups in wet or dry kidney weights (data not shown).

    DISCUSSION
Top
Abstract
Introduction
Methods
Results
Discussion
References

The main finding of the present study was that adult neonatally enalapril-treated rats showed an intact urinary acidification and renal net acid excretion despite exhibiting renal histopathological abnormalities, which comprised the inner medulla and were associated with a urine concentration defect. This finding indicates that neonatal ACE inhibition has a differentiated effect on long-term tubular function in which urine concentration is impaired but tubular acidification and acid excretion are preserved.

In the present study, neonatal enalapril treatment induced irreversible renal histological changes, which were qualitatively similar to those previously reported after neonatal ACE inhibition or AT1 receptor antagonism in the rat (10, 13-15, 24, 25, 31), underlining the importance of an intact RAS for the development and maintenance of a normal renal morphology. In accordance with previous studies (10, 13-15), the main functional abnormality in adult rats treated neonatally with enalapril was an impairment in urinary concentrating ability. We have previously determined the pathophysiological mechanisms underlying the impairment in urine concentration in detail and demonstrated that this abnormality was of renal origin and due to a specific defect in tubular free water reabsorption, which may be explained by the atrophy of the papilla (15).

During baseline conditions, i.e., when rats consumed ordinary chow and tap water, neonatally enalapril-treated rats did not develop acidosis and showed a similar urine pH and urinary excretion rate of TA, ammonium, and bicarbonate, as controls. Tubular bicarbonate reabsorption was virtually complete in neonatally enalapril-treated rats during control conditions. Moreover, enalapril-treated rats demonstrated a similar rate of maximal tubular bicarbonate reabsorption as controls during bicarbonate titration experiments, in support of an intact proximal tubular acidification.

Chronic NH4Cl loading resulted in a similar degree of acidemia and reduction in the plasma bicarbonate concentration in neonatally enalapril- and vehicle-treated rats. In addition, minimal urine pH and urinary excretion rates of TA, ammonium, and net acid were similar in the two groups, both before and after administration of Na2SO4. Infusing sodium with the poorly reabsorbable anion sulfate increases the lumen negative voltage in the collecting duct, thereby accelerating the secretion of hydrogen ions and potassium (17). Neonatally enalapril-treated rats showed significant increases in net acid and potassium excretion after Na2SO4 administration, suggesting intact secretory mechanisms for these cations in the collecting duct. To further characterize the consequences of neonatal ACE inhibition on distal tubular acidification, we measured the U-B PCO2 gradient in highly alkaline urine during NaHCO3 infusion, which serves as a reliable qualitative index of hydrogen ion secretion by the collecting duct under these circumstances (5, 6). Neonatally enalapril-treated rats were able to increase the U-B PCO2 gradient to the same level as the one seen in controls, at comparable urine bicarbonate concentrations, providing additional support for an intact hydrogen ion secretion in the collecting duct. Notably, urinary ammonium excretion was normal in neonatally enalapril-treated rats. This finding suggests that these rats were able to accumulate ammonia in the renal medullary interstitium secondary to countercurrent multiplication, despite displaying papillary damage and interstitial changes in the medulla. Furthermore, the rate of ammonium excretion is largely dependent on the amount of ammonia secreted along the collecting duct, which in turn is partially determined by hydrogen ion secretion and the ability to reduce luminal pH in this nephron segment. Thus the observation of a normal ammonium excretion in chronically acidotic neonatally enalapril-treated rats corroborates with the other findings of an intact hydrogen ion secretion in the collecting duct. Taken together, neonatal enalapril treatment did not produce any defects in urinary acidification or renal net acid excretion in adult rats, despite inducing irreversible renal histopathological changes.

Intriguingly, although in vivo microcatheterization experiments in rats have suggested that acid secretion along the inner medullary collecting duct contributes ~80% of net acid excreted during control conditions (12) and that the absolute rate of acid secretion in this nephron segment may increase fivefold during chronic metabolic acidosis (4), neonatally enalapril-treated rats with papillary damage and a urine concentrating defect showed intact distal tubular acidification. This finding suggests that a normal distal tubular acidification does not critically depend on a structurally intact papilla and that undamaged nephron segments are able to increase their rate of acidification, thereby compensating for papillary defects. In strong support of this notion, papillary necrosis induced by bromoethylamide hydrobromide has been shown to be associated with intact urinary acidification and acid secretion, both during control conditions and after chronic acid loading (2, 29). In contrast, Finkelstein and Hayslett (9) demonstrated a reduced urinary ammonium excretion in acutely acid-loaded, unilaterally nephrectomized rats with papillectomy performed on the remaining kidney compared with controls with a similar degree of nephrectomy but with an intact papilla. However, in keeping with the hypothesis that a lack of tubular acidification and acid secretion in the papilla can be compensated for by remaining tubular structures, the impaired ammonium excretion in these rats could be explained by the nephrectomy and the prevailing reduction in GFR.

The present study was not designed to elucidate mechanisms that could be involved in the induction and maintenance of a compensatory increase in urinary acidification and acid excretion in undamaged nephron segments outside of the injured papilla. Still, one might speculate that a small reduction in blood pH in neonatally enalapril-treated rats, which we were unable to detect with conventional methods, could enhance the synthesis or activity of H+-ATPase and/or H+-K+-ATPase in intercalated cells along the collecting duct proximal to the papillary damage, thereby upregulating distal hydrogen ion secretion. Moreover, acidemia could stimulate ammonium synthesis from glutamine in proximal tubular cells and increase the buffer delivery to undamaged acidifying sites in the collecting duct. Clearly, additional studies are needed to resolve these issues.

In conclusion, in concert with previous studies neonatal ACE inhibition in the rat produced irreversible abnormalities in renal morphology, including papillary atrophy and an impairment in urinary concentrating ability. However, despite papillary damage, urinary acidification and renal net acid excretion were intact in adult, neonatally enalapril-treated rats. Thus neonatal enalapril treatment produces a differentiated abnormality in tubular function in which urine concentration is impaired but urinary acidification and net acid excretion are intact.

Perspectives

The use of ACE inhibitors by pregnant women is known to cause renal tubular dysplasia and anuria in the neonate (30), although the pathogenetic mechanisms have not been determined. In addition to regulating perinatal renal function and hemodynamics (22), recent studies in a number of species (8, 10, 13-16, 19, 20, 24, 25, 27, 31), indicate that ANG II may also be essential for normal renal morphogenesis. Nephrogenesis is completed at about gestational week 36 in humans (23), but continues into the second postnatal week in the rat (26). Thus the neonatal rat provides a suitable experimental model for analyzing the effects of pharmacological blockade of the RAS on immature kidneys with ongoing nephrogenesis. We have previously demonstrated that a main characteristic of adult rats treated neonatally with ACE inhibitors or AT1 receptor antagonists is papillary atrophy in association with an impaired urinary concentrating ability (10, 13-15). Thus an intact RAS may be of particular importance for the development and/or maintenance of a normal inner medullary morphology and function. The present study extends our knowledge of the long-term effects of neonatal ACE inhibition on tubular function and indicates that an intact RAS, although essential for the formation of a structurally intact papilla, is not mandatory for the development of normal urinary acidification and net acid excretion.

    ACKNOWLEDGEMENTS

The authors thank Merck Sharp & Dohme, Sollentuna, Sweden, for providing enalapril maleate. The authors are indebted to professor Gerald F. DiBona for providing valuable advice and comments.

    FOOTNOTES

This study was supported by the Göteborg Medical Society, the Swedish Medical Society, and the Swedish Medical Research Council (9047, 11133).

Address for reprint requests: G. Guron, Dept. of Physiology, Institute of Physiology and Pharmacology, Göteborg Univ., Medicinaregatan 11, S-413 90 Göteborg, Sweden.

Received 22 December 1997; accepted in final form 17 February 1998.

    REFERENCES
Top
Abstract
Introduction
Methods
Results
Discussion
References

1.   Aguilera, G., S. Kapur, P. Feuillan, B. Sunar-Akbasak, and A. Bathia. Developmental changes in angiotensin II receptor subtypes and AT1 receptor mRNA in rat kidney. Kidney Int. 46: 973-979, 1994[Medline].

2.   Arruda, J. A. L., S. Sabatini, P. K. Mehta, B. Sodhi, and R. Baranowski. Functional characterization of drug-induced experimental papillary necrosis. Kidney Int. 15: 264-275, 1979[Medline].

3.   Battle, D. C., M. Downer, C. Gutterman, and N. A. Kurtzman. The relationship between the urinary and blood carbon dioxide tension during hypercapnia. J. Clin. Invest. 75: 1517-1530, 1985.

4.   Bengele, H. H., J. H. Schwartz, E. R. McNamara, and E. A. Alexander. Chronic metabolic acidosis augments acidification along the inner medullary collecting duct. Am. J. Physiol. 250 (Renal Fluid Electrolyte Physiol. 19): F690-F694, 1986.

5.   DuBose, T. D., Jr. Hydrogen ion secretion by the collecting duct as a determinant of the urine to blood PCO2 gradient in alkaline urine. J. Clin. Invest. 69: 145-156, 1982.

6.   DuBose, T. D., Jr., and C. R. Caflisch. Validation of the difference in urine and blood carbon dioxide tension during bicarbonate loading as an index of distal nephron acidification in experimental models of distal tubular acidosis. J. Clin. Invest. 75: 1116-1123, 1985.

7.   DuBose, T. D., Jr., M. S. Lucci, R. J. Hogg, L. R. Pucacco, J. P. Kokko, and N. W. Carter. Comparison of acidification parameters in superficial and deep nephrons of the rat. Am. J. Physiol. 244 (Renal Fluid Electrolyte Physiol. 13): F497-F503, 1983.

8.   Esther, R. E., T. E. Howard, E. M. Marino, J. M. Goddard, M. R. Capecchi, and K. E. Bernstein. Mice lacking angiotensin-converting enzyme have low blood pressure, renal pathology and reduced male fertility. Lab. Invest. 74: 953-965, 1996[Medline].

9.   Finkelstein, F. O., and J. P. Hayslett. Role of medullary structures in the functional adaptation of renal insufficiency. Kidney Int. 6: 419-425, 1974[Medline].

10.   Friberg, P., B. Sundelin, S.-O. Bohman, A. Bobik, H. Nilsson, A. Wickman, H. Gustafsson, J. Petersen, and M. A. Adams. Renin-angiotensin system in neonatal rats: induction of a renal abnormality in response to ACE inhibition or angiotensin II antagonism. Kidney Int. 45: 485-492, 1994[Medline].

11.   Gomez, R. A. Molecular biology of components of the renin-angiotensin system during development. Pediatr. Nephrol. 4: 421-423, 1990[Medline].

12.   Graber, M. L., H. H. Bengele, E. Mroz, C. Lechene, and E. A. Alexander. Acute metabolic acidosis augments collecting duct acidification rate in the rat. Am. J. Physiol. 241 (Renal Fluid Electrolyte Physiol. 10): F669-F676, 1981[Abstract/Free Full Text].

13.   Guron, G., M. A. Adams, B. Sundelin, and P. Friberg. Neonatal angiotensin converting enzyme inhibition in the rat induces persistent abnormalities in renal function and histology. Hypertension 29: 91-97, 1997[Abstract/Free Full Text].

14.   Guron, G., A. Nilsson, G. F. DiBona, B. Sundelin, N. Nitescu, and P. Friberg. Renal adaptation to dietary sodium restriction and loading in rats treated neonatally with enalapril. Am. J. Physiol. 273 (Regulatory Integrative Comp. Physiol. 42): R1421-R1429, 1997[Abstract/Free Full Text].

15.   Guron, G., B. Sundelin, A. Nilsson, N. Nitescu, and P. Friberg. Renal defect in urine concentrating ability after neonatal ACE inhibition in the rat (Abstract). J. Am. Soc. Nephrol. 7: 1632, 1996.

16.   Guron, G., B. Sundelin, A. Wickman, and P. Friberg. Angiotensin-converting enzyme inhibition in piglets induces persistent renal abnormalities. Clin. Exp. Pharmacol. Physiol. 25: 88-91, 1998[Medline].

17.   Hamm, L. L., and R. J. Alpern. Cellular mechanisms of renal tubular acidification. In: The Kidney: Physiology and Pathophysiology, edited by D. W. Seldin, and G. Giebisch. New York: Raven, 1992, p. 2581-2626.

18.   Katagawa, K., T. Nagashima, N. Inase, M. Kanayama, M. Chida, S. Sasaki, and F. Marumo. Urinary ammonium measurement by the auto-analyzer method. Kidney Int. 36: 291-294, 1989[Medline].

19.   Kim, H.-S., J. H. Krege, K. D. Kluckman, J. R. Hagaman, J. B. Hodgin, C. F. Best, J. C. Jennette, T. M. Coffman, N. Maeda, and O. Smithies. Genetic control of blood pressure and the angiotensinogen locus. Proc. Natl. Acad. Sci. USA 92: 2735-2739, 1995[Abstract/Free Full Text].

20.   Krege, J. H., S. W. M. John, L. L. Langenbach, J. B. Hodgin, J. R. Hagaman, E. S. Bachman, J. C. Jennette, D. A. O'Brien, and O. Smithies. Male-female differences in fertility and blood pressure in ACE-deficient mice. Nature 375: 146-148, 1995[Medline].

21.   Kurtzman, N. A. Regulation of renal bicarbonate reabsorption by extracellular volume. J. Clin. Invest. 49: 586-595, 1970.

22.   Lumbers, E. R. Functions of the renin-angiotensin system during development. Clin. Exp. Pharmacol. Physiol. 22: 499-505, 1995[Medline].

23.   MacDonald, M. S., and J. L. Emery. The late intrauterine and postnatal development of human renal glomeruli. J. Anat. 93: 331-340, 1959[Medline].

24.   McCausland, J. E., J. F. Bertram, G. B. Ryan, and D. Alcorn. Glomerular number and size following chronic angiotensin II blockade in the postnatal rat. Exp. Nephrol. 5: 201-209, 1997[Medline].

25.   McCausland, J. E., G. B. Ryan, and D. Alcorn. Angiotensin converting enzyme inhibition in the postnatal rat results in decreased cell proliferation in the renal outer medulla. Clin. Exp. Pharmacol. Physiol. 23: 552-554, 1996[Medline].

26.   Neiss, W. F., and K. L. Klehn. The postnatal development of the rat kidney, with special reference to the chemodifferentiation of the proximal tubule. Histochemistry 73: 251-268, 1981[Medline].

27.   Niimura, F., P. A. Labosky, J. Kakuchi, S. Okubo, H. Yoshida, T. Oikawa, T. Ichiki, A. J. Naftilan, A. Fogo, T. Inagami, B. L. M. Hogan, and I. Ichikawa. Gene targeting in mice reveals a requirement for angiotensin in the development and maintenance of kidney morphology and growth factor regulation. J. Clin. Invest. 96: 2947-2954, 1995.

28.   Norwood, V., M. R. Craig, J. M. Harris, and R. A. Gomez. Differential expression of angiotensin II receptors during early renal morphogenesis. Am. J. Physiol. 272 (Regulatory Integrative Comp. Physiol. 41): R662-R668, 1997[Abstract/Free Full Text].

29.   Sabatini, S., V. Alla, A. Wilson, M. Cruz-Soto, A. deWhite, N. A. Kurtzman, and J. A. L. Arruda. The effects of chronic papillary necrosis on acid excretion. Pflügers Arch. 393: 262-268, 1982[Medline].

30.   Shotan, A., J. Widerhorn, A. Hurst, and U. Elkayam. Risks of angiotensin-converting enzyme inhibition during pregnancy: experimental and clinical evidence, potential mechanisms, and recommendations for use. Am. J. Med. 96: 451-456, 1994[Medline].

31.   Tufro-McReddie, A., L. M. Romano, J. M. Harris, L. Ferder, and R. A. Gomez. Angiotensin II regulates nephrogenesis and renal vascular development. Am. J. Physiol. 269 (Renal Fluid Electrolyte Physiol. 38): F110-F115, 1995[Abstract/Free Full Text].

32.   Wall, S. M., J. M. Sands, M. F. Flessner, H. Nonoguchi, K. R. Spring, and M. A. Knepper. Net acid transport by isolated perfused inner medullary collecting ducts. Am. J. Physiol. 258 (Renal Fluid Electrolyte Physiol. 27): F75-F84, 1990[Abstract/Free Full Text].

33.   Wolf, G., and E. G. Neilson. Angiotensin II as a renal growth factor. J. Am. Soc. Nephrol. 3: 1531-1540, 1992[Abstract].

34.   Yosipiv, I. V., and S. S. El-Dahr. Activation of angiotensin-generating systems in the developing rat kidney. Hypertension 27: 281-286, 1996[Abstract/Free Full Text].

Am J Physiol Regul Integr Compar Physiol 274(6):R1718-R1724
0002-9513/98 $5.00 Copyright © 1998 the American Physiological Society



This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Guron, G.
Right arrow Articles by Friberg, P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Guron, G.
Right arrow Articles by Friberg, P.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online