(Hypertension. 1997;30:868-872.)
© 1997 American Heart Association, Inc.
Articles |
From the Departments of Pediatrics and Physiology, State University of New York at Buffalo School of Medicine and Biomedical Sciences; Buffalo Veterans Affairs Medical Center; The Children's Hospital, Buffalo, NY; and Max Delbruck Center for Molecular Medicine, Berlin-Buch, Germany.
| Abstract |
|---|
|
|
|---|
Key Words: glomerulosclerosis albuminuria nitric oxide
| Introduction |
|---|
|
|
|---|
In an earlier study, we described various aspects of renal function in 2- to 8-month-old male rats heterozygous for the mouse renin transgene.4 We found that transgenic rats had significantly higher BPs than control animals at all ages but that the severity of their hypertension diminished over time. Albuminuria and glomerulosclerosis appeared early in life but did not lead to renal insufficiency. This lack of progressive renal injury was associated with elevated preglomerular vascular resistance that protected glomeruli from severe systemic hypertension. In the present study, we determined the effect of long-term treatment with the angiotensin-converting enzyme inhibitor enalapril on kidney function and histology in these animals.
| Methods |
|---|
|
|
|---|
Kidney Function
Indirect systolic BP was measured monthly in
unanesthetized rats by a tail-cuff method (Narco Biosystems).
Urine was collected from animals kept in metabolic cages
for 16 hours with free access to water but deprived of food. Blood
samples were obtained from the tail vein at the end of collection
periods. Hematocrit; plasma sodium, total protein, albumin,
creatinine, cholesterol, high-density
lipoprotein, and triglyceride concentrations; and urinary
sodium, creatinine, albumin, and NOM excretion, a
measure of endogenous nitric oxide
production,5 were determined from these
samples.
Nonfasted 8-month-old TGR and CON rats were anesthetized with thiobutabarbital (80 mg/kg IP) and prepared for micropuncture studies as previously described.4 Isotonic saline containing tritiated inulin (1 mCi/mL) and PAH (0.1 g/dL) were given as a bolus injection (0.5% body weight) followed by continuous infusion at 0.6 mL · h-1 · 100 g body wt-1. After 30 minutes of equilibration, two clearance periods each approximately 45 minutes long were performed for each experiment. During each experiment, timed (3 to 5 minutes) collections of tubular fluid were made in four to six proximal tubules after the tubule lumen was blocked with Sudan blackstained castor oil to determine SNGFR. Four free flow pressures (FFP) and stop flow pressures (SFP) were measured in the earliest convolutions of proximal tubules with a 3- to 7-µm pipette connected to a servonull micropressure system (World Precision Instruments). SFP was determined by blocking the tubule with Sudan blackstained castor oil contained in a 12- to 14-µm pipette. Four hydraulic pressures in efferent star vessels were also measured. Renal vein samples were taken with a 30-gauge needle at the end of clearance periods to determine the extraction ratio for PAH. Heart and kidney weights were measured at the end of each experiment.
Calculations
RPF was estimated from the clearance of and extraction
ratio for PAH. RBF was calculated by dividing RPF by
1-arterial hematocrit. RVR was calculated by MAP/RBF. PGC
was determined indirectly from SFP+Arterial Plasma Colloid
Osmotic Pressure (COPA). Plasma colloid osmotic pressure was determined
with 1.886C+0.206C2+0.005C3, where C is the plasma protein
concentration. The
P was calculated as PGC-FFP. SNPF was estimated
by dividing SNGFR by whole-kidney filtration fraction (GFR/RPF). Mean
effective ultrafiltration pressure (PUF) was calculated by subtracting
[(COPA-COPE)/2] from
P, where COPE was the colloid osmotic
pressure in the efferent arteriole, the protein concentration of which
was estimated from the arterial plasma protein
concentration divided by 1-Whole-Kidney Filtration Fraction.
Kf values were determined by dividing SNGFR by PUF. RA was
calculated using [(MAP-PGC)/ SNBFA]x7.962x1010, where SNBFA was
determined by dividing SNPF by 1-Arterial Hematocrit. RE
was calculated using [(PGC-PE)/SNBFE]x7.962x1010, where PE was the
pressure in the efferent star vessel and SNBFE was calculated as
SNBFA-SNGFR.
Analytic Methods
We used a microcontinuous gradient gel electrophoresis procedure
for the separation and quantification of proteins in plasma and
urine.6 Tritiated samples of plasma, urine, and tubular
fluid were placed in scintillation fluor and counted in a scintillation
counter. PAH and creatinine concentrations were
analyzed using previously described methods.4
Plasma lipid concentrations were measured using a commercial kit
(Boehringer-Mannheim Diagnostics). Sodium
concentrations were determined by flame photometry. NOM concentrations
were measured as described by Feld et al.5
Histology
Kidney tissue samples were fixed in buffered formalin and
embedded in paraffin. Sagittal sections were cut from the middle of
each kidney and stained with periodic acidSchiff. A total of 100
glomeruli were examined by light microscopy in each rat. Each
glomerulus was graded from 1 to 4 using a previously described
glomerular injury score7 : grade 1, normal
glomerulus; grade 2, involvement of up to one third of the glomerulus;
grade 3, involvement of one third to two thirds of the glomerulus; and
grade 4, two thirds to global sclerosis. Each score was then calculated
according to the formula Glomerular Injury Score=[(1xNo.
of Grade 2 Glomeruli)+(2xNo. of Grade 3 Glomeruli)+(3xNo. of Grade 4
Glomeruli)]x100/(No. of Glomeruli Evaluated).
Statistics
Results are expressed as mean±SE. All data obtained from an
individual animal during micropuncture experiments were averaged, and
this single average represented that animal. Data for GFR
and RBF were normalized for the weight of both kidneys. Data for SNGFR
and SNPF were normalized for left (micropunctured) kidney weight.
Differences between group means were analyzed by ANOVA with the
Scheffé post hoc test or by Student's t test and
considered significant at a value of P<.05.
| Results |
|---|
|
|
|---|
|
Plasma and Urine Composition of Conscious Animals
Table 1
also shows the composition of plasma and urine in
conscious animals. Hematocrit and plasma sodium, total protein,
albumin, cholesterol, and high-density lipoprotein
concentrations did not differ among age-matched treated and untreated
TGR and CON rats during the study. Plasma creatinine
concentrations were significantly greater in untreated TGR rats and
decreased with enalapril therapy. Compared with levels in untreated CON
rats, fasting plasma triglyceride levels of untreated TGR
rats were elevated at 3 and 8 months of age. The use of enalapril did
not alter triglyceride levels in either group. Urinary
sodium excretion of 8-month-old TGR rats was lower than in CON rats.
Enalapril did not change this difference. Endogenous
creatinine clearance did not differ in TGR and CON rats at
3 months but fell in TGR relative to CON rats at 8 months of age.
Enalapril did significantly affect creatinine clearance in
either TGR or CON rats. Untreated TGR rats had significantly higher
urinary albumin excretion than CON rats at age 3 and 8 months.
Enalapril significantly reduced the amount of albuminuria
in both TGR and CON rats. Urinary NOM excretion was lower in TGR than
CON rats at 3 months of age. At 8 months, urinary NOM excretion did not
differ in TGR and CON rats. Enalapril significantly increased NOM
excretion in TGR but not CON rats. To reexamine these changes in
urinary NOM excretion, we analyzed frozen samples from an
earlier study.4 NOM excretion was also lower in TGR (n=7)
than CON (n=7) rats at 4 months (43±14 versus 125±23 nmol/h,
P<.05) and similar at 8 months of age (97±30 versus
114±39 nmol/h). Urine flow rate and sodium excretion did not
differ significantly in TGR and CON rats in this group of animals.
Whole-Kidney Function
Table 2
shows whole-kidney function
in TGR and CON rats. Consistent with their awake
systolic BP, untreated anesthetized 8-month-old TGR
rats had significantly higher MAP than untreated CON rats, and
enalapril significantly lowered their MAP. MAP values did not differ
among untreated CON rats, treated CON rats, and treated TGR rats. Urine
flow rate, GFR, RBF, and RVR did not differ in treated and untreated
CON rats. In TGR rats, enalapril produced a significant fall in urine
flow rate and RVR, significantly increased RBF, but did not alter
GFR.
|
Glomerular Hemodynamics
Table 3
shows glomerular
hemodynamics of TGR and CON rats. No significant
differences in SNGFR, SNPF, PGC,
P, Kf, or arteriolar
colloid osmotic pressures or resistances were detected in untreated and
treated CON rats. Treatment with enalapril significantly increased SNPF
without altering SNGFR or Kf in TGR rats. Efferent
arteriolar colloid osmotic pressure fell in treated TGR rats,
reflecting their lower filtration fraction. Enalapril significantly
lowered PGC,
P, RA, RE, and RA/RE of TGR rats.
|
Renal Histology
The glomerular injury score of untreated TGR rats was
approximately eight times greater than that of TGR rats receiving
enalapril (22.7±6.6 versus 2.6±0.8, P<.01) and mainly
affected deep cortical nephrons. The glomerular injury
score of untreated CON rats was 0.8±0.4. No sclerotic glomeruli were
found in enalapril-treated CON rats. There was a significant positive
correlation between the incidence of
glomerulosclerosis and the magnitude of
albuminuria (r=.85, n=22,
P<.001).
| Discussion |
|---|
|
|
|---|
The presence of the mouse renin transgene and use of enalapril did not change somatic growth as judged by absolute body and kidney weights. Increased cardiac weight has been previously reported in this and other genetically hypertensive rat strains.6 8 12 Its regression with enalapril potentially reflects not only lower BP but also a tissue-specific effect because normotension achieved by hydralazine does not reverse cardiac hypertrophy and angiotensin II receptor blockade reduces heart weight without altering BP.8 13
On the basis of elevated serum urea and creatinine concentrations, Lee et al8 noted that heterozygous TGR rats develop renal insufficiency that can be prevented by lisinopril. Enalapril had a similar beneficial effect on TGR serum creatinine concentrations in the present study; however, inulin clearances did not confirm either age-related GFR loss or alteration by enalapril. Acute administration of captopril and losartan to conscious TGR rats has been reported to cause a reduction in sodium excretion and creatinine clearance after salt or water loading.14 We did not perform the experiments needed to determine whether this response persists with long-term use of these medications. Treatment of 3-month-old TGR rats for 9 weeks with an angiotensin II receptor antagonist has been associated with decreased urine volume and sodium excretion.13 Long-term enalapril therapy did not cause these changes in the current study.
We previously reported a significant increase in fasting plasma triglyceride concentrations in transgenic rats.4 This finding was confirmed in the present study. We had speculated that the mouse renin transgene might alter some aspect of triglyceride metabolism because of its increased expression in adipose tissue; however, the inability of enalapril to significantly lower triglyceride levels suggests that neither angiotensin-converting enzyme nor angiotensin is involved in this alteration.4 15
In terms of renal function in anesthetized rats, enalapril significantly reduced urine flow rate in TGR rats. We assume that this reduction reflects a pressure-diuresis response and was responsible for the somewhat higher hematocrit values in untreated TGR rats. Enalapril did not affect whole-kidney and single-nephron filtration rates in CON or TGR animals. Whole-kidney blood flow and SNBF increased only in enalapril-treated TGR rats and were associated with lower filtration fraction, RVR, and arteriolar resistances. Given the significant decrease in RA/RE, this rise in RBF was primarily caused by preglomerular vasodilatation. Prior investigations of the effect of renin-angiotensin system inhibitors on TGR(mRen2)27 renal function have been limited to the acute administration of these medications. In their studies of pressure-natriuresis during which renal perfusion pressures were controlled, Gross et al16 found that treatment of TGR rats with captopril and an angiotensin II receptor blocker increased RBF without changing GFR. In contrast, Mitchell and Mullins17 reported that the angiotensin II antagonist L158,809 reduced GFR without altering effective RPF. Interpretation of this latter study is confounded by evidence that TGR rats appear to autoregulate poorly in the face of abrupt falls in renal perfusion pressure.18 The fact that mechanical reduction of renal perfusion pressure lowers both GFR and RBF while L158,809 only decreases GFR supports the blood flowenhancing effect of angiotensin inhibitors in TGR rats.17
Renal injury, manifested by albuminuria and glomerulosclerosis, appears early in the life span of TGR(mRen2)27 rats.4 Treatment with converting enzyme inhibitors prevented these abnormalities and was associated with mildly diminished glomerular capillary pressure. It is generally accepted that sustained hypertension damages the kidney by preglomerular vascular injury leading to glomerular ischemia or by transmission of increased systemic pressure to glomerular capillaries.19 We have previously shown that renal injury in TGR rats evolves, at least initially, without evidence of glomerular ischemia or hypertension in superficial nephrons.4 Thus, either these mechanisms are not solely responsible for renal injury or they occur predominantly in deeper nephrons. Histological studies indicate that renal injury in TGR rats begins in the inner cortex and then progresses superficially.4 This pattern of injury is similar to that previously documented in spontaneously hypertensive rats, a strain in which progressive proteinuria and sclerosis also develop despite the absence of substantial reduction in superficial glomerular blood flow or elevation in superficial glomerular capillary pressure before 15 to 18 months of age.6 7 20 Because measurement of juxtamedullary glomerular hemodynamics is not feasible in the intact kidney, it is impossible to determine whether glomerular ischemia or hypertension exists in inner cortical nephrons.
Alternatively, the benefit of converting enzyme inhibitors and related medications might not depend on systemic/glomerular normotension. Despite only modest declines in awake systolic BP and no alterations in MAP or glomerular capillary pressure, our enalapril-treated control rats had minimal albuminuria and no glomerulosclerosis. This renal protective effect of chronic converting enzyme inhibition has been previously documented in aging rodents and suggests a mechanism that does not involve hemodynamic changes.21 22 Supporting this hypothesis are studies by Remuzzi et al23 dissociating the antihypertensive and renal protective effects of lisinopril in aging Munich-Wistar rats and by Bohm et al13 showing that low-dose treatment with an angiotensin II receptor antagonist ameliorates kidney damage without substantially lowering systemic BP in TGR(mRen2)27 rats. Suppression of the intrarenal renin-angiotensin system and its stimulation of cell proliferation, production of transforming growth factor-ß, and synthesis of matrix protein could therefore play important roles in preserving function in hypertensive renal disease even when glomerular hemodynamics remain unchanged.24 The status of the intrarenal renin-angiotensin system in heterozygous TGR rats is currently unclear. Although functional studies suggest heightened activity,15 16 kidney angiotensin II levels are significantly lower compared with levels in normotensive Sprague-Dawley control rats.7 Regardless, angiotensin inhibition is clearly not a panacea for all forms of hypertension-associated kidney damage. Chronic treatment of spontaneously hypertensive rats with enalapril, which normalizes systemic BP and presumably suppresses glomerular barotrauma and intrarenal angiotensin II production, only postpones the eventual development of renal injury that is as severe as that found in their untreated hypertensive counterparts.25
In summary, long-term use of enalapril in TGR(mRen2)27 rats controls hypertension and cardiac hypertrophy, increases RBF by preferentially decreasing preglomerular vascular resistance, and prevents glomerulosclerosis and pathological proteinuria. The mechanism of its beneficial effect on the kidney is unclear. Potential explanations include reduction of glomerular capillary pressure and suppression of the intrarenal renin-angiotensin system. These animals represent a unique model for examining the effects of renin-angiotensin system dysfunction on BP regulation and the kidney. Future studies that compare varying doses of angiotensin II antagonists or renin inhibitors with other antihypertensive medications and employ telemetry to assure equivalent levels of BP control should prove useful in separating hypertensive from nonhypertensive actions of the mouse renin transgene on the kidney.
| Selected Abbreviations and Acronyms |
|---|
|
| Acknowledgments |
|---|
| Footnotes |
|---|
Portions of this work were presented at the 1995 Annual Meeting of the Society for Pediatric Research, San Diego, Calif, May 9, 1995, and published in abstract form (Pediatr Res [1995;37:371A]).
Received January 9, 1997; first decision February 17, 1997; accepted March 12, 1997.
| References |
|---|
|
|
|---|
2.
Bader M, Zhao Y, Sander M, Lee M, Bachmann J, Bohm M,
Djavidani B, Peters J, Mullins J, Ganten D. Role of tissue renin
in the pathophysiology of hypertension in TGR(mRen2)27 rats.
Hypertension. 1992;19:681-686.
3.
Rapp J. Use and misuse of control strains for
genetically hypertensive rats. Hypertension. 1987;10:7-10.
4.
Springate J, Feld L, Ganten D. Renal function
in hypertensive rats transgenic for mouse renin gene.
Am J Physiol. 1994;266:F731-F737.
5. Feld L, Cachero S, Ellis E, Dedeoglu O, Ueda Y, Leonard C, Letizia R, Mallon V, Van Liew J. Resistance to glomerular injury in the diabetic biobreeding rat. Exp Physiol. 1995;80:991-1000.[Abstract]
6. Feld L, Van Liew J, Brentjens J, Boylan J. Renal lesions and proteinuria in the spontaneously hypertensive rat made normotensive by treatment. Kidney Int. 1981;20:606-614.[Medline] [Order article via Infotrieve]
7.
Komatsu K, Frohlich E, Ono H, Ono Y, Numabe A, Willis
G. Glomerular dynamics and morphology of aged
spontaneously hypertensive rats. Hypertension. 1995;25:207-213.
8.
Lee M, Bohm M, Kim S, Bachmann S, Bachmann J, Bader M,
Ganten D. Differential gene expression of renin and
angiotensinogen in the TGR(mRen2)27 transgenic rat.
Hypertension. 1995;25:570-580.
9.
Hollenberg N, Fisher N. Renal circulation and
blockade of the renin-angiotensin system.
Hypertension. 1995;26:602-609.
10. Suto T, Losonczy G, Qiu C, Hill C, Samsell L, Ruby J, Charon N, Venuto R, Baylis C. Acute changes in urinary excretion of nitrite + nitrate do not necessarily predict renal vascular NO production. Kidney Int. 1995;48:1272-1277.[Medline] [Order article via Infotrieve]
11. Boger R, Bode-Boger S, Gerecke U, Gutzki F, Tsikas D, Frolich J. Urinary NO3- excretion as an indicator of nitric oxide formation in vivo during oral administration of L-arginine or L-NAME in rats. Clin Exp Pharmacol Physiol. 1996;23:11-15.[Medline] [Order article via Infotrieve]
12. Bachmann S, Peters J, Engler E, Ganten D, Mullins J. Transgenic rats carrying the mouse renin genemorphological characterization of a low-renin hypertension model. Kidney Int. 1992;41:24-36.[Medline] [Order article via Infotrieve]
13. Bohm M, Lee M, Kreutz R, Kim S, Schinke M, Djavidani B, Wagner J, Kaling M, Wienen W, Bader M, Ganten D. Angiotensin II receptor blockade in TGR(mRen2)27: effects on renin-angiotensin system gene expression and cardiovascular functions. J Hypertens. 1995;13:891-899.[Medline] [Order article via Infotrieve]
14.
Hirth-Dietrich C, Stasch J, Ganten D, Luft F.
Renal effects of captopril and nitrendipine in transgenic rats with an
extra renin gene. Hypertension. 1994;23:626-631.
15.
Campbell D, Rong P, Kladis A, Rees B, Ganten D, Skinner
S. Angiotensin and bradykinin peptides in the
TGR(mRen2)27 rat. Hypertension. 1995;25:1014-1020.
16.
Gross V, Lippoldt A, Schneider W, Luft F. Effect
of captopril and angiotensin II receptor blockade on
pressure-natriuresis in transgenic TGR(mRen2)27 rats.
Hypertension. 1995;26:471-479.
17.
Mitchell K, Mullins J. Ang II dependence of
tubuloglomerular feedback responsiveness in
hypertensive ren-2 transgenic rats. Am J
Physiol. 1995;268:F821-F828.
18. Gross V, Roman R, Cowley A. Abnormal pressure-natriuresis in transgenic renin gene rats. J Hypertens. 1994;12:1029-1034.[Medline] [Order article via Infotrieve]
19. Buckalew VM. Pathophysiology of progressive renal failure. South Med J. 1994;87:1028-1033.[Medline] [Order article via Infotrieve]
20. Feld L, Zamlauski-Tucker M, Springate J, Van Liew J. Single nephron hemodynamics in spontaneously hypertensive rats. Proc Soc Exp Biol Med. 1995;209:185-189.[Medline] [Order article via Infotrieve]
21.
Heudes D, Michel O, Chevalier J, Scalbert E, Ezan E,
Bariety J, Zimmerman A, Corman B. Effect of chronic ANG
I-converting enzyme inhibition on aging processes, I: kidney structure
and function. Am J Physiol. 1994;266:R1038-R1051.
22. Ferder L, Inserra F, Romano L, Ercole L, Pszenny V. Decreased glomerulosclerosis in aging by angiotensin-converting enzyme inhibitors. J Am Soc Nephrol. 1994;5:1147-1152.[Abstract]
23.
Remuzzi A, Imberti O, Puntorieri S, Malanchini B,
Macconi D, Magrini L, Bertani T, Remuzzi G. Dissociation between
antiproteinuric and antihypertensive effect of angiotensin
converting enzyme inhibitors in rats. Am
J Physiol. 1994;267:F1034-F1044.
24. Ruiz-Ortega M, Gonzalez S, Seron D, Condom E, Bustos C, Largo R, Gonzalez E, Ortiz A, Egido J. ACE inhibition reduces proteinuria, glomerular lesions and extracellular matrix production in a normotensive rat model of immune complex nephritis. Kidney Int. 1995;48:1778-1791.[Medline] [Order article via Infotrieve]
25.
Feld L, Cachero S, Van Liew J, Zamlauski-Tucker M,
Noble B. Enalapril and renal injury in spontaneously
hypertensive rats. Hypertension. 1990;16:544-554.
This article has been cited by other articles:
![]() |
E. VanBavel and M. J. Mulvany Integrins in Hypertensive Remodeling Hypertension, February 1, 2006; 47(2): 147 - 148. [Full Text] [PDF] |
||||
![]() |
S. M. Jacinto, J. J. Mullins, and K. D. Mitchell Enhanced renal vascular responsiveness to angiotensin II in hypertensive ren-2 transgenic rats Am J Physiol Renal Physiol, February 1, 1999; 276(2): F315 - F322. [Abstract] [Full Text] [PDF] |
||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Hypertension Home | Subscriptions | Archives | Feedback | Authors | Help | AHA Journals Home | Search Copyright © 1997 American Heart Association, Inc. All rights reserved. Unauthorized use prohibited. |