(Hypertension. 1999;34:176-180.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
From the Departments of Pediatrics (S.D., S.S.E.-D.) and Physiology (L.C., L.M.H.-B., G.P., J.D.I.), Tulane University School of Medicine, New Orleans, La.
Correspondence to Samir S. El-Dahr, MD, Tulane University School of Medicine, Department of Pediatrics, SL-37, 1430 Tulane Ave, New Orleans, LA 70112-2699. E-mail seldahr{at}tmcpop.tmc.tulane.edu
| Abstract |
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Key Words: kallikrein-kinin system renin-angiotensin system receptors, bradykinin blood pressure
| Introduction |
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Although the role of kinins in the regulation of renal and cardiovascular homeostasis in the adult animal has received much attention, the role of the kallikrein-kinin system in developmental physiology is largely unknown. Emerging evidence indicates that the developing distal nephron is endowed with a local kinin-generating system.5 6 7 8 In addition, there is a considerable amount of kallikrein-like activity and bradykinin (BK) B2R mRNA in the developing vascular wall and heart.7 8 Therefore, it is conceivable that endogenous kinins counterbalance the effects of vasoconstrictor and antinatriuretic systems during normal growth and development. Accordingly, the present study was designed to (1) compare the maturational changes in BP in wild-type and B2R null mice, and (2) examine the impact of B2R ablation on the susceptibility to early salt-sensitive hypertension. In addition, to assess the contribution of the renin-angiotensin system, we examined the effects of B2R inactivation and differing salt diets on renal renin and angiotensin type 1 (AT1) gene expression and angiotensin II (Ang II) levels.
| Methods |
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Male and female B2R+/+ and
B2R-/- mice were placed
on either NS 1% (TD#90229) or HS 5% (TD#92102) isocaloric diets
(Harlan Laboratories) 1 day before mating, and the diets were continued
for the duration of pregnancy. After delivery, the pups remained with
their mothers on the respective diets. Four groups of mice were
designated as follows:
NS/B2R+/+ (N=6),
HS/B2R+/+ (N=6),
NS/B2R-/- (N=14), and
HS/B2R-/- (N=12).
Additional B2R-/- mice
received (1) NS during gestation and the diet was switched to HS on day
1 or 10 after birth (end of nephrogenesis in mice): these groups were
designated NS
HS, groups 1 (N=16) and 2 (N=18), respectively; or (2)
HS during gestation and switched to NS on day 10 after birth (HS
NS,
N=10). We used the term NS to indicate a lower salt content than HS
(5-fold difference). The NS diet used in this study contained a
slightly higher NaCl content than the standard laboratory chow
(0.7%).
BP Measurement
Tail-cuff BP was measured in trained prewarmed conscious mice at
2, 3, and 4 months of age with a tail-cuff apparatus (IITC
model 29 pulse amplifier, IITC Inc). Arterial pulsations
detected by the tail-cuff photocell were displayed with output from the
cuff pressure transducer on a digital storage oscilloscope (Tektronix
model 2211, Tektronix, Inc) and measured with a built-in cursor
function. Tail-cuff pressure values were derived from an average of 8
to 10 measurements per animal at each time point. The reproducibility
of this method was tested periodically by repeating the BP measurements
on the same animals on different days.
To measure intra-arterial mean arterial pressure (MAP), 4-month-old mice were anesthetized with a combination of thiobutabarbital (Inactin; 100 mg/kg) and ketamine (Ketaset; 10 mg/kg) IP and placed on a servo-controlled surgical table to maintain body temperature at 37°C. A tracheostomy was performed and a short polyethylene (PE-90) tube was inserted to maintain a patent airway. The right jugular vein was cannulated with a PE-10 catheter for infusion of fluids and drugs. The right carotid artery was cannulated with a short PE-10 catheter connected to PE-50 catheter and a polygraph (Grass Instrument Co) by a polygraph transducer (Gulton-Statham Transducers, Inc) for arterial pressure monitoring and blood sampling. The bladder was exposed and catheterized through a suprapubic incision with the use of a PE-50 tube for urine collections. During surgery, an isotonic saline solution that contained 6% albumin was infused at a rate of 5 µL/min. After surgical preparation, an isotonic saline solution that contained 1% albumin was infused. The animals were allowed to breath air enriched with O2 by placing the exterior end of the tracheal cannula inside a small plastic chamber into which humidified 95% O2/5% CO2 is continuously passed. This procedure improves the cardiovascular stability of animals. After completion of the surgical procedure, a 20-minute equilibration period was allowed. Control MAP under anesthesia was monitored for a 20-minute period. Vasodepressor dose of BK (50 ng IV) followed by vasopressor dose of Ang II (50 ng IV) was administered as a bolus in 100 µL volume. The aim of BK injection was to confirm the functional absence of B2R in -/- mice. The aim of Ang II injection was to evaluate whether the absence of functional B2R leads to increased vasopressor sensitivity to exogenous Ang II. Drug testing occurred in 5-minute intervals. At the end of the experiment, the animals were euthanized with excess IV anesthesia.
Northern Blot Analysis
RNA extraction, gel electrophoresis, RNA transfer to membrane,
and hybridization procedures were performed as previously
described.5 7 The Northern blots were hybridized with
random-primed 32P-labeled cDNAs for rat renin or
mouse AT1 that were stripped and reprobed with
human GAPDH. Signals were detected by autoradiography
and quantified by scanning densitometry (Ultroscan, Pharmacia LKB).
Measurement of Plasma and Kidney Ang II
Plasma and kidney Ang II levels were measured in
anesthetized mice by a radioimmunoassay with rabbit antiAng
II antibody (Jean Sealy, Cornell School of Medicine, New
York),9 monoiodinated
125I-labeled Ang II (Amersham), and Ang II
standard as previously described.10 Results are reported
in femtomoles per gram kidney weight or mL of plasma. The sensitivity
of the Ang II assay is <5 fmol/g. The percentage of specific binding
for Ang II averaged 39%, with nonspecific binding of 2.7%.
Data Analysis and Statistics
Comparisons among the groups were performed by Student
t test or ANOVA followed by Tukey test. P<0.05
was considered statistically significant. All data are reported as
mean±SEM.
| Results |
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HS, groups 1) or 10 (NS
HS,
group 2) after birth (Figure 2B). The results revealed that SBP
in both B2R-/- NS
HS
groups were comparably elevated and values were not significantly
different from the
HS/B2R-/- mice that
received lifelong HS. In contrast, discontinuation of HS on day 10
after birth (HS
NS) was associated with a gradual decrease in SBP to
normal values by 4 months of age (Figure 2).
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Anesthetized MAP, which was measured at 4 months of age, is similar in NS/B2R+/+, HS/B2R+/+, and NS/B2R-/- mice (Figure 3A). In contrast, HS/B2R-/- mice have a significantly higher MAP compared with HS/B2R+/+, NS/B2R+/+, and NS/B2R-/- mice (91±3 versus 75±5, 70±2, and 74±2 mm Hg, respectively), which is consistent with the results of SBP obtained in the conscious state.
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As shown in Figure 3B, BK (50 ng IV) caused a significant drop of MAP in B2R+/+ mice (-12±3 mm Hg) but did not alter MAP in B2R-/- mice, which confirmed the functional absence of B2R. Ang II (50 ng IV) increased MAP to the same extent (40±3 to 55±4 mm Hg) in all the groups, thus indicating that Ang IIinduced vascular responses are preserved in B2R-/- mice (Figure 3C). Urinary sodium excretion, which was measured in anesthetized mice, was 3-fold higher in HS/B2R-/- than NS/B2R-/- mice (0.98±0.1 versus 0.32±0.05 µEq · min-1 · g-1, P<0.001, N=6 and 5, respectively), which reflected the higher salt intake.
Effects of B2R Ablation and Lifelong HS on the
Renin-Angiotensin System
To evaluate whether increased expression of renin and/or
AT1 receptors contributes to hypertension in
B2R-/- mice on lifelong
HS, we measured kidney renin and AT1 mRNA levels
by Northern blot analysis (Figure 4A). The results showed that kidney renin
mRNA levels were not different between
NS/B2R-/- and
HS/B2R-/- groups.
However, renin mRNA levels were significantly lower in
B2R mutants than wild-type controls irrespective
of the salt intake (Figure 4A). Additionally, no significant
differences were observed in renal AT1 mRNA
levels between HS/B2R-/-
and HS/B2R+/+ mice (Figure 4B).
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Plasma and kidney Ang II levels were measured at 5 months of age in 2
groups of B2R-/- mice
that were switched from NS
HS or HS
NS on day 10 after birth
(Figure 5). Plasma Ang II levels were not
different between the 2 groups. In contrast, kidney Ang II levels were
significantly suppressed in the NS
HS group versus the HS
NS group
(306±38 versus 143±36 fmol/g, P<0.001).
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| Discussion |
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Borkowski et al1 developed a mouse strain with a targeted disruption of the B2R gene in which the open reading frame of the B2R gene was replaced by a neomycin cassette. These mice lack all physiological responses to exogenous BK. With the use of adult B2R-/- mice (weight 26 to 30 g) derived from an inbred strain on a 129Sv genetic background, Alfie et al2 examined the effects of permanent loss of endogenous kinin activity on BP and renal hemodynamics. In B2R-/- mice maintained on a HS diet (3% in food and 1% saline in drinking water) for 8 weeks, tail-cuff BP and anesthetized MAP were 15 and 35 mm Hg higher than in null mice on NS, respectively. In contrast, there was no difference in tail-cuff BP or MAP in control mice fed either a NS or HS diet. In addition, renal blood flow was reduced by 20% and renal vascular resistance was doubled in B2R-/- on HS compared with controls. Subsequent studies by Madeddu et al3 in adult B2R null mice (weight 23 g) bred onto 129Sv genetic background reported that SBP and MAP were 15 mm Hg higher in B2R-/- than B2R+/+ mice on NS. B2R-/- mice showed exaggerated vasopressor responses to Ang II, and chronic administration of an AT1 receptor antagonist reduced the BP of B2R-/- to the levels of B2R+/+ mice.3 However, renin and AT1 gene expression were not different between the groups. In addition, chronic salt loading (0.84 mmol/g chow for 15 days) increased tail-cuff BP of B2R-/- mice by 35 mm Hg, whereas it was ineffective in B2R+/+.3 Additional studies by the same group of investigators found that B2R null mice are highly susceptible to mineralocorticoid-induced hypertension.4 Together, these studies indicate that kinins play an important role in the prevention of salt-sensitive hypertension and that this may be achieved by maintaining renal blood flow under conditions of HS intake.
To our knowledge, the BP phenotype in B2R
mutants of younger age groups has not been reported. Likewise, the
effect of HS intake on the maturation of BP in
B2R null mice is unknown. Madeddu et
al11 administered the B2R
antagonist icatibant (formerly known as Hoe 140) to
pregnant Wistar rats and subsequently to their offspring maintained on
NS diet. At 9 weeks of age, rats that were administered icatibant
during prenatal and postnatal phases of life showed a modest BP
elevation (
8 mm Hg) versus vehicle-treated controls. In the
present study, we found that permanent inactivation of
B2R does not cause adult hypertension nor does it
alter the maturation of BP under conditions of NS intake. We observed a
small but statistically significant rise in tail-cuff SBP of
NS/B2R-/- compared with
NS/B2R+/+ at 2 months of
age. However, this difference in BP was not sustained and could no
longer be observed at 3 and 4 months of age, which indicated that the
B2R null mutation did not have a long-lasting
effect on the maturation of BP. In this regard, our findings are in
agreement with those of Alfie et al2 who found that the BP
of adult B2R null mice maintained on normal
sodium intake is not different from wild-type controls. A technical
limitation of investigations with mice is that measurement of BP is not
feasible in young animals (
1 month of age). Therefore, the earlier
changes in BP during the evolution of hypertension in
HS/B2R-/- mice were not
determined. The BL6 strain used in this study is known to have a
reduced BP when compared with other mouse strains, which suggests that
protective genes might overcome the lack of B2R
signaling and mask hypertension during early development.
The development of salt-sensitive hypertension in young B2R null mice suggests that the kallikrein-kinin system plays an important role in the hemodynamic adaptation to postnatal life under conditions of HS intake. We have demonstrated previously that the renal and cardiovascular systems express local, developmentally regulated, kinin-generating systems. Vascular and renal kallikrein-like activity increases 10- to 20-fold from birth to adulthood in the rat.8 In addition, B2R and kininogen gene expression in the kidney, aorta, and heart is 10- to 30-fold higher in developing animals versus adult animals.11 12 Accordingly, we speculate that lack of kinin's natriuretic and vasodilator activities contribute to the pathogenesis of the salt-sensitive hypertension in B2R-deficient mice. In addition, we tested whether the renin-angiotensin system contributes to the hypertension in B2R null mice on lifelong HS. No significant differences were detected in renal renin or AT1 receptor mRNA levels in the mutant animals on HS, and kidney Ang II levels were appropriately suppressed in mutant mice that received HS from early postnatal life. We also found that the BP responses to intravenous Ang II were similar in B2R null mutants and wild-type mice. However, the latter data should be interpreted with caution because of the small number of animals in the NS/B2R-/- group, the possible influence of anesthesia on Ang II activity, and that a full dose-response curve was not performed. As suggested previously,3 unopposed activity of Ang II in the absence of kinin activity may contribute to hypertension in B2R null mice on HS. The role of the renin-angiotensin system in mediating salt sensitivity in B2R mutants deserves additional study.
Barker12 has proposed that adult arterial hypertension has its roots during early development. Developmental insults or stressors may alter the maturation of blood pressure and/or predispose to salt sensitivity. For example, intrauterine protein undernutrition is associated with later development of hypertension in the offspring.13 In the present study, we found that exposure to a HS environment only during fetal life is not sufficient in itself to cause long-term hypertension in B2R null mice. Altered renal development is less likely to be a factor in the development of hypertension because delaying salt loading until the end of nephrogenesis in mice (day 10) was associated with the same magnitude of hypertension as in null mice on lifelong HS.
On the basis of these and other results,2 3 4 we conclude that the kallikrein-kinin system protects both developing and mature animals from salt-sensitive hypertension. The B2R-deficient mouse is a monogenetic model that can be used to study the developmental aspects of salt-sensitive hypertension.
| Acknowledgments |
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| Footnotes |
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Received January 8, 1999; first decision February 8, 1999; accepted April 13, 1999.
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