Hypertension is a cardiovascular risk factor present in over two thirds of people
over age 60. Elevated blood pressure(BP) correlates with increased risk of heart attack,
stroke, and progression to heart and kidney failure and current therapies are insufficient
to control BP in almost half of these patients
1,2
. Mineralocorticoid receptors(MR) in the kidney are known to regulate BP through aldosterone
binding and stimulation of sodium retention
3
. Recent studies support extra-renal actions of MR
4–7
as sodium handling alone cannot fully explain the development of hypertension and
associated cardiovascular mortality
8,9
. We and others have identified functional MR in human vascular smooth muscle cells(SMC)
10,11
, supporting the potential for vascular MR to play a direct role in regulating BP.
Here we show that mice with SMC-specific deletion of MR have decreased BP as they
age without defects in renal sodium handling or vascular structure. Aged mice lacking
SMC-MR have reduced vascular myogenic tone, agonist-dependent contraction, and expression
and activity of L-type calcium channels. Moreover, SMC-MR contributes to Angiotensin
II-induced vascular oxidative stress, vascular contraction, and hypertension. This
study identifies a novel role for SMC-MR in BP control and in vascular aging and supports
the emerging hypothesis that vascular tone contributes directly to systemic BP.
The renin-angiotensin-aldosterone system(RAAS) is a hormonal cascade with a well-recognized
role in BP regulation. Angiotensin II(AngII) acts via the angiotensin type 1 receptor(AT1R)
on vascular cells to cause vasoconstriction and on adrenal cells to cause release
of the hormone aldosterone that activates renal MR to modulate sodium balance
12
. Mice deficient in MR in all tissues die in the neonatal period from salt wasting
consistent with the known role of MR in regulating vascular volume
13,14
. However, mice with renal tubule-specific MR deletion survive unless challenged with
low-salt conditions
15,16
, supporting that loss of extra-renal MR contributes to the hypotension and mortality
associated with complete MR-deficiency. MR antagonists prevent activation of MR
11,12
and decrease BP and cardiovascular mortality
17–19
. A recent analysis of MR antagonist trials for the treatment of hypertension concluded
that a component of the BP-lowering effect of MR antagonism can be distinguished from
effects on urinary electrolyte excretion, supporting renal-independent regulation
of BP by MR in humans
6,20
. Mice over-expressing MR in endothelial cells(EC) have increased BP
7
, but the role of endogenous vascular MR in the regulation of BP has not been explored.
We developed a model specifically deficient in SMC-MR by creating and breeding loxP
flanked MR(MRf/f) mice with mice containing the smooth muscle actin promoter driving
expression of the tamoxifen-inducible Cre-ERT2 recombinase
21
(SMA-Cre-ERT2, Supplementary Fig. 1a). Comparisons are made between tamoxifen-induced
MRf/f/SMA-Cre-ERT2+ (“Cre+”) and tamoxifen-induced MRf/f/SMA-Cre-ERT2− littermate
controls (“Cre−“). In this model, there are no alterations in feeding or growth(Supplementary
Table 1 and data not shown) and MR remains intact in tissues without significant SMC
populations(Fig. 1a). Following induction of Cre+ mice, vascular SMC display near
complete MR gene excision and loss of MR(Nr3c2) mRNA expression(Figs. 1b–c and Supplementary
Fig. 1) without alteration in aortic mRNA expression of angiotensin receptors (data
not shown).
Telemetric BP monitoring reveals lower BP in male Cre+ mice compared to Cre− littermates(Fig.
1d). The BP difference increases with age becoming significant by age 7 months. Contraction
and relaxation of mesenteric resistance arteries(MRA) was assessed in adult(3–4 months-old)
and aged(>9 months-old) mice. Cre+ vessels from adult mice show a modest increase
in contraction to phenylephrine(PE) and enhanced endothelial-dependent vasodilatation
compared to Cre− controls consistent with overall preserved BP(Fig. 1e and Supplementary
Fig. 2). With aging, MRA from Cre− mice demonstrate significantly increased contraction
to KCl and the thromboxane receptor(TP) agonist U46619 compared to adult mice. This
age-dependent contraction increase is lost in aged Cre+ littermates and is associated
with a modest decrease in EC-independent vasodilatation. PE-induced contraction increases
with age regardless of genotype. These data support a direct role for SMC-MR in BP
regulation by modulating intrinsic vascular function of resistance vessels, particularly
with aging.
Since MR contributes to BP control by modulating renal sodium handling
3
, we investigated salt-sensitivity of BP. In adult mice, there is no BP difference
between genotypes, regardless of dietary sodium intake(Fig. 2a). In aged mice, the
lower BP in Cre+ mice is maintained under high- or low-sodium conditions(Fig. 2b)
and throughout the diurnal cycle(Supplementary Fig. 3). No difference is observed
in serum or urine electrolytes or serum aldosterone levels between Cre+ and Cre− mice(Supplementary
Table 1). Infusion of aldosterone with 1% NaCl drinking water significantly increased
BP in adult and aged mice to similar levels, independent of genotype(Figs. 2c–d).
On a low-sodium diet, aged mice demonstrate the expected decrease in urinary sodium
excretion and fractional excretion of sodium regardless of genotype(Figs. 2e–f). These
studies are consistent with intact MR function in the kidney tubules and support an
extra-renal, SMC-MR-dependent mechanism for BP regulation.
We next examined the effect of SMC-MR deletion on vascular structure by several methods.
Aortic medial area and collagen content increase with age independent of the presence
of SMC-MR(Figs. 3a–b). In 18 month-old mice, cardiac hypertrophy is prevented in Cre+
animals(Supplementary Fig. 4), consistent with end-organ effects of longstanding exposure
to lower BP. Since resistance vessels play a critical role in modulating BP, we examined
structural characteristics of pressurized MRA from aged mice. There is no difference
between Cre+ and Cre− animals in passive vessel or lumen diameter, area, stress, strain,
distensibility or stiffness over a range of intraluminal pressures(Fig. 3c–d, and
Supplementary Table 1). However, MRA from Cre+ mice develop significantly less spontaneous
myogenic tone compared to Cre− mice(Fig. 3d). These data support that SMC-MR contributes
to vascular tone and BP regulation in aged mice independent of vascular structural
changes.
Voltage-gated calcium (Ca)-channel activation contributes to myogenic tone while large
conductance Ca-activated potassium channels(BKCa) counter-regulate myogenic constriction
22
. Since coronary BKCa expression is reduced in mice with cardiac-specific aldosterone
synthase overexpression
23
, BKCa activity was first explored. Patch clamp studies of mesenteric SMC from aged
Cre+ and Cre− mice reveal no difference in total potassium(K+) current, the proportion
of K+ current from BKCa or voltage-activated K+ channels, or the response of BKCa
to activation with the agonist, NS1619(Fig. 3e). BKCa subunits Kcnma1 and Kcmnb1 mRNA
expression were not different in aged Cre− and Cre+ vessels but, expression of the
L-type Ca-channel Cacna1c (Cav1.2) was significantly decreased in Cre+ vessels(Fig.
3f). In addition, the contractile response of MRA to the L-type Ca-channel agonist
BayK8644 was significantly decreased in aged Cre+ mice(Fig. 3g). These data support
regulation of L-type Ca-channels by SMC-MR as a mechanism underlying age-associated
alterations in myogenic tone, agonist-induced contraction, and BP.
RAAS signaling is recognized to be enhanced in the aging vasculature, contributing
to the vascular aging phenotype
24–26
. Since we previously demonstrated direct AT1R-dependent activation of MR by AngII
in human SMC
11
, we examined the responsiveness to in vivo AngII infusion. As expected
27
, AngII infusion produced a robust hypertensive response in Cre− mice that was significantly
enhanced in aged compared to adult mice(Figs. 4a–b). In adult Cre+ mice, the maximal
pressor response to AngII is reduced 31% correlating with a 44% reduction in maximal
vascular contraction to AngII(Figs. 4a and 4c). In aged Cre+ mice, there is no significant
AngII pressor response correlating with a lack of significant MRA contraction to AngII
in aged Cre+ vessels(Figs. 4b and 4d). The pressor response to AngII involves production
of reactive oxygen species(ROS)
28
as confirmed by inhibition of the AngII pressor response by the SOD-mimetic TEMPOL(Supplementary
Fig. 5). Basal and AngII-stimulated vascular ROS-production was quantified by whole
vessel dihydroethidium staining. In adult mice, there is no difference in basal vascular
ROS production but AngII-stimulated ROS is attenuated in Cre+ mice(Fig. 4e). These
findings are consistent with the lack of difference in basal BP in adult mice with
attenuation of the AngII pressor response in Cre+ mice. In aged mice, Cre+ vessels
produce significantly less vascular ROS that does not increase in response to AngII(Fig.
4f) correlating directly with the decreased basal BP and loss of AngII pressor and
contractile response in aged mice lacking SMC-MR. These data support that SMC-MR contributes
substantially to AngII-induced vascular oxidative stress, vascular constriction, and
BP elevation, particularly in the aging vasculature, and provides in vivo relevance
for bidirectional crosstalk between MR and AT1R signaling in SMC
11,29
.
Together, these studies demonstrate that SMC-MR modulates vascular contractile function
and tone independent of changes in vascular structure or defects in renal sodium handling
and plays a role in the BP response to AngII and the age-associated increase in BP.
These findings support the emerging hypothesis that direct regulation of vascular
tone contributes to regulation of systemic BP
30
and are consistent with clinical studies suggesting extra-renal mechanisms underlying
the development of hypertension and associated cardiovascular diseases
8,9
. These studies also support that vascular MR may be important in the electrolyte-independent
anti-hypertensive effects of AT1R- and MR-antagonist drugs
6,20
. Alternatively, or in addition, modulation of renal vascular tone could alter renal
function as novel treatments for hypertension are targeting renal vascular sympathetic
tone with some success
31
. Thus, although this study demonstrates intact renal MR function, regulation of renal
vascular tone by SMC-MR could have secondary effects on sodium handling that also
contribute to BP control.
Vessels from aged SMC-MR-deficient mice have profound defects in myogenic tone and
contraction to the GPCR agonists thromboxane and AngII (but not to the GPCR agonist
PE, which has some divergent signaling mechanisms). TP antagonism inhibits AngII-induced
hypertension
32
supporting overlap between AT1R and TP signaling pathways. Both pathways modulate
Ca-channel activity
22,33
, which is decreased in this model. Angiotensin-mediated ROS production has been shown
to regulate vascular Cav1.2 expression suggesting a potential mechanistic link between
reduced vascular ROS and Cav1.2 expression in aged SMC-MR-deficient mice
34
. Further exploration of the role of SMC-MR in regulation of vascular GPCR signaling,
Ca-channel expression, and oxidative stress is warranted.
Aging-associated hypertension is common, with an incidence approaching 80% in people
over 80 years of age
1
. Although there is potential for some adaptive benefit to the rise in BP with age,
this phenomenon contributes substantially to the incidence of heart attack, stroke,
atrial fibrillation, and kidney and heart failure
1,2
. Deficiency in SMC-MR prevented many aspects of cardiovascular aging including increased
BP, cardiac hypertrophy, vascular contraction, BP responsiveness to AngII, and oxidative
stress supporting SMC MR as a global regulator of vascular aging. Current MR antagonists
are beneficial but hyperkalemia mediated by renal MR inhibition is one important limitation.
Further study of the diverse roles of MR, and the signaling pathways it regulates
in the vasculature, will be valuable to identify novel therapies for common cardiovascular
disorders, particularly in the aging population.
Online Methods
Generation of inducible SMC-specific MR KO mouse
All animals were handled in accordance with National Institutes of Health standards
and all procedures were approved by the Tufts Medical Center Institutional Animal
Care and Use Committee. A genomic region encompassing exons 5 and 6 of the MR gene,
encoding the hinge region and the N-terminal part of the ligand binding domain, was
flanked by loxP sites via homologous recombination in ES cells, and floxed MR mice
(MRf/f) were generated according to standard procedures
35
. SMC-MR KO mice were generated by crossing MRf/f mice with SMA-Cre-ERT2 mice (smooth
muscle actin promoter driving expression of Cre-ERT2 recombinase that is activated
by tamoxifen)
21
. Mice were born in Mendellian frequencies. For all studies, male MRf/f/SMA-Cre-ERT2+
(Cre+) and MRf/f/SMA-Cre-ERT2− littermates (Cre−) were induced by intraperitoneal
injection of 1 mg Tamoxifen daily for 5 days at age 6–8 weeks and all studies were
performed at least 4 weeks post-induction to allow for MR excision and degradation.
SMA-Cre-ERT2 mice were also crossed with the Rosa26 reporter mouse (Jackson labs)
and induced with vehicle or tamoxifen as described above and sacrificed two weeks
following the last injection. Tissues were removed, fixed and embedded in paraffin,
and stained with X-gal as described
21
.
PCR for MR genomic DNA
DNA was extracted and PCR was performed resulting in a smaller LoxP-MR DNA band and
a larger excised MR band in induced Cre+ tissues using a combination of three primers:
5’-CCACTTGTATCGGCAATACAGTTTAGTGTC-3’; 5’-CACATTGCATGGGGACAACTGACTTC-3’; 5’-CTGTGATGCGCTCGGAA
ACGG -3’.
QRT-PCR
RNA was extracted, reverse transcribed and QRT-PCR was performed with gene-specific
primers as previously described
36
. CT values were normalized to β2-microglobulin (B2m) and Cre+ mRNA levels were expressed
as a percent of Cre− levels. Specific primers for QRT-PCR for MR (nuclear receptor
subfamily 3, group C, member 2; Nr3c2), BKCaα1 (potassium large conductance calcium-activated
channel, subfamily M, alpha member 1; Kcnma1), BKCaβ1 (potassium large conductance
calcium-activated channel, subfamily M, beta member 1; Kcnmb1), Cav1.2 (calcium channel,
voltage-dependent, L type, alpha 1C subunit; Cacna1c) and B2m are listed in Supplementary
Table 2.
Telemetric blood pressure (BP)
All BP studies were performed using implantable BP transmitters (Data Sciences International,
TA11PA-C10) with n = 4–8 mice per group. BP was recorded for 60 sec every 30 min as
previously described
37
. Animals were maintained on a 12:12 hour light:dark cycle, with normal chow (0.3%
NaCl; Harlan diet TD8604) and water available ad libitum. For salt challenges, mice
with telemetric devices were fed a low salt diet (0.02% NaCl; Harlan diet TD90228)
or a high salt diet (6% NaCl; Harlan diet TD90230) for 5 days and BP on days 3–5 were
averaged. For aldosterone and salt administration, osmotic minipumps were implanted
(Alzet) to infuse aldosterone (Sigma) at 240 μg kg−1 d−1, for two weeks. After 1 week,
1% NaCl was added to the drinking water. For AngII administration, osmotic minipumps
were implanted (Alzet) to infuse AngII (Sigma) at 800 ng kg−1 min−1 for two weeks.
After 1 week, TEMPOL (1mmol, Sigma) was added to the drinking water.
Chemistries
Serum aldosterone was measured by RIA (Diagnostic Products Inc.). 24-hour urine and
simultaneous serum samples were collected from mice fed normal (0.3%) or low salt
chow, electrolytes were quantified (IDEXX Preclinical Services) and FENa was calculated:
FENa = (serumCr * urineNa ) / (serumNa × urineCr)*100.
Histology
Formalin fixed, paraffin embedded sections of thoracic aorta from 3, 9, and 18 month-old
mice, were stained with elastin stain or Masson’s trichrome and medial area and collagen
content were quantified using computerized morphometric analysis (Image-Pro software)
by a blinded investigator as previously described
6
. n = 3 aortas per age group and genotype.
Mesenteric vessel wire myograph studies
Rings from second order mesenteric resistance arteries (MRA) were mounted (Danysh
MyoTechnology) for isometric tension recordings using PowerLab software (AD Instruments).
A total of eight rings per mouse were used with n = 5–8 mice in each wire myograph
study. Rings were placed under a resting tension of 2 mN in tissue baths containing
warmed (37°C), aerated (95% O2, 5% CO2) standard PSS (in mM: 130 NaCl, 4.7 KCl, 1.17
MgSO4, 0.03 EDTA, 1.6 CaCl2, 14.9 NaHCO3, 1.18 KH2PO4, and 5.5 glucose). Administration
of 10 μM phenylephrine (PE) was used to test arterial viability, and presence of intact
endothelium was verified by acetylcholine (Ach, 1 μM)-induced relaxation of a half-maximal
PE-induced contraction. Concentration-response curves for Ang II, PE, U46619, KCl,
Ach and sodium nitroprusside (SNP) were built. For relaxation studies, vessels were
pre-contracted with U46619 at EC90 prior to administration of Ach and SNP (Average
pre-contracted force = 6.6 mN Cre- adult, 7.2 mN Cre-aged, 6.9 mN Cre+ adult, 7.2
mN Cre+ aged, P = not significant).
Mesenteric vessel structural and reactivity studies
For structure and distensibility studies, MRA from n = 5–8 mice for each genotype
were cannulated in a pressure myograph (Living System Instrumentation) and incubated
in calcium-free PSS containing 2 mM EGTA and 1 μM SNP for analysis of passive structure
over a range of intralumenal pressures (0 to 180 mm Hg) as described previously
38
. The elastic modulus (β-coefficient) was calculated from the stress/strain curves
for the individual vessels, and these curves were fitted to an exponential model (y
= ae
β
x
), where β is the slope of the curve: the higher the β-coefficient the stiffer the
vessel. Distensibility was calculated as the percent change in LD at a given intralumenal
pressure (LDp) from LD at 3 mm Hg (LDo): [(LDp-LDo)/LDo]*100 as described
38
. Myogenic reactivity was measured over a pressure range of 10–120 mm Hg in Ca2+-containing
PSS. After active tone measurements, vessels were superfused with buffer lacking added
Ca2+ and containing 2 mM EGTA. Passive diameter responses were then recorded over
the pressure range, 10–120 mm Hg. Tone was calculated as the percent decrease in lumen
diameter (LD) from the passive LD at 70 mmHg: % tone = [1-(active diameter/passive
diameter)]*100 as described
39
. Contraction to BayK8644 (10−7M) and PE (10−6M), was assessed by video microscopy
in pressurized (70 mmHg) MRA.
Whole Cell K+ Channel Recordings
Mesenteric artery SMC were isolated as previously described
40
. K+ currents were measured using standard whole cell recording techniques
40
. To obtain current voltage relationships, cells were set at a holding potential of
−70 mV and voltage steps applied from −70 to +70 mV at 300 ms intervals. Components
of the total K+ current were pharmacologically isolated using iberiotoxin (BKCa inhibitor;
IBTX,10−7M), 4-aminopyridine (voltage-gated K+ channels inhibitor; 4AP, 10−3M) and
NS1619 (BKCa agonist, 10−7M).
Dihydroethidium (DHE) staining
Superoxide accumulation in carotid arteries was measured using DHE staining. Vessels
were incubated with vehicle (saline) or 200 nM AngII for 30 min (37 °C), rinsed and
treated with 2 × 10−6 M DHE (Molecular Probes) for 45 minutes (37 °C in the dark).
Vessels were washed, mounted on slides with ProLong Gold reagent (Invitrogen) and
images were obtained over the length of the vessel using a fluorescent microscope
(Nikon Optiphot-2) and SPOT advanced software, and average mean fluorescence intensity
was calculated by a blinded investigator for each vessel using ImageJ software.
Statistical Analysis
Values are reported as mean ± standard error of the mean. Within-group differences
were assessed with two-factor or three-factor ANOVA or RM ANOVA (mesenteric vessel
contraction studies) with Student-Newman-Keuls post-test. P < 0.05 was considered
significant.
Supplementary Material
1