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-estradiol in absence of NO in
ovariectomized rats: role of angiotensin II
Departamento de Fisiología y Farmacología, Facultad de Medicina, Universidad de Murcia, Campus de Espinardo, 30100, Murcia, Spain
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ABSTRACT |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Previous reports correlate plasma levels of
estrogen with increased nitric oxide (NO) production. To investigate
whether the hemodynamic effects of estrogens are mediated by NO, we
compared the hemodynamic changes induced by 17
-estradiol (100 µg/kg) in the absence and presence of the NO synthesis inhibitor
N
-nitro-L-arginine methyl ester
(L-NAME). All protocols were
performed in ovariectomized, conscious rats. Estradiol alone resulted
in no significant changes in cardiac index (CI) or mean arterial pressure (MAP). However, in the presence of
L-NAME, estradiol induced a
significant increase in total peripheral resistance (TPR) of 37.3 ± 11.7% and a decrease in CI of 27 ± 4.9%, without changes in MAP.
Previous blockade of angiotensin II
AT1 receptors with losartan
prevented any change in CI and TPR induced by 17-estradiol in the
presence of L-NAME. These
observations suggest that NO is necessary to offset a vasoconstrictor
action of angiotensin II, which is stimulated by estradiol
administration.
cardiac output; vascular resistance; estrogen; N-nitro-L-arginine methyl ester
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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A SIGNIFICANT ROLE for estrogen has been demonstrated in cardiovascular regulation. Emerging data suggest that estrogen may account for the reduced incidence of cardiovascular disease in premenopausal women. Estrogen therapy is associated with a reduction in the incidence of coronary heart disease (10a, 41) and reduction of blood pressure in postmenopausal women (32). In addition, estradiol treatment attenuated the development of hypertension in female spontaneously hypertensive rats (40) and transgenic hypertensive rats expressing the mouse Ren-2 gene (6). Although the estrogen-dependent mechanisms contributing to the regulation of arterial pressure remain unclear, some evidence indicates a possible relationship between estrogen, endothelial function, and the renin-angiotensin system.
Estrogen receptors were demonstrated in smooth muscle cells of the
aorta of dog, rat, and human coronary arteries (25, 26), and
high-affinity binding sites for estrogen were also reported in cytosols
isolated from endothelium of rabbit (10) and bovine aortas (3),
suggesting that the vascular endothelium is also estrogen sensitive.
According to these data, local and systemic responses of estradiol
administration are interpreted to mean that there is local production
of one or more vasodilator substances such as prostaglandin and nitric
oxide (NO) (9, 46). Thus Sudhir et al. (43) reported that in
perimenopausal women, estrogen supplementation enhances basal NO
release in forearm resistance arteries. Furthermore, studies performed
by Van Buren et al. (46) in animals reported that local uterine artery
injection of N-nitro-L-arginine
methyl ester (L-NAME)
suppressed, in a dose-dependent manner, the estrogen-induced increase
in uterine blood flow, suggesting that such a vasodilatory effect is
mediated mainly by NO. In addition, pregnancy and estradiol treatment
increase the amount of mRNA for nitric oxide synthase (NOS) isozymes in
the skeletal muscle and heart of female guinea pigs (47).
On the other hand, during pregnancy, decreased vascular reactivity to angiotensin II (1, 8) is associated with increased activity of the renin-angiotensin system (18). In addition, after estradiol administration to chronically instrumented sheep, there are significant increases in plasma renin activity (PRA) and cardiac output and a decrease in vascular resistance (27, 29) associated with a reduced pressor effect of angiotensin II (39). This attenuated pressor response to angiotensin II seen in pregnancy may simply reflect downregulation of the vascular AT receptor, as suggested by Beilin et al. (5). This explanation, however, is not supported by recent studies of angiotensin II binding characteristics and AT receptor density in arteries from pregnant animals (13, 37).
Bearing in mind the above data, we hypothesized that NO plays an
important role in countermodulating the angiotensin II vasoconstrictor effect induced by estrogen administration. Therefore, the purpose of
this study was to determine the role of NO in modulating the actions of
17-estradiol on systemic hemodynamic effects and the implication of
angiotensin II on these hemodynamic effects. To achieve
this goal, we compared the hemodynamic effect of estradiol in the
presence and absence of the NO synthesis inhibitor
L-NAME and evaluated whether or
not angiotensin II is implicated in the hemodynamic effect of estrogen
when NO synthesis is blocked. These last experiments were also
performed in the presence of AT1
angiotensin receptor blockade.
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METHODS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Experiments were performed on female ovariectomized Sprague-Dawley rats (330-400 g). Castration was done under anesthesia with Thalamonal (mixture of 16 mg/kg fentanyl and 0.83 mg/kg droperidol), and rats were placed in their cages for 2 mo until the day of the experiment. All the experimental protocols were carried out in previously intrumentized conscious rats.
Surgical procedures. Catheters were placed into the left femoral artery for measurement of mean arterial pressure (MAP) and heart rate (HR) and into the left femoral vein for infusion. A right atrial catheter and a thoracic aortic thermocouple were implanted via the right external jugular vein and right carotid artery, respectively. The catheters were brought out through the skin on the dorsal side of the neck. Finally, the distal ends of these lines were threaded through a lightweight flexible spring connected to a swivel. All surgical procedures were performed under aseptic conditions. Rats were placed in plastic cages with the swivels mounted above, allowing complete freedom of movement and free access to chow and tap water. Two full days were permitted for recovery from surgery.
Cardiac output was measured by thermodilution as previously described in our laboratory (23). The thermodilution curve and the pressure signal were processed with a microcomputer system (Cardiomax IIR, Columbus Instruments). Hemodynamic values were the mean of three determinations. Cardiac output was measured by rapid injection of 200 µl 0.9% saline at room temperature (20°C) through the jugular catheter, using a spring-loaded, constant-rate, constant-volume syringe (Hamilton CR 700-200). Cardiac index (CI) was calculated by dividing cardiac output by animal weight (100 g), and total peripheral resistance (TPR) was calculated by dividing MAP by CI.Experimental protocols. Four protocols were designed to evaluate systemic hemodynamic interactions among estrogens, NO, and angiotensin II (Fig. 1).
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-estradiol/kg
in 25 µl of absolute ethyl alcohol) in
group
I (n = 9). Vehicle for estrogen was infused in another group of rats
(group II,
n = 5). Hemodynamic parameters were
measured before the administration of 17-estradiol or vehicle (time
0) and 1, 2, and 3 h after the
administration of 17-estradiol.
Protocol
II was designed to evaluate how NO
synthesis blockade modified the hemodynamic responses to
17-estradiol. In group III
(n = 7),
L-NAME (a bolus of 3 mg/kg plus
continuous infusion of 50 µg · kg
1 · min1
in a 0.5% bovine albumin solution) was administered 30 min before 17-estradiol. In group
IV (n = 5), the same dose of L-NAME
was supplied and vehicle was administered instead of 17-estradiol. Basal hemodynamic measurements were performed 30 min after
L-NAME administration
(time
0), and hemodynamic measurements
were performed again 1, 2, and 3 h after administration of
17-estradiol or vehicle.
Protocol
III was designed to determine the
hemodynamic effects of estradiol in the presence of a vasoconstrictor
other than L-NAME, such as
phenylephrine. In group
V (n = 5), phenylephrine (15 µg · kg1 · min1)
was continuously infused, and 1 h was necessary to achieve a steady
state; 17-estradiol was then administered as described in
protocols
I and
II. In
group
VI (n = 5), the same dose of phenylephrine was given and vehicle instead of
estradiol was administered. Basal hemodynamic measurements were
performed 60 min after phenylephrine administration
(time
0), and hemodynamic measurements
were performed again 1, 2, and 3 h after administration of
17-estradiol (group V) or vehicle
(group VI).
Protocol
IV was designed to determine the role
of angiotensin II on the cardiovascular effects of 17-estradiol
administration in rats given
L-NAME
(group VII,
n = 7). In this group, losartan (10 mg/kg) was administered 30 min before the onset of
L-NAME infusion. After another
30-min equilibration period, hemodynamic measurements were taken before
the administration of 17-estradiol (time
0) and 1, 2, and 3 h after
17-estradiol. To investigate the role of angiotensin II on the
hemodynamic effects of 17-estradiol in normal conditions, losartan
was administered to another group of rats
(group VIII,
n = 5) 30 min before estradiol
infusion. The rest of protocol was the same as in
group
VII.
Hematocrit was measured in all protocols before any infusion (control
time), after L-NAME or losartan + L-NAME
(time
0), and 3 h after 17-estradiol or
vehicle administration.
To test inhibition of the angiotensin II receptor in the presence of
losartan, angiotensin II (50 ng/kg; Sigma, St. Louis, MO)
was administered as a bolus 10 min before and 1 h after losartan administration and also at the end of the experiment.
Evidence of the magnitude and persistence of receptor blockade is
presented in Table 1.
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Blood sample collection and analysis. One milliliter of whole blood was obtained at the end of the experiment in groups I and II. PRA was determined by using an angiotensin I radioimmunoassay kit (RENCTC, P2721). Plasma level of estradiol was measured by microparticle enzyme immunoassay (IMx estradiol assay; Abbott Laboratories, North Chicago, IL).
Data analysis. Analysis of variance for repeated measures was used to examine changes over time within and between groups at each time period, and Bonferroni's multiple-range test was used to determine differences between means. Changes were considered significant at P < 0.05. All values are reported as means ± SE.
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Hemodynamic responses to systemic 17-estradiol administration in
protocol
I are illustrated in Fig.
2. Baseline values at time
0 for all cardiovascular measurements
did not differ among two groups treated with 17-estradiol or vehicle
and were similar to those previously reported in our laboratory (23,
24). In group
I, estradiol alone had no significant
effect on hemodynamic parameters such as MAP, CI, stroke volume index
(SVI), and TPR. However, HR achieved a significant increase (10 ± 5%) at the third hour of 17-estradiol administration. Similar
results were obtained in group
II, in which vehicle was administered
instead of 17-estradiol, although HR remained unchanged in these
animals. Baseline hematocrit in groups
I and
II was 39.2 ± 1.4 and 37.7 ± 1.1%, respectively. Neither 17-estradiol nor vehicle had a
significant effect (38.7 ± 1.5 and 35.3 ± 1.8% 3 h after
17-estradiol and vehicle, respectively).
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Control data obtained before infusion of phenylephrine in
protocol III and of antagonists
(L-NAME and/or losartan)
in protocols II and
IV, as well as the systemic responses
after their administration and before 17-estradiol infusion, are
presented in Table 2. A significant increase in MAP was
observed after L-NAME,
accompanied by a significant fall in CI and HR and a rise in TPR
(P < 0.01). AT1 receptor blockade with
losartan resulted in slight but significant hemodynamic changes, a
decrease in MAP and TPR associated with increases in CI and HR.
However, L-NAME in the presence
of losartan (group
VII) produced hemodynamic changes
similar to those of L-NAME alone
(group III).
Infusion of phenylephrine resulted in a similar increase in blood
pressure and TPR as observed after
L-NAME. CI and HR significantly
decreased with phenylephrine to values that were not different from the
L-NAME-treated
group.
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Data obtained in protocol II are shown
in Fig. 3, in which the systemic responses
to 17-estradiol or vehicle in presence of
L-NAME are compared
(groups III
and IV, respectively). Vehicle did not
produce any hemodynamic changes after
L-NAME. When we examined the
response to 17-estradiol in presence of NO synthesis inhibition, we
found that MAP was unchanged over the 3-h period of the experiment.
However, 1 h after systemic 17-estradiol administration, CI
significantly decreased by 27 ± 4.9% and TPR increased by 37.3 ± 11.7% (P < 0.01), and these changes were maintained throughout the 3 h of the
experiment (P < 0.01). This fall in
CI and rise in TPR were also significantly different from vehicle
administration in presence of
L-NAME (Fig. 3). Furthermore,
the hemodynamic changes induced by 17-estradiol in the presence of
L-NAME were significantly different from those seen with 17-estradiol alone
(group I, Fig. 2) (P < 0.05). Of note, the
increases in TPR by 17-estradiol in the presence of
L-NAME were associated with
alterations in HR throughout the study. HR increased 1 h after systemic
administration of 17-estradiol and continued to increase, reaching
353 ± 17 beats/min at 3 h (P < 0.001). In the presence of
L-NAME, SVI significantly decreased from 82.1 ± 5.3 to 48.4 ± 7.3, 51.7 ± 7.2, and
44.4 ± 3.8 µl · beat1 · 100 g1 1, 2, and 3 h after
17-estradiol administration, respectively. In addition, hematocrit
increased from 42.5 ± 0.7% at
time
0 after L-NAME to 45.2 ± 0.9% after
17-estradiol (P < 0.05). In contrast, in
group
IV, vehicle administration in the
presence of L-NAME did not
induce changes in SVI or hematocrit (SVI was 74.3 ± 5.1 µl · beat1 · 100 g1 after
L-NAME and 73.5 ± 5.1, 59.7 ± 6.4, and 66.1 ± 3.1 µl · beat1 · 100 g1 1, 2, and 3 h after
vehicle administration, respectively; hematocrit was 40.4 ± 1.4%
before and 40.1 ± 1.9% 3 h after injection of vehicle).
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Data obtained in protocol III are shown in Fig. 4. Neither estradiol nor vehicle administration during phenylephrine infusion induced any significant changes in hemodynamic parameters. When group III (L-NAME + estradiol) and group V (phenylephrine + estradiol) were compared, no differences between basal values were seen. However, CI and TPR were significantly different 1 and 3 h after estradiol administration, between the L-NAME- and phenylephrine-pretreated groups.
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When the effects of both AT1
receptor and NO synthesis blockade on the 17-estradiol response were
examined in protocol IV, a significant
inhibition of the hemodynamic effects induced by 17-estradiol in
presence of L-NAME was observed.
As shown in Fig. 5, in
group
VII (losartan + L-NAME + estradiol), losartan markedly prevented the decrease in CI and the rise in TPR. Accordingly, losartan also significantly attenuated the fall in SVI seen after L-NAME plus 17-estradiol
administration [SVI decreased from 87.7 ± 3.4 to 80.2 ± 7.6, 64.1 ± 5.6, and 65.5 ± 2.9 µl/beat 1, 2, and 3 h after
estradiol administration, respectively; values were significantly
higher than in untreated animals of
group
III
(L-NAME + estradiol)].
Furthermore, pretreatment with losartan prevented the hemoconcentration
induced by 17-estradiol administration in the presence of
L-NAME (hematocrit was 42.5 ± 1.1 and 42.4 ± 1.1% in animals that received losartan + L-NAME before and after 17-estradiol administration, respectively). However, pretreatment with losartan did not prevent the increase in HR seen in
group III
(L-NAME + 17-estradiol; Fig.
5).
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Hemodynamic responses to 17-estradiol administration in the presence
of losartan (group
VIII) did not differ from those seen with 17-estradiol alone (group
I). After losartan, the changes in MAP were not significant: from a basal value of 87 ± 2 to 87 ± 3, 86 ± 3, and 82 ± 5 mmHg 1, 2, and 3 h after estradiol
administration, respectively. Similarly, the changes in HR were not
significant: from 386 ± 22 beats/min before to 381 ± 17, 409 ± 20, and 400 ± 23 beats/min after 1, 2, and 3 h of estradiol
infusion, respectively. CI after losartan was 30.8 ± 1.5 ml · min1 · 100 g1 and was 29.9 ± 1.7, 31.1 ± 1.3, and 33 ± 1.9 ml · min1 · 100 g1 1, 2, and 3 h after
estradiol administration, respectively. After losartan, TPR values were
2.9 ± 0.2, 2.9 ± 0.2, and 2.8 ± 0.2 mmHg · ml1 · min · 100 g 1, 2, and 3 h after estradiol administration, respectively. In the
presence of losartan, estradiol administration again did not change
hematocrit (41.6 ± 0.8 and 40.7 ± 0.9% before and after estradiol, respectively).
As expected, PRA was unchanged by estradiol administration (PRA was
1.34 ± 0.28 ng ANG
I · ml1 · h1
in the estradiol-treated group and 1.84 ± 0.43 ng ANG
I · ml1 · h1
in the group treated with vehicle). These data are in agreement with
data from group
VIII, in which losartan failed to
unmask a vasodilatory action of estradiol.
Plasma 17-estradiol concentration 3 h after intravenous
administration (100 µg/kg) was 103.3 ± 21.6 pg/ml.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Because estrogen replacement therapy appears to be beneficial for the
prevention of cardiovascular disease in postmenopausal women (17), the
cardiovascular response to estrogen administration is of substantial
interest. Furthermore, increased estrogen levels during pregnancy play
a key role in cardiovascular adaptation (44), but other vasoregulatory
systems as NO and the renin-angiotensin system are also activated (11,
18, 48). The present study was designed to evaluate the role of NO in
modulating the actions of estradiol on systemic hemodynamics and the
implication of angiotensin II in these hemodynamic effects. We found
that acute 17-estradiol (100 µg/kg) administration in rats given
L-NAME results in substantial systemic vasoconstriction, as evidenced by increases in TPR and decreases in CI. Moreover, these hemodynamic changes appear to be
mediated by potentiation of angiotensin II, because they were abolished
in the presence of an angiotensin
AT1 receptor antagonist (losartan).
Acute estrogen infusion to ovariectomized rats resulted in no
significant changes in TPR and MAP. This differs from other studies
performed in ovariectomized ewes in which estrogen administration increased CO and decreased TPR with little change in MAP (29). These
discrepancies may be due to species differences. In our study, the
light vasodilator effect of estrogens could have been modulated by
other compensatory systems, such as enhanced activation of the
autonomic system. In view of this, in the future, it might be
interesting to investigate whether or not pretreatment with a
ganglionic blocker or an 
-receptor antagonist might unmask a
vasodilatory action of 17-estradiol.
However, one major new finding in the present study is that
17-estradiol administration to rats pretreated with
L-NAME induced an unexpected
further increase of TPR and decrease of CI without changes in MAP.
These hemodynamic changes are due to estradiol administration, because
hemodynamic variables did not change after the administration of
L-NAME plus vehicle. Infusion of
L-NAME alone resulted in a rise
of MAP and TPR and a decrease in CI and HR, as expected after
inhibition of NO synthesis. The further vasoconstriction observed after
estradiol in the presence of
L-NAME seems to be due to the
suppression of NO and not to the increased peripheral resistance,
because estradiol did not induce further vasoconstriction in the
presence of a continuous phenylephrine infusion that increased TPR to
similar levels as those of
L-NAME. On the
other hand, from the results of
protocol
II (group
III, L-NAME + estradiol), a mechanism
other than vasoconstriction seems to contribute to produce the decrease
in CI seen after administration of estradiol in the absence of NO. One
possibility is a decrease in plasma volume and stroke volume, because
estradiol administration after
L-NAME significantly increased
hematocrit. It is well accepted that in the face of a constant red cell
volume (RCV), rising hematocrit signifies a fall in plasma volume. Van
Beaumont (45), in an elegant mathematical derivation based on actual
and theoretical data, illustrates the relationship between hematocrit
and plasma volume in a nomogram. Because the hematocrit is actually the
ratio of RCV and total blood volume (RCV + plasma volume), the change in plasma volume must always be larger than the change reflected by
hematocrit. Thus, from that nomogram, an increase of 2.7 points in the
hematocrit represents an 11% drop in plasma volume, and such a fall
could be responsible at least in part for the decrease in stroke volume
observed after estradiol administration in the presence of
L-NAME.
The underlying mechanism by which estrogen induced vasoconstriction
when NO synthesis was inhibited by
L-NAME is difficult to explain.
It appears that 17-estradiol, either directly or mediated by a
vasoconstrictor substance, increased vascular tone and TPR. One
candidate is angiotensin II, because angiotensin II
AT1 receptor blockade with
losartan suppressed the vasoconstrictor effect of 17-estradiol in
these circumstances. Nevertheless, the role of angiotensin II in
maintaining hemodynamics in the presence of 17-estradiol is
controversial. On one hand, several studies observed greater decreases
in blood pressure in response to an angiotensin II receptor antagonist,
saralasin, in anesthetized gravid compared with nongravid rats (18). In
addition, comparable results with a converting enzyme inhibitor,
captopril, in pregnant rabbits and goats have been reported (15, 35).
In contrast, Baylis and Collins (4) reported that in awake, chronically instrumented, pregnant rats, neither saralasin nor captopril decreased arterial pressure. Furthermore, Pan et al. (36) also showed that the
contribution of the renin-angiotensin system to maintaining basal blood
pressure is similar in pregnant and virgin rats. The reasons explaining
these discrepancies are unknown, but the results of our experiment in
which the AT1 receptor was blocked
are in basic agreement with the major findings of Baylis and Collins (4), because no hemodynamic differences were seen between the group
treated with AT1 receptor
antagonist plus 17-estradiol and the group treated with
17-estradiol alone. This may be because under resting conditions in
conscious, sodium-replete animals and humans, the renin-angiotensin
system does not play a very important role in the maintenance of normal
blood pressure and vascular tone. This explanation is consistent with
our measurement of PRA data from group
I, treated with estradiol alone,
suggesting a minor role of the renin-angiotensin system. On the other
hand, the administration of an angiotensin
AT1 receptor antagonist to rats
treated with estradiol in presence of
L-NAME prevented the estradiol-induced vasoconstriction and the rise in hematocrit and
decreased the fall in stroke volume observed in
group
III (L-NAME + estradiol), suggesting
a key role of angiotensin II in mediating these effects in the absence
of NO. Schricker et al. (42) have recently reported that inhibition of
NO synthesis led to an attenuation of basal renin secretion and to an
increase in blood pressure. Therefore, because administration of
L-NAME increased blood pressure
in this study, we can expect a decrease or no change in PRA. In this
case, one may speculate that the hemodynamic effects of estradiol in
the absence of NO would more likely be due to a potentiation of
angiotensin II than to an increase in renin secretion. Thus our results
showing that administration of 17-estradiol induced a further
increase of TPR and decrease of CI in the presence of
L-NAME indicate that NO is
necessary to offset a vasoconstrictor action of angiotensin II enhanced by estradiol administration. Furthermore, losartan inhibited the increase in hematocrit and the fall in stroke volume observed after
L-NAME plus estradiol,
indicating that angiotensin II may contribute to the decrease in stroke
volume and CI seen in the group treated with
L-NAME plus estradiol. These
data are in agreement with other reports that support a role for
angiotensin II in the pathogenesis of vascular injury and permeability
via mechanisms that are independent of its pressor activity (38, 48).
The remaining fall in stroke volume seen after estradiol administration in the group treated with losartan plus
L-NAME may be due to a decrease
in ventricular filling time as a result of the rise in HR, because
venous return was maintained constant.
An increase in HR was observed in all groups in which estradiol was administered, independent of changes in other hemodynamic variables. The tachycardia seen in rats given estradiol alone probably reflects a baroreceptor-mediated tachycardia, because MAP tended to fall. Another possibility is that estrogen may have a direct effect on the sinoatrial node, as has been proposed by others. Estradiol has been shown to accumulate in the nuclei of atrial myocytes (31) and to cause positive chronotropism in isolated perfused rabbit heart (2); thus estrogen may have a direct effect on the sinoatrial node. On the other hand, estrogen may also induce positive chronotropism via activation of the renin-angiotensin system (34), but it is unlikely that this last mechanism may explain the rise in HR by estradiol, because HR still increased in the presence of AT1 receptor blockade with losartan. It remains unclear whether or not estrogen has direct chronotropic effects.
From the data discussed above, we can speculate that there is an
interaction among estrogen, angiotensin, and NO. Recently, studies have
been reported indicating that endothelial NO synthase may be regulated
by estrogen. In this regard, it has been shown that pregnancy and
chronic treatment with estrogen increased calcium-dependent NOS
activity in the heart, kidney, and skeletal muscle (47) of guinea pigs.
In addition, nitrite/nitrate serum levels are increased in women after
estrogen replacement therapy (40). These reports are in concordance
with the decreased vascular reactivity to angiotensin II described in
normal pregnancy, in which the pressor response to infused angiotensin
II becomes attenuated (11, 28), probably due to an increase in NO
activity. However, there is controversy about the role of estradiol in
mediating this response. Conrad et al. (12) showed no change in
angiotensin II pressor responsiveness in rats during administration of
17-estradiol. Nevertheless, other investigators (8) have shown a
reduced response to angiotensin II in aortic rings of rats after
chronic estradiol treatment. Furthermore, in conscious ovariectomized ewes, acute administration of estradiol produced attenuation of systemic pressor responses to angiotensin II (33). However, from our
results we cannot exclude the possibility that an increased NO
production after administration of estrogen could modulate the
vasoconstrictor effect of angiotensin II, and removing NO allows for a
potentiation of angiotensin II-induced vasoconstriction.
In summary, we found that estradiol administration to ovariectomized rats produced a vasoconstrictor effect, seen as an increase in TPR, when NO synthesis is inhibited by L-NAME. Furthermore, these hemodynamic changes may be mediated by angiotensin II, because they were prevented by angiotensin AT1 receptor blockade. These results indicate that NO is necessary to maintain a normal hemodynamic state when estradiol is administered. Furthermore, the vasoconstrictor effect of estradiol when NO synthesis is inhibited appears to be due to potentiation of angiotensin II.
Perspectives
There are numerous reports indicating that estrogen replacement therapy has a protective effect in postmenopausal women. The cardiovascular protective action of estrogen is reported to be mediated indirectly by an effect on lipoprotein metabolism and directly by an effect on the blood vessel wall itself (7, 17, 20, 22). This effect appears to be mediated by enhancing the endothelial function, as indicated in several studies in which physiological levels of estrogen increased the endothelium-dependent vasorelaxation in the coronary vasculature in postmenopausal women (20). Estrogen supplementation has also been shown to augment endothelial NOS activity and mRNA for NOS isozymes in the uterine artery, heart, and skeletal muscle in female guinea pigs (47). On the other hand, preeclampsia is a disease characterized by hypertension in which the normal vascular adaptations of pregnancy are compromised (19). The attenuated response to angiotensin II seen in normal pregnancy is lost in women destined to develop preeclampsia. Then, an abnormal endothelial function, as has been described in the arteries of women with preeclampsia (30), may cause a decrease in urinary nitrite/nitrate (14), and the resulting imbalance between vasodilator and vasoconstrictor systems may be responsible for this characteristic hypertensive state with augmented sensitivity to angiotensin II (16). Thus it is of interest to determine the quantitative importance of NO in protecting the systemic hemodynamics in these physiological and pathophysiological states, and more studies will be necessary in the future.| |
ACKNOWLEDGEMENTS |
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We are grateful to Dr. F. J. Fenoy for help in the preparation of this manuscript.
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FOOTNOTES |
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This investigation was supported by Dirección General de Investigación Científica y Técnica grant P/PB94-1150 from el Ministerio de Educación y Ciencia and Fondo de Investigaciones Sanitarias de la Seguridad Social Grant 95/1972.
Address for reprint requests: I. Hernandez, Departamento de Fisiología y Farmacología, Facultad de Medicina, Universidad de Murcia, Campus de Espinardo, 30100, Murcia, Spain.
Received 14 May 1997; accepted in final form 4 December 1997.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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60 min, before infusion
of phenylephrine, represents control for
protocol III and before infusion of losartan
represents control for protocol IV. Time period at 
, 100 µg/kg) and vehicle (
,
ethanol) administration in ovariectomized conscious rats on cardiac
index, mean arterial pressure, heart rate, and total peripheral
resistance (TPR). Values are means, and bars show SE.
* P < 0.05 vs.
time 0.
, 100 µg/kg)
or vehicle (
, ethanol) in presence of
L-NAME at dose of 3 mg/kg + 50 mg · kg
) in ovariectomized
conscious rats on cardiac index, mean arterial pressure, heart rate,
and TPR. Values are means, and bars show SE.
* P < 0.05 vs.
time 0.