(Stroke. 1997;28:1821-1829.)
© 1997 American Heart Association, Inc.
Articles |
The Role of Blood Pressure and Aldosterone in the Production of Hemorrhagic Stroke in Captopril-Treated Hypertensive Rats
From the Division of Basic Medical Sciences, Memorial University, St John's, Newfoundland, Canada.
Correspondence and reprint requests to Dr J.S. Smeda, Division of Basic Medical Sciences, Rm H4354, Memorial University, Health Science Centre, St John's, Newfoundland, Canada A1B 3V6.
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Abstract |
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Background and Purpose We tested the hypothesis that the lowering of plasma aldosterone levels contributed to the antistroke effects of captopril treatment in Wistar Kyoto stroke-prone spontaneously hypertensive rats (SHRSP).
Methods The suppression of plasma aldosterone by captopril treatment (50 mg · kg–1 · d–1) was prevented by the subcutaneous infusion of aldosterone into captopril-treated SHRSP. We studied the effect this had on blood pressure (BP) and stroke development.
Results SHRSP fed a Japanese-style diet containing 4% NaCl developed hypertension and a 100% mortality associated with intracerebral hemorrhage by 14 weeks of age. Captopril treatment from 6 weeks of age did not lower the BP but increased survival past 35 weeks of age. Hydralazine treatment (40 to 80 mg/L of drinking water) lowered BP in SHRSP but was less effective than captopril in retarding stroke. Plasma aldosterone levels were elevated with age in SHRSP after 10 weeks and were higher in poststroke versus prestroke SHRSP. Captopril treatment suppressed plasma aldosterone. When we elevated plasma aldosterone in captopril-treated SHRSP to levels between those present in untreated pre- and poststroke SHRSP, the ability of captopril to retard stroke development was negated. The effects of aldosterone were mimicked by deoxycorticosterone (40 mg/kg, SC2 times/wk) but not by dexamethasone (0.1 mg · kg–1 · d–1, SC). Spironolactone treatment (20 mg · kg–1 · d–1, SC) of SHRSP reduced BP but had little effect on stroke development.
Conclusions Elevations in plasma aldosterone enhance stroke development within captopril-treated SHRSP through mechanisms that do not involve stimulation of mineralocorticoid receptors or the enhancement of hypertension. The antistroke effects of captopril treatment may be partially mediated through the suppression of plasma aldosterone.
Key Words: aldosterone • angiotensins • cerebral hemorrhage • captopril • rats
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Introduction |
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Studies involving Wistar Kyoto SHRSP have shown that both plasma renin levels and activity increase with age1 2 3 and that plasma renin activity and Ang II levels are elevated in SHRSP with established hypertension when compared with normotensive WKY.4 5 6 The renin angiotensin system responds to dietary NaCl in a unique manner in SHRSP. Whereas elevations in dietary sodium suppress plasma renin activity in normal rats,5 renin activity5 7 and stroke development8 are enhanced in SHRSP.
Treatment of salt-loaded SHRSP with ACEIs, or the Ang II receptor antagonist losartan, produces a long-term delay in the onset of stroke development.3 5 9 10 11 12 Despite the fact that plasma renin activity and Ang II levels are elevated in these animals,1 3 5 10 the latter drugs are ineffective antihypertensive agents when administered to salt-loaded SHRSP, and the antistroke effects of the drugs occur under conditions where only a small decrease in blood pressure is observed.3 5 9 10 11 12
The mechanisms by which ACEIs and losartan inhibit stroke development in SHRSP have not been elucidated. Speculation has centered around the possibility that the small decrease in blood pressure often produced during ACEI treatment11 and/or the prevention of short-term elevations in blood pressure could play a role in the prevention of stroke development.11 12 Other researchers have speculated that ACEIs produce an antistroke effect independent of a change in blood pressure.5 9 13 It has been suggested that such treatment may protect against vascular damage produced by the elevations in plasma Ang II. A reduction in neutrophil chemoattraction to vascular endothelium, decreased vascular smooth muscle hypertrophy and hyperplasia, decreased vascular permeability, and a reduction in the incidence of cerebrovascular fibrinoid necrosis have all been suggested as possible beneficial mechanisms that could act to retard stroke development in response to the lowering of Ang II levels through ACEI or losartan treatment.9 11 13 To date little consideration has been given to the possibility that elevations in plasma aldosterone may play a role in stroke development in SHRSP. Plasma aldosterone increases in a parallel manner to renin activity with increasing age in SHRSP14 ; and during established hypertension, the levels of plasma aldosterone and Ang II are both four to six times the levels present in normotensive rats.15
In the present study we assessed the hypothesis that the antistroke effects of ACEI treatment with captopril are, in part, mediated by a lowering of plasma aldosterone levels. We measured plasma aldosterone levels before and after stroke development in untreated SHRSP and SHRSP subjected to captopril treatment. Subsequently, we replaced the levels of plasma aldosterone suppressed by captopril treatment through a subcutaneous infusion of aldosterone into captopril-treated SHRSP and determined the effect that this had on stroke development. We also tested the ability of the mineralocorticoid deoxycorticosterone to duplicate the effects of aldosterone. In the design of the study we anticipated that the above manipulations could modify blood pressure within SHRSP. To assess the potential influence of altered blood pressure during treatment, hydralazine was used to lower blood pressure to levels below those observed during captopril treatment, and the glucocorticoid hypertensive agent dexamethasone was administered to raise blood pressure in captopril-treated SHRSP. We reasoned that if captopril treatment produced an antistroke effect by lowering blood pressure, then other more effective antihypertensive agents should be able to duplicate or exceed the antistroke effects of captopril. Conversely, elevations in blood pressure through the use of hypertensive drugs should nullify the effects of captopril treatment. Finally, we assessed the ability of the type I aldosterone receptor antagonist spironolactone to inhibit stroke development in SHRSP.
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Methods |
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Animals
Only male SHRSP were used in the study. These were taken from a colony maintained at Memorial University of Newfoundland (St John's, Newfoundland, Canada). The rats were fed a Japanese-style diet containing 4% NaCl (Zeigler Brothers). Individual treatments were initiated within the 6th week (wk) of life and complied with the institutional guidelines of Memorial University of Newfoundland and the Canadian Council on Animal Care.
Treatments
Groups of SHRSP were subjected to the following treatments: (1)
captopril, 50 mg · kg–1 · d–1,
administered orally in the drinking water (drinking rates were
monitored and the concentration of the drug in the drinking water was
modified to achieve the correct dose); (2) hydralazine, 20, 40,
60, 80, and 100 mg/L of drinking water; (3)
aldosterone, 0.66 µg/hr administered
subcutaneously by implanted osmotic pumps (Alzet) within polyethylene
glycol vehicle (300 molecular weight) infused at a rate of 0.5 µL/h
(pumps were implanted under anesthesia produced by 10 mg
xylazine and 50 mg ketamine per kg, IM); (4)
dexamethasone, 0.1 mg · kg–1 ·
d–1 injected subcutaneously within olive oil at 0.1 mL/100
g; (5) deoxycorticosterone (acetate salt), 40 mg/kg injected
subcutaneously 2 times/wk within olive oil at 0.1 mL/100 g; and (6)
spironolactone, 20 mg · kg–1 ·
d–1 injected subcutaneously within olive oil at 0.1 mL/100
g. In addition, three groups of SHRSP were treated with combined
treatments of captopril plus aldosterone, captopril plus
dexamethasone, and captopril plus deoxycorticosterone at
the dosages and modes of administration described above. Cotreatment
with aldosterone, deoxycorticosterone, or
dexamethasone was started on average 3 days after the
initiation of captopril treatment. Control groups consisted of
untreated SHRSP, SHRSP subjected to polyethylene glycol infusion by
osmotic pump at 0.5 µL/h (volume used to infuse
aldosterone), and SHRSP injected subcutaneously with olive
oil at 0.1 mL · 100 g–1 · d–1
(volume of vehicle used to inject spironolactone and
dexamethasone). In addition, captopril-treated SHRSP were
infused with aldosterone (polyethylene glycol) or injected
with dexamethasone/spironolactone (olive oil) vehicles
equivalent to the amounts used in the combined treatment studies
described above. All the above drugs were purchased from Sigma
Chemical Co.
Monitoring of Stroke Development
The rats were monitored daily for the occurrence of stroke. The
symptoms associated with stroke development within SHRSP have been
previously described.16 17 Initially, SHRSP develop
convulsive repetitive forearm movement followed by inappropriate
posture during which the rat sits with its legs hyperextended in a
"kangaroo-type" posture. The latter symptom was often associated
with lethargy and poor grooming. There is typically a 1.5-week period
between the onset of the first behavioral symptoms of stroke and death.
Some animals died abruptly after the first behavioral symptoms of
stroke. Other animals were killed at a point when death was likely to
occur within a day. These latter animals were anesthetized, and
a blood sample was taken by cardiac puncture. The brain was removed
from all animals and fixed with 84 mmol/L PO4
buffer containing 4% formaldehyde and 1%
glutaraldehyde, pH 7.4. In most cases, the presence of
intracerebral hemorrhage could be clearly seen
within one or both cerebral hemispheres. In cases in which this did not
occur, the entire brain, including the brain stem, was sectioned and
examined histologically for the presence of
intracerebral hemorrhage.
Physiological Measurements
The systolic blood pressure of all the rats was measured
weekly using a tail-cuff compression method (IITC Life Science
Instruments model 29, pulse/pressure amplifier). Radioimmunoassay
techniques were used to measure plasma aldosterone levels
within untreated SHRSP and in SHRSP treated with captopril in the
presence and absence of aldosterone infusion. These were
performed by the Memorial University, Health Science Centre Renal
Diagnostic Laboratory (St John's, Newfoundland, Canada) by
qualified personnel that perform these assays on a routine basis. The
assays were performed using the Coat-A-Count radioimmunoassay kit
(Diagnostic Products Corp). The personnel analyzing
plasma aldosterone were blinded to the identity of the
samples.
Statistical Analysis
Alterations of blood pressure with age were analyzed
using a general linear model of multivariant ANOVA. The effects of
varying treatments on blood pressures were determined with respect to
age over common ages between treatment groups. Mortality with age was
assessed using a Mann-Whitney rank order test. Plasma
aldosterone levels between multiple groups were
analyzed using a one-way ANOVA to determine whether
between-group differences existed. Subsequently, Student's
t test compensated for multiple comparisons with the
Bonferroni method was used to determine which groups differed from each
other. Changes in plasma aldosterone levels with age were
analyzed using linear regression analysis coupled with
the determination of a Pearson product of correlation (r
value). Results were considered significant at P<.05. All
results are expressed as the mean±one standard error measurement.
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Results |
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Figure 1A
and 1B, respectively,
shows the alterations in blood pressure and mortality with age in
control and captopril-treated SHRSP. Control SHRSP developed stroke and
either died or were euthanized at a stage of near death between 10 to
14 weeks of age. Treatment with captopril 50 mg ·
kg–1 · d–1 dramatically increased the
survival of the SHRSP. At 18 weeks of age none of the 12 SHRSP treated
with captopril demonstrated behavioral evidence of stroke. To determine
that intracerebral hemorrhage was absent in
these animals, 5 of the 12 SHRSP treated with captopril were randomly
selected and sampled between 18 to 20 weeks of age. No
histological evidence of intracerebral
hemorrhage was observed. The remaining 7 SHRSP were allowed to
live and continued with captopril treatment up to a maximum of 39 weeks
of age after which the experiment was terminated. Between 34 and 39
weeks of age, 3 of 7 SHRSP exhibited behavioral signs of stroke and
either died or were euthanized near death. All exhibited
histological evidence of intracerebral
hemorrhage. At 39 weeks of age the 4 surviving SHRSP were
sampled. These animals appeared healthy and lacked behavioral symptoms
of stroke; however, 2 of the 4 SHRSP exhibited
histological evidence of intracerebral
hemorrhage. The above results suggested that captopril
treatment not only retarded the onset of intracerebral
hemorrhage in SHRSP but may have also enhanced survival after
intracerebral hemorrhage had developed. When
assessed over the life span of the control SHRSP, captopril treatment
did not significantly alter the blood pressure of SHRSP.
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Previous studies involving the treatment of SHRSP with ACEIs or
losartan3 5 9 10 11 12 have demonstrated a statistically
significant depression in blood pressure during the early phases of
hypertension development (9 to 13 weeks). It has been suggested that a
small suppression of blood pressure during this critical period of
blood pressure development may produce a profound effect in retarding
subsequent stroke development in SHRSP.12 Although we did
not observe a significant suppression of hypertension with captopril
treatment, we further explored this possibility. SHRSP were subjected
to antihypertensive treatment by introducing hydralazine into
the drinking water at doses of 20, 40, 60, and 100 mg/L. The
antihypertensive effects of hydralazine varied among animals.
It was not uncommon for the animals to initially exhibit a suppression
in blood pressure only to have their blood pressure re-elevate to
levels that were present in control animals. Since the objective of
the study was to achieve an antihypertensive effect and not to study
the effects of hydralazine treatment per se, mortality was only
studied in hydralazine-treated SHRSP that exhibited average
weekly systolic blood pressures of less than 220 mm Hg
from 10 weeks of age, a time when hypertension is fully established, to
the time of death. Three SHRSP treated with 40 mg hydralazine/L
in their drinking water, one 60 mg/L–treated SHRSP, two 80
mg/L–treated SHRSP, and one 100 mg/L–treated SHRSP met
the latter criteria, and the blood pressure and mortality data
pertaining to this group of animals are plotted in Fig 1
. As shown in
Figure 1A, when compared over common life spans, hydralazine
treatment significantly suppressed blood pressure development in the
SHRSP when compared with nontreated control SHRSP and captopril-treated
SHRSP. Such treatment retarded mortality associated with stroke
development within SHRSP by about 7 weeks (Fig 1B). Despite the
presence of higher blood pressures, captopril treatment was
considerably more effective than hydralazine treatment in
retarding the onset of mortality associated with stroke. These results
would suggest that captopril treatment retards the onset of mortality
associated with stroke through mechanisms that are independent of its
potential ability to lower blood pressure.
In subsequent experiments we attempted to determine whether the ability
of captopril treatment to retard stroke development in SHRSP was
related to the suppression of plasma aldosterone levels. We
observed that within prestroke SHRSP, plasma aldosterone
levels tended to rise with age in SHRSP after 10 weeks of age (Fig 2). Also, as shown in Fig 3, SHRSP (10.5 to 13.5 weeks of age) that
developed stroke had, on average, more than five times the levels of
plasma aldosterone that was present in comparably aged
prestroke SHRSP. Captopril treatment suppressed plasma
aldosterone to 25% of the levels observed in untreated
prestroke SHRSP and to 4% of the levels present in untreated
poststroke SHRSP. To investigate the influence of plasma
aldosterone levels on stroke development, SHRSP were
treated with captopril from 6 weeks of age. After 3 days of captopril
treatment aldosterone was continuously infused
subcutaneously at 0.66 µg/h into captopril-treated animals by
implanted osmotic pumps. Through preliminary studies we determined that
this rate of aldosterone infusion would elevate plasma
aldosterone levels in 10- to 13-week-old captopril-treated
SHRSP to levels between those present in comparably aged nontreated
pre- and poststroke SHRSP. As shown in Fig 4, SHRSP that received captopril
treatment plus aldosterone infusion had blood pressures
that did not significantly differ from those present in SHRSP that
received no treatment when these groups were compared over common life
spans. When compared over common life spans captopril plus
aldosterone–treated SHRSP did have higher blood pressures
than SHRSP treated only with captopril. However, at ages past those
where a 100% mortality occurred in SHRSP treated with captopril plus
aldosterone, SHRSP treated with captopril alone achieved
blood pressures equal to or higher than those of the former group and
still survived. As indicated in Fig 4B, the infusion of
aldosterone into SHRSP treated with captopril prevented the
ability of captopril treatment to retard the onset of stroke
development. An analysis of plasma aldosterone
levels of the SHRSP treated with captopril plus aldosterone
infusion (Fig 3) indicated levels that were less than those of
untreated SHRSP that developed stroke and greater than those
present in comparably aged prestroke SHRSP. The data were
consistent with the hypothesis that the ability of captopril to
retard the onset of stroke development in SHRSP may be produced by a
lowering of plasma aldosterone levels.
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In other experiments, we tested the ability of deoxycorticosterone and
dexamethasone to mimic the effects of
aldosterone within captopril-treated SHRSP.
Deoxycorticosterone is a mineralocorticoid with properties similar to
aldosterone, whereas dexamethasone is a
glucocorticoid that lacks mineralocorticoid activity. Both drugs have
the ability to elevate blood pressure. SHRSP were treated with
captopril from 6 weeks of age, and deoxycorticosterone (40 mg/kg
2 times/wk) or dexamethasone (0.1 mg ·
kg–1 · d–1) was injected into the
animals. As shown in Fig 5A, when
compared over common life spans the injection of deoxycorticosterone or
dexamethasone did not significantly elevate the blood
pressure of SHRSP over that present in control animals, but
deoxycorticosterone and dexamethasone injections both
elevated the blood pressure of captopril-treated animals. Like
aldosterone infusion, the injection of deoxycorticosterone
into captopril-treated SHRSP prevented captopril treatment from
retarding mortality associated with stroke development, whereas
dexamethasone did not (Fig 5B). None of the 6 rats treated
with captopril plus dexamethasone died or exhibited
symptoms related to stroke development (Fig 5B). The captopril plus
dexamethasone–treated SHRSP were sampled between 24 and 30
weeks of age. None of the rats exhibited behavioral symptoms of stroke;
however, histological examination revealed the presence
of intracerebral hemorrhage in 2 of the 6
animals. The conclusions reached were that despite the ability of
dexamethasone to elevate blood pressure in
captopril-treated SHRSP, the glucocorticoid did not reverse the effects
of captopril treatment on mortality to the extent of that observed
during combined captopril plus mineralocorticoid treatment with either
deoxycorticosterone or aldosterone. The experiments also
suggested that the small elevations in blood pressure observed during
captopril plus mineralocorticoid treatment, compared with that of
captopril treatment alone, likely did not contribute to the reversal of
the antistroke effects of captopril treatment.
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We also tested the individual effects of aldosterone,
dexamethasone, and deoxycorticosterone on blood pressure
and mortality associated with stroke within SHRSP not treated with
captopril. The results of this experiment are shown in Fig 6. The blood pressures of SHRSP subjected
to aldosterone infusion, deoxycorticosterone, or
dexamethasone injections did not significantly differ from
untreated control SHRSP when compared over corresponding life spans
(Fig 6A). Aldosterone infusion and deoxycorticosterone
injections accelerated the onset of mortality associated with stroke on
average by 2.2 to 2.5 weeks when compared with untreated SHRSP (Fig 6B). The injection of dexamethasone did not significantly
alter stroke development in SHRSP.
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In other experiments, we tested the ability of spironolactone to
duplicate the effects of captopril treatment. Canrenone, a
metabolic by-product of spironolactone, acts as a
competitive antagonist for the type I
aldosterone receptor present within the renal tubule
and other cells. Spironolactone was injected subcutaneously into SHRSP
at 20 mg · kg–1 · d–1 from 6
weeks of age. As shown in Fig 7A, spironolactone treatment significantly suppressed blood pressure
development within SHRSP over the life span of the control SHRSP. Such
treatment slightly delayed mortality associated with stroke within
SHRSP (Fig 7B).
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The potential effects of the infusion/injection vehicles on stroke
development within SHRSP were also studied. The injection vehicle for
the administration of spironolactone, dexamethasone, or
deoxycorticosterone was olive oil that was administered at a volume of
0.1 mL/100 g body wt. SHRSP injected daily with the above vehicle died
at an average age of 12.2±0.3 weeks (range, 11.2 to 13.0 weeks; n=4).
SHRSP infused with polyethylene glycol (aldosterone
vehicle) at 0.5 µL/h by implanted osmotic pumps from 6 weeks of age
died at an average age of 13.6±0.5 weeks (range, 13.0 to 15.0 weeks).
When mortality profiles were compared with those of untreated SHRSP
(average age of death, 12.4±0.36 weeks; range 10.3 to 13.7 weeks;
n=11), no significant differences were observed (Mann-Whitney rank
order test). In other experiments, captopril-treated SHRSP (50 mg
· kg–1 · d–1) were injected with
olive oil (the deoxycorticosterone vehicle, n=4) or infused with
polyethylene glycol (the aldosterone vehicle) by osmotic
pumps (n=4). None of these animals developed stroke. The latter
experiments were terminated when SHRSP reached an average age of 17
weeks, which surpassed the age at which a 100% mortality occurred
within captopril-treated SHRSP receiving either aldosterone
infusion (0.66 µg/h, see Fig 4B) or deoxycorticosterone
injections (40 mg/kg 2 times/wk, see Fig 5B). When the
alterations in blood pressure with age were statistically
analyzed over comparable life spans, no significant differences
were observed between untreated SHRSP, SHRSP treated with captopril
alone, and SHRSP injected with olive oil or infused with polyethylene
glycol vehicle in the presence or absence of captopril treatment (group
effect of treatments, P=.436). It was concluded that the
acceleration in stroke development observed in SHRSP treated with
deoxycorticosterone or aldosterone, the inhibition of the
antistroke effects of captopril treatment observed during cotreatment
with aldosterone or deoxycorticosterone, and the
antihypertensive effects observed during spironolactone treatment could
not be attributed to the vehicle used.
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The key finding of the present study was the observation that the reconstitution of plasma aldosterone levels to levels observed before the suppression of this hormone by captopril treatment negated the antistroke effects of captopril treatment. Plasma aldosterone levels were elevated with age in SHRSP after 10 weeks of age and were about five times higher in SHRSP that developed stroke when compared with similarly aged SHRSP that had not yet developed stroke. This is a remarkable elevation in plasma aldosterone levels when one considers that the prestroke animals already had twice the plasma aldosterone levels typically observed within normotensive 5- to 25-week-old WKY.15 Captopril treatment of SHRSP suppressed plasma aldosterone to levels comparable to those previously reported in WKY15 and prevented mortality associated with stroke up to 34 weeks of age. The reinfusion of aldosterone into captopril-treated SHRSP to achieve levels between those present in pre- and poststroke nontreated SHRSP totally negated the effects of captopril treatment and caused mortality to be comparable to that observed in untreated SHRSP. The effects of aldosterone could be duplicated by injecting captopril-treated SHRSP with the mineralocorticoid deoxycorticosterone. These results suggest that under conditions in which Ang II levels should be suppressed by captopril, elevations in plasma aldosterone represent a significant risk factor for stroke development within SHRSP.
One of our concerns was that aldosterone was infused into SHRSP from an early age (6.5 weeks of age). Since the osmotic pumps delivering aldosterone do so at a constant delivery (0.66 µg aldosterone/h), younger and smaller SHRSP might have been subjected to higher plasma aldosterone levels than those observed in 10- to 13-week-old captopril plus aldosterone–treated SHRSP. Recently we have started treating SHRSP with captopril from 10 weeks of age, a point at which we have shown in the present study that plasma aldosterone levels rise with age. We observed that captopril treatment (50 mg · kg–1 · d–1) totally prevented mortality associated with stroke up to the termination of the experiment (24 weeks of age), whereas the co-treatment of captopril-treated SHRSP with aldosterone starting at 10 weeks of age (3 days after the initiation of captopril treatment) largely negated the antistroke effects of captopril treatment (captopril plus aldosterone–treated SHRSP mean age of death, 16.5±0.8 weeks, n=6; untreated SHRSP, mean age of death, 14.0±0.4 weeks, n=8; captopril-treated SHRSP, no death up to 24 weeks of age, n=7; unpublished results, 1997). This would suggest that elevations in aldosterone within prestroke SHRSP from 10 weeks of age enhance stroke development with SHRSP.
It is difficult to explain the antistroke effects of captopril treatment (50 mg · kg–1 · d–1) on the basis of an antihypertensive effect or to explain the ability of aldosterone to reverse the antistroke effects of captopril on the basis of the very small increases in blood pressure observed. When blood pressure was compared over the life span of untreated SHRSP, captopril treatment did not produce a statistically significant depression in blood pressure. On the other hand, hydralazine treatment significantly depressed blood pressure within SHRSP but only extended the life span of SHRSP by about one third of that produced by captopril treatment. The highest average blood pressures within the study were observed in captopril-treated SHRSP co-treated with dexamethasone. At the termination of the experiment (24 weeks of age) they had systolic blood pressures that in many cases exceeded 300 mm Hg. These blood pressures were equal to or higher than the highest blood pressures present within captopril-treated SHRSP co-treated with aldosterone or deoxycorticosterone, yet none of these animals died from stroke at 24 weeks, whereas a 50% mortality associated with stroke was observed before 13 weeks of age in captopril-treated SHRSP supplemented with either aldosterone or deoxycorticosterone. The above experiments support the argument that the ability of aldosterone or deoxycor- ticosterone to reverse the effects of captopril treat- ment occurs through mechanisms other than a slight elevation in blood pressure produced by the above mineralocorticoid.
Other studies have indicated that the inhibition of stroke development as a result of captopril and other ACEI treatments was associated with the inhibition of cerebrovascular fibrinoid necrosis development.5 13 It is possible that the occurrence of vascular fibrinoid necrosis could weaken the cerebral vascular wall causing such sites to burst or leak vascular fluids and that the prevention of the development of this vascular pathology might act to inhibit stroke development. However, some studies of salt-loaded SHRSP have noted the occurrence of cerebral hemorrhage in SHRSP under conditions where histological examination indicated the virtual absence of fibrinoid necrosis within the cerebrovasculature.18 Our most recent studies indicate that captopril treatment (50 mg · kg–1 · d–1) of SHRSP prolongs their life span (by 5 to 9 weeks) even when treatment is started 1 to 5 days after the initial signs of stroke, at a time when cerebrovascular fibrinoid necrosis might be present (unpublished results, 1997). This would suggest that captopril treatment can increase the life span of SHRSP via mechanisms beyond the inhibition of cerebrovascular fibrinoid necrosis development.
At the present time we are unclear as to the mechanisms through which elevations in plasma aldosterone induce stroke development within captopril-treated SHRSP. An important finding in the present study was the observation that treatment of SHRSP with spironolactone (20 mg · kg–1 · d–1) could not duplicate the effects of captopril treatment. Spironolactone is quickly metabolized to canrenone, and both spironolactone and canrenone are potent competitive antagonists of the type 1 mineralocorticoid receptor.19 20 21 Within distal segments of the rat nephron and proximal regions of the collecting ducts, aldosterone binds to the intracellular type 1 receptor within the epithelium and initiates an increase in Na+ permeability and the de novo synthesis of Na+/K+ ATPase, both of which facilitate the uptake of Na+ from the urine into the blood.19 Initially we felt that elevations in plasma aldosterone may aggravate sodium loading within SHRSP by enhancing the uptake of this ion from the urine. Since these animals were already being fed a high salt diet, which is a risk factor for stroke development,8 such a modification could further aggravate stroke. However, the uptake of Na+ from the urine should have been inhibited not only by lowering plasma aldosterone levels with captopril treatment but also by spironolactone treatment. Despite the fact that the dose of spironolactone used in the present study (20 mg · kg–1 · d–1, SC) was about four times the per kilogram daily oral dose used to treat Conn's syndrome patients, we considered the possibility that the dose was too small or delivered too infrequently (once a day). In this regard, canrenone has a half-life of 10 to 35 hours in humans21 and the dose of spironolactone used did produce an antihypertensive effect in SHRSP suggesting that the drug was physiologically active. In recent experiments that we have performed, higher doses of spironolactone (150 mg · kg–1 · d–1, SC) initiated at 10 weeks of age did not alter stroke development in SHRSP (unpublished results, 1997). Hence, we believe that the ineffectiveness of spironolactone was not due to insufficient quantities of the drug being administered. The conclusion we have reached is that the ability of aldosterone to accelerate stroke development in captopril-treated SHRSP likely occurs through a mechanism that does not involve activation of the type 1 mineralocorticoid receptors. A possible speculative mechanism by which this could occur is through the stimulation of nongenomic plasma membrane aldosterone receptor.20 22 23 These receptors have been recently discovered and their actions characterized on the surface of vascular smooth muscles,24 25 26 endothelium,25 and human leukocytes.20 22 27 Within vascular smooth muscle, aldosterone has been shown to produce a rapid rise in free intracellular Ca2+,25 a phospholipase C–mediated increase in diacylglycerol production and protein kinase C activation,24 an increase in Na+ efflux thought to be mediated by enhanced Na+/H+ porter activity, and a stimulation of Na+/K+ ATPase.26 These effects were observed within seconds to 4 minutes after the application of doses of aldosterone or deoxycorticosterone <1 nmol/L. The effects could not be inhibited by canrenone and could not be duplicated by high (0.1 to 1 µmol/L) levels of hydrocorticosterone or dexamethasone. How such mechanisms might act to facilitate stroke development within captopril-treated SHRSP is unclear. An important mechanism involved in promoting cerebral blood-flow autoregulation is the occurrence of cerebral vascular constriction in response to elevated pressure. The loss of such function could promote over perfusion of the cerebral vasculature during hypertension and thus lead to the production of intracerebral hemorrhage.17 The occurrence of smooth muscle depolarization28 and the activation of protein kinase C29 are both thought to be important second messenger events linking the sensing of pressure to the promotion of vascular constriction. One could speculate that elevations in plasma aldosterone, perhaps through the stimulation of the nongenomic aldosterone receptor, might alter smooth muscle ionic fluxes, protein kinase C function, and/or produce cytotoxic effects that could alter cerebral blood-flow autoregulation in a manner conducive to the development of stroke under hypertensive conditions. In this regard, previous studies have indicated that the middle cerebral arteries of prestroke SHRSP lose their ability to constrict to pressure after 10 weeks of age, at about the time plasma aldosterone levels elevate.17 More recent studies have indicated that such function is preserved in captopril-treated SHRSP but not captopril plus aldosterone–or captopril plus deoxycorticosterone–supplemented SHRSP.30
It is tempting to extend the interpretation of the results of this study to suggest that elevations in plasma aldosterone levels, independent of any direct effect of Ang II, are responsible for stroke development in SHRSP. The results we have presented are consistent with the above hypothesis but such a conclusion is premature. Support for the above view would be strengthened by the observation that stroke development is retarded in adrenalectomized SHRSP subjected to aldosterone infusions that maintain plasma aldosterone below the levels observed in prestroke SHRSP. In such an instance, the independence of an Ang II effect with respect to stroke development would dictate that the coinfusion of Ang II should not alter the onset of stroke development. Until such experiments are performed we can only conclude that by elevating plasma aldosterone levels during captopril treatment you inhibit the ability of captopril to act as an antistroke agent.
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Acknowledgments |
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This study was supported by a grant from the Heart and Stroke Foundation of Newfoundland and Labrador.
Received October 17, 1996; revision received May 15, 1997; accepted June 11, 1997.
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References |
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