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-receptor regulation
Department of Physiology and Pharmacology, Auburn University, Auburn, Alabama 36849
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ABSTRACT |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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The objectives of this study were
1) to determine whether ANG
II-induced myocardial damage (ANG Dam) is mediated via the
1-adrenergic receptor,
2) to elucidate whether adrenal
medulla or cardiac sympathetic neuron catecholamines are responsible
for ANG Dam, and 3) to determine whether the lack of damage after 3 days of elevated ANG II levels is
due to 1-receptor
downregulation. To this end, ANG II was administered to rats
1) that were treated with a
-receptor blocker, 2) after
adrenal medullectomy and/or cardiac sympathectomy, and 3) for 3 or 8 days. ANG II caused
both myocyte necrosis and coronary vascular damage after adrenal
medullectomy but not after cardiac sympathectomy. There was a 38 and
55% decrease in -receptor density after 3 and 8 days, respectively,
of ANG II infusion, and an upregulation to control levels 5 days after
a 3-day ANG II infusion was stopped. We conclude that cardiac
sympathetic neuron catecholamines are responsible for ANG Dam and that
the acute nature of this damage is associated with a downregulation of
1-adrenergic receptors.
myocyte necrosis; atenolol; adrenal medullectomy; cardiac sympathectomy; coronary arteriolar damage
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INTRODUCTION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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PHARMACOLOGICAL and pathophysiological levels of ANG II
and catecholamines have been shown to produce myocyte necrosis and coronary vascular damage (5, 6, 9, 10, 13, 17, 25). Our laboratory has
recently demonstrated that 1) ANG
II-induced myocyte necrosis and coronary vascular damage are mediated
by the ANG II type 1 (AT1)
receptor (17); 2) ANG II-related
myocardial damage is significantly reduced by -adrenergic receptor
blockade with propranolol, suggesting that
AT1-receptor-mediated
catecholamine release is responsible for the damage (14);
3) serum norepinephrine levels were
not significantly elevated until days
4-6 of a 14-day ANG II infusion, whereas serum
epinephrine levels were at or below control values (14), suggesting
that a local norepinephrine release from the cardiac sympathetic
neurons may be responsible for ANG II-related myocardial damage; and
4) the myocardial damage associated
with chronic elevations of ANG II occurs within the first 3 days of ANG
II infusion, with little additional damage occurring after 3 days even
when ANG II levels remain elevated (17). This would suggest that
-adrenergic receptor downregulation occurs during chronic ANG II
elevation and protects the myocardium from further damage.
From these observations and to further elucidate the mechanisms of ANG
II-induced myocardial damage, the overall objectives of this study were
1) to determine whether ANG
II-induced myocardial damage is mediated via the
1- or
2-adrenergic receptor,
2) to elucidate whether
catecholamines derived from the adrenal medulla and/or the
cardiac sympathetic neurons are responsible for ANG II-related
myocardial damage, and 3) to
determine whether the lack of further damage after 3 days of
chronically elevated ANG II levels is due to -adrenergic receptor
downregulation.
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MATERIALS AND METHODS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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All experiments were performed using adult male Sprague-Dawley rats housed under standard environmental conditions and maintained on commercial rat laboratory diet and tap water ad libitum. The protocols were approved by the Institution's Animal Care and Use Committee, and the care and use of the rats conformed to the National Institutes of Health guidelines.
ANG II (Sigma, St. Louis, MO) was delivered at a rate of 150 ng/min by an osmotic minipump (model 2002, Alza, Palo Alto, CA) implanted subcutaneously in the nape of the neck with the use of aseptic techniques. The dose of 150 ng/min and this method of delivery have been shown to raise circulating ANG II levels to ~70 pg/min (25) and are known to cause myocyte necrosis and coronary vascular damage (17, 25). The blood levels of ANG II measured during heart failure have been reported to range between 28 and 155 pg/ml (19, 24). Thus the 150 ng/min dose of ANG II delivered subcutaneously via the osmotic minipump is considered to be a pathophysiological dose.
Surgical procedures were performed with the rats under xylazine hydrochloride (7 mg/kg)-ketamine hydrochloride (60 mg/kg) anesthesia administered by intraperitoneal injection. Buprenorphine hydrochloride (0.05 mg/kg sc) was given to the rats at the end of surgery for postoperative relief of pain. After all surgical procedures, the rats were alert and resumed normal activity within 24 h. To maintain normokalemia, rats receiving an ANG II infusion were given drinking water that contained 0.3% KCl (8).
Surgical adrenal medullectomy. To remove the adrenal medullary source of catecholamines, a group of anesthetized rats underwent bilateral adrenal medullectomy. Through a midline abdominal incision, a small incision was made in the adrenal cortex. The outer surface of the adrenal gland was compressed by use of curved mosquito forceps with sufficient force to extrude the adrenal medulla. The adrenal medulla tissue was then removed, the incision in the abdominal muscle was closed with 2-0 gut suture, and the skin incision was approximated with staples. On completion of the adrenal medullectomy, each rat was randomly assigned to one of seven groups. One group received ANG II for 4 days, as described previously, beginning 1 day after medullectomy (n = 6). A second group received ANG II for 4 days beginning 5 days after medullectomy (n = 11). The remaining five groups received no ANG II and were killed 1 (n = 2), 2 (n = 2), 3 (n = 5), 4 (n = 3), and 8 (n = 3) days after medullectomy. At the end of the study periods, rats were anesthetized, and blood samples were drawn via catheters placed in the carotid artery for the subsequent determination of serum catecholamines to ensure that the medullectomy was successful. After the removal of the heart, a section of left ventricle (LV) was frozen in liquid N2 for analysis of cardiac catecholamines by use of glyoxylic acid labeling of endogenous catecholamines (see Histological examination). Other sections of LV and right ventricle (RV) were fixed and stained for subsequent histological examination (see Histological examination).
Surgical sympathectomy. Cardiac sympathectomy was performed to determine the role of the cardiac sympathetic neurons in ANG II-related myocardial damage. The sympathetic ganglia innervating the heart were exposed through a bilateral thoracotomy. An incision was made between the first and second rib to gain access to the sympathetic chain ganglia. The middle cervical ganglion was exposed by grasping just above the inferior cervical ganglion and pulling caudally, and all of the nerve bundles from the middle cervical ganglion were severed. The nerve branches coming from the superior thoracic ganglion were then cut by pulling the nerve cranially. After the nerve branches were cut, the middle and inferior cervical ganglia were removed. The thoracic and skin incisions were then surgically closed with standard techniques. Because these sympathetic ganglia also innervate the eyelids, successful sympathectomy was evidenced by the development of ptosis, or Horner's syndrome. For further confirmation of successful sympathectomy, a portion of the LV just basal to the section that was removed for histological assessment was frozen in liquid N2, and cardiac catecholamine content was analyzed by labeling cardiac catecholamines with glyoxylic acid (see Histological examination). Five groups of sympathectomized rats were studied. Four groups began receiving ANG II 5 days after sympathectomy; the fifth group did not receive ANG II. The four groups receiving ANG II were killed after 1 (n = 6), 2 (n = 6), 4 (n = 6), or 10 (n = 4) days of ANG II infusion. On these days, rats were anesthetized, carotid blood pressure was measured, serum samples were taken for catecholamine analysis, and the hearts were excised and prepared for histological analysis as described below.
1-Adrenergic receptor
blockade.
A set of rats was given ANG II at 150 ng/min, as above, while receiving
the 1-adrenergic receptor
antagonist atenolol (n = 6). Atenolol
was given 1 day before and throughout a 4-day ANG II infusion period at
100 mg · kg
1 · day1
via the drinking water. Sprague-Dawley rats drink ~10 ml
water · l00 g body
wt1 · day1.
From this estimate, the concentration in the drinking water was
adjusted to ensure correct dosing. At the end of the 4-day infusion
period, the rats were anesthetized, and the hearts were removed and
fixed, as described below for subsequent histological analysis.
Histological examination. In all groups, the animals were killed by removal of the heart while under deep pentobarbital anesthesia. After removal of the heart and before fixation in buffered formaldehyde solution, the RV was separated from the septum and LV. The midportion of each ventricle was embedded in paraffin, and a 5-µm-thick cross section, stained with hematoxylin and eosin, was used to delineate necrotic areas (i.e., areas of abnormal cellular infiltrate) and the cell types present in the areas of damage. Only one cross section per heart was analyzed, since we have previously reported that ANG II-related myocardial damage was evenly distributed throughout the heart (17). With an overall microscope magnification of ×400, the extent of myocyte necrosis was assessed by counting the number of necrotic sites present in the entire cross section and measuring the area of each necrotic site in both the RV and LV without knowing the source of tissue. The average necrotic area per site was calculated by dividing the total necrotic area by the total number of necrotic sites. Coronary vascular damage was assessed by counting the number of arteries and arterioles in the entire cross section of LV and RV having either abnormal endothelia and/or adventitia. Again, the source of tissue was not known until after all data were collected. The percentage of damaged vessels was calculated by dividing the number of damaged vessels by the total number of arteries and arterioles in the section.
Glyoxylic acid labeling was used to identify catecholamine containing neurons in the myocardium. Frozen, 16-µm histological sections were dipped in 1% glyoxylic acid for 3 s. The slides were then placed under a stream of warm air for 15 min until dry and then placed in a 95°C oven for 6 min. Slides were subsequently covered using light mineral oil and a glass coverslip. Sections were viewed with a fluorescence microscope and qualitatively analyzed for the presence or absence of neurons containing catecholamines.-Adrenergic receptor density.
Four groups of rats were used for -adrenergic receptor analysis as
follows: 1) untreated control
(n = 6);
2) ANG II infused as described above
for 3 days (n = 5);
3) ANG II infused for 3 days and the
osmotic minipump removed for 5 days (n = 4); and 4) ANG II infused for 8 days (n = 4). At the end of the four
experimental periods, the hearts were excised via a thoracotomy, the RV
was separated from the LV and septum, the entire LV was frozen in liquid N2, and the tissue was
stored at 
80°C. Frozen LV tissue was placed in ice-cold
homogenization buffer [25 mM Tris (pH 7.5), 1 mM EGTA, 1 mM
MgCl2, and 10 M
phenylmethylsulfonyl fluoride] and minced with scissors. The
tissue was homogenized with a Polytron tissue homogenizer (setting 10)
and centrifuged for 15 min at 500 g at
40°C. The supernatant was centrifuged at 48,000 g for 30 min at 40°C, and the
final pellet was suspended in assay buffer [25 mM Tris (pH 7.5),
1 mM EGTA, and 1 mM MgCl2]
and stored at 80°C. Protein content was measured with the
Lowry method (20). Membranes stored in this fashion retain at least
97% of the -adrenergic receptor population for up to 3 mo (4).
-Adrenergic receptor density was determined by
125I-iodocyanopindolol
([125I]CYP, 2,000 Ci/mmol) saturation binding using serial dilutions (5-200 pM) of
radioligand with 20 µg of crude membrane in a total volume of 150 µl. All experiments were performed in triplicate. Nonspecific binding
was determined by using propranolol (10 µM). Membranes were incubated
with radioligand alone or with radioligand and propranolol for 2 h at
30°C. The experiment was terminated by rapid vacuum filtration
through Whatman BF/C filters, and the filters were rinsed three times
with 4 ml of ice-cold assay buffer. The radioactivity remaining on the
filters was determined in a gamma counter (Micromedix Systems). Also,
the relative number of
1-adrenergic receptors was
determined by using the highly selective
2-adrenergic receptor
antagonist ICI-118551 (100 nM, 100× dissociation constant). The
equilibration constants and maximal number of
[125I]CYP binding
sites were calculated from plots according to Scatchard.
Statistics. All grouped and averaged results are presented as means ± SD. Group comparisons were made by using the Student's t-test for unpaired variates. Multigroup comparisons were made with one-way ANOVA with Bonferroni bounds. The level of statistical significance was taken at the P < 0.05/k level, where k is the number of comparisons.
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RESULTS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Ability of atenolol to prevent ANG II-induced myocardial damage.
Chronic ANG II infusion via a subcutaneous osmotic minipump (150 ng/min) caused marked myocyte necrosis and coronary vascular damage.
1-Adrenergic receptor
blockade (atenolol, 100 mg · kg1 · day1) significantly
decreased the number of necrotic sites per section from 15 ± 9 to 3 ± 1 and the area of each necrotic site from 0.04 ± 0.03 to 0.01 ± 0.01 mm2 compared with the
untreated, ANG II hearts. It also reduced the percentage of ANG
II-induced coronary vascular damage from 49 ± 16 to 16 ± 9%.
This reduction in myocardial damage was similar to that obtained with
propranolol treatment (14).
ANG II infusion after adrenal medullectomy. Adrenal medullectomy was created either 1 or 5 days before a 4-day ANG II infusion (150 ng/min). Plasma epinephrine was at the lowest detectable concentration (10 pg/ml) for both adrenal medullectomy groups, indicating that the medullectomy was complete. Figures 1 and 2 depict the amount of ANG II-related myocardial damage after adrenal medullectomy. When an ANG II infusion was initiated 1 day after adrenal medullectomy, there was significant myocardial damage in both ventricles. The number of necrotic sites (6 ± 5 sites/section) and the total area of necrosis (0. 12 ± 0.15 mm2) were not significantly different from unoperated, ANG II-infused hearts (9 ± 4 sites/section and 0.14 ± 0.06 mm2, respectively). The percentage of damaged coronary arterioles in this group (32 ± 12%) was similar to the percentage of damaged arterioles seen in the unoperated ANG II-infused group (47 ± 6%) and significantly greater than that in adrenal medullectomy without ANG II infusion group (control group, 15 ± 6%, in Fig. 2). In contrast, when adrenal medullectomy was performed 5 days before the beginning of the ANG II infusion, no myocyte necrosis was observed, and the percentage of coronary arterioles damaged was not statistically different from that of the control group. Histological sections were analyzed for catecholamines with the glyoxylic acid technique, and the following observations were made. 1) Catecholamine-positive areas were present in the groups that received no ANG II and were killed 1, 2, and 3 days postmedullectomy and in the group in which an ANG II infusion was begun 1 day after adrenal medullectomy; 2) two of the three hearts from rats that received no ANG II and were killed 4 days after medullectomy had no catecholamine-positive areas; and 3) no catecholamine-positive areas were observed in the two groups (i.e., with and without an ANG II infusion) killed 8 days after medullectomy. An example of catecholamine-positive and -negative histological sections is presented in Fig. 3.
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ANG II infusion after surgical sympathectomy. All rats were surgically sympathectomized 5 days before ANG II infusion. The rats were killed after 1, 2, 4, or 10 days of ANG II infusion. The results are shown in Figs. 4 and 5. At 1, 2, and 4 days of ANG II infusion after sympathectomy, the number of necrotic sites and the average necrotic area were significantly reduced compared with the unoperated, ANG II-infused group. The number and size of necrotic sites in the 10 day ANG II infusion group were lower than, but not statistically different from, the unoperated ANG II infused group.
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-Adrenergic receptor response to elevated plasma ANG
II levels.
Crude membrane preparations of left ventricles were made for the
following groups: 1) control; 2) ANG
II infused for 3 days (ANG II 3 day);
3) ANG II infused for 3 days
followed by removal of the ANG II for 5 days (ANG II 3 on/5 off); and
4) ANG II infused for 8 days (ANG II
8 day). The -adrenergic receptor densities for the four groups are
shown in Fig. 6. Three days of ANG II infusion caused a 38% decrease in -adrenergic receptor density compared with control (112 ± 30 fmol/mg protein for control vs. 70 ± 20 fmol/mg protein for ANG II 3 day,
P < 0.05). When ANG II was infused
for 8 consecutive days, -adrenergic receptor density was
significantly less than control (55% decrease,
P < 0.05) and ANG II 3-day (29%
decrease, P < 0.05) groups,
indicating continuing downregulation of the -adrenergic receptors.
Finally, when the osmotic minipump was removed after 3 days of ANG II
infusion and the rats were killed 5 days later, the -adrenergic
receptor density was measured to be 133 ± 29 fmol/mg protein, a
value that was significantly greater than those obtained in the 3- and
8-day ANG II infusion groups but not statistically different from that of the control group.
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1-Adrenergic receptor density
was decreased after 3 and 8 days of ANG II infusion (Fig.
7). On removal of the ANG II pumps, 1-adrenergic receptor density
was significantly higher compared with densities in the ANG II 3-day
and ANG II 8-day groups but not statistically different from control
group 1-adrenergic receptor density.
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DISCUSSION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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The aims of the present study were
1) to determine whether ANG
II-induced myocardial damage is mediated via the
1-adrenergic receptor,
2) to elucidate whether adrenal
medulla catecholamines or cardiac sympathetic neuron catecholamines are
responsible for ANG II-related myocardial damage, and
3) to determine whether the lack of
damage after 3 days of chronically elevated ANG II levels was due to
-adrenergic receptor downregulation. Myocardial damage associated
with chronic ANG II infusion was significantly reduced by
1-adrenergic receptor blockade.
ANG II caused both myocyte necrosis and coronary vascular damage after
adrenal medullectomy but not after cardiac sympathectomy. There was a
progressive decrease in
1-adrenergic receptor density
associated with a continuing ANG II infusion and an upregulation to
control levels 5 days after the ANG II infusion was stopped. We
conclude that cardiac sympathetic neuron catecholamines are responsible
for ANG II-related myocardial damage and that the acute nature of this
damage is associated with the downregulation of
1-adrenergic
receptors.
The myocyte necrosis associated with chronic elevations of ANG II is
acute with most of the damage occurring within the first 1-3 days
(17, 25). The fact that losartan totally prevents the ANG II-induced
myocyte necrosis and coronary vascular damage (16) indicates that the
damage is mediated through the AT1
receptor. However, ANG II is known to stimulate the release of
norepinephrine and epinephrine from the adrenal glands and sympathetic
nerve endings (29), and we recently reported that the myocardial damage associated with chronic elevations in ANG II is due to
AT1-receptor-mediated catecholamine release and not due to ANG II directly (14). The results
presented herein indicate that the myocyte necrosis associated with
chronic elevations in ANG II is mediated via
1-adrenergic receptors or by a
1-adrenergic receptor-mediated
pathway.
Coronary vascular damage is associated with abnormal elevations in
circulating ANG II and norepinephrine (3, 11, 17). In a recent study
from our laboratory (16), in which rat models of either renovascular
hypertension or ANG II infusion were used, approximately one-half of
the coronary arterial vessels were categorized as damaged. In that
study, AT1-receptor blockade
prevented the coronary vascular damage. In the present series of
studies, 1-adrenergic receptor
blockade was able to prevent coronary vascular damage, indicating that
the damage was due to ANG II-induced catecholamine release and that
these catecholamines elicit damage via the
1-adrenergic receptors.
To examine the contribution of adrenal catecholamines to the myocardial damage observed with chronic elevations in circulating ANG II, the adrenal medulla glands were removed before ANG II infusion. Myocyte necrosis and coronary vascular damage were observed when ANG II was infused 1 day but not 5 days after adrenal medullectomy. The fact that plasma levels of epinephrine were not detectable suggests that adrenal medullectomy was complete. These observations suggest that catecholamines from the cardiac sympathetic neurons are responsible for the early myocardial damage associated with chronic ANG II infusion, since damage occurred in the absence of the adrenal medulla glands. The myocardial damage was significantly less when the ANG II infusion was started 5 days after adrenal medullectomy, suggesting that the cardiac sympathetic neurons rely on the adrenal medulla to supply a major part of their catecholamine stores. This was substantiated histochemically in that catecholamine-positive areas were present in all the hearts from rats that were medullectomized for <4 days, but such areas were not detected in two of the three hearts from rats medullectomized for 4 days and all of the hearts from rats that were medullectomized for 8 days. It has been reported that as much as 79% of the norepinephrine entering the coronary circulation is sequestered by the heart (7). Ratajska et al. (23) reported that ANG II-induced myocyte necrosis was due to increased circulating catecholamines of adrenal origin. The infusion technique used in their study was the same as the one used in the studies presented herein (150 ng/min via a subcutaneous osmotic minipump). In their study, however, blood samples were only taken when the animals were killed (i.e., 1 wk after medullectomy followed by 2 wk of ANG II infusion). Serum norepinephrine levels are known to be elevated at day 14 of ANG II infusion; however, this increase in norepinephrine levels does not occur until day 4 (14), although the myocardial damage is observed before this time. Thus, without the temporal profile of serum norepinephrine, the conclusion of Ratajska et al. (23) that circulating catecholamines are the cause of the acute episode of necrosis cannot be substantiated.
When the neural source of catecholamines was removed by surgical
sympathectomy and ANG II was infused for 1 day, myocardial damage was
prevented. This was not the case if ANG II was infused for longer than
1 day. Although the damage observed after 2 and 4 days of ANG II
infusion was significantly less than that seen in unoperated, ANG II
infused animals, the damage in the 10-day group was similar (Figs. 4
and 5). The damage with longer ANG II infusion periods in hearts from
sympathectomized rats in all likelihood was due to the gradual ANG
II-induced increase in plasma catecholamines described previously (14)
and possibly to the phenomenon of adrenergic supersensitization. The
first time derivative of LV pressure response to norepinephrine has
been shown by Vatner et al. (28) to be significantly greater 2-4
wk after cardiac denervation in dogs compared with sham-operated dogs.
This increase in adrenergic sensitivity was attributed to a slight
increase in -adrenergic receptor density and the absence of
norepinephrine uptake. The findings of Gilbert et al. (12) indicate
that catecholamine supersensitivity is a presynaptic event (i.e., lack
of norepinephrine uptake) and not a postsynaptic event (i.e., increased
1-adrenergic receptor number).
However, Valette et al. (26) demonstrated that surgical sympathectomy
caused a 190% increase in -adrenergic receptor density 9 days after
sympathectomy and a 219% increase 3 wk after sympathectomy and that
the increased sensitivity was blunted by -adrenergic receptor
blockade. Similarly, Jo et al. (15) reported a 29% increase in
-adrenergic receptor number 4 days after sympathectomy. Therefore
there are conflicting data as to whether an increase in -adrenergic
receptor number or a decrease in norepinephrine uptake is responsible
for the supersensitivity seen after cardiac sympathectomy.
The results presented herein indicate that myocardial damage due to
chronic elevations in ANG II is significantly reduced by
1-adrenergic receptor blockade.
With chronic exposure to elevated levels of norepinephrine,
1-adrenergic receptors quickly
become downregulated by receptor internalization, and the
subsequent signaling events (e.g.,
Gs protein coupling with adenylyl
cyclase activity) are desensitized (21). This downregulation is quickly reversed when the level of catecholamines is returned to normal. This
provides a plausible mechanism for the absence of ANG II-related myocardial damage after 3 days even when ANG II levels remain chronically elevated. It is conceivable that the downward trend in
-adrenergic receptor density would continue with longer periods of
exposure to elevated plasma levels of ANG II. Indeed, over an 8-day
infusion period most of the myocyte necrosis occurs in the first few
days of infusion (17). There could also be catecholamine-induced alterations in transcriptional and posttranscriptional regulation of
the -adrenergic receptors secondary to the chronic ANG II infusion,
which then contribute to the decreased -adrenergic receptor density
and the acute nature of the myocardial damage (21). Finally, the
observation that -adrenergic receptor density returned to baseline
levels 5 days after cessation of the ANG II infusion would explain the
findings of Kabour et al. (17), in which de novo necrosis was reported
to occur during a second 2-day ANG II infusion that was started 5 days
after an initial 2-day ANG II infusion.
Our results could also be explained by a downregulation of the
AT1 receptor. However, this does
not seem likely, since we have previously shown that
1) ANG II-induced increases in
plasma aldosterone and norepinephrine remain elevated throughout a
14-day infusion of ANG II, 2)
-adrenergic blockade prevented ANG II-induced myocardial damage, and
3) when this blockade was removed
after 4 days of ANG II infusion, myocardial damage occurred during the next 3 days of ANG II infusion (14). Thus, if the
AT1 and/or -adrenergic
receptors had downregulated during the 4-day period of ANG II infusion
and -adrenergic blockade, the hearts would not be expected to have
myocardial damage after removal of the -adrenergic blockade and
continuation of the ANG II infusion. AT1-receptor activation has been
shown to inhibit -adrenergic receptors in cell culture (2) by
stimulating Gi protein and thereby
inhibiting adenylyl cyclase activity. Also as stated above, when ANG II
was chronically infused during -adrenergic blockade and then the
blockade was removed, significant myocardial damage occurred as a
result of the continuing ANG II infusion (14). Thus, despite the
continuous stimulation of the AT1
receptor, the degree of the "cross talk" inhibition of the
-adrenergic receptor, if present, was insufficient to protect the
myocardium. However, this does not affect the ability of ANG II to
induce apoptosis of cardiac myocytes via the
AT1 receptor (18).
The fact that myocardial damage due to chronic elevations in ANG II is caused by cardiac sympathetic neuron norepinephrine does not mean that elevated circulating catecholamines do not contribute to this phenomenon. Myocardial damage is known to occur with intravenous infusions of norepinephrine (10) and epinephrine (22) and in pheochromocytoma (1, 27). Also, as discussed above, the myocardial damage seen in the sympathectomized group that received an ANG II infusion for 10 days was probably due to increased plasma catecholamines. However, even though elevations in circulating catecholamines can cause myocardial damage, catecholamine-induced damage secondary to elevated ANG II levels appears to be due to catecholamine release from the cardiac sympathetic neurons, since a significant increase is not seen in circulating catecholamines until days 4-6 of ANG II infusion (14) and since cardiac sympathectomy can prevent the damage.
In conclusion, myocyte necrosis and coronary vascular damage associated
with chronic elevations of ANG II are due primarily to locally released
catecholamines rather than catecholamines released from the adrenal
medulla. The myocardial damage is mediated via the
1-adrenergic receptor. The
acute nature of the myocardial damage is due to a progressive
downregulation of the
1-adrenergic receptor over the
ANG II infusion period. Removal of the ANG II results in an
upregulation of 1-adrenergic
receptors making the heart once again susceptible to subsequent
elevations of ANG II.
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ACKNOWLEDGEMENTS |
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We are grateful to Lisa Henegar for the preparation of the
histological material and to Leslie Allen for assistance with the -adrenergic receptor analysis.
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FOOTNOTES |
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This study was supported by National Heart, Lung, and Blood Institute Grant R01-HL-46461 and American Heart Association Alabama Affiliate Grant-in-Aid AL-G960041.
Address for reprint requests: J. S. Janicki, Physiology and Pharmacology, 106 Greene Hall, Auburn University, Auburn, AL 36849-5517.
Received 27 June 1997; accepted in final form 20 April 1998.
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REFERENCES |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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