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Regulation of β-Adrenoceptors in a Rat Model of Cardiac Failure: Effect of Perindopril

Gu, X. H.; Kompa, A. R.; Summers, R. J.

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Journal of Cardiovascular Pharmacology: July 1998 - Volume 32 - Issue 1 - p 66-74
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Abstract

Congestive heart failure in humans is characterized by alterations in the β-adrenoceptor (β-ar) signal-transduction system, with a reduction of β-ar number (1-3), loss of G-protein function, and inhibition of adenylate cyclase activity (4). Until recently, attention was focused on downregulation of β- ars (1-3,5,6) associated with all types of human cardiac failure. Reductions of β1-ars or both β1- and β2-ars are associated with particular pathologic conditions (for review, see 1). The loss of β1-ars is associated with a selective reduction in β1-ar messenger RNA (mRNA) in human failing heart (2,5).

In ∼30-40% of patients with heart failure, the cause is myocardial infarction (MI; 7). Therefore a commonly used model of experimental heart failure has been produced by surgical ligation of rat coronary artery (8,9). The reports of the effects of this procedure on β-ar density in this animal model are contradictory. Thus 3 weeks after infarction, increases (10) or decreases (4) in β-ars have been described, whereas 7 days after MI, no change in β-ar number (11) was reported. In left ventricular (LV) sarcolemmal membranes, β-ars were reduced by 30% 4 weeks after MI and failure (6). No further decrease in β-ar density was observed when the experimental period was extended to 16 weeks (6). The affinity of binding in all reports was unchanged. There are two major limitations to these studies. The use of homogenates or purified membranes does not allow the study of the regional localization of the changes, and despite a wealth of information of the changes in β-ar subtypes in humans in different pathologic conditions (3), all previous animal studies have not delineated the β-ar subtypes.

Our studies were designed to investigate β-ar subtype changes in anatomically defined regions of infarcted and noninfarcted LV free wall and right ventricle in rats with cardiac failure by using autoradiographic techniques. Angiotensin-converting enzyme (ACE) inhibitors are known to improve the response of the infarcted rat heart to (−)-isoprenaline stimulation (12,13), and captopril was reported to inhibit the loss of β-ars from isoprenaline-treated guinea pigs (14). We examined whether the effect of ACE inhibitors on responsiveness of the infarcted heart may be explained by inhibition of the loss of β-ars by using a long-acting ACE inhibitor, perindopril.

MATERIALS AND METHODS

Experimental heart-failure model

The rat heart-failure model was prepared by surgical ligation of the coronary artery (8,9). Female Sprague-Dawley rats weighing 220-280 g were anesthetized with Saffan (1.5 ml/kg, i.v.), a steroid anesthetic that has few inhibitory effects on respiration. The rats were intubated and ventilated with a respirator. Left thoracotomy was performed, and the heart exteriorized through the intercostal space after removing the pericardium. The left coronary artery was ligated with a 6-0 blue monofilament polypropylene suture, ∼3 mm from its origin, just below the left atrium. Before ligating the coronary artery, we gave lidocaine (10 mg/kg) by intramuscular injection to prevent ventricular arrhythmias. The heart was repositioned and thorax closed by using a previously placed purse-string suture after expelling air from the cavity to avoid pneumothorax. When spontaneous respiration was established, the rats were extubated and allowed to recover. By using this method, there was 70% survival within the first 24 h after occlusion of the left coronary artery. Sham-operated controls were prepared similarly except that the ligation around the coronary artery was not tied.

Perindopril treatment

One week after the ligation of the coronary artery, rats with reduced systolic blood pressure (≤120 mm Hg) were considered to have a significant MI (8,15). These rats were then randomly divided into two groups: one group received perindopril at a dose of 2 mg/kg/day in drinking water, and the other, normal drinking water. A similar procedure was carried out for the sham-operated rats. The dose of perindopril used is known to provide cardioprotection in infarcted or failing rat heart (16-18) and is in the therapeutic range used clinically (19). This treatment regimen also is known to cause marked and significant reduction in ACE activity in plasma, heart, lung, and kidney (20). The concentration of perindopril solution was adjusted to the individual drinking habits of the rats to ensure the appropriate dose based on the measurement of body weight and daily fluid intake. All rats had free access to food and drinking water.

Measurement of blood pressure

Systolic blood pressure was measured in conscious rats by the tail-cuff method (Phenmatic pulse transducer, Bioscience, Sherness, Kent, U.K.; and a Rikadenki chart recorder, Rikadenki, Kogyo Co. Ltd., Tokyo, Japan) between 10 a.m. and 12 noon, and the mean value of at least four measurements was taken. The blood pressure was measured on the day before surgery to familiarize the animals with the procedure and then on the day of surgery immediately before the operation. Subsequently blood pressure was recorded weekly throughout the experimental period.

Measurement of organ weights

Five weeks after the ligation of the coronary artery, rats were anesthetized with pentobarbital (50 mg/kg, i.p.). The heart and lungs were removed and perfused with cold solution comprising equal parts of sucrose (0.32 M) and Krebs buffer containing (in mM) NaCl, 118.4; KCl, 4.70; MgSO4, 1.20; CaCl2, 1.27; and NaH2PO4, 10.0; pH 7.2, and then with the same buffer containing 0.1% paraformaldehyde. The free wall of the right and LVs was dissected and weighed. Organ weights were expressed as a proportion of body weight.

Estimation of infarct size

The infarct size was measured by placing cooled and opened LV under a transparent plate and directly tracing whole LV and infarcted area. The infarct area was then measured by using a computerized image-analysis system (MCID; Imaging Research, St. Catharines, Ontario, Canada) and the ratio of the mean of the endocardial and epicardial scar area divided by the total LV area calculated. Only rats with an infarct size >42% of the endocardial area of the LV free wall were included in the study and had overt heart failure, which is known to be characterized by increased ventricular pressure, reduced cardiac output, and myocardial hypertrophy, especially right ventricular hypertrophy (8,9,21). Because area measurement could underestimate infarct size as a result of resorption of necrotic tissue and subsequent wall thinning (22), rats with >42% infarct area accompanied by decreased blood pressure and hypertrophied right ventricle were considered to be in heart failure.

Autoradiography

Autoradiography was performed as previously described (23). Right and LV free wall from either sham-operated or MI rats was frozen in isopentane, cooled in liquid nitrogen, and stored at −80°C until required. Frozen tissues were mounted in O.C.T. (Tissue-Tek; Miles, Elkhart, IN, U.S.A.), cut into longitudinal sections (10 μm) on a Reichert-Jung cryostat (Heidelberg, Germany) at −25°C, and mounted onto gelatin/chromic potassium sulfate-coated microscope slides. Every fifteenth slide was stained with hematoxylin and eosin for histologic examination. Tissue sections were preincubated in Krebs buffer containing ascorbic acid (0.1 mM), phenylmethylsulfonylfluoride (PMSF; 10 μM), and guanosine triphosphate (GTP; 0.1 mM) for 30 min at 25°C to remove endogenous catecholamines (24). Sections were then incubated at 37°C for 150 min in Krebs with 0.1 mM ascorbic acid, 10 μM PMSF, and 50 pM (−)[125I]cyanopindolol [(−)[125I]CYP] in the absence or presence of the selective β1-ar antagonist CGP 20712A (100 nM) or the β2-ar antagonist ICI 118,551 (70 nM) or (−)-propranolol (1 μM) to define nonspecific binding. After incubation, labeled sections were quickly rinsed, followed by 2 × 15-min washes in Krebs buffer at 37°C to achieve low nonspecific binding with no loss of specific binding. The slides then were quickly dipped in distilled water (22-25°C) and dried with a stream of cold air. Dried labeled sections were apposed to x-ray film (Kodak NMC100, Melbourne, Australia) in light-tight boxes for 14 days at 4°C. The film was developed with Kodak D19 at 17-21° for 5 min, rinsed briefly in water, and fixed in Kodak Rapid Fix for 5 min (17-21°C). All the experiments were carried out with six replicates. Film images were quantitated by using a computerized image system (MCID; Imaging Research) and standardized by using (−)[125I]CYP standards prepared in rat-heart paste. Previous studies confirmed that protein concentration is not uniform throughout the heart (25), and MI also may alter protein density in affected regions of the LV. Therefore protein was determined in the same section used for autoradiography by a densitometric method based on Coomassie brilliant blue, as described previously (25). Receptor density was quantitated in defined areas and corrected for protein concentration in the same area.

Saturation studies

To determine the affinity (KD) of (−)[125I]CYP binding to rat ventricles, saturation studies were performed under the experimental conditions described earlier. Sections (10 μm) of normal rat ventricular free wall were incubated with 10-150 pM (−)[125I]CYP for 150 min. Nonspecific binding was defined in the presence of 1 μM (−)propranolol. After washing, the labeled sections were wiped from the slides with Whatman GF/B filters and counted in a Packard gamma counter (model 5301, Meriden, CT, U.S.A.) at an efficiency of 79%. Saturation data were analyzed by using "EBDA" (26), which performed preliminary Scatchard, Hill, and Hofstee analysis. A file was then produced and the data analyzed by a weighted, nonlinear, least-square computer-curve-fitting programme, "Ligand" (27) to obtain final binding parameters (KD and Bmax).

Statistics

Results are given as mean ± SEM of six experiments conducted in sextuplicate. A multiple-group comparison was performed by analysis of variance (ANOVA) followed by Student's t test with Bonferroni's corrections (28).

Reagents

(−)[125I]CYP (specific activity, 2,000 Ci/mM) was from Amersham International, Buckinghamshire, U.K. CGP 20712A was obtained from Ciba-Geigy Ag (Basel, Switzerland) and ICI 118,551 from ICI (Wilmslow, Cheshire, U.K.). Perindopril was a gift from Technologies Servier (Neuilly, Sur Seine, France). All other reagents were from Sigma Chemical Company (St. Louis, MO, U.S.A.).

RESULTS

Organ weights and infarct size

Table 1 shows the body and organ weights (expressed as a proportion of body weight) in MI rats and sham-operated controls given water or perindopril solution. There was no significant difference in body weight between the four groups. Heart, lung, right ventricular, and left atrial weights were significantly higher in control MI rats than in sham-operated controls. This indicates cardiac hypertrophy and suggests the development of cardiac dysfunction (8,21). There was a tendency for the cardiac hypertrophy and increased lung weight to be decreased in perindopril-treated rats with MI compared with untreated rats, but this was not significant. Unlike untreated rats, however, perindopril-treated MI rats showed no significant alterations in heart, right ventricular, and lung weights compared with corresponding sham-operated controls. For sham-operated rats, perindopril treatment decreased heart (p < 0.03) and LV weights (p < 0.003) compared with those of untreated controls. Infarct size was not significantly different (p = 0.67) in the perindopril-treated (46 ± 3%) and untreated (48 ± 2%) rats and ranged between 42 and 61%.

TABLE 1
TABLE 1:
Body weight, organ weights, and infarct size in sham-operated and myocardial infarcted rats with or without perindopril treatment

Systolic blood pressure

Seven days after surgery, rats with MI exhibited significant decreases in systolic blood pressure compared with sham-operated controls (Fig. 1). With perindopril treatment, MI rats maintained a lower blood pressure (∼10 mm Hg) than untreated MI rats throughout the experimental period. The magnitude of this hypotensive response to perindopril was similar during the first and final weeks of treatment and did not correlate with the size of the infarct. After 2 weeks of perindopril treatment, sham-operated rats exhibited a small but significant reduction in systolic blood pressure (p < 0.005) compared with untreated controls (Fig. 1).

FIG. 1
FIG. 1:
Effects of myocardial infarction and perindopril treatment on systolic blood pressure during the experimental period. Blood pressure (mm Hg) was recorded immediately before operation in both sham-operated controls (circles) and myocardial infarcted rats (triangles) and then at weekly intervals with (solid symbols) or without (open symbols) perindopril treatment. Treatment (at the arrow) with perindopril or vehicle was commenced immediately after the second blood pressure recording. Each point is a mean ± SEM for six experiments. *p < 0.05 relative to the appropriate sham-operated controls. p < 0.05 relative to untreated controls.

Protein density in tissue sections

Protein density (in μg/mm2) in slide-mounted tissue sections was similar in right ventricles between groups (Table 2). Compared with LVs of sham-operated controls, no significant change in protein concentration was observed in the noninfarcted and border areas of the LVs for both untreated and perindopril-treated rats (Table 2). However, the protein density was reduced in the infarcted regions compared with the noninfarcted ventricular region of the same section (Table 2) for both the untreated (p = 0.004) and perindopril-treated groups (p = 0.018).

TABLE 2
TABLE 2:
Protein concentration in slide-mounted ventricular tissue sections (10 μm) from sham-operated and MI rats with or without perindopril treatment

Histologic changes in infarcted tissue

Examination of hematoxylin and eosin-stained tissue sections indicated that coronary artery ligation produced replacement of myocardium by fibroelastic connective tissue. Under the microscope, the noninfarcted, border, and infarcted regions could be precisely defined for autoradiographic analysis. The border area was defined as the 1-mm rim bordering the edge of the infarct.

Characterization of (−)[125I]CYP binding in rat heart sections

Tissue sections from LV free wall of normal rat heart contained a single population (nH, 0.964 ± 0.012; n = 3) of high-affinity (−)[125I]CYP binding sites (Fig. 2B), with a KD (dissociation constant) of 56.2 ± 1.4 pM and Bmax of 43.9 ± 3.0 fmol/mg protein. The KD value for (−)[125I]CYP binding is similar to that recently reported in rat LV sections (29) and homogenates (4). Binding was saturable with increasing concentrations of (−)[125I]CYP (Fig. 2A) and equilibrated within 150 min. Nonspecific binding was ∼10% of the total (−)[125I]CYP binding.

FIG. 2
FIG. 2:
Representative saturation curve (A) for (−)[125I]CYP binding to tissue sections of rat left ventricular free wall. Specific binding (open squares) was calculated as the difference between total binding (solid circles) and nonspecific binding (open circles). Scatchard plot (B) shows that (−)[125I]CYP binds to single population of high-affinity sites with a KD of 57.0 pM and Bmax of 45.6 fmol/mg protein.

Ventricular β-adrenoceptor changes in heart failure

In the sham-operated animals, total β-ar, β1-ar, and β2-ar subtypes were measured in right and LVs. No heterogeneity was noted in the distribution of the β-ar subtypes across the myocardium, but LV had slightly higher concentrations of β-ar compared with right ventricle (see later). In the MI animals, similar regions were identified, but in addition, the β-ar subtypes were measured in the area of the infarct and in an area of histologically normal myocardium within 1 mm of the edge of the infarct (the border region).

Noninfarcted tissues. In both right ventricle and LV free wall away from the area of the infarct, MI-induced heart failure had no significant effect on either total, β1-, or β2-ar density or on the proportion of β1- and β2-ars or their distribution (Table 3). In right ventricle, the total β-ar density did not change significantly and was 62.4 ± 3.7 fmol/mg protein in sham-operated controls and 57.7 ± 5.5 fmol/mg protein in MI rats (p = 0.50; n = 6). Similarly, in intact LV, the β-ar density was 71.5 ± 4.5 fmol/mg protein in sham-operated controls and 78.0 ± 11.5 fmol/mg protein in MI rats (p = 0.61; n = 6; Fig. 3). When the β-ars were delineated into β1-ar and β2-ar, it was seen that the lack of change in the overall β-ar population was not due to reciprocal changes in the subtypes (p > 0.45 for comparison of both β1-ar and β2-ar). The percentage of β1- to β2-ars in untreated control animals (79:21, right ventricles; 74:26, LVs) was similar to that previously reported (30).

TABLE 3
TABLE 3:
Effect of MI and perindopril treatment on rat ventricular β-adrenoceptor density and subtype distribution
FIG. 3
FIG. 3:
Effects of myocardial infarction and perindopril treatment on total, β1-, and β2-adrenoceptors in right (A-C) and left (D-F) ventricles from sham-operated (Sham) and myocardial infarcted (MI) rats with (crosshatch) or without (open bar) perindopril (P) treatment. Note the marked reduction in total β-adrenoceptors (D) due entirely to loss of β1-adrenoceptors (E) in border and infarcted (MI) regions of the left ventricle. There was no reduction in β-adrenoceptors in intact areas of left ventricle or in right ventricle. Perindopril treatment had no effect on β-adrenoceptors in any region of the ventricles. *p < 0.05 relative to appropriate sham-operated rat left ventricle and noninfarcted regions (Non-MI) of left ventricle in the same group. Each histogram is mean ± SEM of six experiments.

Border region. In untreated MI rats, the total β-ar density in the border region was significantly lower (Bmax = 44.0 ± 5.2 fmol/mg protein) than in the remaining noninfarcted ventricle (Bmax = 78.0 ± 11.5 fmol/mg protein; p < 0.05; n = 6; Fig. 3D). Delineation of receptor changes showed that almost all of the reduction was due to loss of β1-ars, and the Bmax value for this subtype decreased from 56.3 ± 9.4 fmol/mg protein in the noninfarcted region to 21.8 ± 2.4 fmol/mg protein (p < 0.01; n = 6) in the border region, a decrease of 61%. In contrast, the Bmax for β2-ar remained unchanged, being 21.7 ± 3.1 fmol/mg protein in LV myocardium away from the infarcted area and 22.2 ± 3.6 fmol/mg protein (p = 0.42; n = 6) in the border region (Fig. 3F), so that the proportion of β1-ars to β2-ars in the border area compared with noninfarcted regions decreased to 50:50 (Table 3).

Infarcted region. As expected for the area of the infarct, the total β-ar (Fig. 3D) density in MI rats decreased further in the infarcted region of LVs to a Bmax value of 26.8 ± 5.5 fmol/mg protein compared with 78.0 ± 11.5 fmol/mg protein (p < 0.003; n = 6) in unaffected LV myocardium. All of the change in β-ar density was due to loss of β1-ar from this region because this subtype density was reduced to 5.4 ± 1.8 fmol/mg protein from 56.3 ± 3.1 fmol/mg protein (p < 0.001; n = 6) in unaffected myocardium (Table 3). In contrast and rather surprisingly, the β2-ar population did not change, being 21.4 ± 4.3 fmol/mg protein in the infarcted region and 21.7 ± 3.1 fmol/mg protein in healthy myocardium. (p = 0.63; n = 6). Thus relative to the noninfarcted and border area, infarcted regions lost 66% and 39% of total β-ars and 90% and 75% of β1-ars. In contrast to the change in β1-ars, β2-ars showed no significant change in either the border or the infarcted region (Fig. 3F). Thus the proportion of β1-ars to β2-ars changed from 71:29 in unaffected myocardium to 50:50 in the border region and 19:81 in the infarcted region.

Effects of perindopril

Perindopril treatment was started 1 week after the production of the infarct, and although the mean weights of the heart, left ventricle, right ventricle, and left atrium were reduced with treatment, these changes were not significant. In addition, 4 weeks of perindopril treatment had no significant effect on total β-ar density or on β1- or β2-ar subtypes or on the proportion of subtypes in left and right ventricles of sham-operated rats (p > 0.18; n = 6; Table 3). In MI rats, perindopril treatment had no significant effect on the reduction in total or β1-ar density in the right ventricle or in the noninfarcted region of the left ventricle (p > 0.81; n = 6). In addition, in the same group, perindopril treatment had no significant effect on total, β1-ar, or β2-ar density in the border and infarcted regions of left ventricles compared with untreated MI rats (p > 0.54; n = 6; Table 3; Fig. 3D and E).

DISCUSSION

The experimental model of cardiac failure used in this study has been widely used and well characterized (8). The ventricular dysfunction in this rat model is proportional to the infarct size (8), and in our study, the infarct size was >42% of the LV free wall. The development of cardiac hypertrophy, especially in the right ventricles, with the decrease in systolic blood pressure occurs only in the decompensated stage of cardiac failure (8,15). Our autoradiographic experiments determining the location and density of β-ar subtypes were performed 5 weeks after MI when the right ventricular hypertrophy was well developed, and this, together with the decreased systolic blood pressure, increased lung and left atrial weights (indicative of lung congestion) and suggested the presence of overt cardiac failure.

Previous studies using membrane preparations found a reduction in β-ar density in left ventricles of MI rats, without any change in binding affinity. The most interesting finding in our study is that a selective downregulation of β1-ars did occur but only in border and infarcted regions of the left ventricle. Receptor changes were not found in surviving noninfarcted regions of the same left ventricle or in the hypertrophied right ventricle. The border region in this study was defined as the area within 1 mm of the edge of the infarct. Microscopically this region appeared normal and did not contain the large amounts of connective tissue observed in the region of the infarct. It is therefore unlikely that the reduction in β-ar numbers resulted from a "dilution" of myocardial cells with connective tissue. This view is in agreement with studies showing that myocytes isolated from the border region of LV infarcts in dogs have a markedly lower β-ar population (31) and that these cells have unusual electrophysiologic properties (32). Not surprisingly, there was a major loss of β-ar from the infarcted area because the myocardium in this region was replaced by connective tissue with few scattered surviving myocardial cells. The loss of <90% of β1-ars almost certainly reflects the loss of myocytes (which contain predominantly β1-ars) from this infarcted region. However, rather surprisingly, there was no change in the β2-ar population, even in the infarcted area, probably indicating that these receptors are not on myocytes. The mechanisms involved in the loss of β1-ars in the border region is uncertain. Because the right ventricle and noninfarcted portion of the left ventricle did not show the same change, it could not be simply explained by receptor downregulation associated with the increased sympathetic activity (reflected in increased plasma noradrenaline levels) found in most human cases of heart failure (33). Although the plasma noradrenaline levels do not show a significant increase in this animal model (34), the possibility of locally increased synaptic concentrations of noradrenaline cannot be excluded and could be involved in the regional downregulation. Nevertheless, the fact that we observed changes in β1-ars only in border and infarcted regions, and not in other noninfarcted areas, suggests the involvement of local rather than systemic or neuronal factors in modulating receptor density in this animal model.

Steinberg et al. (31) recently found that >50% of β1-ars were lost on myocytes isolated from border regions of infarcts of left ventricle in dogs. Myocytes from border regions also have abnormal electrophysiologic properties (32). These findings are in accord with our results showing a reduction in β1-ars in the border region. The reduction of total β-ar number by 45% in this region is much higher than in previous reports using cardiac membrane preparations, which reported a 20-30% decrease in β-ar density (4,6,10). This difference may be explained by the inclusion in our studies of the border region, which appears to be normal tissue under histologic and gross examination. In addition, the procedure of membrane preparation could mask actual loss of β-ars (35). It has been reported that loss of myocytes occurs from the surviving noninfarcted LV free wall (36), but whether this change is also nonuniform between infarcted and border or noninfarcted areas remains unknown. Because β1-ars are predominantly in myocytes, the loss of myocytes may contribute to the reduction of β1-ars seen in the border region. However, the unchanged protein concentration in this region does not support this possibility. One point should be emphasized here, and this is that although no apparent changes were found in β-ars in RV and in the noninfarcted region of LV, changes in postreceptor signaling components may be responsible for the change in β-ar responsiveness (4,37). The time course of the change in β-ars during the development of heart failure has been investigated in this animal model (6). It will be important to know whether the postreceptor changes share the same time course.

The recent report from Xiao and Lakatta (38) stated that functional β1- and β2-ars coexist in rat ventricular myocytes, but that stimulation of these subtypes elicits qualitatively different cell responses at the level of ionic channels, the myofilaments, and sarcoplasmic reticulum. The significance is unknown. Our studies confirmed the coexistence of β1- and β2-ars in rat ventricles by using highly selective β1- and β2-ar antagonists, which is consistent with previous reports (29). Our studies also showed that despite large reductions in β1-ars, β2-ars did not change in any region of ventricles in rat MI. Thus the β1-ar to β2-ar ratio shifted from 70:30 in noninfarcted to 50:50 in the border region. Similar situations also are found in human failing heart (39), but as yet relatively little is known of the physiologic role of cardiac β2-ars in health or disease.

This investigation shows that treatment with an ACE inhibitor perindopril attenuated cardiac hypertrophy for MI rats compared with corresponding sham-operated controls but did not reach significance compared with untreated MI rats. The consistent reduction in weight of cardiac regions observed with perindopril treatment probably did not reach significance because of the variations in the size of the infarcts, which resulted in larger standard errors in the infarcted rats. A study using a treatment regimen similar to that used here found similar effects on heart weights (20), but others using the same dose of perindopril showed a significant improvement in cardiac hypertrophy for MI rats (17,18). This discrepancy could be related to the different infarct sizes of MI rats. Rats with MI in this study had large infarct size of ∼45%, whereas other studies used MI rats with smaller infarct size, determined by either visual inspection (18) or weight of infarct area (17). The severity of hypertrophy in infarcted myocardium parallels infarct size (8). Additionally, perindopril treatment in our study was not initiated until 7 days after MI at a time when cardiac hypertrophy had already occurred (40). The histomorphologic effect of an ACE inhibitor on hypertrophied myocardium in the delayed treatment (41) takes a longer time than when the drug is given immediately after MI (17). The dose of perindopril given in this study was adequate to cause significant inhibition of ACE. Similar studies (20) showed that 2 mg/kg/day perindopril in drinking water causes marked and significant reductions in ACE activity in plasma, heart, lung, and kidney, as well as the reductions in blood pressure observed here.

The autoradiographic findings show that perindopril treatment had no effect on β-ar density or subtype distribution in any region of ventricles in both sham-operated and infarcted rats. Similar results were also obtained in the perindopril-treated hypertensive rats (30). Maisel et al. (14) reported that the ACE inhibitor captopril prevented the isoprenaline infusion-induced downregulation of β-ars in guinea-pig heart. It is possible that the differences relate to the prevention of (−)-isoprenaline-induced cardiac injury by captopril, to the fact that different ACE inhibitors were used, or to a species difference in β-ar response to cardiac failure. It was reported that β-ars increase in a guinea-pig model of pressure overload heart failure (42). However, ACE inhibition does improve the response of infarcted rat heart to isoprenaline stimulation (12,13). Our results suggest that the improved response of the infarcted rat heart to (−)-isoprenaline after treatment with ACE inhibitors does not result from changes in the numbers of β-adrenoceptors but is more likely to be associated with changes in the signal-transduction pathway. Captopril has been shown to improve cardiac adenylate cyclase activity in the noninfarcted myocardium without any effect on the density or affinity of β-ars in the rat model of cardiac failure (37). Even so, the cardioprotective effect of ACE inhibition in vivo may be accounted for by the effect on ventricular remodeling and reduction in workload associated with peripheral vasodilation (43) or possible decreased cardiac ACE activity and angiotensin II (44), rather than by the direct effect on the sympathetic β-ar system. ACE inhibitors are also known to affect the functioning of the heart by increasing kinin levels, which then in turn release nitric oxide (45). However, the marked change that occurs in the function of the atrial and ventricular tissue after infarction (Kompa et al., unpublished data) is, as we have shown here, not accompanied by a corresponding change in β-ar number. Thus it is not likely that the desensitization results from an increase in sympathetic activity. Current clinical evidence does not support the idea that ACE inhibitors improve cardiac function by inhibiting sympathetic activity in patients with congestive heart failure (46).

In conclusion, the results reported here establish that β1-ars were decreased in the border region of the infarcted LV from rats with cardiac failure. Such a change was not found in the noninfarcted area of the same LV or in the right ventricle. The β2-ars did not alter in any region of the ventricles at a time when β1-ars showed a large reduction in the border or infarcted regions of LV. Treatment with the ACE inhibitor perindopril failed to show any effect on the β-ar density in any region. Further studies are required to determine the significance of the reduction in β1-ars and the precise mechanisms of the enhancement of β-adrenergic responsiveness after ACE inhibition in both animal and human heart failure.

Acknowledgment: This study was supported by the National Heart Foundation of Australia and Technologies Servier. We thank Dr. Elizabeth Scalbert for useful advice and suggestions and for gifts of perindopril.

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Keywords:

Autoradiography; β-Adrenoceptor; Myocardial infarction; Cardiac failure; Angiotensin-converting enzyme inhibition; Perindopril

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