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Angiotensin Type 1 Receptor Antagonism with Irbesartan Inhibits Ventricular Hypertrophy and Improves Diastolic Function in the Remodeling Post-Myocardial Infarction Ventricle

Ambrose, Jayaseelan; Pribnow, David G.*; Giraud, George D.; Perkins, Keith D.; Muldoon, Leslie*; Greenberg, Barry H.

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Journal of Cardiovascular Pharmacology: March 1999 - Volume 33 - Issue 3 - p 433-439
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Ventricular remodeling after transmural myocardial infarction (MI) is seen in both human patients and experimental animal models (1,2). This process initially involves infarct scar bulging and thinning. Later, myocyte hypertrophy, fibroblast hyperplasia and deposition of interstitial matrix occur in spared segments of myocardium distant from the infarct zone. These pathologic changes result in eccentric left ventricular (LV) hypertrophy. Although the increase in muscle mass and conformational changes in chamber geometry may help maintain cardiac performance, the long-term consequences are detrimental, leading to LV dysfunction and the onset of heart failure (3).

Abnormalities in diastolic function have been observed in the remodeling LV and contribute to the development of heart failure (4-6). Sarcoendoplasmic reticulum ATPase (SERCA) activity mediates calcium reuptake into the sarcoplasmic reticulum (SR) of cardiac myocytes and plays a major role in the process of myocardial relaxation (7). In addition to SERCA, phospholamban (PLB), a 27,000-Dalton proteolipid, is important for the regulation of active diastolic relaxation. In the dephosphorylated state, PLB prevents calcium uptake by inhibiting SERCA (8). Both animal models of hypertrophy and human patients with end-stage heart failure have been shown to have decreased levels of myocardial SERCA messenger ribonucleic acid (mRNA; 9,10). Thus abnormalities in SERCA and PLB gene expression may be involved in the mediation of diastolic dysfunction observed in post-MI ventricular remodeling.

Angiotensin II (AII), through its interaction with angiotensin type 1 (AT1) receptor, has been implicated as one of the major factors responsible for post-MI remodeling. Angiotensin II stimulates myocyte hypertrophy and fibroblast hyperplasia (11,12). Moreover, interruption of renin-angiotensin system (RAS) activity has been shown to inhibit LV hypertrophy, diminish the remodeling process, delay the onset of heart failure, and improve survival (13-15). The effects of chronic blockade of AII on diastolic function during post-MI remodeling, however, are uncertain. Consequently, we evaluated the effects of AT1 blockade on cardiac hypertrophy, diastolic function, and SERCA and PLB gene expression in the remodeling heart.


Study design

Male Sprague-Dawley rats weighing between 200 and 225 g were randomized to receive either distilled water or Irbesartan, 40 mg/kg/day, dissolved in distilled water, beginning 24 h before surgical intervention. They were maintained on this regimen till being killed at 6 weeks after surgery. As described later, some animals underwent coronary ligation, whereas others underwent sham operation. This yielded four experimental groups: group 1, sham operated without therapy (SH); group 2, sham operated, pretreated with irbesartan (SH-IRB); group 3, coronary ligation without therapy (MI); and group 4, coronary ligation pretreated with irbesartan (MI-IRB). Irbesartan is a newly developed nonpeptidic AII blocker with high specificity and affinity for the AII, type 1 (AT1) receptor. The dose was selected based on reports demonstrating inhibition of the pressor response to AII infusion in rats (16).

Experimental infarction

Myocardial infarction or sham operation was performed by standard techniques (17). Rats were deeply anesthetized with an intramuscular injection of ketamine (2.5 mg/kg), xylazine (0.2 mg/kg), and acepromazine (0.15 μg/kg). After endotracheal intubation, mechanical ventilation, and left thoracotomy, pericardial stripping and proximal left anterior descending artery (LAD) ligation were performed. Sham-operated rats underwent the identical procedure but without LAD ligation. The chest wall was then closed, and the animals were allowed to recover.

Physiologic studies of hemodynamic variables and cardiac hypertrophy

At 6 weeks after thoracotomy, rats were weighed and anesthetized with a mixture of ketamine HCl (1 mg/kg) and xylazine (0.2 mg/kg) dissolved in distilled water. A cutdown was performed of the right carotid artery, which was cannulated with a solid-state micromanometer-tipped catheter (Millar Instruments, Houston, TX, U.S.A.). Aortic and LV systolic and diastolic pressures and LV dP/dt were recorded directly onto on a PC hard drive by using a customized hemodynamic analysis program (courtesy Drs. Thomas E. Raya and Steven Goldman, University of Arizona, Tucson). When pressures were stabilized, 100 consecutive beats of LV pressure were recorded for computer-assisted analysis. Zero reference pressures were obtained initially and at the conclusion of the studies. The decay in LV pressure with time can be closely approximated by the exponential relation: Equation (1) where Pa + PA is ventricular pressure at maximum negative dP/dt, PA is LV asymptote pressure, assuming LV pressure decays to infinity, and a is the constant of the exponential relation. LV relaxation [i.e., the time constant of isovolumic relaxation (τ)] was calculated with standard methods (18,19). LV pressure of 100 consecutive cardiac cycles was digitized with an IBM AT computer at frequency of 1,000 Hz. For each cardiac cycle, maximal positive and maximum negative dP/dt, and peak systolic and end-diastolic pressure points were identified. The value of end-diastolic pressure for each cycle were subtracted from each pressure point on the pressure-decay curve of that cycle beginning at maximal negative dP/dt and ending at a point equal to the end-diastolic pressure. The resulting values were then fit to the relation: Equation (2) where a is calculated by the least-squares method and the time constant of isovolumic relaxation or τ = −1/a.

This method was originally described in a canine model by Weiss et al. (18). Raya et al. (19) successfully applied a modification of this technique to the rat infarct model, and this method was used in our experiments. Determination of the rate of decrease in pressure over time (τ) and maximal negative dP/dt were used as markers of active LV relaxation.

While still under anesthesia, the animals were killed by exsanguination. The hearts were quickly removed, and the ventricles separated from the atria. Incisions were made in the LV, and the chamber was flattened. The right ventricular (RV) free wall was then separated from the LV and interventricular septum, and the chambers were weighed. In animals randomized to the coronary-ligation groups, infarct size was estimated by using a technique previously described (20). In brief, the area of LV scar and the total LV free wall and septal areas were traced on plastic transparencies. These traced segments were cut out and weighed. The ratio of the traced scar area transparency weight to the total traced LV free wall and septum area transparency weight gave an estimate of the percentage of infarction. These tissue samples were then snap-frozen in liquid nitrogen and kept at −70°C until analysis. The presence of hypertrophy in the ventricles was determined by the measurement of the chamber weight/body weight ratio (17,21).

Molecular biology studies

Total RNA was extracted from the noninfarcted LV free wall and septum and purified by using the acid guanidinium/phenol/chloroform method (22). The quantity of RNA was determined by optical density, and the quality assessed by ethidium bromide visualization under ultraviolet light after gel electrophoresis. Based on the experimental data of others and our own preliminary results, the quantity of mRNA was sufficient to perform Northern blot analysis (23). Samples containing 10 μg of total RNA were denatured, electrophoresed in 1.4% agarose gels, and transferred to charged nylon membranes by capillary action. A standard RNA sample also was incorporated into each blot to allow normalization of signal intensity between blots. Hybridization was carried out by using gene-specific 32P-DNA probes labeled by standard in vitro random priming kits. Fragments of cDNA for SR-ATPase, atrial natriuretic peptide (ANP), PLB, and glyceraldehyde-3-phosphate dehydrogenase (GADPH; supplied by Dr. Wolfgang Dillmann, University of California, San Diego, and Dr. Arthur Feldman, University of Pittsburgh) were excised from purified plasmids by treatment with restriction endonucleases and purified on agarose gels. Prehybridization was performed with 10 ml of "AndyHyb" [5% sodium dodecylsulfate (SDS), 400 nM NaPO4, 1 mM EDTA, 1 mg/ml bovine serum albumin (BSA), and 50% formamide] for 2 h at 42°C. Hybridization was carried out for 18 h at 42°C. The blot was then washed in 2× SSC/0.1% SDS with moderate agitation at room temperature, followed by a similar wash at 50°C. Higher-stringency washes were performed as needed to maximize signal-to-noise ratio. The blots were exposed on x-ray film (Kodak X-Omat AR5; Eastman Kodak Co., Rochester, NY, U.S.A.) with an image intensifier for 12-36 h at −37°C. The films were developed, laser-scanned by computer, and analyzed with a standard laser scanner and image-quantitation program. The signal density of each lane was measured as pixels per region of interest, and this value was proportional to the amount of message per lane. Each Northern blot was rehybridized with several probes including the GAPDH probe, which was used to normalize the amount of RNA in each lane of the Northern blot. Subsequently this ratio (i.e., mRNA signal/GAPDH signal) was then normalized to the standard incorporated on each blot. These individual lanes were then averaged and expressed as whole-number arbitrary units for statistical analysis. Independent evidence that hypertrophy had developed was obtained from the Northern blot assessment of mRNA quantities for ANP (24). Messenger RNA levels of cardiac SR-ATPase and PLB were used to determine if there is an associated regulation of these transcripts in the post-MI ventricle and whether these were associated with abnormalities in diastolic relaxation (i.e., τ), the time constant of isovolumic relaxation.

Analysis of results

Results from the rats treated with irbesartan, the AT1 inhibitor, were compared with those obtained in the untreated rats to determine the response of AII type 1 receptor antagonism on post-MI remodeling. Comparisons of the effects of infarction and of irbesartan on hemodynamic variables, heart weights, and mRNA levels were performed by using two-way analysis of variance (ANOVA), in which the results of the four experimental groups (i.e., SH vs. SH-IRB, SH vs. MI, MI vs. MI-IRB, and MI-IRB vs. SH-IRB) were compared. Tukey-Kramer's test was used for multiple comparisons. A value of p < 0.05 is considered to be statistically significant.


Morphometric measurements are summarized in Table 1. Animals that underwent coronary ligation gained significantly less weight than their respective sham controls. None of the sham-treated animals had evidence of MI by gross inspection. The coronary ligation animals had macroscopic evidence of transmural infarction involving, on average, 37% of the LV free wall. There was no difference in the extent of infarction between animals pretreated with irbesartan versus those given no drug.

Morphometric measurements

Myocardial hypertrophy was evaluated by comparing heart weight/body weight ratios among the various groups. Values for RV and LV mass uncorrected for body weight also are included in Table 1. In the untreated MI group, there was a substantial increase in heart weight/body weight ratio that was primarily related to an increase in RV weight. Among the irbesartan-treated post-MI rats, the increased heart weight/body weight ratio was essentially normalized with no significant difference between this group and both sham groups. This was due to a reduction in both RV/BW and LV/BW ratios as well as in uncorrected RV and LV weights in post-MI rats receiving irbesartan. AT1-receptor antagonism also resulted in a small but significant reduction of LV mass in sham rats (SH-IRB), but there was no substantial change in overall heart weight/body weight ratio in this group.

Hemodynamic variables are summarized in Table 2. Treatment with irbesartan did not substantially affect hemodynamic parameters among the sham-operated rats. Heart rate was not significantly different between the study groups. Mean arterial pressure (MAP) was reduced in rats with MI. There was, however, no difference between MAPs of the MI group versus the MI-IRB group. LV systolic pressure (LVSP) followed a similar trend, being lower in the infarcted rats, but not significantly different in the MI group versus the MI-IRB group. Left ventricular end-diastolic pressure (LVEDP) was significantly increased after MI, and although there was a downward trend in the MI-IRB group, this did not differ significantly from the MI group. Positive dP/dt also was reduced after MI, and it was not significantly altered by irbesartan treatment.

Hemodynamic variables

Derived measurements of relaxation were abnormal after MI. There was a significant decrease in negative dP/dt after MI. Although there was partial normalization of this variable in the MI-IRB group, negative dP/dt was still significantly lower in the MI-IRB than the SH-IRB controls. The time constant of isovolumic relaxation (τ) was significantly prolonged after MI. However, among the animals treated with irbesartan after MI, this value was partially normalized and not significantly different compared with sham-operated rats.

The Northern blot data (Table 3) reflect total RNA from the noninfarcted LV free wall and septum. The expression of ANP mRNA was substantially increased after infarction. However, there was a significant reduction in ANP in the MI-IRB group, so that the value measured was essentially back to the baseline level of expression and not significantly different from the SH-IRB (Fig. 1). There was no significant effect of irbesartan on ANP mRNA expression in the SH-IRB group compared with the SH group. Despite changes in τ and -dP/dt after MI, the mRNA expressions of SERCA and PLB were not significantly depressed after infarction. Furthermore, although the measures of diastolic function were improved in rats receiving irbesartan, expressions SERCA and PLB were not altered by this treatment.

Northern Blot data
FIG. 1
FIG. 1:
Autoradiographs of Northern blot analysis messenger RNA (mRNA) derived from the noninfarcted left ventricular myocardium for atrial natriuretic peptide (ANP) and glyceraldehyde-3-phosphate dehydrogenase (GADPH) demonstrating marked upregulation of ANP 6 weeks after myocardial infarction (MI) and normalization to baseline levels with irbesartan treatment. A: Sham without treatment (SH). B: Sham pretreated with irbesartan (SH-IRB). C: MI without treatment. D: MI with irbesartan pretreatment (MI-IRB).


Post-MI ventricular remodeling is characterized by the development of eccentric myocardial hypertrophy and abnormalities in cardiac function. Because AII appears to play an important role in this process, we studied the effects of AT1-receptor antagonism on cardiac remodeling, diastolic function, and the expression of genes related to hypertrophy and myocyte calcium reuptake. Our results show that AT1-receptor antagonism with irbesartan reduces post-MI ventricular hypertrophy and the accompanying expression of ANP. These changes occur even in the absence of significant changes in LV loading conditions. Furthermore, there was an improvement in LV relaxation with irbesartan, which occurred without concomitant alteration in the mRNA levels of SERCA or PLB. Overall, these findings demonstrate that AT1-receptor antagonism has beneficial effects on post-MI remodeling and LV function that are independent of changes in loading conditions.

Hemodynamic effects after MI

In these experiments, rats undergoing MI resulting from coronary artery ligation developed increases in LVEDPs as well as reductions in MAP and LVSP (2,13,17,19,20,25). Pretreatment with irbesartan in rats that underwent coronary ligation resulted in a slight but insignificant reduction in LVEDP and essentially no change in LVSP and MAPs. Other investigators reported more substantial effects of AT1 blockade on post-MI hemodynamic variables (26-30). These differences could be related to variations in drug dosing or to pharmacologic differences between the various AT1-receptor antagonists.

Myocardial hypertrophy

Myocardial infarction secondary to coronary ligation resulted in a significant degree of ventricular hypertrophy in these studies. Similar findings have been noted by other investigators who studied post-MI remodeling in rat (2,3,13,19,20,25). The majority of the increased myocardial mass was due to RV hypertrophy, which likely occurs in response to pulmonary hypertension as a consequence of increased LVEDP (31). In this study and others previously reported (2), LV mass was not significantly increased despite evidence that there is hypertrophy of myocytes from spared segments of LV in this experimental model of post-MI cardiac remodeling (32,33). These findings can explained by the fact that eccentric hypertrophy of distant segments of the LV is balanced by replacement of myocardium with scar in the infarcted zone. The result is little or no increase in overall LV mass.

Our results demonstrate that AT1-receptor inhibition with irbesartan results in a striking reduction in RV hypertrophy and a somewhat smaller reduction in LV mass. These findings are similar to those reported by other investigators (23,27-30,32,34). Although post-MI remodeling can be diminished by angiotensin-converting enzyme inhibitors (13-15), there has been controversy regarding the mechanism, because these agents can increase bradykinin production (35-37) as well as inhibit AII generation. AT1-receptor antagonists, however, offer a more direct way of evaluating the involvement of AII in post-MI remodeling, because alterations in the levels of other potentially important growth factors are not seen with these agents. The results of this study provide further evidence that blockade of angiotensin effects alone is sufficient to inhibit the development of hypertrophy. It has been suggested that the regression of post-MI hypertrophy with AT1 blockers is related primarily to improvement in loading conditions and is mediated by changes in either MAP, LVSP, or LVEDP. The absence of significant changes in LV systolic or diastolic filling pressures in this study, however, suggests that irbesartan ameliorated both LV and RV hypertrophy by blocking direct AII effects on myocardial growth, which are mediated through the AT1 receptor.

Because infarct size was measured as a percentage of LV mass in this study, inhibition of hypertrophy by irbesartan could have magnified our determination of this variable. Although we measured similar infarct sizes in the two study groups, it is possible that irbesartan-treated rats may actually have had somewhat smaller infarcts than rats in the control group. Although this is a potential limitation of the study, the extent of hemodynamic abnormalities in the irbesartan group and the similarity in magnitude of the hemodynamic derangements in the two groups of infarcted rats suggest that infarct size in the irbesartan group was both substantial and similar to that in the control group. Thus it is unlikely that this factor accounts for the significant changes in chamber weights, cardiac gene expression, and active diastolic relaxation that were seen with irbesartan.

Diastolic function in the remodeling ventricle

It has been recognized that significant derangements in diastolic function develop during the remodeling process (6,38,39). This may be due to one of several mechanisms: alteration in the passive viscoelastic properties of the ventricular walls, persistence throughout diastole of actin-myosin interactions due to abnormalities in the active relaxation process, and mechanical interference between the cardiac chambers and the pericardium. Whatever the cause, the reduction in LV distensibility augments filling pressures and contributes to pulmonary congestion. These abnormalities also increase myocardial oxygen demand and, thus, augment the progression of myocardial dysfunction (40,41). We demonstrated a substantial prolongation in the time constant of isovolumic relaxation (τ) 6 weeks after MI. This finding is consistent with a study by Raya et al. (19) in the same experimental model.

The mechanism by which isovolumic relaxation is prolonged remains unclear. In the normal heart, myocardial relaxation is influenced by several factors including loading conditions, asynchrony, and deactivation (42-45). A number of animal models of myocardial hypertrophy and heart failure demonstrated a decrease in the expression of mRNA for SERCA, which controls calcium reuptake, and PLB, which regulates SERCA activity by phosphorylation/dephosphorylation (46), as well as decreases in protein expression and calcium-uptake function (47-49). These phenomena, however, have not been studied extensively in the post-MI ventricle. One preliminary study did appear to suggest that the SERCA mRNA levels are reduced 2 days after MI, but this decrease was not sustained at 4 weeks. However, in this same report, the mRNA level of SERCA in the control group also was reduced at 4 weeks by nearly 50%, thereby minimizing any true differences in gene expression (50). Our results demonstrate that there is no significant change in the expression of SERCA or PLB at 6 weeks after MI, and this expression is unaltered by pretreatment with irbesartan. These findings, however, do not exclude qualitative changes in SERCA pump function that alters calcium reuptake and thereby disrupts diastolic relaxation (51). Although our study did not specifically evaluate calcium uptake, previous studies demonstrated a decrease in calcium uptake at 4, 8, and 16 weeks in the LV of rats after MI (52).

Our study demonstrated that AT1-receptor antagonism produces a significant improvement in myocardial relaxation with improvement of negative dP/dt and near normalization of τ at 6 weeks after MI. Because neither the prolongation of τ 6 weeks after MI nor the normalization of this variable by pretreatment with irbesartan was associated with changes in SERCA or PLB gene expression, it is unlikely that this mechanism was involved. It is possible, however, that increases in SERCA protein levels due to RNA stability or posttranslational events could account for the improvement in relaxation seen with irbesartan.

Experimental evidence from a number of investigators demonstrated that AII has an acute effect on diastolic relaxation. Abnormal relaxation patterns in cardiomyocytes from spontaneously hypertensive and normotensive rats were completely abolished by AII-receptor blocker (53). A recent study in 25 patients with significant myocardial hypertrophy demonstrated an immediate improvement in the time constant of isovolumic relaxation (τ) in response to intracoronary enalaprilat (54). Conversely, Capasso et al. (55) demonstrated impairment in the time to one-half relaxation in isolated rat papillary muscles exposed directly to AII. Furthermore, this decrement in relaxation was more marked in muscles obtained from infarcted hearts. Thus an effect of irbesartan in blocking AII-mediated diastolic dysfunction is a possible explanation for our results. An effect of irbesartan on diastolic function related to a reduction in hypertrophy could also account for the beneficial effects seen in these experiments. Alternatively, intrinsic changes within the myocyte or interstitium may be the major contributors to diastolic abnormalities after MI, and AT1-receptor blockade may have had a favorable impact on myocardial relaxation through alternative sites within the heart.


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Myocardial infarction; Remodeling; Diastolic function; Irbesartan; Hypertrophy; SERCA

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