Effect of atenolol on myocardial infarct expansion in a nonreperfused rat model

Comparison of Pyrroloquinoline Quinone and/or Metoprolol on Myocardial Infarct Size and Mitochondrial Damage in a Rat Model of Ischemia/Reperfusion Injury

The cardioprotective effectiveness of low-dose pyrroloquinoline quinone (PQQ, 3 mg/kg) was compared with metoprolol, a β1-selective adrenoceptor antagonist. Rats underwent 30 minutes of left anterior descending coronary artery occlusion and 2 hours of reperfusion. Metoprolol and/or PQQ were given at the onset of reperfusion to mimic clinical treatment. Metoprolol and/or PQQ reduced infarct size and protected against ischemia-induced left ventricular dysfunction after 2 hours of reper-fusion. Combined therapy augmented left ventricular developed pressure at the end of reperfusion. Metoprolol or PQQ alone enhanced mitochondrial respiratory ratios in ischemic and nonischemic myocardium. Although the PQQ/metoprolol combination therapy increased respiratory ratio values, the effects were small when compared with PQQ alone. Only PQQ decreased lipid peroxidation. Metoprolol and/or PQQ given at the onset of reperfusion reduce infarct size and improve cardiac function. Combination therapy further reduces infarct size. PQQ is superior to metoprolol in protecting mitochondria from ischemia/reperfusion oxidative damage

Comparison of Metoprolol With Low, Middle and High Doses of Carvedilol in Prevention of Postinfarction Left Ventricular Remodeling in Rats

The dose-related beneficial effects of carvedilol on survival in heart failure have been verified, however, the effects on left ventricular remodeling (LVRM) after acute myocardial infarction (AMI) have not been defined. This experiment was designed to compare the effects of low, middle, and high doses of carvedilol (LD-car, MD-car, and HD-car) with metoprolol (Meto) in preventing postinfarction LVRM in rats. After the left coronary artery was ligated, 177 surviving female SD rats were randomized to: (1) AMI (n = 35), (2) LD-car (0.1 mg x kg(-1) x d(-1), n = 35), (3) MD-car (1 mg x kg(-1) x d(-1), n = 35), (4) HD-car (10 mg x kg(-1) x d(-1), n = 37) and (5) Meto (2 mg x kg(-1) x d(-1), n = 35) groups. A sham-operated group (n = 16) was also randomly selected. Gastric gavage therapy lasted for 4 weeks. After hemodynamic studies, the rat hearts were fixed and pathologically analyzed. After exclusion of rats which died or had an infarct size < 35% or > 55%, complete data were obtained in 69 rats, comprising AMI (n = 11), LD-car (n = 11), MD-car (n = 12), HD-car (n = 12), Meto (n = 11) and sham (n = 12) groups. There were no significant differences in MI size among the five AMI groups (44.5-46.3%, all P > 0.05). Compared with the sham group, left ventricular (LV) end-diastolic pressure (LVEDP), volume (LVV), weight (LVW) and septal thickness (STh) were all significantly increased, while +/- dp/dt was significantly decreased in the AMI group (all P < 0.001). Compared with the AMI group, heart rate was significantly decreased in all except the LD-car treatment groups (P < 0.05-0.01); LVEDP, LVV, LVW, and STh in the four treatment groups were also significantly decreased (P < 0.05-0.001) except LVW and STh in the Meto group (both P > 0.05)(LVEDP: 14.5 +/- 4.6, 12.1 +/- 2.4, 7.7 +/- 1.9 and 13.0 +/- 6.7 mmHg vs 24.1 +/- 5.2 mmHg; LVV: 0.82 +/- 0.1, 0.79 +/- 0.1, 0.72 +/- 0.1 and 0.72 +/- 0.1 mL vs 0.92 +/- 0.1 mL; LVW: 666 +/- 57, 622 +/- 70, 589 +/- 57 and 699 +/- 78 mg vs 730 +/- 79 mg; STh: 1.14 +/- 0.12, 1.18 +/- 0.21, 1.19 +/- 0.15 and 1.35 +/- 0.20 mm vs 1.33 +/- 0.29 mm; P < 0.05-0.001); while +/- dp/dt was significantly increased in each therapy group (P < 0.05-0.001). There were dose-effect relations in LVEDP and LVV in the carvedilol groups. The results indicate that low, middle and high dose carvedilol has dose-related effects in the prevention of postinfarction LVRM with respect to volume expansion and segmental hypertrophy in rats, while metoprolol prevents only LV dilatation but not hypertrophy.

Growth hormone alone or combined with metoprolol preserves cardiac function after myocardial infarction in rats

BACKGROUND AND OBJECTIVE

Beta-adrenoreceptor blocking agents are important for the treatment of myocardial infarction (MI). Accumulating evidence also indicates that growth hormone (GH) improves cardiac function after MI in rats. We aimed to investigate the cardiovascular effects of combined treatment in an animal model of MI.

METHODS

MI was induced in rats by ligation of the left coronary artery. Three days after MI, animals were randomly assigned to one of four groups: controls (C) (n=19); GH (n=19) receiving s.c. 2 mg/kg per day rhGH; metoprolol (M) group (n=19) receiving 24 mg/kg per day and combined group (GHM) (n=20) treated with both GH (2 mg/kg per day s.c.) and M (24 mg/kg per day) for 9 days. Transthoracic echocardiography was performed before and after treatment.

RESULTS

Serum levels of insulin-like growth factor I were significantly elevated in the GH-group but not in the GHM group compared to controls. Left ventricular volumes, cardiac index, systolic blood pressure, were similar in all groups. Percent changes in ejection fraction compared to baseline were; GH (6.1+/-5.0%) and GHM (6.1+/-4.2%) vs. C (-12.5+/-3.0%), P<0.01, M (-7.3+/-4.2%). The occurrence of aneurysms was not significantly different between the various treatment regimes.

CONCLUSION

Treatment with growth hormone alone or in combination with metoprolol preserved left ventricular function after MI.

Long-term effects of beta-adrenergic blocking agent treatment on hemodynamic function and left ventricular remodeling in rats with experimental myocardial infarction: importance of timing of treatment and infarct size

OBJECTIVES

This study was designed to assess the long-term effects of a beta1-selective beta-adrenergic blocking agent on mortality, in vivo hemodynamic function, left ventricular volume and wall stress in post-myocardial infarction (MI) rats.

BACKGROUND

Beta-blockers have shown beneficial results in clinical studies after MI. However, the underlying mechanism is not yet understood, and experimental studies have shown conflicting results.

METHODS

Bisoprolol (60 mg/kg body weight per day) was given 30 min or 14 days after MI or sham operation.

RESULTS

The mortality rate was reduced only in early bisoprolol-treated rats (29% vs. 46% in untreated rats, p < 0.05). Heart rate was equally reduced in all treatment groups, and the maximal rate of rise of left ventricular systolic pressure (dP/dt(max)) decreased in sham rats and in rats with a small to moderate infarct size. Stroke volume index was unchanged in sham rats and in rats with a small to moderate infarct with early or late bisoprolol treatment and increased in rats with a large infarct in the late bisoprolol group. Left ventricular volume was increased by bisoprolol in sham rats and rats with a small infarct but not in rats with a large infarct.

CONCLUSIONS

Treatments starting early (30 min) or late (14 days) after coronary artery ligation with bisoprolol increased left ventricular volume in sham rats and in rats with a small infarct but not in rats with a large infarct. Late bisoprolol treatment improved stroke volume index, and early bisoprolol treatment reduced diastolic wall stress, in rats with a large myocardial infarct. Thus, bisoprolol effects on remodeling and cardiac performance after myocardial infarction strongly depend on infarct size and timing of treatment. This finding may explain previous controversial results that did not consider infarct size and timing of treatment.

Beta-Adrenergic Mechanisms in Arterial Hemodynamics: A Comparison between Normotensive and Hypertensive Rats

The purpose of this study was to determine whether beta-adrenergically mediated cardiovascular functions such as arterial pressure (AP), heart rate (HR), stroke volume (SV), cardiac output (CO), peripheral resistance (R(p)), arterial impedance (Z(c)), mean arterial compliance (C(m)) and pulse wave reflection (P(b)) were altered in the spontaneously hypertensive rat (SHR) compared to the normotensive Wistar Kyoto rat (WKY). In pentobarbital-anesthetized and artificially ventilated rats, the aortic pressure wave was recorded with a high-fidelity Millar sensor, and aortic flow wave with an electromagnetic flow probe. The pressure and flow waves were subjected to Fourier transform so as to analyze impedance spectra. Acute beta-adrenergic blockade was produced by an intravenous injection of propranolol (nonselective) and atenolol (selective beta(1)-blocker) at doses of 2 and 5 mg/kg, respectively. Steady-state parameters were obtained 15-20 min after intravenous administration. The SHR had higher AP, HR, R(p) and Z(c) than the WKY. SV and CO remained unaltered while C(m) was lower. In response to propranolol, the mean AP was increased by 7 mm Hg in the WKY, but did not change in the SHR. Moreover, significant decreases occurred in HR, CO and C(m) in addition to increases in R(p), Z(c) and P(b). These changes between the SHR and WKY were only slight. Atenolol caused decreases in AP, HR and CO in both SHR and WKY, but did not significantly alter the R(p), Z(c), C(m) and P(b). Again, the atenolol-induced changes in AP, HR and CO did not appear to be significantly different between SHR and WKY. The results indicate that beta-adrenergic effects on the heart, Windkessel and resistance vessels are neither greatly enhanced nor impaired during the development of hypertension. In the hypertensive state, significant beta-adrenergic mechanisms still exert tonic vasodilatory effects on the large and small arterial system. Copyright 1996 S. Karger AG, Basel

Effects of metoprolol on early infarct expansion after acute myocardial infarction

The effects of metoprolol on early infarct expansion after acute myocardial infarction were studied in rats (n = 54) that underwent either left coronary artery ligation (MI) or sham operation. Immediately after surgery, the rats received either metoprolol (M) by mouth, which had been dissolved in drinking water, for 72 hours supplemented with three intraperitoneal doses over the first 24 hours or no treatment (H2O). Three days after the initial surgery, hemodynamic measurements were made before and after volume loading. The rats were killed, the hearts were removed, and passive pressure-volume curves were obtained. The hearts were then fixed at a constant pressure and analyzed morphometrically. Infarct size was nonsignificantly lower in the metoprolol-treated group compared with the untreated group (38% +/- 5% MI-M vs 48% +/- 3% MI-H2O, p = 0.10) Compared with infarcted untreated rats, infarcted metoprolol-treated rats had a lower heart rate (322 +/- 13 beats/min MI-M vs 452 +/- 19 beats/min MI-H2O, p < 0.001), lower left ventricular systolic pressure (63 +/- 4 mm Hg MI-M vs 90 +/- 6 mm Hg MI-H2O, p = 0.004), and lower +dp/dt (1340 +/- 169 mm Hg/sec MI-M vs 2872 +/- 273 mm Hg/sec MI-H2O, p < 0.001), but left ventricular end-diastolic pressure and cardiac index did not differ between the two groups. Left ventricular weight corrected for body weight was higher in infarcted rats treated with metoprolol compared with infarcted untreated rats (2.76 +/- 0.07 gm/kg MI-M vs 2.41 +/- 0.09 gm/kg MI-H2O, p < 0.05). The initial slope of the pressure-volume relationship Ki, an index of operative volume stiffness, was lower in infarcted rats treated with metoprolol compared with infarcted untreated rats (p = 0.03). There were, however, no significant differences in the expansion index, thinning ratio, or left ventricular volume between the two infarcted groups. Thus metoprolol therapy begun in the immediate postinfarction period promotes an increase in left ventricular weight and reduces operative volume stiffness but has no significant effect on indexes of early infarct expansion.