Effects of metoprolol and amiodarone combination on heart rate, myocardial contractility and coronary flow: Study in isolated perfused rat hearts

Abstract

BACKGROUND:

Beta-blockers and amiodarone have been used concomitantly to treat arrhythmias associated with congestive heart failure. However, the combination of metoprolol and amiodarone has only been studied restrospectively, and its potential effects in congestive heart failure remain to be properly elucidated in prospective trials.

OBJECTIVE:

The present investigation focused on evaluating the pharmacological interaction between metoprolol and amiodarone in an isolated perfused rat heart preparation.

MATERIALS AND METHODS:

Adult male Wistar rats (n=24) were divided into four groups of six animals, and the effects of the metoprolol/amiodarone combination on systolic pressure, myocardial contractility (dP/dt), coronary flow (CF) and heart rate were analyzed, and the interdependent variables were compared.

RESULTS:

There was a negative chronotropic effect by both metoprolol and the metoprolol/amiodarone combination in isolated rat hearts. However, the acute effects of the metoprolol/amiodarone combination showed no myocardial contractility depression or bradycardia accentuation compared with metoprolol alone. CF increased by 9.2% at minute 1 through minute 5 (P=0.004) with the metoprolol/amiodarone combination. There was no difference in systolic pressure or myocardial contractility among the groups.

CONCLUSIONS:

The acute effects of the metoprolol/amiodarone combination in the isolated rat heart were an increase in CF, and no myocardial contractility depression or bradycardia accentuation.

Various pharmacological agents have been used to promote blockade of the neurohumoral system involved in the pathophysiology of congestive heart failure (CHF). Interest in metoprolol, a selective beta1-adrenergic receptor blocker (1), as an option to treat patients with CHF began in 1993 with a classic publication (2) that evaluated New York Heart Association class II and III patients undergoing dilated cardiomyopathy. In 1999, the Metoprolol CR/XL Randomised Intervention Trial in Congestive Heart Failure (MERIT-HF) (3) showed a decrease in total mortality, cardiovascular morbidity, sudden death and death due to worsening of CHF compared with placebo in New York Heart Association class II to class IV dilated cardiomyopathy patients.

Due to the high incidence of ventricular and/or supraventricular arrhythmias associated with CHF, pharmacological interventions – particularly amiodarone – have been used in these patients. Amiodarone is a class III antiarrhythmic agent (4) indicated for the treatment of atrial fibrillation, sustained ventricular tachycardia and ventricular fibrillation, and has produced successful results in randomized studies (5). Amiodarone has also demonstrated satisfactory effects in the treatment of supraventricular and ventricular arrhythmias in CHF patients (6,7).

The concomitant use of beta-blockers and amiodarone has been a reason for additional clinical caution due to possible outcomes from their interaction such as myocardial depression and proarrhythmic effects, including bradycardia and atrioventricular block. However, this theoretical misconception has been challenged because the combination of these drugs seems to be otherwise potentially beneficial based on the analysis of previous studies (8–10). Some studies (8,9) showed that the use of beta-blockers, in particular carvedilol, and amiodarone can be efficacious and safe for treating arrhythmias associated with CHF. Previous studies (10,11) in post-acute myocardial infarction patients showed benefits of amiodarone in patients on beta-blocker therapy. The combination of metoprolol and amiodarone has been studied in few retrospective clinical trials and not at all in experimental studies. Therefore, the potential effects of a metoprolol and amiodarone combination remain to be properly elucidated.

The present investigation focused on evaluating the pharmacological interaction between metoprolol and amiodarone in an isolated perfused rat heart preparation. We examined the effects of a metoprolol/amiodarone combination on dynamic parameters, such as systolic pressure (SP), myocardial contractility (dP/dt), coronary flow (CF) and heart rate (HR), and compared interdependent variables.

MATERIALS AND METHODS

Animals

Adult male Wistar rats (n=24) were kept in the animal holding facilities at the São Francisco de Assis Cardiovascular Foundation according to the experimental guidelines of the Brazilian College of Animal Experimentation (12).

Body and heart weights ranged from 400 g to 490 g (mean 442 g) and 0.9 g to 1.3 g (mean 1.03 g), respectively. There were no significant differences among the study groups.

The Langendorff technique (13) for isolated perfused rat hearts was used due to its reproducibility, quantitative and qualitative criteria, methodological parameters and low cost (14).

Rats were divided into four groups of six animals, anesthetized using sulfuric ether inhalation (15) and subsequently submitted to thoracotomy. Heart resection and assembling of Langendorff’s apparatus followed methods described previously (16). Heart perfusion was carried out with 5% CO₂/95% O₂ Krebs-Henseleit solution (NaKCl 126 mmol/L; NaHCO3 25 mmol/L; KH2PO4 1.2 mmol/L; KCl 4.8 mmol/L; MgSO4 1.2 mmol/L; CaCl2 5.0 mmol/L; glucose 11.0 mmol/L) (17) with a pH of 7.4±0.5 and a temperature of 37.0±0.5°C. The perfusion pressure was held constant at 90±10 cm H₂O for 15 min and the heart was connected to a contractility recording system.

Study groups

Group I (control) comprised six hearts that were perfused with Krebs-Hensleit solution for 15 min for stabilization and resumption of physiological activity. Electrogram (EG), CF and contractility parameters were recorded at minute 1. The control group was obtained by injecting the animals with a bolus of 1 mL Krebs-Hensleit solution, and records were obtained at minutes 1, 2, 3, 5, 10 and 15.

Group II comprised six hearts that were perfused with Krebs-Hensleit solution and injected with a bolus of 200 μg/mL metoprolol (AstraZeneca, Brazil), and records were obtained at minutes 1, 2, 3, 5, 10 and 15.

Group III comprised six hearts that were perfused with Krebs-Hensleit solution and injected with a bolus of 50 μg/mL amiodarone (Libbs Farmacêutica Ltd, Brazil), and records were obtained at minutes 1, 2, 3, 5, 10 and 15.

Group IV comprised six hearts that were perfused with Krebs-Hensleit solution and injected simultaneously with a bolus of 200 μg/mL metoprolol and 50 μg/mL amiodarone using distinct pathways of administration, and records were obtained at minutes 1, 2, 3, 5, 10 and 15.

The metoprolol dose of 200 μg/mL was based on a 20% HR reduction standard for heart beta-blockade according to data described previously (3,18). The amiodarone dose was based on methods used by Moraes et al (19) and Carvalho et al (20).

HR

HR was obtained by an EG register using a biomonitor (model DH 073; Bese Bioengineer, Brazil).

Heart contractility recording system

Previous studies have shown that left ventricle contractility and dP/dt determination are well correlated (21). The contractility record was acquired through a 4 mm diameter latex balloon (22) inserted into the left ventricle via the mitral valve orifice and connected to a biomonitor (model DH 073; Bese Bioengineer). Systolic, diastolic and mean pressure, as well as dP/dt, HR and EG tracings were recorded.

CF

CF was estimated by collecting the Krebs-Henseleit solution that drained into the right atrium after myocardial perfusion into a graduated beaker (16,23) .

Statistical analysis

Sample size estimation and statistical methods were based on previously published experimental studies with metoprolol (24,25) and amiodarone (26) in isolated rat hearts. The effects of the treatments on the 1 min, 2 min, 3 min, 5 min, 10 min and 15 min HR variation were determined, and the results were expressed as mean ± SD and variance was assessed by repeated measures ANOVA. Additionally, the least significant difference test was used where the analysis indicated a significant difference. If the difference observed was greater than the least significant difference value, this difference was considered significant (27). The nonparametric Kruskal-Wallis and Friedman (28) tests were used where repeated measures ANOVA was not applicable. The comparison among SP, CF and dP/dt was done using the Kruskal-Wallis test. The Friedman test was used to analyze the treatments at different periods of time within the treatment group. The Pearson correlation was used to analyze the linear correlations among the variable measures (r). Results were considered significant at P<0.05, and 95% CI was used for all the results.

RESULTS

Analyses of the four groups showed a significant decrease in HR in the metoprolol group (group II) compared with the control group (group I) at minute 1 (100% versus 81.2%, P=0.002) to minute 15 (97.9% versus 88.3%, P=0.002). HR in the amiodarone group (group III) was not significantly different than that in the control group. The metoprolol/amiodarone group (group IV) demonstrated a significant reduction in HR in comparison with the control group (P=0.002) and the amiodarone group (P=0.002). The metoprolol/amiodarone group (group IV) showed a nonsignificant reduction in HR compared with the metoprolol group (group II) (Figure 1).

Figure 1).

Descriptive and comparative analysis of the influence of time and treatment on heart rate (HR) variation. bpm Beats/min

SP and dP/dt did not change significantly among the groups (Figure 2 and Figure 3, respectively).

Figure 2).

Descriptive and comparative analysis of the influence of time and treatment on systolic pressure (SP) variation

Figure 3).

Descriptive and comparative analysis of the influence of time and treatment on myocardial contractility (dP/dt) variation

CF varied significantly with time (minute 1 to minute 15) within the groups: control group (P<0.001), metoprolol group (P<0.001), amiodarone group (P=0.001) and metoprolol/amiodarone group (P=0.004). In addition, when CF was compared among groups, the amiodarone group showed no significant differences compared with the control group, whereas the metoprolol and metoprolol/amiodarone groups revealed an increase in CF compared with the control group; however, no significant difference between the metoprolol and metoprolol/amiodarone groups was observed (Figure 4).

Figure 4).

Descriptive and comparative analysis of the influence of time and treatment on coronary flow (CF) variation

The HR, SP and dP/dt results within the metoprolol/amiodarone group yielded a discrepant correlation compared with CF. CF increased by 9.2% between minute 1 and minute 5 (P=0.004). In contrast, HR, SP and dP/dt showed a marked propensity for decrease, particularly HR (23.9% decrease within minute 1, P=0.002) (Figure 5).

Figure 5).

Mean comparative analysis of heart rate (beats/min), systolic pressure (mmHg), myocardial contractility (dP/dt) (g/seg-1) and coronary flow (mL/min) in the metoprolol/amiodarone group

DISCUSSION

Our results demonstrate that the acute effects of a metoprolol/amiodarone combination, assessed in isolated rat hearts, showed no myocardial contractility depression or bradycardia accentuation by metoprolol. CF increased with metoprolol and no difference among groups was observed in SP or myocardial contractility.

To the best of our knowledge, there is a lack of basic experimental investigations on the combination of metoprolol/amiodarone in the literature. This combination has been studied in only a few clinical cases and low casuistic studies targeting the management of complex ventricular arrhythmias associated with ventricular dysfunction (29,30).

The present investigation found that metoprolol significantly decreased HR and did not alter myocardial contractility, similar to results reported by Nakasone et al (25); however, the metoprolol-mediated increase in CF observed in the present study conflicts with the data reported by Tosaki et al (31), which demonstrated that the metoprolol-mediated antiarrhythmic effect on reperfusion-induced arrhythmias had no effect on CF. In view of these findings, we hypothesize that the metoprolol-stimulated CF increase may be an experimental model-based occurrence. The ischemic/reperfusion model seems to attenuate the metoprolol-mediated CF increase effect due to coronary perfusion conditions. The seriousness of ischemic injury, as well as its reversibility, depend on the duration and severity of CF restriction (32). We also assume that if metoprolol protects the heart against ischemic reperfusion lesions, as a previous study has shown (33), one of the possible mechanisms to explain this beneficial effect may be related to CF improvement. Furthermore, our findings suggest an inverse relationship between HR and CF because we observed a metoprolol-mediated CF improvement for both the metoprolol and metoprolol/amiodarone groups despite a decrease in HR.

The present study found no significant differences in HR, CF and myocardial contractility in the amiodarone group compared with the control group, similar to results found in previous studies (24,34) in rats where amiodarone did not change CF or heart mechanical function; in contrast, they found it decreased HR significantly. They further concluded that the actions of amiodarone depend on the time of use and are related to its accumulation in cardiac muscle. Kojima et al (35) studied the electrophysiological mechanisms of amiodarone in isolated rat hearts and observed that this drug did not interfere with myocardial contractility or CF. However, in contrast to previous results (24,34), neither Kojima et al (35) nor Moraes et al (19) observed a significant decrease in HR. Contrary to previously published results (24,34), HR was measured in the present study to minute 15 with no significant reduction observed; however, this time frame did not allow for the assessment of late bradycardic effects of amiodarone.

Beta-blockers and amiodarone interaction mechanisms have not yet been fully clarified. Amiodarone has been shown to affect the hepatic metabolism of certain beta-blockers, such as propranolol and metoprolol (30). Moreover, the pharmacological properties common to both amiodorone and beta-blockers, such as the blockade of beta-adrenergic receptors, might be intensified. According to data published previously (36,37), the use of a metoprolol/amiodarone combination may be clinically safe.

In physiological conditions, the higher the myocardial oxygen consumption, the greater the CF. However, despite a bradycardia-induced myocardial oxygen consumption and contractility decrease, we observed a paradoxical increase in CF. This occurrence possibly resulted from an add-on effect on the coronary arteries. Amiodarone is a coronary and peripheral vasodilator (1). Metoprolol is a selective beta1-blocker and exerts a slight vasoconstrictor effect on coronary arteries due to its lack of beta2 stimulation (1).

The results of the present investigation were obtained in vitro. As an isolated heart model, the Langendorff method is known to present certain limitations, eg, hearts are not exposed to neurohumoral and vascular resistance influences, and are limited to the direct effects of the drugs. Therefore, additional investigations using different models must be carried out to cautiously corroborate our findings.

CONCLUSIONS

The acute effect of a metoprolol/amiodarone combination in the isolated rat heart was an increase in CF, but no myocardial contractility depression or bradycardia accentuation was observed, suggesting this combination may be safe for clinical use.