Novel simple approach for detection of regional perturbations of cardiac function in mouse models of cardiovascular disease

Age and sex hormones modulate left ventricle regional response to Angiotensin II in male and female mice

Age, hypertension, and the female sex are among risk factors in the development of heart failure with preserved ejection fraction. We studied by standard and speckle-tracking echocardiography (STE), the response of the left ventricle (LV) to aging and angiotensin II (AngII; 1.5 mg/kg/day for 28 days) in 2-month-old and 12-month-old male and female C57Bl6/J mice. We also investigated the effects of the loss of sex steroids by gonadectomy (GDX). We used STE data from 48 points or regions of interest (ROIs) around the LV endocardium from B-mode images and generated profiles of maximal strain, strain rate (SR) and reverse SR for each experimental group of mice. In young mice, LV strain, strain rate (SR) and reverse SR profile levels were higher in females than in males. Aging was characterized by concentric LV remodeling and a decrease of strain, SR and reverse SR. GDX at 6 weeks of age slowed normal cardiac growth in male mice. In females, GDX reduced LV strain, SR and reverse SR but did not influence cardiac growth. AngII caused similar levels of hypertrophy in young and older mice. In young mice, AngII had little effects on STE parameters, whereas in older animals, strain, SR and reverse SR were reduced, mainly for the LV posterior wall. In older GDX mice, hypertrophic response to AngII was decreased compared to intact animals. Generating detailed STE profile for the LV wall can help detect differences linked to sex, age or to a stressor better than global strain measurements.

Segmental analysis by speckle-tracking echocardiography of the left ventricle response to isoproterenol in male and female mice

We studied by conventional and speckle-tracking echocardiography, the response of the left ventricle (LV) to a three-week continuous infusion of isoproterenol (Iso), a non-specific beta-adrenergic receptor agonist in male and female C57Bl6/J mice. Before and after Iso (30 mg/kg/day), we characterized LV morphology and function as well as global and segmental strain. We observed that Iso reduced LV ejection in both male (−8.7%) and female (−14.7%) mice. Several diastolic function parameters were negatively regulated in males and females such as E/A, E/E′, isovolumetric relaxation time. Global longitudinal (GLS) and circumferential (GCS) strains were reduced by Iso in both sexes, GLS by 31% and GCS by about 20%. For the segmental LV analysis, we measured strain, strain rate, reverse strain rate, peak speckle displacement and peak speckle velocity in the parasternal long axis. We observed that radial strain of the LV posterior segments were more severely modulated by Iso than those of the anterior wall in males. In females, on the other hand, both posterior and anterior wall segments were negatively impacted by Iso. Longitudinal strain showed similar results to the radial strain for both sexes. Strain rate, on the other hand, was only moderately changed by Iso. Reverse strain rate measurements (an index of diastolic function) showed that posterior LV segments were negatively regulated by Iso. We then studied the animals 5 and 17 weeks after Iso treatment. Compared to control mice, LV dilation was still present in males. Ejection fraction was decreased in mice of both sex compared to control animals. Diastolic function parameters, on the other hand, were back to normal. Taken together, our study indicates that segmental strain analysis can identify LV regions that are more negatively affected by a cardiotoxic agent such as Iso. In addition, cessation of Iso was not accompanied with a complete restoration of cardiac function after four months.

The importance of heart rate in isoprenaline-induced takotsubo-like cardiac dysfunction in rats

Aims

Takotsubo syndrome (TS) is an acute cardiac syndrome characterized by regional myocardial akinesia that cannot be attributed to a culprit lesion in coronary arteries. Cardiac overstimulation by catecholamines in the setting of stress is implicated in the pathogenesis of TS. While catecholamine‐induced alterations in cardiac contractility have been studied as part of the causal pathway in TS, the importance of catecholamine‐mediated tachycardia has not been studied. Our aim was to explore whether the reduction in heart rate, either by pharmacological suppression of the sinoatrial node with ivabradine or by surgical induction of third‐degree atrioventricular block, prevents isoprenaline‐induced TS‐like akinesia in an experimental animal model.

Methods and results

We used 142 female Sprague–Dawley rats in two separate protocols. The TS‐like phenotype was induced by an intraperitoneal bolus dose of isoprenaline (ISO) 50 mg/kg. In the first protocol, we randomized 54 rats to ivabradine 10 min before ISO (IVAB1), ivabradine 10 min after ISO (IVAB2), or saline 10 min before ISO (CONTROL). In the second protocol, we randomized 88 rats to surgically induced complete heart block (CHB) or sham operation (CTRL) 10 min before the administration of ISO. All drugs were administered intraperitoneally. We recorded heart rate and blood pressure invasively in the right carotid artery. Cardiac morphology and function were evaluated by high‐resolution echocardiography (VisualSonics 770 VEVO, Toronto, Ontario, Canada) 90 min after ISO injection. IVAB1 and IVAB2 rats had significantly lower heart rate and less pronounced TS‐like cardiac dysfunction than CONTROL. CHB rats had a lower (54%) heart rate, and no animal developed left ventricular akinesia. In the first protocol, the CONTROL group had a median degree of akinesia of 10.2 [inter‐quartile range (IQR) 0.0–18.6]. The IVAB1 group showed a median of akinesia of 0% (IQR 0.0–0.0, P < 0.001 vs. CONTROL). In the IVAB2 group, 5% had TS‐like dysfunction (P = 0.001). Ejection fraction was higher in both the IVAB1 (92%, IQR 89–95) and IVAB2 groups (93%, IQR 87–96) than in the CONTROL group (78%, IQR 63–87, P < 0.05). In the second protocol, the median degree of akinesia in the CTRL group was 21.9% (IQR 8.9–24.6). In the CHB group, no rat developed akinesia (median 0%; IQR 0.0–0.0, P < 0.001 vs. CONTROL). Ejection fraction was higher in the CHB group (90%, IQR 87–92) than in the CTRL group (51%, IQR 87–92, P < 0.05).

Conclusions

Isoprenaline‐induced TS‐like cardiac dysfunction can be prevented by lowering heart rate. Tachycardia may be an important part of the causal pathway in TS.