Development of a new animal model of chronic mitral regurgitation in rats under transesophageal echocardiographic guidance

Dapagliflozin Improves Cardiac Hemodynamics and Mitigates Arrhythmogenesis in Mitral Regurgitation-Induced Myocardial Dysfunction

Background

Mitral regurgitation (MR) is a major contributor for heart failure (HF) and atrial fibrillation. Despite the advancement of MR surgeries, an effective medical therapy to mitigate MR progression is lacking. Sodium glucose cotransporter 2 inhibitors, a new class of antidiabetic drugs, has shown measurable benefits in reduction of HF hospitalization and cardiovascular mortality but the mechanism is unclear. We hypothesized that dapagliflozin (DAPA), a sodium glucose cotransporter 2 inhibitor, can improve cardiac hemodynamics in MR‐induced HF.

Methods and Results

Using a novel, mini‐invasive technique, we established a MR model in rats, in which MR induced left heart dilatation and functional decline. Half of the rats were randomized to be administered with DAPA at 10 mg/kg per day for 6 weeks. After evaluation of electrocardiography and echocardiography, hemodynamic studies were performed, followed by postmortem tissue analyses. Results showed that DAPA partially rescued MR‐induced impairment including partial restoration of left ventricular ejection fraction and end‐systolic pressure volume relationship. Despite no significant changes in electrocardiography at rest, rats treated with DAPA exhibited lower inducibility and decreased duration of pacing‐induced atrial fibrillation. DAPA also significantly attenuated cardiac fibrosis, cardiac expression of apoptosis, and endoplasmic reticulum stress‐associated proteins.

Conclusions

DAPA was able to suppress cardiac fibrosis and endoplasmic reticulum stress and improve hemodynamics in an MR‐induced HF rat model. The demonstrated DAPA effect on the heart and its association with key molecular contributors in eliciting its cardio‐protective function, provides a plausible point of DAPA as a potential strategy for MR‐induced HF.

Hemodynamic and transcriptomic studies suggest early left ventricular dysfunction in a preclinical model of severe mitral regurgitation

OBJECTIVE

Primary mitral regurgitation is a valvular lesion in which the left ventricular ejection fraction remains preserved for long periods, delaying a clinical trigger for mitral valve intervention. In this study, we sought to investigate whether adverse left ventricular remodeling occurs before a significant fall in ejection fraction and characterize these changes.

METHODS

Sixty-five rats were induced with severe mitral regurgitation by puncturing the mitral valve leaflet with a 23-G needle using ultrasound guidance. Rats underwent longitudinal cardiac echocardiography at biweekly intervals and hearts explanted at 2 weeks (n = 15), 10 weeks (n = 15), 20 weeks (n = 15), and 40 weeks (n = 15). Sixty age- and weight-matched healthy rats were used as controls. Unbiased RNA-sequencing was performed at each terminal point.

RESULTS

Regurgitant fraction was 40.99 ± 9.40%, with pulmonary flow reversal in the experimental group, and none in the control group. Significant fall in ejection fraction occurred at 14 weeks after mitral regurgitation induction. However, before 14 weeks, end-diastolic volume increased by 93.69 ± 52.38% (P < .0001 compared with baseline), end-systolic volume increased by 118.33 ± 47.54% (P < .0001 compared with baseline), and several load-independent pump function indices were reduced. Transcriptomic data at 2 and 10 weeks before fall in ejection fraction indicated up-regulation of myocyte remodeling and oxidative stress pathways, whereas those at 20 and 40 weeks indicated extracellular matrix remodeling.

CONCLUSIONS

In this rodent model of mitral regurgitation, left ventricular ejection fraction was preserved for a long duration, yet rapid and severe left ventricular dilatation, and biological remodeling occurred before a clinically significant fall in ejection fraction.

Mechanism of Eccentric Cardiomyocyte Hypertrophy Secondary to Severe Mitral Regurgitation

BACKGROUND

Primary valvular heart disease is a prevalent cause of morbidity and mortality in both industrialized and developing countries. Although the primary consequence of valvular heart disease is myocardial dysfunction, treatment of valvular heart diseases centers around valve repair or replacement rather than prevention or reversal of myocardial dysfunction. This is particularly evident in primary mitral regurgitation (MR), which invariably results in eccentric hypertrophy and left ventricular (LV) failure in the absence of timely valve repair or replacement. The mechanism of LV dysfunction in primary severe MR is entirely unknown.

METHODS

Here, we developed the first mouse model of severe MR. Valvular damage was achieved by severing the mitral valve leaflets and chords with iridectomy scissors, and MR was confirmed by echocardiography. Serial echocardiography was performed to follow up LV morphology and systolic function. Analysis of cardiac tissues was subsequently performed to evaluate valve deformation, cardiomyocyte morphology, LV fibrosis, and cell death. Finally, dysregulated pathways were assessed by RNA-sequencing analysis and immunofluorescence.

RESULTS

In the ensuing 15 weeks after the induction of MR, gradual LV dilatation and dysfunction occurred, resulting in severe systolic dysfunction. Further analysis revealed that severe MR resulted in a marked increase in cardiac mass and increased cardiomyocyte length but not width, with electron microscopic evidence of sarcomere disarray and the development of sarcomere disruption. From a mechanistic standpoint, severe MR resulted in activation of multiple components of both the mammalian target of rapamycin and calcineurin pathways. Inhibition of mammalian target of rapamycin signaling preserved sarcomeric structure and prevented LV remodeling and systolic dysfunction. Immunohistochemical analysis uncovered a differential pattern of expression of the cell polarity regulator Crb2 (crumbs homolog 2) along the longitudinal axis of cardiomyocytes and close to the intercalated disks in the MR hearts. Electron microscopy images demonstrated a significant increase in polysome localization in close proximity to the intercalated disks and some areas along the longitudinal axis in the MR hearts.

CONCLUSIONS

These results indicate that LV dysfunction in response to severe MR is a form of maladaptive eccentric cardiomyocyte hypertrophy and outline the link between cell polarity regulation and spatial localization protein synthesis as a pathway for directional cardiomyocyte growth.

The characteristics of a porcine mitral regurgitation model

The porcine mitral regurgitation (MR) model is a common cardiovascular animal model. Standardized manufacturing processes can improve the uniformity and success rate of the model, and systematic research can evaluate its potential use. In this study, 17 pigs were divided into an experimental group (n=11) and a control group (n=6). We used a homemade retractor to cut the mitral chordae via the left atrial appendage to establish a model of MR; the control group underwent a sham surgery. The model animals were followed for 30 months after the surgery. Enlargement and fibrosis of the left atrium were significant in the experimental group compared with those in the control group, and left atrial systolic function decreased significantly. In addition, model animals showed preserved left ventricular systolic function. There were no differences in left atrial potential or left ventricular myocardial fibrosis between the two groups. Atrial fibrillation susceptibility in the experimental group was higher than that in the control group. Our method enables the simple and effective production of a MR model with severe reflux that can be used for pathophysiological studies of MR, as well as for the development of preclinical surgical instruments and their evaluation. This model could also be used to study atrial fibrillation and myocardial fibrosis but is not suitable for studies of heart failure.

Effects of early, late, and long-term nonselective β-blockade on left ventricular remodeling, function, and survival in chronic organic mitral regurgitation

BACKGROUND

Mitral regurgitation (MR) produces sympathetic nervous system activation which is detrimental in other causes of heart failure. However, whether β-blockade is beneficial in MR has not been determined.

METHODS AND RESULTS

Eighty-seven rats with significant organic MR were randomized to the β-blockade group (n=43) or the control group (n=44). Carvedilol was started in week 2 post MR induction and given for 23 to 35 weeks in the β-blockade group. Echocardiography was performed at baseline and at weeks 2, 6, 12, 24, 30, and 36 after MR induction. After 23 weeks of β-blockade, heart rates were significantly reduced by carvedilol (308 ± 25 versus 351 ± 31 beats per minute; P<0.001). Left ventricular end-diastolic (2.2 ± 0.7 versus 1.59 ± 0.6 mL; P<0.001), end-systolic volumes (0.72 ± 0.42 versus 0.40 ± 0.19 mL; P<0.001), and mass index (2.40 ± 0.55 versus 2.06 ± 0.62 g/kg; P<0.001) were significantly higher, and left ventricular fraction shortening (33 ± 7% versus 38 ± 7%; P<0.001) and ejection fraction (69 ± 11% versus 75 ± 7%; P<0.001) were significantly lower in the β-blockade group than in the control group. Systolic blood pressure was lower in the β-blockade group than in the control group (114 ± 10 versus 93 ± 12 mm Hg; P<0.005). Survival probability was significantly lower in the early β-blockade group than in the control group (88% versus 96%; P=0.03).

CONCLUSIONS

Early and long-term nonselective β-blockade was associated with adverse left ventricular remodeling, systolic dysfunction, and a reduction in survival in the experimental rat model of organic MR.

Increased expression of adenosine triphosphate-sensitive K+ channels in mitral dysfunction: mechanically stimulated transcription and hypoxia-induced protein stability?

OBJECTIVES

The aim of this study was to test whether adenosine triphosphate-sensitive K(+) (KATP) channel expression relates to mechanical and hypoxic stress within the left human heart.

BACKGROUND

The KATP channels play a vital role in preserving the metabolic integrity of the stressed heart. However, the mechanisms that govern the expression of their subunits (e.g., potassium inward rectifier [Kir] 6.2) in adult pathologies are mostly unknown.

METHODS

We collected biopsies from the 4 cardiac chambers and 50 clinical parameters from 30 surgical patients with severe mitral dysfunction. Proteins and messenger ribonucleic acids (mRNAs) of KATP pore subunits and mRNAs of their known transcriptional regulators (forkhead box [FOX] F2, FOXO1, FOXO3, and hypoxia inducible factor [HIF]-1α) were measured respectively by Western blotting, immunohistochemistry, and quantitative real-time polymerase chain reaction, and submitted to statistical analysis.

RESULTS

In all heart chambers, Kir6.2 mRNA correlated with HIF-1α mRNA. Neither Kir6.1 nor Kir6.2 proteins positively correlated with their respective mRNAs. The HIF-1α mRNA related in the left ventricle to aortic pressure, in the left atrium to left atrial pressure, and in all heart chambers to a decreased Kir6.2 protein/mRNA ratio. Interestingly, in the left heart, Kir6.2 protein and its immunohistochemical detection in myocytes were maximal at low venous PO(2). In the left ventricle, the Kir6.2 protein/mRNA ratio was also significantly higher at low venous PO(2), suggesting that tissue hypoxia might stabilize the Kir6.2 protein.

CONCLUSIONS

Results suggest that post-transcriptional events determine Kir6.2 protein expression in the left ventricle of patients with severe mitral dysfunction and low venous PO(2). Mechanical stress mainly affects transcription of HIF-1α and Kir6.2. This study implies that new therapies could aim at the proteasome for stabilizing the left ventricular Kir6.2 protein.

Survival, exercise capacity, and left ventricular remodeling in a rat model of chronic mitral regurgitation: serial echocardiography and pressure-volume analysis

Background and Objectives

The aims of this study were to establish a reliable model of chronic mitral regurgitation (MR) in rats and verify the pathophysiological features of this model by evaluating cardiac function using serial echocardiography and a pressure-volume analysis.

Materials and Methods

MR was created in 37 Sprague-Dawley rats by making a hole with a 23 gauge needle on the mitral leaflet through the left ventricular (LV) apex under the guidance of transesophageal echocardiography.

Results

Serial echocardiograms revealed that the LV began to dilate immediately after the MR operation and showed progressive dilation until the 14th week (LV end-systolic dimension at 14 weeks, 4.71±0.25 mm vs. 6.81±0.50 mm for sham vs. MR, p<0.01; LV end-diastolic dimension, 8.32±0.42 mm vs. 11.01±0.47 mm, p<0.01). The LV ejection fraction tended to increase immediately after the MR operation but started to decrease thereafter and showed a significant difference with the sham group from the 14th week (70.0±2.2% vs. 62.1±3.1% for sham vs. MR). In a pressure-volume analysis performed at the 14th week, the LV end-systolic pressure-volume relationship and +dp/dt decreased significantly in the MR group. A serial treadmill test revealed that exercise capacity remained in the normal range until the 14th week when it began to decrease (exercise duration, 406±45 seconds vs. 330±27 seconds, p<0.01). A pathological analysis showed no significance difference in interstitial fibrosis between the two groups.

Conclusion

We established a small animal model of chronic MR and verified its pathophysiological features. This model may provide a useful tool for future research on MR and volume overload heart failure.