Development of left ventricular hypertrophy in a novel porcine model of mitral regurgitation

Effect of insulin on natriuretic peptide gene expression in porcine heart

Gut hormones affect cardiac function and contractility. In this study, we examined whether insulin affects the cardiac atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) gene expression and release of proANP-derived peptides in pigs. Anaesthetized pigs were included in an experimental study comparing the effect of hyperinsulinemia in 15 pigs submitted to two different protocols versus 11 control pigs receiving saline infusion. Phosphorylation of Akt on Thr308 was determined by western blotting with a pAkt-Thr308 antibody. The mRNA contents of ANP and BNP were determined with real-time PCR; plasma and cardiac tissue proANP was measured with an immunoluminometric assay targeted against the mid-region of the propeptide and a processing-independent assay. Insulin stimulation increased phosphorylation of Akt Thr308 in both left atrium and left ventricle of porcine hearts (p < 0.005). No change was observed in ANP and BNP mRNA contents in the right or left atrium. BNP mRNA contents in the left ventricle, however, decreased 3-fold (p = 0.02) compared to control animals, whereas the BNP mRNA content in the right ventricle as well as ANP mRNA content in the right and left ventricle did not change following hyperinsulinemia. Moreover, the peptide contents did not change in the four cardiac chambers. Finally, proANP concentrations in plasma did not change during the insulin infusion compared to the control animals. These results suggest that insulin does not have direct effect on atrial natriuretic peptide expression but may have a role in the left ventricle.

Inhibition of KCa2 and Kv11.1 Channels in Pigs With Left Ventricular Dysfunction

Background

Inhibition of K_Ca2 channels, conducting I_KCa, can convert atrial fibrillation (AF) to sinus rhythm and protect against its induction. I_KCa inhibition has been shown to possess functional atrial selectivity with minor effects on ventricles. Under pathophysiological conditions with ventricular remodeling, however, inhibiting I_KCa can exhibit both proarrhythmic and antiarrhythmic ventricular effects. The aim of this study was to evaluate the effects of the I_KCa inhibitor AP14145, when given before or after the I_Kr blocker dofetilide, on cardiac function and ventricular proarrhythmia markers in pigs with or without left ventricular dysfunction (LVD).

Methods

Landrace pigs were randomized into an AF group (n = 6) and two control groups: SHAM1 (n = 8) and SHAM2 (n = 4). AF pigs were atrially tachypaced (A-TP) for 43 ± 4 days until sustained AF and LVD developed. A-TP and SHAM1 pigs received 20 mg/kg AP14145 followed by 100 µg/kg dofetilide whereas SHAM2 pigs received the same drugs in the opposite order. Proarrhythmic markers such as short-term variability of QT (STV_QT) and RR (STV_RR) intervals, and the number of premature ventricular complexes (PVCs) were measured at baseline and after administration of drugs. The influence on cardiac function was assessed by measuring cardiac output, stroke volume, and relevant echocardiographic parameters.

Results

I_KCa inhibition by AP14145 did not increase STV_QT or STV_RR in any of the pigs. I_Kr inhibition by dofetilide markedly increased STV_QT in the A-TP pigs, but not in SHAM operated pigs. Upon infusion of AP14145 the number of PVCs decreased or remained unchanged both when AP14145 was infused after baseline and after dofetilide. Conversely, the number of PVCs increased or remained unchanged upon dofetilide infusion. Neither AP14145 nor dofetilide affected relevant echocardiographic parameters, cardiac output, or stroke volume in any of the groups.

Conclusion

I_KCa inhibition with AP14145 was not proarrhythmic in healthy pigs, or in the presence of LVD resulting from A-TP. In pigs already challenged with 100 µg/kg dofetilide there were no signs of proarrhythmia when 20 mg/kg AP14145 were infused. K_Ca2 channel inhibition did not affect cardiac function, implying that K_Ca2 inhibitors can be administered safely also in the presence of LV dysfunction.

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.

Characterization of Atrial and Ventricular Structural Remodeling in a Porcine Model of Atrial Fibrillation Induced by Atrial Tachypacing

Background: Atrial fibrillation (AF) is characterized by electrical and structural remodeling. Irregular and/or fast atrio-ventricular (AV) conduction during AF can result in AV dyssynchrony, tachymyopathy, pressure and volume overload with subsequent dilatation, valve regurgitation, and ventricular dysfunction with progression to heart failure. Objective: To gain further insight into the myocardial pathophysiological changes induced by right atrial tachypacing (A-TP) in a large animal model. Methods: A total of 28 Landrace pigs were randomized as 14 into AF-induced A-TP group and 14 pigs to control group. AF pigs were tachypaced for 43 ± 4 days until in sustained AF. Functional remodeling was investigated by echocardiography (after cardioversion to sinus rhythm). Structural remodeling was quantified by histological preparations with picrosirius red and immunohistochemical stainings. Results: A-TP resulted in decreased left ventricular ejection fraction (LVEF) accompanied by increased end-diastolic and end-systolic left atrium (LA) volume and area. In addition, A-TP was associated with mitral valve (MV) regurgitation, diastolic dysfunction and increased atrial and ventricular fibrotic extracellular matrix (ECM). Conclusions: A-TP induced AF with concomitant LV systolic and diastolic dysfunction, increased LA volume and area, and atrial and ventricular fibrosis.

Serotonin markers show altered transcription levels in an experimental pig model of mitral regurgitation

Serotonin (5-hydroxytryptamine, 5-HT) signalling is implicated in the pathogenesis of myxomatous mitral valve disease (MMVD) through 5-HT1B receptor (R), 5-HT2AR and 5-HT2BR-induced myxomatous pathology. Based on increased tryptophan hydroxylase-1 (TPH-1) and decreased serotonin re-uptake transporter (SERT) in MMVD-affected valves, increased valvular 5-HT synthesis and decreased clearance have been suggested. It remains unknown how haemodynamic changes associated with mitral regurgitation (MR) affect 5-HT markers in the mitral valve, myocardium and circulation. Twenty-eight pigs underwent surgically induced MR or sham-operation, resulting in three MR groups: control (CON, n = 12), mild MR (mMR, n = 10) and severe MR (sMR, n = 6). The gene expression levels of 5-HT1BR, 5-HT2AR, 5-HT2BR, SERT and TPH-1 were analysed using quantitative PCR (qPCR) in the mitral valve (MV), anterior papillary muscle (AP) and left ventricle (LV). MV 5-HT2BR was also analysed with immunohistochemistry (IHC) in relation to histological lesions and valvular myofibroblasts. All 5-HTR mRNAs were up-regulated in MV compared to AP and LV (P <0.01). In contrast, SERT and TPH-1 were up-regulated in AP and LV compared to MV (P <0.05). In MV, mRNA levels were increased for 5-HT2BR (P = 0.02) and decreased for SERT (P = 0.03) in sMR vs. CON. There were no group differences in 5-HT2BR staining (IHC) but co-localisation was found with α-SMA-positive cells in 91% of all valves and with 33% of histological lesions. In LV, 5-HT1BR mRNA levels were increased in sMR vs. CON (P = 0.01). In conclusion, these data suggest that MR may affect mRNA expression of valvular 5-HT2BR and SERT, and left ventricular 5-HT1BR in some pigs.