Ionic Mechanisms Underlying the Effects of Vasoactive Intestinal Polypeptide on Canine Atrial Myocardium

The Liver-Heart Connection: A Literature Review of Liver Disease as a Risk Factor for Atrial Fibrillation

Atrial fibrillation (AFib) is a common type of cardiac arrhythmia, characterized by disorganized atrial electrical activity with features of irregularly irregular heart rhythm and often with rapid ventricular response increasing the risk of stroke and heart failure due to tachyarrhythmia. The pathophysiology mechanism of AFib is either triggered by atrial distension, abnormality in conducting system, catecholamine excess, or increased atrial irritation or automaticity. Risk factors include uncontrolled diabetes, obesity, obstructive sleep apnea, hypothyroidism, and certain stimulants. Based on recent research, liver disease has recently been identified as a risk factor for AFib. Considering the progression of chronic liver disease, this literature review aims to investigate and summarize the relationship between liver disease and AFib and explore clinical interventions that can be utilized to prevent AFib aggravation.

Liver fibrosis scores and atrial fibrillation incidence in heart failure with preserved ejection fraction

AIM

Non-alcoholic fatty liver disease (NAFLD)-related advanced liver fibrosis (Stage 3 or 4) was reported to be linked to worse prognosis in patients with heart failure with preserved ejection fraction (HFpEF). This study aims to assess the relationship between liver fibrosis scores and new-onset atrial fibrillation (AF) incidence in patients with HFpEF in the Treatment of Preserved Cardiac Function Heart Failure with an Aldosterone Antagonist (TOPCAT) trial.

METHODS AND RESULTS

Baseline liver fibrosis levels, assessed by NAFLD fibrosis score (NFS) or Fibrosis-4 index (FIB-4), with AF incidence were expressed as hazard ratios (HRs) using the Cox proportional hazard model. The risk for advanced fibrosis was estimated to be 21.5% (447/2072) as assessed by FIB-4 (>3.25) and 4.2% (88/2072) as assessed by NFS (>0.676) in HFpEF patients without baseline AF. After a median follow-up of 3.11 years, 106 new-onset AF cases occurred. In multivariate analysis, elevated NFS [NFS = -1.455-0.676: HR 2.44, 95% confidence interval (CI) 1.27-4.68; NFS > 0.676: HR 3.36, 95% CI 1.27-6.80; per 1 unit increase: HR 1.15, 95% CI 1.01-1.32], not FIB-4 (FIB-4 = 1.45-3.25: HR 1.02, 95% CI 0.67-1.55; FIB-4 > 3.25: HR 1.69, 95% CI 0.76-3.79; per 1 unit increase: HR 1.13, 95% CI 0.93-1.37), was associated with increased AF incidence. The NFS (C-index 0.662), not FIB-4 (C-index 0.531), had a moderate predictive ability in predicting incident AF.

CONCLUSIONS

Among patients with HFpEF, the risk of advanced liver fibrosis is associated with an increased incidence of new-onset AF and may be a novel predictor for new-onset AF. Additional studies are needed to confirm our results.

Liver Disease as a Predictor of New-Onset Atrial Fibrillation

Background Impact of liver disease on development of atrial fibrillation ( AF ) is unclear. The purpose of the study was to evaluate prevalence of AF in the setting of liver disease and whether increasing severity of liver disease, using Model for End-Stage Liver Disease ( MELD ), is independently associated with increased risk of AF . Methods and Results Retrospective data analysis of 1727 patients with liver disease evaluated for liver transplantation between 2006 and 2015 was performed, and patient characteristics were analyzed from billing codes and review of medical records. Multivariable time-dependent Cox proportional hazards model was performed to determine effect of increasing MELD score on risk of developing AF . Prevalence of AF was 11.2%. Incidence of AF at median follow-up time of 1.04 years was 8.5%. Both prevalence and incidence of AF increased with increasing MELD scores. Prevalence of AF was 3.7%, 6.4%, 16.7%, and 20.2% corresponding with MELD quartiles 1 to 10, 11 to 20, 21 to 30, and >30, respectively. Compared with patients with MELD quartile 1 to 10, patients with MELD quartile of 11 to 20 had hazard ratio of 2.73 (confidence interval, 1.47-5.07), those in the MELD quartile of 21 to 30 had a hazard ratio of 5.17 (confidence interval, 2.65-10.09), and those with MELD values >30 had hazard ratio of 9.33 (confidence interval, 3.93-22.14) for development of new-onset AF . Other significant variables associated with new-onset AF were age, sleep apnea, valvular heart disease, hemodynamic instability, and reduced left ventricular ejection fraction <50% (hazard ratio, of 1.06, 2.17, 3.21, 2.00, and 2.44, respectively). Conclusions Prevalence and incidence of AF in patients with liver disease is high. Severity of liver disease, as measured by MELD , is an important predictor of new-onset AF . This novel finding suggests an interaction between inflammatory and neurohormonal changes in liver disease and pathogenesis of AF .

The Effect of Substrate Stiffness on Cardiomyocyte Action Potentials

The stiffness of myocardial tissue changes significantly at birth and during neonatal development, concurrent with significant changes in contractile and electrical maturation of cardiomyocytes. Previous studies by our group have shown that cardiomyocytes generate maximum contractile force when cultured on a substrate with a stiffness approximating native cardiac tissue. However, effects of substrate stiffness on the electrophysiology and ion currents in cardiomyocytes have not been fully characterized. In this study, neonatal rat ventricular myocytes were cultured on the surface of flat polyacrylamide hydrogels with elastic moduli ranging from 1 to 25 kPa. Using whole-cell patch clamping, action potentials and L-type calcium currents were recorded. Cardiomyocytes cultured on hydrogels with a 9 kPa elastic modulus, similar to that of native myocardium, had the longest action potential duration. Additionally, the voltage at maximum calcium flux significantly decreased in cardiomyocytes on hydrogels with an elastic modulus higher than 9 kPa, and the mean inactivation voltage decreased with increasing stiffness. Interestingly, the expression of the L-type calcium channel subunit α gene and channel localization did not change with stiffness. Substrate stiffness significantly affects action potential length and calcium flux in cultured neonatal rat cardiomyocytes in a manner that may be unrelated to calcium channel expression. These results may explain functional differences in cardiomyocytes resulting from changes in the elastic modulus of the extracellular matrix, as observed during embryonic development, in ischemic regions of the heart after myocardial infarction, and during dilated cardiomyopathy.

Generation of electrophysiologically functional cardiomyocytes from mouse induced pluripotent stem cells

Induced pluripotent stem (iPS) cells can efficiently differentiate into the three germ layers similar to those formed by differentiated embryonic stem (ES) cells. This provides a new source of cells in which to establish preclinical allogeneic transplantation models. Our iPS cells were generated from mouse embryonic fibroblasts (MEFs) transfected with the Yamanaka factors, the four transcription factors (Oct4, Sox2, Klf4 and c-Myc), without antibiotic selection or MEF feeders. After the formation of embryoid bodies (EBs), iPS cells spontaneously differentiated into Flk1-positive cardiac progenitors and cardiomyocytes expressing cardiac-specific markers such as alpha sarcomeric actinin (α-actinin), cardiac alpha myosin heavy chain (α-MHC), cardiac troponin T (cTnT), and connexin 43 (CX43), as well as cardiac transcription factors Nk2 homebox 5 (Nkx2.5) and gata binding protein 4 (gata4). The electrophysiological activity of iPS cell-derived cardiomyocytes (iPS-CMs) was detected in beating cell clusters with optical mapping and RH237 a voltage-sensitive dye, and in single contracting cells with patch-clamp technology. Incompletely differentiated iPS cells formed teratomas when transplanted into a severe combined immunodeficiency (SCID) mouse model of myocardial infarction. Our results show that somatic cells can be reprogrammed into pluripotent stem cells, which in turn spontaneously differentiate into electrophysiologically functional mature cardiomyocytes expressing cardiac-specific makers, and that these cells can potentially be used to repair myocardial infarction (MI) in the future.

Treatment of Atrial and Ventricular Arrhythmias Through Autonomic Modulation

This paper reviews the contribution of autonomic nervous system (ANS) modulation in the treatment of arrhythmias. Both the atria and ventricles are innervated by an extensive network of nerve fibers of parasympathetic and sympathetic origin. Both the parasympathetic and sympathetic nervous system exert arrhythmogenic electrophysiological effects on atrial and pulmonary vein myocardium, while in the ventricle the sympathetic nervous system plays a more dominant role in arrhythmogenesis. Identification of ANS activity is possible with nuclear imaging. This technique may provide further insight in mechanisms and treatment targets. Additionally, the myocardial effects of the intrinsic ANS can be identified through stimulation of the ganglionic plexuses. These can be ablated for the treatment of atrial fibrillation. New (non-) invasive treatment options targeting the extrinsic cardiac ANS, such as low-level tragus stimulation and renal denervation, provide interesting future treatment possibilities both for atrial fibrillation and ventricular arrhythmias. However, the first randomized trials have yet to be performed. Future clinical studies on modifying the ANS may not only improve the outcome of ablation therapy but may also advance our understanding of the manner in which the ANS interacts with the myocardium to modify arrhythmogenic triggers and substrate.

Methods for isolating atrial cells from large mammals and humans

The identification of disturbances in the cellular structure, electrophysiology and calcium handling of atrial cardiomyocytes is crucial to the understanding of common pathologies such as atrial fibrillation. Human right atrial specimens can be obtained during routine cardiac surgery and may be used for isolation of atrial myocytes. These samples provide the unique opportunity to directly investigate the effects of human disease on atrial myocytes. However, atrial myocytes vary greatly between patients, there is little if any access to truly healthy controls and the challenges associated with assessing the in vivo effects of drugs or devices in man are considerable. These issues highlight the need for animal models. Large mammalian models are particularly suitable for this purpose as their cardiac structure and electrophysiology are comparable with humans. Here, we review techniques for obtaining atrial cardiomyocytes. We start with background information on solution composition. Agents shown to increase viable cell yield will then be explored followed by a discussion of the use of tissue-dissociating enzymes. Protocols are detailed for the perfusion method of cell isolation in large mammals and the chunk digest methods of cell isolation in humans.

Neuronally released vasoactive intestinal polypeptide alters atrial electrophysiological properties and may promote atrial fibrillation

BACKGROUND

Vagal hyperactivity promotes atrial fibrillation (AF), which has been almost exclusively attributed to acetylcholine. Vasoactive intestinal polypeptide (VIP) and acetylcholine are neurotransmitters co-released during vagal stimulation. Exogenous VIP has been shown to promote AF by shortening action potential duration (APD), increasing APD spatial heterogeneity, and causing intra-atrial conduction block.

OBJECTIVE

The purpose of this study was to investigate the effects of neuronally released VIP on atrial electrophysiologic properties during vagal stimulation.

METHODS

We used a specific VIP antagonist (H9935) to uncover the effects of endogenous VIP released during vagal stimulation in canine hearts.

RESULTS

H9935 significantly attenuated (1) the vagally induced shortening of atrial effective refractory period and widening of atrial vulnerability window during stimulation of cervical vagosympathetic trunks (VCNS) and (2) vagal effects on APD during stimulation through fat-pad ganglion plexus (VGPS). Atropine completely abolished these vagal effects during VCNS and VGPS. In contrast, VGPS-induced slowing of local conduction velocity was completely abolished by either VIP antagonist or atropine. In pacing-induced AF during VGPS, maximal dominant frequencies and their spatial gradients were reduced significantly by H9935 and, more pronouncedly, by atropine. Furthermore, VIP release in the atria during vagal stimulation was inhibited by atropine, which may account for the concealment of VIP effects with muscarinic blockade.

CONCLUSION

Neuronally released VIP contributes to vagal effects on atrial electrophysiologic properties and affects the pathophysiology of vagally induced AF. Neuronal release of VIP in the atria is inhibited by muscarinic blockade, a novel mechanism by which VIP effects are concealed by atropine during vagal stimulation.