Aging-associated changes in L-type calcium channels in the left atria of dogs

Impact of age-related gut microbiota dysbiosis and reduced short-chain fatty acids on the autonomic nervous system and atrial fibrillation in rats

Objective

Aging is the most significant contributor to the increasing prevalence of atrial fibrillation (AF). Dysbiosis of gut microbiota has been implicated in age-related diseases, but its role in AF development remains unclear. This study aimed to investigate the correlations between changes in the autonomic nervous system, short-chain fatty acids (SCFAs), and alterations in gut microbiota in aged rats with AF.

Methods

Electrophysiological experiments were conducted to assess AF induction rates and heart rate variability in rats. 16S rRNA gene sequences extracted from fecal samples were used to assess the gut microbial composition. Gas and liquid chromatography-mass spectroscopy was used to identify SCFAs in fecal samples.

Results

The study found that aged rats exhibited a higher incidence of AF and reduced heart rate variability compared to young rats. Omics research revealed disrupted gut microbiota in aged rats, specifically a decreased Firmicutes to Bacteroidetes ratio. Additionally, fecal SCFA levels were significantly lower in aged rats. Importantly, correlation analysis indicated a significant association between decreased SCFAs and declining heart rate variability in aged rats.

Conclusions

These findings suggest that SCFAs, as metabolites of gut microbiota, may play a regulatory role in autonomic nervous function and potentially influence the onset and progression of AF in aged rats. These results provide novel insights into the involvement of SCFAs and autonomic nervous system function in the pathogenesis of AF. These results provide novel insights into the involvement of SCFAs and autonomic nervous system function in the pathogenesis of AF.

Ionic current changes underlying action potential repolarization responses to physiological pacing and adrenergic stimulation in adult rat ventricular myocytes

This study aimed to simulate ventricular responses to elevations in myocyte pacing and adrenergic stimulation using a novel electrophysiological rat model and investigate ion channel responses underlying action potential (AP) modulations. Peak ion currents and AP repolarization to 50% and 90% of full repolarization (APD_50-90 ) were recorded during simulations at 1-10 Hz pacing under control and adrenergic stimulation conditions. Further simulations were performed with incremental ion current block (L-type calcium current, I_Ca ; transient outward current, I_to ; slow delayed rectifier potassium current, I_Ks ; rapid delayed rectifier potassium current, I_Kr ; inward rectifier potassium current, I_K1 ) to identify current influence on AP response to exercise. Simulated APD_50-90 closely resembled experimental findings. Rate-dependent increases in I_Ks (6%-101%), I_Kr (141%-1339%), and I_Ca (0%-15%) and reductions in I_to (11%-57%) and I_K1 (1%-9%) were observed. Meanwhile, adrenergic stimulation triggered moderate increases in all currents (23%-67%) except I_K1 . Further analyses suggest AP plateau is most sensitive to modulations in I_to and I_Ca while late repolarization is most sensitive to I_K1 , I_Ca , and I_Ks , with alterations in I_Ks predominantly stimulating the greatest magnitude of influence on late repolarization (35%-846% APD₉₀ prolongation). The modified Leeds rat model (mLR) is capable of accurately modeling APs during physiological stress. This study highlights the importance of I_Ca , I_to , I_K1, and I_Ks in controlling electrophysiological responses to exercise. This work will benefit the study of cardiac dysfunction, arrythmia, and disease, though future physiologically relevant experimental studies and model development are required.

Targeted Atrial Fibrillation Therapy and Risk Stratification Using Atrial Alternans

Atrial fibrillation (AF) is the most persistent arrhythmia today, with its prevalence increasing exponentially with the rising age of the population. Particularly at elevated heart rates, a functional abnormality known as cardiac alternans can occur prior to the onset of lethal arrhythmias. Cardiac alternans are a beat-to-beat oscillation of electrical activity and the force of cardiac muscle contraction. Extensive evidence has demonstrated that microvolt T-wave alternans can predict ventricular fibrillation vulnerability and the risk of sudden cardiac death. The majority of our knowledge of the mechanisms of alternans stems from studies of ventricular electrophysiology, although recent studies offer promising evidence of the potential of atrial alternans in predicting the risk of AF. Exciting preclinical and clinical studies have demonstrated a link between atrial alternans and the onset of atrial tachyarrhythmias. Here, we provide a comprehensive review of the clinical utility of atrial alternans in identifying the risk and guiding treatment of AF.

Consequences and Mechanisms of Left Atrium Remodeling in Aging Rabbits

To investigate the consequences and mechanisms of myocardium remodeling of aging left atrium, we analyzed the main cardiac electrophysiological parameters such as rest membrane potential, action potential amplitude, maximum rate of action potential increase (max dV/dt), action potential plateau, and 30, 50, and 90% action potential duration (APD₃₀, APD₅₀, and APD₉₀, respectively), as well as the inducibility and duration of atrial arrhythmias in adult and aging rabbits. L-type calcium current was also recorded. The collagen content in the myocardium and ultrastructure of left atrial cells were also studied. Significant changes were detected in the electrophysiological parameters and structure in aged left atrium, which can contribute to atrial susceptibility to arrhythmia in aged rabbits.

Effects and pharmacological mechanism of Zhigancao Decoction on electrical and structural remodeling of the atrium of rabbits induced by rapid atrial pacing

BACKGROUND

Zhigancao decoction (ZD) has a long history in China as a traditional Chinese medicine compound for the treatment of tachyarrhythmias. This study mainly explored the pharmacological mechanism of Zhigancao Decoction in preventing atrial fibrillation by altering the electrical and structural remodeling of the atrial in rabbits.

METHODS

In total, 30 male New Zealand white rabbits were randomly divided into 3 groups (ten rabbits for each). The first group was sham-operated (control group). The second group was intervened by the rapid right atrium pacing (RAP) to induce atrial fibrillation (AF group), while the third group was given ZD gavage and RAP (AF + ZD group). All rabbits were anesthetized before two monophasic action potential (MAP) catheters were sequentially inserted into the right atrium. After 8 h of rapid right atrial pacing, the electrophysiological indexes and the induction rate of atrial fibrillation were observed in the three groups of rabbits, and the left atrial myocardium samples were taken to observe the ultrastructure. Single atrial myocytes were separated by enzymolysis, and the L-type calcium current (I_Ca-L) of atrial myocytes in different experimental groups was observed by whole-cell patch clamp technique. The fluorescence intensity of Ca²⁺ in atrial myocytes was observed after Fluo-3/AM fluorescent staining. The main components of ZD were identified by liquid chromatography-mass spectrometry-mass spectrometry (LC-MS/MS) method.

RESULTS

Compared with the AF group, the maximum ascent rate (Max dV/dt) and plateau potential were significantly reduced in the ZD group, the action potential duration at 10% and 20% (APD₁₀, APD₂₀) were significantly shortened (P < 0.01), action potential duration at 50%, 70%, and 90% (APD₅₀, APD₇₀, APD₉₀) were significantly prolonged, and atrial effective refractory period (AERP) was significantly prolonged (P < 0.01) in the ZD group. In the ZD group, the I_Ca-L amplitudes of rabbit atrial myocytes under each clamping voltage were significantly smaller than those in the AF group (P < 0.01) and the control group (P < 0.05). The Ca²⁺ fluorescence intensity in the rabbit atrial myocytes in the ZD group was significantly weaker than that in the AF group (P < 0.01) and the control group (P < 0.05). Electron microscopy displayed that the control group had neatly arranged atrial tissue myofilaments and intact mitochondria. However, the ultrastructural damage of the AF group was severe compared with that of the ZD group. LC-MS/MS analysis confirmed that ZD contained several antiarrhythmic compounds including ginsenoside, isoliensinine, catalpol, glycyrrhizinate and hesperetin.

CONCLUSION

Rapid atrial pacing (RAP) could cause the electrical and structural remodeling of rabbit atrial myocytes. ZD might reverse the atrial electrical remodeling but could have little effect on structural remodeling, which might be the mechanism of ZD treatment on atrial fibrillation.

Action potential responses to changes in stimulation frequency and isoproterenol in rat ventricular myocytes

PURPOSE

Current understanding of ventricular action potential adaptation to physiological stress is generally based on protocols using non-physiological rates and conditions isolating rate effects from escalating adrenergic stimulation. To permit refined understanding, ventricular action potentials were assessed across physiological pacing frequencies in the presence and absence of adrenergic stimuli. Isolated and combined effects were analyzed to assess their ability to replicate in-vivo responses.

METHODS

Steady-state action potentials from ventricular myocytes isolated from male Wistar rats (3 months; N = 8 animals) were recorded at 37°C with steady-state pacing at 1, 2, 4, 6, 8 and 10 Hz using whole-cell patch-clamp. Action potential repolarization to 25, 50, 75, 90 and 100% of full repolarization (APD25-100 ) was compared before and after 5 nM, 100 nM and 1 µM isoproterenol doses.

RESULTS

A Repeated measures ANOVA found APD50-90  shortened with 5 nM isoproterenol infusion by 6-25% (but comparable across doses) (p ≤ 0.03). Pacing frequencies emulating a normal rat heart rate (6 Hz) prolonged APD50 23% compared with 1 Hz pacing. Frequencies emulating exercise or stress (10 Hz) shortened APD90 (29%).

CONCLUSION

These results demonstrate modest action potential shortening in response to adrenergic stimulation and elevations in pacing beyond physiological resting rates. Our findings indicate changes in action potential plateau and late repolarization predominantly underlie simulated exercise responses in the rat heart. This work provides novel action potential reference data and will help model cardiac responses to physiological stimuli in the rat heart via computational techniques.

Reduced cardiac response to the adrenergic system is a key limiting factor for physical capacity in old age

Ageing is associated with a progressive reduction in physical capacity reducing quality of life. One key physiological limitation of physical capacity that deteriorates in a progressive age-dependent manner is cardiac reserve. Peak cardiac output falls progressively with advancing age such that in extreme old age there is limited ability to enhance cardiac output beyond basal function as is required to support the increased metabolic needs of physical activity. This loss of dynamic range in cardiac output associates with a progressive reduction in the heart's response to adrenergic stimulation. A combination of decreases in the expression and functioning of beta1 adrenergic receptors partially underlies this change. Changes in end effector proteins also have a role to play in this decline. Alterations in the efficiency of excitation-contraction coupling contribute to the reduced chronotropic, inotropic and lusitropic responses of the aged heart. Moderate to vigorous endurance exercise training however has some potential to counter elements of these changes. Further studies are required to fully elucidate the key pivotal mechanisms involved in the age-related loss of response to adrenergic signalling to allow targeted therapeutic strategies to be developed with the aim of preserving physical capacity in advanced old age.

Atrial remodeling and atrial fibrillation in acquired forms of cardiovascular disease

Atrial fibrillation (AF) is prevalent in common conditions and acquired forms of heart disease, including diabetes mellitus (DM), hypertension, cardiac hypertrophy, and heart failure. AF is also prevalent in aging. Although acquired heart disease is common in aging individuals, age is also an independent risk factor for AF. Importantly, not all individuals age at the same rate. Rather, individuals of the same chronological age can vary in health status from fit to frail. Frailty can be quantified using a frailty index, which can be used to assess heterogeneity in individuals of the same chronological age. AF is thought to occur in association with electrical remodeling due to changes in ion channel expression or function as well as structural remodeling due to fibrosis, myocyte hypertrophy, or adiposity. These forms of remodeling can lead to triggered activity and electrical re-entry, which are fundamental mechanisms of AF initiation and maintenance. Nevertheless, the underlying determinants of electrical and structural remodeling are distinct in different conditions and disease states. In this focused review, we consider the factors leading to atrial electrical and structural remodeling in human patients and animal models of acquired cardiovascular disease or associated risk factors. Our goal is to identify similarities and differences in the cellular and molecular bases for atrial electrical and structural remodeling in conditions including DM, hypertension, hypertrophy, heart failure, aging, and frailty.

Ca2+ ionophore A23187 inhibits ATP generation reducing mouse sperm motility and PKA-dependent phosphorylation

Male infertility is a global problem in modern society of which capacitating defects are a major cause. Previous studies have demonstrated that Ca²⁺ ionophore A23187 can make mouse sperm capable of fertilizing in vitro, which may aid in clinical treatment of capacitating defects. However, the detailed role and mechanism of Ca²⁺ in the capacitating process are still unclear especially how A23187 quickly renders sperm immotile and inhibits cAMP/PKA-mediated phosphorylation. We report that A23187 induces a Ca²⁺ flux in the mitochondria enriched sperm tail and excess Ca²⁺ inhibits key metabolic enzymes involved in acetyl-CoA biosynthesis, TCA cycle and electron transport chain pathways resulting in reduced ATP and overall energy production, however this flux does not destroy the structure of the sperm tail. Due to the decrease in ATP production, which is the main phosphate group donator and the power of sperm, the sperm is rendered immobile and PKA-mediated phosphorylation is inhibited. Our study proposed a possible mechanism through which A23187 reduces sperm motility and PKA-mediated phosphorylation from ATP generation, thus providing basic data for exploring the functional roles of Ca²⁺ in sperm in the future.

Integrative Analysis of miRNA and mRNA Expression Profiles Associated With Human Atrial Aging

Background

Limited findings have been reported to systematically study miRNA and mRNA expression profiles in aged human atria. In this study, we aimed to identify miRNAs, genes, and miRNA-mRNA interaction networks for human atrial aging (AA).

Methods

Right atrial appendages from twelve patients who received aortic valve replacement were subjected to miRNA-seq and RNA-seq. All the patients were in sinus rhythm (SR) and stratified by age into four groups. Differential expression analysis was carried out to identify miRNAs and genes for human AA. The miRNA-mRNA interactions for human AA were identified by Pearson correlation analysis and miRNA target prediction programs.

Results

Seven miRNAs (4 upregulation and 3 downregulation) and 42 genes (23 upregulation and 19 downregulation) were differentially expressed in human right atrial tissues between older samples and younger samples. Bioinformatic analysis identified 114 pairs of putative miRNA-mRNA interactions on AA and four types of correlation. Pathway enrichment analysis identified over 40 significant pathways and the top three pathways included rhythmic process (P = 7.5 × 10^–5, Q = 0.034), senescence and autophagy in cancer (P = 9.0 × 10^–5, Q = 0.034), and positive regulation of cytokine biosynthetic process (P = 1.1 × 10^–4, Q = 0.034).

Conclusion

Our study revealed novel miRNA-mRNA interaction networks and signaling pathways for AA, providing novel insights into the development of human AA. Future studies are needed to investigate the potential significance of these miRNA-mRNA interactions in human AA or AA-related cardiovascular diseases.

Differentially expressed genes for atrial fibrillation identified by RNA sequencing from paired human left and right atrial appendages

Atrial fibrillation is a significant worldwide contributor to cardiovascular morbidity and mortality. Few studies have investigated the differences in gene expression between the left and right atrial appendages, leaving their characterization largely unexplored. In this study, differential gene expression was investigated in atrial fibrillation and sinus rhythm using left and right atrial appendages from the same patients. RNA sequencing was performed on the left and right atrial appendages from five sinus rhythm (SR) control patients and five permanent AF case patients. Differential gene expression in both the left and right atrial appendages was analyzed using the Bioconductor package edgeR. A selection of differentially expressed genes, with relevance to atrial fibrillation, were further validated using quantitative RT-PCR. The distribution of the samples assessed through principal component analysis showed distinct grouping between left and right atrial appendages and between SR controls and AF cases. Overall 157 differentially expressed genes were identified to be downregulated and 90 genes upregulated in AF. Pathway enrichment analysis indicated a greater involvement of left atrial genes in the Wnt signaling pathway whereas right atrial genes were involved in clathrin-coated vesicle and collagen formation. The differing expression of genes in both left and right atrial appendages indicate that there are different mechanisms for development, support and remodeling of AF within the left and right atria.

Altered Intracellular Calcium Homeostasis and Arrhythmogenesis in the Aged Heart

Aging of the heart is associated with a blunted response to sympathetic stimulation, reduced contractility, and increased propensity for arrhythmias, with the risk of sudden cardiac death significantly increased in the elderly population. The altered cardiac structural and functional phenotype, as well as age-associated prevalent comorbidities including hypertension and atherosclerosis, predispose the heart to atrial fibrillation, heart failure, and ventricular tachyarrhythmias. At the cellular level, perturbations in mitochondrial function, excitation-contraction coupling, and calcium homeostasis contribute to this electrical and contractile dysfunction. Major determinants of cardiac contractility are the intracellular release of Ca²⁺ from the sarcoplasmic reticulum by the ryanodine receptors (RyR2), and the following sequestration of Ca²⁺ by the sarco/endoplasmic Ca²⁺-ATPase (SERCa2a). Activity of RyR2 and SERCa2a in myocytes is not only dependent on expression levels and interacting accessory proteins, but on fine-tuned regulation via post-translational modifications. In this paper, we review how aberrant changes in intracellular Ca²⁺ cycling via these proteins contributes to arrhythmogenesis in the aged heart.

Increased Ca buffering underpins remodelling of Ca2+ handling in old sheep atrial myocytes

KEY POINTS

Ageing is associated with an increased risk of cardiovascular disease and arrhythmias, with the most common arrhythmia being found in the atria of the heart. Little is known about how the normal atria of the heart remodel with age and thus why dysfunction might occur. We report alterations to the atrial systolic Ca2+ transient that have implications for the function of the atrial in the elderly. We describe a novel mechanism by which increased Ca buffering can account for changes to systolic Ca2+ in the old atria. The present study helps us to understand how the processes regulating atrial contraction are remodelled during ageing and provides a basis for future work aiming to understand why dysfunction develops.

ABSTRACT

Many cardiovascular diseases, including those affecting the atria, are associated with advancing age. Arrhythmias, including those in the atria, can arise as a result of electrical remodelling or alterations in Ca2+ homeostasis. In the atria, age-associated changes in the action potential have been documented. However, little is known about remodelling of intracellular Ca2+ homeostasis in the healthy aged atria. Using single atrial myocytes from young and old Welsh Mountain sheep, we show the free Ca2+ transient amplitude and rate of decay of systolic Ca2+ decrease with age, whereas sarcoplasmic reticulum (SR) Ca content increases. An increase in intracellular Ca buffering explains both the decrease in Ca2+ transient amplitude and decay kinetics in the absence of any change in sarcoendoplasmic reticulum calcium transport ATPase function. Ageing maintained the integrated Ca2+ influx via ICa-L but decreased peak ICa-L . Decreased peak ICa-L was found to be responsible for the age-associated increase in SR Ca content but not the decrease in Ca2+ transient amplitude. Instead, decreased peak ICa-L offsets increased SR load such that Ca2+ release from the SR was maintained during ageing. The results of the present study highlight a novel mechanism by which increased Ca buffering decreases systolic Ca2+ in old atria. Furthermore, for the first time, we have shown that SR Ca content is increased in old atrial myocytes.

Age-dependent electrocardiographic and echocardiographic changes in German Shepherd dogs

By causing numerous changes in the cardiovascular system, ageing leads to a decreased threshold for clinical manifestation of heart disease. The aim of this study was to define the existence of cardiac structural and functional changes in healthy dogs of different age. Radiographic, electrocardiographic (ECG) and echocardiographic examinations of 20 clinically healthy German Shepherd dogs were performed in order to define the values of relevant parameters. Afterwards, the values of cardio examinations were compared between young and old dogs and statistically analyzed. The ECG recordings did not show the appearance of clinically significant arrhythmias, nor was their appearance significantly different between dogs of different age. Statistically significant differences in QRS duration (P<0.05) and R wave amplitude (P<0.05) existed between groups, but all values were within the range of ECG reference values for healthy dogs. Concerning structural changes, the left ventricle wall thickness at end-diastole (LVWd) and end-systole (LVWs), and the relative wall thickness (RWT) between young and old dogs differed significantly (P<0.001, P<0.05, P<0.05, respectively). These differences in heart structure were not accompanied by systolic dysfunction, estimated by a left ventricle shortening fraction. The diastolic wall stress index (WSID) was significantly decreased in old dogs (P<0.05). Young and old dogs showed similar electrical and systolic function. Old dogs had different cardiac structure compared to the young dogs, which could result in diastolic function change.

Nkx2.5/CARP signaling pathway contributes to the regulation of ion channel remodeling induced by rapid pacing in rat atrial myocytes

Remodeling of atrial electrophysiology and structure is the primary feature of atrial fibrillation (AF). Evidence suggests that abnormalities in the expression levels of embryological cardiovascular development‑associated transcription factors, including Nkx2.5, are crucial for the development of AF. Rat atrial myocardial cells (AMCs) in culture dishes were placed in an electric field and stimulated. Transmission electron microscopy was used to observe the ultrastucture prior to and following rapid pacing. The action potential durations and effective refractory periods were measured. RT‑PCR and western blotting were performed to investigate the effect of rapid pacing on the expression levels of ion channel and nuclear proteins in AMCs. Nkx2.5 short interfering RNA (siRNA) transfection was performed. Through this in vitro rat AMC culture and rapid pacing model, it was demonstrated that rapid pacing shortened the action potential and downregulated the expression levels of L‑type calcium and potassium channels. Expression levels of Nkx2.5 and cardiac ankyrin repeat protein (CARP) were significantly upregulated by rapid pacing at the mRNA and protein levels. siRNA‑mediated Nkx2.5 silencing attenuated the rapid pacing‑induced downreglation of ion channel levels. These results suggest that the Nkx2.5/CARP signaling pathway contributes to the early electrical remodeling process of AF.