Age-related changes in electrophysiological responses to muscarinic receptor stimulation in rat myocardium

Aging and atrial fibrillation: A vicious circle

Atrial fibrillation (AF) is the commonest sustained cardiac arrhythmia observed in clinical practice. Its prevalence increases dramatically with advancing age. This review article discusses the recent advances in studies investigating the relationship between aging and AF and the possible underlying mechanisms.

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.

Age as a Critical Determinant of Atrial Fibrillation: A Two-sided Relationship

The incidence of atrial fibrillation (AF), the most common sustained arrhythmia and a major public health burden, increases exponentially with age. However, mechanisms underlying this long-recognized association remain incompletely understood. Experimental and human studies have demonstrated the involvement of aging in several arrhythmogenic processes, including atrial electrical and structural remodelling, disturbed calcium homeostasis, and enhanced atrial ectopic activity/increased vulnerability to re-entry induction. Given this wide range of putative mechanisms, the task of delineating the specific effects of aging responsible for AF promotion is not simple, as aging is itself associated with increasing prevalence of a host of AF-predisposing conditions, including heart failure, coronary artery disease, and hypertension. Although we usually think of old age promoting AF, there is also evidence that young age may actually have a protective effect against AF occurrence. For example, the low AF incidence among populations of young patients with significant structural congenital heart disease and substantial atrial enlargement/remodelling suggests that younger age might protect against fibrillation in the diseased atrium; efforts at understating how younger age may prevent AF might be helpful in elucidating missing mechanistic links between AF and age. The goal of this paper is to review the epidemiologic and pathophysiologic evidence regarding mechanisms underlying age-related AF. Although the therapeutic options for AF have recently improved, major gaps still remain and a better understanding of the special relationship between age and AF may be important for the identification of new targets for therapeutic innovation.

Altered expression of hyperpolarization-activated cyclic nucleotide-gated channels and microRNA-1 and -133 in patients with age-associated atrial fibrillation

Hyperpolarization-activated cyclic nucleotide-gated (HCN) cation channels mediate pacemaker currents in the atrium. The microRNA (miR) families miR-1 and miR-133 regulate the expression of multiple genes involved in myocardial function, including HCN channels. It was hypothesized that age‑dependent changes in HCN2, HCN4, miR‑1 and miR‑133 expression may contribute to age‑associated atrial fibrillation, and therefore the correlation between expression levels, among adult (≤65 years) and aged patients (≥65 years), and sinus rhythm was determined. Right atrial appendage samples were collected from 60 patients undergoing coronary artery bypass grafting. Reverse transcription-quantitative polymerase chain reaction (PCR) and western blot analyses were performed in order to determine target RNA and protein expression levels. Compared with aged patients with sinus rhythm, aged patients with atrial fibrillation exhibited significantly higher HCN2 and HCN4 channel mRNA and protein expression levels (P<0.05), but significantly lower expression levels of miR‑1 and miR‑133 (P<0.05). In addition, aged patients with sinus rhythm exhibited significantly higher expression levels of HCN2 and HCN4 channel mRNA and protein (P<0.05), but significantly lower expression levels of miR‑1 and ‑133 (P<0.05), compared with those of adult patients with sinus rhythm. Expression levels of HCN2 and HCN4 increased with age, and a greater increase was identified in patients with age‑associated atrial fibrillation compared with that in those with aged sinus rhythm. These electrophysiological changes may contribute to the induction of ectopic premature beats that trigger atrial fibrillation.

Comparison of Atrial Fibrillation in the Young versus That in the Elderly: A Review

The incidence and prevalence of atrial fibrillation (AF) are projected to increase significantly worldwide, imposing a significant burden on healthcare resources. The disease itself is extremely heterogeneous in its epidemiology, pathophysiology, and treatment options based on individual patient characteristics. Whilst ageing is well recognised to be an independent risk factor for the development of AF, this condition also affects the young in whom the condition is frequently symptomatic and troublesome. Traditional thinking suggests that the causal factors and pathogenesis of the condition in the young with structurally normal atria but electrophysiological "triggers" in the form of pulmonary vein ectopics leading to lone AF are in stark contrast to that in the elderly who have AF primarily due to an abnormal substrate consisting of fibrosed and dilated atria acting in concert with the pulmonary vein triggers. However, there can be exceptions to this rule as there is increasing evidence of structural and electrophysiological abnormalities in the atrial substrate in young patients with "lone AF," as well as elderly patients who present with idiopathic AF. These reports seem to be blurring the distinction in the pathophysiology of so-called idiopathic lone AF in the young versus that in the elderly. Moreover with availability of improved and modern investigational and diagnostic techniques, novel causes of AF are being reported thereby seemingly consigning the diagnosis of "lone AF" to a rather mythical existence. We shall also elucidate in this paper the differences seen in the epidemiology, causes, pathogenesis, and clinical features of AF in the young versus that seen in the elderly, thereby requiring clearly defined management strategies to tackle this arrhythmia and its associated consequences.

Basic Mechanisms Mediating Cardiomyopathy and Heart Failure in Aging

Biological aging represents the major risk factor for the development of heart failure (HF), malignancies, and neurodegenerative diseases. While risk factors such as lifestyle patterns, genetic traits, blood lipid levels, and diabetes can contribute to its development, advancing age remains the most determinant predictor of cardiac disease. Several parameters of left ventricular function may be affected with aging, including increased duration of systole, decreased sympathetic stimulation, and increased left ventricle ejection time, while compliance decreases. In addition, changes in cardiac phenotype with diastolic dysfunction, reduced contractility, left ventricular hypertrophy, and HF, all increase in incidence with age. Given the limited capacity that the heart has for regeneration, reversing or slowing the progression of these abnormalities poses a major challenge. In this chapter, we present a discussion on the molecular and cellular mechanisms involved in the pathogenesis of cardiomyopathies and HF in aging and the potential involvement of specific genes identified as primary mediators of these diseases.

Aged atria: electrical remodeling conducive to atrial fibrillation

The incidence of atrial fibrillation (AF) increases with age. Alterations in structure and function of atrial ion channels associated with aging provide the substrate for AF. In this review we provide an overview of current knowledge regarding these age-related changes in atria, focusing on intrinsic ion channel function, impulse initiation and conduction. Studies on the action potentials (APs) of atria have shown that the AP contour is altered with age and the dispersion of AP parameters is increased with age. However, studies using human tissues are not completely consistent with experimental animal studies, since specimens from humans have been obtained from hearts with concomitant cardiovascular diseases and/or that are under the influence of pharmacologic agents. Ionic current studies show that while there are no age-related changes in sodium currents in atrial tissue, the calcium current is reduced and the transient outward and sustained potassium currents are increased in aged cells. While sinoatrial node firing is reduced with age, enhanced impulse initiation may occur in aged atrial cells, for example in the pulmonary veins and coronary sinus. Fibrous tissue is increased in aged atria, which is associated with an increased likelihood of abnormal electrical conduction. Thus, age-related AF involves alterations in the substrate as well as in the passive properties of aged atria.

Increased expression of Gi-coupled muscarinic acetylcholine receptor and Gi in atrium of elderly diabetic subjects

In an ongoing investigation of the effects of age on G protein-coupled receptor signaling in human atrial tissue, we have found that the density of atrial muscarinic acetylcholine receptor (mAChR) increases with age but reaches statistical significance only in patients with diabetes. Moreover, we find that in elderly subjects of similar ages, those with diabetes have 1.7-fold higher levels of Galpha(i2) and twofold higher levels of Gbeta(1). Diabetes does not affect other atrial G proteins, including Galpha(i3,) Galpha(s), Galpha(o), and Gbeta(2). These data represent the first demonstration of an increase in a G(i)-coupled receptor, Galpha(i2), and Gbeta(1), in atrium of patients with diabetes. These findings suggest a molecular explanation for the increased risk of cardiac disease in patients with diabetes, because increased signaling through G(i) has been shown to lead to the development of dilated cardiomyopathy.