Reduced density and altered regulation of rat atrial L-type Ca2+ current in heart failure

HIF1α/MIF/CD74 signaling mediated OSA-induced atrial fibrillation by promoting M1 macrophages polarization

BACKGROUND

Obstructive sleep apnea (OSA) is known to contribute to the occurrence and recurrence of atrial fibrillation (AF). However, the mechanism remains unknown.

METHODS

Chronic OSA rat model was established to elucidate the role of macrophages in OSA-induced AF. Moreover, to reveal the mechanisms underlying the abnormal polarization of macrophages induced by chronic OSA, co-culture cell model of macrophages and atrial myocytes was created.

RESULTS

Chronic OSA altered the pathological phenotype of atrial myocardial tissues, rendering it more susceptible to AF. Furthermore, chronic OSA promoted the polarization of M1 macrophages in the atrial tissue, and the AF susceptibility induced by chronic OSA was reversed upon clearance of macrophages. Then, we found that macrophages induced an atrial fibrillation-like phenotype in atrial myocytes, while atrial myocytes promoted M1 polarization of macrophages, under hypoxia/reoxygenation treatment. Moreover, hypoxia/reoxygenation upregulated the expression of hypoxia-inducible factor 1-α (HIF1α) in atrial myocytes, which subsequently promoted the expression of macrophage migration inhibitory factor (MIF) by binding to the promoter region. The increased expression of MIF further activated the expression of nuclear factor-kappa B (NF-κB) through interaction with CD74, ultimately leading to M1 macrophages polarization.

CONCLUSIONS

Increased polarization of M1-type macrophages was involved in the increased susceptibility to AF induced by OSA. In mechanism, OSA increased MIF expression by HIF1α in atrial myocytes. Then, MIF activated NF-κB expression by CD74 in macrophages, consequently driving the polarization of M1-type macrophages. Finally, M1 macrophages exacerbated atrial remodeling through the secretion of inflammatory cytokines, which contributed to the increased susceptibility to AF.

Mechanisms underlying pathological Ca2+ handling in diseases of the heart

Cardiomyocyte contraction relies on precisely regulated intracellular Ca²⁺ signaling through various Ca²⁺ channels and transporters. In this article, we will review the physiological regulation of Ca²⁺ handling and its role in maintaining normal cardiac rhythm and contractility. We discuss how inherited variants or acquired defects in Ca²⁺ channel subunits contribute to the development or progression of diseases of the heart. Moreover, we highlight recent insights into the role of protein phosphatase subunits and striated muscle preferentially expressed protein kinase (SPEG) in atrial fibrillation, heart failure, and cardiomyopathies. Finally, this review summarizes current drug therapies and new advances in genome editing as therapeutic strategies for the cardiac diseases caused by aberrant intracellular Ca²⁺ signaling.

Influence of mitochondrial and systemic iron levels in heart failure pathology

Iron deficiency or overload poses an increasingly complex issue in cardiovascular disease, especially heart failure. The potential benefits and side effects of iron supplementation are still a matter of concern, even though current guidelines suggest therapeutic management of iron deficiency. In this review, we sought to examine the iron metabolism and to identify the rationale behind iron supplementation and iron chelation. Cardiovascular disease is increasingly linked with iron dysmetabolism, with an increased proportion of heart failure patients being affected by decreased plasma iron levels and in turn, by the decreased quality of life. Multiple studies have concluded on a benefit of iron administration, even if just for symptomatic relief. However, new studies field evidence for negative effects of dysregulated non-bound iron and its reactive oxygen species production, with concern to heart diseases. The molecular targets of iron usage, such as the mitochondria, are prone to deleterious effects of the polyvalent metal, added by the scarcely described processes of iron elimination. Iron supplementation and iron chelation show promise of therapeutic benefit in heart failure, with the extent and mechanisms of both prospects not being entirely understood. It may be that a state of decreased systemic and increased mitochondrial iron levels proves to be a useful frame for future advancements in understanding the interconnection of heart failure and iron metabolism.

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.

Ion currents, action potentials, and noradrenergic responses in rat pulmonary vein and left atrial cardiomyocytes

The electrophysiological properties of pulmonary vein (PV)-cardiomyocytes, and their responses to the sympathetic neurotransmitter, noradrenaline (NA), are thought to differ from those of the left atrium (LA) and contribute to atrial ectopy. The aim of this study was to examine rat PV cardiomyocyte electrophysiology and responses to NA in comparison with LA cells. LA and PV cardiomyocytes were isolated from adult male Wistar rat hearts, and membrane potentials and ion currents recorded at 36°C using whole-cell patch-clamp techniques. PV and LA cardiomyocytes did not differ in size. In control, there were no differences between the two cell-types in zero-current potential or action potential duration (APD) at 1 Hz, although the incidence of early afterdepolarizations (EADs) was greater in PV than LA cardiomyocytes. The L-type Ca2+ current (ICaL ) was ~×1.5 smaller (p = .0029, Student's t test) and the steady-state K+ current (IKss ) was ~×1.4 larger (p = .0028, Student's t test) in PV than in LA cardiomyocytes. PV cardiomyocyte inward-rectifier current (IK1 ) was slightly smaller than LA cardiomyocyte IK1 . In LA cardiomyocytes, NA significantly prolonged APD30 . In PV cells, APD30 responses to 1 μM NA were heterogeneous: while the mean percentage change in APD30 was not different from 0 (16.5 ± 9.7%, n cells/N animals = 12/10, p = .1177, one-sample t test), three cells showed shortening (-18.8 ± 6.0%) whereas nine showed prolongation (28.3 ± 10.1%, p = .008, Student's t test). NA had no effect on IK1 in either cell-type but inhibited PV IKss by 41.9 ± 4.1% (n/N = 23/11 p < .0001), similar to LA cells. NA increased ICaL in most PV cardiomyocytes (median × 2.2-increase, p < .0001, n/N = 32/14, Wilcoxon-signed-rank test), although in 7/32 PV cells ICaL was decreased following NA. PV cardiomyocytes differ from LA cells and respond heterogeneously to NA.

Atrial fibrillation promotion in a rat model of heart failure induced by left ventricle radiofrequency ablation

Background

Atrial fibrillation (AF) frequently coexists with congestive heart failure (CHF). The increased susceptibility to AF in CHF has been attributed to a variety of structural and electrophysiological changes in the atria, particularly dilation and interstitial fibrosis. We evaluated atrial remodeling and AF vulnerability in a rat model of CHF induced by left ventricle (LV) radiofrequency (RF) ablation.

Methods

Wistar rats were divided into 3 groups: RF-induced CHF (Ab, n = 36), CHF animals treated with spironolactone (AbSpi, n = 20) and sham controls (Sham, n = 29). After 12 weeks, animals underwent echocardiographic and electrophysiological evaluation and were sacrificed for histological (atrial fibrosis) and Western blotting (TGF-β1, collagen I/III, connexin 43 and Ca_V1.2) analysis.

Results

Mild LV dysfunction and marked atrial enlargement were noted in both ablated groups. AF inducibility (episodes ≥2 s) increased in the Ab group compared to sham animals (31/36, 86%; vs. 15/29, 52%; p = 0.005), but did not differ from the AbSpi group (16/20, 80%; p = NS). Sustained AF (>30 s) was also more frequent in the Ab group compared to shams (56% vs. 28%; p = 0.04). Spironolactone reduced atrial fibrosis (p < 0.01) as well as TGF-β1 (p < 0.01) and collagen I/III (p < 0.01) expression but did not affect connexin 43 and Ca_V1.2 expression.

Conclusions

Rats with RF-induced CHF exhibit pronounced atrial structural remodeling and enhanced AF vulnerability. This model may be useful for studying AF substrate in CHF.

Statistical considerations in reporting cardiovascular research

The problem of inadequate statistical reporting is long standing and widespread in the biomedical literature, including in cardiovascular physiology. Although guidelines for reporting statistics have been available in clinical medicine for some time, there are currently no guidelines specific to cardiovascular physiology. To assess the need for guidelines, we determined the type and frequency of statistical tests and procedures currently used in the American Journal of Physiology-Heart and Circulatory Physiology. A PubMed search for articles published in the American Journal of Physiology-Heart and Circulatory Physiology between January 1, 2017, and October 6, 2017, provided a final sample of 146 articles evaluated for methods used and 38 articles for indepth analysis. The t-test and ANOVA accounted for 71% (212 of 300 articles) of the statistical tests performed. Of six categories of post hoc tests, Bonferroni and Tukey tests were used in 63% (62 of 98 articles). There was an overall lack in details provided by authors publishing in the American Journal of Physiology-Heart and Circulatory Physiology, and we compiled a list of recommended minimum reporting guidelines to aid authors in preparing manuscripts. Following these guidelines could substantially improve the quality of statistical reports and enhance data rigor and reproducibility.

Loss of caveolin-3-dependent regulation of ICa in rat ventricular myocytes in heart failure

β₂-Adrenoceptors and L-type Ca²⁺ current (I_Ca) redistribute from the t-tubules to the surface membrane of ventricular myocytes from failing hearts. The present study investigated the role of changes in caveolin-3 and PKA signaling, both of which have previously been implicated in this redistribution. I_Ca was recorded using the whole cell patch-clamp technique from ventricular myocytes isolated from the hearts of rats that had undergone either coronary artery ligation (CAL) or equivalent sham operation 18 wk earlier. I_Ca distribution between the surface and t-tubule membranes was determined using formamide-induced detubulation (DT). In sham myocytes, β₂-adrenoceptor stimulation increased I_Ca in intact but not DT myocytes; however, forskolin (to increase cAMP directly) and H-89 (to inhibit PKA) increased and decreased, respectively, I_Ca at both the surface and t-tubule membranes. C3SD peptide (which decreases binding to caveolin-3) inhibited I_Ca in intact but not DT myocytes but had no effect in the presence of H-89. In contrast, in CAL myocytes, β₂-adrenoceptor stimulation increased I_Ca in both intact and DT myocytes, but C3SD had no effect on I_Ca; forskolin and H-89 had similar effects as in sham myocytes. These data show the redistribution of β₂-adrenoceptor activity and I_Ca in CAL myocytes and suggest constitutive stimulation of I_Ca by PKA in sham myocytes via concurrent caveolin-3-dependent (at the t-tubules) and caveolin-3-independent mechanisms, with the former being lost in CAL myocytes.

NEW & NOTEWORTHY In ventricular myocytes from normal hearts, regulation of the L-type Ca²⁺ current by β₂-adrenoceptors and the constitutive regulation by caveolin-3 is localized to the t-tubules. In heart failure, the regulation of L-type Ca²⁺ current by β₂-adrenoceptors is redistributed to the surface membrane, and the constitutive regulation by caveolin-3 is lost.