lipopolysaccharide induces atrial arrhythmogenesis via down-regulation of L-type Ca2+ channel genes in rats

Metabolic remodelling in atrial fibrillation: manifestations, mechanisms and clinical implications

Atrial fibrillation (AF) is a continually growing health-care burden that often presents together with metabolic disorders, including diabetes mellitus and obesity. Current treatments often fall short of preventing AF and its adverse outcomes. Accumulating evidence suggests that metabolic disturbances can promote the development of AF through structural and electrophysiological remodelling, but the underlying mechanisms that predispose an individual to AF are aetiology-dependent, thus emphasizing the need for tailored therapeutic strategies to treat AF that target an individual's metabolic profile. AF itself can induce changes in glucose, lipid and ketone metabolism, mitochondrial function and myofibrillar energetics (as part of a process referred to as 'metabolic remodelling'), which can all contribute to atrial dysfunction. In this Review, we discuss our current understanding of AF in the setting of metabolic disorders, as well as changes in atrial metabolism that are relevant to the development of AF. We also describe the potential of available and emerging treatment strategies to target metabolic remodelling in the setting of AF and highlight key questions and challenges that need to be addressed to improve outcomes in these patients.

Association of lipopolysaccharide with new-onset atrial fibrillation in ST-segment elevation myocardial infarction

Background

Lipopolysaccharide (LPS) is related to various cardiovascular diseases. However, the relationship between LPS and new-onset atrial fibrillation (NOAF) after ST-segment elevation myocardial infarction (STEMI) has yet to be elucidated. This study aimed to evaluate the impact of LPS on NOAF in STEMI patients.

Methods

This was a single-center retrospective observational study including 806 patients diagnosed with STEMI. LPS levels were determined using a commercial ELISA kit. NOAF was characterized by postadmission AF with the absence of any prior history of AF.

Results

A total of 806 participants were enrolled, with 752 individuals in the non-AF group (93.3%) and 54 individuals in the AF group (6.7%). Multivariable analysis showed that LPS (OR = 1.047; 95% CI: 1.029-1.065, P < 0.001) was an independent risk marker for NOAF. The analysis of the ROC demonstrated that LPS had an AUC of 0.717 in predicting NOAF. When LPS was added to the conventional model, the ability of the risk model to discriminate and reclassify NOAF was improved significantly (IDI 0.053, P = 0.001; NRI 0.510, P < 0.001).

Conclusion

Elevated LPS is associated with an increased risk of NOAF in STEMI patients. The integration of LPS can improve the ability to predict NOAF in STEMI patients.

The relationship between atrial fibrillation and NLRP3 inflammasome: a gut microbiota perspective

Atrial fibrillation (AF) is a common clinical arrhythmia whose pathogenesis has not been fully elucidated, and the inflammatory response plays an important role in the development of AF. The inflammasome is an important component of innate immunity and is involved in a variety of pathophysiologic processes. The NLRP3 inflammasome is by far the best studied and validated inflammasome that recognizes multiple pathogens through pattern recognition receptors of innate immunity and mediates inflammatory responses through activation of Caspase-1. Several studies have shown that NLRP3 inflammasome activation contributes to the onset and development of AF. Ecological dysregulation of the gut microbiota has been associated with the development of AF, and some evidence suggests that gut microbiota components, functional byproducts, or metabolites may induce or exacerbate the development of AF by directly or indirectly modulating the NLRP3 inflammasome. In this review, we report on the interconnection of NLRP3 inflammasomes and gut microbiota and whether this association is related to the onset and persistence of AF. We discuss the potential value of pharmacological and dietary induction in the management of AF in the context of the association between the NLRP3 inflammasome and gut microbiota. It is hoped that this review will lead to new therapeutic targets for the future management of AF.

Gut microbiome relationship with arrhythmias and conduction blocks: A two-sample Mendelian randomization study

INTRODUCTION

Past research based on observations has suggested that the gut microbiome (GM) could play a role in developing arrhythmias and conduction blocks. Nonetheless, the nature of this association remains uncertain due to the potential for reverse causation and confounding factors in observational research. The aim of this investigation is to elucidate the causal relationship between GM and the development of arrhythmias as well as conduction blocks.

METHODS

This study collected summary statistics regarding GM, arrhythmias, and conduction blocks. Two-sample Mendelian randomization (MR) analysis was carried out employing various methods, with inverse variance weighted being the primary approach, followed by weighted median, simple mode, MR-Egger, and MR-PRESSO. Moreover, the MR findings were corroborated through multiple sensitivity analyses.

RESULTS

Among them, for atrial fibrillation and flutter (AF), phylum_Actinobacteria and genus_RuminococcaceaeUCG004 demonstrated a negative correlation, while order_Pasteurellales, family_Pasteurellaceae, and genus_Turicibacter were associated with an increased risk. In the case of paroxysmal tachycardia (PT), genus_Holdemania and genus_Roseburia were found to reduce risk. For atrioventricular block (AVB), order_Bifidobacteriales, family_Bifidobacteriaceae, and genus_Alistipes exhibited a negative correlation, whereas genus_CandidatusSoleaferrea showed a positive correlation. Concerning the left bundle-branch block (LBBB), family_Peptococcaceae appeared to decrease the risk, while genus_Flavonifractor was linked to an increased risk. Lastly, no causative GM was identified in the right bundle-branch block (RBBB) context.

CONCLUSION

We have uncovered potential causal links between some GM, arrhythmias, and conduction blocks. This insight may aid in designing microbiome-based interventions for these conditions and their risk factors in future trials. Additionally, it could facilitate the discovery of novel biomarkers for targeted prevention strategies.

Gut microbiota and cardiac arrhythmia

One of the most prevalent cardiac diseases is cardiac arrhythmia, however the underlying causes are not entirely understood. There is a lot of proof that gut microbiota (GM) and its metabolites have a significant impact on cardiovascular health. In recent decades, intricate impacts of GM on cardiac arrythmia have been identified as prospective approaches for its prevention, development, treatment, and prognosis. In this review, we discuss about how GM and its metabolites might impact cardiac arrhythmia through a variety of mechanisms. We proposed to explore the relationship between the metabolites produced by GM dysbiosis including short-chain fatty acids(SCFA), Indoxyl sulfate(IS), trimethylamine N-oxide(TMAO), lipopolysaccharides(LPS), phenylacetylglutamine(PAGln), bile acids(BA), and the currently recognized mechanisms of cardiac arrhythmias including structural remodeling, electrophysiological remodeling, abnormal nervous system regulation and other disease associated with cardiac arrythmia, detailing the processes involving immune regulation, inflammation, and different types of programmed cell death etc., which presents a key aspect of the microbial-host cross-talk. In addition, how GM and its metabolites differ and change in atrial arrhythmias and ventricular arrhythmias populations compared with healthy people are also summarized. Then we introduced potential therapeutic strategies including probiotics and prebiotics, fecal microbiota transplantation (FMT) and immunomodulator etc. In conclusion, the GM has a significant impact on cardiac arrhythmia through a variety of mechanisms, offering a wide range of possible treatment options. The discovery of therapeutic interventions that reduce the risk of cardiac arrhythmia by altering GM and metabolites is a real challenge that lies ahead.

Gut Microbiota and Atrial Fibrillation: Pathogenesis, Mechanisms and Therapies

Over the past decade there has been an interest in understanding the role of gut microbiota in the pathogenesis of AF. A number of studies have linked the gut microbiota to the occurrence of traditional AF risk factors such as hypertension and obesity. However, it remains unclear whether gut dysbiosis has a direct effect on arrhythmogenesis in AF. This article describes the current understanding of the effect of gut dysbiosis and associated metabolites on AF. In addition, current therapeutic strategies and future directions are discussed.

Intestinal Barrier Dysfunction and Microbial Translocation in Patients with First-Diagnosed Atrial Fibrillation

BACKGROUND

According to the leaky gut concept, microbial products (e.g., lipopolysaccharide, LPS) enter the circulation and mediate pro-inflammatory immunological responses. Higher plasma LPS levels have been reported in patients with various cardiovascular diseases, but not specifically during early atrial fibrillation (AF).

METHODS

We studied data and blood samples from patients presenting with first-diagnosed AF (FDAF) (n = 80) and 20 controls.

RESULTS

Circulating biomarkers that are suggestive of mucosal inflammation (zonulin, mucosal adhesion molecule MAdCAM-1) and intestinal epithelium damage (intestinal fatty acid binding protein, IFABP) were increased in the plasma of patients with FDAF when compared to patients with chronic cardiovascular diseases but without AF. Surrogate plasma markers of increased intestinal permeability (LPS, CD14, LPS-binding protein, gut-derived LPS-neutralising IgA antibodies, EndoCAbs) were detected during early AF. A reduced ratio of IgG/IgM EndoCAbs titres indicated chronic endotoxaemia. Collagen turnover biomarkers, which corresponded to the LPS values, suggested an association of gut-derived low-grade endotoxaemia with adverse structural remodelling. The LPS concentrations were higher in FDAF patients who experienced a major adverse cardiovascular event.

CONCLUSIONS

Intestinal barrier dysfunction and microbial translocation accompany FDAF. Improving gut permeability and low-grade endotoxaemia might be a potential therapeutic approach to reducing the disease progression and cardiovascular complications in FDAF.

Gut microbiota and cardiac arrhythmia: a pharmacokinetic scope

BACKGROUND

Dealing with cardiac arrhythmia is a difficult challenge. Choosing between different anti-arrhythmic drugs (AADs) while being cautious about the pro-arrhythmic characteristics of some of these drugs and their diverse interaction with other drugs is a real obstacle.

MAIN BODY

Gut microbiota (GM), in our bodies, are now being considered as a hidden organ which can regulate our immune system, digest complex food, and secrete bioactive compounds. Yet, GM are encountered in the pathophysiology of arrhythmia and can affect the pharmacokinetics of AADs, as well as some anti-thrombotics, resulting in altering their bioavailability, therapeutic function and may predispose to some of their unpleasant adverse effects.

CONCLUSIONS

Knowledge of the exact role of GM in the pharmacokinetics of these drugs is now essential for better understanding of the art of arrhythmia management. Also, it will help deciding when to consider probiotics as an adjunctive therapy while treating arrhythmia. This should be discovered in the near future.

Serum Lipopolysaccharide Is Associated with the Recurrence of Atrial Fibrillation after Radiofrequency Ablation by Increasing Systemic Inflammation and Atrial Fibrosis

Objectives

The gut microbiota and its metabolites are linked to inflammation and contribute to the progression of atrial fibrillation (AF), but the predictive value of the gut microbiota-derived metabolite lipopolysaccharide (LPS) for AF recurrence (RAF) is unknown. This study is aimed at investigating (1) the correlation between LPS and RAF and (2) its relationship with inflammation and atrial fibrosis.

Method

We performed a single-centre retrospective analysis in 159 AF patients. Fasting plasma samples were collected, and an enzyme-linked immunosorbent assay was used to determine the levels of serum LPS, interleukin-6 (IL-6), collagen type-1 C-terminal telopeptide (CITP), and transforming growth factor-β1 (TGFβ1). The cumulative risk for RAF was evaluated with Kaplan–Meier analysis. Cox proportional hazard analysis was carried out to predict the hazard of RAF. The correlations among LPS and IL-6, CITP, TGFβ1, and left atrial diameter (LAD) were analysed by Pearson's correlation coefficient. Subsequent univariate and multivariable linear regression analyses were carried out to evaluate the connection between clinical variables and Log-LPS.

Results

All 159 AF patients were included in this study. The proportion of persistent atrial fibrillation was 40.3%, the mean age was 61.9 ± 10.1 years, the proportion of males was 61.6%, and the mean LPS was 56.5 ± 29.5 pg/mL. After all patients were divided into tertiles according to the circulating LPS level, a total of 44 RAF occurred: 10 in the first tertile, 15 in the second tertile, and 19 in the third tertile (log-rank test P = 0.037). Heart failure (hazard ratio 2.029, P = 0.041), LAD (hazard ratio 1.064, P = 0.022), Log-LPS (hazard ratio 5.686, P = 0.043), and CITP (hazard ratio 6.841, P = 0.033) independently predicted the risk of RAF. In all patients, univariate analysis showed that heart failure, LAD, hs-CRP, IL-6, CITP, and TGF-β1 were connected with Log-LPS. Multivariate linear regression analysis indicated that IL-6 and hs-CRP were independently and positively connected with Log-LPS.

Conclusions

Our results indicated that circulating LPS was a predictor of RAF and may contribute to RAF incidence after ablation by increasing systemic inflammation and atrial fibrosis.

Gut microbiota, dysbiosis and atrial fibrillation. Arrhythmogenic mechanisms and potential clinical implications

Recent preclinical and observational cohort studies have implicated imbalances in gut microbiota composition as a contributor to atrial fibrillation (AF). The gut microbiota is a complex and dynamic ecosystem containing trillions of microorganisms, which produces bioactive metabolites influencing host health and disease development. In addition to host-specific determinants, lifestyle-related factors such as diet and drugs are important determinants of the gut microbiota composition. In this review, we discuss the evidence suggesting a potential bidirectional association between AF and gut microbiota, identifying gut microbiota-derived metabolites as possible regulators of the AF substrate. We summarize the effect of gut microbiota on the development and progression of AF risk-factors, including heart failure, hypertension, obesity and coronary artery disease. We also discuss the potential antiarrhythmic effects of pharmacological and diet-induced modifications of gut microbiota composition, which may modulate and prevent the progression to AF. Finally, we highlight important gaps in knowledge and areas requiring future investigation. Although data supporting a direct relationship between gut microbiota and AF are very limited at the present time, emerging preclinical and clinical research dealing with mechanistic interactions between gut microbiota and AF is important as it may lead to new insights into AF pathophysiology and the discovery of novel therapeutic targets for AF.

Systemic inflammation in patients with Takotsubo syndrome: a review of mechanistic and clinical implications

Over recent decades, systemic inflammation  as  quantified  with  inflammation  markers  or  indices has been extensively  investigated  in the setting of  various  cardiovascular  conditions  including heart failure (HF),  acute coronary syndromes (ACS). In contrast, systemic inflammation  in patients with  takotsubo syndrome (TTS) has been an underrated  phenomenon in clinical practice. On the other hand, experimental and clinical data  have been  rapidly  accumulating  in the recent years  regarding   pathogenetic, prognostic as well as therapeutic implications of  systemic inflammation in TTS.  Accordingly, the present article  aims to provide a general perspective  on mechanistic and  clinical aspects of  systemic  inflammation in the setting of  TTS.

Gut microbial composition in patients with atrial fibrillation: effects of diet and drugs

Atrial fibrillation (AF) reduces the quality of life by triggering stroke and heart failure. The association between AF onset and gut metabolites suggests a causal relationship between AF and gut microbiota dysbiosis; however, the relationship remains poorly understood. We prospectively enrolled 34 hospitalized patients with AF and 66 age-, sex-, and comorbidity-matched control subjects without a history of AF. Gut microbial compositions were evaluated by amplicon sequencing targeting the 16S ribosomal RNA gene. We assessed differences in dietary habits by using a brief-type self-administered diet history questionnaire (BDHQ). Gut microbial richness was lower in AF patients, although the diversity of gut microbiota did not differ between the two groups. At the genus level, Enterobacter was depleted, while Parabacteroides, Lachnoclostridium, Streptococcus, and Alistipes were enriched in AF patients compared to control subjects. The BDHQ revealed that the intake of n-3 polyunsaturated fatty acids and eicosadienoic acid was higher in AF patients. Our results suggested that AF patients had altered gut microbial composition in connection with dietary habits.

Potential Impact of Oral Inflammations on Cardiac Functions and Atrial Fibrillation

Inflammation may be a risk factor for atrial fibrillation (AF). Oral infections frequently lead to chronic inflammation, such as gingivitis, periodontitis, and endodontic lesions. In this narrative review, we consider five basic pathogenic mechanisms that involve oral infections and inflammations in the pathogenesis of AF: (1) low level bacteremia by which oral bacteria enter the blood stream at inflamed sites of the oral cavity and invade the heart; (2) Systemic inflammation induced by inflammatory mediators, which are released from the sites of oral inflammation into the blood stream, affecting cardiac remodeling; (3) autoimmunity against molecular structures expressed in the heart caused by the host immune response to specific components of oral pathogens; (4) potentially arrhythmic effects mediated by activation of the autonomous nervous system triggered by oral inflammations; and (5) arrhythmic effects resulting from specific bacterial toxins that are produced by oral pathogenic bacteria. A number of studies support the involvement of all five mechanisms, suggesting a potentially complex contribution of oral inflammations to the pathogenesis of AF.

Systemic Inflammation and Arrhythmogenesis: A Review of Mechanistic and Clinical Perspectives

In the recent decades, systemic inflammation, as a clinical phenomenon, has been the focus of extensive research particularly with regard to its potential association with a variety of cardiovascular diseases including atherogenesis and acute coronary syndromes. Within this context, there also exists a potential link between systemic inflammation and cardiac arrhythmogenesis in various aspects. Accordingly, systemic inflammation response as measured with inflammation markers (cytokines, etc) has been investigated in the setting of well-known cardiac arrhythmias including atrial fibrillation and ventricular tachycardia. Based on current literature, clinical utility of these markers might potentially yield important prognostic implications in the setting of certain arrhythmogenic conditions. On the other hand, there exists limited data regarding therapeutic implications including clinical benefit of primary anti-inflammatory agents (corticosteroids, colchicine, etc) in the setting of arrhythmia management. The present review primarily aims to discuss potential triggers and fundamental mechanisms of inflammation-related arrhythmias along with a particular emphasis on clinical implications of systemic inflammation in the setting of cardiac arrhythmogenesis.

Excavatolide B Modulates the Electrophysiological Characteristics and Calcium Homeostasis of Atrial Myocytes

Severe bacterial infections caused by sepsis always result in profound physiological changes, including fever, hypotension, arrhythmia, necrosis of tissue, systemic multi-organ dysfunction, and finally death. The lipopolysaccharide (LPS) provokes an inflammatory response under sepsis, which may increase propensity to arrhythmogenesis. Excavatolide B (EXCB) possesses potent anti-inflammatory effects. However, it is not clear whether EXCB could modulate the electrophysiological characteristics and calcium homeostasis of atrial myocytes. This study investigated the effects of EXCB on the atrial myocytes exposed to lipopolysaccharide. A whole-cell patch clamp and indo-1 fluorimetric ratio technique was employed to record the action potential (AP), ionic currents, and intracellular calcium ([Ca²⁺]_i) in single, isolated rabbit left atrial (LA) cardiomyocytes, with and without LPS (1 μg/mL) and LPS + EXCB administration (10 μM) for 6 ± 1 h, in order to investigate the role of EXCB on atrial electrophysiology. In the presence of LPS, EXCB-treated LA myocytes (n = 13) had a longer AP duration at 20% (29 ± 2 vs. 20 ± 2 ms, p < 0.05), 50% (52 ± 4 vs. 40 ± 3 ms, p < 0.05), and 90% (85 ± 5 vs. 68 ± 3 ms, p < 0.05), compared to the LPS-treated cells (n = 12). LPS-treated LA myocytes showed a higher late sodium current, Na⁺/Ca²⁺ exchanger current, transient outward current, and delayed rectifier potassium current, but a lower l-type Ca²⁺ current, than the control LA myocytes. Treatment with EXCB reversed the LPS-induced alterations of the ionic currents. LPS-treated, EXCB-treated, and control LA myocytes exhibited similar Na⁺ currents. In addition, the LPS-treated LA myocytes exhibited a lower [Ca²⁺]_i content and higher sarcoplasmic reticulum calcium content, than the controls. EXCB reversed the LPS-induced calcium alterations. In conclusion, EXCB modulates LPS-induced LA electrophysiological characteristics and calcium homeostasis, which may contribute to attenuating LPS-induced arrhythmogenesis.

Lipopolysaccharide Compromises Human Sperm Function by Reducing Intracellular cAMP

A worldwide decline in the quality of human semen is currently occurring. In mammals, sperm are produced from diploid stem-cell spermatogonia by spermatogenesis in testes and become mature in epididymis. Nevertheless, these biological processes can be affected by Gram-negative bacterial infection mediated by lipopolysaccharide (LPS), the major endotoxin of Gram-negative bacteria. It is well known that LPS can disturb spermatogenesis and affect sperm maturation and quality in vivo. However, the effect of LPS on the ejaculated mature sperm in vitro remains unclear. Thus, this study aimed to assess the in vitro toxicity of LPS on human sperm function and to elucidate the underlying mechanism. Human sperm were incubated with LPS (0.1-100 μg/ml) for 1-12 h in vitro and, subsequently, sperm viability, motility and capacitation, and the acrosome reaction were examined. LPS dose-dependently inhibited total and progressive motility and the ability to move through a viscous medium of the sperm but did not affect sperm viability, capacitation, and the acrosome reaction. To explore the underlying mechanism of LPS's actions, we examined the effects of LPS on the intracellular concentrations of cyclic adenosine monophosphate (cAMP) and calcium ([Ca(2+)]i) and protein-tyrosine phosphorylation of human sperm, which are key regulators of human sperm function. LPS decreased intracellular cAMP dose-dependently but had no effect on [Ca(2+)]i and protein-tyrosine phosphorylation of human sperm. These findings suggest that LPS inhibits human sperm motility by decreasing intracellular cAMP.

Importance of Pulmonary Vein Preferential Fibrosis for Atrial Fibrillation Promotion in Hypertensive Rat Hearts

BACKGROUND

Hypertension is one of the independent risk factors for atrial fibrillation (AF). Pulmonary veins (PVs) play an important role as the substrate for AF and triggers of AF. The purpose of this study was to determine the structural remodelling of the PVs and its effect on promoting AF in hypertensive (HT) rat hearts.

METHODS

Eighteen-week-old Dahl salt-sensitive HT rats and their controls were used for histological and immunohistological analyses, and electrophysiological studies were performed in Langendorff perfused hearts.

RESULTS

Masson-trichrome staining revealed that hypertension significantly increased the fibrosis in the PVs, particularly in subendocardial and perivascular areas, compared with that in control rats, however, at this early stage of hypertension, left atrial fibrosis was not prominent. In the HT rat hearts with PVs, electrical stimulation significantly increased the number of repetitive atrial firing and atrial tachycardia inducibility, which significantly diminished after the excision of the PVs. An immunofluorescent analysis revealed that HT rats had PV specific endocardial smooth muscle actin (αSMA)-positive cells with remarkable proliferation of platelet-derived growth factor (PDGF)-C and vascular endothelial growth factor (VEGF), which was lacking in the left atrial structures of the control and the HT rats. Pretreatment with imatinib, a PDGF receptor activity blocker, in HT rats reduced the αSMA-positive cell proliferation and fibrosis in the PVs and also induced a significant reduction in VEGF expression. Also, the drug pretreatment effectively prevented repetitive atrial firing promotion without affecting the blood pressure.

CONCLUSIONS

PV preferential fibrosis might play an important role in the arrhythmogenic substrate of AF in HT rat hearts.

The evolutionary significance of depression in Pathogen Host Defense (PATHOS-D)

Given the manifold ways that depression impairs Darwinian fitness, the persistence in the human genome of risk alleles for the disorder remains a much debated mystery. Evolutionary theories that view depressive symptoms as adaptive fail to provide parsimonious explanations for why even mild depressive symptoms impair fitness-relevant social functioning, whereas theories that suggest that depression is maladaptive fail to account for the high prevalence of depression risk alleles in human populations. These limitations warrant novel explanations for the origin and persistence of depression risk alleles. Accordingly, studies on risk alleles for depression were identified using PubMed and Ovid MEDLINE to examine data supporting the hypothesis that risk alleles for depression originated and have been retained in the human genome because these alleles promote pathogen host defense, which includes an integrated suite of immunological and behavioral responses to infection. Depression risk alleles identified by both candidate gene and genome-wide association study (GWAS) methodologies were found to be regularly associated with immune responses to infection that were likely to enhance survival in the ancestral environment. Moreover, data support the role of specific depressive symptoms in pathogen host defense including hyperthermia, reduced bodily iron stores, conservation/withdrawal behavior, hypervigilance and anorexia. By shifting the adaptive context of depression risk alleles from relations with conspecifics to relations with the microbial world, the Pathogen Host Defense (PATHOS-D) hypothesis provides a novel explanation for how depression can be nonadaptive in the social realm, whereas its risk alleles are nonetheless represented at prevalence rates that bespeak an adaptive function.

Phosphoinositide-3-kinase/akt - dependent signaling is required for maintenance of [Ca(2+)](i), I(Ca), and Ca(2+) transients in HL-1 cardiomyocytes

The phosphoinositide 3-kinases (PI3K/Akt) dependent signaling pathway plays an important role in cardiac function, specifically cardiac contractility. We have reported that sepsis decreases myocardial Akt activation, which correlates with cardiac dysfunction in sepsis. We also reported that preventing sepsis induced changes in myocardial Akt activation ameliorates cardiovascular dysfunction. In this study we investigated the role of PI3K/Akt on cardiomyocyte function by examining the role of PI3K/Akt-dependent signaling on [Ca²⁺]_i, Ca²⁺ transients and membrane Ca²⁺ current, I_Ca, in cultured murine HL-1 cardiomyocytes. LY294002 (1–20 μM), a specific PI3K inhibitor, dramatically decreased HL-1 [Ca²⁺]_i, Ca²⁺ transients and I_Ca. We also examined the effect of PI3K isoform specific inhibitors, i.e. α (PI3-kinase α inhibitor 2; 2–8 nM); β (TGX-221; 100 nM) and γ (AS-252424; 100 nM), to determine the contribution of specific isoforms to HL-1 [Ca²⁺]_i regulation. Pharmacologic inhibition of each of the individual PI3K isoforms significantly decreased [Ca²⁺]_i, and inhibited Ca²⁺ transients. Triciribine (1–20 μM), which inhibits AKT downstream of the PI3K pathway, also inhibited [Ca²⁺]_i, and Ca²⁺ transients and I_Ca. We conclude that the PI3K/Akt pathway is required for normal maintenance of [Ca²⁺]_i in HL-1 cardiomyocytes. Thus, myocardial PI3K/Akt-PKB signaling sustains [Ca²⁺]_i required for excitation-contraction coupling in cardiomyoctyes.

Bordetella adenylate cyclase toxin promotes calcium entry into both CD11b+ and CD11b- cells through cAMP-dependent L-type-like calcium channels

Adenylate cyclase toxin (ACT), a 200 kDa protein, is an essential virulence factor for Bordetella pertussis, the bacterium that causes whooping cough. ACT is a member of the pore-forming RTX (repeats-in-toxin) family of proteins that share a characteristic calcium-binding motif of Gly- and Asp-rich nonapeptide repeats and a marked cytolytic or cytotoxic activity. In addition, ACT exhibits a distinctive feature: it has an N-terminal calmodulin-dependent adenylate cyclase domain. Translocation of this domain into the host cytoplasm results in uncontrolled production of cAMP, and it has classically been assumed that this surge in cAMP is the basis for the toxin-mediated killing. Several members of the RTX family of toxins, including ACT, have been shown to induce intracellular calcium increases, through different mechanisms. We show here that ACT stimulates a raft-mediated calcium influx, through its cAMP production activity, that activates PKA, which in turn activates calcium channels with L-type properties. This process is shown to occur both in CD11b(+) and CD11b(-) cells, suggesting a common mechanism, independent of the toxin receptor. We also show that this ACT-induced calcium influx does not correlate with the toxin-induced cytotoxicity.