Neutrophil Extracellular Traps: Potential Prothrombotic State Markers and Therapeutic Targets for Atrial Fibrillation

BACKGROUND

 Recently, the mechanism of thrombogenesis has taken a new direction with the involvement of neutrophil extracellular traps (NETs). However, little is known about the relationship between NETs and thrombogenesis in atrial fibrillation (AF).

OBJECTIVE

 Our study aimed to evaluate NETs in AF patients and their potential association with thrombogenesis. In addition, we studied the effect of NETs on thrombogenesis in rat models.

METHODS

 A total of 125 AF patients and 172 controls were studied. Spontaneous echo contrast (SEC) was examined using transesophageal echocardiography to assess the prothrombotic state. We used rapid atrial pacing (RAP) rat models to study NETs' formation and their effects on thrombogenesis. The levels of NETs were analyzed by flow cytometry. To deeply understand the regulatory mechanism of NET formation, the transcriptional characteristics of the left atrial appendage (LAA) tissue from RAP rats were analyzed.

RESULTS

 We found that NETs were increased significantly in AF patients and positively correlated with SEC grades. And inserting the NET level could significantly enhance the predictivity of CHA2DS2-VASc scores for the AF prothrombotic state. In the RAP models, we observed that NET levels increased significantly in the LAA and promoted thrombosis. Meanwhile, we found that these changes could be suppressed by the NET formation inhibitor. Transcriptomic analysis of the LAA tissue from RAP rats suggested that RAP might stimulate the NET formation by promoting the expression of inflammatory cytokine and adhesion genes.

CONCLUSION

 NETs may constitute useful thrombogenesis risk markers in AF patients and provide a potential therapeutic strategy for AF management.

Diabetes induces remodeling of the left atrial appendage independently of atrial fibrillation in a rodent model of type-2 diabetes

BACKGROUND

Diabetic patients have an increased predisposition to thromboembolic events, in most cases originating from thrombi in the left atrial appendage (LAA). Remodeling of the LAA, which predisposes to thrombi formation, has been previously described in diabetic patients with atrial fibrillation, but whether remodeling of the LAA occurs in diabetics also in the absence of atrial fibrillation is unknown. To investigate the contribution of diabetes, as opposed to atrial fibrillation, to remodeling of the LAA, we went from humans to the animal model.

METHODS

We studied by echocardiography the structure and function of the heart over multiple time points during the evolution of diabetes in the Cohen diabetic sensitive rat (CDs/y) provided diabetogenic diet over a period of 4 months; CDs/y provided regular diet and the Cohen diabetic resistant (CDr/y), which do not develop diabetes, served as controls. All animals were in sinus rhythm throughout the study period.

RESULTS

Compared to controls, CDs/y developed during the evolution of diabetes a greater heart mass, larger left atrial diameter, wider LAA orifice, increased LAA depth, greater end-diastolic and end-systolic diameter, and lower E/A ratio-all indicative of remodeling of the LAA and left atrium (LA), as well as the development of left ventricular diastolic dysfunction. To investigate the pathophysiology involved, we studied the histology of the hearts at the end of the study. We found in diabetic CDs/y, but not in any of the other groups, abundance of glycogen granules in the atrial appendages , atria  and ventricles, which may be of significance as glycogen granules have previously been associated with cell and organ dysfunction in the diabetic heart.

CONCLUSIONS

We conclude that our rodent model of diabetes, which was in sinus rhythm, reproduced structural and functional alterations previously observed in hearts of human diabetics with atrial fibrillation. Remodeling of the LAA and of the LA in our model was unrelated to atrial fibrillation and associated with accumulation of glycogen granules. We suggest that myocardial accumulation of glycogen granules is related to the development of diabetes and may play a pathophysiological role in remodeling of the LAA and LA, which predisposes to atrial fibrillation, thromboembolic events and left ventricular diastolic dysfunction in the diabetic heart.

Reduced left atrial cardiomyocyte PITX2 and elevated circulating BMP10 predict atrial fibrillation after ablation

BACKGROUNDGenomic and experimental studies suggest a role for PITX2 in atrial fibrillation (AF). To assess if this association is relevant for recurrent AF in patients, we tested whether left atrial PITX2 affects recurrent AF after AF ablation.METHODSmRNA concentrations of PITX2 and its cardiac isoform, PITX2c, were quantified in left atrial appendages (LAAs) from patients undergoing thoracoscopic AF ablation, either in whole LAA tissue (n = 83) or in LAA cardiomyocytes (n = 52), and combined with clinical parameters to predict AF recurrence. Literature suggests that BMP10 is a PITX2-repressed, atrial-specific, secreted protein. BMP10 plasma concentrations were combined with 11 cardiovascular biomarkers and clinical parameters to predict recurrent AF after catheter ablation in 359 patients.RESULTSReduced concentrations of cardiomyocyte PITX2, but not whole LAA tissue PITX2, were associated with AF recurrence after thoracoscopic AF ablation (16% decreased recurrence per 2-(ΔΔCt) increase in PITX2). RNA sequencing, quantitative PCR, and Western blotting confirmed that BMP10 is one of the most PITX2-repressed atrial genes. Left atrial size (HR per mm increase [95% CI], 1.055 [1.028, 1.082]); nonparoxysmal AF (HR 1.672 [1.206, 2.318]), and elevated BMP10 (HR 1.339 [CI 1.159, 1.546] per quartile increase) were predictive of recurrent AF. BMP10 outperformed 11 other cardiovascular biomarkers in predicting recurrent AF.CONCLUSIONSReduced left atrial cardiomyocyte PITX2 and elevated plasma concentrations of the PITX2-repressed, secreted atrial protein BMP10 identify patients at risk of recurrent AF after ablation.TRIAL REGISTRATIONClinicalTrials.gov NCT01091389, NL50069.018.14, Dutch National Registry of Clinical Research Projects EK494-16.FUNDINGBritish Heart Foundation, European Union (H2020), Leducq Foundation.

Genome-wide profiling reveals atrial fibrillation-related circular RNAs in atrial appendages

Atrial fibrillation (AF) is an abnormal heart rhythm characterized by rapid and irregular beating of the atria. The non-coding RNAs (ncRNAs) have attracted much attention of AF researchers, as they play a critical role in the transcriptional and post-transcriptional regulation, which could greatly benefit the interpretation of the pathogenesis of AF. However, circRNAs, as a special member of the ncRNAs, and their role in the pathogenesis of AF is less understood. In the present study, we detected a total of 14,215 circRNAs in AF patients and healthy controls. Differential expression analysis of these circRNAs revealed 20 upregulated and 3 downregulated circRNAs, which were differentially expressed in both left and right atrial appendages. The association analysis of the AF-related circRNAs and their parental genes revealed that hsa_circ_0003965 had significantly negative correlation with its parental gene TMEM245 (PCC = -0.51), suggesting that the dysregulation of hsa_circ_0003965 was not regulated by the transcription of its parental gene, but could be associated with glucagon signaling pathway. The competing endogenous RNA (ceRNA) network analysis revealed two upregulated genes, IFNG and GDF7, and one downregulated gene, BMP7, all of which were involved in TGF-beta signaling pathway, which further suggested that these circRNAs, namely hsa_circ_0000075 and hsa_circ_0082096, participated in the AF pathogenesis via TGF-beta signaling pathway. Consistently, TGF-beta signaling pathway was a well-recognized player for its association with atrial fibrosis in AF. In summary, we aimed to discover and provide key circRNAs involved in AF for AF-related researchers, which had the potential to greatly improve our understanding of the underlying mechanism behind circRNAs and AF.

Absence of Functional Nav1.8 Channels in Non-diseased Atrial and Ventricular Cardiomyocytes

PURPOSE

Several studies have indicated a potential role for SCN10A/NaV1.8 in modulating cardiac electrophysiology and arrhythmia susceptibility. However, by which mechanism SCN10A/NaV1.8 impacts on cardiac electrical function is still a matter of debate. To address this, we here investigated the functional relevance of NaV1.8 in atrial and ventricular cardiomyocytes (CMs), focusing on the contribution of NaV1.8 to the peak and late sodium current (INa) under normal conditions in different species.

METHODS

The effects of the NaV1.8 blocker A-803467 were investigated through patch-clamp analysis in freshly isolated rabbit left ventricular CMs, human left atrial CMs and human-induced pluripotent stem cell-derived CMs (hiPSC-CMs).

RESULTS

A-803467 treatment caused a slight shortening of the action potential duration (APD) in rabbit CMs and hiPSC-CMs, while it had no effect on APD in human atrial cells. Resting membrane potential, action potential (AP) amplitude, and AP upstroke velocity were unaffected by A-803467 application. Similarly, INa density was unchanged after exposure to A-803467 and NaV1.8-based late INa was undetectable in all cell types analysed. Finally, low to absent expression levels of SCN10A were observed in human atrial tissue, rabbit ventricular tissue and hiPSC-CMs.

CONCLUSION

We here demonstrate the absence of functional NaV1.8 channels in non-diseased atrial and ventricular CMs. Hence, the association of SCN10A variants with cardiac electrophysiology observed in, e.g. genome wide association studies, is likely the result of indirect effects on SCN5A expression and/or NaV1.8 activity in cell types other than CMs.

Mitochondrial and Contractile Function of Human Right Atrial Tissue in Response to Remote Ischemic Conditioning

Background Remote ischemic preconditioning ( RIPC ) by repeated brief cycles of limb ischemia/reperfusion attenuates myocardial ischemia/reperfusion injury. We aimed to identify a functional parameter reflecting the RIPC -induced protection in human. Therefore, we measured mitochondrial function in right atrial tissue and contractile function of isolated right atrial trabeculae before and during hypoxia/reoxygenation from patients undergoing coronary artery bypass grafting with RIPC or placebo, respectively. Methods and Results One hundred thirty-seven patients under isoflurane anesthesia underwent RIPC (3×5 minutes blood pressure cuff inflation on the left upper arm/5 minutes deflation, n=67) or placebo (cuff uninflated, n=70), and right atrial appendages were harvested before ischemic cardioplegic arrest. Myocardial protection by RIPC was assessed from serum troponin I/T concentrations over 72 hours after surgery. Atrial tissue was obtained for isolation of mitochondria ( RIPC /placebo: n=10/10). Trabeculae were dissected for contractile function measurements at baseline and after hypoxia/reoxygenation (60 min/30 min) and for western blot analysis after hypoxia/reoxygenation ( RIPC /placebo, n=57/60). Associated with cardioprotection by RIPC (26% decrease in the area under the curve of troponin I/T), mitochondrial adenosine diphosphate-stimulated complex I respiration (+10%), adenosine triphosphate production (+46%), and calcium retention capacity (+37%) were greater, whereas reactive oxygen species production (-24%) was less with RIPC than placebo. Contractile function was improved by RIPC (baseline, +7%; reoxygenation, +24%). Expression and phosphorylation of proteins, which have previously been associated with cardioprotection, were not different between RIPC and placebo. Conclusions Cardioprotection by RIPC goes along with improved mitochondrial and contractile function of human right atrial tissue. Clinical Trial Registration URL: https://www.clinicaltrials.gov . Unique identifier: NCT 01406678.

Occurrence of Incomplete Endothelialization Causing Residual Permeability After Left Atrial Appendage Closure

AIMS

Percutaneous left atrial appendage (LAA) occlusion is occasionally incomplete, with residual permeability of the LAA on cardiac computed tomography. The cause for this is unclear. Our objective was to determine if residual permeability was related to incomplete endothelialization.

METHODS

A total of 35 consecutive patients contraindicated for anticoagulant therapy admitted for LAA occlusion were included; 12 patients received a Watchman device (Boston Scientific) and 23 patients received an Amplatzer Cardiac Plug (St. Jude Medical). Incomplete endothelialization was defined as residual permeability on cardiac computed tomography without peridevice leak on transesophageal echocardiography at follow-up.

RESULTS

Five patients did not receive cardiac computed tomography. After 10 ± 6 months of follow-up, residual permeability of the LAA (at least partial) was recorded on cardiac computed tomography in 21 of 30 patients (70%). Seven of 30 patients presented with a peridevice leak on transesophageal echocardiography. Among the remaining 23 patients, 14 (61%) presented with incomplete endothelialization and 9 (39%) presented with complete endothelialization. There was no statistical difference between the patients presenting with complete vs incomplete endothelialization.

CONCLUSION

We found that incomplete endothelialization, defined as residual permeability on cardiac computed tomography without peridevice leak on transesophageal echocardiography, occurred in 61% of the patients after 10 ± 6 months of percutaneous LAA closure. Predisposing factors and appropriate monitoring of LAA patients remain to be determined in larger cohorts.

Circulating Galectin-3 is Associated With Left Atrial Appendage Remodelling and Thrombus Formation in Patients With Atrial Fibrillation

BACKGROUND

Left atrial appendage (LAA) is gaining increasing attention in patients with atrial fibrillation (AF) in the context of cardioembolic stroke. Galectin-3 (Gal-3) is a mediator of profibrotic pathways and is associated with an increased incidence of heart failure. However, the role of Gal-3 in LAA remodelling and thrombus formation in AF has not been evaluated.

METHODS

This prospective study included 153 consecutive patients with paroxysmal (n=58), persistent (n=55) or permanent (n=40) nonvalvular AF. The serum level of Gal-3 was measured by enzyme-linked immunosorbent assay. The morphology and function of LAA were determined by transoesophageal echocardiography.

RESULTS

Left atrial appendage thrombus was observed in 22 patients (2 in paroxysmal AF, 11 in persistent AF and 9 in permanent AF). Significant differences among patients with different types of AF were found in terms of LAA morphology (orifice diameter and depth) and function (flow velocity and tissue Doppler contracting velocity) as well as serum levels of Gal-3. Furthermore, patients with persistent or permanent AF had higher levels of Gal-3. High Gal-3 level was closely related to LAA flow velocity and occurrence of LAA thrombus. Multivariate logistic regression analysis revealed that Gal-3 was an independent determinant of LAA thrombus in patients with AF. Receiver operating characteristic (ROC) curves related to LAA thrombus formation established a cut-off point for Gal-3 >18.95ng/ml.

CONCLUSIONS

Cardiac rhythm disturbances caused by AF may lead to morphologic and functional remodelling of LAA. The serum level of Gal-3 was significantly correlated with LAA remodelling in patients with AF. High levels of Gal-3 were also a predicator for LAA thrombus formation.

Omega-3 fatty acids do not alter P-wave parameters in electrocardiogram or expression of atrial connexins in patients undergoing coronary artery bypass surgery

AIMS

We previously reported omega-3 polyunsaturated fatty acids (n-3PUFAs) supplementation does not reduce atrial fibrillation (AF) following coronary artery bypass graft (CABG) surgery. The aim of the present study is to evaluate the impact of n-3 PUFAs on electrocardiogram (ECG) atrial arrhythmic markers and compare with expression of gap-junction proteins, Connexins.

METHODS AND RESULTS

Subset of clinical trial subjects with right atrial sampling during CABG surgery included. Twelve-lead ECG performed at recruitment and at surgery [after supplementation with n-3 PUFA (∼1.8 g/day) or matched placebo] for ∼14 days. Electrocardiograms analysed for maximum P-wave duration (P-max) and difference between P-max and minimum P-wave duration, P-wave dispersion (PWD). Right atrial specimens analysed for expression of Connexins 40 and 43 using real-time quantitative polymerase chain reaction (qPCR) and western blot. Serum levels of n-3 PUFA at baseline, at surgery, and atrial tissue levels at surgery collated from file. Postoperative AF was quantified by analysing data from stored continuous electrograms. A total of 61 patients (n-3 PUFA 34, Placebo 27) had ECG analysis and AF burden, of which 52 patients (26 in each group) had qPCR and 16 (8 in each group) had western blot analyses for Connexins 40 and 43. No difference between the two groups in ECG parameters or expression of Connexin 40 or 43. P-wave dispersion in the preoperative ECG independently predicted occurrence of AF following CABG surgery.

CONCLUSIONS

Omega-3 polyunsaturated fatty acids supplementation does not alter pro-arrhythmic P-wave parameters in ECG or connexin expression in human atrium with no effect on the incidence of AF following CABG surgery.

ISOLATION AND CHARACTERIZATION OF CARDIAC PROGENITOR CELLS OBTAINING FROM MYOCARDIAL RIGHT ATRIAL APPENDAGE TISSUE

Resident stem cells of the heart are denoted as heterogeneous population of immature cells, which reside in the myocardium and characterized by their ability to self-renewal and are multipotent differentiation capacity into cardiomyocyte-like and vascular like cells. CSCs were originally isolated directly by long enzymatic digestion of heart tissue and selection using stem cell markers. However, long exposure to enzymatic digestion and small myocardial sample size can affect the possibility of obtaining a significant amount of viable cells. To avoid these problems, we developed a method consisting of growing of the CPC in explant culture and subsequent immunomagnetic selection.

The role of connexin40 in developing atrial conduction

Connexin40 (Cx40) is the main connexin expressed in the murine atria and ventricular conduction system. We assess here the developmental role of Cx40 in atrial conduction of the mouse. Cx40 deficiency significantly prolonged activation times in embryonic day 10.5, 12.5 and 14.5 atria during spontaneous activation; the severity decreased with increasing age. In a majority of Cx40 deficient mice the impulse originated from an ectopic focus in the right atrial appendage; in such a case the activation time was even longer due to prolonged activation. Cx40 has thus an important physiological role in the developing atria.

Atrial Fibrillation associated chromosome 4q25 variants are not associated with PITX2c expression in human adult left atrial appendages

Atrial Fibrillation (AF), the most common sustained arrhythmia, has a strong genetic component, but the mechanism by which common genetic variants lead to increased AF susceptibility is unknown. Genome-wide association studies (GWAS) have identified that the single nucleotide polymorphisms (SNPs) most strongly associated with AF are located on chromosome 4q25 in an intergenic region distal to the PITX2 gene. Our objective was to determine whether the AF-associated SNPs on chromosome 4q25 were associated with PITX2c expression in adult human left atrial appendages. Analysis of a lone AF GWAS identified four independent AF risk SNPs at chromosome 4q25. Human adult left atrial appendage tissue was obtained from 239 subjects of European Ancestry and used for SNP analysis of genomic DNA and determination of PITX2c RNA expression levels by quantitative PCR. Subjects were divided into three groups based on their history of AF and pre-operative rhythm. AF rhythm subjects had higher PITX2c expression than those with history of AF but in sinus rhythm. PITX2c expression was not associated with the AF risk SNPs in human adult left atrial appendages in all subjects combined or in each of the three subgroups. However, we identified seven SNPs modestly associated with PITX2c expression located in the introns of the ENPEP gene, ∼54 kb proximal to PITX2. PITX2c expression in human adult left atrial appendages is not associated with the chromosome 4q25 AF risk SNPs; thus, the mechanism by which these SNPs are associated with AF remains enigmatic.

Autophagy gene fingerprint in human ischemia and reperfusion

OBJECTIVE

Autophagy is an evolutionary conserved adaptive response that is believed to promote cell survival in response to stressful stimuli via recycling of precursors derived from the degradation of endogenous cellular components. The autophagic molecular machinery is controlled by a large family of autophagy-related genes (ATGs) and downstream regulators. We sought to define the autophagy gene fingerprint associated with human ischemia and reperfusion (IR) injury using an intraoperative model developed by Sellke and colleagues.

METHODS

Right atrial appendages, collected from human hearts before and after cardioplegic arrest and after reperfusion, were submitted for polymerase chain reaction (PCR) array, quantitative real-time PCR, and immunoblot analysis for autophagy proteins and their associated upstream regulators.

RESULTS

Perioperative IR significantly upregulated 11 (13.1%) and downregulated 3 (3.6%) of 84 ATGs. Specifically, there were increases in the autophagy machinery components ATG4A, ATG4C, and ATG4D; tumor necrosis factor-related apoptosis-inducing ligand, MAPK8 and BCL2L1; and chaperone-mediated autophagy activity with increased heat shock protein (HSP) A8, HSP90AA1, and a-synuclein. Autophagy activity was confirmed through observations of higher LC3-I levels and an increase in the LC3-II/LC3-I ratio. Autophagy activation coincided with increased AMPK activation and decreased protein levels of the mammalian target of rapamycin, the latter a key negative regulator of autophagy.

CONCLUSIONS

We provide the first human cardiac fingerprint of autophagy gene expression in response to IR. These findings may inform on appropriate cell- and gene-based therapeutic approaches to limit aberrant cardiac injury.

Reduced connexin40 protein expression in the right atrial appendage of patients bearing the minor connexin40 allele (-44 G --> A)

AIMS

The occurrence of connexin40 (Cx40) minor polymorphism (-44 G → A) was increased in patients with idiopathic atrial fibrillation (AF), although its effect on atrial Cx40 protein expression is unknown. We aimed to evaluate whether alterations in Cx40 are directly linked to the development of AF, we studied the effect of this polymorphism on Cx40 expression and distribution in patients without any history of AF and in patients who developed post-operative AF.

METHODS AND RESULTS

Hundred and eight patients (mean age 67 ± 9 years), without a history of AF or conditions that predispose to AF, were included. During heart surgery, 10 cc blood was collected for DNA genotyping and the right atrial appendage was partly excised. Ten patients (9%) were homozygous for the minor allele (AA, Group 1), 30 (28%) were heterozygous (AG, Group 2), and 68 (63%) were non-carriers (GG, Group 3). Ten age- and sex-matched tissue samples per group were analysed for Cx40 expression by: (i) real-time quantitative polymerase chain reaction (Q-PCR), (ii) western blotting, and (iii) immunohistochemistry on cryosections. Real-time quantitative polymerase chain reaction showed no significant differences of Cx40 mRNA among the groups. Western blot analysis, however, revealed a reduction in Cx40 protein in Groups 1 (-36.4%) and 2 (-39.5%) as compared with Group 3. Immunohistochemistry confirmed this reduction but indicated an unaltered subcellular distribution of the remaining Cx40. Incidence of post-operative AF (28%) was age-dependent but unrelated to the presence of the polymorphism or fibrosis.

CONCLUSION

Presence of the Cx40 minor allele (-44 G → A) results in a uniform down-regulation of right atrial appendage Cx40 protein which was not significantly related to development of post-operative AF.

Natriuretic peptides and right atrial fibrosis in patients with paroxysmal versus persistent atrial fibrillation

Natriuretic peptides (NPs) are excellent diagnostic and prognostic markers of heart failure, but their roles in atrial fibrillation (AF), particularly of isolated cardiac valvular origin, are unclear. We assessed the mRNA and protein content of pro-atrial natriuretic peptide (pro-ANP) and pro-brain natriuretic peptide (pro-BNP) in right atrial appendages (RAAs) and their N-terminal fragments (nt-proANP and nt-proBNP) in the plasma of 30 patients with paroxysmal AF (PaAF) and 40 patients with persistent AF (PeAF) matched with 34 patients in sinus rhythm (SR) undergoing isolated valvular replacement. To explore the underlying mechanism, fibrosis related examinations were simultaneously carried out in RAAs. Unexpectedly, atrial expression of pro-NPs mRNA was notably augmented in the PaAF subgroup, but not so pronounced in the PeAF subgroup. Atrial content of pro-NPs proteins and plasma nt-proNPs, between which surprisingly strong positive correlations were found (pro-ANP and nt-proANP: r=0.918, p<0.001; pro-BNP and nt-proBNP: r=0.913, p<0.001), were increased analogously in PaAF and PeAF subgroups. We identified significantly increasing gradients of atrial collagen volume fraction (CVF), levels of collagen I and III in the SR, PaAF and PeAF groups, and convincing negative linear correlations between CVF, levels of collagen I and III, and atrial transcripts of pro-NPs. These findings suggest that the discordance between transcripts and protein contents of pro-NPs was possibly due to the more outstanding atrial fibrosis in PeAF, and that circulating nt-proNPs levels could reflect the corresponding atrial pro-NPs contents in this report.

[The cholinergic non-excitability phenomenon in the atrial myocardium of lower vertebrates]

Changes of electric activity induced by acetylcholine were studied in atrial myocardium of fishes (cod and carp) and reptilians (lizard and grass-snake). Standart microelectrode technique and novel method of optical mapping were used in the study. Acetylcholine (1-50 microM) provoked decrease of the action potential amplitude down to full inhibition of electrical activity in wide regions of atrium of cod and carp. We define this phenomenon as cholinergic inexcitability. In other regions excitation persisted even during action of 500 microM acetylcholine. In atria of lizard and grass-snake acetylcholine caused shortening of action potential without changes in it's amplitude. Local cholinergic inexcitability, shown in the atrial myocardium of fishes, is quite similar to the phenomenon, that was described earlier in the atria of frogs. It presents the heart of fish as an interesting model for study of mechanisms of cholinergic atrial arrhythmias initiation.

Atrial fibrillation is associated with cardiac hypoxia

BACKGROUND

Atrial fibrillation (AF), the most common human arrhythmia, is responsible for substantial morbidity and mortality and may be promoted by selective atrial ischemia and atrial fibrosis. Consequently, we investigated markers for hypoxia and angiogenesis in AF.

METHODS

Right atrial appendages (n=158) were grouped according to heart rhythm [sinus rhythm (SR) or AF]. The degree of fibrosis and microvessel density of all patients were determined morphometrically using Sirius-Red- and CD34/CD105-stained sections, respectively. Next, sections (n=77) underwent immunostaining to detect hypoxia- and angiogenesis-related proteins [hypoxia-inducible factor (HIF)1 alpha, HIF2 alpha, vascular endothelial growth factor (VEGF), VEGF receptor 2 (KDR), phosphorylated KDR (pKDR), carboanhydrase IX, platelet-derived growth factor] and the apoptosis-related B-cell lymphoma 2 protein.

RESULTS

Fibrosis progressed significantly from 14.7+/-0.8% (SR) to 22.3+/-1.4% (AF). While the positive cytoplasmic staining of HIF1 alpha, HIF2 alpha, VEGF, KDR, and pKDR rose significantly from SR to AF, their nuclear fractions fell (only pKDR significantly). The median CD34/CD105-positive microvessel size increased significantly from SR to AF.

CONCLUSIONS

AF is closely associated with an atrial up-regulation of hypoxic and angiogenic markers. Whether this is cause, effect, or co-phenomenon of fibrosis remains to be investigated. It is conceivable that fibrosis might lead to an increased O(2) diffusion distance and thus induce ischemic signaling, which, in turn, leads to angiogenesis.

Connexin40 imparts conduction heterogeneity to atrial tissue

Impulse propagation in cardiac tissue is a complex process in which intercellular coupling through gap junction channels is a critical component. Connexin40 (Cx40) is an abundant gap junction protein that is expressed in atrial myocytes. Alterations in the expression of Cx40 have been implicated in atrial arrhythmogenesis. The purpose of the current study was to assess the role of Cx40 in atrial impulse propagation. High-resolution optical mapping was used to study conduction in the right and left atrial appendages of isolated Langendorff-perfused murine hearts. Wild-type (Cx40(+/+)), heterozygous (Cx40(+/-)), and knockout (Cx40(-/-)) mice, both adult and embryonic, were studied to assess the effects of reduced Cx40 expression on sinus node function and conduction velocity at different pacing cycle lengths (100 and 60 ms). In both adult and late-stage embryonic Cx40(+/+) mice, heterogeneity in CV was found between the right and left atrial appendages. Either partial (Cx40(+/-)) or complete (Cx40(-/-)) deletion of Cx40 was associated with the loss of conduction heterogeneity in both adult and embryonic mice. Additionally, sinus node impulse initiation was found to be ectopic in Cx40(-/-) mice at 15.5 days postcoitus, whereas Cx40(+/+) mice showed normal activation occurring near the crista terminalis. Our findings suggest that Cx40 plays an essential role in establishing interatrial conduction velocity heterogeneity in the murine model. Additionally, we describe for the first time a developmental requirement for Cx40 in normal sinus node impulse initiation at 15.5 days postcoitus.

[Gene expression analysis in myocytes of right atrial appendages in patients with atrial fibrillation using cDNA microarray technique]

Gene expression level of 2900 genes was studied by cDNA microarray in patients with atrial fibril-lation (AF) or sinus rhythm. Gene transcripts were analysed in samples of right atrial appendages from 47 patients undergoing surgery for valve repair or coronary artery bypass. Standard correlation analysis and two dimensional hierarchical clustering were used for study of differentially expressed genes in patient groups. A highly positive correlation of gene expression with AF was shown for cardiac muscle LIM domain protein (CSRP3), cardiac muscle myosin heavy chain beta isoform (MYH7), calmodulin (CALMS) and homeobox protein (PKNOXl) genes (r > 0.77, p < 0.007). In contrast, metallothionein (MT1/2), mitochondrial aldehyde dehydrogenase 2 (ALDH2), ras-related protein (RaplA) and guanine nucleotide binding protein G (GNAL) genes revealed highly negative correlation with AF (r < -0.75, p < 0.002). Alterations of gene activity were more evident at permanent as compared with paroxysmal AF. In addition, genes overexpressed in AF patients demonstrated underexpression in coronary artery disease patients (r=-0.8, p=0.0002) and conversely. Genes correlating with AF belong to different functional categories, including sarcomere organization, contraction, Ca2+ homeostasis, signaling and transcription regulation, extracellular matrix interactions and oxidative stress. Downregulation of MT1/2 and ALDH2 genes, known protectors against oxidative stress, may contribute to maintenance of oxidative stress in myocardial tissues of AF patients. The identification of novel genes - participants of pathological process in AF may open new perspective for search of therapeutic agents.

Selective activation of human atrial Galpha12 and Galpha13 by Galphaq-coupled angiotensin and endothelin receptors

Galphaq-coupled receptors such as alpha1-adrenergic, angiotensin, and endothelin receptors, play key roles in cardiac physiology. These receptors have also been shown to couple to G proteins of the G12 family, including Galpha12 and Galpha13. In this report, we determined whether these G proteins interact with endothelin, angiotensin, and alpha1-adrenergic receptors in the human heart. We find that these receptors activate cardiac Galpha12 and Galpha13 differentially; endothelin receptors activate only Galpha12 (to 218 +/- 22% of unstimulated levels), angiotensin receptors activate only Galpha13 (to 236 +/- 49% of unstimulated levels), and alpha1-adrenergic receptors activate neither Galpha12 (123 +/- 18% of unstimulated levels) nor Galpha13 (113 +/- 12% of unstimulated levels). Consistent with these data, translocation of guanine nucleotide exchange factor p115RhoGEF, which responds to Galpha13, occurs only after stimulation of angiotensin receptors (shifting from 73 +/- 12% to 41 +/- 10% cytosolic). These differences in the activation of Galpha12 and Galpha13 by Galphaq-coupled receptors may underlie reported differences in the functions of these receptors.

Differential phosphorylation-dependent regulation of constitutively active and muscarinic receptor-activated IK,ACh channels in patients with chronic atrial fibrillation

OBJECTIVE

In chronic atrial fibrillation (cAF) the potassium current IK,ACh develops agonist-independent constitutive activity. We hypothesized that abnormal phosphorylation-dependent regulation underlies the constitutive IK,ACh activity.

METHODS

We used voltage-clamp technique and biochemical assays to study IK,ACh regulation in atrial appendages from 61 sinus rhythm (SR), 11 paroxysmal AF (pAF), and 33 cAF patients.

RESULTS

Compared to SR basal current was higher in cAF only, whereas the muscarinic receptor (2 micromol/L carbachol)-activated IK,ACh was smaller in pAF and cAF. In pAF the selective IK,ACh blocker tertiapin abolished the muscarinic receptor-activated IK,ACh but excluded agonist-independent constitutive IK,ACh activity. Blockade of type-2A phosphatase and the subsequent shift to increased muscarinic receptor phosphorylation (and inactivation) reduced muscarinic receptor-activated IK,ACh in SR but not in cAF, pointing to an impaired function of G-protein-coupled receptor kinase. Using subtype-selective kinase inhibitors we found that in SR the muscarinic receptor-activated IK,ACh requires phosphorylation by protein kinase G (PKG), protein kinase C (PKC), and calmodulin-dependent protein kinase II (CaMKII), but not by protein kinase A (PKA). In cAF, constitutive IK,ACh activity results from abnormal channel phosphorylation by PKC but not by PKG or CaMKII, whereas the additional muscarinic receptor-mediated IK,ACh activation occurs apparently without involvement of these kinases. In cAF, the higher protein level of PKCepsilon but not PKCalpha, PKCbeta1 or PKCdelta is likely to contribute to the constitutive IK,ACh activity.

CONCLUSIONS

The occurrence of constitutive IK,ACh activity in cAF results from abnormal PKC function, whereas the muscarinic receptor-mediated IK,ACh activation does not require the contribution of PKG, PKC or CaMKII. Selective drug targeting of constitutively active IK,ACh channels may be suitable to reduce the ability of AF to become sustained.

Altered expression of FHL1, CARP, TSC-22 and P311 provide insights into complex transcriptional regulation in pacing-induced atrial fibrillation

Atrial fibrillation (AF) is the most common progressive disease in patients with cardiac arrhythmia. AF is accompanied by complex atrial remodeling and changes in gene expression, but only a limited number of transcriptional regulators have been identified. Using a low-density cDNA array, we identified 31 genes involved in transcriptional regulation, signal transduction or structural components, which were either significantly upregulated or downregulated in porcine atria with fibrillation (induced by rapid atrial pacing at a rate of 400-600 bpm for 4 weeks that was then maintained without pacing for 2 weeks). The genes for four and a half LIM domains protein-1 (FHL1), transforming growth factor-beta (TGF-beta)-stimulated clone 22 (TSC-22), and cardiac ankyrin repeat protein (CARP) were significantly upregulated, and chromosome 5 open reading frame gene 13 (P311) was downregulated in the fibrillating atria. FHL1 and CARP play important regulatory roles in cardiac remodeling by transcriptional regulation and myofilament assembly. Induced mRNA expression of both FHL1 and CARP was also observed when cardiac H9c2 cells were treated with an adrenergic agonist. Increasing TSC-22 and marked P311 deficiency could enhance the activity of TGF-beta signaling and the upregulated TGF-beta1 and -beta2 expressions were identified in the fibrillating atria. These results implicate that observed alterations of underlying molecular events were involved in the rapid-pacing induced AF, possibly via activation of the beta-adrenergic and TGF-beta signaling.

Atrial Appendage Transcriptional Profile in Patients with Atrial Fibrillation with Structural Heart Diseases

During the last few years DNA microarray studies of gene expression changes in human atrial tissues from patients with and without atrial fibrillation (AF) have been performed. For this purpose, tissue samples are usually collected from AF patients undergoing open heart surgery. These investigations have limitations associated with the unavoidable heterogeneity of compared groups which is due to the presence of various structural changes accompanying different sets of underlying heart diseases in both groups. It is thus reasonable to compare the atrial tissue samples from AF patients with those from individuals without signs of cardiovascular disease. To address this, we selected the atrial tissue samples from 12 AF patients (who underwent open heart surgery) and compared them with control atrial tissue samples from 10 individuals with no signs of cardiovascular diseases (those who died due to street accident). cDNA microarray method and reverse transcription-polymerase chain reaction (RT-PCR) analysis were used to identify genes which can discriminate between control and pathologically altered atrial tissues. Thirty-nine genes were found to be differentially expressed in pathologically altered tissues samples independently of the type of the underlying structural heart disease. These genes are involved in signal transduction, gene transcription regulation, cell proliferation, and apoptosis. The greatest alterations were observed for NOR1, DEC1, MSF, and Bcl2A1 genes (5 to 28-fold decrease, P < 0.05). Additional studies are needed to determine the specific role of each selected gene in pathophysiological changes leading to AF.

Activation of the calcineurin signaling pathway induces atrial hypertrophy during atrial fibrillation

Atrial tachyarrhythmia (AF) alters intracellular calcium homeostasis and induces cellular hypertrophy of atrial myocytes. The impact of the calcium-dependent calcineurin pathway on the development of AF-induced atrial hypertrophy has not yet been analyzed. In this study, atrial tissue samples from patients with sinus rhythm and chronic persistent atrial fibrillation (CAF) were used to determine changes in expression and activity of calcineurin A (CnA), and its relation to CnA-regulated transcription factors NFATc1-4, and hypertrophic markers ANP, troponin I, and beta-MHC. CnA phosphatase activity and CnAbeta protein contents were significantly upregulated in patients with CAF. Calcineurin activation led to dephosphorylation, redistribution, and subsequent accumulation of NFATc3 in nuclei during CAF, and expression of hypertrophic genes was increased. CAF-dependent changes were reproduced by ex vivo pacing (2-4 Hz) of human atrial tissue slices. FK506 abolished the hypertrophic response induced by electrical-field stimulation. Atrial tachyarrhythmia causes atrial hypertrophy by activation of the CnA signal pathway, which thereby contributes to structural remodeling of human atria.

Interaction between simvastatin and metoprolol with respect to cardiac beta-adrenoceptor density, catecholamine levels and perioperative catecholamine requirements in cardiac surgery patients

Beta-blockade is a standard cardiovascular therapy known to induce the up-regulation of beta-adrenoceptor density. Upon ligand-binding, beta-adrenoceptors are normally internalised via the arrestin pathway, and after dissociation they are re-inserted into the membrane. This means that at high catecholamine levels the adrenoceptor density is low and under beta-blockade it is high. The insertion of receptors into the membrane is often dependent on farnesylation processes that can be inhibited by statins. We carried out a prospective, controlled, observational study to determine whether beta-blockade-induced up-regulation of beta-adrenoceptor density is attenuated by statin therapy and whether this would subsequently affect catecholamine consumption during surgery. We obtained pre-operative blood samples and intra-operative biopsies of the right atrial appendage from 39 patients (age: 65+/-5 years; BMI: 28+/-1) undergoing coronary bypass surgery with or without simvastatin (20 mg/day) therapy and with or without concomitant metoprolol therapy (50 mg/day). The atrial tissue was used for radioligand-binding studies with (-)-[125I]-iodocyanopindolol (ICYP) and for assessment of the beta-adrenoceptor subtype distribution following standard protocols. In the blood samples, plasma adrenaline and noradrenaline concentrations were determined using HPLC. In all tissue samples, we found a total beta-adrenoceptor density of 38+/-4 fmol/mg protein in untreated controls; this which was up-regulated to 55+/-5 fmol/mg protein in patients receiving metoprolol. This increase in receptor number was nearly prevented completely by simvastatin therapy (42+/-5 fmol/mg protein). The up-regulation could be attributed to increases in the beta1-adrenoceptor subtype. In contrast, simvastatin alone had no effect on beta-adrenoceptor density. Pre-operative adrenaline levels were slightly reduced in all drug therapy groups (nonsignificant differences), while the levels noradrenaline were not significantly different among the groups. With respect to the perioperative catecholamine requirements, patients on metoprolol needed significantly less dopamine than control patients, while patients undergoing simvastatin/metoprolol therapy needed as much as the controls. The post-operative total catecholamine requirements were not different among the four groups of patients. There were no differences in plasma metoprolol concentration between patients receiving metoprolol alone and those receiving a combination of metoprolol and simvastatin.

IN CONCLUSION

Simvastatin therapy seems to counter-regulate the up-regulation of beta-adrenoceptor density. In the up-regulated state induced by metoprolol therapy, the patients seemed to need less catecholamines during cardiac surgery, which may be due to the higher number of beta-adrenoceptors. Additional simvastatin therapy did not reduce post-operative catecholamine consumption.

Is cyclic AMP formation desensitized in patients with end-stage renal failure?

1 Cyclic AMP formation has consistently been reported to be desensitized in various tissues including heart of animal models of end-stage renal failure (ESRF). In contrast, reports on desensitization of cAMP formation in ESRF patients remain contradictory. Whether this discrepancy results from a difference between human ESRF and its animal models or from the use of circulating blood cells in the human and various solid tissues in the animal studies, remains unclear. Therefore, we performed three studies with heart and platelets of ESRF patients undergoing haemodialysis or continuous ambulatory peritoneal dialysis and age- and gender-matched controls with normal renal function (n = 11-13 each). 2 In platelets from haemodialysis patients adenylyl cyclase activity in response to receptor-dependent and -independent agonists was reduced by approximately 30%, and this could be explained by an alteration at the level of adenylyl cyclase itself. However, no such desensitization was seen in platelets from peritoneal dialysis patients. 3 In hearts from ESRF patients undergoing haemodialysis, beta-adrenoceptor density and subtype distribution, cAMP formation in response to the beta-adrenoceptor agonist isoprenaline or various receptor-independent stimuli, were very similar to those in control patients but activity of G-protein-coupled receptor kinase was increased by approximately 20%. 4 We conclude that conflicting reports on the desensitization of cAMP formation between ESRF patients and ESRF animal models are not explained by the use of solid tissues in animal studies vs. circulating blood cells in patient studies. Rather desensitization of cAMP formation seems to be a less consistent feature of human ESRF than of its animal models.

Transcriptional profile of genes induced in human atrial myocardium with pressure overload

BACKGROUND

The molecular response of human myocardium to mechanical stimuli, particularly the difference between pressure or volume overload cardiac hypertrophy, remains incompletely defined.

METHODS

We investigated the transcriptional profile of genes induced in human pressure- or volume-overloaded myocardium with DNA microarray technology. We used right atrial tissue from patients who underwent cardiac surgery. On the basis of pressure data and echocardiographic findings, the patients were divided into three groups: control group (n=3), pressure overload group (mean right atrial pressure of >7 mm Hg, n=3), and volume overload group (moderate or severe tricuspid regurgitation, n=3). Expression profiles of 2139 human genes were investigated with mRNA obtained from the samples.

RESULTS

In the pressure overload group, expression of genes of cyclin-dependent kinase inhibitor 1A (CDKI1A, 11.7+/-3.1-fold vs. control), and mitogen-activated protein kinase phosphatase-1 (MKP-1, 26.2+/-2.1-fold) was significantly increased compared with those in control or volume overload group (P<0.05). The specificity of these gene expressions was confirmed by a quantitative "real-time" polymerase chain reaction (PCR) analysis. In addition, mechanical strain induced CDKI1A and MKP-1 protein expressions in neonatal rat cardiac myocytes in an amplitude-dependent manner. In contrast, transcripts of growth factors did not significantly increase.

CONCLUSIONS

This study demonstrated that gene expressions of CDKI1A and MKP-1, but not growth factors, are induced in chronic pressure-overloaded myocardium. These findings suggest that suppressors of the cell cycle or cell proliferation may play a critical role in the pathophysiology of pressure overload.

Peptidyl-Glycine α-Amidating Monooxygenase Targeting and Shaping of Atrial Secretory Vesicles

ANP (atrial natriuretic peptide) is widely recognized as an important vasorelaxant, diuretic, and cardioprotective hormone. Little is known, however, about how ANP-secretory vesicles form within the atrial myocytes. Secretory vesicles were visualized by fluorescence microscope imaging in live rat atrial myocytes expressing proANP–enhanced green fluorescent protein (EGFP), or N-terminal–mutated fusion proteins thought to suppress the calcium-dependent aggregation of proANP. Results showed the following: (1) aggregates of proANP and coexpressed proANP-EGFP recruited peptidylglycine α-amidating monooxygenase (PAM)-1, an abundant atrial integral vesicle membrane protein; (2) coexpressed N-terminal–mutated (Glu23,24→Gln23,24) and N-terminal–deleted proANP-EGFP inhibited recruitment of PAM-1 by up to 60%; (3) 4-phenyl-3-butenoic acid (PBA) (10 μmol/L), a pharmacological inhibitor of the lumenal peptidylglycine α-hydroxylating monooxygenase domain of PAM proteins, inhibited recruitment of endogenous PAM-1 and of coexpressed pro-EGFP–PAM-1; (4) PBA had no effect on exocytosis of the potassium inward rectifier KIR2.1; (5) PBA induced a deformation of the secretory vesicles but did not inhibit docking. These findings suggest that recruitment of PAM-1 to secretory vesicles depends on intact N-terminal proANP and on the lumenal domain of PAM-1. Conversely, PAM-1 participates in shaping the proANP-secretory vesicles. The full text of this article is available online at http://circres.ahajournals.org.

Reconstruction of the patterns of gene expression in the developing mouse heart reveals an architectural arrangement that facilitates the understanding of atrial malformations and arrhythmias

Firm knowledge about the formation of the atrial components and of the variations seen in congenital cardiac malformations and abnormal atrial rhythms is fundamental to our understanding of the normal structure of the definitive atrial chambers. The atrial region is relatively inaccessible and has continued to be the source of disagreement. Seeking to resolve these controversies, we made three-dimensional reconstructions of the myocardial components of the developing atrium, identifying domains on the basis of differential expression of myocardial markers, connexin40, and natriuretic precursor peptide A. These reconstructions, made from serial sections of mouse embryos, show that from the outset of atrial development, the systemic and pulmonary veins are directly connected to the atrium. Relative to the systemic junctions, however, the pulmonary venous junction appears later. Our experience shows that three-dimensional reconstructions have three advantages. First, they provide clear access to the combined morphological and molecular data, allowing clarification and verification of morphogenetic concepts for nonmorphological experts and setting the scene for further discussion. Second, they demonstrate that, from the outset, the myocardium surrounding the pulmonary veins is distinct from that clothing the systemic venoatrial junctions. Third, they reveal an anatomical and molecular continuity between the entrance of the systemic venous tributaries, the internodal atrial myocardium, and the atrioventricular region. All these regions are derived from primary myocardium, providing a molecular basis for the observed nonrandom distribution of focal right atrial tachycardias.

Gene expression profiling of oxidative stress on atrial fibrillation in humans

Atrial Fibrillation (AF) is thought be caused by oxidative stress. Oxidative stress at the cellular level results from many factors, including exposure to alcohol, medications, cold, toxins or radiation. In this study we investigated gene transcriptional profiles on the human myocardial tissues from AF and oxidative stress conditions. Right atrial appendages were obtained from AF patients (n = 26) undergoing the Maze procedure, and from control patients (n = 26) who were in normal sinus rhythm and undergoing coronary artery bypass graft operation. To examine the effects of oxidative stress on AF, we used radioactive complementary DNA (cDNA) microarrays to evaluate changes in the expression of 1,152 known genes. This technology, which monitors thousands of genes simultaneously, gives us a better picture of the interactions between AF and oxidative stress. Total RNAs prepared from the retrieved tissues were used to synthesize 33P-labeled cDNAs by reverse transcription and hybridized to cDNA microarrays. Gene expression profiles showed that 30 genes were upregulated and 25 were downregulated in AF patients compared with control patients. Moreover, comparison rank analysis revealed that the expression of five genes related to reactive oxygen species (ROS)-including flavin containing monooxygenase 1, monoamine oxidase B, ubiquitin specific protease 8, tyrosinase-related protein 1, and tyrosine 3-monooxygenase-increased by more than 2.0 of the Z-ratio, and two genes related to antioxidants including glutathione peroxidase 1, and heme oxygenase 2-decreased to the Z-ratio levels of < or = -2.0. Apparently, a balanced regulation of pro- and anti-oxidation can be shifted toward pro-oxidation and can result in serious damage similar to that of human AF. Western blotting analysis confirmed the upregulation of tyrosinase-related protein 1 and tyrosine 3-monooxygenase and the downregulation of heme oxygenase 2. These results suggested that the gene expression pattern of myocardial tissues in AF patients can be associated with oxidative stress, resulting in a significant increase in ROS. Thus, the cDNA microarray technique was useful for investigating transcription profiles in AF. It showed that the intracellular mechanism of oxidative stress plays a pivotal role in the pathologic progression of AF and offers novel insight into potential treatment with antioxidants.

Neuropeptide Y inhibits acetylcholine release in human heart atrium by activation of Y2-receptors

Congestive heart failure and other cardiac diseases are characterized by increased activity of the sympathetic nervous system, whereas at the same time parasympathetic activity is often suppressed. Such imbalance may be a result of or at least enhanced by presynaptic inhibitory effects of sympathetic neurotransmitters on acetylcholine release. We investigated whether the sympathetic cotransmitters neuropeptide Y (NPY), norepinephrine (NE), and ATP are capable of modulating acetylcholine release in human heart atrium. Human atrial appendages were incubated with [(3)H]-choline to label cholinergic transmitter stores and placed in superfusion chambers. Electrical field stimulations (S1, S2) induced a tetrodotoxin-dependent [(3)H]-release, which was taken as an index of endogenous acetylcholine release. NE, NPY, ATP, and a P2-receptor analogue were added before S2. NPY (0.05-1.0 micromol/l) concentration dependently inhibited acetylcholine release. This effect was prevented by the NPY-Y(2)-receptor antagonist BIIE 0246 (0.1 micromol/l) but not by the NPY-Y(1)-receptor antagonist BIBP 3226 (10 micromol/l). ATP (10 micromol/l), a stable analogue ADP-beta S (3 micromol/l), and NE (1 micromol/l) had no effect on acetylcholine release. m-RNA for the NPY-receptor subtypes Y(1), Y(2), Y(4), Y(5), and y(6) was demonstrated by reverse transcription-polymerase chain reaction (RT-PCR). The results suggest that the sympathetic neurotransmitter NPY inhibits parasympathetic neurotransmission in the human heart through activation of presynaptic Y(2)-receptors. NE and ATP seem not to play a role. Since NPY plasma levels are high in chronic heart failure patients, NPY may be one component leading to impaired parasympathetic neurotransmission in those patients.

Expression of the von Willebrand Factor in Atrial Endocardium is Increased in Atrial Fibrillation Depending on the Extent of Structural Remodeling

BACKGROUND

The incidence of stroke in patients suffering atrial fibrillation (AF) is increased when left atrial enlargement occurs. Recently, the platelet adhesive molecule, von Willebrand factor (vWF), located in the atrial endocardium, has been shown to be increased in patients with a variety of heart diseases compared with patients who have no cardiac problems.

METHODS AND RESULTS

We investigated the expression of vWF mRNA and protein in the endocardium as a possible prothrombotic alteration of AF in association with atrial structural remodeling. Atrial appendage specimens were obtained during either heart surgery or at an autopsy from AF patients with and without underlying heart disease. The immunohistochemical and in situ hybridization signals for vWF in the endocardium were well correlated and varied widely among the individual atrial appendages examined. The increased expression of vWF in the endocardium was associated with enlarged left atrial dimensions in mitral valvular disease or increased myocyte diameters in the underlying myocardium. Platelet adhesion/aggregation on the endocardium was always found under the fresh thrombi and was colocalized with strong vWF staining, but not necessarily with fibrinogen and/or fibrin staining.

CONCLUSIONS

Endocardial overexpression of vWF may occur during the process of atrial structural remodeling contributing to the thrombotic predilection of AF in association with underlying heart disease.

Norepinephrine release is reduced in cardiac tissue of Type 2 diabetic patients

AIMS/HYPOTHESIS

The aim of this study was to assess whether cardiac catecholamine release is affected in patients with Type 2 diabetes mellitus.

METHODS

A trial tissue was obtained from 19 diabetic (Type 2) and 43 non-diabetic patients undergoing coronary surgery. Endogenous norepinephrine release was examined under baseline conditions as well as during electrical field stimulation (effective voltage 5 V, stimulation frequency 4 Hz, pulse width 2 msec) by high performance liquid chromatography and electrochemical detection. Cardiac function and arterial blood pressure was assessed from coronary angiography.

RESULTS

In atrial tissue from diabetic patients, stimulation-induced norepinephrine release was reduced by 25% compared with non-diabetic patients, while baseline norepinephrine release did not differ between both groups. Preoperative plasma glucose and haemoglobin A(1C) concentrations were increased in patients with diabetes, however, no relation was found to catecholamine release. Diabetic and non-diabetic patients did not differ regarding left ventricular ejection fraction and arterial blood pressure.

CONCLUSION/INTERPRETATION

Cardiac norepinephrine release is suppressed in patients with Type 2 diabetes which could contribute to sympathetic neuropathy. The difference of norepinephrine release in diabetic and non-diabetic patients was independent of cardiac function and arterial blood pressure.

Electromechanical impairment of human auricle and rat myocardial strip subjected to exogenous oxidative stress

OBJECTIVE

Animal myocardial dysfunction induced by remote ischemia-reperfusion (IR) was shown to be partly accomplished via a direct effect of the pro-oxidant xanthine oxidase (XO). This direct remote effect was not tested in humans. We now assessed the performance of human auricles in the presence of solutions containing XO and/or allopurinol and compared them to those of rat myocardial strips.

METHODS

Human and rat specimens (n=64) were separately exposed for 2h to Krebs-Henseleit solution that either (1) exited from rat livers that were earlier perfused for 2h (control-human or control-rat), (2) exited from livers that were earlier made ischemic for 2h (IR-human, IR-rat), (3) contained xanthine (X) 3.8 microM + XO 3 mU ml(-1) (X+XO-human, X+XO-rat), or (4) exited from post 2h-ischemic livers and contained 100 microM allopurinol (human or rat IR + allopurinol groups).

RESULTS

Unlike the unchanged electromechanical performance in the control and IR+allopurinol auricles and strips, the rates of contraction, maximal force of contraction and working index of either preparation were reduced by 75-98% (P<0.01) when exposed to the IR reperfusate or to the X+XO-enriched Krebs. The basal amplitudes of contraction in these four latter groups increased twofold (P<0.01). XO activity was similarly low in the control and in the IR+allopurinol groups, but four- to 45-fold (P<0.001) higher in the IR and the X+XO groups, both in the rat and human organs. The reduced glutathione was reduced by approximately 50% (P<0.01) in either preparation in the IR and the X+XO groups compared to the control and IR+allopurinol groups.

CONCLUSIONS

Remotely and exogenously originated oxidative burst directly induces electromechanical dysfunction and disrupts oxidant/antioxidant balance in human auricles as it does in the rat myocardial strip.

Plasmalemmal KATP channels shape triggered calcium transients in metabolically impaired rat atrial myocytes

The relative role of plasmalemmal and mitochondrial ATP-sensitive K(+) (K(ATP)) channels in calcium homeostasis of the atrium is little understood. Electrically triggered (1 Hz) cytoplasmic calcium transients were measured by 340-to-380-nm wavelength fura 2 emission ratios in cultured rat atrial myocytes. CCCP, a mitochondrial protonophore (100-400 nmol/l), dose dependently reduced the transient amplitude by up to 85%, caused a slow rise in baseline calcium, and reduced the recovery time constant of the transient from 143 to 91 ms (P < 0.05). However, neither 5-hydroxydecanoate, a mitochondrial K(ATP) channel blocker, nor diazoxide (500 micromol/l) affected the amplitude, baseline, or time constant in CCCP-treated cells. HMR-1098 (30 micromol/l), a plasmalemmal K(ATP) channel blocker, and glibenclamide (1 micromol/l) increased the amplitude in CCCP-treated myocytes by 69-82%, sharply elevated the calcium baseline, and prolonged the recovery time constant to 181-193 ms (P < 0.01). Thus opening of plasmalemmal but not mitochondrial K(ATP) channels reduces the calcium overload in metabolically compromised but otherwise intact atrial myocytes. Mitochondrial K(ATP) channels probably operate through a different mechanism to afford ischemic protection.

Alteration of gap junctions and connexins in the right atrial appendage during cardiopulmonary bypass

OBJECTIVES

We investigated the influence of cardiopulmonary bypass on cardiomyocyte gap junctions and connexins.

METHODS

Samples were collected at intervals during operation from the right atrial appendage in 21 patients (mean [+/- SD] age 55 +/- 21 years). Immunodetection of connexins was conducted by Western blotting and confocal microscopy with parallel electron microscopic examination of gap junctions.

RESULTS

Downregulation of connexin 43 during the course of operation occurred in more than half of the patients. The mean densitometric value of connexin 43 decreased by 23%, with samples from patients with coronary artery disease showing a greater reduction than seen in those from patients with other diseases (31% +/- 22% vs 10% +/- 24%, P =.04). Such alterations were confirmed by confocal microscopy, which also demonstrated reduced connexin 45 immunolabeling in most patients. Electron microscopy revealed a reduction in the dimensions of cell membrane-located gap junctions and more frequent intracytoplasmic gap junctional membrane in samples from later time points (P =.04).

CONCLUSIONS

Downregulation of connexins accompanied by a reduction in gap junctions is common in the cardiomyocytes of the right atrial appendage during cardiopulmonary bypass. The association of a marked reduction in connexin 43 with coronary artery disease may imply inadequate intraoperative cardiac protection in patients with this disease.

Dedifferentiation of atrial myocytes during atrial fibrillation: role of fibroblast proliferation in vitro

OBJECTIVES

Severe myocyte alterations, characterized by enlarged myocytes and myolysis, is observed in fibrillating and dilated atria and contributes to atrial fibrillation. The aim of this study was to determine the nature of this cellular remodeling process and factors involved in its regulation.

METHODS

In vivo, contractile proteins were studied in 24 human right atrial specimens by means of immunohistochemical techniques. In an attempt to reproduce in vitro the myocyte remodeling and to study its regulation, human atrial myocytes were cultured (n=27) and analyzed immunocytochemically; intracellular Ca(2+) transients (Ca(i)-tr) in response to electrical stimulation were monitored using Fura-2/AM.

RESULTS

In diseased specimens, sarcomeres, seen at the periphery of myolytic myocytes, stained positively with antibodies against sarcomeric proteins of the Z-band (alpha-actinin and titin epitope T12) but not with antibodies against titin epitope T11 (I-band) or desmin (intermediate filament). beta-myosin heavy chain (MHC) and smooth muscle alpha-actin, two proteins of the fetal program, were re-expressed. In culture, diseased myocytes also showed myolysis and glycogen accumulation; their sarcomeres stained positively with anti-alpha-actinin, anti-T12, anti-beta-MHC and anti-smooth muscle alpha-actin but not with anti-titin T11 or anti-desmin antibodies. At confluence, myocytes regained a normal sarcomeric apparatus and were excitable, as shown by electrical Ca(i)-tr triggering. This redifferentiation process was inhibited by fibroblast proliferation.

CONCLUSION

In diseased atria, myolytic myocytes are in a dedifferentiated state resembling that of immature muscle cells. In vitro, fibroblast proliferation prevents the reversibility of this cellular alteration.

Activation of proteolysis by calpains and structural changes in human paroxysmal and persistent atrial fibrillation

OBJECTIVE

Atrial fibrillation (AF) is accompanied by electrical, structural and ion-channel protein remodeling. We tested if proteolysis by calpain and proteasome is activated during AF, and studied the relation with the remodeling processes.

METHODS

Right atrial appendages were obtained from patients with paroxysmal (n=7) or persistent (n=10) lone AF and compared to controls (n=10) in sinus rhythm undergoing coronary artery bypass grafting (CABG). Proteolysis was measured using Suc-Leu-Leu-Val-Tyr-7-amino-4-methyl-coumarin. Protein expression of calpain I and II was assessed by Western-blot and calpain I localization by immunohistochemistry. Structural changes were quantified by counting atrial myocytes with contraction bands or hibernation.

RESULTS

Calpain activity was significantly increased in paroxysmal AF (2-fold, P<0.001) and persistent AF (3-fold, P<0.001), mainly due to calpain I activation. Increased calpain I protein expression was found in AF with Western blot and immunohistochemistry. Myocytes from all AF groups showed increased contraction bands, whereas hibernation was only found in persistent AF. Calpain activity correlated with L-type Ca(2+) channel and Kv1.5 protein amounts (r=-0.80, P<0.001 and r=-0.72, P<0.001, respectively), degree of structural changes (r=0.90, P<0.001), shortening of atrial effective refractory period (AERP) (basic cycle length 500 ms, r=-0.60, P<0.001) and AERP rate adaptation (r=-0.80, P<0.001).

CONCLUSIONS

Calpain activity is induced during AF and correlates with parameters of ion-channel protein, structural and electrical remodeling. The results suggest that calpain activation represents an important mechanism linking calcium overload to cellular adaptation mechanisms in human AF.

Does recombinant human interleukin-11 exert direct electrophysiologic effects on single human atrial myocytes?

Recombinant human interleukin-11 (rhIL-11) treatment given to alleviate side effects of cancer therapy is associated with an increased susceptibility to atrial arrhythmias in elderly patients. To elucidate the mechanism underlying this action, we investigated the direct electrophysiologic effect of rhIL-11 on single human atrial myocytes (HuAM) using the patch-clamp technique. Action potentials (AP) at different driving rates were recorded in the perforated-patch configuration, and L-type calcium current (I(Ca,L)), outward potassium currents (I(to) and I(K)), and the hyperpolarization-activated pacemaker current If were measured in the disrupted whole-cell configuration. At therapeutic concentrations (i.e., 10-100 ng/ml), rhIL-11 did not modify AP parameters and cycle-length dependence of AP duration. I(Ca,L) (measured at 0 mV) was 370 +/- 45 pA in control and 379 +/- 48 pA and 368 +/- 42 pA in the presence of 10 and 50 ng/ml rhIL-11, respectively (p = NS). The amplitude and activation of I(to) were not modified by rhIL-11 (i.e., I(to) was at +60 mV: 2.1 +/- 0.2 nA in control vs. 1.9 +/- 0.2 nA and 2.1 +/- 0.2 nA in the presence of 10 and 50 ng/ml rhIL-11, respectively, p = NS). Similarly, late currents measured at the end of the pulse were unchanged in the presence of 10 or 50 ng/ml of rhIL-11. If activation was not modified by rhIL-11: maximal current was 173 +/- 34 pA in control and 159 +/- 35 pA and 117 +/- 14 pA in the presence of 10 and 50 ng/ml of rhIL-11, respectively; midpoint activation was -99 +/- 3 mV in control and -98 +/- 4 mV and -94 +/- 2 mV in the presence of 10 and 50 ng/ml of rhIL-11, respectively (p = NS). Thus, it is unlikely that direct alterations of membrane potential and currents of HuAM caused by rhIL-11 are the basis for the genesis of atrial arrhythmias observed in patients treated with this agent.

Atrial fibrillation-induced atrial contractile dysfunction: a tachycardiomyopathy of a different sort

OBJECTIVE

Although AF-induced atrial contractile dysfunction has significant clinical implications the underlying intracellular mechanisms are poorly understood.

METHODS

From the right atrial appendages of 59 consecutive patients undergoing mitral valve surgery (31 in SR, 28 in chronic AF) thin muscle preparations (diameter<0.7 mm) were isolated. Isometric force of contraction was measured in the presence of different concentrations of Ca(2+) and isoprenaline. To assess the function of the sarcoplasmic reticulum, the force-frequency relationship and the post-rest potentiation were studied. The myocardial density of the ryanodine-sensitive calcium release channel (CRC) of the sarcoplasmic reticulum was determined by [3H]ryanodine binding. Myocardial content of SR-Ca(2+)-ATPase (SERCA), phospholamban (Plb), calsequestrin (Cals) and the Na(+)/Ca(2+)-exchanger (NCX) were analyzed by Western blot analysis. Adenylyl cyclase activity was measured with a radiolabeled bioassay using [32P]ATP as a tracer.

RESULTS

In 72 muscle preparations of SR patients contractile force was 10.9+/-1.8 mN/mm(2) compared to 3.3+/-0.9 mN/mm(2) (n=48, P<0.01) in AF patients. The positive inotropic effect of isoprenaline was diminished but the stimulatory effect on relaxation and the adenylyl cyclase were not altered in AF patients. The force-frequency relation and the post-rest potentiation were enhanced in atrial myocardium of AF patients. The protein levels of CRC, SERCA, Plb, and Cals were not different between the two groups. In contrast, the Na(+)/Ca(2+)-exchanger was upregulated by 67% in atria of AF patients.

CONCLUSIONS

AF-induced atrial contractile dysfunction is not due to beta-adrenergic desensitization or dysfunction of the sarcoplasmic reticulum and thus is based on different cellular mechanisms than a ventricular tachycardia-induced cardiomyopathy. Instead, downregulation or altered function of the L-type Ca(2+)-channel and an increased Ca(2+) extrusion via the Na(+)/Ca(2+)-exchanger seem to be responsible for the depressed contractility in remodeled atria.

Impaired myofibrillar energetics and oxidative injury during human atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is associated with severe contractile dysfunction and structural and electrophysiological remodeling. Mechanisms responsible for impaired contractility are undefined, and current therapies do not address this dysfunction. We have found that myofibrillar creatine kinase (MM-CK), an important controller of myocyte contractility, is highly sensitive to oxidative injury, and we hypothesized that increased oxidative stress and energetic impairment during AF could contribute to contractile dysfunction. Methods and Results-- Right atrial appendages were obtained from AF patients undergoing the Maze procedure and from control patients who were in normal sinus rhythm and undergoing cardiac surgery. MM-CK activity was reduced in AF patients compared with controls (25.4+/-3.4 versus 18.2+/-3.8 micromol/mg of myofibrillar protein per minute; control versus AF; P<0.05). No reduction in total CK activity or myosin ATPase activity was detected. This selective reduction in MM-CK activity was associated with increased relative expression of the beta-myosin isoform (25+/-6 versus 63+/-5%beta, CTRL versus AF; P<0.05). Western blotting of AF myofibrillar isolates demonstrated no changes in protein composition but showed increased prevalence of protein oxidation as detected by Western blotting for 3-nitrotyrosine (peroxynitrite biomarker) and protein carbonyls (hydroxyl radical biomarker; P<0.05). Patterns of these oxidative markers were distinct, which suggests discrete chemical events and differential protein vulnerabilities in vivo. MM-CK inhibition was statistically correlated to extent of nitration (P<0.01) but not to carbonyl presence.

CONCLUSIONS

The present results provide novel evidence of oxidative damage in human AF that altered myofibrillar energetics may contribute to atrial contractile dysfunction and that protein nitration may be an important participant in this condition.

Ultrastructural and morphometric features of nodal and impulse-conducting cardiac myocytes of the bat Pipistrellus pipistrellus

Cells of the impulse-generating and conducting tissues of the insect-eating bat Pipistrellus pipistrellus were studied and evaluated using ultrastructural morphometry. Sinoatrial node cells are smaller than working atrial cells and measure about 6.5 microm in diameter. Their mitochondira and myofibril content constitute 23% and 19% of cytoplasmic volume, respectively. Corresponding values for working atrial cells are 23% and 52%. Atrioventricular node cells are 4.2 microm in diameter and contain abundant glycogen in the cytoplasm. The fractional volume of mitochondria in about 24% while that of myofibrils is 7%. Cells of the bundle of His are larger (6-8 microm diameter) and contain more cellular organelles than do nodal cells. Their mitochondria and myofibril contents are 25% and 25%, respectively. Cells in the proximal part of the right bundle branch are slender with diameters averaging 3.4 microm. Mitochondrial content is 23% while myofibrils occupy 20% of the cytoplasmic volume of these cells. Distally located bundle branch cells measure 7-10 microm in diameter with mitochondria and myofibril volumes of 30% and 33%. Subendocardial cells in the ventricular free wall are large reaching 28 microm in diameter (cf. 14-18 microm in working ventricular cells) and have mitochondira and myofibril volume fractions of 32% and 29%, respectively (35% & 40% for working ventricular cells).

Structural changes in the auricles of the rat heart after local ionizing irradiation

BACKGROUND AND PURPOSE

Irradiation of the heart may lead to late cardiovascular complications and depending on the dose to cardiac-related death. There is increasing evidence that left atrial appendages play an important role in left ventricular filling especially in cardiac disease. The aim of the present study was to investigate the radiation response of the atria of the rat heart (auricles in particular) at morphological, histological and transcriptional level.

MATERIAL AND METHODS

Sprague-Dawley rats were irradiated with a single dose locally on the heart (0-22.5 Gy). End-diastolic diameters of left auricles were measured during evaluation of cardiac function. Histopathological evaluations were performed at various time points up to 16 months post irradiation. Changes in mRNA expression of procollagen types I and III and pro-fibrogenic cytokines (TGF-beta1 and IL-1beta) were investigated using competitive PCR.

RESULTS

Irradiation leads to a dose-dependent decrease in end-diastolic diameter of the left auricles. This decrease was observed at 4 months post-irradiation, where no gross damage of the ventricle has been reported. Histologically, epicardial fibrosis was found already 1 month post irradiation, and the frequency/severity of the structural changes appeared to be dose-dependent and progressive with time post irradiation. At 9 months, fibrosis was observed in all three layers (epicardium, myocardium and endocardium) of both auricles. On the level of gene expression, increases in procollagen types I and III were observed at 12 and 3 months post irradiation, respectively. Increases in IL-1beta and TGF-beta1, cytokines known to influence collagen deposition at different levels, preceded the upregulation of procollagen mRNA.

CONCLUSIONS

Auricles of the rat heart show a marked pathological response to ionizing radiation, characterized by generalized accumulation of collagen (fibrosis) and a reduction of end-diastolic diameter. The reduction of auricular volume and loss of elasticity will negatively contribute to the pump function of the irradiated ventricle.

Cloning and characterization of G protein-gated inward rectifier K+ channel (GIRK1) isoforms from heart and brain

A G protein-gated inward rectifier potassium (K+) channel (GIRK1a) has been cloned from different tissues (Kubo et al., 1993b; Dascal et al., 1993). Here we report the cloning of three additional novel isoforms of GIRK1a from rat atria and and one from human brain. These isoform cDNAs code for proteins that have identical N-termini, M1-H5-M2 (predicted transmembrane and pore domains), and post-M2 amino acid regions to GIRK1a (1-501 amino acids), but they have shorter C-termini (GIRK1b (1-309), GIRK1c (1-308), GIRK1d (1-235), and GIRK1e (1-253). These results indicated that isoforms were generated by alternative splicing and partial genomic analysis confirmed the presence of exons and introns in the rat GIRK1 gene. RNase protection analysis and immunoblot analysis indicated that the isoforms were expressed in both rat atria and brain but at lower levels versus GIRK1a. The physiological role that the isoforms may play in atrial and brain physiology remains to be determined.

Ingrowth of sympathetic innervation occurs concomitantly with a decrease of ANP in the growing rat cardiac ventricles

The relationship between the immunohistochemical expression of atrial natriuretic peptide and the occurrence of sympathetic nerve fibres, as visualized by staining for tyrosine hydroxylase, in the growing rat heart was evaluated. Rats were investigated at four different stages from birth to 21 days postnatally. The effects of chemical destruction of sympathetic nerve terminals in neonatal rats on the cardiac atrial natriuretic peptide content were furthermore examined by use of radioimmunoassay. There was in principle a reciprocal pattern of immunoreaction for atrial natriuretic peptide and tyrosine hydroxylase positive innervation in the ventricular myocardium, atrial natriuretic peptide reaction becoming less and less pronounced with the ingrowth of innervation positive for tyrosine hydroxylase. Furthermore, in the peripheral Purkinje fibre network, there was a marked atrial natriuretic peptide immunoexpression and scarce or no nerve fibres throughout the examination period. The radioimmunoassay measurements showed that chemical sympathectomy lead to elevated cardiac levels of atrial natriuretic peptide. The study shows that sympathetic innervation grows into the ventricular parts concomitantly with the occurrence of a decline in atrial natriuretic peptide expression during development of the heart. Furthermore, it is shown that a reversion of the in growth of sympathetic innervation by destruction of cardiac sympathetic nerves at an early stage leads to increased levels of atrial natriuretic peptide in the heart. The results give new evidence to the phenomenon that the atrial natriuretic peptide levels in the ventricular myocardium and the peripheral parts of the conduction system are under influence of the presence of sympathetic innervation.

Beta(2)-adrenergic and several other G protein-coupled receptors in human atrial membranes activate both G(s) and G(i)

Cardiac G protein-coupled receptors that couple to Galpha(s) and stimulate cAMP formation (eg, beta-adrenergic, histamine, serotonin, and glucagon receptors) play a key role in cardiac inotropy. Recent studies in rodent cardiac myocytes and transfected cells have revealed that one of these receptors, the beta(2)-adrenergic receptor (AR), also couples to the inhibitory G protein Galpha(i) (activation of which inhibits cAMP formation). If beta(2)ARs could be shown to couple to Galpha(i) in the human heart, it would have important ramifications, because levels of Galpha(i) increase with age and in failing human heart. Therefore, we investigated whether beta(2)ARs in the human heart activate Galpha(i). By photoaffinity labeling human atrial membranes with [(32)P]azidoanilido-GTP, followed by immunoprecipitation with antibodies specific for Galpha(i), we found that Galpha(i) is activated by stimulation of beta(2)ARs but not of beta(1)ARs. In addition, we found that other Galpha(s)-coupled receptors also couple to Galpha(i), including histamine, serotonin, and glucagon. When coupling of these receptors to Galpha(i) is disrupted by pertussis toxin, their ability to stimulate adenylyl cyclase is enhanced. These data provide the first evidence that beta(2)AR and many other Galpha(s)-coupled receptors in human atrium also couple to Galpha(i) and that abolishing the coupling of these receptors to Galpha(i) increases the receptor-mediated adenylyl cyclase activity.

Atrial high energy phosphate content and mitochondrial enzyme activity during chronic atrial fibrillation

OBJECTIVE

Prolonged atrial fibrillation (AF) results in (ultra)structural remodelling of atrial cardiomyocytes resembling alterations seen in ischemia-induced ventricular hibernation. The mechanisms underlying these changes are incompletely understood. In the present study we explored the hypothesis that a profound imbalance in energy status during chronic AF acts as a stimulus for structural remodelling.

METHODS AND RESULTS

The content of high energy-phosphates and related compounds together with a selected number of mitochondrial enzymes, known to be altered under ischemic conditions, were determined in tissue samples taken from atria of goats in sinus rhythm (SR) and after 1, 2, 4, 8 and 16 weeks of AF maintained by burst pacing. Atrial remodelling was quantified by counting the percentage of cells with >10% myolysis. During AF structural remodelling developed progressively, after 8 weeks about 40% of the atrial myocytes were affected. The concentration of adenine nucleotides and their degradation products did not change significantly during AF. Also the activity of mitochondrial cytochrome c oxidase activity was similar during AF and SR. Mitochondrial NADH-oxidase and proton-translocating ATPase activities were not induced by AF. The tissue content of phosphocreatine decreased during the first week by 60%, but completely recovered between 8 and 16 weeks of AF.

CONCLUSIONS

The analysis of adenine nucleotides during AF provided no indication for the development of severe atrial ischemia. This notion is supported by enzyme cytochemical findings. However, AF-induced atrial remodelling was associated with a transient lowering of phosphocreatine content, suggesting an increase in energy demand during the early phase of AF. The subsequent recovery of the phosphocreatine pool indicates restoration of the balance between energy demand and supply in chronically fibrillating atria.

Gap junctional remodeling in relation to stabilization of atrial fibrillation in the goat

OBJECTIVE

It has been postulated that high atrial rate induced changes at the level of the gap junctions ('gap junctional remodeling', i.e. changes in distribution, intercellular orientation and expression of gap junction proteins), could be part of the vicious circle of electrophysiologic and structural changes leading to sustained atrial fibrillation (AF). To obtain experimental evidence in favour of such a postulate the timing of this remodeling process was studied in relation to the development of sustained AF in a goat model.

METHODS AND RESULTS

Thin sections from the left (LAA) and right atrial appendage (RAA) from goats in sinus rhythm (SR) or AF, induced through programmed endocardial burst pacing for time periods between 0 and 16 weeks, were immunolabeled with antibodies against connexin(Cx)40 and Cx43 and analysed by immunofluorescence and confocal laser scanning microscopy. During SR the distribution pattern for Cx43 was completely homogeneous (LAA and RAA) and for Cx40 mostly homogeneous (LAA: all five goats, RAA: three out of five goats). The distribution pattern for Cx43 remained stable during AF, while the Cx40 distribution pattern became increasingly heterogeneous, both in the LAA and RAA, with increasing duration of pacing. This increase in heterogeneity in Cx40 distribution correlated (Spearman rank order) with an increase in stability of AF and the occurrence of structural changes (myolysis) in atrial myocytes. The Cx40/Cx43 immunofluorescence signal ratio in both the LAA and RAA appeared to be significantly lower in AF (1-16 weeks) as compared to SR (0 weeks); going from 0 to 16 weeks average ratios decreased 54.5% (n=5; P=0.026) in the LAA and 35.8 (n=5; P=0.034) in the RAA. Western blot analyses revealed similar decreases in the total Cx40/Cx43 protein ratio, on average 50.0% (n=5; P=0.008) and 47.8% (n=5; P=0.02) in the LAA and RAA, respectively. No changes were measured in the levels of Cx40 or Cx43 mRNA, as was assessed through RT-PCR.

CONCLUSION

The time course of changes in the distribution and content of Cx40 gap junctions as observed during endocardial burst pacing of the goat atrium suggests that Cx40 gap junctional remodeling might be involved in the pathogenesis of sustained atrial fibrillation.