Left Atrial Myocytes Research Articles

Left atrial strain differences between hypertensive and sarcomeric left ventricular hypertrophy: a sign of sarcomeric auricular myopathy?

Abstract Introduction In clinical practice, left ventricular (LV) hypertrophy is a common finding that can be caused by physiologic or pathologic conditions. An etiologic diagnosis is extremely important because it may change the prognosis and management of the patient and his family. Left atrial (LA) dysfunction is common due to diastolic dysfunction but, in patients with sarcomeric LV hypertrophy, primary cardiomyopathy disease could also be present. Purpose To evaluate if there are differences in LA function between arterial hypertension (HT) and hypertrophic cardiomyopathy (HCM) patients. Methods 73 patients with HCM (according to ESC guidelines criteria) and 82 patients with HT (under treatment with ≥1 hypertensive drug for ≥3 months) and ≥13mm of LV hypertrophy underwent an echocardiography to assess LA function with strain analysis and LV diastolic function. Continuous normally distributed variables were compared with T-student test and categoric variables with square-chi or exact Fisher test if required, accepting as significant a p-value <0.05. Results Globally, HCM patients showed thicker LV walls and larger LA indexed volumes (18.7±4.7 vs. 13.3±0,8 mm and 44.9±20.7 vs. 30.7±8.8 ml/m2, p<0.000 for both). Also, HCM patients presented diastolic dysfunction and atrial fibrillation history in a higher frequency (31 vs. 7.6% and 16.4% vs. 0%, p<0.000 for both). LA strain was significantly lower in HCM patients as compared to the HT group (LA contractile 11.5±5.8 vs. 17.2±4.4, p<0.000; LA conduit 12.2±5.4 vs. 13.3±3.8, p=0.041; and LA reservoir 23.9±8.7 vs. 31.1±5.9, p<0.000). In the subgroup of patients with moderate-severe LV hypertrophy (<17mm) and no significant mitral regurgitation (n=103; HCM=21, HT=82), LA contractile strain remained lower in HCM as compared to HT patients (13.2±4.8% vs. 17.2±4.4%, p<0.000) (figure 1); without differences in diastolic dysfunction, LA volumes, significant mitral regurgitation or atrial fibrillation history. Conclusions Our results show that LA strain is more impaired in HCM patients as compared to HT patients. Specifically, contractile LA strain remained lower even in those patients with moderate-severe LV hypertrophy, despite having similar LV diastolic function, LA indexed volumes, significant mitral regurgitation, or atrial fibrillation history. These findings could be caused by HCM involvement of the LA myocytes. In the context of moderate-severe hypertrophy of unknown origin with non-significant mitral regurgitation, the detection of abnormally reduced LA strain could indicate the presence of an HCM.Figure 1

Alterations of local Ca2+ signaling by high pacing frequency and shear stress in right and left atrial myocytes

Alterations of local Ca2+ signaling by high pacing frequency and shear stress in right and left atrial myocytes

From cells to circulating biomarker: BMP10 is a myocyte-secreted peptide with potential to detect atrial fibrillation

Abstract Background Early onset detection and recurrence prediction of atrial fibrillation (AF) may improve therapeutic decision-making, prognosis and long-term outcome. Plasma proteome analyses and mechanistic studies identified bone morphogenetic protein 10 (BMP10) as an atrial-specific secreted biomolecule for AF associated with reduced paired-like homeodomain transcription factor 2 (PITX2) expression. Purpose Our aim is 1) to confirm BMP10 synthesis and secretion by human atrial isolated myocytes, 2) to compare BMP10 secreted levels between right atrial, left atrial and left ventricular myocytes, and 3) to assess if BMP10 secretion is indeed associated with AF in human myocytes. Methods Right atrial (RA), left atrial (LA) and left ventricular (LV) myocytes were isolated from 16 tissues samples of patients undergoing open heart surgery which were currently in sinus rhythm (SR) or AF. Freshly isolated myocytes were resuspended in minimal essential medium (MEM, M1) containing fetal bovine serum (FBS) and plated in laminin coated dishes. M1 was collected after 3 hours of culture and substituted by FBS-free medium (M2), which was collected after 48 hours of culture. Cardiomyocyte-released BMP10 levels were measured in M1 and M2 by ELISA. Results Cardiomyocytes from twelve patients (68±2.7 years, 52±2.1 % ejection fraction, 17 % women, 50 % with AF, 50 % valvular surgery and 75 % bypass) were cultured and BMP10 secreted levels analyzed. BMP10 concentrations were high in medium from atrial cardiomyocytes (2.76±1.8 ng/µl) and negligible in medium from ventricular cardiomyocytes (0.01±0.005 ng/µl). Left atrial cardiomyocytes released less BMP10 than right atrial cardiomyocytes (LA 0.02±0.007 ng/µl; RA 3.50±2.3 ng/µl). In addition, BMP10 concentrations were higher in right atrial cardiomyocytes from patients with AF (3.76±3.6 ng/µl) than in the right atrial cardiomyocytes from patients in SR (0.97±0.9 ng/µl). Conclusion Our results provide a direct proof that BMP10 is secreted by human atrial myocytes, mainly right atrial myocytes. Furthermore, atrial myocytes from patients with AF secreted significantly more BMP10 than myocytes from patients in sinus rhythm. These observations validate the atrial cardiomyocyte origin of circulating BMP10, making BMP10 a potentially attractive atrial-specific cardiac biomarker.

BS-452682-4 EFFECTS OF CHRONIC ANGIOTENSIN CONVERTING ENZYME INHIBITION ON ATRIAL FUNCTION AND ARRHYTHMOGENESIS IN AGED AND FRAIL MICE

Aging is associated atrial remodeling and increased susceptibility to atrial fibrillation (AF); however, aging-dependent changes can be variable. Frailty measures health status independent of chronological age, can be quantified using a frailty index (FI) and can account for heterogeneity in aging individuals. Angiotensin converting enzyme inhibition (ACEi) can impact frailty however its effects on AF and atrial remodeling are unknown. To determine the effects of chronic ACEi on atrial structure, function, and arrhythmogenesis in aged and frail mice. Wildtype mice were treated with the ACEi enalapril (20 mg/kg/day) or placebo for 6 months starting at 75 weeks of age. Mice aged 10-15 weeks were used as young controls. Mice were used in intracardiac electrophysiology, high resolution optical mapping, patch-clamping, and histology. ACEi reduced FI in aged mice (0.24 vs 0.36; n=27-31; P<0.05) without altering blood pressure. AF susceptibility was increased in aged vs young mice (100 vs 17%; n=6; P<0.05) and reduced in ACEi vs aged mice (43 vs 100%; n=6-7; P<0.05). P wave duration was reduced in ACEi vs aged mice (27.4 vs 32.5 ms; n=7; P<0.05), showed variability within age/treatment groups, and was correlated and graded by FI (R=0.91; n=20; P<0.05). Optical mapping revealed an age dependent reduction in atrial conduction velocity (CV; 35.3 vs 40.2 cm/s; n=6-9; P<0.05) while ACEi improved CV in aged mice (37.7 vs 35.3 cm/s; n=6-9; P=0.05). CV was also correlated with FI score (R=0.72; n=24; P<0.05). Patch-clamp studies in isolated left atrial (LA) myocytes identified a reduction in action potential upstroke velocity (Vmax; 150.8 vs 165.4 V/s; n=18-32; P<0.05) that was associated with a reduction in peak sodium current (INa; -38.3 vs -52.1 pA/pF; n=7-19; P<0.05) in aged vs young mice. INa was increased in ACEi vs aged (-47.2 vs -38.3 pA/pF; n=14–19; P=0.09) and not different (P=0.62) vs young mice. Importantly, Vmax (R=0.32; n=73; P<0.05) and INa (R=0.5; n=36; P < 0.05) were graded by FI. LA fibrosis was increased in aged mice vs young (10.8 vs 3.9%; n=5-8; P<0.05), reduced in ACEi vs aged mice (8.9 vs 10.8%; n=5-8; P<0.05), and strongly correlated with FI (R=0.89; n=19; P<0.05). ACEi reduced frailty in aging mice and attenuated age-related AF and atrial remodeling. Aging was associated with substantial variability within groups. Frailty was a strong predictor of changes atrial function in aging mice including after ACEi treatment, independent of chronological age.

Pitx2c deficiency confers cellular electrophysiological hallmarks of atrial fibrillation to isolated atrial myocytes

AimsAtrial fibrillation (AF) has been associated with altered expression of the transcription factor Pitx2c and a high incidence of calcium release-induced afterdepolarizations. However, the relationship between Pitx2c expression and defective calcium homeostasis remains unclear and we here aimed to determine how Pitx2c expression affects calcium release from the sarcoplasmic reticulum (SR) and its impact on electrical activity in isolated atrial myocytes. MethodsTo address this issue, we applied confocal calcium imaging and patch-clamp techniques to atrial myocytes isolated from a mouse model with conditional atrial-specific deletion of Pitx2c. ResultsOur findings demonstrate that heterozygous deletion of Pitx2c doubles the calcium spark frequency, increases the frequency of sparks/site 1.5-fold, the calcium spark decay constant from 36 to 42 ms and the wave frequency from none to 3.2 min−1. Additionally, the cell capacitance increased by 30% and both the SR calcium load and the transient inward current (ITI) frequency were doubled. Furthermore, the fraction of cells with spontaneous action potentials increased from none to 44%. These effects of Pitx2c deficiency were comparable in right and left atrial myocytes, and homozygous deletion of Pitx2c did not induce any further effects on sparks, SR calcium load, ITI frequency or spontaneous action potentials. ConclusionOur findings demonstrate that heterozygous Pitx2c deletion induces defects in calcium homeostasis and electrical activity that mimic derangements observed in right atrial myocytes from patients with AF and suggest that Pitx2c deficiency confers cellular electrophysiological hallmarks of AF to isolated atrial myocytes.

Sarcomere remodeling of left atrial myocytes in non-failing atrial fibrillation patients.

Atrial fibrillation (AF) significantly contributes to adverse cardiac events like stroke. To identify novel pathways to target therapeutically, we performed mass spectrometry on left atrial (LA) tissue from patients in Sinus Rhythm (SR, n = 3) or with AF (n = 3), with no other cardiovascular disease that could confound interpretation. Pathway analysis identified inflammatory, metabolic, and extracellular matrix pathways as highly altered in AF, all of which are currently active areas of AF research. However, contractile proteins were also significantly altered, and the role of the sarcomere in AF remains mostly unstudied. To understand the functional impact of AF on the sarcomere, we measured force-calcium relationships in single cardiomyocytes from LA and left ventricular (LV) tissue from SR (n = 6) and AF (n = 3) patients. Surprisingly, maximal calcium-activated force (Fmax) was almost doubled in AF compared to SR patients. This result contradicts previous work which observed depressed contractile function in AF patients; however, these patients had heart failure and valve disease. Ventricular cells from these patients showed no change in Fmax (expected in non-failing patients), but a slight calcium sensitization in AF due to decreased troponin I (TnI) phosphorylation, possibly in compensation for elevated heart rate. Mass spec analysis showed that AF atria had decreased myosin-binding protein H-like, slow skeletal TnI, and increased alpha tropomyosin and beta-MHC (confirmed by gel electrophoresis). These changes suggest that AF drives a more “ventricular-like” or mature sarcomere in atrial myocytes, resulting in increased force production that is also similar to ventricular myocytes. Together, these data provide insight into the development and outcomes of atrial fibrillation in absence of other cardiovascular conditions in humans and suggests that the sarcomere may be a feasible therapeutic target in atrial fibrillation.

Different roles of P2X4- and P2Y1-receptors in the atrial pathogenesis during prolonged pressure overload.

Pressure overload (PO) involve increase in interstitial fluid shear in cardiac muscle during cardiac cycles due to detachment of myocytes from the extracellular matrix and myocyte slippage. However, the effects of shear stress on cardiac muscle and their pathophysiological roles are poorly understood. We have recently discovered that shear stress induces global Ca2+ waves via activation of P2 purinergic receptors in atrial myocytes by ATP released from the cells, and that this ATP release is mediated by connexin 43 (Cx43) gap junction hemichannels. In this study, we investigated role of shear stress signaling in PO-induced atrial remodeling and failure. We used a transverse aortic constriction (TAC) rat model to increase afterload and examined shear stress responses and remodeling of their signaling molecules during the development of pathological phenotypes by progressive TAC. After 5-week TAC, atrial dimension was enlarged with ventricular hypertrophy, which was followed by severe atrial dilation as well as ventricular failure at long-term (>20-week) TAC. Atrial myocytes were enlarged following 5-week TAC, which was maintained during more prolonged TAC. In the 5-week-TAC left atrial (LA) cells, ATP release was increased with no changes in Cx43 expression and in Cx43 hemichannel-mediated dye (calcein) flux. At this stage, however, function (cation current) of P2X4 receptor (P2X4R) and their protein expressions were largely enhanced. In failed LA cells, reduction in the Cx43 expression and smaller Cx43 hemichannel-mediated dye flux were observed. In failed LA myocytes, shear-induced longitudinal Ca2+ waves that were mediated by P2Y1 receptor (P2Y1R), were enhanced with higher P2Y1R expression. The opposite changes in the shear-induced responses mediated by P2X4R and P2Y1R and in their expressions suggest that P2X4R and P2Y1R may play beneficial and harmful role, respectively, in the PO-dependent atrial pathogenesis.

Regional and temporal progression of atrial remodeling in angiotensin II mediated atrial fibrillation

Atrial fibrillation (AF) is associated with electrical and structural remodeling in the atria; however, the regional and temporal progression of atrial remodeling is incompletely understood. The objective of this study was to investigate the regional and temporal progression of atrial remodeling leading to changes in AF susceptibility in angiotensin II (Ang II) mediated hypertension. Mice were infused with Ang II for 3, 10 or 21days. AF susceptibility and atrial electrophysiology were studied in vivo using intracardiac electrophysiology. Right and left atrial myocyte electrophysiology was studied using patch-clamping. Atrial fibrosis was assessed histologically. P wave duration and atrial effective refractory period increased progressively from 3 to 21days of Ang II. AF susceptibility tended to be increased at 10days of Ang II and was elevated at 21days of Ang II. Left, but not right, atrial AP upstroke velocity and Na+ current were reduced at 10 and 21days of Ang II. Left atrial action potential (AP) duration increased progressively from 3 to 21days of Ang II due to reductions in repolarizing K+ current. Right atrial AP prolongation was increased only after 21days of Ang II. Left and right atrial fibrosis developed progressively from 3 to 21days, but increases were larger in the left atrium. In conclusion, Ang II mediated atrial electrical and structural remodeling develop earlier and more extensively in the left atrium compared to the right atrium, providing insight into how atrial remodeling leads to enhanced AF susceptibility in Ang II mediated hypertension.

Abstract P1101: Mitochondrial Ca Uniporter Complex, ROS And CaT Alternans

Background: Atrial calcium transient (CaT) alternans is defined as beat-to-beat alternation in CaT amplitude and is causally linked to atrial fibrillation (AF). Inhibition of mitochondrial Ca uptake via the mitochondrial Ca uniporter complex (MCUC) facilitates CaT alternans and can lead to an increase in mitochondrial reactive oxygen species(ROS) production that compromises the cellular antioxidative capacity. The objective was to determine the mechanisms through which mitochondrial Ca uptake and ROS production determine the susceptibility of atrial cells to develop CaT alternans. Methods: CaT alternans was measured in single rabbit left atrial myocytes with fluorescence microscopy using the fluorescent Ca indicator Cal-520/AM, and for simultaneous measurements of ROS and alternans, cells were loaded with CM-DCFDA and Cal-590/AM. CaT alternans was induced by increasing the pacing frequency until stable CaT alternans was observed. The degree of alternans was quantified as the alternans ratio (AR = 1 – S/L, where S/L is the ratio of the small to the large amplitude of a pair of alternating CaTs). Results: Exposure of LA myocytes to the specific MCUC inhibitor Ru360 increased the AR by 86%. However, the subsequent stimulation of mitochondrial Ca uptake with the MCUC activator spermine (Spm) decreased AR by 79% (p=0.028). Quercetin (a O2 .- scavenger) and Tempol (a superoxide dismutase mimetic) lowered CaT AR by 96% (p=0.0007) and 79% (p=0.0029), respectively. An oxidative challenge with tert-butylhydroperoxide (tBHP) increased AR by 140% (p=&lt;0.0001), and the reducing agent dithiothreitol (DTT) decreased AR by 39% (p=0.0194). Moreover, DTT treatment rescued CaT alternans previously enhanced by tBHP. During CaT alternans the rate of ROS production increased by 23% (p=0.026), however Spm treatment attenuated the increased ROS production. Conclusion: An oxidizing environment facilitates the development of CaT alternans. During CaT alternans mitochondrial ROS formation is increased and involves MCUC activity.

Renal denervation reduces atrial remodeling in hypertensive rats with metabolic syndrome

Atrial fibrillation (AF) is highly prevalent in hypertensive patients with metabolic syndrome and is related to inflammation and activation of the sympathoadrenergic system. The multi-ligand Receptor-for-Advanced-Glycation-End-products (RAGE) activates inflammation-associated tissue remodeling and is regulated by the sympathetic nervous system. Its counterpart, soluble RAGE (sRAGE), serves as anti-inflammatory decoy receptor with protective properties. We investigated the effect of sympathetic modulation by renal denervation (RDN) on atrial remodeling, RAGE/sRAGE and RAGE ligands in metabolic syndrome. RDN was performed in spontaneously hypertensive obese rats (SHRob) with metabolic syndrome compared with lean spontaneously hypertensive rats (SHR) and with normotensive non-obese control rats. Blood pressure and heart rate were measured by telemetry. The animals were killed 12 weeks after RDN. Left atrial (LA) and right atrial (RA) remodeling was assessed by histological analysis and collagen types. Sympathetic innervation was measured by tyrosine hydroxylase staining of atrial nerve fibers, RAGE/sRAGE, RAGE ligands, cytokine expressions and inflammatory infiltrates were analyzed by Western blot and immunofluorescence staining. LA sympathetic nerve fiber density was higher in SHRob (+44%) versus controls and reduced after RDN (-64% versus SHRob). RAGE was increased (+718%) and sRAGE decreased (− 62%) in SHRob as compared with controls. RDN reduced RAGE expression (− 61% versus SHRob), significantly increased sRAGE levels (+162%) and induced a significant decrease in RAGE ligand levels in SHRob (− 57% CML and − 51% HMGB1) with reduced pro-inflammatory NFkB activation (− 96%), IL-6 production (− 55%) and reduced inflammatory infiltrates. This led to a reduction in atrial fibrosis (− 33%), collagen type I content (− 72%), accompanied by reduced LA myocyte hypertrophy (− 21%). Transfection experiments on H9C2 cardiomyoblasts demonstrated that RAGE is directly involved in fibrosis formation by influencing cellular production of collagen type I. In conclusion, suppression of renal sympathetic nerve activity by RDN prevents atrial remodeling in metabolic syndrome by reducing atrial sympathetic innervation and by modulating RAGE/sRAGE balance and reducing pro-inflammatory and pro-fibrotic RAGE ligands, which provides a potential therapeutic mechanism to reduce the development of AF.

Stretch-induced sarcoplasmic reticulum calcium leak is causatively associated with atrial fibrillation in pressure-overloaded hearts.

Despite numerous reports documenting an important role of hypertension in the development of atrial fibrillation (AF), the detailed mechanism underlying the pathological process remains incompletely understood. Here, we aim to test the hypothesis that diastolic sarcoplasmic reticulum (SR) Ca2+ leak in atrial myocytes, induced by mechanical stretch due to elevated pressure in the left atrium (LA), plays an essential role in the AF development in pressure-overloaded hearts. Isolated mouse atrial myocytes subjected to acute axial stretch displayed an immediate elevation of SR Ca2+ leak. Using a mouse model of transverse aortic constriction (TAC), the relation between stretch, SR Ca2+ leak, and AF susceptibility was further tested. At 36 h post-TAC, SR Ca2+ leak in cardiomyocytes from the LA (with haemodynamic stress), but not right atrium (without haemodynamic stress), significantly increased, which was further elevated at 4 weeks post-TAC. Accordingly, AF susceptibility to atrial burst pacing in the 4-week TAC mice were also significantly increased, which was unaffected by inhibition of atrial fibrosis or inflammation via deletion of galectin-3. Western blotting revealed that type 2 ryanodine receptor (RyR2) in left atrial myocytes of TAC mice was oxidized due to activation and up-regulation of Nox2 and Nox4. Direct rescue of dysfunctional RyR2 with dantrolene or rycal S107 reduced diastolic SR Ca2+ leak in left atrial myocytes and prevented atrial burst pacing stimulated AF. Our study demonstrated for the first time the increased SR Ca2+ leak mediated by enhanced oxidative stress in left atrial myocytes that is causatively associated with higher AF susceptibility in pressure-overloaded hearts.

Distinct shear-induced Ca2+ signaling in the left and right atrial myocytes: Role of P2 receptor context

Atrial myocytes are continuously exposed to shear stress during cardiac cycles. Previous reports have shown that shear stress induces two different types of global Ca2+ signaling in atrial myocytes–longitudinal Ca2+ waves (L-waves) and action potential-involved transverse waves (T-waves), and suggested an underlying role of the autocrine activation of P2 receptors. We explored the correlations between ATP release and Ca2+ wave generation in atrial myocytes and investigated why the cells develop two Ca2+-wave types during the same shear force. We examined whether ATP release correlates with different shear-stress (~16 dyn/cm2)-mediated Ca2+ signaling by simultaneous measurement of local Ca2+ and ATP release in individual atrial myocytes using two-dimensional confocal imaging and sniffer patch techniques, respectively. Functional P2X7-receptor-expressing HEK293 cells were established as sniffer cells, which generated currents in real time in response to ATP released from a closely positioned atrial myocyte. Both shear-stress-induced L- and T-waves were preceded by sniffer currents with no difference in the current magnitude. Left atrial (LA) myocytes had two- to three-fold larger sniffer currents than right atrial (RA) cells, as was confirmed by ATP chemiluminescence assay. Shear-stress-induced ATP release was eliminated by connexin (Cx) 43 hemichannel inhibition using La3+, Gap19, or knock-down of Cx43 expression. The level of phosphorylated Cx43 at Ser386 (p-Cx43Ser368), but not total Cx43, was higher in LA versus RA myocytes. Most LA cells (~70%) developed L-waves, whereas most RA myocytes (~80%) presented T-waves. Shear-stress-induced T-waves were completely removed by inhibition of P2X4R, which were most abundant in rat atrial cells. Expression of P2X4R was higher in RA than LA myocytes, whereas expression of P2Y1R, the mediator of L-waves, was higher in LA than RA myocytes. ATP release mainly triggers L-waves in LA myocytes and T-waves in RA myocytes under the same shear force, partly because of the differential expression of P2Y1R and P2X4R between LA and RA myocytes. Higher ATP release in LA myocytes under shear stress may not contribute to determination of the wave pattern.

Reduced Left Atrial Emptying Fraction and Chymase Activation in Pathophysiology of Primary Mitral Regurgitation

Increasing left atrial (LA) size predicts outcomes in patients with isolated mitral regurgitation (MR). Chymase is plentiful in the human heart and affects extracellular matrix remodeling. Chymase activation correlates to LA fibrosis, LA enlargement, and a decreased total LA emptying fraction in addition to having a potential intracellular role in mediating myofibrillar breakdown in LA myocytes. Because of the unreliability of the left ventricular ejection fraction in predicting outcomes in MR, LA size and the total LA emptying fraction may be more suitable indicators for timing of surgical intervention.

Different Context for Shear Signaling in Left Versus Right Atrial Myocytes: Differential Roles of P2Y1- and P2X4-Purinoceptors

Different Context for Shear Signaling in Left Versus Right Atrial Myocytes: Differential Roles of P2Y1- and P2X4-Purinoceptors

NPR-C (Natriuretic Peptide Receptor-C) Modulates the Progression of Angiotensin II-Mediated Atrial Fibrillation and Atrial Remodeling in Mice.

Atrial fibrillation (AF) commonly occurs in hypertension and in association with elevated Ang II (angiotensin II) levels. The specific mechanisms underlying Ang II-mediated AF are unclear, and interventions to prevent the effects of Ang II are lacking. NPs (natriuretic peptides), which elicit their effects through specific NP receptors, including NPR-C (natriuretic peptide receptor-C), are cardioprotective hormones that affect cardiac structure and function. This study used wild-type and NPR-C knockout (NPR-C-/-) mice to investigate the effects of Ang II (3 mg/kg per day for 3 weeks) on AF susceptibility and atrial function using in vivo electrophysiology, high-resolution optical mapping, patch clamping, and molecular biology. In some experiments, wild-type mice were cotreated with Ang II and the NPR-C agonist cANF (0.07-0.14 mg/kg per day) for 3 weeks. In wild-type mice, Ang II increased susceptibility to AF in association with a prolongation of P-wave duration, increased atrial refractory period, and slowed atrial conduction. These effects were exacerbated in Ang II-treated NPR-C-/- mice. Ang II prolonged action potential duration and reduced action potential upstroke velocity (Vmax). These effects were greater in left atrial myocytes from Ang II-treated NPR-C-/- mice. Ang II also increased fibrosis in both atria in wild-type mice, whereas Ang II-treated NPR-C-/- mice exhibited substantially higher fibrosis throughout the atria. Fibrotic responses were associated with changes in expression of profibrotic genes, including TGFβ and TIMP1. Cotreating wild-type mice with Ang II and the NPR-C agonist cANF dose dependently reduced AF inducibility by preventing some of the Ang II-induced changes in atrial myocyte electrophysiology and preventing fibrosis throughout the atria. NPR-C may represent a new target for the prevention of Ang II-induced AF via protective effects on atrial electrical and structural remodeling.

The role of Natriuretic Peptide Receptor C in atrial electrophysiological remodelling in hypertensive heart disease

Atrial fibrillation (AF) is prevalent in hypertension in association with alterations in angiotensin II (Ang II) signaling. Natriuretic peptides (NPs) are a family of hormones with potent effects on atrial electrophysiology mediated in part through the NPR-C receptor. The goal of this study was to determine the role of NPR-C in atrial electrophysiological remodelling in Ang II treated mice (3 mg/kg/day for 3 weeks). Ang II treatment increased the susceptibility to AF in WT (P = 0.001) and was more severe in NPR-C-/- (P = 0.03) mice vs. saline controls. Action potential (AP) upstroke velocity (Vmax; P = 0.01) and peak Na+ current (INa; P < 0.001) were reduced in left atrial (LA) myocytes from Ang II treated WT mice, which was attributed to enhanced PKC⍺ signalling. Ang II differentially prolonged the AP in right atrial (RA; APD50; P < 0.001) and LA (P < 0.001) myocytes. In addition, Ang II reduced Ito in RA myocytes (P < 0.001) and to a greater extent in LA myocytes (P = 0.045). In the RA of NPR-C-/‑ mice, Ang II prolonged (P < 0.05) APD50 to a similar extent vs. WT. In contrast, Vmax was reduced and APD50 was prolonged by Ang II to a greater extent (P < 0.05) in the LA of NPR-C-/- vs. WT mice. Ito was similarly reduced (P<0.05) in the RA and LA of Ang II treated WT and NPR-C-/- mice. Co-treatment with cANF (NPR-C agonist) reduced the susceptibility to AF (P = 0.04) in WT mice vs. Ang II alone. Strikingly, APD50 was reduced (P = 0.01) and Ito was increased (P = 0.001) in the RA, but not LA, by cANF co-treatment. Collectively, these data demonstrate that NPR-C plays a critical role in modulating disease progression in the atria elicited by Ang II treatment.

Distinct patterns of atrial electrical and structural remodeling in angiotensin II mediated atrial fibrillation

Atrial fibrillation (AF) is prevalent in hypertension and elevated angiotensin II (Ang II); however, the mechanisms by which Ang II leads to AF are poorly understood. Here, we investigated the basis for this in mice treated with Ang II or saline for 3 weeks. Ang II treatment increased susceptibility to AF compared to saline controls in association with increases in P wave duration and atrial effective refractory period, as well as reductions in right and left atrial conduction velocity. Patch-clamp studies demonstrate that action potential (AP) duration was prolonged in right atrial myocytes from Ang II treated mice in association with a reduction in repolarizing K+ currents. In contrast, APs in left atrial myocytes from Ang II treated mice showed reductions in upstroke velocity and overshoot, as well as greater prolongations in AP duration. Ang II reduced Na+ current (INa) in the left, but not the right atrium. This reduction in INa was reversible following inhibition of protein kinase C (PKC) and PKCα expression was increased selectively in the left atrium in Ang II treated mice. The transient outward K+ current (Ito) showed larger reductions in the left atrium in association with a shift in the voltage dependence of activation. Finally, Ang II caused fibrosis throughout the atria in association with changes in collagen expression and regulators of the extracellular matrix. This study demonstrates that hypertension and elevated Ang II cause distinct patterns of electrical and structural remodeling in the right and left atria that collectively create a substrate for AF.

Increased thin filament activation enhances alternans in human chronic atrial fibrillation.

Action potential duration (APD) alternans (APD-ALT), defined as beat-to-beat oscillations in APD, has been proposed as an important clinical marker for chronic atrial fibrillation (cAF) risk when it occurs at pacing rates of 120-200 beats/min. Although the ionic mechanisms for occurrence of APD-ALT in human cAF at these clinically relevant rates have been investigated, little is known about the effects of myofilament protein kinetics on APD-ALT. Therefore, we used computer simulations of single cell function to explore whether remodeling in myofilament protein kinetics in human cAF alters the occurrence of APD-ALT and to uncover how these mechanisms are affected by sarcomere length and the degree of cAF-induced myofilament remodeling. Mechanistically based, bidirectionally coupled electromechanical models of human right and left atrial myocytes were constructed, incorporating both ionic and myofilament remodeling associated with cAF. By comparing results from our electromechanical model with those from the uncoupled ionic model, we found that intracellular Ca2+ concentration buffering of troponin C has a dampening effect on the magnitude of APD-ALT (APD-ANM) at slower rates (150 beats/min) due to the cooperativity between strongly bound cross-bridges and Ca2+-troponin C binding affinity. We also discovered that cAF-induced enhanced thin filament activation enhanced APD-ANM at these clinically relevant heart rates (150 beats/min). In addition, longer sarcomere lengths increased APD-ANM, suggesting that atrial stretch is an important modulator of APD-ALT. Together, these findings demonstrate that myofilament kinetics mechanisms play an important role in altering APD-ALT in human cAF. NEW & NOTEWORTHY Using a single cell simulation approach, we explored how myofilament protein kinetics alter the formation of alternans in action potential duration (APD) in human myocytes with chronic atrial fibrillation remodeling. We discovered that enhanced thin filament activation and longer sarcomere lengths increased the magnitude of APD alternans at clinically important pacing rates of 120-200 beats/min. Furthermore, we found that altered intracellular Ca2+ concentration buffering of troponin C has a dampening effect on the magnitude of APD alternans.

Ultrastructural Morphological Characterization of Right Atrial and Left Ventricular Rat Cardiomyocytes during Postreperfusion Period.

Immunocytochemistry and transmission electron microscopy were employed to examine the ultrastructural morphometric parameters of the left ventricular cardiomyocytes and right atrial secretory myocytes in rats during early and delayed postreperfusion periods. The revealed alterations in these cells are stereotypical, but differed by their severity, probably due to specific morphofunctional peculiarities of these heart structures.

Protein Phosphatase Inhibitor-1 Gene Therapy in a Swine Model of Nonischemic Heart Failure

BackgroundIncreased protein phosphatase-1 in heart failure (HF) induces molecular changes deleterious to the cardiac cell. Inhibiting protein phosphatase-1 through the overexpression of a constitutively active inhibitor-1 (I-1c) has been shown to reverse cardiac dysfunction in a model of ischemic HF. ObjectivesThis study sought to determine the therapeutic efficacy of a re-engineered adenoassociated viral vector carrying I-1c (BNP116.I-1c) in a preclinical model of nonischemic HF, and to assess thoroughly the safety of BNP116.I-1c gene therapy. MethodsVolume-overload HF was created in Yorkshire swine by inducing severe mitral regurgitation. One month after mitral regurgitation induction, pigs were randomized to intracoronary delivery of either BNP116.I-1c (n = 6) or saline (n = 7). Therapeutic efficacy and safety were evaluated 2 months after gene delivery. Additionally, 24 naive pigs received different doses of BNP116.I-1c for safety evaluation. ResultsAt 1 month after mitral regurgitation induction, pigs developed HF as evidenced by increased left ventricular end-diastolic pressure and left ventricular volume indexes. Treatment with BNP116.I-1c resulted in improved left ventricular ejection fraction (−5.9 ± 4.2% vs. 5.5 ± 4.0%; p < 0.001) and adjusted dP/dt maximum (−3.39 ± 2.44 s-1 vs. 1.30 ± 2.39 s-1; p = 0.007). Moreover, BNP116.I-1c-treated pigs also exhibited a significant increase in left atrial ejection fraction at 2 months after gene delivery (−4.3 ± 3.1% vs. 7.5 ± 3.1%; p = 0.02). In vitro I-1c gene transfer in isolated left atrial myocytes from both pigs and rats increased calcium transient amplitude, consistent with its positive impact on left atrial contraction. We found no evidence of adverse electrical remodeling, arrhythmogenicity, activation of a cellular immune response, or off-target organ damage by BNP116.I-1c gene therapy in pigs. ConclusionsIntracoronary delivery of BNP116.I-1c was safe and improved contractility of the left ventricle and atrium in a large animal model of nonischemic HF.