1,721,055 research outputs found
Myofibroblasts induce ectopic activity in cardiac tissue
No full textFocal ectopic activity in cardiac tissue is a key factor in the initiation and perpetuation of tachyarrhythmias. Because myofibroblasts as present in fibrotic remodeled myocardia and infarct scars depolarize cardiomyocytes by heterocellular electrotonic interactions via gap junctions in vitro, we investigated using strands of cultured ventricular cardiomyocytes coated with myofibroblasts, whether this interaction might give rise to depolarization-induced abnormal automaticity. Whereas uncoated cardiomyocyte strands were invariably quiescent, myofibroblasts induced synchronized spontaneous activity in a density dependent manner. Activations appeared at spatial myofibroblast densities >15.7% and involved more than 80% of the preparations at myofibroblast densities of 50%. Spontaneous activity was based on depolarization-induced automaticity as evidenced by: (1) suppression of activity by the sarcolemmal K(ATP) channel opener P-1075; (2) induction of activity in current-clamped single cardiomyocytes undergoing depolarization to potentials similar to those induced by myofibroblasts in cardiomyocyte strands; and (3) induction of spontaneous activity in cardiomyocyte strands coated with connexin 43 transfected Hela cells but not with communication deficient HeLa wild-type cells. Apart from unveiling the mechanism underlying the hallmark of monolayer cultures of cardiomyocytes, ie, spontaneous electromechanical activity, these findings open the perspective that myofibroblasts present in structurally remodeled myocardia following pressure overload and infarction might contribute to arrhythmogenesis by induction of ectopic activity
Electrotonic modulation of cardiac impulse conduction by myofibroblasts
No full textStructural remodeling of the myocardium associated with mechanical overload or cardiac infarction is accompanied by the appearance of myofibroblasts. These fibroblast-like cells express alpha-smooth muscle actin (alphaSMA) and have been shown to express connexins in tissues other than heart. The present study examined whether myofibroblasts of cardiac origin establish heterocellular gap junctional coupling with cardiomyocytes and whether ensuing electrotonic interactions affect impulse propagation. For this purpose, impulse conduction characteristics (conduction velocity [theta] and maximal upstroke velocity [dV/dtmax]) were assessed optically in cultured strands of cardiomyocytes, which were coated with fibroblasts of cardiac origin. Immunocytochemistry showed that cultured fibroblasts underwent a phenotype switch to alphaSMA-positive myofibroblasts that expressed connexin 43 and 45 both among themselves and at contact sites with cardiomyocytes. Myofibroblasts affected theta and dV/dtmax in a cell density-dependent manner; a gradual increase of myofibroblast-to-cardiomyocyte ratios up to 7:100 caused an increase of both theta and dV/dtmax, which was followed by a progressive decline at higher ratios. On full coverage of the strands with myofibroblasts (ratio >20:100), theta fell <200 mm/s. This biphasic dependence of theta and dV/dtmax on myofibroblast density is reminiscent of "supernormal conduction" and is explained by a myofibroblast density-dependent gradual depolarization of the cardiomyocyte strands from -78 mV to -50 mV as measured using microelectrode recordings. These findings suggest that myofibroblasts, apart from their role in structural remodeling, might contribute to arrhythmogenesis by direct electrotonic modulation of conduction and that prevention of their appearance might represent an antiarrhythmic therapeutic target
Atrial fibrillation and fibrosis: Beyond the cardiomyocyte centric view
No full textAtrial fibrillation (AF) associated with fibrosis is characterized by the appearance of interstitial myofibroblasts. These cells are responsible for the uncontrolled deposition of the extracellular matrix, which pathologically separate cardiomyocyte bundles. The enhanced fibrosis is thought to contribute to arrhythmias "indirectly" because a collagenous septum is a passive substrate for propagation, resulting in impulse conduction block and/or zigzag conduction. However, the emerging results demonstrate that myofibroblasts in vitro also promote arrhythmogenesis due to direct implications upon cardiomyocyte electrophysiology. This electrical interference may be considered beneficial as it resolves any conduction blocks; however, the passive properties of myofibroblasts might cause a delay in impulse propagation, thus promoting AF due to discontinuous slow conduction. Moreover, low-polarized myofibroblasts reduce, via cell-density dependence, the fast driving inward current for cardiac impulse conduction, therefore resulting in arrhythmogenic uniformly slow propagation. Critically, the subsequent reduction in cardiomyocytes resting membrane potential in vitro significantly increases the likelihood of ectopic activity. Myofibroblast densities and the degree of coupling at cellular border zones also impact upon this likelihood. By considering future in vivo studies, which identify myofibroblasts "per se" as a novel targets for cardiac arrhythmias, this review aims to describe the implications of noncardiomyocyte view in the context of AF
Mitochondrial mechanosensor microdomains in cardiovascular disorders
No full textThe cardiomyocytes populating the ‘working myocardium’ are highly organized and such organization ranges from macroscale (e.g. the geometrical rod shape) to microscale (dyad/t-tubules) domains. This meticulous level of organization is imperative for assuring the normal and physiological pump-function of the heart. In the pathological cardiac tissue, the domains-related architecture is partially lost, resulting in morphological, electrical and metabolic remodeling and promoting cardiovascular diseases including heart failure and arrhythmias. Indeed, arrhythmogenesis during heart failure is a major clinical problem. Arrhythmias have been extensively studied from an electrical etiology, but only recently, physiologists and scientists have focused their attention on cellular and subcellular mechanosensors. We and others have investigated whether the nanoscale mechanosensitive properties of cardiomyocytes from failing hearts have a bearing upon the initiation of abnormal electrical activity. This chapter highlights the recent findings in the field, especially the role of mitochondria function and alignment in failing cardiomyocytes interrogated via nanomechanical stimuli
Altered Mitochondrial Metabolism and Mechanosensation in the Failing Heart: Focus on Intracellular Calcium Signaling
No full textThe heart consists of millions of cells, namely cardiomyocytes, which are highly organized in terms of structure and function, at both macroscale and microscale levels. Such meticulous organization is imperative for assuring the physiological pump-function of the heart. One of the key players for the electrical and mechanical synchronization and contraction is the calcium ion via the well-known calcium-induced calcium release process. In cardiovascular diseases, the structural organization is lost, resulting in morphological, electrical, and metabolic remodeling owing the imbalance of the calcium handling and promoting heart failure and arrhythmias. Recently, attention has been focused on the role of mitochondria, which seem to jeopardize these events by misbalancing the calcium processes. In this review, we highlight our recent findings, especially the role of mitochondria (dys)function in failing cardiomyocytes with respect to the calcium machinery
Heart rate variability in neonatal seizures: Investigation and implications for management
No full textMany factors acting during the neonatal period can affect neurological development
of the infant. Neonatal seizures (NS) that frequently occur in the immature brain
may influence autonomic maturation and lead to detectable cardiovascular signs. These
autonomic manifestations can also have significant diagnostic and prognostic value. The
analysis of Heart Rate Variability (HRV) represents the most used and feasible method to
evaluate cardiac autonomic regulation. This narrative review summarizes studies investigating
HRV dynamics in newborns with seizures, with the aim of highlighting the potential
utility of HRV measures for seizure detection and management. While HRV analysis in critically
ill newborns is influenced by many potential confounders, we suggest that it can
enhance the ability to better diagnose seizures in the clinical setting. We present potential
applications of the analysis of HRV, which could have a useful future role, beyond the
research setting
Coupling of cardiac electrical activity over extended distances by fibroblasts of cardiac origin
No full textRoughly half of the cells of the heart consist of nonmyocardial cells, with fibroblasts representing the predominant cell type. It is well established that individual cardiomyocytes and fibroblasts in culture establish gap junctional communication at the single cell level (short-range interaction). However, it is not known whether such coupling permits activation of cardiac tissue over extended distances (long-range interaction). Long-range interactions may be responsible for electrical synchronization of donor and recipient tissue after heart transplantation and may play a role in arrhythmogenesis. This question was investigated using a novel heterocellular culture model with strands of cardiomyocytes interrupted by cardiac fibroblasts over defined distances. With use of optical recording techniques, it could be shown that impulse propagation along fibroblast inserts was successful over distances up to 300 μm and was characterized by length-dependent local propagation delays ranging from 11 to 68 ms (apparent local "conduction velocities" 4.6±1.8 mm/s, n=23). Involvement of mechanical stretch in this phenomenon was excluded by showing that inserts consisting of communication-deficient HeLa cells were incapable of supporting propagation. In contrast, HeLa cells expressing connexin43 permitted impulse conduction over distances as long as 600 μm. Immunocytochemistry showed that fibroblasts and cardiomyocytes expressed connexin43 and connexin45, whereas connexin40 was absent. These results illustrate that fibroblasts of cardiac origin are capable of synchronizing electrical activity of multicellular cardiac tissue over extended distances through electrotonic interactions. This synchronization is accompanied by extremely large local conduction delays, which might contribute to the generation of arrhythmias in fibrotic hearts
Effects of combustion-derived ultrafine particles and manufactured nanoparticles on heart cells in vitro
No full textEvidence from epidemiological studies indicates that acute exposure to airborne pollutants is associated with an increased risk of morbidity and mortality attributed to cardiovascular diseases. The present study investigated the effects of combustion-derived ultrafine particles (diesel exhaust particles) as well as engineered nanoparticles (titanium dioxide and single-walled carbon nanotubes) on impulse conduction characteristics, myofibrillar structure and the formation of reactive oxygen species in patterned growth strands of neonatal rat ventricular cardiomyocytes in vitro. Diesel exhaust particles as well as titanium dioxide nanoparticles showed the most pronounced effects. We observed a dose-dependent change in heart cell function, an increase in reactive oxygen species and, for titanium dioxide, we also found a less organized myofibrillar structure. The mildest effects were observed for single-walled carbon nanotubes, for which no clear dose-dependent alterations of theta and dV/dt(max) could be determined. In addition, there was no increase in oxidative stress and no change in the myofibrillar structure. These results suggest that diesel exhaust as well as titanium dioxide particles and to a lesser extent also single-walled carbon nanotubes can directly induce cardiac cell damage and can affect the function of the cells
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