The susceptibility of the rat pulmonary and caval vein myocardium to the catecholamine-induced ectopy changes oppositely in postnatal development.

KEY POINTS: The developmental changes of the caval (SVC) and pulmonary vein (PV) myocardium electrophysiology are traced throughout postnatal ontogenesis. The myocardium in SVC as well as in PV demonstrate age-dependent differences in the ability to maintain resting membrane potential, to manifest automaticity in a form of ectopic action potentials in basal condition and in responses to the adrenergic stimulation. Electrophysiological characteristics of two distinct types of thoracic vein myocardium change in an opposite manner during early postnatal ontogenesis with increased proarrhythmicity of pulmonary and decreased automaticity in caval veins. Predisposition of PV cardiac tissue to proarrhythmycity develops during ontogenesis in time correlation with the establishment of sympathetic innervation of the tissue. The electrophysiological properties of caval vein cardiac tissue shift from a pacemaker-like phenotype to atrial phenotype in accompaniment with sympathetic nerve growth and adrenergic receptor expression changes. ABSTRACT: The thoracic vein myocardium is considered as a main source for atrial fibrillation initiation due to its high susceptibility to ectopic activity. The mechanism by which and when pulmonary (PV) and superior vena cava (SVC) became proarrhythmic during postnatal ontogenesis is still unknown. In this study, we traced postnatal changes of electrophysiology in a correlation with the sympathetic innervation and adrenergic receptor distribution to reveal developmental differences in proarrhythmicity occurrence in PV and SVC myocardium. A standard microelectrode technique was used to assess the changes in ability to maintain resting membrane potential (RMP), generate spontaneous action potentials (SAP) and adrenergically induced ectopy in multicellular SVC and PV preparations of rats of different postnatal ages. Immunofluorescence imaging was used to trace postnatal changes in sympathetic innervation, ß1- and α1A-adrenergic receptor (AR) distribution. We revealed that the ability to generate SAP and susceptibility to adrenergic stimulation changes during postnatal ontogenesis in an opposite manner in PV and SVC myocardium. While SAP occurrence decreases with age in SVC myocardium, it significantly increases in PV cardiac tissue. PV myocardium starts to demonstrate RMP instability and proarrhythmic activity from the 14th day of postnatal life which correlates with the appearance of the sympathetic innervation of the thoracic veins. In addition, postnatal attenuation of SVC myocardium automaticity occurs concomitantly with sympathetic innervation establishment and increase in ß1-ARs, but not α1A-AR levels. Our results support the contention that SVC and PV myocardium electrophysiology change during postnatal development, resulting in higher PV proarrhythmicity in adults.

Rat caval vein myocardium undergoes changes in conduction characteristics during postnatal ontogenesis.

The electrophysiological properties of the superior vena cava (SVC) myocardium, which is considered a minor source of atrial arrhythmias, were studied in this study during postnatal development. Conduction properties were investigated in spontaneously active and electrically paced SVC preparations obtained from 7-60-day-old male Wistar rats using optical mapping and microelectrode techniques. The presence of high-conductance connexin 43 (Cx43) was evaluated in SVC cross-sections using immunofluorescence. It was found that SVC myocardium is excitable, electrically coupled with the atrial tissue, and conducts excitation waves at all stages of postnatal development. However, the conduction velocity (CV) of excitation and action potential (AP) upstroke velocity in SVC were significantly lower in neonatal than in adult animals and increased with postnatal maturation. Connexins Cx43 were identified in both neonatal and adult rat SVC myocardium; however, the abundance of Cx43 was significantly less in neonates. The gap junction uncoupler octanol affected conduction more profound in the neonatal than in adult SVC. We demonstrated for the first time that the conduction characteristics of SVC myocardium change from a slow-conduction (nodal) to a high-conduction (working) phenotype during postnatal ontogenesis. An age-related CV increase may occur due to changes of AP characteristics, electrical coupling, and Cx43 presence in SVC cardiomyocyte membranes. Observed changes may contribute to the low proarrhythmicity of adult caval vein cardiac tissue, while pre- or postnatal developmental abnormalities that delay the establishment of the working conduction phenotype may facilitate SVC proarrhythmia.

Extracellular ATP and ß-NAD alter electrical properties and cholinergic effects in the rat heart in age-specific manner.

Extracellular ATP and nicotinamide adenine dinucleotide (ß-NAD) demonstrate properties of neurotransmitters and neuromodulators in peripheral and central nervous system. It has been shown previously that ATP and ß-NAD affect cardiac functioning in adult mammals. Nevertheless, the modulation of cardiac activity by purine compounds in the early postnatal development is still not elucidated. Also, the potential influence of ATP and ß-NAD on cholinergic neurotransmission in the heart has not been investigated previously. Age-dependence of electrophysiological effects produced by extracellular ATP and ß-NAD was studied in the rat myocardium using sharp microelectrode technique. ATP and ß-NAD could affect ventricular and supraventricular myocardium independent from autonomic influences. Both purines induced reduction of action potentials (APs) duration in tissue preparations of atrial, ventricular myocardium, and myocardial sleeves of pulmonary veins from early postnatal rats similarly to myocardium of adult animals. Both purine compounds demonstrated weak age-dependence of the effect. We have estimated the ability of ATP and ß-NAD to alter cholinergic effects in the heart. Both purines suppressed inhibitory effects produced by stimulation of intracardiac parasympathetic nerve in right atria from adult animals, but not in preparations from neonates. Also, ATP and ß-NAD suppressed rest and evoked release of acetylcholine (ACh) in adult animals. ß-NAD suppressed effects of parasympathetic stimulation and ACh release stronger than ATP. In conclusion, ATP and ß-NAD control the heart at the postsynaptic and presynaptic levels via affecting the cardiac myocytes APs and ACh release. Postsynaptic and presynaptic effects of purines may be antagonistic and the latter demonstrates age-dependence.

Association Between Antiarrhythmic, Electrophysiological, and Antioxidative Effects of Melatonin in Ischemia/Reperfusion.

Melatonin is assumed to confer cardioprotective action via antioxidative properties. We evaluated the association between ventricular tachycardia and/or ventricular fibrillation (VT/VF) incidence, oxidative stress, and myocardial electrophysiological parameters in experimental ischemia/reperfusion under melatonin treatment. Melatonin was given to 28 rats (10 mg/kg/day, orally, for 7 days) and 13 animals received placebo. In the anesthetized animals, coronary occlusion was induced for 5 min followed by reperfusion with recording of unipolar electrograms from ventricular epicardium with a 64-lead array. Effects of melatonin on transmembrane potentials were studied in ventricular preparations of 7 rats in normal and "ischemic" conditions. Melatonin treatment was associated with lower VT/VF incidence at reperfusion, shorter baseline activation times (ATs), and activation-repolarization intervals and more complete recovery of repolarization times (RTs) at reperfusion (less baseline-reperfusion difference, ΔRT) (p < 0.05). Superoxide dismutase (SOD) activity was higher in the treated animals and associated with ΔRT (p = 0.001), whereas VT/VF incidence was associated with baseline ATs (p = 0.020). In vitro, melatonin led to a more complete restoration of action potential durations and resting membrane potentials at reoxygenation (p < 0.05). Thus, the antioxidative properties of melatonin were associated with its influence on repolarization duration, whereas the melatonin-related antiarrhythmic effect was associated with its oxidative stress-independent action on ventricular activation.

Redox differences between rat neonatal and adult cardiomyocytes under hypoxia.

It is generally accepted that oxidative stress plays a key role in the development of ischemia-reperfusion injury in ischemic heart disease. However, the mechanisms how reactive oxygen species trigger cellular damage are not fully understood. Our study investigates redox state and highly reactive substances within neonatal and adult cardiomyocytes under hypoxia conditions. We have found that hypoxia induced an increase in H2O2 production in adult cardiomyocytes, while neonatal cardiomyocytes experienced a decrease in H2O2 levels. This finding correlates with our observation of the difference between the electron transport chain (ETC) properties and mitochondria amount in adult and neonatal cells. We demonstrated that in adult cardiomyocytes hypoxia caused the significant increase in the ETC loading with electrons compared to normoxia. On the contrary, in neonatal cardiomyocytes ETC loading with electrons was similar under both normoxic and hypoxic conditions that could be due to ETC non-functional state and the absence of the electrons transfer to O2 under normoxia. In addition to the variations in H2O2 production, we also noted consistent pH dynamics under hypoxic conditions. Notably, the pH levels exhibited a similar decrease in both cell types, thus, acidosis is a more universal cellular response to hypoxia. We also demonstrated that the amount of mitochondria and the levels of cardiac isoforms of troponin I, troponin T, myoglobin and GAPDH were significantly higher in adult cardiomyocytes compared to neonatal ones. Remarkably, we found out that under hypoxia, the levels of cardiac isoforms of troponin T, myoglobin, and GAPDH were elevated in adult cardiomyocytes, while their level in neonatal cells remained unchanged. Obtained data contribute to the understanding of the mechanisms of neonatal cardiomyocytes' resistance to hypoxia and the ability to maintain the metabolic homeostasis in contrast to adult ones.

The role of M3 receptors in regulation of electrical activity deteriorates in the rat heart during ageing.

Ageing is a complex process which affects all systems of the organism and therefore changes the environment where the heart is working. In this study we demonstrate the ageing-related changes in the mechanisms of parasympathetic regulation of mammalian heart. Electrophysiological effects produced by selective activation of M3-cholinoreceptors were compared in isolated cardiac preparations from young adult (4 months), adult (1 year) and ageing (2 years) rats using sharp glass microelectrode technique. M3-receptors were activated with muscarinic agonist pilocarpine (10-5M) in the presence of selective M2 antagonist AQ-RA741 (10-7M). In atrial and ventricular myocardium from young rats M3 stimulation induced shortening of action potentials(APs), while no significant effect was observed in both elder groups. The main mechanism of M3-induced AP shortening is inhibition of L-type Ca2+ current, estimated using whole-cell patch-clamp. It was negligible in atrial myocytes from ageing animals in comparison with young rats. The loss of sensitivity to stimulation of M3-receptors is due to decrease in M3 gene expression, shown by RT-PCR both in atrial and ventricular samples from ageing rats. Thus, in ageing rat heart M3-receptors are down-regulated and not involved in regulation of electrical activity.

Real-time fiber-optic recording of acute-ischemic-stroke signatures.

We present an experimental framework and methodology for in vivo studies on rat stroke models that enable a real-time fiber-optic recording of stroke-induced hydrogen peroxide and pH transients in ischemia-affected brain areas. Arrays of reconnectable implantable fiber probes combined with advanced optogenetic fluorescent protein sensors are shown to enable a quantitative multisite time-resolved study of oxidative-stress and acidosis buildup dynamics as the key markers, correlates and possible drivers of ischemic stroke. The fiber probes designed for this work provide a wavelength-multiplex forward-propagation channel for a spatially localized, dual-pathway excitation of genetically encoded fluorescence-protein sensors along with a back-propagation channel for the fluorescence return from optically driven fluorescence sensors. We show that the spectral analysis of the fiber-probe-collected fluorescence return provides means for a high-fidelity autofluorescence background subtraction, thus enhancing the sensitivity of real-time detection of stroke-induced transients and significantly reducing measurement uncertainties in in vivo acute-stroke studies as inherently statistical experiments operating with outcomes of multiply repeated measurements on large populations of individually variable animal stroke models.

Micro-RNA 133a-3p induces repolarization abnormalities in atrial myocardium and modulates ventricular electrophysiology affecting ICa,L and Ito currents.

Mir-133a-3p is the most abundant myocardial microRNA. The impact of mir-133a-3p on cardiac electrophysiology is poorly explored. In this study, we investigated the effects of mir-133a-3p on the main ionic currents critical for action potential (AP) generation and electrical activity of the heart. We used conventional ECG, sharp microelectrodes and patch-clamp to clarify a role of mir-133a-3p in normal cardiac electrophysiology in rats after in vivo and in vitro transfection. Mir-133a-3p caused no changes to pacemaker APs and automaticity in the sinoatrial node. No significant changes in heart rate (HR) were observed in vivo; however, miR transfection facilitated HR increase in response to ß-adrenergic stimulation. Mir-133a-3p induced repolarization abnormalities in the atrial working myocardium and the L-type calcium current (ICa,L) was significantly increased. The main repolarization currents, including the transient outward (Ito), ultra-rapid (IK,ur), and inward rectifier (IK1) remained unaffected in atrial cardiomyocytes. Mir-133a-3p affected both ICa,L and Ito in ventricular cardiomyocytes. Systemic administration of mir-133a-3p induced QT-interval prolongation. Bioinformatic analysis revealed protein phosphatase 2 (PPP2CA/B) and Kcnd3 (encoding Kv4.3 channels generating Ito) as the main miR-133a-3p targets in the heart. No changes in mRNA expression of Cacna1c (encoding Cav1.2 channels generating ICa,L) and Kcnd3 were seen in mir-133a-3p treated rats. However, the expression of Ppp2cA, encoding PPP2CA, and Kcnip2 encoding KChIP2, a Kv4.3 regulatory protein, were significantly decreased. The accumulation of mir-133a-3p in cardiac myocytes causes chamber-specific electrophysiological changes. The suppression of PPP2CA, involved in adrenergic signal transduction, and Kchip2 may indirectly mediate mir-133a-3p-induced augmentation of ICa,L and attenuation of Ito.

Attenuation of inward rectifier potassium current contributes to the α1-adrenergic receptor-induced proarrhythmicity in the caval vein myocardium.

AIM: This study is aimed at investigation of electrophysiological effects of α1-adrenoreceptor (α1-AR) stimulation in the rat superior vena cava (SVC) myocardium, which is one of the sources of proarrhythmic activity. METHODS: α1-ARs agonists (phenylephrine-PHE or norepinephrine in presence of atenolol-NE + ATL) were applied to SVC and atrial tissue preparations or isolated cardiomyocytes, which were examined using optical mapping, glass microelectrodes or whole-cell patch clamp. α1-ARs distribution was evaluated using immunofluorescence. Kir2.X mRNA and protein level were estimated using RT-PCR and Western blotting. RESULTS: PHE or NE + ATL application caused a significant suppression of the conduction velocity (CV) of excitation and inexcitability in SVC, an increase in the duration of electrically evoked action potentials (APs), a decrease in the maximum upstroke velocity (dV/dtmax ) and depolarization of the resting membrane potential (RMP) in SVC to a greater extent than in atria. The effects induced by α1-ARs activation in SVC were attenuated by protein kinase C inhibition (PKC). The whole-cell patch clamp revealed PHE-induced suppression of outward component of IK1 inward rectifier current in isolated SVC, but not atrial myocytes. These effects can be mediated by α1A subtype of α-ARs found in abundance in rat SVC. The basal IK1 level in SVC was much lower than in atria as a result of the weaker expression of Kir2.2 channels. CONCLUSION: Therefore, the reduced density of IK1 in rat SVC cardiomyocytes and sensitivity of this current to α1A-AR stimulation via PKC-dependent pathways might lead to proarrhythmic conduction in SVC myocardium by inducing RMP depolarization, AP prolongation, CV and dV/dtmax decrease.

In vivo dynamics of acidosis and oxidative stress in the acute phase of an ischemic stroke in a rodent model.

Ischemic cerebral stroke is one of the leading causes of death and disability in humans. However, molecular processes underlying the development of this pathology remain poorly understood. There are major gaps in our understanding of metabolic changes that occur in the brain tissue during the early stages of ischemia and reperfusion. In particular, it is generally accepted that both ischemia (I) and reperfusion (R) generate reactive oxygen species (ROS) that cause oxidative stress which is one of the main drivers of the pathology, although ROS generation during I/R was never demonstrated in vivo due to the lack of suitable methods. In the present study, we record for the first time the dynamics of intracellular pH and H2O2 during I/R in cultured neurons and during experimental stroke in rats using the latest generation of genetically encoded biosensors SypHer3s and HyPer7. We detect a buildup of powerful acidosis in the brain tissue that overlaps with the ischemic core from the first seconds of pathogenesis. At the same time, no significant H2O2 generation was found in the acute phase of ischemia/reperfusion. HyPer7 oxidation in the brain was detected only 24 h later. Comparison of in vivo experiments with studies on cultured neurons under I/R demonstrates that the dynamics of metabolic processes in these models significantly differ, suggesting that a cell culture is a poor predictor of metabolic events in vivo.

Identification of Key Small Non-Coding MicroRNAs Controlling Pacemaker Mechanisms in the Human Sinus Node.

Background The sinus node (SN) is the primary pacemaker of the heart. SN myocytes possess distinctive action potential morphology with spontaneous diastolic depolarization because of a unique expression of ion channels and Ca2+-handling proteins. MicroRNAs (miRs) inhibit gene expression. The role of miRs in controlling the expression of genes responsible for human SN pacemaking and conduction has not been explored. The aim of this study was to determine miR expression profile of the human SN as compared with that of non-pacemaker atrial muscle. Methods and Results SN and atrial muscle biopsies were obtained from donor or post-mortem hearts (n=10), histology/immunolabeling were used to characterize the tissues, TaqMan Human MicroRNA Arrays were used to measure 754 miRs, Ingenuity Pathway Analysis was used to identify miRs controlling SN pacemaker gene expression. Eighteen miRs were significantly more and 48 significantly less abundant in the SN than atrial muscle. The most interesting miR was miR-486-3p predicted to inhibit expression of pacemaking channels: HCN1 (hyperpolarization-activated cyclic nucleotide-gated 1), HCN4, voltage-gated calcium channel (Cav)1.3, and Cav3.1. A luciferase reporter gene assay confirmed that miR-486-3p can control HCN4 expression via its 3' untranslated region. In ex vivo SN preparations, transfection with miR-486-3p reduced the beating rate by ≈35±5% (P<0.05) and HCN4 expression (P<0.05). Conclusions The human SN possesses a unique pattern of expression of miRs predicted to target functionally important genes. miR-486-3p has an important role in SN pacemaker activity by targeting HCN4, making it a potential target for therapeutic treatment of SN disease such as sinus tachycardia.

Electrophysiological characteristics of the rat azygos vein under electrical pacing and adrenergic stimulation.

Rodent thoracic veins are characterized by an extended myocardial coating. In the present study, the electrical activity in the cardiac tissue of the rat azygos vein (AZV) was investigated for the first time. The atrial-like action potentials (AP) and atrial-like conduction of the excitation were observed in the rat AZV under continuous electrical pacing. Termination of electrical pacing resulted in spontaneous positive shift of resting membrane potential (RMP) in AZV. Boradrenaline induced biphasic effects on RMP in all quiescent AZV preparations but only in 25% preparations-bursts of spontaneous AP, which were suppressed by both α- and ß-adrenoreceptor antagonists. Phenylephrine induced additional depolarization of RMP in quiescent AZV preparations, while isoproterenol caused hyperpolarization. In conclusion, bioelectrical properties of the rat AZV resemble those of atrial myocardium under continuous electrical pacing; however, depolarized RMP and NA-induced spontaneous AP characterize AZV as a tissue prone to rare automaticity.