Effect of Shenfu Injection on Differentiation of Bone Marrow Mesenchymal Stem Cells into Pacemaker-Like Cells and Improvement of Pacing Function of Sinoatrial Node.

Sick sinus syndrome (SSS), a complex type of cardiac arrhythmia, is a major health threat to humans. Shenfu injection (SFI), a formula of traditional Chinese medicine (TCM), is effective in improving bradyarrhythmia. However, the underlying mechanism of SFI's therapeutic effect is subject to few systematic investigations. The purpose of the present research is to examine whether SFI can boost the differentiation effectiveness of bone marrow mesenchymal stem cells (BMSCs) into pacemaker-like cells and whether the transplantation of these cells can improve the pacing function of the sinoatrial node (SAN) in a rabbit model of SSS. BMSCs from New Zealand rabbits were extracted, followed by incubation in vitro. The flow cytometry was utilized to identify the expression of CD29, CD44, CD90, and CD105 surface markers. The isolated BMSCs were treated with SFI, and the whole-cell patch-clamp method was performed to detect hyperpolarization-the activated cyclic nucleotide-gated potassium channel 4 (HCN4) channel current activation curve. The SSS rabbit model was established using the formaldehyde wet dressing method, and BMSCs treated with SFI were transplanted into the SAN of the SSS rabbit model. We detected changes in the body-surface electrocardiogram and recorded dynamic heart rate measurements. Furthermore, transplanted SFI-treated BMSCs were subjected to HE staining, TUNEL staining, qPCR, western blotting, immunofluorescence, immunohistochemistry, and enzyme-linked immunosorbent assay to study their characteristics. Our results indicate that the transplantation of SFI-treated BMSCs into the SAN of SSS rabbits improved the pacing function of the SAN. In vitro data showed that SFI induced the proliferation of BMSCs, promoted their differentiation capacity into pacemaker-like cells, and increased the HCN4 expression in BMSCs. In vivo, the transplantation of SFI treated-BMSCs preserved the function of SAN in SSS rabbits, improved the expression of the HCN4 gene and gap junction proteins (Cx43 and Cx45), and significantly upregulated the expression of cAMP in the SAN, compared to the SSS model group. In summary, the present research demonstrated that SFI might enhance the differentiation capacity of BMSCs into pacemaker-like cells, hence offering a novel approach for the development of biological pacemakers. Additionally, we confirmed the effectiveness and safety of pacemaker-like cells differentiated from BMSCs in improving the pacing function of the SAN.

Tongyang Huoxue Decoction (TYHX) Ameliorating Hypoxia/Reoxygenation-Induced Disequilibrium of Calcium Homeostasis and Redox Imbalance via Regulating Mitochondrial Quality Control in Sinoatrial Node Cells.

Sick sinus syndrome (SSS) is a disease with bradycardia or arrhythmia. The pathological mechanism of SSS is mainly due to the abnormal conduction function of the sinoatrial node (SAN) caused by interstitial lesions or fibrosis of the SAN or surrounding tissues, SAN pacing dysfunction, and SAN impulse conduction accompanied by SAN fibrosis. Tongyang Huoxue Decoction (TYHX) is widely used in SSS treatment and amelioration of SAN fibrosis. It has a variety of active ingredients to regulate the redox balance and mitochondrial quality control. This study mainly discusses the mechanism of TYHX in ameliorating calcium homeostasis disorder and redox imbalance of sinoatrial node cells (SANCs) and clarifies the protective mechanism of TYHX on the activity of SANCs. The activity of SANCs was determined by CCK-8 and the TUNEL method. The levels of apoptosis, ROS, and calcium release were analyzed by flow cytometry and immunofluorescence. The mRNA and protein levels of calcium channel regulatory molecules and mitochondrial quality control-related molecules were detected by real-time quantitative PCR and Western Blot. The level of calcium release was detected by laser confocal. It was found that after H/R treatment, the viability of SANCs decreased significantly, the levels of apoptosis and ROS increased, and the cells showed calcium overload, redox imbalance, and mitochondrial dysfunction. After treatment with TYHX, the cell survival level was improved, calcium overload and oxidative stress were inhibited, and mitochondrial energy metabolism and mitochondrial function were restored. However, after the SANCs were treated with siRNA (si-ß-tubulin), the regulation of TYHX on calcium homeostasis and redox balance was counteracted. These results suggest that ß-tubulin interacts with the regulation of mitochondrial function and calcium release. TYHX may regulate mitochondrial quality control, maintain calcium homeostasis and redox balance, and protect SANCs through ß-tubulin. The regulation mechanism of TYHX on mitochondrial quality control may also become a new target for SSS treatment.

Sodium Hydroxide Pinpoint Pressing Permeation Method for the Animal Modeling of Sick Sinus Syndrome.

Sodium hydroxide pinpoint pressing permeation (SHPPP) was investigated in order to build a rat model of sick sinus syndrome (SSS), which is easy to operate and control the degree of damage, with fewer complications and applicable for large and small animals.Thirty healthy Wistar rats (15 males and 15 females, weighing 250-350 g) were randomly divided into 3 groups, namely a formaldehyde thoracotomy wet compressing group (FTWC), formaldehyde pinpoint pressing permeation group (FPPP) group, and SHPPP group. The number of surviving rats, heart rate (HR), sinoatrial node recovery time (SNRT), corrected SNRT (CSNRT), and sinoatrial conduction time (SACT) were recorded 3 days, one week, and two weeks after modeling.The achievement ratio of modeling was 10% in the FTWC group, 40% in the FPPP group, and 70% in the SHPPP group, and the differences were statistically significant (χ(2) = 7.250, P = 0.007). Meanwhile, the HR was reduced by about 37% in these 3 groups 3 days after modeling, while the reduction was maintained only in SHPPP (P > 0.05) and the HR was re-elevated in the FTWC and FPPP groups 2 weeks after modeling (P < 0.05). Additionally, the SNRT, cS-NRT, and SACT were significantly prolonged compared with pre-modeling in all 3 groups (P < 0.01).SHPPP was the best method with which to build an SSS model with stable and lasting low HR and high success rate of modeling, which might be helpful for further studies on the SSS mechanisms and drugs.

Effects of ivabradine on cardiac electrophysiology in dogs with age-related atrial fibrillation.

BACKGROUND: Ivabradine is an inhibitor of mixed Na+-K+ current that could combine with HCN channels to reduce the transmembrane velocity of funny current (If), heart rate, and cardiac efficiency, and thus be used for the treatment of cardiovascular diseases such as chronic heart failure. As an ion channel blocker, Ivabradine is also a potential antiarrhythmic agent. MATERIAL/METHODS: Twelve aging dogs (8-10 years old) underwent rapid atrial pacing for 2 months to induce age-related AF in this study. The dogs were randomly divided into the Ivabradine group and aging-AF group. The effects of Ivabradine on the electrophysiological parameters, including the effective refractory period (ERP) of the pulmonary veins and atrium, duration of AF, and inducing rate of AF, were investigated. RESULTS: As compared to the aging-AF group, the ERPs of the left superior pulmonary vein (139.00±4.18 ms vs. 129.00±4.08 ms, P=0.005) and left auricle (135.00±3.53 ms vs. 122.00±4.47 ms, P=0.001) were significantly increased, while the duration of AF (46.60±5.07 s vs. 205.40±1.14 s, P=0.001) and inducing rate of AF (25% vs. 60%, P=0.001) were significantly decreased. CONCLUSIONS: Ivabradine could effectively reduce the inducing rate of AF, and thus be used as an upstream drug for the prevention of age-related AF.

Selective and non-selective non-steroidal anti-inflammatory drugs differentially regulate pulmonary vein and atrial arrhythmogenesis.

BACKGROUND: Non-steroidal anti-inflammatory drugs (NSAIDs) increase the risk of atrial fibrillation (AF). This study investigated whether selective and non-selective NSAIDs differentially regulate the arrhythmogenesis of pulmonary veins and atria. METHODS: Conventional microelectrodes were used to record action potentials (APs) in isolated rabbit PVs, sinoatrial node (SAN), left atrium (LA), and right atrium (RA) preparations before and after celecoxib or indomethacin administration. A whole-cell patch clamp was used to record the sodium-calcium exchanger (NCX) current, L-type calcium current (ICa-L), and late sodium current (INa-late) before and after celecoxib administration in isolated PV cardiomyocytes. RESULTS: Celecoxib (0.3, 1, and 3 µM) reduced PV spontaneous beating rates, and induced delayed afterdepolarizations and burst firings in four of eight PV preparations (50%, p<0.05). Celecoxib also reduced SAN beating rates and decreased AP durations (APDs) in RA and LA, but did not change the resting membrane potential. Indomethacin (0.3, 1, 3, and 10 µM) changed neither the PV or SAN beating rates nor RA APDs, but it reduced LA APDs. Celecoxib (3 µM) significantly increased the NCX current and decreased the ICa-L, but did not change the INa-late. Ranolazine (10 µM) suppressed celecoxib (3 µM)-induced PV burst firings in 6 (86%, p<0.05) of 7 PVs. KB-R7943 (10 µM) suppressed celecoxib (3 µM)-induced PV burst firings in 5 (71%, p<0.05) of 7 PVs. CONCLUSIONS: Selective and non-selective NSAIDs differentially modulate PV and atrial electrophysiological characteristics. Celecoxib increased PV triggered activity through enhancement of the NCX current, which contributed to its arrhythmogenesis.

Insights from preclinical ultra high-density electroanatomical sinus node mapping.

AIMS: Although sinus node modification by catheter ablation is an established therapy for the treatment of inappropriate sinus tachycardia, there is incomplete understanding of sinus node anatomy and function but also limited electroanatomical mapping data. Recently, an automatic, ultra high-resolution mapping system has been presented to accurately and quickly identify right atrial (RA) geometry and activation patterns but detailed assessment of sinus node activation has not been performed which was one aim of this study. Preclinical experiences are compared with previous sinus node mapping studies in animals and humans, and potential clinical implications for catheter ablation are discussed. METHODS AND RESULTS: In anaesthetized and ventilated 14 pigs, 30 endocardial and 2 eipcardial RA maps were generated using the Rhythmia™ mapping system without complications and earliest activation sites (EAS) and sinus break-out (SBO) were determined. At baseline, EAS and SBO were located anterior to the middle (n = 6) or lower third (n = 8) of the crista terminalis exhibiting a unicentric activation pattern in all cases. Conduction pathways were directed anterior, posterior, superior, or inferior with substantial inter-individual variation in direction, pathway distance, and conduction time. Orciprenaline, propranolol, or amiodarone shifted endocardial activation with considerable variation between animals with inconsistent patterns. Multicentric activation was found in one case after orciprenaline infusion. Sequential endocardial and epicardial high-density mapping of the RA was performed in two animals and showed a high congruence of the sinus node activation in the endo- and the epicardial map. CONCLUSION: Ultra high-density mapping allows fast, simple, and very detailed assessment of sinus node activation. Future studies are clearly needed to evaluate novel mapping and ablation strategies for an improved understanding of sinus node disease and better outcomes.

Berberine attenuates spontaneous action potentials in sinoatrial node cells and the currents of human HCN4 channels expressed in Xenopus laevis oocytes.

The present study investigated the electropharmacological effects of a traditional Chinese herbal drug, berberine, on the spontaneous activity of sinoatrial nodes (SANs) of the rabbit heart and on human hyperpolarization-activated cyclic nucleotide-gated 4 (hHCN4) channels, which are heterologously expressed in xenopus oocytes, and which contribute to pacemaker currents (Ifs). A standard microelectrode technique and standard two­electrode voltage­clamp recordings were employed to examine the properties of transmembrane potentials and cloned hHCN4 subunit currents, respectively, under control conditions and berberine administration. Berberine decreased the rate of pacemaker firing and the rate of diastolic depolarization, and modified the action potential parameters. In addition, berberine suppressed the hHCN4 channel currents in a concentration­ (1­300 µM) and use­dependent manner, and simultaneously decreased the activation and deactivation kinetics of the hHCN4 channels. The ability of berberine to modulate the If of cardiac pacemaker cells may contribute to its antiarrhythmic action.

[Effect of tests with dosed physical load on sinus node function and repolarization in children with hereditary long QT syndrome].

The study is devoted to assessment of effect of physical effort of sinus rhythm rate and the state of the process of repolarization in children with 1 and 2 molecular-genetic variants of the long QT syndrome (LQT). We have shown that LQT1 is characterized by inadequate increment of heart rate (HR) and substantial increase of QTc interval during exercise. In LQT2 compared with control reduction of HR increment also takes place while shortening of QTc interval is more pronounced. Delta QTc<0 is an important differential diagnostic criterion in patients with LQT1 and LQT2. Variatins in T wave morphology in LQT1 and LQT2 diminish at the background of physical exercise. The use of ß-blockers influences relationship between length of QT interval and HR in patients with LQT but does not eliminate distinction from a group of patients with analogous variant of the syndrome not taking these drugs.

[Effect of kangxin fulu recipe on electrophysiological functions of the sinoatrial node in rabbits with sick sinus syndrome].

OBJECTIVE: To study the effect of Kangxin Fulu Recipe (KFR) on electrophysiological functions of the sinoatrial node in rabbits with sick sinus syndrome (SSS). METHODS: Sixty big ears white rabbits were randomly divided into six groups, i.e., the normal group, the model group, the atropine group, the high dose KFR group, the middle dose KFR group, and the low dose KFR group, ten in each group. SSS model was established by injecting formaldehyde to the sinoatrial node except those in the normal group. Changes in AA interval, the sinoatrial conduction time (SACT), the sinus node recovery time (SNRT), and the corrected sinus node recovery time (CSNRT) were measured before and after modeling, seven days before and after gastrogavage. RESULTS: (1) The AA interval and SACT could be significantly shortened in the high dose KFR group, the middle-dose KFR group, and the atropine group (P<0.05, P<0.01). Better effects were obtained in the former two groups (P<0.05). (2) SNRT and CSNRT could be shortened in the high dose KFR group and the atropine group, with no statistical difference between the two groups (P>0.05). CONCLUSION: The electrophysiological mechanism of KFR might possibly be correlated with accelerating the recovery of sinus node autorhythmicity and conduction functions.

Botulinum toxin injection in epicardial autonomic ganglia temporarily suppresses vagally mediated atrial fibrillation.

BACKGROUND: Autonomic denervation may suppress atrial fibrillation (AF) vulnerability. This study was designed to assess the short- to mid-term effects of botulinum toxin, a cholinergic neurotransmission blocker, on AF inducibility. METHODS AND RESULTS: A total of 23 mongrel dogs were studied. The sinus node and atrioventricular node epicardial fat pads were exposed through a right lateral thoracotomy. Botulinum toxin (Botox, 50 U per fat pad) or 0.9% normal saline (control) was injected into the center of each of the 2 fat pads. The electrophysiological effects were evaluated at 1, 2, and 3 weeks (7 to 8 animals at each time point) with and without cervical vagal stimulation. The vagal stimulation effects on the sinus and atrioventricular nodes were inhibited, and dispersion of atrial effective refractory period was lower at 1 week in the Botox group. Significant suppression of AF inducibility was observed at 1 week but disappeared at 2 and 3 weeks. These changes were not observed in the control group. CONCLUSIONS: Temporary suppression of vagally mediated AF, for at least 1 week, was achieved with botulinum toxin injection in this canine model. This effect might be associated with reduced dispersion of effective refractory period. A temporary autonomic block using botulinum toxin might be a novel therapeutic option for several clinical conditions such as post-cardiac surgery AF.

Ca2+ clock malfunction in a canine model of pacing-induced heart failure.

The mechanisms of sinoatrial node (SAN) dysfunction in heart failure (HF) remain unclear. We hypothesized that impaired rhythmic spontaneous sarcoplasmic reticulum Ca(2+) release (Ca(2+) clock) plays an important role in SAN dysfunction in HF. HF was induced in canine hearts by rapid ventricular pacing. The location of pacemaking sites was determined in vivo using computerized electrical mapping in acute open-chest preparations (normal, n = 3; and HF, n = 4). Isoproterenol (Iso, 0.2 µg·kg(-1)·min(-1)) infusion increased heart rate and shifted the pacemaking site to the superior SAN in all normal hearts. However, in failing hearts, Iso did not induce superior shift of the pacemaking site despite heart rate acceleration. Simultaneous optical recording of intracellular Ca(2+) and membrane potential was performed in Langendorff-perfused isolated right atrium (RA) preparations from normal (n = 7) and failing hearts (n = 6). Iso increased sinus rate, enhanced late diastolic Ca(2+) elevation (LDCAE), and shifted the pacemaking sites to the superior SAN in all normal but in none of the HF RAs. Caffeine (2 ml, 20 mmol/l) caused LDCAE and increased heart rate in four normal RAs but in none of the three HF RAs. Iso induced ectopic beats from lower crista terminalis in five of six HF RAs. These ectopic beats were suppressed by ZD-7288, a specific pacemaker current (I(f)) blocker. We conclude that HF results in the suppression of Ca(2+) clock, resulting in the unresponsiveness of superior SAN to Iso and caffeine. HF also increases the ectopic pacemaking activity by activating the I(f) at the latent pacemaking sites in lower crista terminalis.

Electrophysiological effects of Chinese medicine Shen song Yang xin (SSYX) on Chinese miniature swine heart and isolated guinea pig ventricular myocytes.

BACKGROUND: Shen song Yang xin (SSYX) is a compound of Chinese medicine with the effect of increasing heart rate (HR). This study aimed to evaluate its electrophysiological properties at heart and cellular levels. METHODS: The Chinese miniature swines were randomly assigned to two groups, administered with SSYX or placebo for 4 weeks (n = 8 per group). Cardiac electrophysiological study (EPS) was performed before and after drug administration. The guinea pig ventricular myocytes were enzymatically isolated and whole cell voltage-clamp technique was used to evaluate the effect of SSYX on cardiac action potential (AP). RESULTS: SSYX treatment accelerated the HR from (141.8 +/- 36.0) beats per minute to (163.0 +/- 38.0) beats per minute (P = 0.013) without changing the other parameters in surface electrocardiogram. After blockage of the autonomic nervous system with metoprolol and atropin, SSYX had no effect on intrinsic HR (IHR), but decreased corrected sinus node recovery time (CSNRT) and sinus atrium conducting time (SACT). Intra cardiac EPS showed that SSYX significantly decreased the A-H and A-V intervals as well as shortened the atrial (A), atrioventricular node (AVN) and ventricular (V) effective refractory period (ERP). In isolated guinea pig ventricular myocytes, the most obvious effect of SSYX on action potential was a shortening of the action potential duration (APD) without change in shape of action potential. The shortening rates of APD(30), APD(50) and APD(90) were 19.5%, 17.8% and 15.3%, respectively. The resting potential (Em) and the interval between the end of APD(30) and APD(90) did not significantly change. CONCLUSIONS: The present study demonstrates that SSYX increases the HR and enhances the conducting capacity of the heart in the condition of the intact autonomic nervous system. SSYX homogenously decreases the ERP of the heart and shortens the APD of the myocytes, suggesting its antiarrhythmic effect without proarrhythmia.

Review of the If selective channel inhibitor ivabradine in the treatment of chronic stable angina.

Coronary heart disease is the major cause of morbidity and mortality in industrialized countries, and its prevalence is predicted to grow as the population ages. Current drugs for chronic stable angina (such as beta-blockers, calcium-channel blockers, long- and short-acting nitrates, and potassium-channel activators) are often effective, either as monotherapy or in combination, but side effects and contraindications may limit their use. The "I(f)" (for "funny") channel, discovered in 1979, is expressed mainly in the membrane of pacemaker cells present in the sinus node, the atrioventricular node, the ventricular conduction pathways, and ventricular myocytes. By determining the slope of diastolic depolarization, which in turn controls action potential frequency, it is a key determinant of heart rate and so provides a new therapeutic target for controlling angina symptoms. A new antiangina drug, ivabradine, has been developed and licensed for clinical use. It exclusively reduces the heart rate by selectively blocking the I(f) channel of the sino-atrial node. As clinical trials have shown it to be remarkably well-tolerated, ivabradine offers an alternative for patients who cannot take, or are intolerant of, beta blockade. This review provides an insight into this new agent, its historical background, mechanism of action, and pathophysiologic basis, and provides up-to-date evidence-based information on its optimum use in stable angina.

[Effect of qiangxin fumai granule contained serum on sinoatrial node cells during Ca2+ overloading induced by simulated ischemia/reperfusion].

OBJECTIVE: To explore the effect of Qiangxin Fumai Granule (QFG, a Chinese herbal preparation for treatment of sick sinus syndrome) contained serum (QFG-S) on sinoatrial node cells during Ca2+ overloading induced by simulated ischemia/reperfusion. METHODS: Model Ca2+ overloading cells were established on sinoatrial node cells from newborn rats, with deprivation of oxygen and glucose to simulate ischemia and with restoration of them to simulate reperfusion. Cells were divided into 5 groups, those in the normal and model control groups were modeled directly and those in the treated groups were pre-cultured with UMEM containing respective medicines in aerobic environment for 30 mm before ischemia/reperfusion simulation. Cell Ca2+ concentration and morphology were observed by invert microscope and fluorescence spectrophotometer. RESULTS: Most cells in the model control group revealed cell edema and deformation, even abscission. By HE staining, many minimal vacuole appeared in cytoplasm, with crumpled nuclear membrane, partially damaged. While cells in the QFG-S treated group, either high-dose or low-dose, were attached grew well, with basically smooth and complete membrane and nuclear membrane, normal in size and shape. The intracellular Ca2+ concentration raised significantly after modeling, but it was much lower in the QFG-S treated group than in the model control group (P < 0.01) and it was not changed obviously in the atropine treated group. CONCLUSION: QFG-S could diminish the injury of cell induced by simulated ischemia/reperfusion, the acting mechanism for treatment of sick sinus syndrome might be related to its effect in relieving Ca2+ overloading and thus protecting cells from injury.

Effect of Qiangxin Fumai Granule () on electrophysiological functions of the sinoatrial node during ischemia-reperfusion of the right coronary artery in rabbits.

OBJECTIVE: To study the effect of the Chinese medicine Qiangxin Fumai Granule (, QFG) on electrophysiological functions of the sinoatrial node during ischemia-reperfusion (IR) of the right coronary artery in rabbits. METHODS: The right coronary artery IR model in rabbits was adopted. The modeled rabbits were randomly divided into 4 groups: the model group, the atropine group, the high-dose QFG group, and the low-dose QFG group, with 8 animals in each group. In addition, twelve rabbits were selected for the sham-operative group. The drugs were administered once via duodenal perfusion after modeling had been stabilized for 10 min. The changes in AA interval, the sinoatrial conduction time (SACT), the sinus node recovery time (SNRT), the corrected sinus node recovery time (CSNRT) and the index of sinus node recovery time (ISNRT) at different time points during ischemia and reperfusion were measured. RESULTS: The AA interval was prolonged for more than 40 ms in the model group during ischemia. Compared with the model group, the four electrophysiological parameters abovementioned in the high-dose QFG group and the low-dose QFG group were decreased to different extents at each time point (P<0.01 or P<0.05), and no statistically significant differences were found between the QFG groups and the atropine group (P>0.05). CONCLUSION: QFG is beneficial for accelerating the recovery of sinus node autorhythmicity and conduction function, so as to protect electrophysiological functions of the sinoatrial node. Accelerating the recovery of autorhythmicity and conduction function in the sinus node is considered its electrophysiological mechanism in the treatment of sinoatrial node injury induced by ischemia.

The effects of dexmedetomidine on cardiac electrophysiology in children.

BACKGROUND: Dexmedetomidine (DEX) is an alpha2-adrenergic agonist that is approved by the Food and Drug Administration for short-term (<24 h) sedation in adults. It is not approved for use in children. Nevertheless, the use of DEX for sedation and anesthesia in infants and children appears to be increasing. There are some concerns regarding the hemodynamic effects of the drug, including bradycardia, hypertension, and hypotension. No data regarding the effects of DEX on the cardiac conduction system are available. We therefore aimed to characterize the effects of DEX on cardiac conduction in pediatric patients. METHODS: Twelve children between the ages of 5 and 17 yr undergoing electrophysiology study and ablation of supraventricular accessory pathways had hemodynamic and cardiac electrophysiologic variables measured before and during administration of DEX (1 microg/kg IV over 10 min followed by a 10-min continuous infusion of 0.7 microg x kg(-1) x h(-1)). RESULTS: Heart rate decreased while arterial blood pressure increased significantly after DEX administration. Sinus node function was significantly affected, as evidenced by an increase in sinus cycle length and sinus node recovery time. Atrioventricular nodal function was also depressed, as evidenced by Wenckeback cycle length prolongation and prolongation of PR interval. CONCLUSION: DEX significantly depressed sinus and atrioventricular nodal function in pediatric patients. Heart rate decreased and arterial blood pressure increased during administration of DEX. The use of DEX may not be desirable during electrophysiology study and may be associated with adverse effects in patients at risk for bradycardia or atrioventricular nodal block.

Comparison of the QT interval response during sinus and paced rhythm in conscious and anesthetized beagle dogs.

INTRODUCTION: The aim of the present study was to compare sensitivity in detecting the drug-induced QT interval prolongation in three dog models: conscious telemetered at sinus rhythm and conscious and anesthetized dogs during atrial pacing. The test substances used represent different chemical classes with different pharmacological and pharmacokinetic profiles. METHOD: Dofetilide and moxifloxacin were tested in all models, whereas cisapride and terfenadine were tested in the conscious telemetered and paced models. All substances were given as two consecutive 1.5-h intravenous infusions (infusions 1 and 2). The individual concentration-time courses of dofetilide, moxifloxacin, and cisapride were linked to the drug-induced effects on the QT interval and described with a pharmacokinetic-pharmacodynamic model to obtain an estimate of the unbound plasma concentrations at steady state that give a 10- and 20-ms drug-induced QT interval prolongation (CE10ms and CE20ms). RESULTS: In the conscious telemetered, conscious paced, and anesthetized dog models, the mean CE10ms values were 1.4, 4.0, and 2.5 nM for dofetilide and 1300, 1800, and 12,200 nM for moxifloxacin. For cisapride, the CE10ms values were 8.0 and 4.4 nM in the conscious telemetered and conscious paced dog models. The drug-induced QT interval prolongation during the last 30 min of infusions 1 and 2 was comparable in the conscious models, but smaller in the anesthetized dog model. Terfenadine displayed a marked delay in onset of response, which could only be detected by the extended ECG recording. DISCUSSION: All dog models investigated detected QT interval prolongation after administration of the investigated test substances with similar sensitivity, except for a lower sensitivity in the anesthetized dogs following moxifloxacin administration. The conscious telemetered dog model was favorable, mainly due to the extended continuous ECG recording, which facilitated detection and quantification of delayed temporal differences between systemic exposure and drug-induced QT interval prolongation.

A cardiovascular monitoring system used in conscious cynomolgus monkeys for regulatory safety pharmacology. Part 2: Pharmacological validation.

INTRODUCTION: This project addresses the validation study design of a test system using a telemetered non-human primate model for cardiovascular safety pharmacology evaluation. METHODS: In addition to non-pharmacological validation including installation and operation qualifications, performance qualification (locomotor activity and cardiovascular evaluations) was completed on free-moving cynomolgus monkeys by quantifying the degree of cardiovascular response measured by the telemetric device to various positive control drugs following their intravenous administration. Remifentanil (0.0005, 0.001, 0.002, 0.004, 0.008 and 0.016 mg/kg) was given to induce bradycardia and hypotension. Medetomidine (0.04 mg/kg) was used to induce an initial phase of hypertension followed by hypotension and bradycardia. Esmolol (0.5, 1.0 and 2.0 mg/kg) was used to induce bradycardia. Dopamine (0.002, 0.008, 0.01, 0.02, 0.03 and 0.05 mg/kg/min) was infused over 30 min to induce an increase in arterial and pulse pressures and tachycardia. Amiodarone (0.4, 0.8 and 1.6 mg/kg/min) was infused over 10 min to induce QT interval prolongation. Potassium chloride (0.08 mEq/kg/min) was infused for periods of less than 30 min to induce electrocardiographic (EKG) changes characteristic of hyperkalemia. Reliability was evaluated over 60 days. RESULTS: Monitoring with a reference methodology and the telemetry system was important in order to evaluate precision and accuracy of the test system. Positive control drugs produced a wide range of cardiovascular effects with different amplitudes, which were useful in identification of the limits of the test system. DISCUSSION: Reference monitoring methods and selection of a battery of positive control drugs are important to ensure proper test system validation. Drugs inducing not only QT prolongation but also positive and negative chronotropic effects, positive and negative systemic arterial pressure changes and ECG morphology alterations were useful to identify test system limitations during performance qualification. ECG data processing at significantly elevated heart rates revealed that a trained observer should review all cardiac cycles evaluated by computer.

Electrophysiological effects of carvedilol on rabbit heart pacemaker cells.

The electrophysiological effects of carvedilol, a beta-blocking agent with vasodilating actions, have been studied on rabbit pacemaker cells using the whole-cell patch clamp technique. Nystatin-perforated patch recordings from the sinoatrial (SA) and atrioventricular (AV) nodes demonstrated that 1-3 microM of carvedilol caused a decrease in the spontaneous firing frequency, depolarization of the maximal diastolic potential, and prolongation of the action potential duration in both species. Voltage clamp experiments were performed using SA and AV node myocytes to identify and define the carvedilol-induced changes in the Ca(2+) current, I(Ca), delayed rectifier K(+) current, I(K), and hyperpolarization-activated inward current, I(f). In the SA node cells, 1 microM of carvedilol blocked I(K), I(Ca), and I(f) by 72%, 47%, and 22%, respectively. In the AV node cells, the corresponding reductions were 64% (I(K)) and 46% (I(Ca)), respectively. In both the SA and AV nodes the decrease in I(K) appeared to be mainly due to the rapidly activating component of the delayed rectifier, I(Kr), since the high dose of carvedilol blocked I(K) in the SA and AV nodes to a submaximal degree. In conclusion, effective doses of carvedilol have classical class III antiarrhythmic actions and a negative chronotropic effect resulting from the inhibition of I(K) and I(Ca). Both actions may be efficacious for treating supraventricular tachyarrhythmias.