Endurance training-induced cardiac remodeling in a guinea pig athlete's heart model

Besides the health benefits of regular exercise, high-level training-above an optimal level-may have adverse effects. In this study, we investigated the effects of long-term vigorous training and its potentially detrimental structural-functional changes in a small animal athlete's heart model. Thirty-eight 4-month-old male guinea pigs were randomized into sedentary and exercised groups. The latter underwent a 15-week-long endurance-training program. To investigate the effects of the intense long-term exercise, in vivo (echocardiography, electrocardiography), ex vivo, and in vitro (histopathology, patch-clamp) measurements were performed. Following the training protocol, the exercised animals exhibited structural left ventricular enlargement and a significantly higher degree of myocardial fibrosis. Furthermore, resting bradycardia accompanied by elevated heart rate variability occurred, representing increased parasympathetic activity in the exercised hearts. The observed prolonged QTc intervals and increased repolarization variability parameters may raise the risk of electrical instability in exercised animals. Complex arrhythmias did not occur in either group, and there were no differences between the groups in ex vivo or cellular electrophysiological experiments. Accordingly, the high parasympathetic activity may promote impaired repolarization in conscious exercised animals. The detected structural-functional changes share similarities with the human athlete's heart; therefore, this model might be useful for investigations on cardiac remodeling.

Assessment of proarrhythmogenic risk for cannabidiol using dog and rabbit cardiac preparations: the electrophysiological effects on action potential and transmembrane potassium currents

Funding Acknowledgements

Type of funding sources: Public Institution(s). Main funding source(s): Economic Development and Innovation Operative Programme GINOP-2.3.2-15-2016-00012, the National Research Development and Innovation Office (NKFIH K 135464 and NKFIH K 128851), the Ministry of Human Capacities Hungary (20391-3/2018/FEKUSTRAT and EFOP-3.6.2-16-2017-00006), and from the Eötvös Loránd Research Network

Background

Cannabidiol (CBD), a major active phytogenic cannabinoid, is one of the main constituents of cannabis. Cannabis has been widely used as recreational drug over the decades and its use is constantly increasing as hallucinogenic and/or medicinal agent. However, significant cardiovascular side effects can accompany its use ranging from arrhythmia to sudden cardiac death.

Purpose

The aim of the present work was to investigate the possible cardiac adverse electrophysiological effects of cannabidiol (CBD) on action potentials and various transmembrane potassium currents, such as the rapid (IKr) and slow (IKs) delayed rectifier, the transient outward (Ito) and inward rectifier (IK1) potassium currents in rabbit and dog cardiac preparations to assess the cardiac safety profile and proarrhythmic risk.

Methods

In the current study, conventional microelectrode and voltage clamp techniques were used to record the action potential and transmembrane ionic currents in dog and rabbit ventricular tissue preparations and enzymatically isolated myocytes, respectively.

Results

The results show that CBD lengthens APD90 significantly at the concentration of 5 µM both in dog and rabbit ventricular tissues without changing other action potential parameters significantly. To further investigate the APD90 lengthening effect of CBD, transmembrane potassium currents (IKr, IKs, Ito and IK1) were investigated in dog and/or rabbit ventricular myocytes using voltage clamp technique. CBD significantly inhibited IKr and IKs currents in rabbit ventricular myocytes with an estimated EC50 values of 4.9 and 3.1 µM, respectively. The effect of CBD on rabbit’s Ito current was not significant while it was significant on dog’s Ito current with an estimated EC50 value of 5 µM. IK1 was not responsive to CBD even at high concentration.

Conclusion

In conclusion, looking at the inhibitory effects of CBD on repolarizing potassium currents, despite of the fact that these calculated EC50 values are higher than pharmacokinetics based Cmax values of CBD recorded after smoking and oral intake, it can be speculated that in the presence of certain cardio active drugs or co-morbidity where CBD metabolism or cardiac repolarization reserve is impaired CBD can have an additive and proarrhythmic effect.

Cardiac remodeling accompanied by increased arrhythmia susceptibility in a dog model of chronic high-intensity endurance training

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): This work was supported by NKFIH grants (K-19992, K-135464) and GINOP-2.3.2-15-2016-00047.

Background

Despite the cardiovascular benefits of regular physical exercise, chronic high-level exercise can evoke malignant arrhythmias, including ventricular fibrillation and even sudden cardiac death, especially in young top athletes. In some cases the underlying mechanisms are unclear.

Objectives

The goal of this study was to assess mechanisms underlying cardiac structural-electrical changes and arrhythmia vulnerability by high-level vigorous exercise training in animal species that are electrophysiologically relevant to the human heart.

Methods

Beagle dogs were randomly assigned to matched sedentary (Sed) or intensive exercise-training (Ex) groups (n=12-12). Ex dogs underwent a 4-month-long intensive treadmill-running protocol (5 days a week, 6 hours a day at a speed of 14-21 km/h with an inclination from 5% to 12%). In vivo echocardiography and electrophysiological measurements were performed. Proarrhythmic sensitivity was tested and the autonomic alterations were examined. At study end, arrhythmia susceptibility was tested with high-frequency burst stimulation in open-chest anaesthetized dogs. This was followed by cardiac excision and cardiomyocyte isolation, formalin preservation for histology, snap-freezing in liquid nitrogen for molecular biology.

Results

The vigorous endurance training was resulted in increased left ventricular end-diastolic diameter, increased septal wall thickness and greater left ventricular mass index (Ex vs. Sed: 98±12 vs. 136±7 g/m2, p<0.05). Some degree of enhanced fibrosis was observed. Endurance training decreased heart rate both in whole animal and in vitro dog experiments. ECG recordings presented enhanced heart rate variability parameters, prolonged PQ (Ex vs. Sed: 98.3±2.9 vs. 116.7±3.6 ms, p<0.05), QRS (Ex vs. Sed : 60.5±2.4 vs 70.8±1.6 ms, p<0.05), QTc (Ex vs. Sed: 213.6±2.8 vs. 237.1±3.4 ms, p<0.05), Tp-Te (Ex vs. Sed: 27.9±2.5 vs.36.5±1.7 ms, p<0.05) intervals associated with significantly enhanced QT interval variability (eg. Ex vs. Sed: QT-STV, 2.5±0.2 vs. 3.6±0.4 ms, p<0.05), reflecting elevated level of repolarization dispersion. Ectopic activity was also enhanced in the exercised dog ventricle. Atropine treatment resulted in moderate heart rate increase in the Ex animals. Chronic endurance exercise elevated the proarrhythmic risk and consequent ventricular fibrillation in dogs subjected to burst electrical stimulation.

Conclusion

We developed a new animal model that shares similarities with the human endurance-trained athlete’s heart. The model represents increased arrhythmia susceptibility, an important clinical paradigm, and explores potential underlying mechanisms, including vagal enhancement, increased repolarization dispersion and enhanced fibrotic changes. Increased arrhythmia susceptibility is supported by the enhanced arrhythmia incidence in the exercised group. Similar changes may be present in young human top athletes, however further investigations are required.

Is selective late sodium current inhibition different from class I/B antiarrhythmic action? Comparison of the effects of GS967 to mexiletine in canine ventricular myocardium

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): National Research, Development and Innovation Office New National Excellence Programme

Enhancement of the late Na+ current (INa,late) increases arrhythmia propensity in the heart, while suppression of the current is antiarrhythmic. GS-458967 (GS) is an agent considered to be a selective blocker of INa,late. In the present study, effects of GS967 on INa,late, on L-type calcium current (ICaL), and on action potential (AP) morphology were studied in canine ventricular myocytes by using conventional voltage clamp, action potential voltage clamp and sharp microelectrode techniques. These effects of GS were compared to tetrodotoxin (TTX) and to the class I/B antiarrhythmic compound mexiletine.

GS (1 μM), mexiletine (40 μM) and TTX (10 μM) dissected largely similarly shaped inward currents under action potential voltage clamp conditions. In case of GS and mexiletine, the amplitude and integral of this inward current was significantly smaller when measured in the presence of 1 μM nisoldipine, while no difference was observed in case of TTX. Under conventional voltage clamp conditions, INa,late was significantly reduced by 1 μM GS and 40 μM mexiletine (about 79% and 63% reduction of current integrals, respectively). The integral of ICa,L was moderately but significantly decreased by both drugs (reduction of 9% and 14%, respectively). These changes were associated with a faster inactivation of ICa,L.

Drug effects on early Na+ current (INa,early) were assessed by analyzing the maximal rate of depolarization (V + max) in multicellular preparations. Both GS and mexiletine showed fast onset and offset kinetics: 110 ms and 289 ms offset time constants, respectively, as determined from V + max measurements in right ventricular papillary muscles, while the onset kinetics was characterized by 5.3 AP and 2.6 AP lengths, respectively, at 2.5 Hz.

Effects on beat-to-beat variability of AP duration (APD) was studied in isolated myocytes. Short-term variability was significantly decreased by both GS and mexiletine (average reduction of 42% and 24%, respectively) while they caused similar shortening of the APD.

The electrophysiological effects of GS are similar to those of mexiletine, but with a somewhat faster offset kinetics of V + max block. However, since GS reduced V+ max and INa,late in the same concentration, the currently accepted view that GS that selectively blocks INa,late has to be questioned and it is suggested that GS should be classified as a class I/B (or I/B + IV) antiarrhythmic agent.