ß-Adrenergic receptor signalling pathway mediated antiarrhythmic activity of s-limonene in the rat heart.

S-Limonene (s-Lim) is a monocyclic monoterpene found in a variety of plants and has been shown to present antioxidant and cardioprotective activity in experimental models of myocardial infarction. The aim of this study was to evaluate the potential mechanism by which s-Lim exerts its antiarrhythmic effect, focusing on the blockade of ß-adrenoceptor (ß-AR) and its effects on various in vivo and in vitro parameters, including electrocardiogram (ECG) measurements, left ventricular developed pressure (LVDP), the ß-adrenergic pathway, sarcomeric shortening and L-type calcium current (ICa,L). In isolated hearts, 10 µM of s-Lim did not alter the ECG profile or LVPD. s-Lim increased the heart rate corrected QT interval (QTc) (10.8%) at 50 µM and reduced heart rate at the concentrations of 30 (12.4%) and 50 µM (16.6%). s-Lim (10 µM) also inhibited the adrenergic response evoked by isoproterenol (ISO) (1 µM) reducing the increased of heart rate, LVDP and ECG changes. In ventricular cardiomyocyte, s-Lim antagonized the effect of dobutamine by preventing the increase of sarcomeric shortening, demonstrating a similar effect to atenolol (blocker ß1-AR). In vivo, s-Lim antagonized the effect of ISO (agonists ß1-AR), presenting a similar effect to propranolol (a non-selective blocker ß-AR). In ventricular cardiomyocyte, s-Lim did not alter the voltage dependence for ICa,L activation or the ICa,L density. In addition, s-Lim did not affect changes in the ECG effect mediated by 5 µM forskolin (an activator of adenylate cyclase). In an in vivo caffeine/ISO-induced arrhythmia model, s-Lim (1 mg/kg) presented antiarrhythmic action verified by a reduced arrhythmia score, heart rate, and occurrence of ventricular premature beats and inappropriate sinus tachycardia. These findings indicate that the antiarrhythmic activity of s-Lim is related to blockade of ß-AR in the heart.

Exploring the interplay between extracellular pH and Dronedarone's pharmacological effects on cardiac function.

Dronedarone (DRN) is a clinically used drug to mitigate arrhythmias with multichannel block properties, including the sodium channel Nav1.5. Extracellular acidification is known to change the pharmacological properties of several antiarrhythmic drugs. Here, we explore how modification in extracellular pH (pHe) shapes the pharmacological profile of DRN upon Nav1.5 sodium current (INa) and in the ex vivo heart preparation. Embryonic human kidney cells (HEK293T/17) were used to transiently express the human isoform of Nav1.5 α-subunit. Patch-Clamp technique was employed to study INa. Neurotoxin-II (ATX-II) was used to induce the late sodium current (INaLate). Additionally, ex vivo Wistar male rat preparations in the Langendorff system were utilized to study electrocardiogram (ECG) waves. DRN preferentially binds to the closed state inactivation mode of Nav1.5 at pHe 7.0. The recovery from INa inactivation was delayed in the presence of DRN in both pHe 7.0 and 7.4, and the use-dependent properties were distinct at pHe 7.0 and 7.4. However, the potency of DRN upon the peak INa, the voltage dependence for activation, and the steady-state inactivation curves were not altered in both pHe tested. Also, the pHe did not change the ability of DRN to block INaLate. Lastly, DRN in a concentration and pH dependent manner modulated the QRS complex, QT and RR interval in clinically relevant concentration. Thus, the pharmacological properties of DRN upon Nav1.5 and ex vivo heart preparation partially depend on the pHe. The pHe changed the biological effect of DRN in the heart electrical function in relevant clinical concentration.

Resistance training improves cardiac function and cardiovascular autonomic control in doxorubicin-induced cardiotoxicity.

Doxorubicin (DOX) is an anticancer chemotherapy drug that is widely used in clinical practice. It is well documented that DOX impairs baroreflex responsiveness and left ventricular function and enhances sympathetic activity, cardiac sympathetic afferent reflexes and oxidative stress, which contribute to hemodynamic deterioration. Because resistance training (RT)-induced cardioprotection has been observed in other animal models, the objective of this study was to assess the effects of RT during DOX treatment on hemodynamics, arterial baroreflex, cardiac autonomic tone, left ventricular function and oxidative stress in rats with DOX-induced cardiotoxicity. Male Wistar rats were submitted to a RT protocol (3 sets of 10 repetitions, 40% of one-repetition maximum (1RM) of intensity, 3 times per week, for 8 weeks). The rats were separated into 3 groups: sedentary control, DOX sedentary (2.5 mg/kg of DOX intraperitoneal injection, once a week, for 6 weeks) and DOX + RT. After training or time control, the animals were anesthetized and 2 catheters were implanted for hemodynamic, arterial baroreflex and cardiac autonomic tone. Another group of animals was used to evaluate left ventricular function. We found that RT in DOX-treated rats decreased diastolic arterial pressure, heart rate, sympathetic tone and oxidative stress. In addition, RT increased arterial baroreflex sensitivity, vagal tone and left ventricular developed pressure in rats with DOX-induced cardiotoxicity. In summary, RT is a useful non-pharmacological strategy to attenuate DOX-induced cardiotoxicity.

NOX-dependent reactive oxygen species production underlies arrhythmias susceptibility in dexamethasone-treated rats.

Dexamethasone is the most clinically used glucocorticoid with an established role in the treatment of a wide spectrum of inflammatory-related diseases. While the therapeutic actions are well known, dexamethasone treatment causes a number of cardiovascular side effects, which are complex, frequent and, in some cases, clinically unnoticeable. Here, we investigated whether a therapeutic regimen of dexamethasone affects cardiac arrhythmogenesis, focusing on the contribution of Nox-derived reactive oxygen species (ROS). Male Wistar rats were treated with dexamethasone (2 mg/kg, i.p.) for 7 days. Afterward, hemodynamic measurements, autonomic modulation, left ventricular function, cardiac fibrosis, reactive oxygen species (ROS) generation, Nox protein expression, superoxide dismutase (SOD) and catalase activities, and arrhythmias incidence were evaluated. Here, we show that dexamethasone increases blood pressure, associated with enhanced cardiac and vascular sympathetic modulation. Moreover, a marked increase in the cardiac ROS generation was observed, whereas the enhanced SOD activity did not prevent the higher levels of lipid peroxidation in the dexamethasone group. On the other hand, increased cardiac Nox 4 expression and hydrogen peroxide decomposition rate was observed in dexamethasone-treated rats, while Nox 2 remained unchanged. Interestingly, although preserved ventricular contractility and ß-adrenergic responsiveness, we found that dexamethasone-treated rats displayed greater interstitial and perivascular fibrosis than control. Surprisingly, despite the absence of arrhythmias at basal condition, we demonstrated, by in vivo and ex vivo approaches, that dexamethasone-treated rats are more susceptible to develop harmful forms of ventricular arrhythmias when challenged with pharmacological drugs or burst pacing-induced arrhythmias. Notably, concomitant treatment with apocynin, an inhibitor of NADPH oxidase, prevented these ectopic ventricular events. Together, our results reveal that hearts become arrhythmogenic during dexamethasone treatment, uncovering the pivotal role of ROS-generating NADPH oxidases for arrhythmias vulnerability.