Atrial-selective block of sodium channels by acehytisine in rabbit myocardium

Aconitum coreanum Rapaics: Botany, traditional uses, phytochemistry, pharmacology, and toxicology

The present review summarizes the multifaceted uses and recent findings regarding the phytochemistry, traditional use, pharmacology, and toxicity of the extracts and compounds of Aconitum coreanum Rapaics (Ranunculaceae) for the first time to facilitate further research and exploitation of these types of compounds and the utilization of A. coreanum plants. A. coreanum is one of the most important medicinal Aconitum species and has been traditionally and popularly used in China and other Asian countries for the treatment of headaches and migraines, Bi syndrome induced by wind, cold and dampness, and facial paralysis. Phytochemical studies have led to the isolation of 55 distinct small molecule compounds from A. coreanum, most of which are diterpenoid alkaloids. Related pharmacological studies have focused primarily on the antiarrhythmic, anti-inflammatory, analgesic, and anticancer activities of A. coreanum and its derived drugs. Alkaloids have been demonstrated to be the main active ingredients in this plant. In particular, hetisine-type DAs, mainly Guan-fu base A and its analogues, which possess prominent antiarrhythmic effects, other effects, and hypotoxicity, could be regarded as the representative constituents of A. coreanum. Polysaccharides from A. coreanum also displayed broad bioactivities, demonstrating great potential for further research and exploitation. However, few of the current studies have examined the main active components in A. coreanum from different regions. In addition, most of the pharmacological studies on A. coreanum polysaccharides were carried out using crude or poorly characterized fractions. Finally, reliable analytical methods and deeper studies on the toxicity of the compounds from A. coreanum are needed to ensure the safe usage of these products.

Protective effect of low-intensity treadmill exercise against acetylcholine-calcium chloride-induced atrial fibrillation in mice

Atrial fibrillation (AF) is the most common supraventricular arrhythmia, and it corresponds highly with exercise intensity. Here, we induced AF in mice using acetylcholine (ACh)-CaCl₂ for 7 days and aimed to determine the appropriate exercise intensity (no, low, moderate, high) to protect against AF by running the mice at different intensities for 4 weeks before the AF induction by ACh-CaCl₂. We examined the AF-induced atrial remodeling using electrocardiogram, patch-clamp, and immunohistochemistry. After the AF induction, heart rate, % increase of heart rate, and heart weight/body weight ratio were significantly higher in all the four AF groups than in the normal control; highest in the high-ex AF and lowest in the low-ex (lower than the no-ex AF), which indicates that low-ex treated the AF. Consistent with these changes, G protein-gated inwardly rectifying K⁺ currents, which were induced by ACh, increased in an exercise intensity-dependent manner and were lower in the low-ex AF than the no-ex AF. The peak level of Ca²⁺ current (at 0 mV) increased also in an exercise intensity-dependent manner and the inactivation time constants were shorter in all AF groups except for the low-ex AF group, in which the time constant was similar to that of the control. Finally, action potential duration was shorter in all the four AF groups than in the normal control; shortest in the high-ex AF and longest in the low-ex AF. Taken together, we conclude that low-intensity exercise protects the heart from AF, whereas high-intensity exercise might exacerbate AF.

Increased atrial effectiveness of flecainide conferred by altered biophysical properties of sodium channels

Atrial fibrillation (AF) affects over 1% of the population and is a leading cause of stroke and heart failure in the elderly. A feared side effect of sodium channel blocker therapy, ventricular pro-arrhythmia, appears to be relatively rare in patients with AF. The biophysical reasons for this relative safety of sodium blockers are not known. Our data demonstrates intrinsic differences between atrial and ventricular cardiac voltage-gated sodium currents (INa), leading to reduced maximum upstroke velocity of action potential and slower conduction, in left atria compared to ventricle. Reduced atrial INa is only detected at physiological membrane potentials and is driven by alterations in sodium channel biophysical properties and not by NaV1.5 protein expression. Flecainide displayed greater inhibition of atrial INa, greater reduction of maximum upstroke velocity of action potential, and slowed conduction in atrial cells and tissue. Our work highlights differences in biophysical properties of sodium channels in left atria and ventricles and their response to flecainide. These differences can explain the relative safety of sodium channel blocker therapy in patients with atrial fibrillation.

Chemistry and biological activities of hetisine-type diterpenoid alkaloids

Hetisine-type C20-diterpenoid alkaloids (DAs) are one of the most important DA subtypes. During the past decades, a total of 157 hetisine-type DAs were obtained from plants from seven genera in three families, most of which were isolated from the genera Aconitum and Delphinium in the Ranunculaceae family. Structurally, hetisine-type DAs are characterized by a heptacyclic hetisane skeleton formed by the linkage of C(14)-C(20) and N-C(6) bonds in an atisine-type DA, and their structural diversity is created by the states of the N atom and various substituents. Pharmacological studies have revealed a wide range of pharmacological actions for hetisine-type DAs, including antiarrhythmic, antitumor, antimicrobial and insecticidal activities, as well as effects on peripheral vasculature, which are closely related to their chemical structures. In particular, the prominent antiarrhythmic effects and low toxicity of hetisine-type DAs highlight their potential in antiarrhythmic drug discovery. Hetisine-type DAs with diverse bioactivities are promising lead structures for further development as commercial agents in medicine.

Acehytisine suppresses atrial fibrillation in rats with dilated atria caused by chronic volume overload

Atrial dilation is an independent risk factor for the development of atrial fibrillation (AF) and modulates the efficacy of anti-AF drugs, leading to the unsatisfactory control of AF. Pre-clinical studies showed anti-AF effects of acehytisine, a multi-ion channel inhibitor, in atria without structural and/or electrophysiological abnormalities, but information is limited regarding its anti-AF efficacy in dilated atria. We evaluated anti-AF effects of acehytisine at 4 and 10 mg/kg intravenously infused over 10 min using 8-week-old Wistar rats (n = 5; male) with atrial dilation caused by aorto-venocaval shunt (AVS). Echocardiography showed that atria were enlarged by +26.9% after one month of operation in AVS rats compared with sham-operated rats (n = 4; male). Electrophysiological examinations indicated burst pacing-induced AF reached 206 s. Acehytisine at doses of 4 and 10 mg/kg decreased the duration of burst pacing-induced AF with prolongation of Wenckebach cycle length and P wave duration in a dose-dependent manner. Importantly, the drug effectively terminated the persistent AF that was resistant to multiple programmed electrical stimulations in one rat. Therefore, these results provide in vivo evidence that acehytisine may be beneficial for preventing and terminating persistent AF in dilated atria.

Electropharmacological profile of an atrial-selective sodium channel blocker acehytisine assessed in the isoflurane-anesthetized guinea-pig model

Experimental evidence regarding the risk of proarrhythmic potential of acehytisine is limited. We assessed its electropharmacological effect together with proarrhythmic potential at intravenous doses of 4 and 10 mg/kg (n = 6) using isoflurane-anesthetized guinea pigs in comparison with that of bepridil at 1 and 3 mg/kg, intravenously (n = 6). Acehytisine at therapeutic dose (4 mg/kg) decreased the heart rate, prolonged P wave duration, QRS width, QT interval, QTc, MAP_90(sinus), MAP_90(CL300) and MAP_90(CL250). At supratherapeutic dose (10 mg/kg), it prolonged the PR interval besides enhancing the changes induced by the therapeutic dose. Quantitative assessment showed that peak changes in P wave duration by acehytisine at 10 mg/kg were 1.7 times longer than bepridil, and in MAP_90(sinus), MAP_90(CL300) and MAP_90(CL250) by acehytisine were 1.9, 1.5 and 1.5 times shorter than bepridil, respectively. Importantly, qualitative assessment indicated that bepridil increased beat-to-beat variability and J-T_peakc in a dose-related manner, confirming a higher proarrhythmic risk, whereas such dose-related responses were not observed in acehytisine, suggesting a lower proarrhythmic risk. These results suggest that acehytisine exhibits favorable pharmacological characters, i.e. potent atrial inhibition and lower proarrhythmic toxicity compared with bepridil, being a promising candidate for the treatment of paroxysmal supraventricular tachycardia.

Synergistic Anti-arrhythmic Effects in Human Atria with Combined Use of Sodium Blockers and Acacetin

Atrial fibrillation (AF) is the most common cardiac arrhythmia. Developing effective and safe anti-AF drugs remains an unmet challenge. Simultaneous block of both atrial-specific ultra-rapid delayed rectifier potassium (K⁺) current (I_Kur) and the Na⁺ current (I_Na) has been hypothesized to be anti-AF, without inducing significant QT prolongation and ventricular side effects. However, the antiarrhythmic advantage of simultaneously blocking these two channels vs. individual block in the setting of AF-induced electrical remodeling remains to be documented. Furthermore, many I_Kur blockers such as acacetin and AVE0118, partially inhibit other K⁺ currents in the atria. Whether this multi-K⁺-block produces greater anti-AF effects compared with selective I_Kur-block has not been fully understood. The aim of this study was to use computer models to (i) assess the impact of multi-K⁺-block as exhibited by many I_Kur blokers, and (ii) evaluate the antiarrhythmic effect of blocking I_Kur and I_Na, either alone or in combination, on atrial and ventricular electrical excitation and recovery in the setting of AF-induced electrical-remodeling. Contemporary mathematical models of human atrial and ventricular cells were modified to incorporate dose-dependent actions of acacetin (a multichannel blocker primarily inhibiting I_Kur while less potently blocking I_to, I_Kr, and I_Ks). Rate- and atrial-selective inhibition of I_Na was also incorporated into the models. These single myocyte models were then incorporated into multicellular two-dimensional (2D) and three-dimensional (3D) anatomical models of the human atria. As expected, application of I_Kur blocker produced pronounced action potential duration (APD) prolongation in atrial myocytes. Furthermore, combined multiple K⁺-channel block that mimicked the effects of acacetin exhibited synergistic APD prolongations. Synergistically anti-AF effects following inhibition of I_Na and combined I_Kur/K⁺-channels were also observed. The attainable maximal AF-selectivity of I_Na inhibition was greatly augmented by blocking I_Kur or multiple K⁺-currents in the atrial myocytes. This enhanced anti-arrhythmic effects of combined block of Na⁺- and K⁺-channels were also seen in 2D and 3D simulations; specially, there was an enhanced efficacy in terminating re-entrant excitation waves, exerting improved antiarrhythmic effects in the human atria as compared to a single-channel block. However, in the human ventricular myocytes and tissue, cellular repolarization and computed QT intervals were modestly affected in the presence of actions of acacetin and I_Na blockers (either alone or in combination). In conclusion, this study demonstrates synergistic antiarrhythmic benefits of combined block of I_Kur and I_Na, as well as those of I_Na and combined multi K⁺-current block of acacetin, without significant alterations of ventricular repolarization and QT intervals. This approach may be a valuable strategy for the treatment of AF.