Amiodarone inhibits sarcolemmal but not mitochondrial KATP channels in Guinea pig ventricular cells

Role of Oxidative Stress in the Genesis of Ventricular Arrhythmias

Ventricular arrhythmias, mainly lethal arrhythmias, such as ventricular tachycardia and fibrillation, may lead to sudden cardiac death. These are triggered as a result of cardiac injury due to chronic ischemia, acute myocardial infarction and various stressful conditions associated with increased levels of circulating catecholamines and angiotensin II. Several mechanisms have been proposed to underlie electrical instability of the heart promoting ventricular arrhythmias; however, oxidative stress which adversely affects ion homeostasis due to changes in the ion channel structure and function, seems to play a critical role in eliciting different types of ventricular arrhythmias. Prevention or mitigation of the severity of ventricular arrhythmias due to antioxidants has been indicated as the fundamental contribution in the field of preventive cardiology; however, novel interventions have to be developed for greater effectiveness and specificity in attenuating the adverse effects of oxidative stress. In this review, we have attempted to discuss proarrhythmic effects of oxidative stress differing in time and concentration dependence and highlight a molecular and cellular concept how it alters cardiac cell automaticity and conduction velocity sensitizing the probability of ventricular arrhythmias with resultant sudden cardiac death due to ischemic heart disease and other stressful situations. It is concluded that pharmacological approaches targeting multiple mechanisms besides oxidative stress might be more effective in the treatment of ventricular arrhythmias than current antiarrhythmic therapy.

Neuroprotective effects of amiodarone in a mouse model of ischemic stroke

Background

Ion channels play a crucial role in the development of ischemic brain injury. Recent studies have reported that the blockade of various types of ion channels improves outcomes in experimental stroke models. Amiodarone, one of the most effective drugs for life-threatening arrhythmia, works as a multiple channel blocker and its characteristics cover all four Vaughan-Williams classes. Although it is known that amiodarone indirectly contributes to preventing ischemic stroke by maintaining sinus rhythm in patients with atrial fibrillation, the direct neuroprotective effect of amiodarone has not been clarified. The purpose of this study was to investigate the direct effect of amiodarone on ischemic stroke in mice.

Methods

Focal cerebral ischemia was induced via distal permanent middle cerebral artery occlusion (MCAO) in adult male mice. The amiodarone pre-treatment group received 50 mg/kg of amiodarone 1 h before MCAO; the amiodarone post-treatment groups received 50 mg/kg of amiodarone immediately after MCAO; the control group received vehicle only. In addition, the sodium channel opener veratrine and selective beta-adrenergic agonist isoprotelenol were used to elucidate the targeted pathway. Heart rate and blood pressure were monitored perioperatively. Infarct volume analysis was conducted 48 h after MCAO. The body asymmetry test and the corner test were used for neurological evaluation.

Results

Amiodarone pre-treatment and post-treatment reduced the heart rate but did not affect the blood pressure. No mice showed arrhythmia. Compared with the control group, the amiodarone pre-treatment group had smaller infarct volumes (8.9 ± 2.1% hemisphere [mean ± SD] vs. 11.2 ± 1.4%; P < 0.05) and improved functional outcomes: lower asymmetric body swing rates (52 ± 17% vs. 65 ± 18%; P < 0.05) and fewer left turns (7.1 ± 1.2 vs. 8.3 ± 1.2; P < 0.05). In contrast, amiodarone post-treatment did not improve the outcomes after MCAO. The neuroprotective effect of amiodarone pre-treatment was abolished by co-administration of veratrine but not by isoproterenol.

Conclusions

Amiodarone pre-treatment attenuated ischemic brain injury and improved functional outcomes without affecting heart rhythm and blood pressure. The present results showed that amiodarone pre-treatment has neuroprotective effects, at least in part, via blocking the sodium channels.

Functional Role of Mitochondria in Arrhythmogenesis

Growing evidence indicate that mitochondria play a functional role in arrhythmogenesis. We report here the molecular mechanisms underlying the action of these highly dynamic organelles in the regulation of cell metabolism, action potential and, overall, heart excitability. In particular, we examine the role of cardiac mitochondria in linking metabolism and cell excitability. The importance of the main mitochondrial channels is evaluated as well, including the recently identified calcium uniporter. Promises and pitfalls of potential therapeutic strategies targeting mitochondrial pathways are also assessed.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Hypokalemia promotes late phase 3 early afterdepolarization and recurrent ventricular fibrillation during isoproterenol infusion in Langendorff perfused rabbit ventricles

BACKGROUND

Hypokalemia and sympathetic activation are commonly associated with electrical storm (ES) in normal and diseased hearts. The mechanisms remain unclear.

OBJECTIVE

The purpose of this study was to test the hypothesis that late phase 3 early afterdepolarization (EAD) induced by IKATP activation underlies the mechanisms of ES during isoproterenol infusion and hypokalemia.

METHODS

Intracellular calcium (Cai) and membrane voltage were optically mapped in 32 Langendorff-perfused normal rabbit hearts.

RESULTS

Repeated episodes of electrically induced ventricular fibrillation (VF) at baseline did not result in spontaneous VF (SVF). During isoproterenol infusion, SVF occurred in 1 of 15 hearts (7%) studied in normal extracellular potassium ([K(+)]o, 4.5 mmol/L), 3 of 8 hearts (38%) in 2.0 mmol/L [K(+)]o, 9 of 10 hearts (90%) in 1.5 mmol/L [K(+)]o, and 7 of 7 hearts (100%) in 1.0 mmol/L [K(+)]o (P <.001). Optical mapping showed that isoproterenol and hypokalemia enhanced Cai transient duration (CaiTD) and heterogeneously shortened action potential duration (APD) after defibrillation, leading to late phase 3 EAD and SVF. IKATP blocker (glibenclamide, 5 μmol/L) reversed the post-defibrillation APD shortening and suppressed recurrent SVF in all hearts studied despite no evidence of ischemia. Nifedipine reliably prevented recurrent VF when given before, but not after, the development of VF. IKr blocker (E-4031) and small-conductance calcium-activated potassium channel blocker (apamin) failed to prevent recurrent SVF.

CONCLUSION

Beta-adrenergic stimulation and concomitant hypokalemia could cause nonischemic activation of IKATP, heterogeneous APD shortening, and prolongation of CaiTD to provoke late phase 3 EAD, triggered activity, and recurrent SVF. IKATP inhibition may be useful in managing ES during resistant hypokalemia.

Effects of the novel amiodarone-like compound SAR114646A on cardiac ion channels and ventricular arrhythmias in rats

Amiodarone is the "gold standard" for current antiarrhythmic therapy because it combines efficacy with good hemodynamic and electrophysiological tolerance. Amiodarone is effective against both atrial and ventricular arrhythmias by intravenous (i.v.) or oral route. However, the i.v. formulation has limitations. Therefore, we identified SAR114646A, an amiodarone-like antiarrhythmic agent with good aqueous solubility suitable for i.v. application. Patch-clamp experiments were performed with isolated cardiomyocytes from guinea pigs and rats. In guinea pig ventricular cardiomyocytes, the fast Na(+) channel and the L-type Ca(2+) channels were blocked by SAR114646A with half-maximal concentrations (IC(50)) of 2.0 and 1.1 μM, respectively. The tail current of the fast activating rectifying potassium channel I(Kr) was blocked with an IC(50) value of 0.6 μM, whereas the IC(50) values for inhibition of the I(Ks) and I(K1) channels was >10 μM. ATP-sensitive K(+) channels were evoked by application of the channel opener rilmakalim (3 μM). SAR114646A blocked this current with an IC(50) value of 2.8 μM. In guinea pig atrial cardiomyocytes, carbachol (1 μM) was used to activate the I(KACh) and SAR114646A inhibited this current with IC(50) of 36.5 nM. The transient outward current I(to) and the sustained current I(sus) were investigated in rat ventricular myocytes. SAR114646A blocked these currents with IC(50) of 1.8 and 1.2 μM, respectively. When expressed in Chinese hamster ovary cells, the currents hKv1.5 and hHCN4 were inhibited with IC(50) values of 1.1 and 0.4 μM, respectively. Micropuncture experiments in isolated rabbit left atria revealed that SAR114646A prolonged the 50% repolarization significantly at 3 and 10 μM. In guinea pig papillary muscle, the APD at 90% of repolarization was slightly prolonged at 3 and 10 μM. SAR114646A demonstrates antiarrhythmic activity in anaesthetised rats, subjected to 5 min ischemia followed by 10 min reperfusion, where 1 mg/kg of SAR114646A applied as i.v. bolus 5 min prior to ischemia, decreased mortality to 0% compared to 80% under control conditions. In conclusion, SAR114646A is a multichannel blocker with improved water solubility, compared to amiodarone. In contrast to amiodarone, SAR114646A also blocks the K(+) channels I(to) and I(sus). A potent antiarrhythmic effect as observed in rats can also be expected in other animal models.

Inhibition of ATP-sensitive K+ channels and L-type Ca2+ channels by amiodarone elicits contradictory effect on insulin secretion in MIN6 cells

Some class I antiarrhythmic drugs induce a sporadic hypoglycemia by producing insulin secretion via inhibition of ATP-sensitive K(+) (K(ATP)) channels of pancreatic β-cells. It remains undetermined whether amiodarone produces insulin secretion by inhibiting K(ATP) channels. In this study, effects of amiodarone on K(ATP) channels, L-type Ca(2+) channel, membrane potential, and insulin secretion were examined and compared with those of quinidine in a β-cell line (MIN6). Amiodarone as well as quinidine inhibited the openings of the K(ATP) channel in a concentration-dependent manner without affecting its unitary amplitude in inside-out membrane patches of single MIN6 cells, and the IC(50) values were 0.24 and 4.9 µM, respectively. The L-type Ca(2+) current was also inhibited by amiodarone as well as quinidine in a concentration-dependent manner. Although glibenclamide (0.1 µM) or quinidine (10 µM) significantly potentiated the insulin secretion from MIN6 cells, amiodarone (1-30 µM) failed to increase insulin secretion. Amiodarone (30 µM) and nifedipine (10 µM) significantly inhibited the increase in insulin secretion produced by 0.1 µM glibenclamide. Amiodarone (30 µM) produced a gradual decrease of the membrane potential, but did not produce repetitive electrical activity in MIN6 cells. Glibenclamide (1 µM) produced a slow depolarization, followed by spiking activity which was inhibited by 30 µM amiodarone. Thus, amiodarone is unlikely to produce hypoglycemia in spite of potent inhibitory action on K(ATP) channels in insulin-secreting cells, possibly due to its Ca(2+) channel-blocking action.

Cardiac mitochondria and arrhythmias

Despite a high prevalence of sudden cardiac death throughout the world, the mechanisms that lead to ventricular arrhythmias are not fully understood. Over the last 20 years, a growing body of evidence indicates that cardiac mitochondria are involved in the genesis of arrhythmia. In this review, we have attempted to describe the role that mitochondria play in altering the heart's electrical function by introducing heterogeneity into the cardiac action potential. Specifically, we have focused on how the energetic status of the mitochondrial network can alter sarcolemmal potassium fluxes through ATP-sensitive potassium channels, creating a 'metabolic sink' for depolarizing wave-fronts and introducing conditions that favour catastrophic arrhythmia. Mechanisms by which mitochondria depolarize under conditions of oxidative stress are characterized, and the contributions of several mitochondrial ion channels to mitochondrial depolarization are presented. The inner membrane anion channel in particular opens upstream of other inner membrane channels during metabolic stress, and may be an effective target to prevent the metabolic oscillations that create action potential lability. Finally, we discuss therapeutic strategies that prevent arrhythmias by preserving mitochondrial membrane potential in the face of oxidative stress, supporting the notion that treatments aimed at cardiac mitochondria have significant potential in attenuating electrical dysfunction in the heart.

K(ATP) channel current increases in postinfarction remodeled cardiomyocytes

Adenosintriphosphate-sensitive potassium channels (K(ATP) channels) are an important linkage between the metabolic state of a cell and electrophysiological membrane properties. In this study, K(ATP) channels were studied in myocytes of normal and remodeled myocardium of the rat. Myocardial infarction was induced by ligature of the left anterior descending artery. Remodeled myocytes were obtained from the hypertrophied posterior left ventricular wall and interventricular septum 3 months after infarction. The current through K(ATP) channels was measured in whole-cell and inside-out patches by using the patch-clamp technique. After myocardial infarction, the heart weight/body weight ratio was doubled and the myocytes were hypertrophied yielding a cell capacitance of 266+/-16 pF compared to 122+/-12 pF in control cells. The amount of Kir6.2 protein was indistinguishable in corresponding regions of control and remodeled hearts. The ATP sensitivity of K(ATP) channels in remodeled cells was significantly lower than in control cells (half maximum block at 115 micromol/l ATP in remodeled and at 71 mumol/l ATP in control cells). The maximum I (KATP) density induced by metabolic inhibition was higher in small remodeled (176+/-15 pA/pF) than in control cells (127+/-11 pA/pF), but was unchanged in large remodeled cells. Both, the higher I (KATP) density and the lower sensitivity of the K(ATP) channels to ATP suggest that remodeled cardiomyocytes develop an improved tolerance to ischemia by stabilizing the resting potential and decreasing excitability.

Bepridil, an antiarrhythmic drug, opens mitochondrial KATP channels, blocks sarcolemmal KATP channels, and confers cardioprotection

Bepridil, which is clinically useful in the treatment of arrhythmias, has been reported to inhibit sarcolemmal ATP-sensitive K(+) (sarcK(ATP)) channels. However, the effect of bepridil on mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channels remains unclear. The objective of the present study was to determine whether bepridil activates mitoK(ATP) channels and confers cardioprotection. SarcK(ATP) channels composed of Kir6.2+SUR2A in human embryonic kidney (HEK) 293 cells were examined using the patch-clamp technique. Flavoprotein fluorescence in guinea pig ventricular cells and matrix volume in isolated rat heart mitochondria were measured to assay mitoK(ATP) channel activity. Mitochondrial Ca(2+) concentration ([Ca(2+)](m)) was measured by loading cells with rhod-2 fluorescence. Coronary-perfused guinea pig ventricular muscles were subjected to 35-min no-flow ischemia followed by 60-min reperfusion. Bepridil (10 microM) completely inhibited the pinacidil-induced Kir6.2+SUR2A channel current expressed in HEK 293 cells. Bepridil reversibly oxidized the flavoprotein and increased mitochondrial matrix volume in a concentration-dependent manner. Furthermore, bepridil significantly attenuated the ouabain-induced increase of [Ca(2+)](m). Pretreatment with bepridil for 5 min before ischemia improved the recovery of developed tension measured after 60 min of reperfusion. These effects of bepridil were abolished by the mitoK(ATP) channel blocker 5-hydroxydecanoate (500 microM) and by the nonselective K(ATP) channel blocker glisoxepide (10 microM). Our results indicate that bepridil is an opener of mitoK(ATP) channels but an inhibitor of sarcK(ATP) channels and exerts a direct cardioprotective effect on native cardiac myocytes. This is the first report of a unique modulator of K(ATP) channels; bepridil would be expected to mitigate ischemic injury while blunting arrhythmias.

Antiarrhythmic Properties of a Prior Oral Loading of Amiodarone in In Vivo Canine Coronary Ligation/Reperfusion-Induced Arrhythmia Model: Comparison With Other Class III Antiarrhythmic Drugs

Amiodarone, which is generally classified as class III antiarrhythmic drug in the Vaughan Williams classification, is widely used for the treatments of refractory arrhythmias. However, we previously reported that intravenous infusion of amiodarone (6.67 mg/kg per hour) did not suppress arrhythmias induced by coronary ligation/reperfusion in dogs. In this study, we examined effects of a prior oral loading of amiodarone on arrhythmias induced by coronary ligation/reperfusion. Sixteen female beagle dogs (8.5 - 12.5 kg) were divided into two groups; one group was given amiodarone (40 mg/kg, orally, n = 8), and the other was given empty gelatin capsules (n = 8) 2 h before the operation. Dogs were anesthetized with pentobarbital and artificially ventilated. The left chest was opened, and the left anterior descending coronary artery was ligated for 30 min and then reperfused. The mean plasma concentration of amiodarone was over 1.3 mug/ml. Although the prior oral loading of amiodarone did not change the QT interval, amiodarone suppressed the number of ectopic beats during coronary ligation and the incidence of ventricular fibrillation during coronary ligation and reperfusion periods (P<0.05 vs control group). In conclusion, a prior oral loading of amiodarone suppressed arrhythmias induced by coronary ligation/reperfusion with a dose that did not prolong the QT interval. This antiarrhythmic property of amiodarone is different from those of the other class III drugs in that antiarrhythmic effects were accompanied by QT prolongation in our all previous studies.

Minoxidil opens mitochondrial K(ATP) channels and confers cardioprotection

1. ATP-sensitive potassium channel in the mitochondrial inner membrane (mitoK(ATP) channel) rather than in the sarcolemma (sarcK(ATP) channel) appears to play an important role in cardioprotection. We examined the effect of minoxidil, a potent antihypertensive agent and hair growth stimulator, on sarcK(ATP) and mitoK(ATP) channels in guinea-pig ventricular myocytes. 2. Minoxidil activated a glybenclamide-sensitive sarcK(ATP) channel current in the whole-cell recording mode with an EC(50) of 182.6 microm. Minoxidil reversibly increased the flavoprotein oxidation, an index of mitoK(ATP) channel activity, in a concentration-dependent manner. The EC(50) for mitoK(ATP) channel activation was estimated to be 7.3 microm; this value was notably approximately 25-fold lower than that for sarcK(ATP) channel activation. 3. Minoxidil (10 microm) significantly attenuated the ouabain-induced increase of mitochondrial Ca(2+) concentration, which was measured by loading cells with rhod-2 fluorescence. Furthermore, pretreatment with minoxidil (10 microm) before 20-min no-flow ischaemia significantly improved the recovery of developed tension measured after 60 min of reperfusion in coronary perfused guinea-pig ventricular muscles. These cardioprotective effects of minoxidil were completely abolished by the mitoK(ATP) channel blocker 5-hydroxydecanoate (500 microm). 4. Our results indicate that minoxidil exerts a direct cardioprotective effect on heart muscle cells, an effect mediated by the selective activation of mitoK(ATP) channels.

Nicorandil attenuates the mitochondrial Ca2+ overload with accompanying depolarization of the mitochondrial membrane in the heart

The anti-anginal drug nicorandil has been demonstrated to protect the myocardium against ischemic injury in both experimental and clinical studies. Although nicorandil seems to protect the myocardium via activation of mitochondrial ATP-sensitive K+ (mitoKATP) channels, the mechanisms underlying its cardioprotection have remained elusive. We therefore examined whether nicorandil depolarizes the mitochondrial membrane and attenuates the mitochondrial Ca2+ overload. With the use of a Nipkow confocal system, the mitochondrial Ca2+ concentration ([Ca2+]m) and the mitochondrial membrane potential (DeltaPsim) in rat ventricular myocytes were measured by loading cells with rhod-2 and JC-1 respectively. The number of cell hypercontractures resulting from mitochondrial Ca2+ overload was counted. Exposing cells to ouabain (1 mM) evoked mitochondrial Ca2+ overload and increased the intensity of rhod-2 fluorescence to 180+/-15% of baseline ( p<0.001). Nicorandil (100 microM) significantly attenuated the ouabain-induced mitochondrial Ca2+ overload (129+/-4% of baseline; p<0.001 vs. ouabain). Nicorandil decreased the DeltaPsim during application of ouabain, thereby reducing the intensity of JC-1 fluorescence to 89+/-2% of baseline ( p<0.05). Exposure of myocytes to ouabain eventually resulted in cell hypercontracture (51+/-2%). This ouabain-induced cell hypercontracture was blunted by application of nicorandil (37+/-2%, p<0.05 vs. ouabain). Moreover, these effects of nicorandil were abolished by 5-hydroxydecanoate (500 microM), a putative mitoKATP channel blocker, and by glibenclamide (10 microM), a nonselective KATP channel blocker. Our results suggest that nicorandil attenuates the matrix Ca2+ overload with accompanying depolarization of the mitochondrial membrane. Such effect might potentially be attributed to the mechanism of cardioprotection afforded by nicorandil.