Puerarin blocks Na~+ current in rat ventricular myocytes

AIM: To study the effect of puerarin (Pue) on Na~+ channel in rat ventricular myocytes. METHODS: Whole-cell patch-clamp technique was applied on isolated cardiomyocytes from rats. RESULTS: Pue inhibited cardiac I_(Na) in a positive rate-dependent and dose-dependent manner, with an IC_(50) of 349 μmol/L. The kinetics of blockage of cardiac sodium channel by Pue resembled the ClassIa/Ic of antiarrhythmic agents. Pue 300 μmol/L did not alter the shape of the I-V curve of I_(Na), but markedly shifted the steady-state inactivation curve of I_(Na) towards more negative potential by 15.9 mV, and postponed the recovery of I_(Na) inactivation state from (21.9±1.6) ms to (54.4±3.4) ms (P<0.01 ). It demonstrated that the steady state of inactivation was affected by Pue significantly. CONCLUSION: Pue protected ventricular myocytes against cardiac damage and arrhythmias by inhibiting recovery from inactivation of cardiac Na~+ channels.     

Biphasic effects of haloperidol on sodium currents in guinea pig ventricular myocytes

Aim： To study the effects of haloperidol on sodium currents （INa） in guinea pig ventricular myocytes. Method： Whole-cell patch clamp technique was employed to evaluate the effects of haloperidol on INa in individual ventricular myocytes. Results： Haloperidol （0.1-3 wnol/L） inhibited INa in a concentration-dependent manner with an IC50 of 0.253±0.015 larnol/L. The inhibition rate of haloperidol （0.3 μmol/L） on INa was 22.14%±0.02%, and the maximum conductance was reduced. Haloperidol significantly reduced the midpoints for the activation and inactivation of INa by 2.09 and 4.09 mV, respectively. The time constant of recovery was increased. The increase in time intervals could only recover by 90.14%±1.4% （n=6）; however, haloperidol at 0.03 μmol/L enhanced INa conductance. The midpoints for the activation and inactivation Of INa were shifted by 1.38 and 5.69 mV, respectively, at this concentration of haloperidol. Conclusion： Haloperidol displayed a biphasic effect on INa in guinea pig cardiac myocytes. High concentrations of haloperidol inhibited INa, while lower concentrations of haloperidol shifted the activation and inactivation curve to the left. Full recovery of recovery curve was not achieved after 0.3 μmol/L haloperidol administration, indicating that the drug affects the inactivated state of sodium channels.     

Metergoline inhibits the neuronal Navl.2 voltage- dependent Na^＋ channels expressed in Xenopus oocytes

Aim： Metergoline is an ergot-derived psychoactive drug that acts as a ligand for serotonin and dopamine receptors. The aim of this study was to investigate the regulatory effects of metergoline on the neuronal Nav1.2 voltage-dependent Na^＋ channels in vitro. Methods： Xenopus oocytes were injected with cRNAs encoding rat brain Nav1.2 α and β1 subunits. Voltage-activated Na^＋ currents were recorded using two-electrode voltage clamp technique. Drugs were applied though perfusion. Results： Both metergoline and lidocaine reversibly and concentration-dependently inhibited the peak of Na^＋ currents with IC50 values of 3.6±4.2 and 916.9±98.8 μmol/L, respectively. Metergoline （3 pmol/L） caused a 6.8±1.2 mV depolarizing shift of the steady-state activation curve of the Na^＋ currents, and did not alter the inactivation curve. In contrast, lidocaine （3 μmol/L） caused a 12.7±1.2 mV hyperpolarizing shift of the inactivation curve of the Na^＋ currents without changing the steady-state activation curve. Both metergoline and lidocaine produced tonic and use-dependent inhibition on the peak of Na^＋ currents. Conclusion： Metergoline exerts potent inhibition on the activity of neuronal Nav1.2 channels, which may contribute to its actions on the central nervous system.     

Inhibitory effects of tetrandrine on the Na^＋ channel of human atrial fibrillation myocardium

Aim： Tetrandrine （Tet） is a Ca^2＋ channel blocker and has antiarrhythmic effects. Less information exists with regard to the mechanisms underlying its antiarrhythmic action other than blocking Ca^2＋ channels. In this study, the effects of Tet on the Na^＋ current （INa） in the atrial myocardium of patients in atrial fibrillation （AF） and sinus rhythm （SR） were investigated, and the characteristics of the Na^＋ current were synchronously compared between the AF and SR patients. Methods： Na^＋ currents were recorded using the whole-cell patch clamp technique in single atrial myocyte of the AF and the normal SR groups. The effects of Tet （40-120μmol/L） on the Na^＋ current in the two groups were then observed. Results： Tet （60-120 pmol/L） decreased INa density in a concentration-dependent manner and made the voltage-dependent activation curve shift to more positive voltages in the SR and AF groups. After exposure to Tet, the voltage-dependent inactivation curve of INa was shifted to more negative voltages in the two groups. Tet delayed the time-dependent recovery of INa in a concentration dependent manner in both AF and SR cells; however, there were no differences in the effects of Tet on INa density and properties in the two groups. The INa density of AF patients did not differ from that of the SR patients. Conclusion： Tet can block sodium channels with slow recovery kinetics, which may explain the mechanisms underlying the antiarrhythmic action of Tet. The decreased conduction velocity （CV） in AF patients is not caused by the Na^＋ current.     

Effects of AMP579 and adenosine on L-type Ca^2＋ current in isolated rat ventricular myocytes

Aim: To compare the effects of AMP579 and adenosine on L-type Ca^2 current (ICa-L) in rat ventricular myocytes and explore the mechanism by which AMP579 acts on ICa-L.Methods: ICa-L was recorded by patch-clamp technique in whole-cell configuration. Results: Adenosine (10nmol/L to 50μmol/L) showed no effect on basal ICa-L, but it inhibited the ICa-L induced by isoproterenol 10nmol/L in a concentration-dependent manner with the IC50 of 13.06μmol/L. Similar to adenosine,AMP579 also showed an inhibitory effect on the ICa-L induced by isoproterenol.AMP579 and adenosine (both in 10μmol/L) suppressed isoproterenol-induced ICa-L by 11.1% and 5.2%, respectively. In addition, AMP579 had a direct inhibitory effect on basal ICa-L in a concentration-dependent manner with IC50(1. 17μmol/L). PD116948 (30μmol/L), an adenosine A1 receptor blocker, showed no action on the inhibitory effect of AMP579 on basal ICa-L. However, GF109203X (0.4μmol/L), a special protein kinase C (PKC) blocker, could abolish the inhibitory effect of AMP579 on basal ICa-L. So the inhibitory effect of AMP579 on basal ICa-L was induced through activating PKC, but not linked to adenosine A1 receptor. Conclusion:AMP579 shows a stronger inhibitory effect than adenosine on the ICa-L induced by isoproterenol. AMP579 also has a strong inhibitory effect on basal ICa-L in rat ventricular myocytes. Activation of PKC is involved in the inhibitory effect of AMP579 on basal ICa-L at downstream-mechanism.     

Tetrandrine and related bis—benzylisoquinoline alkaloids from medicinal herbs：cardiovascular effects and mechanisms of action

Tetrandrine(TET),a bis-benzylisoquinoline alkaloid purified and identified an active ingredient in a Chinese medicinal herb,Radix Stephanae tetrandrae,has been used traditionally for the treatment of congestive circulatory disorder and inflammatory diseases.TET,together with a few of its structural analogues,has long been demonstrated to have antihypertensive action in clinical as well as animal studies.Presumably,the primary anti-hypertensive action of TET is due to its vasodilatory properties.TET prevents or inhibits vascular contraction induced by membrane depolarization with KCl or α-adrenoceptor activation with phenylephrine (PE).TET(30μmol/L) also inhibits the release of endothelium-derived nitric oxide(NO) as well as NO production by inducible NO synthase.TET apparently inhibits multiple Ca^2 entry pathways as demonstrated in cell types lacking the L-type Ca^2 channels.In cardiac muscle cells,TET inhibits both L-and T-type Ca^2 channels.In addition to its actions on cardiovascular tissues,TET may also exert its anti-hypertensive action via a Ca^2 -dependent manner on other tissues intimately involved in the modulation of blood pressure control,such as adrenal grands.In adrenal glomerulosa cells,KCl-or angiotensin II-induced aldosterone synthesis is highly dependent on extracellular Ca^2 .Steroidogenesis and Ca^2 -influx in bovine adrenal glomerulosa cells have been shown to be potently inhibited by TET.In bovine adrenal chromaffin cells,TET inhibits Ca^2 currents via L-and N-type channels as well as other unidentified channels with IC50 of 10μmol/L.Other than the Ca^2 antagonistic effects.TET also interacts with the α-adrenergic receptors and muscarinic receptors based on functional as well as radioligand binding studies.Apart from its functional effects,TET and related compounds also exert effects on tissue structures,such as remodelling of hypertrophied heart and inhibition of angiogenesis,probably by causing apoptotic responses.TET is also known for its anti-inflammatory and anti-fibrogenic actions,which make TET and related compound potentially useful in the treatment of lung silicosis,liver cirrhosis,and rheumatoid arthritis.     

Blockade of L-type calcium channel in myocardium and calcium-induced contractions of vascular smooth muscle by CPU 86017

AIM: To assess the blockade by CPU 86017 on the L-type calcium channels in the myocardium and on the Ca^2 -related contractions of vascular smooth muscle. METHODS: The whole-cell patch-clamp was applied to investigate the blocking effect of CPU 86017 on the L-type calcium current in isolated guinea pig myocytes and contractions by KCl or phenylephrine (Phe) of the isolated rat tail arteries were measured. RESULTS: Suppression of the L-type current of the isolated myocytes by CPU 86017 was moderate, in time- and concentration-dependent manner and with no influence on the activation and inactivation curves. The IC50 was 11.5 μmol/L. Suppressive effect of CPU 86017 on vaso-contractions induced by KCl 100 mmol/L, phenylephrine 1 μmol/Lin KH solution (phase 1),Ca^2 free KH solution ( phase 2), and by addition of CaCl2 into Ca^2 -free KH solution (phase 3) were observed. The IC50 to suppress vaso-contractions by calcium entry via the receptor operated channel (ROC) and Voltage-dependent channel (VDC) was 0.324μmol/L and 16.3μmol/L, respectively. The relative potency of CPU 86017 to suppress vascular tone by Ca^2 entry through ROC and VDC is 1/187 of prazosin and 1/37 of verapamil, respectively.CONCLUSION: The blocking effects of CPU 86017 on the L-type calcium channel of myocardium and vessel are moderate and non-selective. CPU 86017 is approximately 50 times more potent in inhibiting ROC than VDC.     

Eleutheroside B,a selective late sodium current inhibitor,suppresses atrial fibrillation induced by sea anemone toxin Ⅱ in rabbit hearts

Eleutheroside B(EB)is the main active constituent derived from the Chinese herb Acanthopanax senticosus(AS)that has been reported to possess cardioprotective effects.In this study we investigated the effects of EB on cardiac electrophysiology and its suppression on atrial fibrillation(AF).Whole-cell recording was conducted in isolated rabbit atrial myocytes.The intracellular calcium([Ca²⁺]i)concentration was measured using calcium indicator Fura-2/AM fluorescence.Monophasic action potential(MAP)and electrocardiogram(ECG)synchronous recordings were conducted in Langendorff-perfused rabbit hearts using ECG signal sampling and analysis system.We showed that EB dose-dependently inhibited late sodium current(INaL),transient sodium current(INaT),and sea anemone toxin II(ATX II)-increased INaL with IC50 values of 167,1582,and 181μM,respectively.On the other hand,EB(800μM)did not affect L-type calcium current(ICaL),inward rectifier potassium channel current(IK),and action potential duration(APD).Furthermore,EB(300μM)markedly decreased ATX II-prolonged the APD at 90%repolarization(APD90)and eliminated ATX II-induced early afterdepolarizations(EADs),delayed afterdepolarizations(DADs),and triggered activities(TAs).Moreover,EB(200μM)significantly suppressed ATX II-induced Na+-dependent[Ca2+]i overload in atrial myocytes.In the Langendorff-perfused rabbit hearts,application of EB(200μM)or TTX(2μM)substantially decreased ATX II-induced incidences of atrial fibrillation(AF),ventricular fibrillation(VF),and heart death.These results suggest that augmented INaL alone is sufficient to induce AF,and EB exerts anti-AF actions mainly via blocking INaL,which put forward the basis of pharmacology for new clinical application of EB.     

Inhibition of the INa/K and the activation of peak INa contribute to the arrhythmogenic effects of aconitine and mesaconitine in guinea pigs

Aconitine(ACO),a main active ingredient of Aconitum,is well-known for its cardiotoxicity.However,the mechanisms of toxic action of ACO remain unclear.In the current study,we investigated the cardiac effects of ACO and mesaconitine(MACO),a structurally related analog of ACO identified in Aconitum with undocumented cardiotoxicity in guinea pigs.We showed that intravenous administration of ACO or MACO(25μg/kg)to guinea pigs caused various types of arrhythmias in electrocardiogram(ECG)recording,including ventricular premature beats(VPB),atrioventricular blockade(AVB),ventricular tachycardia(VT),and ventricular fibrillation(VF).MACO displayed more potent arrhythmogenic effect than ACO.We conducted whole-cell patch-clamp recording in isolated guinea pig ventricular myocytes,and observed that treatment with ACO(0.3,3μM)or MACO(0.1,0.3μM)depolarized the resting membrane potential(RMP)and reduced the action potential amplitude(APA)and durations(APDs)in a concentration-dependent manner.The ACO-and MACO-induced AP remodeling was largely abolished by an INa blocker tetrodotoxin(2μM)and partly abolished by a specific Na+/K+pump(NKP)blocker ouabain(0.1μM).Furthermore,we observed that treatment with ACO or MACO attenuated NKP current(INa/K)and increased peak INa by accelerating the sodium channel activation with the EC50 of 8.36±1.89 and 1.33±0.16μM,respectively.Incubation of ventricular myocytes with ACO or MACO concentration-dependently increased intracellular Na+and Ca2+concentrations.In conclusion,the current study demonstrates strong arrhythmogenic effects of ACO and MACO resulted from increasing the peak INa via accelerating sodium channel activation and inhibiting the INa/K.These results may help to improve our understanding of cardiotoxic mechanisms of ACO and MACO,and identify potential novel therapeutic targets for Aconitum poisoning.     

Electrophysiological effects of haloperidol on isolated rabbit Purkinje fibers and guinea pigs papillary muscles under normal and simulated ischemia

Aim： Overdoses of haloperidol are associated with major ventricular arrhythmias, cardiac conduction block, and sudden death. The aim of this experiment was to study the effect of haloperidol on the action potentials in cardiac Purkinje fibers and papillary muscles under normal and simulated ischemia conditions in rabbits and guinea pigs. Methods： Using the standard intracellular microelectrode technique, we examined the effects of haloperidol on the action potential parameters [action potential amplitude （APA）, phase 0 maximum upstroke velocity （Vmax）, action potential amplitude at 90% of repolarization （APD90）, and effective refractory period （ERP）] in rabbit cardiac Purkinje fibers and guinea pig cardiac papillary cells, in which both tissues were under simulated ischemic conditions. Results： Under ischemic conditions, different concentrations of haloperidol depressed APA and prolonged APD90 in a concentration-dependent manner in rabbit Purkinje fibers. Haloperidol （3 μmol/L） significantly depressed APA and prolonged APD90, and from 1 μmol/L, haloperidol showed significant depression on Vmax; ERP was not significantly affected. In guinea pig cardiac papillary muscles, the thresholds of significant reduction in APA, Vmax, EPR, and APD90 were 10, 0.3, 1, and 1 μmol/L, respectively, for haloperidol. Conclusion： Compared with cardiac conductive tissues, papillary muscles were more sensitive to ischemic conditions. Under ischemia, haloperidol prolonged ERP and APD90 in a concentration-dependent manner and precipitated the decrease in Vmax induced by ischemia. The shortening of ERP and APD90 in papillary muscle action potentials may be inhibited by haloperidol.     

Electrophysiological effect of l-cis-diltiazem, the stereoisomer of d-cis-diltiazem, on isolated guinea-pig left ventricular myocytes

l-cis-Diltiazem, the stereoisomer of the L-type Ca(2+) channel blocker d-cis-diltiazem, protects cardiac myocytes from ischemia and reperfusion injury in the perfused heart and from veratridine-induced Ca(2+) overload. We determined the effect of l-cis-diltiazem on the voltage-dependent Na(+) current (I(Na)) and lysophosphatidylcholine-induced currents in isolated guinea-pig left ventricular myocytes by a whole-cell patch-clamp technique. l-cis-Diltiazem inhibited I(Na) in a dose-dependent manner without altering the current-voltage relationship for I(Na) (K(d) values : 729 and 9 microM at holding potentials of -140 and -80 mV, respectively). A use-dependent block of I(Na), the leftward shift of the steady-state inactivation curve and the delay of recovery from inactivation suggest that l-cis-diltiazem has a higher affinity for the inactivated state of Na(+) channels. In addition to I(Na), the lysophosphatidylcholine-induced currents were inhibited by l-cis-diltiazem in a similar concentration range. It is suggested that inhibition of both Na(+) channels and lysophosphatidylcholine-activated non-selective cation channels contributes to the cardioprotective effect of l-cis-diltiazem.     

葛根素抑制大鼠心室肌细胞的钠电流

AIM: To study the effect of puerarin (Pue) on Na+ channel in rat ventricular myocytes. METHODS: Whole-cell patch-clamp technique was applied on isolated cardiomyocytes from rats. RESULTS: Pue inhibited cardiac INa in a positive rate-dependent and dose-dependent manner, with an IC(50) of 349 micromol/L. The kinetics of blockage of cardiac sodium channel by Pue resembled the ClassIa/Ic of antiarrhythmic agents. Pue 300 micromol/L did not alter the shape of the I-V curve of INa, but markedly shifted the steady-state inactivation curve of INa towards more negative potential by 15.9 mV, and postponed the recovery of INa inactivation state from (21.9+/-1.6) ms to (54.4+/-3.4) ms (P<0.01). It demonstrated that the steady state of inactivation was affected by Pue significantly. CONCLUSION: Pue protected ventricular myocytes against cardiac damage and arrhythmias by inhibiting recovery from inactivation of cardiac Na+ channels.     

舒必利致心律失常的电生理研究

目的：观察不同浓度舒必利对缺血兔心浦肯野纤维动作电位的影响及舒必利对豚鼠心室肌细胞钠通道电流的作用。方法：采用标准微电极技术,观察不同浓度（1～100μmol／L）舒必利对模拟缺血液灌流的离体兔心浦肯野纤维动作电位0期去极化幅值（APA）、最大除极速率（Vmax）、有效不应期（ERP）及90％动作电位时程（APD50）的影响。应用酶解法分离豚鼠单个心室肌细胞,应用全细胞膜片钳技术记录舒必利对钠通道电流（INa）的影响。结果：不同浓度的舒必利对缺血兔浦肯野纤维动作电位的APA和APD90无明显影响,对Vmax有降低趋势。舒必利（3～300μmol／L）浓度依赖性地抑制INa（IC50=10．79μmol／L,测试电压-35mv）。10μmol／L舒必利降低了INa的最大电导gmax,使半激活、失活电压负值分别减小了1．91mV（P〈0．01）和5．22mV（P〈0．01）;恢复时间常数增加,但最大激活电流可以基本恢复至给药前。结论：舒必利可轻度逆转缺血液灌流造成的心浦肯野纤维动作电位缩短,并浓度依赖性地抑制钠电流,舒必利作用于钠通道的失活态。     

Inhibition of cardiac Na+ current by primaquine

The electrophysiological effects of the anti-malarial drug primaquine on cardiac Na(+) channels were examined in isolated rat ventricular muscle and myocytes. In isolated ventricular muscle, primaquine produced a dose-dependent and reversible depression of dV/dt during the upstroke of the action potential. In ventricular myocytes, primaquine blocked I(Na)(+) in a dose-dependent manner, with a K(d) of 8.2 microM. Primaquine (i) increased the time to peak current, (ii) depressed the slow time constant of I(Na)(+) inactivation, and (iii) slowed the fast component for recovery of I(Na)(+) from inactivation. Primaquine had no effect on: (i) the shape of the I - V curve, (ii) the reversal potential for Na(+), (iii) the steady-state inactivation and g(Na)(+) curves, (iv) the fast time constant of inactivation of I(Na)(+), and (v) the slow component of recovery from inactivation. Block of I(Na)(+) by primaquine was use-dependent. Data obtained using a post-rest stimulation protocol suggested that there was no closed channel block of Na(+) channels by primaquine. These results suggest that primaquine blocks cardiac Na(+) channels by binding to open channels and unbinding either when channels move between inactivated states or from an inactivated state to a closed state. Cardiotoxicity observed in patients undergoing malaria therapy with aminoquinolines may therefore be due to block of Na(+) channels, with subsequent disturbances of impulse conductance and contractility.     

六肽FRCRSFa对大鼠心室肌Na^＋／Ca^＋交换的抑制

AIM: To study the effect of Phe-Arg-Cys-Arg-Ser-Phe-CONH2 (FRCRSFa) on Na+/Ca2+ exchange and its specificity in rat ventricular myocytes. METHODS: Na+/Ca2+ exchange current (INa+/Ca2+) and other currents were measured using whole-cell voltage clamp technique. RESULTS: A concentration-dependent inhibition of hexapeptide FRCRSFa on Na +/Ca2+ exchange was observed in rat ventricular myocytes. IC50 of inward and outward INa+/Ca2+ were 2 and 4 micromol/L, respectively. FRCRSFa 5 micromol/L did not affect L-type Ca2+ current, voltage-gated Na+ current, transient outward K+ current, and inward rectifier K+ current. CONCLUSION: These data indicate that FRCRSFa is an available inhibitor of Na+/Ca2+ exchange with relative selectivity and m ay be valuable for studies of the Na+/Ca2+ exchange in cardiac myocytes.     

Modulation of L-type calcium current in rat cardiac myocytes by sulfur dioxide derivatives.

The effects of sulfur dioxide (SO(2)) derivatives (bisulfite and sulfite, 1:3M/M) on voltage-dependent L-type calcium current (I(Ca,L)) in isolated rat ventricular myocytes were studied using the whole cell patch-clamp technique. SO(2) derivatives increased I(Ca,L) in a concentration-dependent manner. SO(2) derivatives shifted both the steady-state activation and the inactivation curves of I(Ca,L) to more positive potentials, the effect on the latter being more pronounced. SO(2) derivatives markedly accelerated the recovery of I(Ca,L) from inactivation. SO(2) derivatives also significantly shortened the fast and slow time constants of inactivation. These results suggested that SO(2) inhalation might cause cardiac myocyte injury through increasing intracellular calcium via voltage-gated calcium channels.     

Effect of resveratrol on L-type calcium current in rat ventricular myocytes

AIM: To study the effect of resveratrol on L-type calcium current (I(Ca-L)) in isolated rat ventricular myocytes and the mechanisms underlying these effects. METHODS: I(Ca-L) was examined in isolated single rat ventricular myocytes by using the whole cell patch-clamp recording technique. RESULTS: Resveratrol (10-40 micromol/L) reduced the peak amplitude of I(Ca-L) and shifted the current-voltage (I-V) curve upwards in a concentration-dependent manner. Resveratrol (10, 20, 40 micromol/L) decreased the peak amplitude of I(Ca-L) from -14.2+/-1.5 pA/pF to -10.5+/-1.5 pA/pF (P<0.05), -7.5+/-2.4 pA/pF (P<0.01), and -5.2+/-1.2 pA/pF (P<0.01), respectively. Resveratrol (40 micromol/L) shifted the steady-state activation curve of I(Ca-L) to the right and changed the half-activation potential (V0.5) from -19.4+/-0.4 mV to -15.4+/-1.9 mV (P<0.05). Resveratrol at a concentration of 40 micromol/L did not affect the steady-state inactivation curve of I(Ca-L), but did markedly shift the time-dependent recovery curve of I(Ca-L) to the right, and slow down the recovery of I(Ca-L) from inactivation. Sodium orthovanadate (Na(3)VO(4); 1 mmol/L), a potent inhibitor of tyrosine phosphatase, significantly inhibited the effects of resveratrol (P<0.01). CONCLUSION: Resveratrol inhibited I(Ca-L) mainly by inhibiting the activation of L-type calcium channels and slowing down the recovery of L-type calcium channels from inactivation. This inhibitory effect of resveratrol was mediated by the inhibition of protein tyrosine kinase in rat ventricular myocytes.     

The effects of paeonol on the electrophysiological properties of cardiac ventricular myocytes

Previous studies have shown that "Mudanpi", a Chinese herbal medicine, has a significant cardioprotective effect against myocardial ischaemia. Based on these findings we hypothesised that paeonol, the main component of Mudanpi, might have an effect on the cellular electrophysiology of cardiac ventricular myocytes. The effects of paeonol on the action potential and ion channels of cardiac ventricular myocytes were studied using the standard whole-cell configuration of the patch-clamp technique. Ventricular myocytes were isolated from the hearts of adult guinea-pig by enzymic dispersion. The myocytes were continuously perfused with various experimental solutions at room temperature and paeonol applied in the perfusate. Action potentials and membrane currents were recorded using both current and voltage clamp modes of the patch-clamp technique. Paeonol, at concentrations 160 microM and 640 microM, decreased the action potential upstroke phase, an action associated with the blockade of the voltage-gated, fast sodium channel. The effects of paeonol on both action potential and Na(+) current were concentration dependent. Paeonol had a high affinity for inactivated sodium channels. Paeonol also shortened the action potential duration, in a manner not associated with the blockade of the calcium current, or the enhancement of potassium currents. These findings suggest that paeonol, and therefore Mudanpi, may possess antiarrhythmic activity, which may confer its cardioprotective effects.     

Inhibition of cardiac sodium currents by toluene exposure

Toluene is an industrial solvent widely used as a drug of abuse, which can produce sudden sniffing death due to cardiac arrhythmias. In this paper, we tested the hypothesis that toluene inhibits cardiac sodium channels in Xenopus laevis oocytes transfected with Nav1.5 cDNA and in isolated rat ventricular myocytes. In oocytes, toluene inhibited sodium currents (INa+) in a concentration-dependent manner, with an IC50 of 274 microm (confidence limits: 141-407 microm). The inhibition was complete, voltage-independent, and slowly reversible. Toluene had no effect on: (i). the shape of the I-V curves; (ii). the reversal potential of Na+; and (iii). the steady-state inactivation. The slow recovery time constant from inactivation of INa+ decreased with toluene exposure, while the fast recovery time constant remained unchanged. Block of INa+ by toluene was use- and frequency-dependent. In rat cardiac myocytes, 300 microm toluene inhibited the sodium current (INa+) by 62%; this inhibition was voltage independent. These results suggest that toluene binds to cardiac Na+ channels in the open state and unbinds either when channels move between inactivated states or from an inactivated to a closed state. The use- and frequency-dependent block of INa+ by toluene might be responsible, at least in part, for its arrhythmogenic effect.