In vivo and in vitro characterization of the novel antiarrhythmic agent SSR149744C: electrophysiological, anti-adrenergic, and anti-angiotensin II effects.

SSR149744C (SSR, 2-butyl-3-[4-[3-(dibutylamino)pro-pyl]benzoyl]-1-benzofuran-5-carboxylate isopropyl fumarate), is a new non-iodinated benzofuran derivative. The aim of this study was to evaluate in vivo its electrophysiological, hemodynamic, and anti-adrenergic properties and to determine its mechanism of action using in vitro studies. In chloralose-anesthetized dogs, SSR149744C (1-10 mg/kg i.v.) prolonged the sinus cycle length, A-H interval, Wenckebach cycle length, atrial effective refractory period (ERP), and atrio-ventricular node ERP in a dose-dependent manner without change of ventricular ERP and HV, QRS, or QTc intervals. Arterial blood pressure and ventricular inotropism were slightly decreased. SSR149744C, which has no or low affinity for alpha 1 and beta 1 adrenergic and angiotensin II AT1 receptors, reduced isoproterenol-induced tachycardia and phenylephrine- or angiotensin II-induced hypertension in anaesthetized dogs. In guinea pig papillary muscle, SSR149744C did not modify the resting potential, action potential amplitude and duration, but reduced the dV/dt max of the depolarization phase in a frequency-dependent manner. In isolated guinea pig cardiomyocytes and transfected CHO cells, SSR149744C (0.01-30 microM) inhibited several potassium currents: IKr (IC50 approximately 10 microM), IKs (IC50 approximately 30 microM), IK(ACh) (IC50 = 0.09 microM), and IKv1.5 (IC50 = 2.7 microM), the L-type calcium current: ICa(L) (IC50 approximately 5 microM) and also the amplitude of [Ca2+]i transient and cell shortening. Therefore, SSR149744C appears to have a multifactorial mechanism of action, which combines the blockade of several ion channels with the inhibition of responses of alpha 1 and beta 1 adrenergic as well as AT1 receptor stimulation. Like amiodarone, SSR149744C possesses the pharmacological effects of class I, II, III, and IV antiarrhythmic agents, which may confer upon this new drug a strong antiarrhythmic potential without ventricular proarrhythmia and iodine-related amiodarone-like side-effects.

Effects of sasanquasaponin on ischemia and reperfusion injury in mouse hearts.

We investigated effects of sasanquasaponin (SQS), a traditional Chinese herb's effective component, on ischemia and reperfusion injury in mouse hearts and the possible role of intracellular Cl- homeostasis on SQS's protective effects during ischemia and reperfusion. An in vivo experimental ischemia model was made in mice (weight 27-45 g) using ligation of left anterior descending coronary artery, and in vitro models were made in perfused hearts by stopping flow or in isolated ventricular myocytes by hypoxia. The in vivo results showed that SQS inhibited cardiac arrhythmias during ischemia and reperfusion. Incidence of arrhythmias during ischemia and reperfusion, including ventricular premature beats and ventricular fibrillation, was significantly decreased in the SQS-pretreated group (P<0.05). Results in perfused hearts showed that SQS suppressed the arrhythmias, prevented against ischemia-induced decrease in contract force and promoted the force recovery from reperfusion. Furthermore, intracellular Cl- concentrations ([Cl-]i) were measured using a MQAE fluorescence method in isolated ventricular myocytes in vitro. SQS slightly decreased [Cl-]i in non-hypoxic myocytes and delayed the hypoxia/reoxygenation-induced increase in [Cl-]i during ischemia and reperfusion (P<0.05). Our results showed that SQS protected against ischemia/reperfusion-induced cardiac injury in mouse hearts and that modulation of intracellular Cl- homeostasis by SQS would play a role in its anti-arrhythmia effects during ischemia and reperfusion.

Potassium channels on cardiomyocytes of selenium iodine deficiency rats and iodine deficiency rats.

Using diet control method to feed the weaning male Wistar rats with selenium and iodine deficiency Keshan endemic area food for 8 weeks to set up animal model. Singal channel recording in cell attached model was used to measure cardiac cell membrane potassium channel conductances, which coincide with the cardiac cell membrane potassium channel conductances in normal Wistar rats. The potassium channel conductance on selenium and iodine deficiency rat cardiac cell membrane is showing current-voltage increasing lineally in the range of clamping voltage 0 +/- 30 mV with channel conductance of 43.4 pS. The channel current does not increase depending on the clamping voltage that is showing the rectifying characteristic and the channel current amplitude can be augmented by added KIOa 5 mmol/L in bath solution. A kind of inward rectifying potassium channel activity was recorded, but this channel activity disappeared after lasting 6-10 minutes only. Then an inward rectifying potassium channel with the conductance of 14.2 pS was activated by KIOa 5 mmol/L introduced to bath solution. Both conductances are less than that of normal Wistar rats.

The mechanism underlying the cardiotoxic effect of the toxin from the jellyfish Chironex fleckeri.

We have investigated the mechanisms underlying the cardiac effects of the toxin from the box jellyfish Chironex fleckeri. Papillary muscles isolated from the hearts of ferrets and ventricular myocytes isolated from the hearts of ferrets and rats were used. Force, intracellular [Ca2+], and membrane potential were monitored in the papillary muscles; contraction, intracellular [Ca2+], intracellular [Na+], and membrane currents were monitored in the isolated myocytes. Application of the toxin to these preparations resulted in a large increase in intracellular [Ca2+] and the adverse symptoms of Ca2+ overload (aftercontractions, spontaneous contractions, a decrease in developed force, and an increase in resting force). The response of papillary muscles to the toxin was not inhibited by blockers of Ca2+ or Na+ channels or by inhibitors of the sarcoplasmic reticulum, Na+/K+ ATPase, or Na+/H+ exchange. The response to the toxin was, however, blocked by prior exposure to a solution which contained no Na+ and by Ni2+. In the isolated myocytes, as well as an increase in intracellular [Ca2+], the toxin also caused an increase in intracellular [Na+] and the appearance of a current which was inward at negative potentials and reversed at about -10 mV. These data can be explained by the toxin increasing Na+ influx into the cell. The increase in intracellular [Na+] will then increase intracellular [Ca2+] via the Na+/Ca2+ exchange mechanism, thus producing the observed Ca2+ overload.

Effects of Ilexonin A on circulatory neuroregulation.

Ilexonin A (IA), a pentacyclic triterpene, has been semisynthesized in china for the first time. It is extracted from the root of Ilicis pubescentis, a commonly used herbal medicine in Guangdong for the treatment of cardiovascular, cerebrovascular and peripheral vascular diseases and heart failure with satisfactory effects. The pharmacokinetic studies indicated that the elimination half-life after oral and i.v. dosing were 17.7 +/- 2.4 h and 22.5 +/- 2.9 h respectively. The total clearance was 4.6 +/- 0.51/h. The bioavailability of IA capsules was 0.39 +/- 0.14 and LD50 was 234 mg/Kg. We have adopted modern techniques, including cellular electrophysiology, isotope tracing methods, molecular biology, electromicroscopy, etc., to probe into the pharmacologic mechanisms of the effects of IA on cardiovascular system. The results indicated that IA can increase the contractility of isolated guinea pig auricular myocardium, attenuate vascular smooth muscle tension induced by noradrenaline in the rabbit aorta. IA can exert a biphasic regulatory effect on arterial blood pressure. IA also can prolong A-V duration of Hiss bundle electrograph (HBE) in rabbits and prolong the action potential duration and the effective refractory period (ERP) of myocardial cells in guinea pigs. The results showed that IA can increase the cAMP content in the smooth muscle of aorta and exert a calcium-blockade effect. Therefore, the peripheral resistance vessels are relaxed and the cardiac afterload is lowered. IA-blocked calcium channels are correlated with both the potential-dependent channel and the receptor operated channel in vascular smooth muscles. IA also increases the cAMP content of myocardium and accelerates the cellular calcium influx and efflux, and this may be responsible for the direct mechanism of the positive inotropic action of IA. Under electron microscopy, it is observed that IA can alleviate the defect of succinate dehydrogenase in the myocardial mitochrondria of rabbit chronic congestive heart failure (CF) model and reduce the microstructural damage of the failed myodardium, therefore the anoxic tolerance of myocardium is increased, the effect of IA on the platelet stretching activity and microstructure in the patients with CF is also studied. It is found that IA can reduce the hypercoagulability of blood, decrease the severity of blood stagnation and improve the status of microcirculation. Effects of IA introventricular and cardiovascular central microinjection (nucleus tractus solitarius, paraventricular nucleus) on arterial blood pressure and heart rate were studied. It demonstrated that IA possess circulatory neuroregular effects by the medium of alpha-receptor and beta-receptor of cardiovascular motoneurons.

Cellular mechanism of the positive inotropic effect of hydralazine in mammalian myocardium.

1. The purpose of this study was to elucidate the cellular mechanism of the positive inotropic effect of hydralazine, a vasodilator widely used for afterload reduction in patients with heart failure that has also been reported to have positive inotropic effects on the heart. After isolation, right ventricular papillary muscles from the ferret were maintained in bicarbonate-buffered salt solution (30 degrees C). A concentration-response relationship was obtained for hydralazine (10(-6) to 10(-3) M). In order to mimic different levels of catecholamine release found in heart failure, we utilized two methods of stimulation: (a) threshold punctate pulses and (b) suprathreshold punctate stimulation with voltage approximately 10% above threshold. 2. In a first group of muscles (n = 16), a maximally effective concentration of hydralazine (10(-3) M) increased peak isometric tension by 39 +/- 9% (P < 0.05). Doses lower than 10(-5) M had no significant effect. The bioluminescent Ca2+ indicator, aequorin, was loaded into a subset of these muscles (n = 7). A significant increase in peak light (i.e., intracellular Ca2+) developed, concurrently with an increase in peak tension (38 +/- 5% to 66 +/- 8%). This inotropic response was associated with a decrease in time to peak tension (ms), 221 +/- 7 to 186 +/- 5 (P < 0.05), and time to peak light, 65 +/- 4 to 52 +/- 2 (P < 0.05). These effects were markedly attenuated by pretreatment with autonomic blocking agents. 3. In a second group of muscles (n = 12), histamine was used to stimulate cyclic AMP production in the presence of propranolol. Hydralazine (3 x 10-4 M) led to a shift in the pD2 (i.e. the negative log of the concentration of histamine producing 50% of the maximal response) from 6.1 +/- 0.1 to 5.9 +/- 0.1(P <0.05), thus increasing the sensitivity of the muscles to histamine. Hydralazine also increased maximum tension from 160 +/- 77% to 195 +/- 57% (P <0.05) above baseline. Thus, hydralazine altered the potency and efficacy of histamine despite the presence of beta-adrenoceptor blockade.4. A third group of muscles were chemically skinned to examine the effects of hydralazine on myofilament Ca2+ responsiveness. Pretreatment of ferret papillary muscles with hydralazine (10-3 M)before skinning did not shift the force-pCa curve after skinning (n = 16). However, hydralazine added to previously skinned fibres desensitized the myofilaments, as indicated by a rightward shift of the force-pCa curve (n = 12). Maximum tension development was not changed.5. The pharmacological effects of hydralazine are characteristic of inotropic drugs that act mainly via cyclic AMP; however, the increase in peak tension demonstrated with histamine in the presence of hydralazine also suggests an effect on cyclic AMP-independent second messenger pathways. These data are consistent with reports that large doses of hydralazine may increase cellular levels of cyclic AMP, as well as other second messengers, by direct cardiac and indirect neuronal mechanisms.