Effects of the calcium sensitizer [+]-EMD 60263 and its enantiomer [-]-EMD 60264 on cardiac ionic currents of guinea pig and rat ventricular myocytes

The thiadiazinone enantiomers [+]-EMD 60263 and [-]-EMD 60264 ((+)-5-(1-(alpha-ethylimino-3,4-dimethoxybenzyl)-1,2,3,4-tetrah ydroquinoline-6-yl)-6-methyl-3,6-dihydro-2H-1,3,4-thiadiazine-2 -on) exhibit distinct stereoselectivity for Ca2+-sensitizing action ([+]-enantiomer) and phosphodiesterase inhibition ([-]-enantiomer). However, in isolated guinea pig papillary muscle, both compounds cause an action-potential prolongation that has been related to a nonselective depression of the delayed rectifier potassium current. Because [-]-EMD 60264 did not increase force of contraction despite phosphodiesterase inhibition, we postulated that one or several additional actions may oppose the anticipated positive inotropic effect. Therefore we investigated whether other membrane currents were also affected in voltage-clamped ventricular cardiomyocytes. Both [+]-EMD 60263 and [-]-EMD 60264 reduced sodium current as well as L-type calcium current in guinea pig ventricular myocytes, but steady-state inactivation or conductance curves of I(Na) and I(Ca) were not shifted along the voltage axis. Inward rectifier and transient outward current were studied in rat myocytes, but neither current was affected. We conclude that the positive inotropic action of [+]-EMD 60263 can be explained by prevalence of the Ca2+-sensitizing effect over its inhibitory actions on Na+ and Ca2+ current, whereas the negative inotropic effect of [-]-EMD 60264 may be caused by inhibition of I(Ca) predominating over PDE inhibition.     

Cardiac electrophysiological actions of NS-21 and its active metabolite, RCC-36, compared with terodiline

Terodiline, an anticholinergic drug with a Ca²⁺ blocking action, is thought to be associated with torsade de pointes, a serious ventricular tachycardia. NS-21 is a newly developed drug for the treatment of urinary frequency and urinary incontinence and it has pharmacological properties similar to those of terodiline. It remains unknown, however, whether NS-21 and its active metabolite, RCC-36, have any proarrhythmic activity. The electrophysiological properties of NS-21 and RCC-36 were examined in guinea pig ventricular myocytes and were compared with those of terodiline using the whole-cell patch-clamp technique. NS-21, RCC-36 and terodiline inhibited L-type Ca²⁺ currents in a concentration-dependent manner with IC₅₀ values of 27.0, 27.0 and 33.5 μM, respectively. At a concentration of 10 μM, terodiline inhibited both the time-dependent current and the tail current of the delayed rectifier K⁺ current, with the latter being significantly inhibited at voltages more positive than +10 mV. In contrast, NS-21 and RCC-36 had almost no effect on either of these currents. Terodiline also inhibited the inward rectifier K⁺ current significantly at voltages more negative than -100 mV, whereas NS-21 and RCC-36 had little effect. If the proarrhythmic activity of terodiline resulted primarily from the combined inhibition of K⁺ and Ca²⁺ currents, one might expect that NS-21 and RCC-36, which inhibit L-type Ca²⁺ currents without affecting either the delayed rectifier K⁺ current or the inward rectifier K⁺ current, would not share the proarrhythmic activities of terodiline. Received: 27 August 1996 / Accepted: 13 February 1997     

5-HT_4受体激动剂兼5-HT_3受体阻断剂2-[1-(4-piperonyl)piperazinyl]-benzothiazole致心律失常机制探析

目的观察5-HT4受体激动剂兼5-HT3受体阻断剂2-[1-(4-piperonyl)piperazinyl]benzothiazole对大鼠离体心脏心律的影响,并探析其电生理学机制。方法采用成年健康SD大鼠建立离体心脏Langendorff主动脉逆行灌流系统,观察0.1～10μmol·L-12-[1-(4-piperonyl)piperazinyl]benzothiazole对离体心脏节律的影响,全程记录心电图的变化。应用全细胞膜片钳技术观察2-[1-(4-piperonyl)piperazinyl]benzothiazole对胶原酶分解的大鼠心室肌细胞膜内向整流钾电流(IK1)、瞬时外向钾电流(Ito)、静息膜电位(RMP)及动作电位(AP)的影响。结果在大鼠离体心脏,0.1～10μmol·L-12-[1-(4-piperonyl)piperazinyl]benzothiazole可诱发明显的心律失常。给药15min内,药物(10μmol.L-1)诱发期前收缩(PVB)236±37个,室速(VT)和室颤(VF)发生率分别达到87.5%和62.5%(n=8,P<0.01)。膜片钳记录结果显示,0.1～10μmol·L-12-[1-(4-piperonyl)piperazi-nyl]benzothiazole可浓度依赖性抑制大鼠心室肌IK1(EC50=0.74μmol·L-1)和Ito(EC50=2.16μmol·L-1),降低膜电位,并明显延长动作电位时程(n=6,P<0.01)。结论作为5-HT4受体激动剂和5-HT3受体阻断剂2-[1-(4-pipero-nyl)piperazinyl]benzothiazole致大鼠心律失常风险的电生理学机制为抑制IK1和Ito,降低膜电位,延长动作电位时程。     

DPI 201-106, a novel cardioactive agent. Combination of cAMP-independent positive inotropic,negative chronotropic,action potential prolonging and coronary dilatory properties

Summary The in vitro cardiac effects of DPI 201-106, a novel piperazinyl-indole, were investigated.DPI 201-106 produced concentration-dependent positive inotropic effects in guinea-pig and rat left atria, kitten, rabbit and guinea-pig papillary muscles and Langendorff perfused hearts of rabbits between 10^–7 and 3×10^–6 mol/l. During isometric twitches, contraction and relaxation phases were prolonged in guinea-pig left atria and right ventricular papillary muscles from kitten and guinea-pigs. Spontaneous sinus rate was decreased in right atria of guinea-pigs and rats. Coronary flow increased in rabbit isolated hearts. Functional refractory period was increased in left atria from guinea-pigs and rats with EC₅₀ values of 1.7 and 0.24 mol/l respectively.In electrophysiological measurements, DPI 201-106 prolonged the action potential duration (APD₇₀) in guinea-pig papillary muscles up to 70% and in rabbit atria up to 120% at 3 mol/l. Other action potential characteristics were not changed in guinea-pig papillary muscles but V _max was decreased in rabbit left atria. The electrophysiological as well as the positive inotropic effects were stereoselective with the activity residing in the S-enantiomer.DPI 201-106 increased the Ca²⁺-sensitivity of skinned fibres from porcine trabecular septomarginalis with an EC₅₀ of 0.2 nmol/l. DPI 201-106 did not change cAMP levels in guinea-pig atria and rabbit papillary muscles. Slow action potentials were not induced by DPI 201-106 in partially depolarized guinea-pig papillary muscles. Phosphodiesterase activity of rat hearts was not inhibited by DPI 201-106 at pharmacologically relevant concentrations. The presence of propranolol did not influence the inotropic potency of DPI 201-106 in guinea-pig atria.In conclusion, DPI 201-106 represents a novel type of positive inotropic agents with a synergistic sarcolemmal and intracellular mechanism of action.     

Pharmacological differentiation of synergistic contribution of L-type Ca2+ channels and Na+/H+ exchange to the positive inotropic effect of phenylephrine,endothelin-3 and angiotensin II in rabbit ventricular myocardium

The present study was designed to delineate pharmacologically the role of sarcolemmal L-type Ca²⁺ channels and Na⁺/H⁺ exchange in the positive inotropic effect (PIE) of phenylephrine mediated by alpha-1 adrenoceptors, endothelin (ET) and angiotensin II (Ang II) that stimulate phosphoinositide (PI) hydrolysis in the rabbit ventricular muscle. The PIE of these receptor agonists was compared with the PIE of isoprenaline that accumulates cyclic AMP. For this purpose, we investigated the influence of a Ca²⁺ antagonist, verapamil, and of an inhibitor of Na⁺/H⁺ exchange, 5-(N-ethyl-N-isopropyl) amiloride (EIPA), alone or in combination, on the cumulative concentration-response curve (CRC) for phenylephrine (with 0.3 μM bupranolol), ET-3 and Ang II in isolated right ventricular papillary muscles of the rabbit, which were electrically stimulated at 1 Hz in Krebs-Henseleit solution at 37°C. Verapamil at 0.3 and 1 μM decreased the basal force of contraction to 37.0 ± 4.0% and 13.2 ± 1.1% of the control, respectively, while EIPA even at 10 μM affected the basal force to much less extent and decreased it to 87.0 ± 1.4%. Verapamil (0.3 and 1 μM) and EIPA (1 and 10 μM), when used alone, each significantly attenuated but did not abolish the PIEs induced by phenylephrine, ET-3 and Ang II, while the simultaneous administration of verapamil (1 μM) and EIPA (10 μM) consistently and almost completely inhibited the PIE induced by these receptor agonists. By contrast, the PIE of isoprenaline was retained even in the presence of verapamil and EIPA. These results indicate that both the influx of Ca²⁺ ions through L-type Ca²⁺ channels and activation of Na⁺/H⁺ exchange contribute synergistically to the PIE that is mediated by alpha-1 adrenergic, ET and Ang II receptor agonists, while these mechanisms are not essential for the beta-adrenoceptor-mediated PIE. Received: 20 February 1996 / Accepted: 20 August 1996     

Influence of thyroid hormone on the positive inotropic effects mediated by α- and β-adrenoceptors in isolated guinea pig atria and rabbit papillary muscles

Effects of thyroxine treatment for 7–11 days on the positive inotropic effects mediated by α- and β-adrenoceptors were studied in isolated guinea pig atria and rabbit papillary muscles. In guinea pig atria, the thyroxine treatment inhibited the positive inotropic effect of lower concentrations of phenylephrine (PHE), and attenuated the inhibitory effect of phentolamine on the PHE response. The effect of isoproterenol (ISO) was potentiated by the thyroxine treatment. In rabbit papillary muscles, the thyroxine treatment shifted the dose—response curve for PHE to the right and attenuated the inhibitory effect of phentolamine on the PHE response. Propranolol, in both guinea pig atria and rabbit papillary muscles, inhibited the PHE response more effectively in preparations from thyroxine-treated animals than in controls. In guinea pig atria, the attenuation of the PHE response mediated by α-adrenoceptors was observed after the thyroxine treatment for only 2 days, whereas the potentiation of the ISO response required the thyroxine treatment for a longer period. It was concluded that the thyroxine treatment attenuated the positive inotropic effect mediated by α-adrenoceptors and potentiated that mediated by β-adrenoceptors in guinea pig atria and rabbit papillary muscles, and that the changes in the α- and β-adrenoceptor-mediated positive inotropic effects due to the thyroxine treatment may be independent of each other.