Peter Heistracher

Proarrhythmic effects of antidepressants and neuroleptic drugs on isolated, spontaneously beating guinea-pig Purkinje fibers

Antidepressants and neuroleptic drugs are sometimes the reason for the occurrence of the polymorphic ventricular arrhythmia torsades de pointes in patients. Therefore, it was of interest to study the actions of some of these drugs such as imipramine, amitriptyline, doxepin, chlorpromazine, trifluoperazine and thioridazine in isolated, spontaneously beating Purkinje fibers of guinea-pig hearts using the intracellular microelectrode technique because experimentally induced early afterdepolarizations (EADs) may be associated with this special type of arrhythmia. If the extracellular K+ concentration was 2.7mM none of these drugs could elicit EADs. For that reason the K+ concentration was lowered to 1. 35mM and EADs were evoked by imipramine (2 and 5 microM). Amitriptyline (2 and 5 microM) and doxepin (2 microM) did not induce EADs. Only a concentration of 5 microM doxepin elicited EADs. Among the neuroleptic drugs, chlorpromazine at a concentration of 2 and 5 microM was responsible for the occurrence of EADs as well as thioridazine in the same concentrations. When trifluoperazine (2 and 5 microM) was applied no EADs could be observed. Tetrodotoxin (0. 2 microMl-1) abolished thioridazine-induced EADs. Several membrane depolarizing currents may participate in the initiation of these EADs. Our results demonstrate that in guinea-pig Purkinje fibers some tricyclic antidepressants and some neuroleptic drugs are responsible for the rare occurrence of EADs under hypokalemic conditions.

Cardiotoxicity of emetine dihydrochloride by calcium channel blockade in isolated preparations and ventricular myocytes of guinea‐pig hearts

1 The cardiotoxic effects of emetine dihydrochloride on mechanical and electrical activity were studied in isolated preparations (papillary muscles, sinoatrial and atrioventricular nodes, ventricular myocytes) of the guinea‐pig heart. 2 Force of contraction was measured isometrically, action potentials and maximum rate of rise of the action potential were recorded by means of the intracellular microelectrode technique. Single channel L‐type calcium current (Ba2+ ions as charge carrier) was studied with the patch‐clamp technique in the cell‐attached mode. 3 Emetine dihydrochloride (8–256 μm) reduced force of contraction in papillary muscles and spontaneous activity of sinoatrial and atrioventricular nodes concentration‐dependently; the negative inotropic effect was abolished when the extracellular Ca2+ concentration was increased. 4 Maximum diastolic potential, action potential amplitude, maximum rate of rise of the action potential and the slope of the slow diastolic depolarization were decreased by emetine in sinoatrial as well as atrioventricular nodes, while action potential duration was prolonged in both preparations (1–64 μm). 5 The amplitude of the L‐type calcium single channel current was not altered by emetine dihydrochloride, while average open state probability was decreased concentration‐dependently (10, 30 and 60 μm). 6 The most prominent effect of emetine dihydrochloride on single channel current was an increase of sweeps without activity. 7 At 60 μm, emetine dihydrochloride caused a decrease of the mean open time and an increase of the mean closed time. The number of openings per record and number of bursts per record were reduced. 8 It is concluded that emetine dihydrochloride produces an L‐type calcium channel block which might contribute to its cardiac side effects.

Electrophysiological properties of the propafenone-analogue GE 68 (1-[3-(phenylethyl)-2-benzofuryl]-2-(propylamino)-ethanol) in isolated preparations and ventricular myocytes of guinea-pig hearts.

Abstract GE 68 ((Rac.)-1-[3-(Phenylethyl)-2-benzofuryl]-2-(propylamino)-ethanol hydrochloride) is structurally related to propafenone, and exerts negative inotropic and negative chronotropic effects similar to the parent drug, but lacks any β-adrenoceptor blocking activity contrary to propafenone. Thus, the electrophysiological effects of GE 68 were studied in papillary muscles, left atria, Purkinje fibres, sinoatrial nodes and ventricular myocytes of the guinea-pig heart with the intracellular microelectrode technique and the patch-clamp technique in the cell-attached mode.The decrease of the maximum upstroke velocity (V˙max) by GE 68 (1 to 10 μM) was use- and frequency-dependent. V˙max recovered from the use-dependent block with a time constant of 4.1 ± 0.6 s. In papillary muscles and Purkinje fibres action potential duration was shortened, while it was prolonged in left atria and sinoatrial nodes. Half-maximal steady-state inactivation of the sodium channels was shifted to more negative membrane potentials (control: –91.5 ± 0.8 mV, 10 μM GE 68: –97.9 ± 2.5 mV). The peak of the current-voltage relationship and the reversal potential were not changed by GE 68. The amplitude of the unitary current remained unaltered, while open state probability was decreased. The most striking effect of GE 68 was an increase of the number of sweeps without single channel openings (1 μM: 2 fold, 10 μM: 6 fold). GE 68 also caused a decrease of the mean open times, and an increase of the mean closed times in unmodified and pronase-modified sodium channels.Besides the lack of β-adrenoceptor blocking activity, data present a faster recovery from the use-dependent block by GE 68 and a lower affinity to inactivated sodium channels compared to the reference drug propafenone, as well as differences in the effect on single channel kinetics.

Mechanism of sodium channel blockade in the cardiotoxic action of emetine dihydrochloride in isolated cardiac preparations and ventricular myocytes of guinea pigs

Emetine is used in the therapy of special forms of amebiasis and is abused as syrup of ipecac by persons with bulimia. Severe cardiac side effects were reported. Thus the intracellular microelectrode technique and the patch-clamp technique in the cell-attached mode were used to study the effects of emetine on the action potential and upstroke velocity (Vmax) in papillary muscles and Purkinje fibers of guinea pigs as well as on macroscopic and (S)-DPI 201-106-modified and unmodified single-sodium-channel current (I(Na)) of guinea-pig ventricular myocytes. Emetine caused a tonic block of Vmax and reduced I(Na) independent of frequency. Hill plots were linear, with slopes ranging from 0.96 to 1.06, suggestive of a first-order reaction. The current-voltage relation was not influenced, indicating a voltage-independent blockade of the sodium channels. The most prominent effects were an increase of sweeps without activity, a decrease of the fast component of the open-time distribution, an increase of the slow component of the closed-time distribution, and a reduction in the number of bursts per record. The amplitude of the unitary current was not changed. From the results, we conclude that I(Na) blockade contributes to the cardiotoxicity of emetine.

Cardiotoxic effects of fenfluramine hydrochloride on isolated cardiac preparations and ventricular myocytes of guinea‐pigs

The cardiotoxic effects of fenfluramine hydrochloride on mechanical and electrical activity were studied in papillary muscles, Purkinje fibres, left atria and ventricular myocytes of guinea‐pigs. Force of contraction (fc) was measured isometrically, action potentials and maximum rate of rise of the action potential (Vmax) were recorded by means of the intracellular microelectrode technique and the sodium current (INa) with patch‐clamp technique in the cell‐attached mode. For kinetic analysis (S)‐DPI‐201‐106‐modified Na+ channels from isolated guinea‐pig ventricular heart cells were used. Fenfluramine (1–300 μM) produced negative chronotropic and inotropic effects; additional extracellular Ca2+ competitively antagonized the negative inotropic effect. Fenfluramine concentration‐dependently reduced Vmax and showed tonic blockade of sodium channels, shortened the action potential duration in papillary muscles and Purkinje fibres. In cell‐attached patches, fenfluramine decreased INa concentration‐dependently (10–100 μM), frequency‐independently (0.1–3 Hz; 30 μM). The h∞ curve was shifted towards hyperpolarizing direction. At 30 μM, fenfluramine blocked the sodium channel at all test potentials to the same degree, and neither changed the threshold and reversal potentials nor the peak of the curve. No effect on single channel availability, but a significant decrease in mean open times and increase in mean closed times was observed. Mean duration of the bursts decreased and number of openings per record increased with increasing drug concentration. It is concluded that the effect on INa plays an important role in the cardiotoxicity of fenfluramine in addition to primary pulmonary hypertension and valvular disorders.

Structure-activity relationships of new thienothiazine derivatives in isolated heart and smooth muscle preparations of guinea pigs.

New thienothiazine derivatives that differ in their side chains on the nitrogen atom of the thienothiazine ring were investigated regarding structure-activity relationships and calcium antagonistic and/or potassium channel opening properties. Isometric contraction force was measured in guinea pig papillary muscles, aortic strips, and terminal ilea. Chronotropic activity was studied in right atria of guinea pigs. The derivatives with a dimethylaminoethylcarboxamide side chain (HO4) and with a dimethoxyphenylethyl-N-methylaminoethylcarboxamide side chain (HO7) had the most potent negative inotropic effects on papillary muscles and spontaneously beating right atria. The negative inotropic and chronotropic effects of the compounds with a methylpiperazinylcarbonyl side chain (HO5) or a diethylaminopropylcarboxamide side chain (HO6) were less pronounced. The negative inotropic action was reversed by increasing the extracellular calcium concentration. It was also reversed by glibenclamide, for concentrations of the compounds up to the EC50, but not at higher compound concentrations. Among all the compounds studied, HO 7 had the strongest relaxing effect on aortic strips and terminal ilea. The effects of the derivatives on the smooth muscles could not be reversed by glibenclamide. The calcium antagonistic effect of the thienothiazine derivatives is more pronounced than the potassium channel opening activity, at least in high drug concentrations. Compounds with an aromatic or heterocyclic ring in the side chain have the weakest negative inotropic and negative chronotropic effects on papillary muscles and right atria. However, HO7 showed a tissue specificity with the most potent relaxing effect on aortic strips and terminal ilea.