Jörg Weirich

Effects of mexiletine on steady-state characteristics and recovery kinetics of V max and conduction velocity in guinea pig myocardium.

Summary We studied the effects of the antiarrhythmic compound mexiletine on the maximal rate of rise (&OV0312;max) of the action potential and on conduction velocity in isolated papillary muscles of guinea pigs. Action potentials and &OV0312;max were measured by means of conventional microelectrodes. The conduction velocity was determined from extracellular recordings. Mexiletine (concentrations, 4.6 × 10−5 and 1.15 × 10−4 mol/L) caused a depression of &OV0312;max that depended on the condition examined. The hx curve relating &OV0312;max of the action potential to the resting membrane potential was shifted along the voltage axis to more negative membrane potentials. Qualitatively equal effects were observed on the curve of membrane responsiveness relating &OV0312;max to the membrane potential during the phase of repolarization of the preceding action potential. Eventually, marked prolongation of the recovery kinetics of &OV0312;max by mexiletine, as well as frequency-dependent reduction of &OV0312;max, could be demonstrated. The action of mexiletine on conduction velocity was very similar to its influence on &OV0312;max. Thus, a prolongation of the recovery kinetics was found of about the same magnitude as that measured in &OV0312;max. Similarly, in experiments with K+-induced depolarizations, a dependence on the membrane potential of the action of mexiletine on the conduction velocity could be demonstrated. The results suggest that mexiletine may exert antiarrhythmic effects mainly in damaged cardiac cells exhibiting a moderately reduced resting potential.

Use dependence of sodium current inhibition by tetrodotoxin in rat cardiac muscle: influence of channel state

Tetrodoxin (TTX) is known to cause a voltage-and frequency-dependent inhibition of the rapid inward sodium current (INa) of cardiac muscle. This effect was studied by means of the loose-patch-clamp method on intact rat papillary muscle. The availability curve of the fast sodium system, determined by variation of the holding potential, is shifted in the presence of TTX (5.5 μmol · l−1) by 17 mV to more negative potentials. With clamp pulses of 5 ms duration to 0 mV, a frequency-dependent reduction of INa by TTX is found above 0.1 Hz that saturates at about 10 Hz. This frequency-dependent block was further analysed using trains of pulses (10 Hz) of various durations (minimum 50 μs), which allow TTX to equilibrate with channel states reached early during activation. The results show that more than 90% of the frequency-dependent block is attained with pulses of 1 ms duration. An analysis according to the guarded receptor hypothesis reveals that these results are well described by TTX binding to inactivated, activated and probably preactivated channel states.

Contribution of store-operated Ca2+ entry to pHo-dependent changes in vascular tone of porcine coronary smooth muscle.

Vascular smooth muscle contracts on increases of extracellular pH (pH(o)) and relaxes on pH(o) decreases possibly resulting from changes in transsarcolemmal Ca(2+) influx. Therefore, we studied store-operated Ca(2+) entry (SOCE; i.e. capacitative Ca(2+) entry (CCE)) during acidification (pH(o)=6.5) and alkalinization (pH(o)=8.0) in isolated porcine coronary smooth muscle cells (SMCs) by monitoring cytoplasmic Ca(2+) ([Ca(2+)](i)) and divalent cation entry (Mn(2+) quench) with fura-2/AM-fluorometry. Additionally, we evaluated the contribution of SOCE to pH(o)-dependent changes in isometric tension of porcine coronary smooth muscle strips. SOCE elicited in SMCs by the SERCA inhibitor BHQ was strongly modulated by pH(o) showing a decrease upon acidification and vice versa an increase upon alkalinization. BHQ-mediated tension of smooth muscle strips also revealed strong pH(o) dependence. In contrast, L-VOC-dependent tension ([K(+)](o)=20 and 40 mmol l(-1)) was remarkably less affected by pH(o) changes. Moreover, refilling of depleted Ca(2+) stores after repeated M(3)-cholinergic receptor stimulation could be almost completely inhibited by SKF 96365 and was markedly reduced by acidification and considerably enhanced by alkalinization pointing to a major role of SOCE in refilling. We conclude that vascular tone particularly responds to alterations in pH(o) whenever SOCE substantially contributes to the amount of activator Ca(2+) for contraction.

Comparative analysis of the action of class I antiarrhythmic drugs (lidocaine, quinidine, and prajmaline) in rabbit atrial and ventricular myocardium

Effects of three class I antiarrhythmic drugs (quinidine, lidocaine, and prajmaline) on transmembrane resting (RMP) and action potentials (AP) of isolated rabbit atrial and ventricular myocardium were studied at different stimulation rates. The frequency-dependent depression of the maximal upstroke velocity (Vmax) of the AP (sodium channel block) was analyzed according to the “guarded receptor” hypothesis. The resting block (Vmax depression after a resting period) induced by prajmaline (10−6 M), quinidine (2.2 × 10−5 M), and lidocaine (4.3 × 10−5 M) was more expressed in the atrium (44, 28, and 19%, respectively) than in the ventricle (32, 9, and 0%, respectively). There were also significant (p < 0.05) atrioventricular differences in the frequency-dependent extra block (Vmax reduction on stimulation at 3.3 Hz) for quinidine (39 vs. 26%) and lidocaine (4 vs. 25%). From the analysis, according to the guarded receptor hypothesis, it follows that the three compounds bind preferentially to inactivated sodium channels with about the same affinity to the atrium and ventricle, except for quinidine which shows a significantly smaller dissociation constant in the atrium (5 × 10−6 M vs. 2.7 × 10−5 M; p < 0.001). We conclude that the atrioventricular differences in the resting block are mainly due to atrioventricular differences in the RMP, whereas the differences in the frequency-dependent extra block are based on the shorter atrial AP duration (lidocaine) or are due to higher affinity to atrial sodium channels (quinidine).

Differential analysis of frequency-dependent effects of antiarrhythmic drugs : importance of the saturation behavior of frequency-dependent sodium- channel blockade

Frequency-dependent effects of class I antiarrhythmic drugs on Vmax reported in the literature are analyzed with respect to periodical drug binding to sodium channels. The analysis reveals that class I action can be differentiated according to the onset kinetics as well as to the saturation behavior of frequency-dependent sodium-channel blockade at increasing frequencies. As will be shown, class I drugs even of the same subclass (Ic) may differ markedly from each other with respect to the saturation behavior of frequency-dependent block. These findings may be of interest in view of the Cardiac Arrhythmia Suppression Trial (CAST) because the results found with flecainide and encainide in this study are usually extrapolated to other Ic drugs. Additionally, the influence of postrepolarization refractoriness caused by a class I drug on the action potential shortening during repetitive premature stimulation is compared with the effects of prolongation of absolute refractoriness induced by a class III drug.

Factors determining the susceptibility of the isolated guinea pig heart to ventricular fibrillation induced by sinusoidal alternating current at frequencies from 1 to 1000 Hz

SummaryWe determined the threshold for ventricular fibrillation by means of sinusoidal alternating current (AC) from 1 to 1000 Hz on isolated perfused guinea pig hearts. Current was applied via electrodes located in the aorta and at the apex of the heart. The duration of current flow was kept constant at 1 s. Electrical activity was recorded with epicardial electrodes attached to the ventricles. Additional experiments were performed in isolated papillary muscles with intracellular microelectrodes. The fibrillation threshold, expressed as peak-to-peak current strength, attains a minimum at about 30 Hz and rises by a factor of 5 at 1 Hz, and by a factor of 14 at 1000 Hz. In the frequency range from 30 to 1000 Hz the rise of the fibrillation threshold can be attributed to the increase of the threshold for stimulation due to the progressive shortening of the AC periods. Thus no change of the fibrillation threshold occurs if DC pulses of constant duration are used in the same range of frequencies. Below 30 Hz there is only a slight increase of the threshold for stimulation, which cannot entirely account for the rise of the threshold for fibrillation. A likely cause of the reduced susceptibility of the heart to fibrillation at the lower frequencies is the reduced number of extrasystoles preceding the onset of fibrillation, which results in a less pronounced state of inhomogeneous excitability.ZusammenfassungAn isolierten perfundierten Meerschweinchenherzen (Langendorff-Präparation) wurden die ventrikuläre Flimmerschwelle und die diastolische Reizschwelle für sinusförmigen Wechselstrom im Frequenzbereich von 1–1000 Hz ermittelt. Die Stromapplikation erfolgte zwischen einer Ringelektrode um die Herzspitze und einer Elektrode in der Perfusionskanüle. Die Flußdauer des Stroms betrug einheitlich 1 s. Die elektrische Aktivität wurde extrazellulär von beiden Ventrikeln abgegriffen.Die Flimmerschwelle, ausgedrückt als Stromstärke des Wechselstroms Spitze-Spitze, zeigt bei Frequenzen um 30 Hz ein Minimum und steigt bei niedrigeren und bei höheren Frequenzen an (Anstieg bei 1 Hz gegenüber 30 Hz um den Faktor 5, bei 1000 Hz um den Faktor 14). Im Frequenzbereich zwischen 30 und 1000 Hz wird das Verhalten der Flimmerschwelle durch den Anstieg der diastolischen Reizschwelle bestimmt, bedingt durch die zunehmende Verküzung der Wechselstromperioden. Dementsprechend erfolgt kein Anstieg der Flimmerschwelle, wenn Rechteckimpulse von konstanter Dauer und vergleichbarer Frequenz verwendet werden. Unterhalb 30 Hz ist die verminderte Flimmerbereitschaft des Herzens gegenüberWechselstromreizen von 1 s Dauer hauptsächlich durch die frequenzbedingte Abnahme der Extrasystolenzahl während der Reizung verursacht, da eine Verminderung der Extrasystolenzahl eine geringeren Grad flimmerbegünstigender Inhomogenität im Erregbarkeitszustand des Herzens zur Folge hat.

Frequency-dependent action of antiarrhythmic drugs: the useful concept of periodical ligand binding.

SummaryThe action of most antiarrhythmic drugs which block cardiac ionic channels depends on heart rate, which is established as use- or frequency-dependence. This property is consistent with periodical drug (ligand) binding to channel binding sites which are transiently available during the excitation sequence of cardiac tissue. Antiarrhythmic drugs differ with respect to their binding and unbinding kinetics, i.e., with respect to their blocking and unblocking kinetics. This gives rise to different block-frequency relations and onset-kinetics of frequency-dependent ion channel blockade. Antiarrhythmic drugs have never been systematically compared with regard to their block-frequency relations. However, both the onset-kinetics as well as the block-frequency relation are essential in characterizing the frequency-dependent drug action, since both may be predictors of the anti- and proarrhythmic potential of antiarrhythmic drugs.

Reverse Use Dependence of Antiarrhythmic Class la, Ib, and Ic: Effects of Drugs on the Action Potential Duration?

The prolongation of the action potential duration (APD) induced by sotalol has been shown to be diminished with increasing heart rate. This phenomenon is called “reverse use dependence.” We examined the la, Ib, and Ic effects of different Class I drugs on the APD under normal and fast stimulation rates (1.0 and 2.5 Hz) in isolated rabbit atria) and ventricular muscles by means of intracelular microelectrodes. Results (n = 98): With 1.0 Hz lidocaine fib, 4.3 ? 10−5 M) shortened the APD at 90% repolarization (APD90) in the atrium by 9% and in the ventricle lay 8% (NS), whereas quinidine (la, 2.2 ? 10 −5 M) and prajmaline (la, 10 −6 M) prolonged the APD90 in the atrium (quinidine +45%; prajmaline +10%, P < 0.001) and in the ventricle (+ 42%, P < 0.001; +17%, P < 0.05J. Propafenone (Ic, 2.6 ? 10 −6 M) showed this effect only in the atrium (APD90 + 33%; P < 0.01). With the faster stimulation rate of 2.5 Hz we could not find a significant influence of any drug on the APD90 in the ventricle and only quinidine prolonged the APD90 in the atrium by 16% (P < 0.05). Conclusions: The subclassification of Class I antiarrhythmic drugs that is based on APD modifying influences is only valid under normal heart rates (1.0 Hz). During tachycardia these actions are absent and the phenomenon of “reverse use dependence” is found in Class I drugs. Therefore, an additional antiarrhythmic effect due to APD modification by the examined drugs should not be expected at rapid heart rates.

Influence of direct current on the electrical activity of the heart and on its susceptibility to ventricular fibrillation

SummaryIsolated perfused guinea pig hearts (Langendorff preparation) were used to study the influence of direct current (DC) on the shape of the electrogram and to determine the thresholds for extrasystoles and for fibrillation. DC impulse durations ranged between 30 ms and 5 s.During DC flow through the heart, marked deformations of the electrogram occur mainly concerning the repolarization phase and increasing with growing current strength. At an intensity of twice the diastolic threshold for stimulation, constant DC induces extrasystoles, which are suggested to result from an enhancement of the intrinsic automatic activity by cathodal polarization.Compared with the diastolic threshold for stimulation, the threshold for fibrillation is higher by a factor of about 15. The curve relating the threshold for fibrillation to the impulse duration shows a marked decline between 30 ms and 60 ms, followed by a slight increase and adjustment to a constant level beyond about 80 ms. In the used setup, the threshold for fibrillation is independent on the reversal of polarity of the electrodes. Long-lasting DC pulses are able to induce fibrillation even if they are switched on in the absolute refractory period of a normal beat.Application of 50-Hz alternating current revealed a higher threshold for stimulation and a three times lower fibrillation threshold compared with the effects of DC in the same heart.ZusammenfassungAn isolierten Meerschweinchenherzen (Langendroff-Präparation) wurden der Einfluß von Gleichstrom auf das Elektrogramm geprüft und die Schwelle zur Auslösung von Extrasystolen bzw. Kammerflimmern durch Gleichstrom bestimmt. Die Dauer der verwendeten Impulse lag zwischen 30 ms und 5 s.Während Gleichstromeinwirkung kommt es zu beträchtlichen Deformierungen des Elektrogramms besonders in der Repolarisationsphase, die mit der Stromstärke zunehmen. Bei einer Intensität, die etwa der doppelten Reizschwelle entspricht, treten während der Einwirkung von konstantem Gleichstrom Extrasystolen auf, deren Auslösung auf der Stimulation der endogenen Erregungsbildung durch Katelektrotonus beruht.Die Flimmerschwelle für Gleichstrom liegt etwa um einen Faktor 15 höher als die Reiszchwelle. Ihre Beziehung zur Impulsdauer zeigt einen deutlichen Abfall zwischen 30 ms und 60 ms, gefolgt von einem leichten Anstieg und der Einstellung auf ein gleichbleibendes Niveau oberhalb etwa 80 ms. Bei der verwendeten Anordnung ist die Flimmerschwelle unabhängig von der Polungsrichtung der Reizelektroden. Langdauernder Gleichstrom kann Flimmern induzieren, auch wenn er in der absoluten Refraktärperiode eines Normalschlags eingeschaltet wird.Beim Vergleich mit technischem Wechselstrom von 50 Hz liegt am gleichen Herzen die Reizschwelle des Wechselstroms über der für Gleichstrom. Dagegen ist die Flimmerschwelle für Gleichstrom von 1 s Dauer um einen Faktor 3 höher als die Flimmerschwelle für Wechselstrom von 1 s.

Pathophysiologische Grundlagen von Vorhofflimmern

Summary Atrial fibrillation has been recognized and studied extensively in the early twentieth century, and since then reentrant excitation waves (reentry) have been favored as the main mechanism underlying this arrhythmia. The prerequisites of reentry are an inhomogeneous excitability and shortening of the excitation wavelength. Because of anisotropic conduction properties due to directional differences in cell coupling, inhomogeneous excitability is an inherent feature of atrial myocardium. Moreover, the inhomogeneity can be exaggerated by dilatation of the atria due to a spatially inhomogeneous increase in refractory period or by vagal stimulation due to a spatially inhomogeneous decrease in the refractory period. Likewise, interstitial fibrosis in congestive heart failure may enhance the anisotropy. Shortening of the excitation wave occurs, for instance, during high vagal or high sympathetic tone, or in hyperthyroidism; all three factors shorten the atrial refractory period, and thus, the excitation wavelength. However, the most important finding over the last several years is the recognition that tachycardiac stimulation induces alterations in the electrical properties of the atria, i.e. electrical remodeling, which results in shortening of the refractory period, possibly in a spatially inhomogeneous manner. Atrial remodeling is associated with a reduction in ion channel gene expression, and as a main consequence, with a reduction in L-type Ca2+ current. This results in a shorter atrial action potential and, hence, a decrease in refractory period. Thus, once initiated atrial fibrillation promotes its maintenance and re-initiation should it terminate. Furthermore, atrial remodeling may explain a variety of clinical observations, the increasing resistance to therapy of longer-standing atrial fibrillation, the tendency of paroxysmal atrial fibrillation to become chronic, and the decreasing risk of recurrence of atrial fibrillation after electrical cardioversion the longer sinus rhythm can be maintained, i.e. the more time has elapsed for remodeling to dissipate. In this context, it is worth mentioning that in rare cases of paroxysmal atrial fibrillation the tachycardia may be due to rapidly discharging foci. Then, radiofrequency ablation of the ectopic foci that trigger tachycardia-induced remodeling and, thus, atrial fibrillation may cure the arrhythmia.Zusammenfassung Als Pathomechanismus von Vorhofflimmern wird allgemein der Wiedereintritt bzw. das Kreisen von Erregungswellen (Reentry) postuliert. Die Grundvoraussetzungen für Reentry sind eine inhomogene Erregbarkeit des Vorhofmyokards und eine Verkürzung der atrialen Erregungswellenlänge. Aufgrund des unterschiedlichen Kopplungsgrad der atrialen Myozyten längs und quer zur Faserrichtung (Anisotropie) weist das Vorhofmyokard eine inhärente Inhomogenität der Erregbarkeit auf. Diese kann durch interstitielle Fibrose z.B. bei kongestiver Kardiomyopathie verstärkt sein. Ebenso führt Dilatation der Vorhöfe zu einer räumlich inhomogenen Verlängerung der Refraktärzeit. Vagale Stimulation verkürzt die Refraktärzeit ebenfalls inhomogen. Erhöhter Vagotonus, erhöhter Sympathicotonus und Hyperthyreose führen über unterschiedliche ionale Mechanismen zu einer Abnahme der atrialen Refraktärzeit und fördern somit Reentry durch die Verkürzung der Erregungswellenlänge. Besonders hervorgehoben werden müssen neuere Befunde, wonach längeranhaltende tachykarde Erregung die elektrische Eigenschaften des Vohofmyokards nachhaltig verändert (elektrisches „Remodeling”) und dadurch Reentry begünstigt: Die ionalen Mechanismen des Remodeling sind eine Reduktion des transienten K+-Auswärts-Stroms (Ito) und eine Reduktion des langsamen Ca2+-Einwärts-Stroms über L-Typ Ca2+-Kanäle (ICa,L), wobei besonders der letztgenannte Mechanismus zu einer Verkürzung der atrialen Refraktärzeit führt. Somit fördert einmal ausgelöstes Vorhofflimmern selbst sein Fortbestehen bzw. seine Re-Induktion. Klinische Beobachtungen wie die relative Therapieresistenz schon länger bestehenden Vohofflimmerns, die Neigung von gehäuft auftretendem paroxysmalem Vorhofflimmern zur Chronizität und die erhöhte Rezidivneigung in den ersten Tagen nach Kardioversion finden ihre Erklärung im atrialen „Remodeling“. In seltenen wenigen Fällen scheint zumindest der Induktion von paroxysmalem Vorhofflimmern frequente fokale Aktivität im Bereich der Pulmonalvenen-Einmündungen zugrunde zu liegen. Dies ist insofern bemerkenswert, als hier einerseits die fokale Aktivität über atriales Remodeling die Voraussetzungen für Reentry schaffen könnte, andererseits mittels Hochfrequenz-Katheterablation der „Foci“ eine ursächliche Therapie des Vorhofflimmerns möglich scheint.