Use-dependent effects of amiodarone on Vmax in cardiac Purkinje and ventricular muscle fibers

Superfusion with 5 micrograms/ml amiodarone for 3-4 h induced use-dependent Vmax block in dog Purkinje and guinea pig ventricular muscle fibers. The recovery from block was exponential with tau of 289 +/- 30 ms in Purkinje (n = 7) and 282 +/- 47 ms in muscle (n = 6) fibers. The onset of frequency-dependent Vmax block was rapid, i.e. reached steady state after 4.2 +/- 0.5 beats (n = 5). The combination of rapid interaction with sodium channel and the reported action potential lengthening make amiodarone unique among Class I antiarrhythmic drugs.     

Electrophysiological effects of bethanidine sulfate on canine cardiac Purkinje fibers and ventricular muscle cells

Bethanidine sulfate is a congener of bretylium tosylate, which has been reported to have antiarrhythmic and antifibrillatory effects. We studied the effects of bethanidine on transmembrane potentials recorded from canine Purkinje fibers and ventricular muscle cells, using standard microelectrode techniques. Normal Purkinje fibers were exposed to bethanidine (10-80 mg/L) for 30-40 min. Bethanidine produced dose-dependent decreases in the maximal rate of depolarization (MRD) and over-shoot of phase 0, but did not affect maximum diastolic potential (MDP). Action potential plateau duration (APD, to -60 mV) was decreased by bethanidine at all cycle lengths of stimulation between 1,000 and 300 ms. Bethanidine depressed the membrane responsiveness curve, and its effects on MRD showed marked use dependence. In ventricular muscle cells, bethanidine 20 mg/L decreased MRD but did not affect MDP or APD. The rate of normal automaticity in Purkinje fibers with MDPs greater than -85 mV was increased to 21.5 +/- 5.6 beats/min after exposure to bethanidine (10 mg/L for 30 min) from control values of 10.2 +/- 5.3 beats/min. Abnormal automaticity (MDPs = -40 to -60 mV) was induced in Purkinje fibers by superfusion with Tyrode solution containing 0.25 mM BaCl2; this activity also was accelerated after exposure to bethanidine 10 mg/L. The effects of bethanidine on automaticity are similar to those of bretylium, and may be caused by release of endogenous catecholamines. In contrast, the effects of bethanidine on action potentials of normal (driven) Purkinje fibers are markedly different from those of bretylium, and resemble those of lidocaine after 30-60 min of exposure.     

Electrophysiological effects of tocainide on canine subendocardial Purkinje fibers surviving infarction

The effects of 10 and 20 mg/l of tocainide on transmembrane action potential characteristics were examined in Purkinje fibers surviving infarction. Infarcted tissue was obtained from canine hearts 24 h after coronary artery ligation. The preparation was stimulated at basic cycle lengths (BCL) of 1000-300 ms. Tocainide reduced the overshoot and amplitude of Purkinje fibers surviving infarction. The maximum upstroke velocity (Vmax) was decreased by tocainide in a dose dependent manner. This effect was more prominent at the shorter BCL. Statistical analysis revealed a significant interaction of the BCL with the drug effect on overshoot, amplitude, Vmax and action potential durations (APD50% and APD90%). Tocainide reduced the effective refractory period (ERP) at the BCL of 1000 ms, but had no significant effect at the BCL of 300 ms. Membrane responsiveness and steady state characteristics of Vmax were shifted significantly to more negative membrane potentials by tocainide. Investigation of the recovery kinetics of Vmax in the presence of tocainide showed an exponential recovery of Vmax with a time constant of 514 ms. These results support the finding that the effect of tocainide on Vmax and conductions is enhanced at faster rates of stimulation. Thus tocainide may be able to depress conduction to produce bidirectional block in the termination of ventricular tachycardia caused by reentry, while having minimal effect on conduction at normal heart rates.     

Differential electrophysiologic effects of mexiletine on normal and hypoxic canine Purkinje fibers

We studied the effects of increasing concentrations of mexiletine on the active membrane characteristics of canine Purkinje fibers under normal and hypoxic conditions. Under normal conditions, there was a progressive shortening of action potential duration and a small decrease in maximum diastolic potential and overshoot potential. Effective refractory period was shortened by low concentrations but lengthened by higher concentrations. Only at the higher concentrations was Vmax decreased. When the tissue was rendered hypoxic, the shortening effect on action potential duration was attenuated. Low concentrations of mexiletine had a more pronounced effect on maximum diastolic potential and overshoot potential. The reversal of the effect on refractory period occurred at lower concentrations. The effect on Vmax was exaggerated. Hypoxia sensitizes Purkinje fibers to some of the electrophysiologic effects of mexiletine, but other effects are, in fact, attenuated in the presence of hypoxia.     

A comparison of the electrophysiologic effects of acute and chronic amiodarone administration on canine Purkinje fibers

The electrophysiologic effects of acutely and chronically administered amiodarone on canine Purkinje fibers were assessed using microelectrode techniques to record intracellular action potentials. Chronically treated dogs received amiodarone for 3 weeks (serum levels, 0.91 +/- 0.09 microgram/ml or 1.42 X 10(-6) M). Acute studies were performed using fibers from untreated dogs superfused for 1 h with 5 X 10(-5) M amiodarone (32 micrograms/ml) in Tyrode's solution (KCl = 4 mM). Acute superfusion shortened the action potential duration to 50 and 90% repolarization by 41 and 8%, respectively (p less than 0.01), and decreased Vmax of phase 0 from 418 +/- 20 to 309 +/- 23 V/s (p less than 0.01) (paced cycle length of 500 ms). Prominent use-dependent depression of Vmax was noted. Acute exposure of fibers from untreated dogs to blood from dogs chronically treated with amiodarone using the blood cross-perfusion technique decreased the action potential duration to 50% repolarization and Vmax, similar to acute exposure in Tyrode's solution. Blood cross-perfusion was used to study fibers from treated dogs superfused with blood from another amiodarone-treated dog. Chronic amiodarone prolonged the action potential duration to 90% repolarization by 13% (p less than 0.02) and did not change Vmax when compared to control studies using fibers obtained from untreated dogs superfused with blood from untreated dogs. Thus, the effects of acutely superfused amiodarone on action potentials of canine Purkinje fibers differ from the effects of chronically administered amiodarone.     

Acute effects of amiodarone on action potentials of isolated canine Purkinje fibers: comparison with tetrodotoxin effects

1. We compared the acute electrophysiological effects of amiodarone (AM) and tetrodotoxin (TTX) on action potentials of isolated canine Purkinje fibers. All two drugs suppressed action potential amplitude, overshoot, and maximum rate of upstroke of action potential, and shortened action potential duration (APD). 2. However, higher concentrations (4.4 x 10(-4) M) of AM showed differential effects on APD, compared with TTX. 3. These differences between effects of AM and TTX suggest that in case of high concentration of AM the APD-shortening by AM might be masked by AM's other action(s), while in low concentration the APD-shortening effect of AM, probably due to decrease in Na+ and slow Ca2+ current, was predominant.     

Differential effects of quinidine on transmembrane action potentials of normal and infarcted canine Purkinje fibers.

Fourteen days after proximal ligation of the left anterior descending coronary artery (LAD) of mongrel dogs, the effects of quinidine on action potentials of normal and infarcted Purkinje fibers were evaluated. The concentration-dependent (10(-7)-3 x 10(-5) M) and frequency-dependent (1 and 3 Hz) actions of quinidine were evaluated by the following parameters: maximum upstroke velocity (Vmax), action potential duration at 50 and 95% repolarization (APD50, APD95), effective refractory period (ERP), resting membrane potential (RMP), and action potential amplitude (APA). Quinidine reduced Vmax in normal and abnormal Purkinje fibers in a concentration- and frequency-dependent manner; these effects were more pronounced in infarcted tissue. The APD50 was shortened significantly at 1 Hz in noninfarcted Purkinje fibers, whereas in infarcted Purkinje fibers quinidine had no effect on APD50. The APD95 was not significantly altered by quinidine in normal Purkinje fibers; in infarcted areas APD95 was significantly prolonged at 1 and 3 Hz. The effective refractory period (ERP) was prolonged in normal and infarcted Purkinje fibers, these effects were more marked in ischemically damaged fibers. No effects were observed on resting membrane potential (RMP). APA was reduced significantly after quinidine at 1 and 3 Hz; there was no difference between normal and infarcted tissue. These data indicate a differential effect of quinidine in normal and infarcted Purkinje fibers which may be an important mechanism of action of quinidine in infarcted tissue.     

Electrophysiological effects of labetalol on canine atrial, cardiac Purkinje fibres and ventricular muscle.

1. Using conventional microelectrode techniques for the intracellular recordings of the membrane potential, the effects of labetalol were studied on cardiac Purkinje, atrial and ventricular muscle fibres of the dog. 2. Labetalol (1-10 microM) reduced, in a concentration-dependent manner, the action potential amplitude (APA) and the maximum rate of rise of the action potential (Vmax) in Purkinje fibres. 3. The action potential duration (APD) was decreased in Purkinje fibres but significantly increased in ventricular fibres after small concentrations of labetalol (1-3 microM). The atrial fibres were not very sensitive to labetalol. 4. Depolarization of the cardiac Purkinje fibres by increasing the external potassium concentration (8-12 mM), potentiated the labetalol effects on APA and Vmax but blocked its effects on the APD. 5. The effects of labetalol on Vmax of Purkinje fibres were dependent on the frequency of stimulation. 6. The ratio of the effective refractory period to the APD was increased both in normally polarized and depolarized Purkinje fibres after treatment with labetalol (10 microM). 7. Labetalol (10 microM) shifted the membrane responsiveness curve of Purkinje fibres by about 10 mV in the hyperpolarizing direction. 8. The slow response obtained in K-depolarized, Ba-treated Purkinje fibres was not significantly affected by labetalol (10-100 microM). 9. It is suggested that labetalol can exert Class I and Class III antiarrhythmic actions in cardiac muscle of the dog in vitro.     

Comparative electrophysiological and mechanical actions of ATP-sensitive potassium channel openers in canine Purkinje fibers

• 1.1. Electrophysiological and mechanical effects of ATP-dependent potassium channel openers on canine Purkinje fibers were examined.
• 2.2. Cromakalim and pinacidil (0.3–10 μM) and nicorandil (0.3–1 mM) shortened the action potential duration (APD), and reduced the contractile force, in a concentration-dependent manner. Other action potential parameters were unaffected. The effects were reversible.
• 3.3. Effects of the openers were antagonized by tetraethylammonium (a non-specific potassium channel blocker) and more potently by glibenclamide (a blocker of ATP-sensitive K⁺ channel).
• 4.4. The APD shortening and the negative inotropic effect induced by a switch of stimulation frequency (from 0.5 to 3 Hz) were potentiated by the openers. At high Ca²⁺ (5.4 and 10.8 mM), the effects of the openers on the APD and contractile force were unaffected.
• 5.5. In the spontaneously beating preparations, the openers hyperpolarized the maximum diastolic potential, and an arrest occurred.
• 6.6. Under the calcium overload condition, the K⁺ openers abolished a delayed afterdepolarization, and enhanced the depressed post-rest potentiation.
• 7.7. These results suggest that the K⁺ channel openers increase the K⁺ conductance and might decrease the cellular Ca²⁺ level in calcium overloading cells, and that nicorandil might also act directly ATP-sensitive K⁺ channels.

     

Electrophysiological effects of Gallanilide on cardiac ventricular and Purkinje fibers

The effects of gallanilide on action potentials were studied in swine ventricular muscles, Purkinje fibers, and guinea pig papillary muscles by conventional microelectrode techniques. Gallanilide (1–10 μM) prolonged the action potential duration (APD) and effective refractory period (ERP) in ventricular muscles and Purkinje fibers. The ratio of ERP/APD₉₀ increased. The slopes of phase 1 and phase 3 of action potential were flattened. Gallanilide (10 μM) decreased the maximal rising rate and amplitude of the action potential. The depressant effect on the maximal rising rate was frequency-dependent. These results suggest that gallanilide possesses both class I and III antiarrhythmic actions, which may be the electrophysiological basis of its antiarrhythmic effect. Drug Dev. Res. 40:94–98, 1997. © 1997 Wiley-Liss, Inc.     

Effects of dauricine and lidocaine alone or combined on electrophysiological properties of canine Purkinje fibers

Dauricine (Dau) 1 to 30 mumol/L produced the concentration-dependent depressions in the APA, Vmax, MDP, and prolongations of APD50 and APD90 as well as ERP of the isolated canine cardiac Purkinje fibers (PF). The automaticity and excitation were significantly reduced at concentration of 30 mumol/L. The effects of Dau on all action potential parameters of PF were observed at all stimulation frequencies (60, 75, 100, 150 beats/min). Lidocaine (Lid) markedly shortened APD50 of PF at concentration of 30 mumol/L and also shortened APD90, ERP and significantly depressed APA, Vmax at 100 mumol/L. When perfused in combination with Dau, Lid appreciably shortened APD50 and APD90, and lightly abbreviated ERP prolonged by Dau.     

Electrophysiological effects of cyclic GMP on canine cardiac Purkinje fibers

The effect of cyclic 3'5'-guanosine monophosphate (8-bromo-cGMP) on action potential characteristics was investigated. Standard microelectrode techniques were used to study the effects of 8-bromo-cGMP on canine cardiac Purkinje fibers in vitro. Canine Purkinje fiber tissue preparations were exposed to increasing concentrations of 8-bromo-cGMP (10(-6), 10(-5), 10(-4) M). The action potential duration at 50% (APD50) and 90% (APD90) repolarization, resting membrane potential (RMP), action potential amplitude (APA), rate of rise of phase 0 (Vmax), spontaneous rate (SR), escape time (ET), and effective refractory period (ERP) did not change at these concentrations of 8-bromo-cGMP. The effect of 8-bromo-cGMP on isoproterenol (10(-7) M) treated Purkinje fibers was tested. Predictably, isoproterenol shortened APD and ERP and increased SR. APD or ERP shortening was not affected by 8-bromo-cGMP, but the increase in SR produced by isoproterenol was prevented. Eleven of sixteen Purkinje fiber preparations treated with isoproterenol alone became spontaneously arrhythmic, whereas none of six treated with 8-bromo-cGMP and isoproterenol became arrhythmic (p less than 0.05). Slow-response action potentials elicited by potassium depolarization and catecholamines were abbreviated and eventually abolished by 8-bromo-cGMP. In conclusion, 8-bromo-cGMP has no effect on action potential characteristics in normally polarized canine Purkinje fibers but depressed slow response action potentials. The effects of isoproterenol on SR are antagonized and the production of arrhythmias in this model are prevented by 8-bromo-cGMP.     

Electrophysiological effects of desmethylimipramine and desmethyldoxepin on canine cardiac Purkinje fibers

Two studies were conducted to assess the toxicity of rosaramicin (a macrolide antibiotic) when given intravenously for 30 consecutive days to beagle dogs with and without a tapetum lucidum (a light reflecting structure within the choroid of the eye). In the initial study, groups of three tapetal dogs/sex were given 20, 40, or 80 mg of rosaramicin/kg, twice daily. Ophthalmoscopic examination during Week 4 revealed dose-related, bilateral ocular changes characterized by a brown-tan discoloration and general pallor or loss of reflectivity of the normally blue-purple or yellow-green, highly reflective tapetum lucidum. These findings were restricted to the tapetal fundus; recovery occurred between Weeks 4 and 10 of the postdose period. To further investigate these changes, a second study was conducted in which groups of three tapetal dogs were given rosaramicin or erythromycin lactobionate (comparative macrolide antibiotic) at 80 mg/kg, twice daily. A third group of atapetal dogs was given 80 mg of rosaramicin/kg, twice daily. A similar change was observed in tapetal dogs given 80 mg of rosaramicin/kg, twice daily, in the follow-up study, but not in the other two groups. No other compound-related changes were observed in either study. The ocular changes observed in dogs given rosaramicin were reversible and structure-specific, occurring only in animals possessing a tapetum lucidum.     

Analysis of the electrophysiological effects of ambasilide, a new antiarrhythmic agent, in canine isolated ventricular muscle and purkinje fibers

The aim of the study was to determine the in vitro rate-dependent cellular electrophysiological effects of ambasilide (10 and 20 microM/l), a new investigational antiarrhythmic agent, in canine isolated ventricular muscle and Purkinje fibers by applying the standard microelectrode technique. At the cycle length (CL) of 1000 ms, ambasilide significantly prolonged the action potential duration measured at 90% repolarization (APD(90)) in both ventricular muscle and Purkinje fibers. Ambasilide (10 microM/l) produced a more marked prolongation of APD(90) at lower stimulation frequencies in Purkinje fibers (at CL of 2000 ms = 56.0 +/- 16.1%, n = 6, versus CL of 400 ms = 15.1 +/- 3.7%, n = 6; p < 0.05), but, in 20 microM/l, this effect was considerably diminished (15.2 +/- 3.6%, n = 6, versus 7.3 +/- 5.1%, n = 6, p < 0.05). In ventricular muscle, however, both concentrations of the drug induced an almost frequency-independent lengthening of APD(90) in response to a slowing of the stimulation rate (in 20 microM/l at CL of 5000 ms = 19.0 +/- 1.5%, n = 9, versus CL of 400 ms = 16.9 +/- 1.4%, n = 9). Ambasilide induced a marked rate-dependent depression of the maximal rate of rise of the action potential upstroke (V(max)) (in 20 microM/l at CL of 300 ms = -45.1 +/- 3.9%, n = 6, versus CL of 5000 ms = -8.5 +/- 3.9%, n = 6, p < 0. 05, in ventricular muscle) and the corresponding recovery of V(max) time constant was tau = 1082.5 +/- 205.1 ms (n = 6). These data suggest that ambasilide, in addition to its Class III antiarrhythmic action, which is presumably due to its inhibitory effect on the delayed rectifier potassium current, possesses I/B type antiarrhythmic properties as a result of the inhibition of the fast sodium channels at high frequency rate with relatively fast kinetics. This latter effect may play an important role in its known less-pronounced proarrhythmic ("torsadogenic") potential.     

Comparative effects of tetrodotoxin, lidocaine, and amiodarone on Vmax in guinea pig cardiac Purkinje and papillary muscle fibers

1. Electrophysiological effects of three drugs (tetrodotoxin, lidocaine, and amiodarone), which are known to depress Na channels in cardiac tissues, were studied on isolated guinea pig Purkinje fibers, by means of microelectrode techniques, and compared the findings with their effects on the ventricular papillary muscle. 2. Special attention was paid with regard to their effects on the maximum upstroke velocity (Vmax) of action potentials. 3. In Purkinje fibers, tetrodotoxin was the most inhibitory for Vmax, then in descending order, amiodarone, and lidocaine. 4. That is, half maximal inhibition (IC50) of Vmax by tetrodotoxin, lidocaine and amiodarone was approximately 1.7 x 10(-5) M, 1.5 x 10(-4) M and 2.8 x 10(-4) M, respectively. 5. As for papillary muscle, higher concentrations of the three drugs were needed to get similar potency. 6. The relationships between the depression in the Vmax and action potential duration are discussed in conjunction with their antiarrhythmic activities.     

Electropharmacological effects of berberine on canine cardiac Purkinje fibres and ventricular muscle and atrial muscle of the rabbit.

1. Conventional microelectrode techniques were used for intracellular recordings of the transmembrane electrical potentials, the effects of berberine were studied on canine cardiac Purkinje and ventricular muscle fibres and on rabbit atrial fibres. 2. Berberine (3-30 microM) increased in a concentration-dependent manner, the action potential duration (APD) in canine Purkinje and ventricular muscle without affecting other parameters of the action potential. 3. The berberine-induced enlargement of the APD showed reverse use-dependence, so that the effect was greater at lower rates of stimulation. 4. Preparations perfused with berberine (30 microM) and driven at rates below 0.5 Hz exhibited early after depolarizations which persisted 3-4 h after washing. 5. The early afterdepolarizations were reversibly abolished by perfusion with lignocaine (3 microM) or by the increase in the rate of stimulation. 6. The effective refractory period (ERP) of Purkinje fibres was greatly increased by berberine (30 microM); however, the ratio ERP/APD was not significantly affected. 7. Berberine (10-100 microM) decreased in a concentration-dependent manner the spontaneous frequency of rabbit sinoatrial cells. The decrease in frequency was accompanied by a depression of the phase 4 depolarization, without significant changes in other parameters of the nodal action potential. 8. Atropine (2.5 microM) did not affect the bradycardic effect of berberine. On the other hand, berberine (30 microM) did not alter the chronotropic effect of isoprenaline. 9. Berberine (30 microM) also increased the duration of slow responses in K-depolarized rabbit atrial muscle fibres, other parameters being unaffected. 10. It is suggested that berberine exerts Class III antiarrhythmic and proarrhythmic actions in cardiac muscle of the dog in vitro.     

Comparative electrophysiological effects of the antidepressants fluvoxamine and amitriptyline in the canine heart after myocardial infarction

We studied the effects of fluvoxamine and amitriptyline on epicardial activation delay of premature excitations, the effective refractory period, and the incidence of ventricular arrhythmias by programmed electrical ventricular stimulation in the canine heart after myocardial infarction. Additionally, we investigated whether the inhibition of norepinephrine reuptake by amitriptyline contributes to epicardial activation delay or arrhythmias by combination with propranolol pretreatment. Amitriptyline, at a dose of 3 mg/kg, significantly prolonged epicardial activation delay of premature excitations in the infarcted zone in a frequency-dependent manner (n = 10). Amitriptyline also prolonged epicardial activation delay of premature excitations in the normal zone (n = 10). The effective refractory period in the infarcted zone was significantly prolonged by amitriptyline at a dose of 3 mg/kg (n = 8). Amitriptyline increased the incidence of ventricular arrhythmias induced by programmed electrical ventricular stimulation (n = 8). Propranolol did not affect the epicardial activation delay caused by amitriptyline or the incidence of ventricular arrhythmias induced by programmed electrical ventricular stimulation (n = 6). Fluvoxamine, on the other hand, had no significant effect on epicardial activation delay of premature excitations (n = 10) or the effective refractory period (n = 8) in both the infarcted and normal zones. Fluvoxamine did not increase the incidence of ventricular arrhythmias induced by programmed electrical ventricular stimulation (n = 8).From the present results, fluvoxamine seems to have lower cardiac toxicity than amitriptyline. Moreover, the electrophysiological effects of amitriptyline in this model may be due to a direct cardiac depressive action, but not to the inhibition of norepinephrine reuptake.