Guinea Pig Purkinje Fibers Research Articles

Direct effects of arsenic trioxide on action potentials in isolated cardiac tissues: Importance of the choice of species, type of cardiac tissue and perfusion time

IntroductionThe aim of the present study was to evaluate direct/acute effects of arsenic trioxide on action potentials (APs) in isolated cardiac tissues, and to investigate if the choice of species and tissue and the duration of the perfusion play a role in arsenic-induced acute/direct prolongation of AP/QT. Methods and resultsDirect electrophysiological effects of arsenic trioxide were measured in cardiac tissues isolated from four different species using micro-electrode recording. Arsenic (after 30 to 95min perfusion at 10μM) significantly prolonged APD90, increased triangulation of the AP and elicited early afterdepolarizations (EADs) only in isolated guinea-pig and dog Purkinje fibers but not in rabbit and porcine (minipig) Purkinje fibers. Arsenic induced a prolongation of the APD90 and increases in triangulation and the occurrence of EADs was not observed in papillary muscles of guinea-pigs and rabbits. Arsenic at 4 increasing concentrations from 0.1μM to 10μM at the standard perfusion-time of 15min per concentration, and after a continuous 90-min perfusion at 1μM and 1Hz did not induce these direct effects on APD90, triangulation and EADs in isolated guinea-pig Purkinje fibers, but it at 1 µM elicited EADs in 2 out of 7 preparations after 90min at 0.2Hz. DiscussionThe present study demonstrates that the choice of species and cardiac tissue as well as perfusion-time play important roles in arsenic-induced direct/acute effects on APD90 and induction of EADs in vitro.

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 2.7 mM none of these drugs could elicit EADs. For that reason the K + concentration was lowered to 1.35 mM and EADs were evoked by imipramine (2 and 5 μM). Amitriptyline (2 and 5 μM) and doxepin (2 μM) did not induce EADs. Only a concentration of 5 μM doxepin elicited EADs. Among the neuroleptic drugs, chlorpromazine at a concentration of 2 and 5 μM was responsible for the occurrence of EADs as well as thioridazine in the same concentrations. When trifluoperazine (2 and 5 μM) was applied no EADs could be observed. Tetrodotoxin (0.2 μM l −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.

Effects of pinacidil, verapamil, and heart rate on afterdepolarizations in the guinea-pig heart in vivo.

Recently, ionic current simulation in the Luo-Rudy model has elucidated putative mechanisms of afterdepolarizations under various experimental conditions. The present study was aimed at gaining insight into the differential mechanism of different types of afterdepolarizations in the guinea-pig heart in vivo. The effects of pharmacological and heart rate perturbations on early (EADs) and delayed (DADs) afterdepolarizations, induced by either digoxin, CsCl, or BayK 8644 were studied, using mid-myocardial left ventricular monophasic action potential (MAP) recordings. Digoxin insignificantly shortened sinus cycle length (SCL) and CsCl and BayK 8644 differentially prolonged SCL and MAP duration. Digoxin induced phase 3-EADs and DADs and CsCl or BayK 8644 induced phase 2- and phase 3-EADs. Pinacidil shortened MAP duration, suppressed almost all the phase 2-EADs and some of the phase 3-EADs, but not the DADs. In a few cases, DADs were manifested following the abolishment of phase 2-EADs by pinacidil, but this phenomenon did not occur in the presence of hexamethonium. Verapamil prolonged SCL, did not significantly affect phase 2-EADs, but suppressed almost all of the DADs, including those which appeared after pinacidil, and all but one of the phase 3-EADs. The effects of pinacidil and verapamil were independent of the mode of afterdepolarization induction. A pacing-induced heart rate increase, which shortened MAP duration, and vagal stimulation, which prolonged MAP duration, attenuated and enhanced phase 2-EADs, respectively. The amplitude of phase 3-EADs was inversely related to the heart rate. These data, taken together, are consistent with those obtained previously by others in a computer model and recent observations on CsCl-induced EADs in the guinea-pig Purkinje fibers in vitro which have indicated that the mechanism of phase 2-EADs is different from that of DADs and that late phase 3-EADs generated under conditions of Ca2+ overload and DADs share similar properties.

Role of calcium loading in early afterdepolarizations generated by Cs+ in canine and guinea pig Purkinje fibers.

Our previous observations indicate that the Na+:Ca2+ exchange current (INa:Ca) plays an important role in early afterdepolarizations occurring at more negative Vm (L-EAD). The purpose of these studies was to examine the role of Ca(2+)-loading, which stimulates INa:Ca, in generation of L-EAD. Purkinje strands and preparations of ventricular myocardium from dogs and guinea pigs were superfused with oxygenated physiologic buffer solutions at 37 degrees C. To induce EADs, [K+]o was reduced to 2.0 to 3.0 mM and [Cs+]o (3.6 to 4.0 mM) was added at slow rates of < or = 0.3 Hz. Isometric contraction in canine Purkinje strands and guinea pig papillary muscles doubled in 1-hour exposure to Cs+ and low [K+]o at slow rates and the uptake of 45Ca2+ was approximately doubled after 30 minutes. Forty-three percent of Purkinje fibers developed L-EAD after a latent period of 17 to 123 minutes of exposure. Ouabain (0.2 microM) suppressed L-EAD within 10 minutes reversibly. Ca(2+)-loading (low [Na+]o or high [Ca2+]o) for 5 to 10 minutes before exposure to Cs+, low [K+]o, and slow rates resulted in rapid development of L-EAD in all preparations during subsequent exposure. In Ca(2+)-loaded preparations, delayed afterdepolarizations (DADs) as well as L-EADs developed. Reduction of K+ currents with Cs+, low [K+]o, and slow rates induced L-EAD in a fraction of Purkinje fibers after a latent period during which Ca(2+)-loading of the sarcoplasmic reticulum occurred, while fibers preloaded with Ca2+ developed L-EAD rapidly and uniformly. These findings indicate that Ca(2+)-loading is a critical condition for the development of L-EAD. Early suppression of L-EAD by ouabian suggests a dependence of L-EAD on low [Na+]i. These findings implicate INa:Ca in the generation of L-EAD.

Tetrodotoxin-sensitive component in action potential plateau of guinea pig Purkinje fibers: Comparison with the papillary muscle

1. The effects of tetrodotoxin on action potentials of isolated guinea pig Purkinje fibers were examined and compared the findings with those obtained in the ventricular papillary muscle, by use of conventional microelectrode techniques. 2. Tetrodotoxin (5 × 10 −7–10 −5 M) decreased the amplitude, overshoot, and maximum upstroke velocity of action potentials of the Purkinje fibers, and shortened the duration of action potential at all levels of repolarization concentration- and stimulus cycle length-dependently. 3. The longer the stimulus cycle length, the greater the shortening by the drug of the action potential duration. 4. In particular, the plateau potential of the Purkinje fibers exposed to tetrodotoxin was remarkably depressed, and which occurred even in case of blockade of K + conductance, using tetraethylammonium. 5. On the other hand, a high concentration (10 −5 M) of tetrodotoxin did not significantly affect the papillary muscle action potentials. 6. These findings suggest that there is a tetrodotoxin-sensitive component of Na + current in plateau voltage range of the Purkinje fibers, but little in the papillary muscle, and that the component plays an important role to maintain the plateau of Purkinje fibers action potential.

Comparative effects of tetrodotoxin, lidocaine, and amiodarone on [formula omitted] in guinea pig cardiac purkinje and papillary muscle fibers

1. 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. 2. Special attention was paid with regard to their effects on the maximum upstroke velocity ( V · max ) of action potentials. 3. 3. In Purkinje fibers, tetrodotoxin was the most inhibitory for V · max , then in descending order, amiodarone, and lidocaine. 4. 4. That is, half maximal inhibition (IC 50) of V · max by tetrodotoxin, lidocaine and amiodarone was approximately 1.7 × 10 −5M, 1.5 × 10 −4M and 2.8 × 10 −4M, respectively. 5. 5. As for papillary muscle, higher concentrations of the three drugs were needed to get similar potency. 6. 6. The relationships between the depression in the V · max and action potential duration are discussed in conjunction with their antiarrhythmic activities.

Electrophysiological effects of lidocaine on isolated guinea pig Purkinje fibers: Comparison with its effects on papillary muscle

1. 1. The effects of lidocaine on action potentials of isolated guinea pig Purkinje fibers were examined and compared with those on the papillary muscle, by use of conventional microelectrode techniques. 2. 2. Lidocaine 2.1 × 10 −4 M significantly depressed the resting membrane potential, amplitude, overshoot, maximum upstroke velocity, and duration of action potential of the Purkinje fibers stimulated at cycle length (CL) 1 sec, while the lower concentration (2.1 × 10 −5 M) produced no significant effect. 3. 3. Various parameters of action potentials from the papillary muscle at CL = 1 sec were also depressed by lidocaine 2.1 × 10 −4 M, but the extent of the depression was less marked than in Purkinje fibers. 4. 4. When the Purkinje fibers were stimulated at various CLs (0.3–10 sec), the shortening of action potential duration induced by lidocaine was more evident at slower stimulus rate.

Triggered activity induced by combined mild hypoxia and acidosis in guinea-pig Purkinje fibers

The effects of prolonged exposure to combined mild hypoxia (Po2 230 +/- 20 mmHg) and acidosis (pH: 6.8 +/- 0.05) were studied in guinea-pig left ventricular myocardium superfused in vitro. Only Purkinje fibers were impaled by microelectrodes. Triggered activity developed in depolarized Purkinje fibers after 48 +/- 9 min of exposure to hypoxic and acid conditions and was initiated either by short periods of rapid electrical driving or by the background slow Purkinje automaticity. Triggered activity occurred when a delayed afterdepolarization attained its threshold potential and terminated after a subthreshold afterdepolarization. Interaction between triggered activity and slow background automaticity was observed until 90 to 180 min of exposure to hypoxic and acid conditions. These effects were reversed by replacement in standard conditions (Po2 510 +/- 20 mmHg; pH 7.35 +/- 0.05). Norepinephrine (1 X 10(-6)M) significantly accelerated the rate of discharge of triggered foci and led to a stable sustained triggered activity. Increasing extracellular Ca2+ concentration aggravated the effects of combined mild hypoxia and acidosis and led to the occurrence of early afterdepolarizations initiating triggered activity. In addition abnormal automaticity developed in quiescent fibers without any triggering action potential. Lidocaine and verapamil suppressed the triggered activity following a subthreshold afterdepolarization. Their effects were reversed on wash-out. It is concluded that prolonged exposure to combined mild hypoxia and acidosis induces triggered activity by a basic mechanism common to other situations leading to a calcium overload and showing such behaviour.

Electropharmacological Effects of a New Dihydropyridine Analog on Isolated Guinea Pig Papillary Muscles and Purkinje Fibers

The effects of a new dihydropyridine, FR-34235, were compared with those of the dihydropyridine, nifedipine, and verapamil on the normal fast action potentials (APs), slow APs, and contractions of guinea pig papillary muscles and Purkinje fibers. FR-34235 (10(-6) M) blocked the contractions of papillary muscles superfused with normal Tyrode solution within 10-12 min. Maximal upstroke velocity (+Vmax) and overshoot of the fast APs were not affected, whereas the AP durations at 50 and 90% repolarization (APD50 and APD90) were shortened. The effects of FR-34235 on the fast APs and contractions were reversed within 10 min on washout. To determine the effects of the calcium antagonists on slow APs, the fast Na+ channels were inactivated by partial depolarization (to approximately -45 mV) by elevated [K]0, and isoproterenol (10(-6) M) or histamine (10(-5) M) was used to induce slow APs on stimulation. Nifedipine (10(-7) M) and verapamil (2 X 10(-6) M) completely blocked the slow APs. FR-34235 depressed (3 X 10(-8) M) and blocked (10(-7) M) the slow APs in a frequency-dependent manner. The effects were reversed by elevated [Ca]0 or washout of the drug. The contractions accompanying the slow APs were depressed and blocked in parallel with the depression of +Vmax. In guinea pig Purkinje fibers, FR-34235 had no significant effect on the fast AP parameters but produced a marked depression of automaticity. FR-34235 also blocked the slow APs of the Purkinje fibers in a frequency-dependent manner; all drug effects were reversed within 10 min on washout.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient increase in the slow inward current following acetylcholine removal in catecholamine-treated guinea-pig Purkinje fibers.

In voltage-clamped guinea-pig Purkinje fibers, removal of acetylcholine (ACh) in the presence of isoproterenol produced a rebound increase in the slow inward current. This effect of ACh was abolished by atropine. These results are consistent with the hypothesis that cessation of the stimulation of myocardial muscarinic receptors transiently leads to enhanced activity of the beta-receptor/slow channel system in this tissue.

Comparison of the effects of several calcium antagonistic drugs on the electrical activity of guinea pig purkinje fibers

The effects of several slow channel blockers were compared on the normal fast action potentials (APs) and the slow APs of guinea pig Purkinje fibers. In spontaneously-firing Purkinje fibers perfused with normal Tyrode solution, mesudipine (analog of nifedipine) at 10 −7 and 10 −6 M, had no significant effect on the fast AP parameters (at 5 × 10 −6 M, excitability was abolished due to about −45min mV). Washout of the drug rapidly (within 3 min) repolarized the preparations and restored automatic activity. In order to determine the effect of the calcium antagonists on the slow APs, the fast Na + channels were inactivated by partial depolarization (to about −45 mV) by perfusing with 20 mM K +-Tyrode solution and isoproterenol (10 −6 M), or histamine (10 −5 M) were used to induce slow APs upon stimulation. Verapamil (10 −6 M) and nifedipine (10 −7 M) completely blocked the slow APs. Mesudipine at 10 −8 and 4 × 10 −8 M depressed the V̇ max, amplitude and duration of the slow APs and abolished excitability within 11 min at 10 −7 M. At 10 −7 M, mesudipine blocked the slow APs earlier (within 3 min) at higher stimulation frequency (from the usual drive rate of 0.5–1.5 Hz). Lowering the stimulation frequency to 0.1 Hz restored the slow APs; however, the mesudipine block was independent of the stimulating frequency after 20 min. The dose/response curve for the mesudipine effect was shifted to the right in high Ca 2+ concentration (5.4 mM). Washout of mesudipine and nifedipine restored the slow APs within 15 min, whereas verapamil required about 1 h. The results indicate that mesudipine has potent Ca 2+ slow channel blocking properties in Purkinje fibers. Mesudipine and nifedifine are about ten times more potent than verapamil. This effect is not mediated by beta-adrenergic receptor antagonism, because the histamine-induced slow APs were also blocked.

Chemicals as tools in the study of excitable membranes.

Chemicals as tools in the study of excitable membranes.