Effect of cardiotonic polypeptide anthopleurin-A on canine Purkinje and ventricular muscle fibers

Choice of cardiac tissue plays an important role in the evaluation of drug-induced prolongation of the QT interval in vitro in rabbit

INTRODUCTION

Regulatory guidelines (CPMP/986/96, ICHS7B) recommend the use of isolated cardiac tissues, including Purkinje fibers, papillary muscles and ventricular trabeculae, for detecting potential drug-induced long QT. However, the differential sensitivity of these tissues in experimental drug-induced long QT is relatively unknown. We investigated the electrophysiological characteristics of these tissue types in vitro together with their different responses to drugs that are known to induce prolongation of the QT interval in man.

METHODS

Electrophysiological parameters were measured in vitro using a micro-electrode technique. The isolated rabbit Purkinje fibers, papillary muscles or ventricular trabeculae were superperfused with Tyrode's solution and stimulated according to different stimulation protocols. The effects of dofetilide (1 x 10(-8) M), sertindole (1 x 10(-6) M), erythromycin (3 x 10(-4) M) and sparfloxacin (1 x 10(-4) M) were evaluated relative to solvent (n=8 to 12 for each group).

RESULTS

In isolated Purkinje fibers, action potential duration at 90% repolarization (APD(90) at 1 Hz) was markedly prolonged by 55% (erythromycin), 103% (dofetilide), 118% (sertindole) and 88% (sparfloxacin) from baseline. The prolongation of APD(90) caused by these 4 compounds was associated with a 28% to 78% incidence of early afterdepolarizations (EADs) at 0.2 Hz only in the Purkinje fiber. In contrast, APD(90) was altered by erythromycin, dofetilide, sertindole and sparfloxacin only by +15%, +6%, -7% or +15%, respectively, in isolated papillary muscles, and by 33%, +28%, +4% or +16%, respectively, in ventricular trabeculae. EADs were not induced by these four compounds in papillary muscles or in trabeculae. Reducing the stimulation rate to 0.2 Hz resulted in a 33% prolongation of APD(90) in Purkinje fibers, while APD(90) was shortened by 10% in papillary muscles and by 20% in ventricular trabeculae.

CONCLUSION

The present study demonstrates that the differential sensitivity of tissue types play an important role in the detection of drug-induced long APD and EADs. Indeed the Purkinje fiber was the only tissue type to display the well known phenomenon associated with I(kr) channel blockade (inverse-use dependence), when the stimulation rate was decreased below 1 Hz. Rabbit Purkinje fibers constitute the most sensitive tissue type for detecting drug-induced long action potential duration and EADs. As such the selection of tissue type needs to be taken into careful consideration in cardiac safety assessments when exploring drug induced long QT.

Antiarrhythmic and Arrhythmogenic Actions of Varying Levels of Extracellular Magnesium: Possible Cellular Basis for the Differences in the Efficacy of magnesium and Lidocaine in Torsade de Pointes

BACKGROUND: In recent years, there has been an increasing use of antiarrhythmic drugs that act predominantly by prolonging myocardial repolarization. An inevitable electrophysiologic consequence of these drugs is the development of torsade de pointes as a proarrhythmic reaction. Both intravenous lidocaine and magnesium sulphate have been used in the acute control of such a proarrhythmia. Their electrophysiologic mechanisms in this setting are not well defined. METHODS AND RESULTS: Using the standard microelectrode techniques, the effects of magnesium (Mg) and lidocaine on action potential duration (APD), and on barium-induced spontaneous action potentials, were studied in canine Purkinje fiber preparations. The objective was to clarify the direct and indirect effects of magnesium on triggered activities due to early afterdepolarizations. Superfusion in media with 0.1 mM Mg and 2.5 mM K produced more pronounced increases in APD measured at -20mV repolarization time [APD(20)] than those in a solution with 5 mM K. This effect was further enhanced at lower stimulation frequencies. The striking prolongation of APD(20) by solutions with low potassium concentrations diminished as the Mg concentration was increased. In solutions with 2.5 mM K, Mg produced concentration-dependent decreases in APD(20). This effect was greater at lower stimulation frequencies. Lidocaine at 4.0 x 10(_5) M produced a marked shortening of the APD in the entire firing frequency of the abnormal automaticity in a concentration-dependent manner. With 10 mM Mg, such action potentials appeared only sporadically. Magnesium also decreased the amplitude and the maximum upstroke velocity of these action potentials. In contrast, lidocaine at 4.0 x 10(-5) M exhibited no significant effects on action potentials due to barium-induced abnormal automaticity, or on additional depolarizations developing from the repolarization phase of these action potentials. CONCLUSIONS: The data indicate that (i) hypomagnesemia may be arrhythmogenic when combined with hypokalemia and bradycardia leading to a prolongation of the plateau phase of the action potential, (ii) magnesium administration may suppress triggered activities mainly by a direct inhibition of the development of triggered potentials, and (iii) lidocaine may suppress triggered potentials only indirectly by preventing the development of early afterdepolarizations due to the shortening effect on the APD. These findings are consistent with the clinical observation of a high incidence of torsade de pointes in the setting of hypokalemia and hypomagnesemia introduced by a chronic diuretic therapy. They are also consistent with the marked effectiveness of intravenous Mg relative to the inconsistent clinical effects of lidocaine in controlling torsade de pointes.

Selective block of Ca(2+)-dependent K+ current in crayfish neuromuscular system and chromaffin cells by sea anemone Bunodosoma cangicum venom

The effects of the nematocyst venom of the sea anemone Bunodosoma cangicum on depolarization-activated currents were studies in opener crayfish muscle fibers and in cultured bovine chromaffin cells. The venom selectively and reversibly blocked the Ca(2+)-dependent K+ current (IK(Ca)) present in crayfish muscle in a dose-dependent manner without affecting voltage-gated Ca2+ or K+ currents. Furthermore, the venom also reduced IK(Ca) in chromaffin cells, without modifying voltage-gated Na+, Ca2+, or K+ currents. Synaptic transmission in crayfish muscle was also affected by the venom. Repetitive excitatory and inhibitory postsynaptic currents (each associated with a presynaptic action potential) were evoked by each nerve stimulus, suggesting that presynaptic IK(Ca) may control the electrical activity of excitatory and inhibitory presynaptic fibers. We conclude that B. cangicum venom includes a toxin that selectively and reversibly blocks Ca(2+)-dependent K+ currents in crayfish muscle and in bovine chromaffin cells, and modifies excitatory and inhibitory synaptic transmission, probably abolishing a similar conductance at the presynaptic fibers.

Modification of cardiac Na+ channels by anthopleurin-A: effects on gating and kinetics

We used the whole cell patch clamp technique to investigate the characteristics of modification of cardiac Na+ channel gating by the sea anemone polypeptide toxin anthopleurin-A (AP-A). Guinea pig ventricular myocytes were isolated enzymatically using a retrograde perfusion apparatus. Holding potential was -140 mV and test potentials ranged from -100 to +40 mV (pulse duration 100 or 1000 ms). AP-A (50-100 nM) markedly slowed the rate of decay of Na+ current (INa) and increased peak INa conductance (gNa) by 38 +/- 5.5% (mean +/- SEM, P < 0.001, n = 12) with little change in slope factor (n = 12) or voltage midpoint of the gNa/V relationship after correction for spontaneous shifts. The voltage dependence of steady-state INa availability (h infinity) demonstrated an increase in slope factor from 5.9 +/- 0.8 mV in control to 8.0 +/- 0.7 mV after modification by AP-A (P < 0.01, n = 14) whereas any shift in the voltage midpoint of this relationship could be accounted for by a spontaneous time-dependent shift. AP-A-modified INa showed a use-dependent decrease in peak current amplitude (interpulse interval 500 ms) when pulse duration was 100 ms (-15 +/- 2%, P < 0.01, n = 17) but showed no decline when pulse duration was 100 ms (-3 +/- 1%). This use-dependent effect was probably the result of a decrease in the recovery from inactivation caused by AP-A which had a small effect on the fast time constant of recovery (from 4.1 +/- 0.3 ms in control to 6.0 +/- 1.1 ms after AP-A, P < 0.05) but increased the slow time constant from 66.2 +/- 6.5 ms in control to 188.9 +/- 36.4 ms (P < 0.002, n = 19) after exposure to AP-A. Increasing external divalent cation concentration (either Ca2+ or Mg2+) to 10 mM abolished the effects of AP-A on the rate of INa decay. These results demonstrate that modification of cardiac Na+ channels by AP-A markedly slowed INa inactivation and altered the voltage dependence of activation; these alterations in gating characteristics, in turn, caused an increase in gNa presumably by increasing the number of channels open at peak INa. AP-A slows the rate of recovery of INa from inactivation which is probably the basis for a use-dependent decrease in peak amplitude. Finally, AP-A binding is sensitive to external divalent cation concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)

The mechanism of the inotropic action of striatoxin, a novel polypeptide toxin from a marine snail, in isolated cardiac muscle

1. Striatoxin (StTX), a novel polypeptide from a marine snail, caused a dose-dependent increase in contractility in the isolated atria of guinea-pig and rat in the concentration-range of 2 x 10(-9) to 3 x 10(-8)M and 3 x 10(-6)M, respectively. 2. In guinea-pig atria, the StTX-induced inotropic effect was inhibited by tetrodotoxin but not by cimetidine or chlorpheniramine. Practolol, propranolol or reserpine caused only partial block of this inotropic action. 3. In isolated single cells from rat hearts, StTX caused an increase in the degree and the rate of contraction. 4. In guinea-pig atria, StTX provoked action potentials with a plateau phase of long duration without affecting the maximum rate of rise, the amplitude of action potential and the resting membrane potential. This prolongation was also reversed by tetrodotoxin. 5. In guinea-pig cardiac myocytes, whole-cell patch-clamp experiments showed that StTX slowed Na channel inactivation without affecting the time course of channel activation. The voltage dependence of Na currents was not altered by StTX. 6. The residual currents, but not peak currents were markedly enhanced by StTX. 7. These results suggest that StTX causes prolongation of the action potential duration probably due to slowed inactivation of Na inward currents and enhanced residual currents and that this may result in an increase in Ca2+ availability in cardiac muscle cells. This could explain the cardiotonic action of StTX.

QTU prolongation and polymorphic ventricular tachyarrhythmias due to bradycardia-dependent early afterdepolarizations. Afterdepolarizations and ventricular arrhythmias

Polymorphic ventricular tachyarrhythmias occurred spontaneously during bradycardia in dogs given the inotropic polypeptide anthopleurin-A (AP-A). The arrhythmia was investigated in in vitro and in vivo experiments. In in vitro experiments, AP-A (50 micrograms/l) produced bradycardia-dependent prolongation of action potential duration that was more pronounced in Purkinje than in muscle fibers. Only Purkinje fibers developed early afterdepolarizations (EAD) and triggered activity. These effects could be abolished by rapid pacing, lidocaine (4 mg/l), or tetrodotoxin (1 mg/l). In vivo experiments were conducted in anesthetized healthy dogs with simultaneous recording of surface ECG, monophasic action potentials from the endocardial and epicardial surface of the left ventricle by contact electrode catheter technique, and transmembrane action potentials from the epicardial surface of the left ventricle with a floating microelectrode technique. AP-A in a dose comparable to that used in vitro (4 micrograms/kg, i.v. bolus) resulted in bradycardia-dependent marked prolongation of both monophasic and transmembrane action potentials. An EAD gradually appeared on both recordings but was more marked in endocardial monophasic action potentials. Eventually, a premature ventricular depolarization arose from or very close to the peak of the EAD. The prolongation of action potentials was associated with similar prolongation of the QTU interval in surface ECG, and in some experiments, the EAD corresponded to a distinct prominent U wave. A ventricular premature depolarization arose from the U or TU complex and initiated polymorphic ventricular tachyarrhythmias that terminated spontaneously or degenerated into ventricular fibrillation. These effects were reversed by rapid pacing or lidocaine (1 mg/kg). The present study provides evidence in support of the hypothesis that AP-A-induced ventricular tachyarrhythmias are due to bradycardia-dependent EAD and triggered activity.

The effects of the Anemonia sulcata toxin (ATX II) on membrane currents of isolated mammalian myocytes

The effects of Anemonia sulcata toxin (ATX II) on action potentials and membrane currents were studied in single myocytes isolated from guinea-pig or bovine ventricles. Addition of ATX II (2-20 nM) prolonged the action potential duration without a significant change in resting membrane potential. Concentrations of 40 nM-ATX II or more induced after-depolarizations and triggered automaticity. The effects were reversible after washing or upon addition of 60 microM-tetrodotoxin (TTX). 5 mM-Ni did not modify the effects. The single patch-electrode voltage-clamp technique of Hamill, Marty, Neher, Sakmann & Sigworth (1981) was applied to record membrane currents in response to 8.4 S long depolarizations starting from a holding potential of -90 mV. Currents flowing later than 5 ms after the depolarizing step were analysed. The fast events could not be considered because of insufficient voltage homogeneity. After 2 min of exposure to ATX II (20 nM) the changes in net membrane currents were measured. The difference between the currents in the presence of ATX II and during control was defined as the 'ATX-II-induced current' (iATX). After 4 min of wash iATX disappeared. Within 10 S of exposure to 60 microM-TTX, iATX was blocked completely. At potentials positive to -60 mV, iATX was inwardly directed and decayed slowly but incompletely during the 8.4 S long depolarizing pulse. The rate of decay was faster during clamp pulses to more positive potentials. A high amplitude noise was superimposed on the current trace; its amplitude decreased with more positive potentials. We analysed the voltage dependence of iATX with 'isochronous' current-voltage relations. The 0.1 S isochrone of iATX was characterized by a 'threshold' for negative currents at -60 mV, a branch with a negative slope (k = -7 mV, potential of half-maximal activation (V0.5) = -38 mV, bovine cells) leading to a maximum inward current at -20 mV, and an ascending branch which led to an apparent reversal potential (Erev) around +40 mV. The values measured in guinea-pig myocytes were similar though not identical (k = -5.5 mV, V0.5 = -30 mV, maximum of inward current at -5 mV, Erev = +50 mV). Erev shifted to less positive potentials in later isochrones. Holding the membrane at -45 mV prevented the induction of extra current by ATX II. When the holding potential was then changed to -85 mV, iATX developed within some 2 min. Returning the holding potential to -45 mV blocked iATX with a similar slow time course.(ABSTRACT TRUNCATED AT 400 WORDS)

Extracellular calcium ions modify the effects of Anemonia sulcata toxin (ATX II) in guinea-pig papillary muscles

In isolated guinea-pig papillary muscle ATX II prolonged the action potential duration to a lesser extent at high extracellular Ca++-concentrations. This is interpreted as an interference of Ca++ with ATX II-binding sites.

The influence of the rate of electrical stimulation on the effects of the Anemonia sulcata Toxin ATX II in guinea pig papillary muscle

In guinea pig papillary muscle, the rate of electrical stimulation (0.1-2 Hz) strongly influenced the effects of the Anemonia sulcata toxin ATX II on action potential duration (APD) and contractile force. In the concentration range studied (10-8-10-7 M), ATX II always produced a larger prolongation in APD at low rates of stimulation. At 0.1 Hz there was a temporal dissociation between the onset of the APD-prolonging and the positive inotropic effect. However, under equilibrium conditions there was a positive relationship between the APD expressed as a fraction of the time during which the membrane was depolarized, and the contractile force irrespective of the change in experimental conditions being variation of stimulation frequency or the addition of ATX II. The results suggest that the positive inotropic effects of both ATX II and increased stimulation frequency could be induced by a similar mechanism, e.g. an increase in sodium of the heart muscle.

Electromechanical effects of anthopleurin-A (AP-A) on rabbit ventricular muscle: influence of driving frequency, calcium antagonists, tetrodotoxin, lidocaine and ryanodine

1 Anthopleurin-A (AP-A 5 x 10(-9) M, 1 x 10(-8) M) caused a prolongation of action potential duration (APD) and an increase of contractile force in rabbit isolated ventricular muscle preparations.2 The prolongation of APD and the positive inotropic effect of AP-A (1 x 10(-8) M) were augmented by lowering the driving frequency from 2.0 to 0.2 Hz, resulting in an apparent negative staircase of contractile force in this frequency range. When the preparation was driven at an extremely low frequency (0.017 Hz), AP-A did not increase the contractile force, but caused a considerable prolongation of APD.3 Verapamil (1 x 10(-6) M) and nifedipine (1 x 10(-6) M) had no apparent influence on the APD prolongation by AP-A (5 x 10(-9) M, 1 x 10(-8) M). The positive inotropic effect of AP-A was also relatively well maintained even in the presence of these calcium antagonistic drugs when the preparation was driven at a lower frequency (0.2 Hz).4 Tetrodotoxin (TTX 2 x 10(-6) M) and lidocaine (1 x 10(-4) M) markedly inhibited both the APD prolongation and the positive inotropic effect of AP-A (1 x 10(-8) M).5 In the presence of ryanodine (2 x 10(-6) M), an agent which is known to interfere with calcium release from the intracellular activator pool, AP-A (1 x 10(-8) M) failed to cause its positive inotropic effect in spite of the marked prolongation of APD.6 These results suggest that the effects of AP-A on cardiac muscle are primarily mediated by the fast sodium inward current. Thus, delayed inactivation of sodium inward current may cause APD prolongation, and probably induces an alteration of intracellular calcium kinetics reflected by an increase of contractile force.

Excitatory effect of a new polypeptide (anthopleurin-B) from sea anemone on the guinea-pig vas deferens

1 Anthopleurin-B (AP-B, greater than 3 x 10(-9) M), a newly isolated polypeptide from sea anemone (Anthopleura xanthogrammica), caused powerful rhythmic contractions in the guinea-pig isolated vas deferens. The other polypeptides anthopleurin-A from A. xanthogrammica and anthopleurin-C from A. elegantissima, elicited similar effects but in higher concentrations ( less than 5 x 10(-8) M). Toxin II (10(-6) M) isolated from the sea anemone, Anemonia sulcata, had no effect. 2 The rhythmic contractions induced by AP-B were inhibited by phentolamine, bretylium, guanethidine, reserpine, 6-hydroxydopamine, tetrodotoxin (TTX) and verapamil. Mecamylamine, atropine, methysergide, chlorpheniramine, and indomethacin had no effect. 3 AP-B (10(-8) M approximately 10(-5) M) caused a dose-dependent increase in the amount of endogenous noradrenaline (NA) released from the vas deferens. AP-B (10-5M) increased the amount of NA released to approximately 310 times (12 micrograms/g tissue) that of untreated tissues. 4 The AP-B-induced release of NA was inhibited or abolished by TTX, verapamil, or incubation in Ca-free medium. 5 These results suggest that the AP-B-induced rhythmic contraction of the vas deferens is mediated through the release of NA from adrenergic nerve endings; AP-B is one of the most potent substances in stimulating NA release from the vas deferens.