The effect of verapamil on potassium fluxes in canine cardiac Purkinje fibres.

Electrophysiological studies of verapamil on isolated cardiac tissue show its effects to occur during Phase 2 and Phase 4 of the cardiac action potential and to be a result of blocking the slow inward Ca2+ current. It is important to know if verapamil effects the transmembrane movement of ions other than Ca2+. By studying uptake and efflux of 42K by canine cardiac Purkinje fibres in the presence and absence of verapamil (1.1 x 10-6 M-4.4 X 10-6 M), it was found that the drug inhibits both 42K uptake (15%) and efflux (30%).     

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.     

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.     

The combined electrophysiological effects of lignocaine and sotalol in canine isolated cardiac Purkinje fibres are rate-dependent.

1. The frequency-dependent electrophysiological effects of lignocaine, sotalol, and their combination were studied in dog isolated cardiac Purkinje fibres, both at various constant rates of stimulation and following abrupt changes in pacing cycle length. 2. The combined effect of 18 microM lignocaine and 30 microM sotalol selectively lengthened duration of premature action potentials evoked at a diastolic interval of 40 ms (from 172.2 +/- 5.4 to 201.7 +/- 4.9 ms, n = 6, P less than 0.01) without significantly changing the durations of action potentials evoked at the basic cycle length of 500 ms (259.1 +/- 7.7 vs 251.9 +/- 3.9 ms, n = 11). 3. The combination of lignocaine with sotalol, like lignocaine alone, displayed a use-dependent depression of Vmax and revealed a slow component for a recovery of Vmax (tau = 173.5 +/- 16.0 ms, n = 5). 4. The kinetics for restitution of action potential duration were also slowed by the combination of the two dwo drugs (tau f = 173.6 +/- 16.7, before, vs 268.5 +/- 8.5 ms, after; n = 5, P less than 0.01), while the maximum action potential duration attained in this relation was not increased as it was by sotalol alone. 5. Lignocaine, therefore, appeared to inhibit the sotalol-induced lengthening of action potential duration at slow pacing rates and at long diastolic intervals. The combination of lignocaine with sotalol also completely abolished the occurrence of sotalol-induced early after depolarizations. 6. Finally, sotalol alone moderately increased the range of premature action potential durations, while the combination of the two drugs significantly decreased this parameter.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electrophysiological effects of flecainide enantiomers in canine Purkinje fibres

Summary Since flecainide is a chiral class I antiarrhythmic agent, we examined the basic electrophysiological effects of its enantiomers (10^–6–3 × 10^–5 mol/l) in isolated canine Purkinje fibres using standard microelectrode techniques. Frequency-dependent block was studied by pacing at cycle lengths of 300 and 2000 ms. Both enantiomers produced a marked, concentration-dependent decrease in maximum upstroke velocity (V _max), with greater depression occurring at a cycle length of 300 ms. The concentrations causing a 50% decrease in V _max (EC₅₀) at this pacing rate were 5.0 ± 0.6 × 10^-6 mol/l for (+)-flecainide and 6.2 ± 0.8 × 10^-6 mol/l for (-)-flecainide, and were not significantly different. Both enantiomers also produced a concentration and frequency-dependent decrease in action potential amplitude and an increase in conduction time with no significant differences between the enantiomers. However, at the highest concentration studied (3 × 10^-5 mol/l), none of the nine tissues exposed to (+)-flecainide could be paced at 300 ms cycle length while five of eight tissues exposed to (-)-flecainide could be paced, suggesting the possibility of a small difference between the effects of the enantiomers on membrane responsiveness. The effect of the enantiomers on action potential duration (APD) also depended on pacing rate. At the longer cycle length, ADP₅₀ was shortened to a maximum of 61 ± 7% and 67 ± 7% from baseline by (+)- and (-)-flecainide, respectively, whereas APD₉₅ was shortened by 30 ± 6% and 32 ± 3% from baseline by (+)-and (-)-flecainide, respectively. At the 300 ms cycle length, there was a modest decrease in both ADP₅₀ and APD₉₅ at low concentrations (up to 3 × 10^-6 mol/l) followed by a return toward baseline at the higher concentrations 10^-5 and 3 × 10^-5 mol/l. No significant differences between the enantiomers were noted at either pacing frequency. The results suggest there is no significant difference between the effects of flecainide enantiomers on basic electrophysiological parameters of canine Purkinje fibres. Although the (+)-enantiomer may produce a slightly greater decrease in cellular responsiveness, the results generally agree with prior in vivo data showing little or no difference in the antiarrhythmic activities of the enantiomers. Send offprint requests to J. K. Smallwood at the above address     

Nitroglycerin on contractile force in canine cardiac Purkinje fibers

The objective of this study was to investigate the effects of nitroglycerin (NG) on the contractile force in order to elucidate regulatory roles of cyclic guanosine monophosphate (cGMP) in the bundle of canine cardiac Purkinje fibers. NG suppressed the developed tension in canine Purkinje fibers immersed in normal Tyrode's solution. The contraction was also elicited in the depolarizing solution containing 30 mM K+ by adding isoproterenol (Iso) or tetraethylammonium (TEA). The suppression caused by NG was more marked in fibers in the depolarizing solution treated with TEA than in the depolarizing solution treated with Iso. 8-Bromo-cGMP and sodium nitroprusside had a slight inhibition on the contraction in the fibers. Methylene blue did not affect the decrease in contractile force induced by NG. The weaker inhibition caused by NG on the contraction in the presence of Iso might result from a possible increase in intracellular cAMP levels in the fibers.     

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.     

Effect of verapamil and diltiazem on calcium-dependent electrical activity in cardiac Purkinje fibres.

The effects of verapamil and diltiazem on normal action potentials, abnormal automaticity at depolarized membrane potential and oscillatory afterpotentials were compared in sheep cardiac Purkinje fibres. Concentrations of verapamil and diltiazem exerting the same action on abnormal automaticity due to slow action potentials, caused different effects on action potential characteristics and on oscillatory afterpotentials. Diltiazem significantly shortened action potential duration whereas verapamil slightly lengthened it (NS). Diltiazem appeared to be more effective than verapamil in preventing the development of oscillatory afterpotentials induced by barium or by strophanthidin. In 50% of barium-treated preparations, verapamil caused the appearance of spontaneous activity due to enhanced normal diastolic depolarization, while diltiazem had no such effect. The observed differences were explained in terms of the different effects of the two drugs on currents other than the slow inward current, since diltiazem was more potent than verapamil in depressing Vmax.     

Evidence for gender differences in electrophysiological properties of canine Purkinje fibres

Women are more prone to develop torsades de pointes, a rare life-threatening polymorphic ventricular tachycardia, than are men during administration of medicines that have the potential to block I(Kr) (rapid delayed rectifier cardiac K(+) current) and to prolong the QT interval. Blockade of I(Kr), hypokalaemia and extreme bradycardia were used to evaluate whether there are gender differences in cardiac repolarisation in canine Purkinje fibres (PFs). Microelectrode techniques were employed to measure action potential (AP) parameters in PFs from adult female and male dogs. Under control conditions, fibres from female animals in normal or low K(+) conditions exhibited significantly longer AP durations at 50% (APD(50)) and 90% (APD(90)) of repolarisation as compared with APDs of fibres from male animals. Gender-related difference to rate adaptation was also present in APD(90) of fibres from female animals compared to males. At a stimulation rate of 0.2 Hz, but not at 1.0 Hz, dofetilide elicited a significantly higher increase in APD(90), incidence of early afterdepolarisations, triggered and sustained-triggered activities (TAs) in fibres from female animals compared to males in either normal or low K(+) conditions. The sustained TAs were reversed by raising the concentration of [K(+)](0) in Purkinje preparations from both male (one out of one) and female (12 out of 12) dogs. In conclusion, our data provide experimental evidence pointing to gender differences in canine AP repolarisation. PFs from female dogs can be used in safety pharmacology studies as a sensitive model for evaluating the potential proarrhythmic events in vitro of a new medicinal product.     

Comparison of the sodium currents in normal Purkinje fibres and Purkinje fibres surviving infarction--a pharmacological study.

1. Purkinje fibres surviving infarction showed a lower maximum upstroke velocity (Vmax) and a longer action potential duration when compared to normal Purkinje fibers. A reduction in the fast sodium current and an increase in the sodium 'window' current may be responsible for the observed alterations in Vmax and action potential duration respectively. 2. Since voltage clamp studies were not feasible, a pharmacological approach was used. The responses to tetrodotoxin (TTX) and lignocaine in normal Purkinje fibres and Purkinje fibres surviving infarction were used to examine the sodium currents in these fibres. 3. Vmax, an indirect measure of the fast sodium current, was more sensitive to lignocaine in Purkinje fibres surviving infarction than in normal Purkinje fibres. The reduction in Vmax by lignocaine was more prominent at the shorter stimulation cycle length. Significant reduction of Vmax was observed with the higher concentration of TTX and no differential effect on Vmax between normal Purkinje fibres and Purkinje fibres surviving infarction was detected. 4. Reduction of action potential duration in the presence of TTX or lignocaine was used as a measure of the sodium 'window' current. A greater reduction of action potential duration by TTX and lignocaine was observed in normal Purkinje fibres than in Purkinje fibres surviving infarction. 5. The results suggested that the fast sodium current in Purkinje fibres surviving infarction is more sensitive to pharmacological agents with local anaesthetic properties and the prolonged action potential duration in these Purkinje fibres cannot be due to an increase in the sodium 'window' current. The results are compatible with an enhanced effect of antiarrhythmic drugs on Vmax and conduction in the ischaemic heart.     

Electrophysiologic interactions of procainamide and N-acetylprocainamide in isolated canine cardiac Purkinje fibers.

The study objective was to characterize the electrophysiologic interactions of procainamide (PA) and its metabolite, N-acetylprocainamide (NAPA), in canine Purkinje fibers. Cell (N = 43) action potentials were measured in Tyrode's solution (K+ = 4.0 mM, 36 degrees C) at a basic cycle length of 1,000 ms using standard microelectrode techniques. Six PA concentrations (0.020-0.32 mM) and six NAPA concentrations (0.010-0.24 mM) were studied alone and in combination. PA caused concentration-dependent decreases in Vmax and APD50 but did not alter APD90, ERP, or RMP. NAPA caused a small but not significant concentration-dependent decrease in Vmax, no change in RMP, and significant concentration-dependent increases in APD50, APD90, and ERP. Low NAPA concentrations increased, intermediate concentrations did not affect, and high NAPA concentrations again increased PA's effect on Vmax. PA-NAPA combinations resulted in concentration-dependent changes in APD50 that were intermediate between the effects of PA or NAPA alone. PA did not significantly alter NAPA's effects on APD90 at NAPA concentrations less than or equal to 0.040 mM, while it antagonized NAPA's effect at higher concentrations. The effects of PA-NAPA combinations on ERP were generally similar to their effects on APD90. The electrophysiologic effects of PA-NAPA combinations in normal canine Purkinje fibers are complex functions of the relative and absolute concentrations of the two compounds.     

Cisapride-induced prolongation of cardiac action potential and early afterdepolarizations in rabbit Purkinje fibres.

Cisapride, a gastrointestinal prokinetic agent, has been associated with cases of Torsades de Pointes but its effects on the cardiac action potential have not been described. We investigated its electrophysiological effects on rabbit isolated Purkinje fibres. The results demonstrated that cisapride (0.01-10 microM) lengthened concentration-dependently the action potential duration without modifying other parameters and induced early after depolarizations and subsequent triggered activity. This typical class III antiarrhythmic effect, that showed "reverse" rate-dependence and was reduced by increasing external K concentration, can account for clinical arrhythmogenesis.     

Effects of caffeine on intracellular sodium activity in cardiac Purkinje fibres: relation to force.

1. An increase in cytoplasmic calcium by caffeine would lead to Ca extrusion via the Na/Ca exchange. The hypotheses were investigated that, as a consequence, caffeine might increase intracellular sodium activity (aiNa) and that the relation between aiNa and force might be conditioned by the Ca load. 2. Action potential, aiNa and contractile force were recorded in sheep Purkinje fibres during exposure to caffeine under conditions that decrease or increase the Ca load by different mechanisms. 3. In Tyrode solution, caffeine (8 mM) increased aiNa from 8.05 +/- 0.20 to 10.52 +/- 0.40 mM (+30.5%) and had a triphasic effect on force: an initial transient increase (+93.6%), a subsequent decrease (-37.1%) (negative inotropy) and slow partial recovery (+8.9%). 4. Decreasing the Ca load by means of manganese (1 mM) decreased aiNa and force. Adding caffeine re-increased aiNa and no longer caused a negative inotropic action. Cadmium (0.2 mM) also decreased aiNa, and caffeine reincreased it although far less than in Tyrode solution. 5. High [K]o (10 mM) and tetrodotoxin (5 microM) decreased aiNa as well as force. In their presence, caffeine re-increased aiNa and no longer had a negative inotropic action. 6. Increasing the Ca load by means of high [Ca]o (8.1 mM) increased force (+195%) and decreased aiNa, (-20.3%). Adding caffeine re-increased aiNa (+28.1%), but immediately decreased force (-32.3%). 7. Addition of pyruvate (10 mM) to caffeine increased force, as it does in the presence of Ca overload. 8. Noradrenaline (0.1-1 microM) decreased aiNa and increased contractile force. In its presence, caffeine decreased aiNa further and increased force.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prediction of the risk of Torsade de Pointes using the model of isolated canine Purkinje fibres

Torsade de Pointes (TdP) is a well-described major risk associated with various kinds of drugs. However, prediction of this risk is still uncertain both in preclinical and clinical trials. We tested 45 reference compounds on the model of isolated canine Purkinje fibres. Of them, 22 are clearly associated and/or labelled with a risk of TdP, and 13 others are drugs with published clinical evidence of QT prolongation, with only one or two exceptional cases of TdP. The 10 remaining drugs are without reports of TdP and QT prolongation. The relevance of different indicators such as APD(90) increase, reverse use dependency, action potential triangulation or effect on V(max) was evaluated by comparison with available clinical data. Finally, a complex algorithm called TDPscreen and based on two subalgorithms corresponding to particular electrophysiological patterns was defined. This latter algorithm enabled a clear separation of drugs into three groups: (A) drugs with numerous or several reports (>2 cases) of TdP, (B) drugs causing QT prolongation and/or TdP only, the latter at a very low frequency (< or =2 cases), (C) drugs without reports of TdP or QT prolongation.The use of such an algorithm combined with a database accrued from reference compounds with available clinical data is suggested as a basis for testing new candidate drugs in the early stages of development for proarrhythmic risk prediction.     

Active and passive electrical properties of isolated canine cardiac Purkinje fibers under conditions simulating ischaemia: effect of diltiazem.

The effect of a calcium channel blocker diltiazem on the electrical properties of canine Purkinje fibers superfused in a milieu similar to that occurring in acute myocardial ischaemia was studied. Action potential parameters, passive electrical properties, and conduction velocity were measured using conventional microelectrode techniques. Superfusion with glucose-free Tyrode's solution containing 9 mM K+, gassed with 100% N2 at pH = 6.5 ('ischemic solution') significantly reduced the maximal diastolic potential, action potential duration, maximal upstroke velocity, conduction velocity and length constant, while input resistance and longitudinal resistance were elevated and membrane resistance remained unchanged. Diltiazem (1 microM) alone reduced only the action potential duration, while all other parameters were unaffected. Pretreatment with diltiazem did not fully prevent the effects of ischemic superfusion; however, the ischaemia-induced decrease in length constant was not significant in the presence of diltiazem. In addition, the increase in longitudinal resistance during ischaemia was significantly reduced following diltiazem pretreatment. This decrease in longitudinal resistance may contribute to the improvement of ischaemia-induced conduction delay observed in intact animals and may be related to a reduction of ischaemia-induced increase in intracellular free Ca2+.     

Effects of the new cardiotonic agent sulmazole on the slow inward current of sheep cardiac Purkinje fibres

The new cardiotonic agent 2-[(2-methoxy-4-methylsulfinyl)-phenyl]-1H-imidazo[4,5-b]pyridine (sulmazole, AR-L 115 BS) has marked positive inotropism but causes a depression in the plateau phase of the action potential of cardiac Purkinje fibres. This loss of plateau is known to occur with calcium antagonists which reduce contractility. In order to identify the mechanism underlying this possibly controversal effect the slow (calcium dependent) inward current (isi) was measured using the double microelectrode voltage clamp technique. In this current system, sulmazole was observed to have a slight effect on the inactivation parameter f infinity of isi by shifting it in hyperpolarizing direction. This increase in inactivation was considered when isi was determined. However, isi itself is reduced quite considerably and the linear instantaneous current voltage relationship is shifted to negative potentials. The kinetics of activation (d infinity) are not affected by sulmazole. From the more or less parallel shifts of isi we conclude that the reversal potential of isi is decreased which in turn strongly indicates an increase of intracellular calcium ion concentration. The reduction of isi by sulmazole is not the result of a specific membrane effect as in the case of some calcium antagonists. Sulmazole does not generate its positive inotropism by way of an increased slow inward current as do beta-adrenoceptor agonists but rather reduces the slow inward current by means of a negative shift of Eisi and a decrease in isi-driving force after it has affected intracellular calcium.     

Effects of seven drugs of abuse on action potential repolarisation in sheep cardiac Purkinje fibres

Seven drugs of abuse have been examined for effects on the action potential in sheep isolated cardiac Purkinje fibres. Phencyclidine (5 microM) induced a significant increase (30.7%) in action potential duration at 90% repolarisation (APD(90)). Similarly, 10 microM 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy') induced a significant increase in APD(90) of 12.1%. Although Delta(9)-tetrahydrocannabinol (0.1 microM) induced a small, but statistically significant, 4.8% increase in APD(90), no effects were observed at 0.01 or 1 microM. Cocaethylene (10 microM) induced a significant shortening of APD(90) (-23.8%). Cocaine (up to 1 microM), (+)-methamphetamine ('Speed'; up to 5 microM), and the heroin metabolite, morphine (up to 5 microM), had no statistically significant effects. The possible significance of these findings is discussed in the context of other recognised cardiac effects of the tested drugs.     

Combined effects of ouabain and hypoxia on the transmembrane potential of of canine cardiac Purkinje fibers

The effects of hypoxia on the transmembrane potential of dog Purkinje cells in isolated, superfused fiber bundles pretreated with ouabain were studied. Cells stimulated electrically at 93/min were exposed to ouabain, 2.1 X 10(-7) M, until the magnitude of phase 4 depolarization increased to 7-12 mV. Arrhythmias did not occur. Following a 10-min washout period, hypoxic solution (PO2 = 15-50 mm Hg) was applied for 2-5 min. This caused decreases in maximum diastolic potential, overshoot, rising velocity of phase 0, and duration of the action potential. The slope of depolarization during phase 4 increased markedly. Arrhythmias characterized by escape rhythms or single and multiple bursts of premature excitations occurred in greater than 90% of the experiments. None of these changes was noted when identical levels of hypoxia were applied for a similar period to normal cells. Blockade of the beta-adrenergic receptors with propranolol, 0.3 mg/L, did not alter the response of ouabain-pretreated cells to hypoxia in any manner, ruling out release of endogenous catecholamines as essential for the observed effects. These results suggest that ouabain and hypoxia have a synergistic effect directly on the cells and produce the observed changes in membrane potential. The ventricular arrhythmias observed in digitalized humans or animals that become hypoxemic may result either from the induction of oscillatory after potentials in Purkinje cells causing triggered spontaneous excitations or from reentry of excitation.     

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.     

Induction of the oscillatory current by low concentrations of caffeine in sheep cardiac Purkinje fibres

Summary The actions of low concentrations of caffeine (0.5–2 mmol/1) on the transient inward oscillatory current (I_os) and the inward tail current (I_ex) were studied in sheep cardiac Purkinje fibres by means of a two microelectrode voltage clamp method. The following results were obtained. Caffeine: 1. induced an I_os when this current was not already present; 2. increased the amplitude (within limits) and consistently decreased the time to peak of an already present I_os; 3. increased I_ex upon which I_os may be superimposed; 4. shifted the depolarizing threshold for the appearance of I_os to more negative and the repolarizing threshold to less negative values; 5. increased the effects of strophanthidin of I_os and I_ex; 6. exaggerated the effects of high [Ca]_o which by itself mimicked some of the actions of caffeine; 7. had a small effect of I_os and I_ex in low [Ca]_o; 8. reversed the effect of norepinephrine on I_ex; 9. enhanced the effects of trains of clamps and of longer clamp steps on I_os and I_ex. It is concluded that low concentrations of caffeine facilitate the manifestations of calcium overload thereby inducing or exaggerating the oscillatory and tail currents and that these effects are modulated by the cellular calcium load but are not mediated through adrenergic mechanisms. Send offprint requests to M. Vassalle at the above address