Strophanthidin inotropy: role of intracellular sodium ion activity and sodium-calcium exchange

The relation among the ratio of extra- and intra-cellular sodium ion activities (aoNa/aiNa), contractile force and action of strophanthidin was studied in cardiac Purkinje fibers when transmembrane Na+ and Ca2+ gradients were changed. The aiNa, contractile force and action potential were simultaneously measured. Simultaneous reduction of [Na+]o and [Ca2+]o to 80.8 and 1.08 mM respectively, decreased aiNa from 8.0 +/- 1.1 mM (mean +/- S.D., n = 17) to 6.0 +/- 0.9 mM (n = 17) whereas contractile force transiently increased and then recovered toward the level similar to that in Tyrode solution. Reduction of [Ca2+]o alone increased aiNa by 1.7 +/- 0.4 mM (n = 5) and decreased contractile force by 87 +/- 5% (n = 5). Raising osmolarity of Tyrode solution with sucrose increased both aiNa and contractile force. Substitution of sucrose with Na+ (high [Na+] solution) increased aiNa by 1.2 +/- 0.3 mM (n = 5) and decreased contractile force by 31 +/- 9% (n = 5). Strophanthidin (2 X 10(-7) M) increased aiNa by 0.4 +/- 0.1 mM (n = 6) and contractile force by 24 +/- 8 (n = 6) in a low [Na+] - [Ca2+] solution. These changes were smaller than those in Tyrode solution (1.1 +/- 0.3 mM); 96 +/- 32%, n = 6). On the other hand, strophanthidin increased aiNa and contractile force more in a low [Ca2+] (2.7 +/- 0.5 mM; 220 +/- 24%, n = 5) or a high [Na+] (2.3 +/- 0.9 mM; 164 +/- 37%, n = 5) solution than in Tyrode solution. In the solutions containing the altered [Na+]o and/or [Ca2+]o, the increases in aiNa and force by strophanthidin were parallel. Therefore, the parallel increase in aiNa and contractile force due to strophanthidin depends on the initial level of aiNa, suggesting the dependence of digitalis inotropy on the rate of Na+ extrusion by the Na+ -K+ pump. The results also indicate that the ratio of aoNa/aiNa is an important and powerful factor in the control of contractile force. Presumably this is mediated through the Na+ -Ca2+ exchange.     

Effects of tetrodotoxin on electrical and mechanical activity of cardiac Purkinje fibers

Canine Purkinje and ventricular muscle fibers were superfused in vitro and the actions of tetrodotoxin (TTX, 1.56 X 10(-5) - 3.2 X 10(-7) M) on both electrical and mechanical events were studied. Tetrodotoxin reduced the amplitude of the upstroke, markedly shortened the plateau, reduced the maximum diastolic potential and decreased contractile force in Purkinje fibers. Citrate (present in the TTX solution) did not have any effect on electrical and mechanical events. The modifications induced by TTX occurred whether external calcium concentration was normal, high or low. Furthermore, high calcium increased force by a similar extent whether TTX was present or not. TTX has little effect on slow action potentials due to the activation of the slow channel. Action potential duration and force were affected similarly by TTX as a function of concentration, of time of exposure and of rate of discharge. Also, TTX abolished action potentials due to a partially activated fast sodium current and reduced action potential duration and force much less in ventricular muscle than in Purkinje fibers. It is concluded that the effects of TTX are primarily related to a reduction of sodium influx during the action potential and that the fall in force is an indirect consequence of reduced sodium influx.     

Regional differences of superoxide dismutase activity enhance the superoxide–induced electrical heterogeneity in rabbit hearts

During myocardial ischemia and the subsequent reperfusion, free radicals are important intermediates of the cellular damage and rhythm disturbances. We examined the effects of superoxide radicals or hydrogen peroxide (H₂O₂) on the action potentials in isolated rabbit Purkinje fibers, atrial muscle and ventricular muscle. Reactive oxygen species (ROS) donors such as adriamycin, xanthine/xanthine oxidase and menadione induced prolongation of APD₉₀ in Purkinje fibers. Menadione (30 µM), the most specific superoxide radical donor, prolonged the action potential duration at 90% repolarization (APD₉₀) by 17% in Purkinje fibers, whereas it shortened the APD by 57% in ventricular muscle, and it did not affect the atrial APD. All these menadione–induced effects were completely blocked by 2,2,6,6–tetramethyl– 1–peperadinyloxy, a superoxide radical scavenger. Superoxide dismutase (SOD) activity was lowest in Purkinje fibers, it was moderate in atrial muscle and highest in ventricular muscle. H₂O₂ shortened the APDs of all three cardiac tissues in a concentration–dependent manner. These results suggest that the different electrical responses to O₂^●– in different cardiac regions may result from the regional differences in the SOD activity, thereby enhancing the regional electrical heterogeneity.Drs. B. H. Choi and K.–Ch. Ha contributed equally to this study.     

The negative inotropic effect of calcium overload in cardiac Purkinje fibers

The role of calcium in increasing or decreasing the force of contraction was studied in canine cardiac Purkinje fibers perfused in vitro under conditions known to alter cellular calcium. The following results were obtained: (1) increasing calcium load by perfusing a low Na solution increases contractile force in the presence of a low concentration of strophanthidin and decreases it in high strophanthidin; (2) decreasing calcium load by decreasing extracellular calcium leads to opposite effects; (3) the positive inotropic effect of low [Na]0 becomes negative in the presence of high [Ca]0; (4) high K decreases contractile force and reverses the negative inotropic effect of low Na in the presence of high strophanthidin; (5) a low Na-Ca solution increases initially the contractile force: high strophanthidin reverses the effect at normal K and exaggerates at high K, as apparently calcium overload is relieved by high K; (6) a low Na-Ca solution slowly removes calcium overload induced by high strophanthidin; (7) a low [Na]0 solution does not decrease force in the presence of high strophanthidin if extracellular calcium is also low; (8) the rebound increase in force is shifted to lower calcium concentrations after exposure to a low sodium solution with or without strophanthidin, as calcium overload becomes greater. It is concluded that in Purkinje fibers calcium overload induced by different means leads to a decrease in contractile force and that the force decline can be reversed by decreasing cellular calcium either during or after calcium overloading.     

Inhibition of the Na pump—a mechanism in the genesis of cardiac arrhythmias

Summary An ATP-driven Na pump maintains the unsymmetrical Na and K distribution across the cell membrane of cardiac cells. An increase of the intracellular Na or extracellular K concentration enhances this active Na transport. About 35 per cent of the actively transported Na is ejected from the cells as a hyperpolarizing outward current. The Na pump influences the cardiac Ca metabolism via the Na–Ca exchange. Inhibition of the pump affects the generation and conduction of the cardiac action potential by various mechanisms. It seems to be involved in the genesis of cardiac arrhythmias.     

缺血对绵羊浦肯野纤维起搏电流影响的研究

采用双微电极电压钳制术测定异丙肾上腺素（ＩＳＯ）对缺血状态下绵羊心脏浦肯野纤维起搏离子流（Ｉｆ）的影响。结果：缺血本身（缺氧、无能量供应、酸中毒）降低Ｉｆ的幅值，使半激活时间延长；此时在模拟缺血液中加入ＩＳＯ１×１０（－６）ｍｏｌ／Ｌ，则Ｉｆ幅值明显增加，半激活时间缩短，但未能达到缺血前基础水平。表明在急性心肌缺血时，虽然局部儿茶酚胺大量积聚产生β－受体过度激活，然而单纯这一因素并不能导致浦肯野纤维的正常自律活动异常增强。     

Electrophysiological effects of lidocaine on distal Purkinje fibers of canine heart

These studies were designed to examine the electrophysiological effects of lidocaine on distal Purkinje fibers of the canine heart. Standard microelectrode techniques were used to record transmembrane action potentials from the right bundle branch, free running strands of Purkinje tissue, and ventricular muscle. The duration of action potentials was maximal in the distal Purkinje fibers. The long duration of action potentials in these distal fibers determined the functional refractory period of the Purkinje-muscle junction. Early premature beats initiated on either side of the distal Purkinje fibers propagated with decrement or were blocked. Lidocaine shortened the action potential of Purkinje fibers but had little effect on muscle. The effect of lidocaine on action potential duration was dose dependent and was maximal in these distal Purkinje fibers. As a consequence of its ability to shorten action potential duration, lidocaine reduced the degree of nonuniformity of recovery of excitability. Lidocaine shortened the functional refractory period of distal Purkinje fibers and abolished decremental conduction of early premature beats. These effects may contribute to the antiarrhythmic actions of lidocaine.     

Rate-force relationship and calcium overload in canine Purkinje fibers

The relationship between driving rate and contractile force was studied in calcium-overloaded canine cardiac Purkinje fibers perfused in vitro. The following results were obtained. In Tyrode solution (2.7 mM Ca), increasing the drive rate ('overdrive') induced an initial decrease followed by a progressive increase in force; the initial decrease was more pronounced and the subsequent increase slower when the driving rate was faster; returning to the basal slow rate caused a transient increase in force above control and this increase was greater when overdrive was faster; a stepwise increase and decrease in rate led to an initial decrease in force with each increment in rate and an initial increase with each decrement in rate; increasing [Ca]o up to 16.2 mM reversed both the initial fall during and the transient increase in force after overdrive; these changes in force were associated with oscillatory potentials, a sign of calcium overload; similar results were obtained by inducing calcium overload with a low [Na]o solution or strophanthidin; the altered force patterns and the oscillatory potentials were reduced or eliminated if calcium load was reduced by decreasing the basal rate or [Ca]o, by increasing [K]o or by administering tetrodotoxin. It is concluded that in Purkinje fibers the force-frequency relationship is markedly altered in the presence of calcium overload because overdrive further increases the calcium load and this results in a reversal of the relation between calcium and force.     

Effect of tetrodotoxin,lidocaine, and quinidine on the transient inward current of sheep Purkinje fibres

Summary The effect of tetrodotoxin (TTX), lidocaine, and quinidine on the transient inward current (TI) was studied in voltage-clamped sheep cardiac Purkinje fibres. The TI was induced by elevation of extracellular Ca or addition of strophanthidin.Reduction of external Na had a biphasic effect on the steady state TI magnitude; a moderate (<50%) reduction of external Na had an enhancing effect on the TI; a further decrease of extracellular Na was accompanied by a decline of TI amplitude. The TI could not be induced in Na-free medium (external Ca9.0 mM).TTX, lidocainc, and quinidine reduced the magnitude of the TI in a dose-dependent way. The blocking effect of these agents could be compensated for by a moderate (<50%) reduction of external Na or an elevation of extracellular Ca.It is suggested that the blocking effect of TTX, lidocaine, and quinidine on the TI is due to a reduction of intracellular Na, which causes a decay of intracellular Ca via the Na–Ca exchange mechanism.     

Effects of Mibefradil, a T-Type Calcium Current Antagonist, on Electrophysiology of Purkinje Fibers that Survived in the Infarcted Canine Heart

INTRODUCTION: We studied the effects of mibefradil (MIB), a nondihydropyridine T-type Ca2+ channel antagonist, on T- and L-type Ca2+ (I(CaT), I(CaL)) currents in Purkinje myocytes dispersed from the subendocardium of the left ventricle of normal (NZPC) and 48-hour infarcted (IZPC) hearts. METHODS AND RESULTS: Currents were recorded with Cs+- and EGTA-rich pipettes and in Na+-K+-free external solutions to eliminate overlapping currents. In all cells, I(Ca) was reduced by MIB (0.1 to 10 microM). No change in the time course of decay of peak I(Ca) was noted. Average peak T/L ratio decreased in NZPCs but not IZPCs with 1 microM MIB. Steady-state availability of I(CaL) was altered with 1 microM MIB in both cell types (mean +/- SEM) (V0.5 = -22 +/- 4 mV for NZPC and -25 +/- 5 mV for IZPC before drug; -63 +/- 9 mV for NZPC and -67 +/- 6 mV for IZPC after drug; P < 0.05). For I(CaT), V0.5 (-50 +/- 3 mV for NZPC and -52 +/- 1 mV for IZPC before drug) shifted to -60 +/- 2 mV (NZPC) and -62 +/- 3 mV (IZPC) (P < 0.05) after drug. We also determined the effects of MIB on spontaneously beating Purkinje normal fibers and on depolarized abnormally automatic fibers from the infarcted heart using standard microelectrode techniques. When NZPC and IZPC fibers were superfused with [K+]o = 2.7 mM, MIB 3 microM and 10 microM had no effect on rate or the maximum diastolic potential, but action potential plateau shifted to more negative values, the slope of repolarization phase 3 decreased, and action potential duration increased. CONCLUSION: MIB blocks L- and T-type Ca2+ currents in Purkinje myocytes but lacks an effect on either normal or abnormal automaticity in Purkinje fibers.     

Alternans of action potential duration after abrupt shortening of cycle length: differences between dog Purkinje and ventricular muscle fibers

The purpose of this study was to determine whether the alternans of action potential duration (APD) occurring in Purkinje and ventricular muscle fibers after an abrupt shortening of cycle length can be explained by the two factors controlling the cycle length-dependent APD changes (i.e., restitution and memory effect). Action potentials were recorded simultaneously from dog Purkinje fibers and ventricular muscle fibers using conventional microelectrode techniques. APD change during alternans was dependent on the preceding diastolic interval in the same manner as during restitution in Purkinje fibers but not in ventricular muscle fibers. The course of memory change was not affected by the presence of alternans in either fiber type. In Purkinje fibers, APD alternans was attenuated by a Ca2+ channel blocker, nisoldipine (2 X 10(-6) M), and augmented by a Ca2+ channel agonist, Bay K 8644 (3 X 10(-8) M). These effects were attributed to the changes in the kinetics and the amplitude of restitution. In ventricular muscle fibers, APD alternans was always preceded and accompanied by alternans of action potential shape. Alternans of both action potential shape and APD was suppressed by nisoldipine (2 X 10(-6) M) and attenuated by Bay K 8644 (3 X 10(-8) M). These results show that in Purkinje fibers, APD during alternans can be explained by restitution and memory effect. However, in ventricular muscle fibers, the mechanism of APD alternans is linked to factors controlling action potential shape. These findings are compatible with the hypothesis that APD alternans in Purkinje fibers depends on the differences in the recovery of membrane currents generated by the preceding action potential and in ventricular muscle fibers on the differences in the concentration and/or handling of intracellular calcium.     

The inotropic effects of strophanthidin in Purkinje fibers and the sodium pump

The role of the inhibition of the Na pump in strophanthidin inotropy was studied in canine Purkinje fibers by correlating changes in contractile force with changes in maximum diastolic potential caused by conditions that enhance the electrogenic extrusion of Na. It was found that a brief exposure to a zero-K or to a zero-K, zero-Ca solution (but not to a zero-Ca solution) is followed by an increase in maximum diastolic potential. This hyperpolarization is reduced if NaCl is substituted by LiCl and in the presence of tetrodotoxin. In quiescent fibers exposed to tetrodotoxin, the hyperpolarization is abolished. A low concentration of strophanthidin (5 X 10(-8)M) increases contractile force but does not modify the hyperpolarization. Larger strophanthidin concentrations (5 X 10(-7)M to 10(-6)M) increase and then decrease contractile force and reduce or abolish the hyperpolarization. Metabolic inhibitors also reduce the hyperpolarization. We conclude that the positive inotropic effect of a low (therapeutic) concentration of strophanthidin is due to a mechanism other than Na pump inhibition.     

Local anesthetic and negative chronotropic effects of diltiazem on canine cardiac Purkinje fibers

The electrophysiologic effects of diltiazem, a drug with antianginal, antihypertensive, and antiarrhythmic actions, were studied on transmembrane action potentials recorded from isolated canine cardiac tissues by means of standard microelectrode techniques. Recordings were made from normal canine cardiac Purkinje fibers in major false tendons, from normal Purkinje fibers partially depolarized by treatment with Tyrode's solution containing 7 mmol/L potassium chloride, from subendocardial Purkinje fibers of the left ventricle of normal hearts, and from subendocardial ventricular muscle preparations. Diltiazem, 1 and 2 mumol/L, exerted local anesthetic effects and decreased the action potential plateau duration in normal Purkinje fibers. In contrast, diltiazem, 1 mumol/L, did not affect the action potentials of potassium-depolarized Purkinje fibers, subendocardial Purkinje fibers, or ventricular muscle cells. Diltiazem, 1 and 2 mumol/L, did not decrease normal or high potential automaticity or catecholamine-enhanced high potential automaticity in canine Purkinje fibers with maximum diastolic potentials greater than -80 mV. In contrast, diltiazem, 1 mumol/L, rapidly terminated low potential automaticity in barium-treated Purkinje fibers with maximum diastolic potentials of -40 to -60 mV. The local anesthetic effects of diltiazem, as well as the effect on low potential automaticity, can explain the antiarrhythmic effects of the drug.     

Role of calcium in the inotropic effects of caffeine in cardiac Purkinje fibers

The inotropic effects of caffeine (1-3 mM) were studied in the presence and absence of strophanthidin in canine Purkinje fibers perfused in vitro. Caffeine (1 mM) induced a similar initial increase in contractile force in different calcium solutions (+22, +23 and +24% in 0.54, 2.7 and 8.1 mM calcium, respectively) and when propranolol (3.4 X 10(-6) M) was present. Also, caffeine increased contractile force in high potassium (16.2 mM) at a time when the slow action potentials were unaltered. After the increase, 1 mM caffeine decreased force by about 50%, and the decrease was larger when caffeine (3 mM) or [Ca]0 (8.1 mM) was higher. In the presence of caffeine, strophanthidin (3 X 10(-7)-1 X 10(-6) M) increased force (+302%) if caffeine (0.3 mM) and Ca concentrations (0.54 mM) were low. If either caffeine or calcium was increased, strophanthidin had no effect or decreased force. Strophanthidin alone increased force and then decreased it; caffeine increased force in the first stage and decreased it during the second stage. The positive inotropic effect (+224%) of low sodium (78.4 mM versus 149.4 mM in Tyrode solution) was also abolished by caffeine (-24%). In ventricular muscle fibers, caffeine increased force more (+59%) and reduced force less in the presence of strophanthidin. The results indicate that caffeine increases force initially by releasing calcium from intracellular stores. The caffeine-induced decline in force is modulated by calcium in that it is exaggerated by agents or procedures which increase cellular calcium (strophanthidin, high calcium, low sodium solutions) and is reversed in a low calcium solution.     

Strontium induces oscillatory potentials in sheep cardiac Purkinje fibers

The induction of strontium overload and its electromechanical manifestations, the factors influencing and the mechanism underlying Sr overload were studied in Purkinje fibers perfused in vitro. Strontium: (1) can induce an oscillatory potential (Vos) and repetitive spontaneous activity at low concentrations (1.35-2.7 mM); (2) at high concentrations (5.4-10.8 mM) less frequently causes a Vos but during recovery in Tyrode solution Vos appears as Sr overload recedes; (3) decreases the maximum diastolic potential by inducing a prolonged depolarization (Vex) which subsides slowly during an interruption of drive; (4) induces a larger Vex after procedures that increase Sr loading (fast driving rates, higher [Sr]o or longer action potentials); (5) does not induce Vos and Vex when the slow channel is blocked; (6) exaggerates Vex (but not Vos) in calcium overloaded fibers; (7) exchanges with Na since in low [Na]o the twitch amplitude increases; (8) is removed from the cell at the resting potential since after a period of quiescence the first resumed twitch decreases as a function of the preceding pause duration; (9) needs Na as charge carrier since the slope of diastolic depolarization decreases in low [Na]o. Thus, Sr causes overload even at low concentrations and induces an oscillatory potential and the prolonged depolarization Vex, whose mechanism appears to be an electrogenic Sr extrusion through Na-Sr exchange.     

Sarcolemma, sarcoplasmic reticulum, and sarcomeres as limiting factors in force production in rat heart

Inotropic interventions were compared with respect to their maximum effect on force of contraction in rat myocardium to identify limiting steps in calcium handling. Peak force, sarcomere length, and action potentials were measured in thin ventricular trabeculae. Relevant control conditions were stimulation frequency, 0.2 Hz; [Ca2+]o, 1 mM; [K+]o, 5 mM; [Na+]o, 150 mM. The inotropic interventions and results were as follows. 1) The interventions of high [Ca2+]o, low [Na+]o, high [K+]o, addition of tetraethylammonium chloride, or postextrasystolic potentiation resulted in approximately the same (within 5%) maximum force (Fmax). Above the respective optimum doses, force declined and aftercontractions were often observed. Combinations of the different interventions never enhanced force to above Fmax. This suggests that Fmax is determined by a maximum level of Ca2+ in the sarcoplasmic reticulum, above which spontaneous release occurs. 2) Sr2+ (10 mM) caused an increase of force to 1.3 X Fmax and lengthening of contraction and action potentials. The force-sarcomere length relation was, then, similar to that in skinned fibers at maximum activation. Hence, 1.3 X Fmax reflects saturation of the sarcomeres. We postulate that a large influx of Sr2+ during the long action potential can circumvent the reticulum and activate the sarcomeres directly. When the reticulum was blocked with ryanodine, maximum force of tetanic contractions was about 1.1 X Fmax. This result supports the above conclusions. 3) Isoproterenol increased force to a maximum that was 20% below Fmax and shortened the contraction. This may be due to a decreased sensitivity of the sarcomeres to Ca2+ or to stimulation of the Ca2+ pump in the reticulum, that is, an increasing fraction of the released Ca2+ is sequestered before it can activate the sarcomeres. Thus, three factors that limit force production were identified, depending on the inotropic stimulus.     

Effects of forskolin on intracellular sodium activity in resting and stimulated cardiac Purkinje fibers from sheep

In the present investigation, the effects of forskolin on intracellular sodium activity were studied in quiescent and electrically stimulated cardiac Purkinje fibers from sheep using Na+-sensitive microelectrodes. Also assessed, were the effects of this promoter of cytosolic cAMP production on resting membrane potential, action potential and twitch tension. In the quiescent fibers, forskolin (12 microM) caused intracellular sodium activity to decrease in the face of cellular depolarization. This cellular depolarization was occasionally accompanied by spontaneous firing of action potentials. In the stimulated fibers, forskolin (10 microM) also caused intracellular sodium activity to decrease. Moreover, it caused a marked acceleration of phase 4 pacemaker depolarization, an elevation of the plateau of the action potential and an increase in twitch tension. When the Na+ pump was inhibited by either strophanthidin (1 microM) or by 0 mM extracellular K+, forskolin had no effect on intracellular sodium activity. In summary, the results of the present study indicate that forskolin, presumably by increasing intracellular cAMP, causes the following to occur in cardiac Purkinje fibers from sheep: (a) a decrease in intracellular sodium activity when the Na+ pump is functioning normally; (b) a promotion of membrane depolarization in quiescent fibers; (c) an increase in the steepness of the pacemaker potential in electrically stimulated fibers, and (d) an increase in the force of contraction. Therefore, forskolin will be a useful tool for investigating the role of cAMP in physiological function of cardiac cells.     

Early afterdepolarizations and triggered activity in rabbit cardiac Purkinje fibers recovering from ischemic-like conditions. Role of acidosis

BACKGROUND. The mechanisms underlying repetitive activity during reperfusion of ischemic myocardium are thought to include triggered responses elicited at short pacing cycle lengths. The potential to generate repetitive responses at longer pacing cycle lengths under similar conditions, however, has not been explored. Thus, the present study examined the role of cycle length on the cellular electrical changes produced during recovery from ischemic-like conditions and identified the major component precipitating nondriven, repetitive activity. METHODS AND RESULTS. Transmembrane potentials were recorded in vitro from isolated rabbit Purkinje fibers exposed to hypoxia (defined as PO2 less than 30 mm Hg, high [K+]o, and zero glucose) plus lactic acidosis (pH 6.7) for 45 minutes and during recovery in normal Tyrode's solution (pH 7.4). Compared with control, action potential duration (90% repolarization) during recovery increased transiently by 40.9 +/- 11.8 and 241.0 +/- 51.1 msec at respective basic cycle lengths of 1,000 and 3,000 msec (both p less than 0.005). In 81% of preparations, action potential prolongation was accompanied by early afterdepolarizations and triggered activity generated from low (positive to -40 mV) or high (negative to -40 mV) membrane potentials. In 62% of experiments, brief periods of abnormal automaticity also occurred. Triggered responses were 1) unaffected by 1 microM ryanodine, 2) abolished by pacing at short basic cycle lengths or by exposing tissues to 2.5 micrograms/ml lidocaine, and 3) more easily induced at long basic cycle lengths or by superfusing 2.5 micrograms/ml quinidine. When tissues were conditioned with hypoxia alone (pH 7.4), action potential prolongation on recovery was comparatively small, and nondriven responses did not develop. Conversely, addition of 10-20 microM amiloride to the hypoxic, acidic test solution augmented recovery-induced action potential prolongation. CONCLUSIONS. We conclude that acidosis, as a component of ischemia, plus slow pacing frequencies may mediate the genesis of early afterdepolarizations and triggered activity in Purkinje fibers on recovery, long after extracellular pH has been restored to normal. These data may have clinical relevance to the mechanisms of reperfusion arrhythmias in the intact human heart.     

Mechanisms by which discharge modulates diastolic depolarization in sheep and dog Purkinje fibers

For reasons unknown, a fast drive is prone to induce overdrive excitation in sheep Purkinje fibers under conditions that still induce overdrive suppression in dog Purkinje fibers. Our aim was to study by means of a microelectrode technique diastolic depolarization (DD) and its changes with overdrive in sheep and dog Purkinje fibers perfused in vitro under identical conditions. The major results are: (a) At a slow rate, diastolic depolarization is much faster and larger in sheep than in dog Purkinje fibers. (b) Faster rates increase DD slope and amplitude in sheep and decrease them in dog Purkinje fibers. (c) DD slope and amplitude increase in sheep and decrease in dog if the same number of action potentials are separated by a shorter diastole. (d) The change in DD slope and amplitude induced by a fast drive persists after a subsequent slow drive of approximately 20 s. (e) The fastest drives can induce an oscillatory potential superimposed on early DD in sheep. (f) In both species, high [Ca(2+)](o) increases and low [Ca(2+)](o) decreases DD slope and amplitude. (g) Neither high nor low [Ca(2+)](o) change the DD rate-dependence patterns peculiar to either species. (h) DD amplitude in dog in high [Ca(2+)](o) is still smaller than that in sheep in Tyrode solution. (i) Caffeine prevents the steepening of early DD by drive, but not the subsequent increase which can lead to overdrive excitation in both species. (j) TTX decreases DD slope and amplitude in both species. (k) Cs(+) markedly reduces DD slope and amplitude and more so at faster rates, especially in the sheep. We conclude that the differences in diastolic depolarization and the different behavior of DD with overdrive in the two species account for the propensity of sheep Purkinje fibers to develop overdrive excitation and for that of dog Purkinje fibers to develop overdrive suppression.     

Effects of nifedipine on electrical activity of cardiac cells

The effects of nifedipine were studied on action potentials recorded from (1) normal canine Purkinje fibers, (2) Purkinje fibers exposed to barium chloride (0.1 to 0.25 mM), which reduces their membrane potential and produces enhanced pacemaker activity (abnormal automaticity), and (3) subendocardial Purkinje fibers underlying infarct zones at 24 hours after ligation of the left anterior descending coronary artery. In concentrations of 0.1 to 0.3 mg/liter, nifedipine did not significantly alter the resting transmembrane potential, action potential amplitude or maximal upstroke velocity of normal Purkinje fibers. The action potential duration (to 50 and 100 percent repolarization) was significantly shortened. These concentrations of nifedipine did not suppress the slope of phase 4 depolarization of automaticity in Purkinje fibers with maximal diastolic potentials between ?85 and ?95 mV.Abnormal automaticity in fibers exposed to barium chloride (Ba⁺⁺), having maximal diastolic potentials of ?40 and ?60 mV, was promptly abolished by nifedipine in concentrations of 20 to 200 μg/liter (n = 12). In slightly higher concentrations (0.2 to 0.6 mg/liter), nifedipine slowed or stopped abnormal automaticity in infarct zone Purkinje fibers (n = 3). In two subendocardial Purkinje fiber preparations, salvos of spontaneous impulses were observed that were terminated with delayed afterdepolarizations; nifedipine (0.1 to 0.2 mg/liter) quickly abolished these impulses.It is concluded that nifedipine can suppress abnormal automatic activity in canine cardiac Purkinje fibers in concentrations of 20 to 200 μg/liter, concentrations that will only shorten the plateau of normal action potentials. Nifedipine may prove to be a useful antiarrhythmic drug.