The effects of tension on acetylstrophanthidin-induced transient depolarizations and aftercontractions in canine myocardial and Purkinje tissues.

Transmembrane potentials and contractile activity were recorded from isolated canine Purkinje and ventricular muscle preparations exposed to acetylstrophantidin (AS) and subjected to a resting tension equal to 80% of that required to elicit peak developed tension. AS induced transient depolarizations (TD's) accompanied by aftercontractions in Purkinje tissue. AS also induced aftercontractions in 11 muscle preparations, and in seven of these the mechanical events were associated with TD's. Aftercontractions and TD's in both and the coupling intervals were directly related to the preceding basic cycle length (BCL). The amplitudes of aftercontractions and TD's reached a maximum at a coupling interval of 600-700 msec. Tension increased the amplitude of TD's in Purkinje tissue and promoted the appearance of TD's in muscle. TD's in muscle occasionally reached threshold in the presence of tension. The results of this study suggest that stretch or increased resting tension may promote the types of cardiac arrhythmias that are causally related to digitalis-induced TD's. The results are compatible with the hypothesis that TD's are caused either by a transmembrane influx of calcium or by an internal release of calcium ions.     

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.     

Adrenergic modulation of the transient outward current in isolated canine Purkinje cells

Single canine Purkinje cells were voltage clamped under Ca2+-free conditions using the patch pipette. Depolarizing pulses from a holding potential of -42 mV induced a time-dependent rapidly activating-slowly inactivating outward current, which was identified as the transient outward current. The current showed two exponential time constants of inactivation (48,352 msec at +58 mV and 53,325 msec at +78 mV). Norepinephrine in concentrations exceeding 10(-9) M modified the inactivation kinetics of this current without affecting the activation kinetics. The half-maximum dose for norepinephrine effect was 1.9 X 10(-8) M, and the effect was saturated at 10(-6) M. Norepinephrine reduced the amplitude of the fast time constant component of inactivation, while increasing the amplitude of the slow component, without changing their time constants. Norepinephrine also increased the amplitude of a time-independent current component. The beta-antagonist sotalol blocked the norepinephrine effect on the transient outward current. On the other hand, both activation of adenyl cyclase by forskolin and increase of intracellular cAMP concentration produced the same effect as exposure to norepinephrine. These results suggest a role for neurotransmitter regulation of the transient outward current in cardiac cells, perhaps by channel phosphorylation.     

Overdrive excitation and cellular calcium load in canine cardiac Purkinje fibers

The induction of spontaneous activity by drive ("overdrive excitation") was studied by means of a microelectrode technique in canine cardiac Purkinje fibers exposed to an enhanced calcium load. The following results were obtained: 1) in quiescent fibers, a single action potential is followed by a prolonged transitory depolarization ("slow afterdepolarization") that may initiate a slow spontaneous rhythm; 2) during short drives, the maximum diastolic potential (Emax) gradually decreases and diastolic depolarization becomes steeper due to a superimposed oscillatory potential of progressively greater amplitude; 3) after the drive, the oscillatory potential either initiates a fast repetitive activity or is followed by a slow repolarization to the original resting level; 4) the cessation of induced activity is associated with an increase in Emax; 5) during longer and faster drives, Emax increases and the oscillatory potential becomes smaller, peaks sooner and may fail to excite; 6) repetitive activity may also be induced at a depolarized level. We conclude that overdrive excitation involves an increased cellular calcium, can occur at normal or depolarized levels, and is induced by an oscillatory potential superimposed on a slow afterdepolarization. It is most easily initiated by a short and fast drive because at that time the oscillatory potential and slow afterdepolarization are optimally combined to induce activity.     

Use of calcium ionophores to determine the effects of intracellular calcium on the action potential of canine cardiac Purkinje fibers.

The effects of calcium ionophores X-537A, A23187, and PR-47 on the action potential of canine cardiac Purkinje fibers were studied using standard microelectrode techniques. The ionophores hyperpolarize the membrane potential, eliminate the notch on the action potential plateau, decrease action potential duration, decrease plateau amplitude and duration, decrease spontaneous diastolic depolarization and excitability, and decrease the rate effect on action potential duration. These effects were shown to be independent of catecholamines, membrane-bound calcium, and the patency of the slow inward current channel. The effects of the ionophores depended on the presence of calcium in the external solution. A plausible interpretation of the data is that an ionophore-mediated increase in intracellular calcium shifts the voltage dependence of transmembrane potassium currents to more negative levels.     

Effect of lidocaine on the early inward transient current in sheep cardiac Purkinje fibers.

We used two experimental techniques to study the effect of lidocaine hydrochloride on the early inward transient (sodium) current as it is reflected by the maximum rate of change of action potential phase 0 (Vmax). We assessed the effect of lidocaine on Vmax as Purkinje fibers were slowly depolarized by increasing the extracellular potassium concentration from 4.0 to 16.0 mM; these voltage-dependent effects were compared with lidocaine's effect on membrane responsiveness (which measures both the time and the voltage dependence of Vmax). We also used a voltage clamp technique to establish the effect of lidocaine on the voltage dependence of Vmax by measuring Vmax 800-1000 msec after transmembrane voltage (Vm) had been changed in small steps. We studied the effect of lidocaine on the time course of early inward transient current reactivation by depolarizing the membrane to -25 +/- 5 mv for 100 msec to inactivate this current, clamping Vm to a repolarized test voltage for various periods, and then measuring phase 0 Vmax of action potentials elicited immediately after termination of the voltage clamp. We showed that lidocaine at 5 mg/liter, but not a 1 mg/liter, shifted the steady-state Vmax- Vm relationship to a more negative position on its voltage axis by about 5 mv and markedly slowed the reactivation of the measure early inward transient current.     

Effects of norepinephine, calcium, and rate of discharge on 42K movements in canine cardiac Purkinje fibers

We studied the effect of norepinephrine, calcium concentration, and rate of discharge in the presence of different [Ca2+]0 on radioactive potassium movements in cardiac Purkinje fibers. The following results were obtained: (1) norepinephrine increases potassium uptake in quiescent fibers and in fibers driven at constant rate, but more in the latter; (2) norepinephrine also increases potassium uptake in quiescent fibers depolarized at the plateau; (3) increasing [Ca2+]0 increases potassium uptake in fibers driven at constant rate; (4) increasing [Ca2+]0 may decrease K+ uptake in quiescent fibers; (5) increasing [Ca2+]0 decreases the rate of loss of tissue radioactivity in quiescent fibers and increases it in a driven fiber; (6) increasing the driving rate increases potassium uptake in low and high [Ca2+]0; (7) high [Ca2+]0 increases K+ uptake, especially at low rates; (8) norepinephrine is less effective in increasing K+ uptake in the presence of a high [Ca2+]0. We conclude that: (a) norepinephrine increases potassium uptake by different mechanisms; (b) calcium affects potassium movements when it is allowed to enter the cell, presumably by affecting potassium conductance; (c) the effect of an increased rate of discharge on K+ uptake may involve stimulation of active K+ uptake and may include a calcium-dependent component which is largest at high [Ca2+]0 and a slow rate of drive; (d) simultaneous application of two interventions results in a summation that is smallest when one of the mechanisms is already substantially activated.     

Comparison of the effects of calcium channel blockers and antiarrhythmic drugs on digitalis-induced oscillatory afterpotentials on canine Purkinje fiber

We studied the effects of Ca channel blockers and 3 antiarrhythmic drugs on the digitalis-induced oscillatory afterpotential (OAP). The OAP was observed in Purkinje fibers stimulated by pulse trains, with cycle lengths ranging from 1,000 to 300 msec. The Ca channel blockers verapamil, diltiazem and nifedipine (2.0 x 10(-6) M) depressed OAP significantly and abolished triggered activity. Verapamil was more effective than diltiazem. However, nicardipine and nitrendipine (2.0 x 10(-6) M) had no depressant effects on OAP or triggered activity. The antiarrhythmic drugs procainamide (1.0 x 10(-4) M), mexiletine (1.0 x 10(-5) M) and propranolol (1.0 x 10(-4) M) depressed both OAP and triggered activity. There were no significant differences in the depressant effects between the Ca2+ antagonists (except for nitrendipine and nicardipine) and the other antiarrhythmic drugs. The OAP coupling interval was prolonged by verapamil, diltiazem, propranolol, procainamide and mexiletine. Although the APD50 was shortened by verapamil, diltiazem and nifedipine, it was prolonged by propranolol. It is concluded that nifedipine, verapamil, diltiazem, procainamide, mexiletine and propranolol may be effective for digitalis-related arrhythmia.     

Effect of electrotonic potentials on pacemaker activity of canine Purkinje fibers in relation to parasystole.

Isolated false tendons excised form dog hearts were mounted in a three-chamber tissue bath. Isotonic sucrose solution was perfused in the central chamber to provide a region of depressed conductivity between the fiber segments in chambers 1 and 3, which were perfused with Tyrode's solution. The electrotonic influence of spontaneous or driven responses evoked in chamber 3 during the first half of the spontaneous cycle of a chamber 1 peacemaker delayed the next spontaneous discharge. This effect changed to acceleration when the chamber 3 segment fired during the second half of the spontaneous cycle. We found that subthreshold depolarizing current pulses 50-300 msec applied across the sucrose gap caused similar degrees of delay or acceleration. Furthermore, hyperpolarizing currents caused the reverse pattern. The results indicate that the discharge pattern of a parasystolic focus may be altered by the electrotonic influence of activity in the surrounding tissue. The significance of these findings is considered in relation to the mechanism of production of parasystolic rhythms.     

Ryanodine as a tool to determine the contributions of calcium entry and calcium release to the calcium transient and contraction of cardiac Purkinje fibers

Our object was to assess the relative roles of transsarcolemmal calcium entry and intracellular calcium release in the contraction of cardiac Purkinje fibers. We observed intracellular calcium transients, membrane potential, and contraction in aequorin-injected canine cardiac Purkinje fibers exposed to highly selective pharmacological modifiers of excitation-contraction coupling. To influence selectively the release of calcium from the sarcoplasmic reticulum, we used the plant alkaloid, ryanodine. To influence calcium entry, selectively, we used either the calcium channel antagonist, nitrendipine, or the calcium channel agonist, Bay k 8644. Ryanodine alone (1 microM) reduced both components of the intracellular aequorin luminescence signal (L1 and L2). In three muscles, the luminescence signals were 3% of control in amplitude (standard error of the mean, 2%) without two distinct components and the twitch tension was 2% of control (standard error of the mean, 3%), whereas the action potential was prolonged. The aequorin signal and twitch remaining in ryanodine were abolished by the calcium antagonist nitrendipine (10 microM), which also lowered the action potential plateau, consistent with the block of functional calcium channels. In two experiments, the calcium-channel agonist, Bay k 8644, in the presence of ryanodine, increased the aequorin luminescence and the contraction, but only to a very small fraction of their control values. Sodium withdrawal in potassium-free, ryanodine-containing solution produced large slow increases in calcium and tension, showing that tension could still be produced, that aequorin remained functional, and that sodium/calcium exchange was not inhibited by ryanodine. Caffeine increased intracellular calcium, showing that calcium stores were not depleted.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhomogeneous spread of excitation in canine Purkinje fibers.

Spread of excitation in isolated canine bundle branches was examined by microelectrode technique in order to investigate the possibility of dissociated impulse transmission in normal Purkinje fibers. In all experiments, local excitation of the preparation evoked either by fine bipolar extracellular electrodes or an intracellular microelectrode proceeded much faster along the longitudinal axis of the fibers than along the transverse axis. As a result, the spread of excitation in the vicinity of stimulating site showed significantly inhomogeneous character. The inhomogeneity of excitation spread became more manifest with more eccentric location of the stimulating site in a given preparation. Larger preparation showed greater degree of inhomogeneity. Nevertheless, the inhomogeneous spread of excitation alone appeared unlikely to provide multiply pathways which were functionally dissociated with each other. When premature stimulation was applied, however, different conductivity among fibers within a single false tendon was oberved, suggesting that each conducting element became more independent.     

Microelectrode study of Wenckebach periodicity in canine Purkinje fibers

Electrophysiologic mechanisms responsible for Wenckebach periodicity produced experimentally in a segment of blocked canine Purkinje fibers were investigated with multiple microelectrode recordings. Transmission through the zone of block was electrotonic. The Wenckebach mechanism was related to a progressive decrease in the efficacy of the transmitted electrotonic potential as a stimulus for the regenerative response at the distal block boundary with successive impulses of the cycle. This was manifested in the transmembrane potentials as a progressive decrease in upstroke velocity and a voltage change that caused the electrotonic potential to become progressively removed from the threshold for stimulation. The immediate cause of this phenomenon was a progressive increase in the voltage level (more negative) from which the electrotonic potential originated and a resulting change in voltage-dependent membrane resistance that further attenuated the signal with successive impulses of the cycle.Two main mechanisms appeared to be responsible for these phenomena: (1) a progressive increase in maximal repolarization voltage (more negative) of the transmembrane potential, and (2) a progressive decrease in the diastolic interval that, in the presence of enhanced phase 4 diastolic depolarization, caused the voltage level from which the electrotonic potential originated to become further removed from the stimulation threshold with successive impulses of the cycle.     

Electrophysiological observations on the digitalis-potassium interaction in canine Purkinje fibers.

We studied the effects of elevating potassium concentration on the membrane potential of Purkinje cells exposed to toxic concentrations of acetylstrophanthidin or ouabain. Conventional intracellular microelectrode techniques were employed. Rapid elevation of [K+]o from 2.7 to 5.4 mEq/liter resulted in an initial increase (more negative) in membrane potential of cells demonstrating ouabain-induced phase 4 depolarization. The increase in maximal diastolic potential occurred initially without suppression of phase 4 depolarization. In cells rendered inexcitable by ouabain or acetylstrophanthidin, elevation of [K+]o consistently increased membrane potential and restored excitability. In four experiments automaticity was initiated within 2 minutes after the increase in [K]o. Although automaticity reappeared, as maximal diastolic potential increased, the automatic rate slowed and then pacemaker activity was suppressed. Studies with 3H-ouabain showed that the increase in membrane potential paralleled K+-induced release of 3H-ouabain from Purkinje cells. These studies suggest that elevation of [K+]o reverses digitalis toxic manifestations in canine Purkinje fibers by causing release of cardiac glycosides bound to the membrane. The observed increase in membrane potential of ouabain-treated Purkinje fibers that occurred after [K+]o elevation was considered to be mediated in part by restoration of the Na pump and by electrogenic pumping.     

Effects of alpha-adrenergic agents on transient inward current in rabbit Purkinje fibers

Reported effects of alpha-adrenergic agents on oscillatory afterpotentials (OAP) are conflicting. Therefore, we used standard two microelectrode voltage clamp techniques to determine the effects of phenylephrine and prazosin on the transient inward current (Iti) responsible for OAP. The Iti was induced in isolated rabbit Purkinje fibers either by acetylstrophanthidin or 8 mM Ca. The magnitude of the Iti was determined at various membrane potentials after activation by steps to -10 mV from a holding potential of -80 mV. When the Iti was induced by acetylstrophanthidin, phenylephrine (10(-7) to 10(-5)M) caused inhibition of Iti at all potentials tested. Phenylephrine also caused a significant decrease in net outward current at plateau voltages. Both effects were blocked by prazosin (5 x 10(-7)M) but not by propranolol (5 x 10(-7)M). Prazosin also strongly inhibited the Iti in the absence of phenylephrine. At 5 x 10(-7)M, prazosin did not affect sodium current activated by voltage steps from -80 to -45 mV or maximum upstroke velocity during interruptions of the voltage clamp. When the Iti was induced by 8 mM Ca, the effect of phenylephrine, but not prazosin, reversed so that phenylephrine increased the amplitude of the Iti. Thus, alpha-adrenergic agonists may exert either inhibitory or stimulatory effects on the Iti depending on the mechanism by which the current is induced. Additional effects on OAP amplitude may be induced by changes in action potential duration mediated through actions on net outward current. Prazosin may suppress OAP by an action on the Iti which is independent of alpha-adrenergic or local anaesthetic actions.