[Origin of Luciani periods during the effects of acetylcholine and eserine on the automation of Purkinje fibers]

Experiments were conducted on the isolated rabbit hearts with a complete atrio-ventricular block. It was shown that under the effect of acetylcholine (1.10(-7)-2.10(-6)g/ml) or eserine (1.10(-7)-2.10(-6)g/ml) there can occur Luciani periods which disappeared under the action of atropine (1.10(-6)g/ml). With the aid of microelectrodes it was revealed that Luciani periods appearing under the effect of acetylcholine or eserine were caused by periodic depression of automation of the Purkinje's fibers. This automation depression is regarded to be the result of a relative insufficiency of the active ionic transport processes.

Manganese amd electrogenic phenomena in canine Purkinje fibers

Studies were performed on canine cardiac Purkinje fibers to evaluate the effects of manganese on membrane electrogenesis. The results indicate that manganese has a calciumlike effect on the excitatory sodium current and inhibitory effects on potassium conductance and slow inward current. The calciumlike effect of manganese on sodium current was reflected through a leftward (toward less negative potentials) and downward shift in the curve relating maximum upstroke velocity to membrane potential. The inhibitory action of manganese on potassium conductance was suggested by the following observations. (1) Manganese caused an initial increase in action potential duration largely due to a lengthening of the plateau and decreases in the rates of phase 3 and terminal repolarization. (2) Manganese increased the rate of diastolic depolarization. (3) Manganes blocked the initial fall in maximum diastolic potential accompanying rapid stimulation. (4) Manganese in high concentrations caused generalized depolarization which was reversed by rapid stimulation and by increased extracellular potassium concentrations. The action of manganese to block slow inward current was indicated by the eventual shortening of the plateau and by the elimination of responses initiated from low levels of membrane potential (less than minus 55 mv). In addition to these effects, manganese also reduced membrane excitability, eliminated arrhythmic beats occurring during low-frequency electrical stimulation, and caused membrane hyperpolarization which was blocked by tetrodotoxin.

Electrophysiology of atropine

The effect of atropine on the refractory periods of the human atrium, A-V node, and His-Purkinje system was studied. The A-V node in man appears more sensitive to atropine than the atria as evidenced by a reduction in the effective and functional refractory periods of the A-V node without alteration of atrial conduction. Although atropine does not directly alter the refractory periods of the His-Purkinje system, atropine-induced facilitation of A-V nodal conduction results in block of premature atrial impulses in the His-Purkinje system, demonstrating an indirect effect of atropine on His-Purkinje conduction. Rarely, atropine may precipitate re-entrant atrial tachycardia (one patient) through facilitation of A-V nodal conduction.

The effect of changes in rate and rhythm on supernormal excitability in the isolated Purkinje system of the dog. A possible role in re-entrant arrhythmias

Microelectrode techniques were used to investigate the effects of rate and rhythm changes upon the period of supernormal excitability during repolarization in isolated canine right and left bundle branch and Purkinje fibers. The same microelectrode was used for both intracellular stimulation and recording. Excitability curves (strength-interval curves) were measured as the minimum depolarizing current required to re-excite the fiber during repolarization of a conducted action potential. During the supernormal period it required an average of 17.0 ± 4.6 SD % less than diastolic current to re-excite the fibers throughout the left and right bundle branch-Purkinje system. The supernormal period of excitability in the bundle branch-Purkinje system was voltage dependent, reaching minimum current requirements at 74.3 ± 5.8 SD mV. Excitability curves of conducted action potentials were determined at basic cycle lengths ranging from 1000 to 300 msec and following single and multiple premature beats. The decrease in action potential durations associated with shorter cycle lengths was not accompanied by a corresponding shortening of the supernormal period of excitability. Therefore, at shorter cycle lengths the supernormal period encompassed a greater proportion of the total action potential; in some cases as much as 50% of the total action potential duration exhibited a period of supernormal excitability. The supernormal period of excitability could be eliminated by perfusing the tissues in elevated potassium (5.0 and 7.5 mM) Tyrode's solution. The possible implications for the generation of re-entrant arrhythmias are discussed.

The clinical pharmacology of lidocaine as an antiarrhythymic drug

This article reviews current knowledge about lidocaine, with reference to its chemistry, metabolism, electrophysiology, hemodynamic effects, antiarrhythmic uses, pharmacokinetics, and side effects. The critical importance of blood levels and their relation to lidocaine's antiarrhythmic and toxic effects is noted, with special emphasis given to patients with compromised clearance due to heart failure. On the basis of this information, we present our current approach to the clinical use of lidocaine in the treatment of ventricular arrhythmias, with particular reference to patients with acute myocardial infarction.

Supernormal excitability and conduction in the His-Purkinje system of the dog

The electrophysiological characteristics of the period of supernormal excitability and supernormal conduction were investigated in the isolated canine His-Purkinje system. Strength-interval curves were determined as the minimum transmembrane current required to bring the impaled fiber to threshold potential following a conducted action potential. During the period of supernormal excitability, 17.0 ± 4.6% (SD) less current than that required during diastole was needed to reexcite fibers throughout the left and right bundle branch-Purkinje system. A period of supernormal excitability was not found in the His bundle proximal to its pseudobifurcation or in ventricular muscle. The period of supernormal excitability was voltage dependent in the bundle branch-Purkinje system; it began during phase 3 at full repolarization (88.8 ± 5.6 [SD] mv) and reached minimum current requirements at about 74.3 ± 5.8 mv. Action potentials evoked during this period were conducted faster than they were during diastole. The maximum rates of depolarization of these supernormally conducted action potentials were not greatly depressed compared with control rates. A period of supernormal conduction was not observed in the His bundle. When the external potassium concentration was increased from 2.7 mM to 5.0 um or 7.5 mM, both the supernormal period of excitability and the period of supernormal conduction were eliminated in Purkinje fibers.

Localization of beta adrenergic receptors, and effects of noradrenaline and cyclic nucleotides on action potentials, ionic currents and tension in mammalian cardiac muscle

1. Isoprenaline and noradrenaline were applied iontophoretically to cardiac Purkinje fibres. Intracellular application of the drugs had no effect, while extracellular application of the same amounts of charge caused acceleration of pace-maker activity and a shift of the plateau level of the action potential. These results indicate that beta-adrenergic receptors are located at the outside of the cardiac cell membrane.2. A systematic comparison of the effects of cyclic AMP derivatives and noradrenaline on action potentials and isometric tension of ventricular myocardial preparations showed that the nucleotides and the catecholamine increase the plateau height and the duration of the action potential and also increase tension. However, there are quantitative differences in the action of these drugs.3. Cyclic AMP derivatives and noradrenaline increase the slow inward current, I(Ca), in ventricular myocardial preparations. Voltage clamp analysis of I(Ca) showed that the kinetic parameters of this membrane current are not affected by these drugs. However, the membrane conductance to Ca ions is greatly increased by noradrenaline and to a smaller extent by dibutyryl cyclic AMP.4. Concentration-response relations of the membrane effects of noradrenaline on plateau height of the action potential and on I(Ca) could be fitted by the same theoretical log concentration-response curve. The Hill plot of this concentration-response curve had a slope of 2. The half maximal response occurred at 5 x 10(-7)M.5. The results are compared with other membrane effects of catecholamines and cyclic nucleotides in cardiac muscle. The effects on I(Ca) are related to the positive inotropic effect of the drugs.

Mode of action of chronotropic agents in cardiac Purkinje fibers. Does epinephrine act by directly modifying the external surface charge?

Hauswirth et al. (1968) proposed that epinephrine acts on i(KK2) by adding its own positive charge to the external membrane surface near the i(KK2) channel. This hypothesis was tested by using noncationic compounds, theophylline and R07-2956, which mimicked epinephrine's effects on pacemaker activity and on i(KK2). In maximally effective doses, theophylline or R07-2956 occluded the effect of epinephrine, indicating a shared final common mechanism. Since theophylline and R07-2956 are noncationic at pH 7.4, the common mechanism cannot be a direct change in external surface charge. On the contrary, epinephrine does not interfere with the voltage shift produced by La(+++), which is thought to modify the external surface charge. The results argue against the original hypothesis but leave open the possibility that an alteration in internal surface charge generates the observed voltage shift. The potency of theophylline and R07-2956 as phosphodiesterase inhibitors suggests that the final common mechanism begins with the elevation of intracellular cyclic AMP, leading to a saturable process which limits the voltage shift's magnitude. This hypothesis is used to generate dose-response curves describing the combined effects of epinephrine and theophylline, and these are compared with experimental data.