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

Digitalis glycosides: a discussion of the similarities and differences in actions and existing controversies

The writing of this review was initiated to answer the question of whether differences in the actions of the various digitalis glycosides exist and to discuss current controversies in the research area of the digitalis glycosides. Data obtained in our laboratory indicated that the effect of digoxin on postganglionic cardiac sympathetic neural discharge in the minute prior to the occurrence of arrhythmia differed from that of ouabain. This raised the question of whether data published in other laboratories would support the contention that differences in glycosides do exist. To answer this question, a review of the literature was begun. Our survey of these studies are cited in the tables of this review. These tables summarize the actions of glycosides in vivo and in vitro in different animal models. The reader should bear in mind that the data included within the tables do not represent an inclusive summary of all studies in the literature. For detailed review articles, the reader is referred to the following references: Gillis et al; Gillis and Quest; Roberts et al; Lathers and Roberts; Farah and Alousi; Benthe; Levitt et al; Smith and Haber; Somberg; Lee and Klaus; Mason; Schwartz. Furthermore the summary of the results for each particular study cited in the table may not, in all cases, include each finding of the published data. Nevertheless, the tables do provide a summary of data obtained in various species with different glycosides in several different areas of research, and as such, represent an abridged compendium for the research working in the field of digitalis glycosides. This review has been organized firstly to consider glycoside-induced alterations in the autonomic nervous system and, secondly, to examine their direct actions on the heart.

Electrophysiologic changes induced by external transthoracic countershock

The purpose of this study was to characterize the electrophysiological effects of suprathreshold transthoracic shocks. Dogs were anesthetized and two consecutive shocks (10 joules/gk) were applied one minute apart. Ventricular extrasystoles were observed in six of the 11 dogs tested, A-V block in three, ventricular tachycardia in three, and asystole in one. Forty eight hours later the hearts were removed. Conventional electrophysiological techniques revealed the following: 1) injured cells had loss of resting potential, triangularization of the action potential and depressed upstrokes; 2) the functional refractory period was shortened, with an increase in time dependence; 3) action potential alternans; 4) discrete areas of marked depression of conduction; 5) re-excitation with premature stimulation and 6) histological evidence of sub-epicardial injury. These studies suggest that the application of suprathreshold shocks may induce electrophysiological changes potentially inducing tachyarrhythmias.

Preferential sensitivity of the left canine purkinje system to cardiac glycosides

Previous studies have shown that the toxic effects of cardiac glycosides are not manifested uniformly throughout the myocardium. The purpose of our study was to determine whether cardiac glycosides exert different effects on the right vs. left peripheral Purkinje systems and to ascertain mechanisms involved. Control in vitro measurements of paired right and left canine Purkinje fibers showed higher spontaneous rates in left (24.2 +/- 1.75 beats/min) than in right (11.6 +/- 1.55 beats/min, P less than 0.01, n = 81) Purkinje fiber bundles. Following overdrive stimulation, left Purkinje fiber bundles also showed earlier escape beats. After ouabain exposure (2 X 10-7 M), left Purkinje fiber bundles showed earlier signs of toxicity in 20 of 28 experiments, as determined by changes in the maximum diastolic potential, the degree of diastolic depolarization, spontaneous escape intervals, and the magnitude of delayed after-depolarizations. The enhanced sensitivity of left Purkinje fiber bundles was independent of the extracellular potassium concentration and glycoside polarity, and was also observed in situ. We conclude that distal Purkinje fibers are functionally dissimilar and that the left Purkinje system shows greater sensitivity to cardiac glycosides than the right Purkinje system. These data also support the observation that digitalis-induced dysrhythmias arise in the left ventricle.

The antiarrhythmic effect of potassium and rubidium in strophanthidin toxicity

The effect of potassium and rubidium on the electrical and mechanical activity of canine Purkinje fibers were studied in vitro in the presence and absence of strophanthidin. High (5.4mM) K or 2.7 Rb decreased the force of contraction. In the presence of these ions, strophanthidin increased the force of contraction as usual but the onset of arrhythmias was delayed. During the toxic stage of strophanthidin, high K or Rb increased the force of contraction, abolished the arrhythias and improved markedly the action potential. In the presence of calcium overload induced by exposure to a K-poor or Na-free solution, K and Rb induced an increase in force of contraction. And in ventricular muscle these ions relaxed the contracture induced by strophanthidin. It is concluded that K and Rb (in addition to other mechanisms) exert an antiarrhythmic action by increasing potassium conductance and by reducing the calcium overload induced by strophanthidin.

Reduction of the equilibrium binding of cardiac glycosides and related compounds to Na+,K+-ATPase as a possible mechanism for the potassium-induced reversal of their toxicity

The influence of potassium ions on the equilibrium state of the binding of cardiac glycosides and their derivatives to partially purified dog heart and rat brain enzyme preparations was studied in vitro. The addition of potassium to the incubation mixture containing enzyme preparation, 3H-ouabain, Na+, Mg2+ and ATP, at the time when the binding reaction is close to equilibrium, caused an immediate reduction of the bound drug concentration; the concentration apparently shifting toward a lower equilibrium state. The degree of the potassium-induced reduction in bound drug concentration was dependent on the potassium concentration and on the chemical structure of the compound. The binding of aglycones, pentacetyl-gitoxin and cassaine was affected to a greater extent than that of the glycosides. These data suggest that one of the mechanisms by which potassium antagonizes the toxic actions of digitalis on the heart is to reduce the drug binding to cardiac Na+,K+-ATPase.

Is pump stimulation associated with positive inotropy of the heart?

A purified sodium and potassium dependent adenosinetriphosphatase isolated from cat heart was not stimulated by any concentration of ouabain that produced positive inotropy of cat papilliary muscle. Only inhibition of enzyme activity was observed. Concentrations of ouabain used ranged from 3.3 x 10(-10) molar to 5 x 10(-7) molar and produced an increased force of contraction without any evidence of toxicity. The results are inconsistent with a concept that stimulation of sodium pump activity is associated with positive inotropy.