Action Of Digitalis Research Articles

A possible molecular mechanism of the action of digitalis: ouabain action on calcium binding to sites associated with a purified sodium-potassium-activated adenosine triphosphatase from kidney.

Calcium binding at 0 degrees C to a purified sheep kidney Na+,K+-ATPase was described by linear Scatchard plots. Binding at saturating free calcium was 65-80 nmol/mg of protein, or 30-40 mol of calcium/mol of enzyme. Aqueous emulsions of lipids extracted from Na+,K+-ATPase yielded dissociation constants and maximum calcium-binding values that were similar to those for native Na+,K+-ATPase. Phospholipase A treatment markedly reduced calcium binding. Pretreatment of native Na+,K+-ATPase with ouabain increased the dissociation constant for calcium binding from 131 +/- 7 to 192 +/- 7 muM without altering maximum calcium binding. Ouabain pretreatment did not affect calcium binding to extracted phospholipids, ouabain-insensitive ATPases, or heat denatured Na+,K+-ATPase, Na+ and K+ (5-20 mM) increased the dissociation constants for calcium, which suggests competition between the monovalent cations and calcium for the binding sites. At higher concentrations of monovalent cations, ouabain increased the apparent affinity of binding sites for calcium. Extrapolation to physiological cation concentrations revealed that the ouabain-induced increase in apparent affinity for calcium may be as much as 2- to 3-fold. These results suggest: (1) calcium binds to phospholipids associated with Na+,K+-ATPase; (2) ouabain interaction with Na+,K+-ATPase induces a perturbation that is transmitted to adjacent phospholipids, altering their affinity for calcium; and (3) at physiological concentrations of Na+ or K+, or both, ouabain interaction with Na+,K+-ATPase may lead to an increased pool of membrane-bound calcium.

Cardiac Physiology for the Clinician

Cardiologists and practical physiology buffs with two or three evenings to spare will be delighted with this treatise that achieves what its preface proclaims: it offers current understanding of cardiac function by experts who have taken pains to simplify language while maintaining the scientific integrity of their subjects and presenting areas of significant controversy. Eight appropriately illustrated and well-referenced chapters cover, in only 238 pages, the remarkable achievements of recent years in cardiac function and control and include the therapeutic implications of topics like arrhythmias and the actions of digitalis. A brief selfassessment test and a good index complete the book. Dr Vassalle has succeeded in arranging the contributions to this book in a logical sequence that will gratify cover-to-cover readers. Yet, each chapter can stand on its own as a minimonograph that will reward spot reading, including quick reference to individual problems. The list of contributors is an indication

NEURAL ACTIONS OF DIGITALIS

NEURAL ACTIONS OF DIGITALIS

Correlation of the electrophysiological and mechanical changes in the dog heart during digitalis administration, and the effect of potassium on it.

The present investigation was designed to study the relationship between mechanical and electrophysiological actions on the heart of digitalis and the effects on it of K administration. Nineteen anesthetized dogs were given acetylstrophanthidin (AS) (30 gamma/Kg, followed by 1 gamma/Kg/min. When ectopic tachycardia occurred, sinus rhythm was restored with an infusion of K (5 mEq/Kg/hour, for 5 min). As the dose of AS was increased beyond 30 approximately 40 gamma/Kg, ventricular rate during electrically induced atrial fibrillation was reduced and PQ interval prolonged, while PP interval, QT interval and QRS duration showed little changes. Ectopic tachycardia occurred at an average dose of 56.9 gamma/Kg of AS. The indices of mechanical properties, dp/dt/IIT, max dp/dt and dp/dt (50), began to increase as soon as AS infusion was started and continued to increase during the administration of AS. The dp/dt (50) and PP interval or ventricular rate during atrial fibrillation did not change in parallel. Inotropic activity was not depressed even during ectopic tachycardia. The administration of K eliminated ectopic tachycardia, but caused no changes in the other electrophysiological and mechanical properties of the digitalized heart. The following conclusions were derived from the experiment. 1) The 2 actions of digitalis, mechanical and electrophysiological ones, are not necessarily related to each other. 2) Digitalis intoxication, represented by ectopic tachycardia, does not mean the depression of the inotropic activity. 3) The administration of K does not affect the inotropic activity of digitalis.

182. Radioreceptor assay for digitalis steroids and its application to studies on the mode of action of digitalis

182. Radioreceptor assay for digitalis steroids and its application to studies on the mode of action of digitalis

Evidence for a dissociation between positive inotropic effect and inhibition of the Na +N +-ATPase by ouabain, cassaine and their alkylating derivatives

An attempt was made to gain further information on the correlation postulated between NaK-ATPase activity and the contractile state of heart muscle. The efficiency of a new alkylating agent, cassic acid mustard (CAM) on contractile force and on the NaK-ATPase was compared with cassaine (C) and ouabain (S). The ED 50 valuess obtained in isolated guinea-pig atria were (× 10 −7 M): C 0.9, CAM 2.7 and S 1.1; the ID 50 values resulting from ATPase inhibition experiments were (× 10 −6 M): C 1.0, CAM 24, and S 0.25. Low concentrations of all three drugs stimulated the ATPase activity up to 20%. The time course of stimulation is characterized by a half-life of 5 min at a concentration of 1 × 10 −0 M ouabain. A considerable discrepancy exists between the time courses of onset of the positive inotropic effect and the development of enzyme inhibition. This is most obvious in the case of ouabain: a drug concentration of 1 × 10 −7 M produces a positive inotropic equilibrium within 30 min, whereas the inhibition of the enzyme still continues after a period of more than 60 min. A positive inotropic effect of 80% can be washed out a half lives of 8.3 min for CAM, 2.8 min for C and 4.4 min for S. In contrast to this ready reversibility of the positive inotropic effect, the enzyme inhibition by S is not reversible and persists for more than 30 min upon dilution of the drug-enzyme complex. The data obtained are consistent with the interpretation, that the Na +K +− ATPase is not the mediator of the positive inotropic action of digitalis and related compounds. It is much more tempting to correlate ATPase inhibition with the toxic effects of these compounds.

Digitalis pharmacology and therapeutics: recent advances.

Abstract The fundamental therapeutic action of digitalis is stimulation of ventricular contractile state. The expression of this increased contractility is dependent on the integrity of cardiocircu...

Digitalis (second of four parts).

Hemodynamic StudiesContractile Response of the Myocardium to Cardiac GlycosidesNineteenth-century investigators generally overlooked what is now considered to be the major action of digitalis, its augmentation of the force of myocardial contraction. They focused instead on alterations in rhythm and rate, effects on the vagus nerve, and an elusive and ill defined concept, "cardiac tone." Clinicians did not recommend the drug for heart failure with slow normal rhythm, but reserved it for patients with rapid heart rates. Gradually, during the early years of the 20th century, a series of clinical observations led to the realization that the beneficial effects . . .

Digitalis intoxication in patients with atrial fibrillation.

The characteristic arrhythmias induced by digitalis glycosides in patients with atrial fibrillation are illustrated, and their mechanisms are described. The two effects of the drug which are most important in the genesis of such disturbances are: production of A-V nodal block and arousal of subsidiary pacemakers. These properties account for slow ventricular responses, escape beats, and nonparoxysmal junctional tachycardia. Less commonly observed arrhythmias include exit block from junctional pacemakers, and bidirectional tachycardia which may reflect blocking actions of digitalis in subnodal tissues. The recent development of His bundle electrocardiography and the immunoassay method of blood level determination permit more accurate appraisal of the clinical status of patients in whom digitoxicity is suspected. Treatment of junctional rhythms due to digitalis intoxication is usually passive. The occasional use of drugs, pacing, or cardioversion is discussed.

Effects of ouabain and diphenylhydantoin on myocardial potassium balance in man

Myocardial potassium balance and hemodynamics were studied in 19 patients. In 12 patients the effect of diphenylhydantoin administration was studied and in 7 the effect of pretreatment with diphenylhydantoin on the ionic and hemodynamic responses to ouabain was observed. Diphenylhydantoin caused no change in myocardial potassium balance, but a significant fall in left ventricular dP dt and a brief increase in left ventricular end-diastolic pressure occurred. Heart rate, cardiac output, stroke work and brachial arterial pressure did not change. In patients pretreated with diphenylhydantoin, ouabain administration was followed by a significant loss of potassium from the myocardium. This was accompanied by a significant increase in left ventricular dP dt and a decrease in left ventricular end-diastolic pressure. No change occurred in heart rate, cardiac output, stroke work or brachial arterial pressure. These ionic and hemodynamic responses to ouabain after pretreatment with diphenylhydantoin are similar in magnitude and timing to those observed when only ouabain is administered. These results in man are compatible with theories of digitalis action which associate inotropic effect with the inhibition of membrane sodium- and potassium-activated adenosine triphosphatase and indicate that diphenylhydantoin acts on the heart by some mechanism other than stimulation of this enzyme.

Breeding experiments on the frequency of adjuvant arthritis in the rat.

In an attempt to resolve the current controversy regarding whether or not myocardial Na,K-ATPase remains inhibited following washout of digitalis inotropy in isolated hearts, the effects of various protein extracting agents used in enzyme isolation procedures were investigated on dissociation of the drug-enzyme complex. The drug-enzyme complex, [³H]ouabain-Na,K-ATPase, was isolated both after <i>in vitro</i> incorporation of [³H]ouabain to crude rabbit heart Na,K-ATPase fraction and after <i>ex vivo</i> binding of [³H]ouabain in isolated Lagendorff rabbit hearts. The drug-enzyme complex was then treated at 0-4°C with 2 M NaI, 0.1% deoxycholate (DOC), or 0.1% DOC plus 2 M NaI, the protein extracting agents used in many laboratories, and compared against treatment with LiBr, the extracting agent used in our laboratory. It was found that a sequential treatment of 0.1% DOC followed by 2 M NaI resulted in over 95% of the [³H]ouabain dissociating from both <i>ex vivo</i> and <i>in vitro</i> bound drug-enzyme complexes in comparison to control. For <i>ex vivo</i> bound drug-enzyme complex, 2 M NaI treatment alone removed approximately 90% of the [³H]ouabain activity and 0.1% DOC removed approximately 45% of the drug. In contrast to these results, treatment of the <i>ex vivo</i> and <i>in vitro</i> labeled drug-enzyme complexes with 1 M LiBr caused no detectable difference in drug dissociation in comparison to control complex treated with 1 mM Tris-HCl. Therefore, these findings may account for the inability of some investigators to demonstrate inhibition of Na,K-ATPase following enzyme isolation from inotropically stimulated Lagendorff rabbit hearts as well as from noninotropic hearts of various species following drug washout due to dissociation of the drug-enzyme complex by NaI and DOC. Since LiBr does not appreciably dissociate the drug-enzyme complex, in comparison to 1 mM Tris-HCl control, the present findings support our original observation that inhibition of Na,K-ATPase is not responsible for the inotropic action of digitalis, because enzyme inhibition is still present in noninotropic rabbit hearts following drug washout.

Singular effects of a new short acting cardiac glycoside in Purkinje cells

A new cardiac glycoside, which possessed all the actions of digitalis, differed in its effects on isolated Purkinje fibers in the 2 following respects: (1) the effects developed very quickly and were quickly reversible even after automaticity and excitability were lost; (2) a given effect could be maintained at a stable level for relatively long periods.

Effects of digitalis on myocardial ionic exchange.

Prior to consideration of the effects of digitalis compounds on ionic exchange the basic mechanisms of active Na + and K + transport are discussed. It is generally accepted that digitalis is a specific inhibitor of the Na + -K + activated ATPase located primarily in the sarcolemmal membrane. Present evidence supports the concept that positive inotropic effects of digitalis are not seen unless evidence of inhibition of the enzyme and, therefore, of the "Na pump" leads to a small increase of intracellular Na + . This leads to augmented activity of a Na + transport system which is coupled, not to K + , but to Ca + + . Evidence for the existence of Na + -Ca + + coupling at excitable membranes is accumulating, and this is reviewed. The possibility that administration of digitalis, through its inhibition of the Na + -K + coupled system produces an increase in Na + - Ca + + coupled transport and thereby an increase of influx of Ca + + to the myofilaments is discussed and is presented as a possible basis for the mechanism of digitalis action. The toxic electrophysiologic effects of digitalis are discussed in terms of the effects on K + exchange induced by the drug.

Effects of Ouabain on Myocardial Potassium and Sodium Balance in Man

Myocardial electrolyte balance and hemodynamics were studied in 14 patients before and after the administration of ouabain. The drug caused a significant loss of potassium from the myocardium. This was accompanied by a significant positive inotropic response, indicated by an increase in the rate of rise of left ventricular pressure and systolic ejection-rate index. Heart rate, cardiac output, left ventricular end-diastolic pressure, brachial artery pressure, and left ventricular stroke-work index remained unchanged. These observations in man are compatible with those theories of digitalis action which associate inotropic effect with the inhibition of membrane sodium- and potassium-activated adenosine triphosphatase.

Cat assay for the emetic action of digitalis and related glycosides (digitoxin, digoxin, lanatoside C, ouabain and calactin).

1. A titration assay with two end points is described for comparison of the emetic and lethal potencies of digitalis-like drugs.2. A drug was infused at constant rate to a conscious, unrestrained cat, through an indwelling venous cannula. At the moment of vomiting the cat was rapidly anaesthetized and infusion continued at the same rate until the moment of cardiac arrest.3. With very slow and very fast infusions, the emetic and lethal doses tended to rise. In the range between these extremes (which varied from drug to drug) they were independent of time.4. The observations could be accounted for by analogue computation, assuming that the drugs entered an initial pool and were distributed at finite rates to receptors in the CNS (vomiting centre) and heart.5. Half times of metabolic loss derived from this computation for digitoxin, digoxin and ouabain (17, 9.9 and 1.8 h, respectively) were in the same ratio as the threefold longer half times reported for these drugs in man.6. When measured with infusion rates in the time independent range, the ratio of lethal to emetic doses did not vary between the drugs studied. All caused vomiting at 40% of the lethal dose.7. From a review of the literature, the emetic and cardiotoxic actions of digitalis-like drugs appear inseparable and probably share a common biochemical mechanism.8. It is concluded that foreseeable improvements in digitalis-like drugs are small and would depend on the elimination of any local emetic effect on gut receptors which they may have.

Current concepts and treatment of digitalis toxicity

Digitalis toxicity is among the most common adverse drug reactions, and may cause arrhythmias and conduction disturbances in as many as 1 of 5 patients. Particularly responsible are the electrophysiologic properties of digitalis in increasing the automaticity of subsidiary pacemakers, reducing the refractory period and prolonging conduction velocity in the atria and ventricles, and delaying conduction in the atrioventricular (A-V) node. Underlying these electrophysiologic effects are digitalis-influenced alterations of cardiac cell transmembrane movements of sodium and potassium; the induction of ectopic impulses due to increased automaticity appears to be caused by inhibition of the activity of the sodium-potassium membrane adenosine triphosphatase (ATPase) pump. Digitalis can provoke every type of cardiac arrhythmia, and no specific disorder of rhythm can be considered absolutely pathognomonic of digitalis toxicity. The factors that predispose to digitalis toxicity, as well as the onset and duration of action of the agent, must be taken into account. Potassium has relatively little influence on the toxic and the contractile actions of digitalis when the cation is administered after the glycoside has been taken up by the myocardium. In contrast, alterations in serum potassium effected before treatment with digitalis may have drastic effects on the electrophysiologic and contractile actions of the glycoside; hyperkalemia reduces the binding of digitalis to the myocardium. Diphenylhydantoin, lidocaine and propranolol are effective in terminating digitalisprovoked tachyarrhythmias, usually without inducing or worsening A-V block. Atrial or ventricular pacing to achieve electrical overdrive of the ectopic focus may be used if standard measures fail. In complete heart block, potassium should not be administered; atropine and electrical pacing should be used.

Beta-receptor blockade and myocardial effects of cardiac glycosides.

Because of suggestions that norepinephrine release from cardiac sympathetic nerves contributes to the myocardial action of digitalis, the effect of betareceptor blockade on these actions was determined. 10 -6 M propranolol did not alter the basal contractility of isolated kitten papillary muscles or of atrial strips from kittens, guinea pigs, rabbits, dogs, and chickens but decreased by 85% their inotropic response to norepinephrine released by tyramine or highintensity electrical stimulation. The same concentration of propranolol had no effect on cumulative inotropic concentration-effect curves for ouabain, acetylstrophanthidin, and digoxin in these preparations. Presence of propranolol did not modify the actions of cardiac glycosides on the time course of isometric contraction. Beta-receptor blockade was without effect on decreases in active tension and increases in resting tension induced by toxic concentrations of cardioactive steroids. The frequency of digitalis-induced automaticity in guinea pig atrial strips and kitten papillary muscles was significantly decreased by propranolol, presumably because of its direct electrophysiologic actions. Exposure of cat hearts to digoxin in vivo and to ouabain in vitro had no effect on myocardial norepinephrine concentrations. It is concluded that release of myocardial norepinephrine plays no role in the actions of cardiac glycosides on the heart.

Clinical studies on the role of aldosterone concerning the edema formation in chronic congestive heart failure.

1. Urinary excretion of aldosterone in the patients with congestive heart failure prior to drug therapy was increased in 23 out of all 45 determinations under the constant diet containing daily 85mEq of Na and 60mEq of K. The increase in urinary aldosterone was seen most frequently in the cases with right-sided or both-sided heart failure, especially with large quantity of edema fluid. But in those with spontaneous diuresis at rest or in the state of so-called de-crescendo decompensation, aldosterone excretion was within normal range. All but one cases of left-sided heart failure gave normal values. 2. There was an inverse correlation between amount of urinary excretion of aldosterone prior to medication and daily urinary Na excretion or urinary Na/K ratio, respectively. No correlation was found between urinary aldosterone and the severity of heart failure, venous pressure, renal function and any serum electrolytes. 3. Almost inversed correlation was found between urinary aldosterone and 17-OHCS. In the cases with high aldosterone values, urinary 1 7-OHCS excretion was low in general. 4. Improvement of cardiac function through bed rest and/or the action of digitalis led to gradual decrease in aldosterone excretion. In the cases with advanced edema or ascites and high venous pressure, prompt diuresis through the administration of thiazide diuretics or triamterene caused a transient decrease in aldosterone excretion and later increase concomitant with decrease in serum Na level, urinary Na and water excretion. This was considered as a rebound phenomenon. 5. When forced diuresis was ensued in two severe cases with repeated congestive heart failure, using powerful diuretics for a long period, showing the state of artificial hyperaldosteronism, the regulation system finally might break, leading to a decrease in aldosterone excretion despite of hyponatremia and of decrease in urinary Na and water excretion. Glucocorticoid excretion also decreased. The living organism may lose its ability for adaptation and probably thus fall living in refractory heart failure. Based on the above results, the role of aldosterone in the pathogenesis of cardiac edema was discussed.

The Mechanism of Action of Digitalis

The ability of digitalis to potentiate cardiac contraction is intimately related to its interference with the active transport mechanism involved in the translocation of sodium and potassium across the cell membrane. This is also the mechanism that explains the toxicity problems so often encountered with use of the cardiac glycosides—myocardial irritability and heart block.

Hemodynamic Studies on the Sensitivity of Dogs to Digitalis with Dye Dilution Method

Although a variety of factors, both exogeneous and endogeneous, has been known to alter the toxic action of digitalis, literatures dealing with the hemodynamic studies on the sensitivity to digitalis are scanty. Present study on the sensitivity of dogs to digitalis from hemodynamic aspects revealed the significant relationship between sensitivity to digitalis and hemodynamics. Furthermore, a close correlation was found between cardiac output and appearance time of dye dilution curve. Materials and Methods Healthy adult dogs were anesthetized by intraperitoneal administration of sodium pentobarbital (35-50 mg/kg). After intubation with endotracheal tube, room air mixed with 100 per cent oxygen was given by respirator. Dye dilution curve was recorded at time intervals after injection of indocyanine green (2-3 mg) into the vena cava near the right atrium and by sampling from the femoral artery. Aortic pressure was measured by the catheter placed in abdominal aorta. Electrocardiogram was monitored and recorded. (1) The determination of cardiac output by the triangle method. In order to determine the cardiac output, Cambridge dye dilution curve recorder was used. The author's modified triangle method was per-formed for the determination of the area of dye dilution curve by the following equation: fcdt = ΔXYZ + 1/10 ΔXYZ since Cambridge dye dilution curve recorder did not show lineality between dye concentration and absorbance in this experiment. (2) Hernodynamic studies on the sensitivity of dogs to digitalis. Nineteen healthy adult dogs weighing 8-16 kg were used. Before administration of strospeside, electrocardiogram, aortic pressure and dye dilution curve were recorded. Thirty minutes after intravenous administration of 0.3 mg/kg of strospeside, the dogs were divided into two groups electrocardiographically; The first group included dogs in which ventricular tachycardia continued more than thirty minutes or ventricular arrest occurred, the second group included dogs in which ventricular tachycardia did not occur or appeared transiently. Then the differences of hemodynamic and electrocardiographic findings before administration of strospeside between two groups were surveyed.