A QSAR study on a series of simplified digitalis-like compounds acting on Na+,K(+)-ATPase

Among the cardiotonics (agents against congestive heart failure), the most important group is of the digitalis cardiac glycosides, but since these compounds suffer from a low therapeutic index, attention has been paid to investigating safer cardiotonic agents through the inhibition of Na+,K(+)-ATPase, the mechanism by which the digitalis cardiac glycosides elicit their action. Recently, a series of perhydroindenes were studied for their Na+,K(+)-ATPase inhibition activity. We report here a QSAR study on them to investigate the physicochemical and structural properties of the molecules that govern their activity in order to rationalize the structural modification to have more potent drugs. A multiple regression analysis reveals a significant correlation between the Na+,K(+)-ATPase inhibition activity of the compounds and Kier's first order valence molecular connectivity index of their R5-substituents and some indicator parameters, suggesting that the R5-substituents of the compounds containing atoms with low valence and high saturation and the R1-substituents having =N-O- moiety will be conducive to the activity.

Progesterone attenuates the inhibitory effects of cardiotonic digitalis on pregnenolone production in rat luteal cells

Previous studies have shown that digoxin decreases testosterone secretion in testicular interstitial cells. However, the effect of digoxin on progesterone secretion in luteal cells is unclear. Progesterone is known as an endogenous digoxin-like hormone (EDLH). This study investigates how digitalis affected progesterone production and whether progesterone antagonized the effects of digitalis. Digoxin or digitoxin, but not ouabain, decreased the basal and human chorionic gonadotropin (hCG)-stimulated progesterone secretion as well as the activity of cytochrome P450 side chain cleavage enzyme (P450scc) in luteal cells. 8-Br-cAMP and forskolin did not affect the reduction. Neither the amount of P450scc, the amount of steroidogenic acute regulatory (StAR) protein, nor the activity of 3beta-hydroxysteroid dehydrogenase (3beta-HSD) was affected by digoxin or digitoxin. Moreover, in testicular interstitial and luteal cells, progesterone partially attenuated the reduction of pregnenolone by digoxin or digitoxin and the progesterone antagonist, RU486, blocked this attenuation. These new findings indicated that (1) digoxin or digitoxin inhibited pregnenolone production by decreasing the activity of P450scc enzyme, but not Na(+)-K(+)-ATPase, resulting in a decrease on progesterone secretion in rat luteal cells, and (2) the inhibitory effect on pregnenolone production by digoxin or digitoxin was reversed partially by progesterone. In conclusion, digoxin or digitoxin decreased progesterone production via the inhibition of pregnenolone by decreasing P450scc activity. Progesterone, an EDLH, could antagonize the effects of digoxin or digitoxin in luteal cells.

Effect of halothane on in vivo and in vitro cardiotoxicity of an aminocardenolide

Halothane opposes cardiotoxicity of neutral-sugar digitalis compounds in intact animals, presumably by depressing a sympathetic component of arrhythmogenesis. However, halothane also produces a dose-related reduction in arrhythmogenicity of ouabain in isolated canine Purkinje fibers, suggesting that the anesthetic may oppose direct mechanisms of cardiotoxicity as well. The present study examined in vivo and in vitro the effect of halothane on the arrhythmogenicity of ASI-222 (3-beta-O[4-amino-4-6-dideoxy-beta-D-galactopyranosyl] digitoxigen in HCl), a highly polar aminocardenolide with no sympathetic component to cardiotoxicity. For in vivo studies, ASI-222 was infused at a rate of 1 microgram/kg/min until appearance of third-degree atrioventricular (AV) block or sustained ventricular arrhythmias in 5 conscious (control) and 6 halothane-anesthetized (1.4% end-tidal) dogs. For in vitro studies, standard microelectrode techniques were used to measure action potentials (AP) in seven excised canine Purkinje fibers superfused with oxygenated Krebs-Henseleit buffer. AP were recorded during control superfusion, after induction of toxicity with 10(-7) M ASI-222, and during exposure to 0.5, 1.0, and 2.0% halothane. Purkinje fibers were paced at 500-ms cycle lengths (CL) for 20 beats, and the amplitude of delayed afterdepolarizations (DAD) were recorded. Pacing at 250 ms CL was used to trigger ectopy. In vivo studies showed no difference in the cardiotoxic dose of ASI-222 between control dogs and those anesthetized with 1.4% halothane. However, in 4 of 6 anesthetized dogs, acutely increasing the inspired halothane concentration suppressed arrhythmias once end-tidal concentration were >2.2%.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a new class I antiarrhythmic drug bidisomide on canine ventricular arrhythmia models

The antiarrhythmic and direct cardiovascular effects of a new antiarrhythmic agent, bidisomide, alpha-(2-[acetyl(1-methylethyl)amino]ethyl)-alpha-(2- chlorophenyl)-1-piperidinebutanamide, were investigated. To determine the anti-arrhythmic effects, spontaneously occurring adrenaline-, digitalis- and two-stage coronary ligation-induced arrhythmias were used. Bidisomide suppressed these three arrhythmia models. The antiarrhythmic plasma concentration, IC50, of bidisomide for digitalis-induced arrhythmia was 22.1 micrograms/ml, and those calculated for intravenous bidisomide in 24 h and 48 h coronary ligation-arrhythmias were 15.1 and 11.6 micrograms/ml and that calculated for oral bidisomide in 24 h coronary ligation-arrhythmia was 5.4 micrograms/ml and that for adrenaline induced arrhythmia was 58.7 micrograms/ml. In the blood perfused sinoatrial node and papillary muscle preparations, bidisomide decreased the sinoatrial rate and contractile force and increased the intraventricular conduction time and coronary blood flow. These results indicate that bidisomide is similar to other class I antiarrhythmic agents such as pirmenol and KW-3401 in its antiarrhythmic profile and is expected to become a clinically useful antiarrhythmic drug.

Metabolic requirement for the inotropic effects of digitalis and BAY K 8644

The positive inotropic effects of ouabain and of BAY K 8644 are not apparent in rabbit atria suspended in substrate free medium or in a medium containing 5 mM pyruvate. Addition of glucose in graded concentrations (1-11 mM) during continued exposure of the preparations to the inotropic agents yields graded inotropic effects. The possible involvement of the glycolytic pathway to the development of the inotropic effect of ouabain and BAY K 8644 was tested by using inhibitors of glycolysis that act at two different steps. Iodoacetic acid completely blocks the inotropic effect of ouabain in atria at 0.1 mM and papillary muscles at 0.05 mM. The inotropic effect of BAY K 8644 was blocked partially in atria and papillary muscles. Iodoacetic acid did not change the inotropic effects of isoproterenol and Ca2+. Addition of 1 mM fluoride did not affect significantly the inotropic effect of either ouabain or BAY K 8644 in atria but partially blocked the effect of BAY K 8644 in papillary muscles. The response of atria to ouabain was not changed significantly when glyceraldehyde (10 mM) was substituted for glucose. We suggest that glycolytic ATP may be important for the full inotropic effect of ouabain and BAY K 8644.

Morphine suppression of digitalis-induced ventricular arrhythmias

The purpose of the study was to investigate the effect of morphine on digitalis-induced arrhythmias. Seventeen normokalemic dogs [serum potassium 3.9 +/- 0.3 mEg/L (+/- 1 S.D.)] and 19 hypokalemic dogs (serum potassium 2.2 +/- 0.3 mEg/L) were allocated to a morphine pretreatment group (morphine sulfate 3 mg/kg) or to a control group without morphine. Ouabain was administered by a constant infusion of 150 micrograms/kg/hr following an initial bolus of 20 micrograms/kg until animals developed ventricular tachycardia. In hypokalemic dogs, morphine pretreatment significantly (p less than 0.05) increased the digitalis dose required to initiate ventricular tachyarrhythmias (66 +/- 14 vs 37 +/- 11 micrograms ouabain/kg). In normokalemic dogs morphine did not affect the relationship between digitalis dose and arrhythmias. Thus, morphine pretreatment reduces the tendency for hypokalemia to potentiate digitalis-induced ventricular arrhythmias.

Effects of low potassium or magnesium concentrations on isolated cardiac tissue

Low potassium and magnesium concentrations not only cause cardiac arrhythmias, but also interfere with the efficacy or enhance the toxicity of drugs commonly used to treat patients with heart disease. Arrhythmias may develop in hypokalemia due to enhanced normal automaticity, abnormal automaticity, or slowed conduction; moreover, hypokalemia is associated with enhanced digitalis toxicity, quinidine-related Torsades de pointes, and interference with the antiarrhythmic activity of quinidine. Hypomagnesemia, especially in the presence of other electrolyte abnormalities, also affects automaticity and is associated with decreased efficacy of digitalis and with quinidine-related Torsades de pointes. Therefore, treatment that controls hypertension without causing electrolyte abnormalities is preferable for patients who are at risk of arrhythmias, or who are receiving drugs such as digitalis or quinidine.

Antiarrhythmic plasma concentrations of cibenzoline on canine ventricular arrhythmias

Using two-stage coronary ligation-, digitalis-, and adrenaline-induced canine ventricular arrhythmias, antiarrhythmic effects of cibenzoline were examined and the minimum effective plasma concentration for each arrhythmia model was determined. Cibenzoline suppressed all the arrhythmias, and the minimum effective plasma concentrations for arrhythmias induced by 24-h coronary ligation, 48-h coronary ligation, digitalis, and adrenaline were 1.9 +/- 0.9 (by 8 mg/kg i.v.), 1.6 +/- 0.5 (by 8 mg/kg i.v.), 0.6 +/- 0.2 (by 2 mg/kg i.v.), and 3.5 +/- 1.3 (by 5 mg/kg i.v.) micrograms/ml, respectively (mean +/- SDM, n = 6-7). The concentration for adrenaline-induced arrhythmia was significantly higher than those for the other types of arrhythmias. This pharmacological profile is similar to those of mexiletine and tocainide, and all three drugs have central nervous system (CNS) stimulant action. Because cibenzoline had only weak hypotensive and sinus node depressive effects and was found to be orally active when given to coronary ligation arrhythmia dogs, its clinical usefulness is expected.

Protector effects of an anti-heart immune serum on guinea-pig atrial and papillary muscle in digitalis intoxication

An anti-heart immune serum (IS) has been tested to prevent digitalis intoxication in guinea-pig atrial and papillary muscle. The results show a good protector effect of the IS against the cardiotoxic effects of digitoxin. The hypothesis of an interaction between the IS and the affinity binding sites of the cardiotonic glycosides can be retained.

[Effects of N-696, a new beta-blocking agent, on canine experimental arrhythmias]

We evaluated antiarrhythmic effects of N-696 using canine digitalis ventricular arrhythmia models and electrophysiological actions of N-696 using canine ventricular muscles and isolated blood perfused A-V node preparations and compared the effects with those of propranolol. 1) N-696 at 30 mg/kg, i.v. was not effective on digitalis arrhythmia, but reduced the total heart rate and atrial rate for 60 min and transiently decreased the blood pressure. Propranolol at 3 mg/kg, i.v. transiently suppressed digitalis ventricular arrhythmia and decreased the total heart rate, atrial rate and blood pressure. The minimum effective plasma concentration of propranolol was 1.7 +/- 0.4 microgram/ml. 2) Ten to 100 micrograms/ml N-696 and 3 to 10 micrograms/ml propranolol did not affect the resting potential and the action potential duration at 75% repolarization of the canine ventricular muscle, but decreased the maximum rate of rise of the action potential in a dose-dependent fashion, and 100 micrograms/ml N-696 decreased significantly the action potential amplitude. The minimum effective drug concentration of N-696 was 60 micrograms/ml, and that of propranolol was 6 micrograms/ml. 3) N-696 (30 micrograms-3mg) dose-dependently delayed the A-V conduction time of the isolated blood perfused A-V node preparation when it was administered into the posterior septal artery (PSA) and the anterior septal artery (ASA). Propranolol (10-600 micrograms) also had similar effects, but the doses of N-696 delaying the A-V conduction time by 15% was about 8 to 21 times higher than those of propranolol.

Immunological probes in cardiovascular disease

The immune system has long been recognised as playing a central role in the organism's defence against infectious diseases and possibly the development of neoplasia. The active stimulation of the immune system by immunisation and the passive administration of antitoxins have a venerable history in medicine. Yet the concept that antibodies may be used to modify physiological or pharmacological effects or may act as diagnostic agents in the living organisms has only recently come to be recognised. Advances, both in an understanding of the structural chemistry of the antibody molecule and in the ability to culture antibody-producing cells, now permit the selection and production of homogeneous antibodies and their smaller fragments in quantity by means other than conventional immunisation. These innovations will allow the development of a new pharmacology based on the remarkable resolving power of the antibody combining site. Antibodies or their fragments are shown to inhibit the pressor action of renin, to neutralise the pharmacological actions of digitalis, to block the beta-adrenergic receptor, and to detect and image myocardial infarcts.

The action of reserpine, 6-hydroxydopamine, and bretylium on digitalis-induced cardiotoxicity

This study determined whether the protective effect of reserpine against ouabain-induced ventricular arrhythmias in the cat is due to an action of the drug on the adrenergic nerve terminal. Reserpine (5 mg/kg i.p.) administered 24 h prior to ouabain (2 micrograms/kg per min i.v., until death) increased the dose of ouabain to produce premature ventricular contractions, ventricular tachycardia, and death from 77.3 +/- 5.2 to 105.0 +/- 6.0; 84.9 +/- 5.2 to 132.7 +/- 9.1; and 108.8 +/- 4.0 to 165.7 +/- 10.4 micrograms/kg, respectively (P less than 0.05). When 6-hydroxydopamine (6OHDA; 20 mg/kg i.v.) was given 3 days prior to the experiment, the protective effect of reserpine was not evident. When bretylium (20 mg/kg i.v., 2 h prior to ouabain) was administered to animals previously treated with reserpine, the dose of ouabain which produced premature ventricular contractions, ventricular tachycardia, and death was increased to 109.0 +/- 7.2; 146.1 +/- 12.6; and 165.8 +/- 7.6 micrograms/kg, respectively (P less than 0.05). However, the magnitude of this protective action was similar to that produced by reserpine alone. Lathers et al. (Fed. Proc. 40, 672, 1981) reported that bretylium alone provides protection of a similar order of magnitude as reserpine. Thus, the effects of reserpine and bretylium were not additive; this indicates that the two agents may be acting on the same locus or they may be acting at different sites with the action of one drug masking or blocking the action of the other. Since 6OHDA prevented the action of reserpine on ouabain-induced ventricular arrhythmia and since 6OHDA only produces degeneration of adrenergic nerve terminals, it is probable that the protective effect of both reserpine and bretylium is due to an action at the adrenergic nerve terminal. The heart rate and blood pressure were not involved in the antiarrhythmic effects of reserpine.

Canrenoate--a spironolactone metabolite. Acute cardiac effects in digitalized patients

Canrenoate (Soldactone)--a spironolactone metabolite for intravenous use--has recently been suggested as a specific antidote in patients suffering from digitalis intoxication. Theoretically, this would imply that canrenoate could counteract the effect of digoxin. The influence of canrenoate on inotropy in 12 digitalized patients was investigated by measurement of sytolic time intervals. Canrenoate improved the left ventricular contractility, since both preejection period (PEP) and the ratio between PEP and left ventricular ejection time (PEP/LVET) decreased significantly. The maximal changes in controlled atrial fibrillation, canrenoate had no effect on ventricular rate, whereas atropine increased the ventricular rate significantly.

The effects of smoking on myocardial conduction in the human heart

Inhalation of a few puffs on a cigarette increases the velocity of conduction and shortens the effective refractory period of the A-V node. These effects are attributed to adrenergic stimulation produced by minute amounts of nicotine absorbed. Wenckebach block is abolished whether induced by atrial pacing or occurring spontaneously. Conduction velocity in the His-Purkinje system and in the anomalous pathways in the WPW syndrome were not affected. Smoking increases the ventricular rate in atrial fibrillation, and antagonizes the cholinergic effects of digitalis.

Neurally mediated and direct effects of acetylstrophanthidin on canine skeletal muscle vascular resistance

The effects of acetylstrophanthidin on the vascular resistance of the isolated canine gracilis muscle were examined in 38 anesthetized mongrel dogs. The neurally mediated effect of intravenous acetylstrophanthidin (0.5 mg) on the vascular resistance of the innervated, separately perfused (constant flow) right gracilis muscle was a transient mean fall of 4.0 ± 0.9 resistance units (RU) ( P < 0.001) at 1 minute followed by a sustained rise to 4 RU ( P < 0.05) above control from 8 to 18 minutes after injection. The sustained increase in resistance was blocked by intra-arterial phenoxybenzamine. The combined direct and neurally mediated effects of intravenous acetylstrophanthidin were directionally similar and greater in magnitude in the innervated, autoperfused (intact circulation) left gracilis. Following denervation of the muscle, however, the initial decrease in vascular resistance was abolished and the prolonged subsequent rise was greatly reduced. Hence the initial fall and most of the subsequent rise in vascular resistance are neurally mediated. There was no consistent change in aortic pressure to explain either of these changes in vascular resistance solely on a reflex basis. Thus, systemically administered acetylstrophanthidin produces skeletal muscle vasoconstriction through both a direct and a neurally mediated effect. The latter is mediated through alphareceptor stimulation and appears to be the predominant mechanism whereby this drug increases muscle vascular resistance.

Effects of chloramphenicol and its isomers and analogues on the mitochondrial respiratory chain

The nature of the inhibition of mitochondrial respiration by chloramphenicol isomers and analogues has been examined further to explain differences in their action in several systems. With coupled rat-liver mitochondria, D-threo-chloramphenicol inhibited β-hydroxybutyrate oxidase 50% at 1.3 mM and succinoxidase at 3.5 mM. L-threo-Chloramphenicol inhibited 50% at 3.6 mM and 3.7 mM, respectively. The sensitivity of succinoxidase to D-chloramphenicol was lost on freezing the mitochondria. Chloramphenicol inhibited dinitrophenol-activated ATPase with 50% inhibition at 6 mM for the D-isomer and at 2.2 mM for the L isomer. The inhibition of NADH oxidase of frozen–thawed beef-heart mitochondria by D-chloramphenicol analogues was dependent on the analogues' solubility properties. A relationship which approximates a straight line was obtained by plotting log (1/concentration50% inhibition) against log (solubilityethanol/solubilitywater) indicating that the inhibition was greater for analogues which would partition into an organic phase. The inhibition did not depend on the orientation of the mitochondrial membrane; the NADH oxidase of submitochondrial particles prepared either by sonication or with digitonin had the same sensitivity to the analogues. The inhibition of NADH oxidase by rotenone and amytal fitted into the straight line relationship obtained with the chloramphenicol analogues. Beef-liver glutamic dehydrogenase was inhibited by some of the analogues of chloramphenicol with the same relationship as for the inhibition of NADH oxidase. It was suggested that the inhibition of NADH oxidase and glutamic dehydrogenase depends on the inhibitors reacting with a hydrophobic site on the enzyme.

Effect of spironolactone and norbolethone on the toxicity of digitalis compounds in the rat

1. In the rat, both spironolactone (an antimineralocorticoid) and norbolethone (an anabolic steroid) inhibit the characteristic neuromuscular disturbances and the mortality produced by digitoxin, gitalin, proscillaridin, digoxin and digitalin. The corresponding effects of strophanthin K, ouabain and digitoxigenin could not be prevented.2. It may be concluded that, in our experimental conditions, both spironolactone and norbolethone counteract the toxicity of some, but not all, digitalis compounds tested and that both the protective steroids affect the toxicity of the same digitalis derivatives.3. This antidigitalis effect is not merely a secondary consequence of either antimineralocorticoid or anabolic potency because spironolactone lacks anabolic and norbolethone lacks antimineralocorticoid properties. Indeed, it appears that the ability to antagonize the toxicity of digitalis compounds represents a pharmacological property independent of all known steroid hormone actions, since spironolactone is virtually devoid of these.