Tritiated digoxin metabolism after prior treatment with propranolol or diphenylhydantoin sodium.

Digitalis-induced arrhythmias can be suppressed by intravenous potassium, diphenylhydantoin sodium and propranolol. As it is known that hyperkalemia can interfere with the myocardial uptake of digoxin, this study was performed to determine whether diphenylhydantoin sodium or propranolol could exert any antiarrhythmic effect by altering the metabolism of 3H-digoxin and, in particular, the accumulation of the glycoside by cardiac muscle. Three groups of anesthetized dogs were given 6 muCi 3H-digoxin per kilogram intravenously, and in two groups 15 mg/kg diphenylhydantoin sodium or 3 mg/kg propranolol were injected intravenously 15 minutes prior to the glycoside. The concentration of labelled digoxin was measured in plasma up to one hour, and then tissues were removed and analyzed for digoxin content. Diphenylhydantoin sodium did not influence myocardial uptake of digoxin. It is considered its suppressant action on digitalis-induced arrhythmias is not due to any effect on the cardiac accumulation of digoxin. Propranolol did reduce the myocardial uptake of digoxin, but this was not considered of sufficient magnitude to be the main factor in suppressing arrhythmias induced by digitalis. These studies provide evidence that both diphenylhydantoin sodium and propranolol do not have a suppressant effect on digitalis-induced arrhythmias by virtue of any significant interference with the uptake of digoxin by myocardial tissue.     

Effect of basic drugs on the hepatic uptake of ouabain by sinusoidal plasma membrane vesicles isolated from rat liver.

Although antiarrhythmic drugs are used to treat digitalis-induced cardiac disorders, some of these drugs have been reported to increase the serum digoxin concentration in patients, causing the severe side-effects. We have previously shown that many basic drugs including antiarrhythmic drugs inhibited the hepatic uptake of cardiac glycosides into isolated rat hepatocytes, which could be a cause for the increased serum digoxin concentration. The present study was designed to examine the mechanism of this inhibition using isolated rat sinusoidal plasma membrane vesicles. The effect of nine basic drugs (dipyridamole, nifedipine, verapamil, chlorpromazine, lidocaine, quinidine, ajmaline, disopyramide, and propranolol) on the uptake of ouabain was studied. Quinidine reduced the initial uptake rate of ouabain (30 s) while it did not change the uptake of ouabain in an equilibrium condition (30 min). Other basic drugs, such as verapamil, dipyridamole, and nifedipine also significantly reduced the initial uptake rate of ouabain. These basic drugs had no effect on the membrane fluidity. The inhibitory effects on the vesicular uptake were significantly correlated with the inhibitory effects on ouabain uptake by the isolated rat hepatocytes. These findings may suggest that the mechanism of the inhibition involves the inhibition of the transport process via the sinusoidal plasma membrane.     

Increase in cardiac toxicity of ouabain in dogs after repetitive treatment with diazepam.

Procedures which diminish adrenergic nervous activity, either by catecholamine depletion sympathectomy, adrenalectomy, inhibition of catecholamine release, or by the use of β-adrenergic-blocking agents, depress ventricular arrhythmias induced by cardiac glycosides and also increase their lethal doses. Since it has been shown that diazepam increases the adrenal catecholamine content of rats, we have studied the influence of such a pretreatment on the cardiac toxicity of ouabain in dogs. The results reported show the various diazepam pretreatments (0.25 to 25 mg/kg/day for 20 days) decrease the LD100 of ouabain as well as the survival time to arrhythmias in dogs. Furthermore, this increase in cardiac toxicity to ouabain in diazepam-treated dogs seems to be related to a greater release of adrenal catecholamines and can be abolished by the use of a β-adrenergic-blocking agent and also by a pseudoadrenalectomy. These data are consistent with reports showing that cardiac arrhythmias produced by ouabain may be partially related to catecholamine release from the adrenal gland.     

Cardiotoxicity of digitalis glycosides: roles of autonomic pathways, autacoids and ion channels

1 Cardiac glycosides have been used for centuries as therapeutic agents for the treatment of heart diseases. In patients with heart failure, digoxin and the other glycosides exert their positive inotropic effect by inhibiting Na(+)-K(+)-ATPase, thereby increasing intracellular sodium, which, in turn, inhibits the Na(+)/Ca(2+) exchanger and increases intracellular calcium levels. As the therapeutic index of digitalis is narrow, arrhythmias are common problems in clinical practice. The mechanisms and mediators of these arrhythmias, however, are not completely understood. 2 The involvement of the sympathetic and parasympathetic nervous system in digitalis cardiac toxicity is reviewed. 3 Receptors, channels, exchange systems or other cellular components involved in digitalis-induced cardiotoxicity are also reviewed. 4 Possible mediators of digitalis-induced cardiac toxicity are discussed. 5 Management of digitalis toxicity in patients is summarized. 6 The determination of the possible mediators of digitalis-induced cardiac toxicity will enhance our knowledge and lead to the development of new therapeutic strategies to treat these lethal arrhythmias.     

Inotropic action,myocardial uptake and subcellular distribution of ouabain,digoxin and digitoxin in isolated rat hearts

In experiments on isolated, electrically driven (240/min) rat hearts, perfused via the aorta at a constant flow (3.8 ml/min), the pharmacologically effective concentration range, the myocardial uptake and the subcellular distribution of three cardiac glycosides (digitoxin, digoxin, ouabain) were determined. The following results were obtained: 1. The effective range varied depending on the cardiac glycoside tested: With digoxin and ouabain very similar results were found- the positive inotropic concentration ranges being within 8x10(-6)M and 6x10(-5)M, the maximum positive inotropic effects attainable being about 100% and the concentration for half maximum effects (ED-50) being 2.4x10(-5)M and 2.3x10(-5)M, respectively. With digitoxin the inotropic concentration range was found to be within 3.6x10(-6)M and 2.4x10(-5)M with a maximum inotropic effect attainable of about 50% only and an ED-50 of 9.5x10(-6)M. The analysis of the time course of the inotropic action revealed extremely short half times for all cardiac glycosides studied (between 48 and 54 sec). 2. The myocardial uptake correlated with the physicochemical behaviour of the three cardiac glycosides studied and was found-depending on the perfusion time (5 to 60 min)-to be in the range of 23 and 36 (ouabain), 66 and 98 (digoxin) and 169 and 264 (digitoxin) nmoles/g wet weight. The respective computed half times for these uptake processes were 2.5 min (digoxin, ouabain) and 3.4 min )digitoxin). 3. Regarding the subcellular distribution an accumulation exceeding an "unspecific" binding (non-perfused hearts) was found mainly in the nuclear-membrane fraction. On the basis of these results (very short half times of either the pharmacological action and the cardiac uptake) the site of action of cardiac glycosides in the rat heart is supposed to be located at the surface membrane of the heart muscle cells. Furthermore, the above results are discussed with respect to those obtained in digitalis-sensitive species.     

The effects of chloroquine and other weak bases on the accumulation and efflux of digoxin and ouabain in HeLa cells.

We have studied the effects of the weak bases chloroquine, NH4Cl and amantadine on the handling of certain cardiac glycosides by HeLa cells. When these weak bases are applied acutely to HeLa cells they have only minor effects on the binding of cardiac glycosides to the sodium pumps and on the recovery of pump function following block. When cells are grown in these weak bases there is a variable (10-30%) reduction in pump numbers. This effect is additive to that of chronic treatment with cardiac glycosides. If all sodium pumps are blocked with ouabain, digoxin or digitoxin then recovery of function recovers with a T1/2 of about 7 h (10% h-1); digoxin and digitoxin molecules are excreted at a similar rate but ouabain excretion occurs at a much slower rate (3% h-1). These weak bases greatly slow (x 3) the rate of excretion of digoxin and digitoxin but do not alter that of ouabain. The process affected by chloroquine was estimated to have a T1/2 of 8 h. Cells grown in the presence of cardiac glycosides accumulate large numbers of glycoside molecules; chloroquine, NH4Cl and amantadine increase the accumulation of digoxin and digitoxin and may decrease that of ouabain. Quantitatively these results fit a model whereby cardiac glycosides are accumulated by HeLa cells bound to the sodium pumps, are processed by the lysosomes and then excreted. The results are consistent with a process of internalisation and renewal of sodium pumps by HeLa cells.     

Antiarrhythmic activity of alpha-tocopherol nicotinate and related compounds and their physico-chemical properties

The effect of alpha-tocopherol nicotinate (Renascin), alpha-tocopherol and dodecanoic acid (lauric acid) on the positive inotropic action of ouabain and digoxin and on cardiac glycoside induced arrhythmias has been tested in isolated guinea-pig left atria and in anaesthetized guinea-pigs. alpha-Tocopherol nicotinate and dodecanoic acid significantly decrease the positive inotropic action of digoxin, but not that of ouabain in isolated guinea-pig atria. Ouabain and digoxin induced arrhythmias are suppressed by the three compounds in isolated guinea-pig atria and in anaesthetized guinea-pigs: alpha-tocopherol nicotinate has the highest antiarrhythmic activity followed by dodecanoic acid and alpha-tocopherol. These results are further evidence for the dissociation between the positive inotropic and arrhythmogenic action of cardiac glycosides. The antiarrhythmic activity of the three compounds and the physico-chemical properties, as measured in liposomes, are compared with that of quinidine and aprindine. As nicotinic acid does not show any effect on cardiac glycoside induced arrhythmias, the ester linkage in alpha-tocopherol nicotinate seems to be of particular importance for its antiarrhythmic activity.     

Suppression of positive inotropic and toxic effects of cardiac glycosides by amiloride

Effects of amiloride on the inotropic and toxic actions of cardiac glycosides were examined using left atrial muscle isolated from guinea pig heart. Preincubation of atrial muscle with amiloride significantly decreased the maximum positive inotropic effect of dihydrodigoxin but failed to reduce that of isoproterenol. Amiloride prevented the contracture and significantly reduced the incidence of arrhythmias induced by 2 microM digoxin. Similar experiments examining 5 microM digoxin-induced arrhythmias showed that amiloride increased both the time required to produce arrhythmias and the fractional occupancy of sarcolemmal Na,K-ATPase by digoxin at the onset of arrhythmias. The antagonism of cardiac glycoside actions was best observed during the decline in developed tension elicited by amiloride subsequent to its initial positive inotropic effect. Amiloride had no effect on binding site concentration for ATP-dependent [3H]ouabain binding but decreased affinity of the binding sites for ouabain in membrane preparations obtained from guinea pig heart. Furthermore, amiloride inhibited Na,K-ATPase activity and increased the IC50 value for ouabain inhibition of the enzyme. These results indicate that amiloride antagonizes the positive inotropic and toxic effects of cardiac glycosides. Possible mechanisms for the antagonism include inhibition of sarcolemmal Na+/Ca2+ or Na+/H+ exchange.     

Accumulation of radioactive cardiac glycosides by various brain regions in relation to the dysrhythmogenic effect.

Ouabain was administered at a loading dose of 3 mug/kg followed by an infusion at a rate of 1 mug/kg-1 min-1 in order to produce severe dysrhythmia in dogs within 60 minutes. Similarly, digitoxin at a loading dose of 9 mug/kg followed by an infusion at a rate of 3 mug kg-1 min-1 was administered to compare its effect with that of ouabain. 2 During the 60 min experimental period, the plasma concentrations gradually rose with the continuous infusion of these drugs. However, in comparison to the 60 min plasma value of 119+/-20 pmol/ml for ouabain and 177+/-68 pmol/ml for digitoxin, the cerebrospinal fluid (CSF) concentrations for these drugs at this time were less than 5 pmol/ml. 3 Upon termination of the experiment at 60 min it was found that kidney, liver, heart, adrenal, and the non-neural tissue in the brain such as pituitary and choroid plexus concentrated ouabain and digitoxin to give high tissue to plasma ratios. However, various neural areas of the brain (cerebellum, mesencephalon, hypothalamus, pons, and medulla) showed no preferential localization or uptake of these two glycosides. 4 Concentration of ouabain and digitoxin by the choroid plexus does not seem to affect the ionic composition of the CSF. 5 It was concluded that sampling the large areas of neural tissue above could provide no evidence for local accumulation of digitalis glycosides that might account for a central nervous system origin of digitalis-induced cardiac arrhythmias.     

Current status of cardiac glycoside drug interactions

The effects of concomitant drug therapy on the absorption, distribution, and elimination of digoxin and digitoxin are reviewed. A number of agents can increase or decrease the absorption of digoxin and digitoxin from the gastrointestinal tract by altering GI motility, binding the drugs through physical adsorption, altering the properties of the intestinal wall, or altering the bacterial flora of the intestine. The steady-state serum concentrations of digoxin and digitoxin can be affected if the changes in absorption are of sufficient magnitude, and adjustments in digoxin or digitoxin dosage may be required. A reduction in digoxin and digitoxin protein binding has occurred during concomitant administration of heparin and cardiac glycosides. Since digitoxin is more highly protein bound than digoxin, interactions that involve changes in protein binding are of much greater clinical importance with digitoxin. A number of drugs increase or decrease the elimination of digoxin and digitoxin, and subtherapeutic or toxic concentrations of the cardiac glycosides often result. Drugs that induce hepatic microsomal enzymes can increase the elimination of digitoxin, which is eliminated mainly by hepatic biotransformation. Digoxin is eliminated mainly by renal excretion; renal clearance of digoxin may be increased by vasodilators and thyroid hormones and decreased by quinidine, verapamil, amiodarone, and potassium-sparing diuretics. The clinical importance of changes in serum concentrations of the cardiac glycosides that result from alterations in glycoside elimination requires further study, as does the importance of preliminary reports of interactions between cardiac glycosides and diazepam, captopril, and combination therapy with quinidine-pentobarbital or quinidine-rifampin. Because the cardiac glycosides have a narrow therapeutic range, patients receiving concomitant therapy with agents that might affect the absorption, distribution, or elimination of the cardiac glycosides should be monitored carefully for symptoms of digitalis toxicity or undertreatment.     

Protective effects of dazmegrel on the PAF potential of ouabain-induced cardiac arrhythmias.

The ouabain threshold to induce cardiac arrhythmias in urethane-anaesthetized guinea-pigs was not modified by the administration of either dazmegrel, 4 mg/kg i.v., or lysine-acetylsalicylate, 13.5 mg/kg i.v., 5 min before the infusion of ouabain,10g/kg per min i.v. The previous administration of platelet-activating factor (PAF), 0.01 to 1 nmol/animal i.v., 2 min prior to ouabain, caused a significant, dose-dependent decrease of the ouabain threshold for the cardiac rhythm disturbances. Lysine-acetylsalicylate lacked any effect on this PAF potentiation. Pretreatment with dazmegrel 5 min before PAF, 0.05 nmol/animal i.v., abolished the PAF potentiation of the digitalis-induced arrhythmias. These results suggest that thromboxane synthesis is involved in this PAF effect and indicate an ability of PAF to induce thromboxane generation even in the case of cyclooxygenase inhibition.     

Pharmacokinetics of tritiated ouabain, digoxin and digitoxin in guinea-pigs

1. Elimination rates of tritiated ouabain, digoxin and digitoxin after single intravenous administrations were investigated in guinea-pigs, the total radioactivity in whole blood being traced for a period of up to 2 weeks. 2. In the initial rapid phase of elimination between 2 and 30 min following intravenous glycoside administration, the concentration decline of radioactivity in the blood was found to be identical for the three glycosides investigated, this part of the elimination curve displaying a hyperbolic shape. 3. During this early elimination phase, rapid metabolic degradation and excretion of digoxin had already taken place. The maximum concentration of radioactivity in the bile was reached 4 min following intravenous administration of 3H-digoxin. The positive inotropic response occurred in the cat heart-lung preparation 1.5 min after intravenous injection of a therapeutic dose of digoxin, indicating a quick occupation of binding sites in the tissues. 4. The biological half-lives of tritiated ouabain, digoxin and digitoxin averaged 11 h, 2.5 days and 4.1 days, respectively, as determined by the terminal exponential elimination phase, in guinea-pigs. This terminal phase was attained 6-12, 7-24, and 24-48 h after administration of ouabain, digoxin and digitoxin, respectively. 5. The findings reveal that in guinea-pig, as has been demonstrated in man, the elimination rates of the three glycosides increase according to their hydrophobic properties. 6. The biological half-lives of tritiated ouabain, digoxin and digitoxin obtained in the guinea-pig closely resemble those found in healthy man.     

Sodium-dependent cardiac glycoside binding: experimental evidence and hypothesis.

1 The influence of increasing Na+ concentrations on the binding of digitoxin, digoxin and ouabain was examined in a Na+-K+-ATPase preparation of guinea-pig hearts. 2 Two distinct processes seem to be involved in this interaction: one binding process was activated at low Na+ concentrations. The maximum binding capacities were different and the K0.5 values were nearly identical for the cardiac glycosides studied. 3 In contrast, the second binding process was activated at appreciably higher Na+ concentrations, the maximum binding capacities were almost identical and the K0.5 values were different for the cardiac glycosides studied. 4 On the basis of these results attempts are made to explain the well known differences in the myocardial accumulation of cardiac glycosides.     

Effect of Reserpine,Desipramine and Phenytoin on Digoxin Induced Arrhythmias and Myocardial Uptake of Digoxin in Guinea Pigs

Abstract: Digoxin was infused intravenously 27.5 μg/min. to guinea pigs. By means of the ECG doses of digoxin needed to cause ventricular extrasystoles (VES), ventricular fibrillation (VF), and asystole (AS) were determined in a control group without any premedication. Three groups were given a pre-treatment with desipramine, phenytoin and reserpine. After AS digoxin concentrations in the heart muscle and in the kidneys were determined by radioimmunoassay. The concentrations of adrenaline and noradrenaline were also determined in these tissues. After reserpine and phenytoin the doses of digoxin needed to induce VF and AS were increased. Desipramine had no effect on digitalis-induced arrhythmias. The relative uptake of digoxin in the heart muscle was decreased after all of the three premedications; there was no change in the kidney. The tissue catecholamine concentrations were decreased after reserpine and desipramine, but remained unchanged after phenytoin. The lethal dose of digoxin seemed not to correlate to the myocardial digoxin concentration after different premedications. The mechanism of the uptake in the heart muscle seemed to be different from that in the kidney. There was no correlation between the catecholamine concentration in myocardium and the ar-rhythmogenic effect of digoxin in the different groups.     

The cardiorespiratory toxicity of ouabain in benzodiazepine-treated rats

The cardiorespiratory toxicity of ouabain and the role of the adrenergic nervous system were evaluated in rats pretreated by metabolites of diazepam and structurally related analogs. All of these benzodiazepine compounds (25 mg/kg/day) reduced the incidence of ouabain-induced respiratory depression and death as compared with control rats receiving the same doses of ouabain. The N₁-H-benzodiazepines (clorazepate, oxazepam, nitrazepam) did not themselves modify the adrenal epinephrine content nor affect the cardiac toxicity and the adrenal epinephrine-releasing property of intravenously injected ouabain. On the other hand, the N₁-substituted benzodiazepines (3-OH-diazepam, flurazepam) produced a dose-related increase in adrenal epinephrine content and the subsequent administration of ouabain resulted in a greater release of adrenal epinephrine and more severe cardiac toxicity as measured by a greater incidence of ventricular arrhythmias and an increase of the index of bradycardia. The incidence of arrhythmias and adrenal epinephrine release were each linearly related to the log doses of either benzodiazepine used for pretreatment, and the incidence of arrhythmias was directly proportional to the adrenal epinephrine release. It is concluded that the increased cardiac toxicity of ouabain in benzodiazepine-treated rats in related to the effect of N₁-substituted benzodiazepines upon the adrenergic nervous system.     

Inhibition of DNA topoisomerases I and II, and growth inhibition of breast cancer MCF-7 cells by ouabain, digoxin and proscillaridin A

We evaluated the cytotoxicity and underlying mechanisms of cardiac glycosides, including digoxin, ouabain and proscillaridin A, on the proliferation of breast cancer MCF-7 cells. In terms of inhibition of cell proliferation of MCF-7 cells, the compounds rank in the order proscillaridin A>digoxin>ouabain. While both digoxin and ouabain inhibited topoisomerase II catalytic activity at nanomolar concentrations (100 nM), neither agent inhibited topoisomerase I catalytic activity even at concentrations as high as 100 microM. On the other hand, proscillaridin A was a potent poison of topoisomerase I and II activity at nanomolar drug concentrations (30 nM, 100 nM, respectively), suggesting that this agent may produce its cytotoxic activity by targeting both enzymes simultaneously. These studies suggest that the stabilization of DNA-topoisomerase II complexes is closely linked to the mechanism of digoxin, ouabain and proscillaridin A cytotoxicity. The potential DNA-binding properties of the cardiac glycosides have been assessed by measuring the displacement of ethidium bromide from calf thymus DNA. These results indicate that digoxin, ouabain and proscillaridin A neither intercalate nor interact with the minor groove of DNA.     

Endogenous and Exogenous Cardiac Glycosides and their Mechanisms of Action

Cardiac glycosides have been used for decades to treat congestive heart failure. The recent identification of cardiotonic steroids such as ouabain, digoxin, marinobufagenin, and telocinobufagin in blood plasma, adrenal glands, and hypothalamus of mammals led to exciting new perspectives in the pathology of heart failure and arterial hypertension. Biosynthesis of ouabain and digoxin occurs in adrenal glands and is under the control of angiotensin II, endothelin, and epinephrine released from cells of the midbrain upon stimulation of brain areas sensing cerebrospinal Na(+) concentration and, apparently, the body's K(+) content. Rapid changes of endogenous ouabain upon physical exercise may favor the economy of the heart by a rise of intracellular Ca(2)(+) levels in cardiac and atrial muscle cells. According to the sodium pump lag hypothesis, this may be accomplished by partial inhibition of the sodium pump and Ca(2+) influx via the Na(+)/Ca(2+) exchanger working in reverse mode or via activation of the Na(+)/K(+)-ATPase signalosome complex, generating intracellular calcium oscillations, reactive oxygen species, and gene activation via nuclear factor-kappaB or extracellular signal-regulated kinases 1 and 2. Elevated concentrations of endogenous ouabain and marinobufagenin in the subnanomolar concentration range were found to stimulate proliferation and differentiation of cardiac and smooth muscle cells. They may have a primary role in the development of cardiac dysfunction and failure because (i) offspring of hypertensive patients evidently inherit elevated plasma concentrations of endogenous ouabain; (ii) such elevated concentrations correlate positively with cardiac dysfunction, hypertrophy, and arterial hypertension; (iii) about 40% of Europeans with uncomplicated essential hypertension show increased concentrations of endogenous ouabain associated with reduced heart rate and cardiac hypertrophy; (iv) in patients with advanced arterial hypertension, circulating levels of endogenous ouabain correlate with BP and total peripheral resistance; (v) among patients with idiopathic dilated cardiomyopathy, high circulating levels of endogenous ouabain and marinobufagenin identify those individuals who are predisposed to progressing more rapidly to heart failure, suggesting that endogenous ouabain (and marinobufagenin) may contribute to toxicity upon digoxin therapy. In contrast to endogenous ouabain, endogenous marinobufagenin may act as a natriuretic substance as well. It shows a higher affinity for the ouabain-insensitive alpha(1) isoform of Na(+)/K(+)-ATPase of rat kidney tubular cells and its levels are increased in volume expansion and pre-eclampsia. Digoxin, which is synthesized in adrenal glands, seems to counteract the hypertensinogenic action of ouabain in rats, as do antibodies against ouabain, for example, (Digibind) and rostafuroxin (PST 2238), a selective ouabain antagonist. It lowers BP in ouabain- and adducin-dependent hypertension in rats and is a promising new class of antihypertensive medication in humans.     

Doxorubicin cardiotoxicity: possible role of digoxin in its prevention

Twenty-nine patients with gynaecological cancers who received over 400 mg of doxorubicin were monitored electrocardiographically to determine whether cardiac glycosides countered the adverse effects of high total doses of doxorubicin. Minor electrocardiographical changes were noted in five out of six patients who were not receiving a cardiac glycoside and four out of six who were receiving ouabain, and none of the 16 who were receiving digoxin. One other patient on digoxin stopped taking it and developed cardiomyopathy. One patient on ouabain also developed cardiomyopathy. So far nine patients on digoxin have received between 550 and 1000 mg/m2 of doxorubicin without ill effect. Cardiac glycosides are thought to prevent doxorubicin cardiomyopathy by competitively inhibiting doxorubicin at its receptor sites, but ouabain has a much shorter half life than doxorubicin and its metabolites and so is less effective than digoxin.     

Effects of basic drugs on the hepatic transport of cardiac glycosides in rats

The effects of some basic and acidic drugs on the hepatic uptake of digoxin and ouabain were studied in isolated rat hepatocytes. Digoxin accumulated against a concentration gradient, and its initial uptake was energy- and temperature-dependent. Digoxin competitively inhibited the uptake of ouabain (Ki = 1.3 microM), which was reported to be transported by a carrier-mediated active transport system. All basic drugs tested (verapamil, dipyridamole, amiodarone, nifedipine, diltiazem, ajmaline, chlorpromazine, imipramine, disopyramide, quinidine, procainamide, propranolol and lidocaine: 50 microM) except for procainamide, propranolol and lidocaine significantly (P less than 0.05) reduced the uptake of digoxin, whereas acidic drugs (salicylic acid and phenytoin) had no effect. The same inhibitory effects were observed for ouabain uptake, whereas the uptake of alanine was not changed by these drugs. Quinidine inhibited the uptake of ouabain in a noncompetitive manner (Ki = 88 microM). These basic drugs had no effect on the permeability of the cells assessed by the trypan blue exclusion test and succinate-simulated oxygen consumption. But carbonylcyanide-m-chlorophenyl hydrazone-stimulated oxygen consumption decreased in the presence of some basic drugs and correlated with their inhibitory effects on digoxin uptake. Therefore, one of the mechanisms of the inhibitory effects of these drugs on digoxin uptake was the inhibition of oxidative phosphorylation. These basic drugs had no effect on the microtubular system, which was assessed by the measurement of tubulin polymerization and colchicine binding to tubulin. The results of our study suggested that many basic drugs have a potential to inhibit the hepatocellular uptake of cardiac glycosides.     

Lack of interaction of spironolactone with ouabain guinea pig isolated heart muscle preparations.

The effect of spironolactone on ouabain action was studied in experiments on guinea pig isolated heart muscle preparations. There was no effect of spironolactone on the ouabain-induced positive inotropic or toxic effects in isolated papillary muscles. This was accompanied by a lack of spironolactone effect on myocardial ouabain uptake. Only rather high spironolactone concentrations (1 X 10(-4) M) led to a reduced cardiac ouabain uptake (about 10%). The subcellular distribution pattern of ouabain, however, remained unchanged under these conditions. Studies on ouabain binding to a cardiac NaK-ATPase supported the conclusion that a direct interaction of spironolactone and cardiac glycosides at the myocardium could be ruled out.