Na(+),K(+)-ATPase isoform selectivity for digitalis-like compounds is determined by two amino acids in the first extracellular loop.

Digitalis-like compounds (DLCs) comprise a diverse group of molecules characterized by a cis-trans-cis ring-fused steroid core linked to a lactone. They have been used in the treatment of different medical problems including heart failure, where their inotropic effect on heart muscle is attributed to potent Na(+),K(+)-ATPase inhibition. Their application as drugs, however, has declined in recent past years due to their small safety margin. Since human Na(+),K(+)-ATPase is represented by four different isoforms expressed in a tissue-specific manner, one of the possibilities to improve the therapeutic index of DLCs is to exploit and amend their isoform selectivity. Here, we aimed to reveal the determinants of selectivity of the ubiquitously expressed α1 isoform and the more restricted α2 isoform toward several well-known DLCs and their hydrogenated forms. Using baculovirus to express various mutants of the α2 isoform, we were able to link residues Met(119) and Ser(124) to differences in affinity between the α1 and α2 isoforms to ouabain, dihydro-ouabain, digoxin, and dihydro-digoxin. We speculate that the interactions between these amino acids and DLCs affect the initial binding of these DLCs. Also, we observed isoform selectivity for DLCs containing no sugar groups.

In vitro propagation and production of cardiotonic glycosides in shoot cultures of Digitalis purpurea L. by elicitation and precursor feeding.

Digitalis purpurea L. (Scrophulariaceae; Foxglove) is a source of cardiotonic glycosides such as digitoxin and digoxin which are commercially applied in the treatment to strengthen cardiac diffusion and to regulate heart rhythm. This investigation deals with in vitro propagation and elicited production of cardiotonic glycosides digitoxin and digoxin in shoot cultures of D. purpurea L. In vitro germinated seedlings were used as a primary source of explants. Multiple shoot formation was achieved for three explant types (nodal, internodal, and leaf) cultured on Murashige and Skoog (MS) medium with several treatments of cytokinins (6-benzyladenine-BA; kinetin-Kin; and thidiazuron-TDZ) and auxins (indole-3-acetic acid-IAA; α-naphthaleneacetic acid-NAA; and 2,4-dichlorophenoxy acetic acid-2,4-D). Maximum multiple shoots (12.7 ± 0.6) were produced from nodal explants on MS + 7.5 µM BA. Shoots were rooted in vitro on MS containing 15 µM IAA. Rooted plantlets were successfully acclimatized. To further maintain the multiple shoot induction, mother tissue was cut into four equal parts and repeatedly sub-cultured on fresh shoot induction liquid medium after each harvest. On adaptation of this strategy, an average of 18 shoots per explant could be produced. This strategy was applied for the production of biomass and glycosides digitoxin and digoxin in shoot cultures on MS medium supplemented with 7.5 µM BA and several treatments with plant growth regulators, incubation period, abiotic (salicylic acid, mannitol, sorbitol, PEG-6000, NaCl, and KCl), biotic (Aspergillus niger, Helminthosporium sp., Alternaria sp., chitin, and yeast extract) elicitors, and precursors (progesterone, cholesterol, and squalene). The treatment of KCl, mycelial mass of Helminthosporium sp., and progesterone were highly effective for the production of cardenolides. In the presence of progesterone (200 to 300 mg/l), digitoxin and digoxin accumulation was enhanced by 9.1- and 11.9-folds respectively.

Selectivity of digitalis glycosides for isoforms of human Na,K-ATPase.

There are four isoforms of the alpha subunit (alpha1-4) and three isoforms of the beta subunit (beta1-3) of Na,K-ATPase, with distinct tissue-specific distribution and physiological functions. alpha2 is thought to play a key role in cardiac and smooth muscle contraction and be an important target of cardiac glycosides. An alpha2-selective cardiac glycoside could provide important insights into physiological and pharmacological properties of alpha2. The isoform selectivity of a large number of cardiac glycosides has been assessed utilizing alpha1beta1, alpha2beta1, and alpha3beta1 isoforms of human Na,K-ATPase expressed in Pichia pastoris and the purified detergent-soluble isoform proteins. Binding affinities of the digitalis glycosides, digoxin, beta-methyl digoxin, and digitoxin show moderate but highly significant selectivity (up to 4-fold) for alpha2/alpha3 over alpha1 (K(D) alpha1 > alpha2 = alpha3). By contrast, ouabain shows moderate selectivity ( approximately 2.5-fold) for alpha1 over alpha2 (K(D) alpha1

Membrane adenosinetriphosphatase: a digitalis receptor?

The enzyme Na+,K+-ATPase is a good model for receptor studies because of its known functional correlates. The binding of digitalis to the enzyme observed in vitro satisfied the criteria for receptor binding. Studies of the relationship between the digitalis binding and the drug action reveal an impressive correlation between these events but fail to provide proof of a causal relationship. Studies with other Na+,K+-ATPase inhibitors and agents that affect transmembrane Na+ movements (steps that would follow Na+,K+-ATPase inhibition) provide further supportive evidence that sodium pump inhibition and the resulting enhancement of intracellular Na+ transients cause the inotropic action of digitalis.

Endogenous digitalis: pathophysiologic roles and therapeutic applications.

Endogenous digitalis-like factors, also called cardiotonic steroids, have been thought for nearly half a century to have important roles in health and disease. The endogenous cardiotonic steroids ouabain and marinobufagenin have been identified in humans, and an effector mechanism has been delineated by which these hormones signal through the sodium/potassium-transporting ATPase. These findings have increased interest in this field substantially. Although cardiotonic steroids were first considered important in the regulation of renal sodium transport and arterial pressure, subsequent work has implicated these hormones in the control of cell growth, apoptosis and fibrosis, among other processes. This Review focuses on the role of endogenous cardiotonic steroids in the pathophysiology of essential hypertension, congestive heart failure, end-stage renal disease and pre-eclampsia. We also discuss potential therapeutic strategies that have emerged as a result of the increased understanding of the regulation and actions of cardiotonic steroids.

The neurogenic vasoconstrictor effect of digitalis on coronary vascular resistance.

The coronary vasoconstrictor properties of digitals were evaluated in 61 anesthetized, openchest dogs after coronary sinus cannulation and under conditions of a constant heart rate (atrioventricular pacing) and near-constant blood pressure. The contribution of alpha adrenergic receptor stimulation to the digitalis-induced increase in coronary vascular resistance (CVR) was examined. With Na pentobarbital anesthesia (16 dogs), intravenous acetylstrophanthidin (0.5 mg) caused a significant (P<0.05) rise in CVR from 1 through 9 min after injection. The peak increase was +11+/-2% SE of the control of 1.8+/-0.2 mm Hg/cm(3)/min. The mean time to peak effect was 3 min, and to recovery was 21 min. Prior alpha adrenergic receptor blockade with phenoxybenzamine in 11 animals reduced (P<0.05) the acetylstrophanthidin-induced peak of CVR and substantially decreased (P<0.05) the time to recovery (5 min). Intravenous digoxin (1.0 mg) with Na pentobarbital anesthesia (five dogs) had no significant effect on CVR. However, with chloralose and urethane anesthesia (nine dogs) the same dose of digoxin produced a significant rise in CVR from 3 through 30 min. The peak increase was +20+/-3% of control (1.4+/-0.1 mm Hg/cm(3)/min). One-third the dose of intravenous digoxin (0.35 mg) produced a 9.5+/-1.0% increase in CVR (five additional dogs). Myocardial oxygen consumption did not change significantly in nine dogs after intravenous digoxin. In 10 additional dogs pretreated with phenoxy-benzamine and in 7 dogs pretreated with mecamylamine, the increase in CVR did not occur after 1.0 mg of intravenous digoxin. Thus there is a coronary vasoconstrictor effect of intravenous acetylstrophanthidin and digoxin, of rapid onset, which is mediated through alpha adrenergic receptor stimulation.

Factors affecting tolerance to digitalis.

A review of factors altering the safety margin between a therapeutic and a toxic dose of digitalis includes the consideration of: clinical conditions to which digitalis action may be undesirable, allergy and hypersensitivity to digitalis, physiologic factors modifying tolerance to digitalis, factors that change the amount of digitalis in the body, nervous and metabolic factors modifying tolerance to digitalis, modifications of digitalis tolerance produced by the status of the myocardium, and modifications of digitalis tolerance produced by diseases of other organs. The problems related to digitalis toxicity are more common than those of resistance to treatment. The most important factors contributing to decreased tolerance and risk of toxicity are: heart disease, poor renal function, hypokalemia and hypothyroidism. The roles of impaired liver function, chronic lung disease, acid-base disturbances, anesthesia, autonomic imbalance, calcium and magnesium are less important and less well established.

Endogenous digitalis-like factors. An historical overview.

The sodium pump is a ubiquitous cell surface enzyme, a Na/K-ATPase, that maintains ion gradients between cells and the extracellular fluid. The extracellular domain of this enzyme contains a highly conserved receptor for a plant-derived family of compounds, the digitalis glycosides, used in the treatment of congestive heart failure, and certain cardiac arrhythmias. The concept that an endogenous modulator of this enzyme, analogous to the cardiac glycosides, emerged from work on two separate areas: the regulation of extracellular fluid (ECF) volume by a natriuretic hormone (NH), and the regulation of peripheral vascular resistance by a circulating inhibitor of vascular Na/K-ATPase. These two areas merged with the hypothesis that natriuretic hormone and the vascular Na/K-ATPase inhibitor were the same factor, and furthermore, that this factor played a causative role in the pathophysiology of certain types of hypertension. In this communication, the development of this field from its beginnings is traced; evidence for the existence of and efforts to identify the structure of this factor are briefly reviewed, and suggestions for future development of the field are put forward.

Developmental increase of digitalis receptors in guinea pig heart.

The apparent differences of digitalis tolerance in human newborns or young animals and adults may be due to the difference in either, the sensitivity of Na+,K+ -ATPase to digitalis, the binding characteristics of the Na+,K+ -ATPase receptor sites or the number of receptor sites itself. Sarcolemmal Na+,K+ -ATPase enriched membrane preparations from guinea pig hearts of various age groups were investigated for Na+,K+ -ATPase activity, the ability of ouabain to inhibit the enzyme and the 3H-ouabain binding characteristics of the receptor sites. There was a continuous developmental increase in Na+,K+ -ATPase activity in fetal (45 days fetus, 1.2 mumol Pi . mg-1 protein . h-1) and newborn (2.4 mumol Pi . mg-1 protein . h-1) reaching maturity at the age of 21 to 25 days (9.1 mumol Pi . mg-1 protein . h-1). The ability of digitalis to inhibit the enzyme, however, was same throughout the development with 50% inhibition occurring at 0.1 mumol . litre-1 concentration of ouabain. The sodium stimulated 3H-ouabain binding was saturable and exhibited a single class of receptor sites. The number of digitalis receptor sites increased progressively in fetal (45 days fetus, 2.2 pmol . mg-1 protein) and newborn (9.1 pmol . mg-1 protein) reaching mature levels at the age of 13 to 25 days. The affinity of the Na+,K+ -ATPase receptor sites for digitalis (Kd, 8.68 - 9.33 x 10(-9) mol . litre-1) remain unchanged during development. The data obtained is consistent with the concept that Na+,K+ -ATPase is the receptor site for digitalis and suggests that factors other than the developmental increase in Na+,K+ -ATPase may be relevant to explain the age-related differences in digitalis sensitivity.

No upregulation of digitalis glycoside receptor (Na,K-ATPase) concentration in human heart left ventricle samples obtained at necropsy after long term digitalisation.

STUDY OBJECTIVE: The aim was to evaluate the hypothesis that digitalis glycosides increase the concentration of their specific receptor (Na,K-ATPase) in human myocardial tissue, thereby possibly reducing the inotropic effect of long term digitalis treatment. DESIGN: Intact samples of left ventricle were obtained at necropsy from patients who had been on long term treatment with digoxin and from patients not previously given digoxin. Digitalis glycoside receptors were quantified using vanadate facilitated 3H-ouabain binding before and after washing samples in buffer containing excess digoxin antibody fragments for 16 h at 30 degrees C. This washing procedure has previously been shown to reduce prior specific digoxin binding in human left ventricle by 95% and to allow subsequent vanadate facilitated complete quantification of 3H-ouabain binding sites. In this context it was performed to reduce occupancy of digitalis glycoside receptors by digoxin, caused by digitalisation before 3H-ouabain binding. SUBJECTS: 11 patients who had been on long term treatment with digoxin and eight who had not previously been given digoxin were studied. Left ventricle samples were obtained at necropsy at around 15 h after death. MEASUREMENTS AND MAIN RESULTS: Standard 3H-ouabain binding was 39% less in samples from digitalised than from undigitalised subjects (p less than 0.001). Washing samples in buffer containing excess digoxin antibody fragments induced an increase in 3H-ouabain binding from 174(SEM 10) to 265(20) pmol.g-1 wet weight (n = 11, p less than 0.001) in samples from digitalised patients. After washing, the digitalis glycoside receptor concentration in left ventricle samples showed a tendency to a lower value (14%, p greater than 0.10) in patients exposed to digoxin compared to left ventricle samples from individuals unexposed to digitalis glycoside treatment. Calculating 3H-ouabain binding relative to dry ventricular muscle weight confirmed the results obtained using wet weight as reference. CONCLUSIONS: The results suggest that digoxin treatment in life is associated with a 34% occupancy of digitalis glycoside receptors with digoxin. In the human heart there was no evidence for upregulation of digitalis glycoside receptor concentration due to long term digitalisation. Thus at receptor level there was no evidence for development of tolerance to digoxin therapy. The lower digitalis glycoside receptor concentration in the left ventricle observed in the heart failure patients may support the report of a relationship between Na,K-ATPase concentration as evaluated by 3H-ouabain binding and left ventricular function.

Capturing clinical expertise. A computer program that considers clinical responses to digitalis.

The administration of digitalis by experienced cardiologists has been examined and a prototypical computer program has been developed which captures portions of their expertise. The new program first constructs a patient-specific model upon which to base the determination of dosage and then uses feedback information about a variety of clinical aspects of the patient's response to modify its recommendations. The model reflects both the program's knowledge of pharmacokinetics and those special features of the patient's condition which may alter his response to therapy. The program makes assessments of the therapeutic and the toxic effects of digitalis on the patient, and bases its subsequent recommendations on the "therapeutic-toxic" state which best describes the evolving clinical situation. A clinical trial was performed in which the program "followed" a series of patients managed by clinicians on a cardiology service. That trial demonstrated the feasibility of this type of program in dealing with acutely ill patients, even those who have increased sensitivity to the toxic effects of digitalis. Each patient in the trial in whom toxicity developed had received more digitalis than would have been recommended by the program. This approach to automated clinical consultation should eventually provide a technology for the distribution of clinical expertise.

Digitalis pharmacokinetics and metabolism.

The pharmacokinetics of the cardiac glycofides have been elucidated as a result of the development of assays of sufficient sensitivity to measure the concentration of digitalis compounds in biological fluids. Digoxin can accumulate in the body without the administration of a loading dose, and a steady state blood concentration will be reached in 5 to 7 days. Digitoxin requires 35 days to accumulate to a plateau. If a loading dose of digoxin is used, it should be approximately three times the estimated daily maintenance dose. Factors that determine the selection of the appropriate maintenance dose of digoxin include renal function and lean body mass. Digitoxin is less dependent on renal function for its elimination than is digoxin. Knowledge of the pharmacokinetics of digitalis preparations is useful in determining how to change from one cardiac glycoside to another, each with different half-lives. One should wait 3 days before starting digoxin therapy when changing from maintenance digitoxin to digoxin (assuming normal renal function). The pharmacokinetics of changing from ouabain to digoxin without loss of digitalis effect are described. The metabolism of the commonly used digitalis preparations are summarized.

Effects of two putative endogenous digitalis-like factors, marinobufagenin and ouabain, on the Na+, K+-pump in human mesenteric arteries.

OBJECTIVES: Recently, in rat aortae, two putative digitalis-like factors, marinobufagenin and ouabain, were shown to interact with alpha-1 (sarcolemma) and alpha-3 (nerve endings) subunits of the sodium pump, respectively, and elicit vasoconstriction via inhibition of the Na+,K+-pump in vascular smooth muscle or norepinephrine release. The purpose of the present study was to investigate the effects of ouabain and marinobufagenin on vascular tone, activity of Na+,K+-pump, and the expression of isoforms of the Na+,K+-ATPase alpha-subunit in human mesenteric arteries. DESIGN AND METHODS: Arteries were obtained from male patients undergoing surgery due to intestinal adenocarcinoma. Vasoconstrictor effects of both inhibitors were studied in isolated vascular rings. Na+,K+-pump activity was measured using the 86Rb technique. Membrane fractions of sarcolemma and nerve endings plasmalemma were prepared via differential centrifugation of membranes in a sucrose density gradient Specific antibodies to the alpha-1 and alpha-3 subunits of Na+,K+-ATPase were used to detect alpha-1 and alpha-3 isoforms in the membrane fractions by Western blotting. RESULTS: Marinobufagenin (EC50 = 88+/-15 nmoles/l) and ouabain (EC50 = 320+/-50 nmoles/l) elicited vasoconstriction in mesenteric artery rings. At a concentration of 1 nmol/l, both compounds stimulated the Na+,K+-pump, but inhibited its activity at 10-1000 nmoles/l. No stimulation of the Na+,K+-pump was observed in the presence of 5 micromol/l phentolamine; rather 1 nmol/l of marinobufagenin and ouabain inhibited the Na+,K+-pump by 30% and 13%, respectively. The alpha-1 polyclonal antibody detected alpha-1 isoform in membrane fractions from both sarcolemma and nerve endings. A monoclonal alpha-1 antibody detected the material in sarcolemmal membranes only. The alpha-3 isoform was detected in both membrane fractions by both antibodies, but staining for alpha-3 was more pronounced in the nerve endings. CONCLUSIONS: These results demonstrate that, in physiologically 'realistic' concentrations, marinobufagenin and ouabain can significantly affect the Na+,K+-pump in human mesenteric artery, and illustrate the importance of interaction of digitalis-like Na+,K+-pump inhibitors with the Na+,K+-ATPase localized to the intravascular adrenergic terminals. Present observations are in accord with the previous data suggesting that marinobufagenin and ouabain display greater affinity to alpha-1 and alpha-3 isoforms of the Na+,K+-pump, respectively, and support the view that the differential responsiveness to endogenous digitalis-like inhibitors is one of the features of alpha-isoforms of Na+,K+-ATPase.

Drug metabolism in thyroid disease.

Thyroid dysfunction can influence the physiological disposition of drugs. Depending on the pharmacokinetic properties of the individual drug, changes in the rate of metabolism ranging from profound to moderate or negligible have been observed. Since renal function is also influenced by thyroid disease, changes in renal elimination of drugs which are excreted in the urine mainly as unchanged drugs have to be considered as another reason for altered drugs disposition in thyroid disease. In patients with thyrotoxicosis lower, and in patients with myxoedema, higher, digitalis plasma levels have been observed. The altered disposition of cardiac glycosides in thyroid dysfunction can be attributed to changes in renal elimination and metabolism. These findings may be the kinetic correlate for the clinical observation that larger than the usual dose of digitalis is required in thyrotoxic patients and lower in hypothyroid patients. Antipyrene half-lives are very much shortened during hyperthyroidism and prolonged appreciably during hypothyroidism. The alterations in the disposition of these drugs seen during thyroid dysfunction can be ascribed to changes in its rate of metabolism which is controlled by the levels of circulating thyroid hormones. N-demethylation of aminopyrine is depressed both in hyper- and hypothyroid patients as compared with euthyroid subjects. Changes in the half-life of this drug were observed only during hypothyroidism. The physiological disposition of the antithyroid drug propylthiouracil is not changed during thyrotoxicosis. A decrease in plasma half-life of methimazole is however, observed during hyperthyroidism, whereas in hypothyroid patients half-life is increased. The few data available so far do not allow general prediction of how thyroid disease could alter drug metabolism in man.

Influence of food on the bioavailability of drugs.

Food intake exerts a complex influence on the bioavailability of drugs. It may interfere not only with tablet disintegration, drug dissolution and drug transit through the gastrointestinal tract, but may also affect the metabolic transformation of drugs in the gastrointestinal wall and in the liver. Different food components can have different effects, and food may interact in opposite ways, even with drugs that are chemically related. Therefore, the net effect of food on drug bioavailability can be predicted only by direct clinical studies of the drug in question. As judged mainly from single meal, single dose studies, food intake enhances the bioavailability of several different drugs, such as propranolol, metoprolol, hydrallazine, hydrochlorothiazide, canrenone (from spironolactone), nitrofurantoin, erythromycin (stearate), dicoumarol, phenytoin and carbamazepine, but reduces that of drugs such as isoniazid, rifampicin, tetracycline, penicillin and ampicillin, while having no consistent effect on the bioavailability of metronidazole, oxazepam, melperone, propylthiouracil, sulphasomidine and sulphonylureas. For some drugs such as digoxin and paracetamol, the rate but not the extent of absorption is reduced. Food may enhance bioavailability even though, or rather because, the rate of gastric emptying is reduced; this is apparently the case with hydrochlorothiazide and nitrofurantoin. The food induced enhancement of bioavailability of propranolol, metoprolol and hydrallazine is probably due to reduced first pass metabolism of these drugs, while food induced improvement of drug dissolution may explain the enhanced bioavailability of carbamazepine, canrenone, dicoumarol and phenytoin. An increased gastrointestinal pH may be in part the cause of the food induced reduction of the bioavailability of drugs such as isoniazid and tetracycline. In addition to single meal effects, repeated intake of protein-rich meals enhance, while carbohydrate-rich meals reduce, the rate of oxidation of antipyrine and theophylline. Moreover, intake of charcoal broiled meat markedly accelerates the oxidation of phenacetin and variably accelerates elimination of theophylline. Thus, food and its components and contaminants may have both short and long term effects on both the absorptive and biotransformation processes influencing systemic availability of drugs.

Species and ionic influences on the accumulation of digitalis glycosides by isolated perfused hearts.

1. The ability of isolated perfused guinea-pig (digitalis-sensitive species) and rat (digitalis-resistant species) hearts to accumulate radio-labelled digitalis glycosides was studied in relation to the ionic composition of the perfusion medium.2. It was observed that in both species much less digoxin was accumulated than was digitoxin or proscillaridin.3. The accumulation of digoxin was markedly inhibited in a low sodium or in high potassium medium. These effects were similar, but relatively less marked, with digitoxin and proscillaridin. Calcium and magnesium removal had relatively smaller effects on the accumulation of both polar and non-polar glycosides.4. The low accumulation of all digitaloids by the rat heart in comparison to the guinea-pig heart may be due to the formation of unstable complexes between the cellular membranes in the rat heart and the various digitaloids used in this study. Although digitaloids have a reduced affinity for rat hearts and rat heart membranes in comparison to guinea-pigs, the order of the accumulation of different glycosides in both species is the same, i.e. much less with polar glycosides than with non-polar glycosides.5. It was concluded that non-polar glycosides such as digitoxin and proscillaridin demonstrate the same ion-dependent accumulation mechanism as do the more polar glycosides such as digoxin and ouabain. In addition, the non-polar glycosides possess high capacity for ion-independent binding presumably due to lipophilic interactions with membranes.

Digitalis structure-activity relationship analyses. Conclusions from indirect binding studies with cardiac (Na+ + K+)-ATPase.

We have performed direct and indirect binding studies with [3H]ouabain or [3H]digitoxin on beef or guinea pig cardiac (Na+ + K+)-ATPase to measure the potencies of a broad range of cardiotonic steroids for structure-activity relationship (SAR) studies for comparison with previously determined positive inotropic potencies. The positive inotropic potencies of twelve compounds on contracting guinea pig left atria correlated well with the equilibrium dissociation constants (KD values) from the inhibition of [3H]ouabain binding to guinea pig cardiac (Na+ + K+)-ATPase (r = 0.98 for seven 5 beta-compounds, r = 0.95 for five 5 alpha-compounds). Further we calculated KD values from the inhibition of [3H]ouabain binding data for a total of 33 digitalis derivatives on the digitalis-sensitive beef cardiac (Na+ + K+)-ATPase. For the 27 compounds tested on both beef cardiac (Na+ + K+)-ATPase and guinea pig left atria, the potencies showed a significant correlation (r = 0.92 for 22 5 beta-compounds, r = 0.96 for five 5 alpha-compounds. For seven compounds, KD values were measured on beef cardiac (Na+ + K+)-ATPase using inhibition of binding of [3H]digitoxin. These values correlated well (r = 0.99) with the KD values from the [3H]ouabain studies. These results show that: (1) The significant correlation observed between KD values on guinea pig cardiac (Na+ + K+)-ATPase and positive inotropic potency in guinea pig left atria is further evidence that the pharmacological receptor for inotropy is part of the enzyme, (2) Inhibition of the binding of [3H]ouabain or [3H]digitoxin can be used to determine the relative potencies of unlabelled digitalis derivatives. The similar relative potencies on beef and guinea pig cardiac (Na+ + K+)-ATPase of a broad range of digitalis derivatives indicate that the binding site is similar for both species; and (3) SAR studies indicate that functional groups on these steroids have the same influence on potency on either the positive inotropy or cardiac (Na+ + K+)-ATPase studies.

Digitalis glycosides.

The digitalis glycosides number among the most commonly used cardiovascular drugs. Despite 200 years of investigation, much new information regarding their mechanisms of action and clinical use continues to be developed. Clinicians dispensing these potent drugs need to be aware of thes advances if they are to make the best possible therapeutic decisions.

Digitalis toxicity.

The principal causes of digitalis toxicity are overdose, reduced volume of distribution, reduced renal elimination, and increased myocardial sensitivity. The metabolic mechanism of digitalis toxicity is intense inhibition of sarcolemma Na-K ATPase, which leads to increases of intracellular Na+ and Ca2+ and arrhythmogenic membrane ionic currents. A variety of cellular electrophysiologic effects and effects on the nervous system are responsible for the array of clinical arrhythmias seen during digitalis toxicity, i.e., sinus bradycardia, atrioventricular block, nonparoxysmal atrioventricular junctional tachycardia, and ventricular tachycardia.

Cardiovascular drug interactions.

Drug interactions may be responsible for certain changes in therapeutic response and toxicity of cardiac drugs. Interactions occur at the sites of drug absorption and elimination as well as at the receptor sites in the pacemaker cells, specialized conducting tissue, and myocardium. Studies of the kinetics of cardiac drugs are being applied clinically in an effort to reduce the danger of adverse drug interactions in heart patients.