Uptake, accumulation and release of ouabain by isolated rat hepatocytes

We investigated uptake of ouabain into isolated rat hepatocytes and release of ouabain from preloaded hepatocytes, thus assessing separately the two membrane transport steps, involved in biliary elimination of the drug. The following results were obtained: Uptake of ouabain was saturable (Km = 263 + 61 microM, V = 3.3 + 1.0 nmol/mgprot. x min), energy-dependent, sensitive to dinitrofluorobenzene and temperature-dependent (E = 80 - 96 kJ/mol). It had no pH-optimum in the physiological pH-range and was independent of the extracellular cation composition. Uptake of ouabain was competitively inhibited by the cardiac glycoside digitoxin (Ki = 0.3 microM). Uptake was not inhibited in the presence of the glycosidic sugar l-rhamnose, but it was competitively inhibited by the steroid taurocholate (Ki = 6.3 microM). Ouabain was accumulated within hepatocytes 170-fold. The predominant fraction of intracellular ouabain being unbound. Release of ouabain from preloaded cells was energy-independent, independent of the Na+-concentration and not susceptible to inhibition by dinitrofluorobenzene or taurocholate. It is concluded, first that hepatocellular uptake of ouabain is mediated by a carrier for steroids and second that the pathway of release is distinct from that of uptake. We assume, that the high bile/plasma concentration-gradient of ouabain in vivo is generated during active uptake into the cell and not during release into bile.

The modifying effect of autonomic blockade on digitoxin-induced changes in monophasic action potential and refractoriness of the right ventricle of the dog heart

The aim of the present investigation was to study the modifying effect of autonomic blockade with atropine plus propranolol and atropine alone on the electrophysiologic response to digitoxin during an observation period of 8 hours. Twenty-five pentobarbital-anaesthetised Labrador dogs were used. Pacing and programmed electrical stimulation were used to determine heart rate independent changes in repolarisation times and the effective (V-ERP) and the functional (V-FRP) refractory periods. The dogs were divided into three groups and all dogs got digitoxin (2.0 mg) intravenously during 5 min. Fourteen dogs were given digitoxin without blockade, while 5 dogs were pretreated with propranolol 0.5 mg/kg plus atropine 0.05 mg/kg every hour for 8 hours and 6 dogs with atropine in the same dose and with the same interval. Digitoxin alone increased 50 and 90% repolarisation times 2-4 hours after injection of the drug. During autonomic blockade digitoxin induced a shortening of action potential duration, and thus digitoxin interacts with autonomic transmitters resulting in prolongation of action potential duration. The "direct" effects of digitoxin, the action potential shortening effect, the decrease in membrane responsiveness (i.e. V-ERP/50% repolarisation time) reached a maximum 2-3 hours after injection of the drug.

ATP-energized Ca2+ pump in isolated transverse tubules of skeletal muscle

A modified protocol for isolation of transverse tubules incorporated an extra stage of purification. The existence of an ATP-energized Ca2+ pump in transverse tubules isolated from rabbit skeletal muscle has been demonstrated. Isolated transverse tubules had a Ca-ATPase activity of 0.78 mu mol/min . mg; this was 300% in excess of that activity attributable to sarcoplasmic reticulum contamination. The distribution of part of the CaATPase activity and ATP-energized Ca2+ uptake coincided with the distribution of transverse tubules in isopycnic sucrose gradients loaded with mechanically disrupted triad junctions. Transverse tubules accumulated over 70 nmol of Ca2+/mg of protein; this uptake was abolished by the Ca2+ ionophore A23187. Neither digitoxin nor monensin inhibited Ca2+ uptake, indicating that Ca2+ accumulation did not occur through a sodium/calcium exchange. Conditions for half-maximal Ca2+ uptake were 5 micro M free Ca2+ and 10 micro M ATP. The Ca2+ pump of isolated transverse tubules was distinguished from the Ca2+ pump of sarcoplasmic reticulum and sarcolemma in that the transverse tubule Ca2+ pump: 1) was not enhanced by oxalate; 2) was not energized by acetyl phosphate, p-nitrophenyl phosphate, or 3-O-methylfluorescein phosphate; and 3) did not hydrolyze p-nitrophenyl phosphate or 3-O-methyl-fluorescein phosphate. Using Ca2+-dependent 3-O-methylfluorescein phosphatase as a marker for sarcoplasmic reticulum, the contamination of the transverse tubule preparation was calculated to be 6%. This agreed with a contamination level of 5% estimated by freeze-fracture electron microscopy.

Hypotensive effects of cardiac glycosides in spontaneously hypertensive rats

The effects of cardiac glycosides on systolic arterial pressure (SAP) and heart rate (HR) were studied in spontaneously hypertensive (SH) and normotensive (NT) Wistar rats. Ouabain (2 mg/kg), given by i.p. injection to unanesthetized rats, produced a significant decrease in SAP in both SH and NT rats with little concomitant HR change. Baroreceptor denervation abolished the response. To analyze the effects more completely, experiments were done on acutely anesthetized rats. Ouabain was administered i.p. 30 min after induction of anesthesia with alpha-chloralose (60 mg/kg i.p.). alpha-Chloralose alone produced a significant SAP decrease in both SH and NT rats and bradycardia in SH but no significant HR change in NT rats. Ouabain i.p. further reduced SAP in SH but had no effect on NT rats. When given by i.a. bolus injection (250 microgram in 0.1 mg of heparin-saline) to anesthetized (pentobarbital, 40 mg/kg i.p.) rats with intact left carotid sinus nerves, ouabain produced a transient but significant reduction in SAP of SH but NT rats or SH rats with sinoaortic denervation. Digitoxin, given by stomach tube (4--5 mg/kg) for 1 week, decreased SAP in SH but not NT rats. Digitoxin produced significant SAP increases in SH rats with sinoaortic denervation. Arterial baroreceptor discharge is increased by cardiac glycosides. The present results suggest that the reflex hypotensive effects of digitalis drugs are due, at least in part, to sensitization of baroreceptors and that SH rats may be more sensitive to these drugs possibly because of altered intrinsic baroreceptor properties.

[Interaction of quinidine and digitoxin in the human (author's transl)]

With a daily maintenance dose of 0.1 mg digitoxin a mean steady state digitoxin serum concentration of 17.0 +/- 3.2 ng/ml was measured in 10 male probands. When 750 mg of quinidine bisulphate were administered at the same time digitoxin concentration increased significantly to 22.4 +/- 4.2 ng/ml (P less than 0.0005). The serum half life of digitoxin during quinidine treatment was significantly increased to 10.8 +/- 2.1 days compared to a control group with 7.6 +/- 1.6 days (P less than 0.0025). Protein binding of digitoxin, renal digitoxin excretion and renal digitoxin clearance were equally uninfluenced by quinidine as were endogenous creatinine clearance and sodium and potassium excretion in urine. In two patient with cardiac insufficiency there was likewise a significant increase in digitoxin serum concentration. For clinical application of combined therapy of quinidine and digitoxin the danger of digitalis intoxication seems to be less in comparison to digoxin as increase of digitoxin concentration in serum is lower than of digoxin.

Digoxin- and digitoxin-induced changes in monophasic action potential of the right ventricle of the dog heart

The effects of digitoxin and digoxin on right ventricular monophasic action potentials were studied in intact dogs during an 8 hour observation period. Pentobarbital anaesthesia was used, and monophasic action potential recordings were obtained with the suction electrode technique. Intravenous injection of 2.0 mg digitoxin to six dogs, 2.0 mg digoxin to two dogs and 1.0 mg digoxin to five dogs caused a rapid increase in the time taken for 90% repolarisation with return to control values after 20 to 30 min. A late increase in 50 and 90% repolarisation times was observed 2 to 4 h after digitoxin, while digoxin had no such effect. The late action potential prolonging effect was inversely correlated to serum concentrations in the early elimination phase (2 to 8 h after the injection) since it reached a maximum towards the end of the observation period. Digitoxin and digoxin thus differed in their late effects on ventricular monophasic action potentials in intact dogs and may possess different antiarrhythmic activity.

Problems of rapid digitalization in severe congestive heart failure

The pharmacodynamic effects (changes of systolic time intervals, STI, reaction of pulmonary arterial pressure) of digitoxin were studied in 7 patients with severe congestive heart failure in comparison with the corresponding plasma level. STI indicated glycoside-dependent changes, i.e. shortening of LVETc and QS2c and normalization of prolonged PEPc, while ICT shortening was less observed. In 2 patients with cor pulmonale a pulmonary oedema occurred accompanied with prolonged LVETc. During the early period of glycoside-dependent recompensation no significant correlation between STI shortening and glycoside plasma level was observed. Because of the retarded normalization of the haemodynamics of the pulmonary circulation and because of possible side-effects, rapid digitalization has to be reconsidered.

[Effective concentration and dosage for cardiac glycosides]

The pharmacokinetic fundaments of the glycoside therapy with strophanthin, digoxin and digitoxin are described and compared with the traditional clinically oriented parameters total body stores, decay ratio and so on. Newly introduced is the parameter medium remaining duration in the circulation. A critical analysis of the hitherto cited values of the total body stores and the decay ratio established from the clinical effect shows that on the basis of pharmacokinetic calculations the corresponding values for digoxin and digotoxin need a great correction. The fully active quantity for digoxin lies between 0.5 and 0.8 mg and the subsiding share is 50%. The values for digitoxin lie at 0.9 mg and 9%. Furthermore the causes of the interindividual variability of the maintenance dose are discussed. According to the already existing international experiences an essential improvement of the therapy is possible, when the pharmacokinetically established effective constants are taken into consideration. Examinations of the inter- and intraindividual differences in the kinetics of the glycosides and the development of digoxin preparations with a high (i.e. constant) bioavailability may contribute to a further decrease of the frequency of toxic side-effects in future.

Effects of autonomic blockade on the inotropic and electrophysiologic response to digitoxin in the intact dog

Inotropic and electrophysiologic effects in dogs of a single dose of digitoxin with and without autonomic blockade with propranolol plus atropine were observed for 8 hr. Left heart catheterization and His bundle registration were performed in 14 Labrador dogs anesthetized with pentobarbital. Pacing and programmed electrical stimulation were used to determine heart rate-independent changes in dP/dt, intra-atrial, AV nodal, His-Purkinje, and intraventricular conduction velocity. The effective and the functional refractory periods of the atrium, and the functional nodal refractory period (AV-FRP) were measured. Six dogs were given propranolol 0.5 mg/kg plus atropine 0.05 mg/kg every hour. After determination of inotropic and electrophysiologic parameters, digitoxin (2.0 mg) was given to 5 of the dogs, while 1 dog served as control. The other 8 dogs were given only digitoxin (2.0 mg). The dP/dt increased after digitoxin in both groups with maximum values after 2 hr. The autonomic nervous system had only minor influence on changes in contractility caused by digitoxin. The marked fall in heart rate and increase in A-H time (i.e., AV nodal conduction time) and AV-FRP after digitoxin were almost completely blocked by atropine and propranolol. Minor increases in A-H time and AV-FRP indicated a slight direct effect of digitoxin on the AV node.

A specific cardiac glycoside for cardiac failure and another for atrial fibrillation?

Ouabain produces a greater degree of prolongation of the P-R interval than digitoxin in rats when dosages which produce similar inotropic responses are used. When digitoxin is administered after pretreatment with propranolol, it produces prolongation of the P-R interval comparable to that produced by ouabain. Indications in the literature that these findings may apply to human beings suggest that in some situations atrial fibrillation may be better controlled with a hydrophilic digitalis preparation (e.g. ouabain), whereas cardiac failure with a tendency to atrioventricular block may be better controlled with a lipophilic preparation (e.g. digitoxin).

Correlation between pharmacokinetics and intropic and electrophysiologic responses to digitoxin in the intact dog

We attempted to correlate inotropic and eletrophysilogic digitoxin effects with serum digitoxin concentrations during 8 hr after a single dose. Eight pentobarbital-anesthetized Labrador dogs were given 2.0 mg digitoxin intravenously. Left heart catheterization and His bundle registration were performed. acing and programed electrical stimulation were used to determine heart rate-independant changes in dP/dt, intra-atrial, atrioventricular (AV), His-Purkinje, and intraventricular conduction velocity. The effective (A-ERP) and functional (A-FRP) refractory periods of the atrium and the functional nodal refractory period (AV-FRP) were measured. Serum digitoxin concentrations were determined by radioimmunoassay. Median serum digitoxin half-time was 5.7 hr. The dP/dt increased after digitoxin, with maximum values after 2 hr. Digitoxin concentration correlated with the inotropic response after 4 hr. Heart rate fell significantly within 2 min and remained below control values for the 8 hr observation period concimitant with an increase in AV-nodal conduction time and AV-ERP increased significantly in the late elimination phase, while A-FRP increased slightly initially and remained high. No consistent correlation was found between the electrophysilogic variables and serum concentrations. We conclude that the two main effects of digitoxin, the inotropic and electrophysiologic, are dissociated in the elimination phase after a single dose.

[Antiaccelerative and antiarrhythmic effect of veratramin on myocardial cells in culture]

It is shown that veratramin (2.5 X 10(-5)M) inhibits, partially or completely, the accelerative effect of adrenalin (5.5 X 10(-7) M), isoproterenol (10(-7) M), prostaglandin E2 (5 X 10(-5) M), and glucagon (10(-5) M) in experiments on cultures of newborn rat cardiac cells and the effect of histamine on 3--4-week embryo cultures. Veratramin (1.25--25 X 10(-5) M) arrests digitoxin-induced (1.25 X10(-5) M--5 X 10(-5) M) fibrillation of individual cells and of cell groups. Veratramin alone and in combination with adrenalin does not change the inotropic parameters of individual cells. Bearing in mind that veratramin (2.5 X 10(-5) M) inhibits the accelerative effect of substances which activate and those which do not activate adenylatecyclse, as well as the fact that it has no effect on the content of cAMP in cultures and does not correct the increase in the cAMP content induced by adrenalin (5.5 X 10(-7) M), the authors conclude that the chronotropic effect of veratramin is not associated with the cell receptor mechanisms but is due to its effect on the transport of ions through the cell membrane, which is a responsible process in the generation of impulses.

The effect of chronic administration of digitoxin on the activity of the myocardial (Na + K)-ATPase in guinea-pigs

When guinea-pigs were treated for 24 days with digitoxin 0.3 mg/kg s.c., the activity of the (Na + K)-ATPase of the heart muscle increased by about 30% compared to controls, whereas the enzymes prepared from kidney and brain showed no significant alteration in their activity. In animal species treated with digitoxin for 1 to 5 days, no increase of enzyme activity was observed. Only after 10 to 15 days of treatment, a significant increase of the (Na + K)-ATPase activity was noted which increased no further with treatment up to 24 days. There was no significant difference in the kinetic properties of the (Na + K)-ATPase prepared from digitoxin-treated animals compared to those from control animals; the KM-values for ATP remained unchanged, and there was the same dependence of the activity on the K+-concentration and the same sensitivity towards digitoxin. As there appears to be no significant change in the specific properties of the enzyme, the increase in activity may possibly be caused by an increase in the amount of enzyme as a result of an adaptive enzyme regulation.

Penta-acetyl-gitoxin: the prototype of a prodrug in the cardiac glycoside series

Penta-acetyl-gitoxin is a suitable prodrug of gitoxin since it shows--side-effect latentiation, due to its inactive application form;--bioavailability, due to its improved solubility and thus good enteral absorption and--bioactivation, due to rapid de-acetylation in the body after absorption.

Studies on drug metabolism and liver ultrastructure after conjoint treatment with pregnenolone-16alpha-carbonitrile and dl-ethionine

Zoxazolamine paralysis, hexobarbital anesthesia and digitoxin convulsions were diminished in rats by 3 days of pretreatment with pregnenolone-16alpha-carbonitrile (PCN). Simultaneous injection of dl-ethionine did not block this effect of PCN. Dl-Ethionine decreased, while PCN or PCN + ethionene increased, zoxazolamine metabolism by the 9,000g supernatant fraction of the liver. Dl-ethionine neutralized the influence of PCN and phenobarbital on zoxazolamine metabolism in rats that were pretreated with PCN or phenobarbital for 1 day, and it inhibited the action of phenobarbital on hexobarbital anesthesia. PCN augmented the liver weight of animals given dl-ehtionine whereas dl-ethionine in itself did not. PCN produced SER proliferation in hepatocytes and dl-ethionine caused lipid accumulation, SER proliferation and myelin-figure formation. When administered conjointly, these compounds had an additive effect on the liver cell ultrastructure.

Some factors affecting a commercial kit for radioimmunoassay of digoxin using tritiated digoxin

Some factors affecting results of digoxin determinations using one commercially available radioimmunoassay kit are described and discussed. Serum of pregnant women, cord blood, amniotic fluid and serum of patients taking spironolactone may show erroneously high digoxin activity due to lack of specificity of the antiserum. Cross-reaction with digitoxin was found to vary substantially with antibody-lot. Haemaccel (5 g/1) in the sample leads to too low results. When ethanol (100 g/1) is present results are too high. The need for testing the specifity of every new lot of antiserum before use is stressed.

The relationship between Na+, K+-ATPase inhibition and cardiac glycoside-induced arrhythmia in dogs

In order to determine if there is a relationship between Na+, K+-ATPase inhibition and cardiac glycoside-induced arrhythmia, the time course of the onset and offset of the arrhythmia induced by the semi-synthetic glycoside, actodigin, and the enzyme activity during arrhythmia and following reversion to normal sinus rhythm was studied in the intact, anethetized dog. An infusion of actodigin(AY22,241) at the rate of 0.1 micronmol/kg/min for 30 min induced a severe and persistent arrhythmia within 13.1 +/- 192 min of 9 dogs. Upon termination of the actodigin infusion, the arrhythmia spontaneously converted to sinus rhythm within 17.5 +/- 2.3 min. Left ventricular tissue was taken from dogs sacrificed at the peak of the actodigin-induced arrhythmic periods or from the dogs that were allowed to recover from the actodigin-induced arrhythmia. These samples were homogenized and the membrane-containing fraction was passed through a Millipore filter. The membrane fraction trapped in the filter was then assayed for Na+ + K+ stimulated, Mg2+ dependent ATPase acctivity. The results showed that, in comparison to the time matched control dogs, the cardiac microsomes prepared from the arrhythmic dogs had a markedly reduced Na+, K+-ATPase activity. On the other hand, actodigin-treated dogs that were allowed to recover from the arrhythmic episode had Na+, K+-ATPase activity that was not significantly different from the control values. The amount of 3H-actodigin bound by the cardiac muscle microsomal fraction was also investigated. The microsomes from left ventricle were isolated with a slight modification of the method of Dutta et al. (1968). The microsomal binding of 3H-actodigin was maximum at 30 min (26.6 mol/mg protein) when the sample was prepared from the dogs at the peak of the arrhythmic effect. However, the binding was significantly reduced (11.5pmol/mg protein) in the microsomal fraction from hearts that had returned to sinus rhythm. These data provide direct evidence that inhibition of Na+, K+-ATPase and cardiac glycosideinduced arrhythmia may have some cause and effect relationship.

Control of brain slice respiration by (Na+ + K+)-activated adenosine triphosphate and the effects of enzyme inhibitors

The involvement of membrane (Na+ + K+)-ATPase (Mg2+-dependent, (Na+ + K+)-activated ATP phosphohydrolase, E.C. 3.6.1.3) in the oxygen consumption of rat brain cortical slices was studied in order to determine whether (Na+ + K+)-ATPase activity in intact cells can be estimated from oxygen consumption. The stimulation of brain slice respiration with K+ required the simultaneous presence of Na+. Ouabain, a specific inhibitor of (Na+ + K+)-ATPase, significantly inhibited the (Na+ + K+)-stimulation of respiration. These observations suggest that the (Na+ + K+)-stimulation of brain slice respiration is related to ADP production as a result of (Na+ + K+)-ATPase activity. However, ouabain also inhibited non-K+ -stimulated respiration. Additionally, ouabain markedly reduced the stimulation of respiration by 2,4-dinitrophenol in a high (Na+ + K+)-medium. Thus, ouabain depresses brain slice respiration by reducing the availability of ADP through (Na+ + K+)-ATPase inhibition and acts additionally by increasing the intracellular Na+ concentration. These studies indicate that the use of ouabain results in an over-estimation of the respiration related to (Na+ + K+)-ATPase activity. This fraction of the respiration can be estimated more precisely from the difference between slice respiration in high Na+ and K+ media and that in choline, K+ media. Studies were performed with two (Na+ + K+)-ATPase inhibitors to determine whether administration of these agents to intact rats would produce changes in brain respiration and (Na+ + K+)-ATPase activity. The intraperitoneal injection of digitoxin in rats caused an inhibition of brain (Na+ + K+)-ATPase and related respiration, but chlorpromazine failed to alter either (Na+ + K+)-ATPase activity or related respiration.

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.

Differential species sensitivity to the inhibitory effect of cardiac glycosides on 3H-1-noradrenaline accumulation by tissue slices

Effects of three cardiac glycosides on the accumulation of 3H-1-noradrenaline (3H-1-NA) by slices of heart and spleen were studied. Ouabain, digitoxin and digoxin, all produced a concentration dependent inhibition of 3H-1-NA uptake in both types of tissue slices. The maximum amount of 3H-1-NA accumulated as well as the rate of uptake was decreased. Digitoxin and ouabain were equipotent; however, digoxin was significantly less potent. Tissues from different mamalian specis did not exhibit the same degree of sensitivity to the inhibitory effect of cardiac glycosides on 3H-1-NA accumulation. Dogs were most sensitive and guinea-pigs an order of magnitude less sensitive. Rats were least sensitive by roughly two orders of magnitude when compared with guinea-pigs. The relationship of the effect of digitalis on 3H-1-NA accumulation to digitalis-induced cardiac arrhythmias is discussed. Finally, the pattern of species sensitivity found here is compared with that observed in relation to inhibition of Na+-K+-ATPase by cardiac glycosides.

Effect of digitoxin on cardiac hypertrophy induced by pericardiectomy and exercise

Removal of the pericardium in combination with a mild exercise programme of swimming resulted in a significant increase in heart weight and heart weight/body weight ratio of young rats. Heart weight/body weight ratios were 3.46+/-0.25 in the sedentary control animals, 4.16+/-0.26 in pericardiectomized animals swimming 2 h each day, and 4.60+/-0.22 in pericardiectomized animals swimming 6h/d. The effect of pericardiectomy on the development of cardiac hypertrophy is additive to that of mild exercise (2h/d) but not to prolonged exercise (6h/d). The administration of digitoxin significantly decreased the development of cardiac hypertrophy in pericardiectomized animals that were exercised for 2 h/d but not those exercised for 6 h/d. These findings further substantiate the physiological effect on the heart of the pericardium. The effects of pericardiectomy should be considered in experimental studies of cardiac hypertrophy and in clinical studies involving cardiac surgery.

The effects of digitoxin and its metabolites on the transmembrane potential and contractile characteristics of guinea-pig ventricle strips

The effects of 10(-7) and 10(-6) M digitoxin, and some of its metabolites, digitoxigenin-bis-digitoxoside, digitoxigenin-mono-digitoxoside and digitoxigenin on the transmembrane potential and contractile characteristics of guinea-pig right ventricle strips were studied to define the role of the sugar side-chain in these cleavage products of digitoxin. Digitoxin and digitoxigenin produced their maximum inotropic responses, without induction of arrhythmias, at about 30 min. However, the bis and mono compounds produced arrhythmias within 12 min, so the inotropy recorded may not be the maximum response. Digitoxin with 3 sugar residues and the bis compound with 2 sugar residues produced a prolongation in the action potenital duration. In contrast, the mono compound with one sugar residue and the digitoxigenin with no sugar residue produced a shortening of the action potential duration. There may be a relationship between the number of sugar moieties and the action potential duration; digitoxin and its bis derivative increased the action potential duration and mono- and digitoxigenin decreased the action potential duration. There also appeared to be an unusual relationship between the number of sugar moieties and induction of arrhythmias; arrhythmogenicity occurred at the doses employed with only the bis- and mono-digitoxoside. Finally, there appeared to be no simple relationship between chemical structure and inotropic potency.

Mechanism of interaction of digitalis with estradiol binding sites in rat uteri

Digitalis preparations have a weak estrogenic effect in man. The data in animals are equivocal. We have studied.the biologic effect of both digitoxin and digoxin on the rat uterus in vivo and the interaction of these drugs with the rat uterus estrogen receptor in vitro. Digitoxin and estradiol significantly increased the uterine weight of immature rats, while digoxin did not. The interaction of digitoxin and digoxin with the rat uterus estrogen cytosol receptor was studied using protamine sulfate precipitation and dextran-coated charcoal (DCC) assays. Both methods gave a Kd for the estradiol-receptor interaction between 0.8-3.1 X 10(-9) M (n = 20). Digitoxin at concentrations of 0.5-2.0 X 10(-6) M significantly inhibited the binding of estradiol to the specific or saturable binding sites with minimal inhibition of hormonal binding to nonspecific sites. The binding was competitive with a calculated Ki for digitoxin of 5.2-7.8 X 10(-7) M (n = 18). Digoxin failed to inhibit estradiol binding to the receptor protein in vitro. We conclude that digitoxin probably acts directly as a weak estrogen and that this effect probably explains the estrogen-like side effects seen with digitoxin therapy in man.

Increase in the (Na+ + K+)-ATPase activity in heart muscle after chronic treatment with digitoxin or potassium deficient diet

In guinea pigs, administration of digitoxin (0.3 mg/kg s.c. for 7-24 days) causes an increase in activity of the (Na+ + K+)-ATPase of the heart. The plasma K+ level and the K+ content of the heart muscle of these animals remains unchanged and there is no significant alteration in the digitoxin toxicity compared to controls. In guinea pigs with potassium deficiency produced by a potassium deficient diet for 12 days, there is a related increase of the (Na+ + K+)-ATPase. The plasma K+ level of these animals is diminished while the K+ content in the heart muscle remains unchanged the toxicity of digitoxin is enhanced. In both test groups the increase in the (Na+ + K+)-ATPase activity is limited to enzymes from heart muscle, those from brain or kidney remaining unaffected. This increase in activity seems to be the result of an adaptive enzyme induction.

[Influence of roentgen rays on electrolyte changes and metabolism of the myocardium. VII. Radiation-induced inhibition of the sodium- and potassium--activated microscomal-trasport ATPase]

The behavior of a Mg2+-dependent microsomal ATPase, which may be activated by Na+ and K+, was investigated in the myocardium of Guinea pigs after local irradiation. The part which can be activated by Na+ and K+ (transport-ATPase) is already slightly inhibited thirty minutes after a dose of 250 R; the inhibition rate with 1500 R amounts to about 60%, the maximal inhibition effect with a dose of 3000 R was about 70%. Four hours after a local irradiation with 1500 R, a restitution of the inhibition effect occurs with overshoot reaction; 15 and 24 hours following irradiation, a significant inhibition of the transport-ATPase activity can again be verified. The k-strophanthidine and digitoxin are inhibiting within the concentration ranges analyzed (2 X 10(-5), 2.5 X 10(-7), 5 X 10(-7) M) the activity of the transport-ATPase. The above-mentioned radiation-induced inhibition effect upon the ATPase activated by Na+ and K+ is additively increased by adding k-strophanthidine (2 X 10(-5) M) and digitoxin (2.5 X 10(-7) M).

The uptake and subcellular distribution of radio-labelled metabolites of digitoxin in the guinea-pig isolated perfused heart

1 Comparisons were made of the uptake and inotropic effects of concentrations of 0.1 muM of digitoxin and its cleavage products digitoxigenin-bis-digitoxoside, digitoxigenin-mono-digitoxoside and digitoxigenin in the isolated perfused hearts of guinea-pigs. 2 Digitoxin produced the greatest inotropic responses in this series, while the sequence of cleavage products produced progressively smaller responses. 3 The uptake of digitoxin was significantly higher than that of the three metabolites, and the uptake of metabolites became progressively less with cleavage. The highest binding in each case was found in the microsomal fraction. 4 The uptake of all four digitaloids was reduced when the potassium in the perfusion medium was increased.

Effect of taurine on responses to noradrenaline, acetylcholine and ouabain in isolated auricles from digitalized guinea pigs

Responses of the isolated heart from the guinea pig treated chronically with digitoxin until the T wave had disappeared were studied. The development of the abnormal ECG pattern was abolished by a simultaneous injection of taurine and digitoxin. Taurine also inhibited the change by digitoxin of the responses to acetylcholine and ouabain but not to noradrenaline. The causal relationship between the inhibitory effect of taurine and prevention of the development of arrhythmias remains obscure.