Novel Digitalis-Like Factor, Marinobufotoxin, Isolated From Cultured Y-1 Cells, and Its Hypertensive Effect in Rats

Marinobufagenin and telecinobufagin have been identified as digitalis-like factors in mammals. In toads, marinobufagenin-related compounds, such as marinobufotoxin (MBT), have been isolated in some tissues but not in mammals, and its biological action has not been elucidated. Herein, we aimed to explore the possible production and/or secretion of MBT and the biological action in rats. First, the MBT in culture supernatant of the adrenocortical-originated cell line Y-1 was analyzed by high-performance liquid chromatography and sensitive ELISA for marinobufagenin-like immunoreactivity. Moreover, the structural information was obtained by mass spectrometry. To determine the biological action, MBT (9.6 and 0.96 μg/kg per day) was intraperitoneally infused via an osmotic minipump for 1 week. Blood pressure and renal excretion of marinobufagenin-like immunoreactivity were measured. Marinobufagenin-like immunoreactivity was found in Y-1 cell culture media, and the concentration increased until 24 hours. The structural analysis suggested that marinobufagenin-like immunoreactivities were marinobufagenin and MBT, and tandem mass spectrum analysis revealed them with the specific daughter ions. The highest sensitive ELISA-positive peak of marinobufagenin-like immunoreactivity in the media was MBT. Continuous administration of MBT in rats for 1 week significantly increased systolic blood pressure and renal excretion of marinobufagenin-like immunoreactivity compared with control rats (135±3.0 versus 126±2.0 mm Hg and 1.41±0.286 versus 0.34±0.064 ng/day, respectively). These data suggest that MBT, arginine-suberoyl ester of marinobufagenin, can be a novel digitalis-like factor with hypertensive action and is secreted from the adrenocortical cells.

The association with receptors regulates the Na+,K(+)-ATPase inhibitory potency of some cardioactive steroids

The onset of inhibition of Na+,K(+)-ATPase from guinea-pig myocardium was quantified with pseudo-first-order rate constants in a series of 14 cardioactive steroids. From these data the association and dissociation rate constants of the steroid-receptor complex were calculated. It was then found that the association of the steroids with receptors but not the dissociation of the steroid-receptor complex determined the largely different inhibitory potencies. Consistent with this finding, at equieffective steroid concentrations the rates of inhibition varied only slightly. The correlation of the association rate with the hydrophobicity of the compounds suggests that hydrophobic interactions facilitate the access of the steroid to the receptor. A conformational transition of the vicinity of the receptor subsequent to the formation of the steroid-receptor complex seems to alter the hydrophobic properties of the receptor environment to make the dissociation rate independent from hydrophobicity.

Cardenolide analogues. 4. (20R)- and (20S)-Cardanolides: on the roles of the 20(22)-ene and 14beta-hydroxyl in genin activity

(20R)-20,22-Dihydrodigitoxigenin (3a) and (20S)-20,22-dihydrodigitoxigenin (3b) were isolated from (20R,S)-20,22-dihydrodigitoxigenin (3) by three fractional crystallizations each from ethyl acetate. The two diastereomers have distinct NMR spectra and similar (Na+,K+)ATPase inhibitory activities (I50 = 1.1-1.4 X 10(-5) M)--about 1/100 as active as digitoxigenin (1). Their activity compared with other cardenolide analogues suggests a passive geometric role for the 20(22) double bond in eliciting (Na+,K+)ATPase inhibition, keeping the lactone carbonyl in the proper orientation. (20S)-3 beta,14 beta-Dihydroxy-22-methylene-5 beta,14 beta-cardanolide (7a) was then synthesized from 3a, and (20R)-3 beta,14 beta-dihydroxy-22-methylene-5 beta,14 beta-cardanolide (7b) from 3b. They were found to be equivalently active in inhibiting (Na+,K+)ATPase, with I50 values of 7.0 x 10(-5) M. Although it has been usually believed that the 14 beta-hydroxyl of cardenolides increases binding to the receptor, 2b (the 14-ene derivative of 7b) was more than twice as active (I50 = 3.0 X 10(-5)) than either 7a or 7b.

Potential antitumor agents: a cytotoxic cardenolide from Coronilla varia L

An alcoholic extract of the seeds of Coronilla varia L. showed inhibitory activity against KB cells in culture and was fractionated through a series of partitions, column chromatography, and preparative layer chromatography to yield hyrcanoside, daphnoretin, scopoletin, and umbelliferone. Hyrcanoside was also tested in the PS mouse leukemia assay and showed borderline activity.

Thiocardenolides I: synthesis and biological actions of 3beta-thiocyanato-14beta-hydroxy-5beta-card-20(22)-enolide

The synthesis of a 3beta-thiocyanatocardenolide is described. The compound exhibited about 0.1 times the cardiotonic effect of digitoxyigenin in the isolated frog heart preparation. At a dosage of 20 mg/kg in the intact rat, it elicited ECG changes similar to those seen with a 10-mg/kg dose of digitoxigenin. Studies also revealed the new cardenolide to be a reversible inhibitor of sodium- and potassium-activated adenosine triphosphatase.

Pharmacological studies of a new 4-aminosugar cardiac glycoside (ASI-222)

ASI-222,3beta-O-(4-amino-4,6-dideoxy-beta-d-galactopyranosyl) digitoxigenin, is a semi-synthetic cardiac glycoside patterned after a natural glycoside obtained from Cambodia. Effects of ASI-222 on contractile force in the isolated rabbit atria, cardiac contractile force, cardiac rate, ventricular excitability and functional refractory period in dogs, and acute toxicity in mice have been compared to those effects of ouabain. Both electrically driven and spontaneously beating atria demonstrated more rapid onset and greater maximum increases in contractile force with ASI-222 than with ouabain in equal bath concentrations. In the dog, ASI-222 increased cardiac contractile force more rapidly and at a lower cumulative dose than ouabain. Moreover, the maximum increase in contractile force obtained with ASI-222 was greater than that obtained with ouabain. The occurrence of ventricular ectopic beats was observed at a higher cumulative dose of ASI-222 than for ouabain. Also, ASI-222 produced a decrease in ventricular excitability and an increase in functional refractory period ot the ventricle. Ouabain, in the same molar dose, produced either no change or a slight increase in these parameters. Our data indicate that ASI-222 has a greater therapeutic index than ouabain. This difference may be partially explained by effects of ASI-222 on electrical properties of the heart.

Reversible inhibition of (Na+ + K+)-ATPase with a cardiac glycoside

The effect of a semisynthetic cardiac glycoside, Actinogen (Ay22241), on Na+ + K+ - ATPase was studied. Ay22241 was found to be as an effective inhibitor of the enzyme as ouabain, Ay22241 inhibition was a time dependent process and was completely reversible. While ouabain inhibition was also time dependent, it was only partially reversible. This reversibility with Ay22241 should make it a useful tool in studying the mode of action of cardiac glycosides.

Cardiotonic steroids II: 3-deoxycardenolides and 3-deoxycardanolides

A series of compounds related to 3-deoxydigitoxigenin was prepared and assayed for inhibition of myocardial Na+,K+- adenosine triphosphatase. Although the relatively high activity of 3-deoxydigitoxin was confirmed, the corresponding 3beta,4beta-epoxide and a mixture of 2,3-olefins and 3,4-olefins were less active. 3-Deoxy compounds with variations at the 14-position and the butenolide ring were much less active than the corresponding 3beta-hydroxy analogs. Thus the activity of 3-deoxydigitoxigenin appears to be particularly susceptible to structural changes elsewhere in the molecule.

Role of catecholamines in the central mechanism of emetic response induced by peruvoside and ouabain in cats

1 Peruvoside, (a glycoside obtained from the plant, Thevetia neriifolia Juss) and ouabain produce emesis in cats. Vomiting is not produced by these drugs in animals pretreated with catecholamine depleting drugs like reserpine, tetrabenazine or syrosingopine. Chloropromazine hydrochloride, mepyramine maleate, or BOL-148 administered intravenously or intracerebro-ventricularly do not afford protection.2 Phenoxybenzamine produces partial protection against peruvoside-induced emesis.3 Haloperidol (1 mg/kg i.v.) prevents vomiting induced by peruvoside or ouabain. Intracerebroventricularly administered haloperidol is ineffective.4 Cats pretreated with SKF-525-A, are not protected by haloperidol. Animals pretreated with phenobarbitone in a dose of 25 mg/kg for a week were protected by haloperidol, 250 mug/kg i.e. one quarter of the effective antiemetic dose in normal cats.5 It is postulated that catecholamines are involved in the mechanism of vomiting induced by cardiac gycosides. Further, a metabolite of haloperidol seems to be responsible for its effective antiemetic action.

Effects of AY-22,241 (Actodigin) on electrical and mechanical activity of cardiac tissues

AY-22,241 (Actodigin) is a new rapid-acting semisynthetic cardiotonic steroid. In experiments on contractility of cat papillary muscle, Actodigin (2 times 10(-7) to 4 times 10(-6) M) produced a dose-dependent positive inotropic effect, a marked increase in the maximum rate of force development, and no change in resting tension. Electrophysiologic studies performed with microelectrode techniques on isolated Purkinje fibers superfused with Tyrode solution, revealed dose-dependent decreases in resting membrane potential, action potential amplitude and duration, and the maximum rate of rise of phase 0 (Vmax). Purkinje fibers superfused with extracorporeally circulated blood from a donor dog receiving 0.075 mg/kg/min Actodigin showed small decreases in resting membrane potential preceding the onset of donor premature ventricular contractions. Progressive decreases in resting membrane potential, action potential amplitude and duration, and Vmax accompanied donor ventricular tachycardia. All effects were rapidly reversible, and compared to ouabain, equi-inotropic concentrations of Actodigin caused significantly less electrophysiologic toxicity.

Cardiotonic steroids: correlation of sodium-potassium adenosine triphosphate inhibition and ion transport in vitro with inotropic activity and toxicity in dogs

1. A new series of cardiotonics based on five steroid nuclei has been evaluated for inhibition of Na-+/K-+-ATPase and Rb uptake by red blood cells, and for inotropic activity and toxicity in dogs. Structure-activity relationships are discussed. 2. The in vitro tests can be used satisfactorily to predict inotropic activity, but not toxicity or therapeutic ratio. 3. Although compounds with greatly improved therapeutic ratios relative to ouabain and tolusin have been obtained, they proved to be strongly emetic in the conscious dog.

The effects of digitalis glycosides on the ventricular fibrillation threshold in innervated and denervated canine hearts

Digitalis glycosides have been implicated in increased vulnerability to ventricular fibrillation in man. In order to investigate the genesis and occurrence of ventricular fibrillation in the presence of digitalis, the effects of both acetylstrophanthidin and ouabain on the ventricular fibrillation threshold (VFT) were studied in the open-chest anesthetized dog. The current required to induce ventricular fibrillation was determined by passing a train of 12 constant current pulses through epicardial electrodes during the vulnerable period of the cardiac cycle. It was found that an intravenous bolus infusion of acetylstrophanthidin (0.050-0.097 mg/kg) or ouabain (0.035-0.075 mg/kg) in intact innervated dog hearts raised the VFT from 40% to 260% above control values. Continuous infusions of acetylstrophanthidin to toxic levels also resulted in an elevated VFT. Vagotomy alone did not qualitatively change the effects of acetylstrophanthidin on VFT. However, following vagotomy and stellate sympathectomy, infusions of both toxic and subtoxic doses of acetylstrophanthidin resulted in a decrease in the VFT from 40 to 80% below control values. In denervated animals in which the peripheral ramifications of the left stellate ganglion nerves were stimulated, the VFT decreased below control values in the absence of acetylstrophanthidin, but during stellate stimulation in the presence of acetylstrophanthidin the VFT was increased above control values. These studies demonstrated that the increase in VFT by digitalis in the healthy, innervated heart was mediated via an associated increase in sympathetic activity; in the absence of neural influences digitalis decreased the VFT.

The Effects of Sodium-Transport Inhibitors and Cooling on Membrane Potentials in Cockroach Central Nervous Connectives

1. Cooling caused axonal depolarization in desheathed and urea-treated connectives, but induced hyperpolarizing responses (measured with micro-electrodes and with the sucrose-gap) in intact preparations. Hyperpolarizing responses were also recorded with extracellularly-located microelectrodes in intact connectives.

2. Strophanthidin (0.2 mM/l) caused axonal depolarization in desheathed preparations, ethacrynic acid being without appreciable effect. Ethacrynic acid (0.2 mM/1) induced apparent hyperpolarizations in intact connectives and abolished or reduced the effects of cooling.

3. It is concluded that the axonal sodium pump is pharmacologically separable from that associated with the perineurial and/or the glial membranes: the former being inhibited by cardiac glycosides, the latter by ethacrynic acid.

4. The results are discussed in relation to extra-axonal sodium regulation and the possible involvement of an electrogenic sodium pump associated with the perineurial or glial membranes.

Reversal of ouabain and acetyl strophanthidin effects in normal and failing cardiac muscle by specific antibody

Isolated cat right ventricular papillary muscles were used to study the effects of antibodies with high affinity for ouabain and acetyl strophanthidin on myocardium exposed to these cardioactive steroids. Antibodies with average intrinsic affinity constants for ouabain and acetyl strophanthidin of the order of 10(8) M(-1) were raised in rabbits challenged by repeated injection of a conjugate of ouabain covalently linked to a poly D,L-alanyl derivative of human serum albumin. Effects were assessed in terms of time-course and extent of inotropy reversal, influence of experimentally induced ventricular failure, digitalis-antibody concentration relations, influence of digitalis-antibody complex on response to additionally added digitalis, and relation of antibody effects on digitalis-induced automaticity and contracture to reversal of inotropy. Specific antibody (but not control antibody) in 1.1-1.5-fold molar excess over cardioactive steroid concentrations blocked positive inotropic effects of ouabain and acetyl strophanthidin, and gradually reversed established contractile effects of these agents with a mean time for half-reversal of ouabain-induced inotropy of 124+/-6 (SEM) min and 37+/-3 min for half-reversal of acetyl strophanthidin-induced inotropy. Papillary muscles from cats with right ventricular failure induced by chronic pulmonary artery constriction responded similarly. Both normal and failing muscles returned to but not below levels of contractility existing before cardioactive steroid exposure, and time for half-reversal of inotropy by antibody was significantly shorter than time for half-reversal after removal of ouabain or acetyl strophanthidin by muscle bath washout alone. Presence of ouabain- or acetyl strophanthidin-antibody complex did not alter the myocardial contractile response to subsequently added cardioactive steroids. Spontaneous automaticity occurring as a toxic response to ouabain or acetyl strophanthidin in eight muscles was rapidly reversed by specific antibody at a time when positive inotropic effects were still fully manifest. Early contracture was also reversed by specific antibody. These studies provide further support for the concept that cardiac glycoside-specific antibodies are capable of reversing established cellular effects of cardioactive steroids.

Digitalis-induced increase in aortic regurgitation and the contrasting effect of glucagon in the sedated dog

The hemodynamic and phasic ascending aortic flow changes induced by acetylstrophanthidin and glucagon were studied in closed-chest sedated dogs with aortic regurgitation. While the positive inotropic effect of both agents was reflected in an increase in peak rate of rise of left ventricular pressure, acetylstrophanthidin increased aortic regurgitation, while glucagon decreased it. With the former, left ventricular end-diastolic pressure rose from 20+/-6 to 27+/-6 mm Hg (P < 0.005), but fell from 18+/-4 to 11+/-3 mm Hg (P < 0.001) with glucagon. Acetylstrophanthidin increased systemic vascular resistance, aortic diastolic pressure, and diastolic regurgitant flow rate, and, heart rate and the duration of regurgitation per beat and per minute being unchanged, regurgitant flow per beat increased 32+/-15% (P < 0.001). Glucagon decreased regurgitant flow per beat 27+/-14% (P < 0.001) because of abbreviation of diastole associated with tachycardia, and because of reduction in regurgitant flow rate. Despite tachycardia, the duration of regurgitation per minute was unchanged, and the small fall in regurgitant blood flow per minute was not significant, but this pertained in the face of 47% increase in effective cardiac output (P < 0.001). In contrast, acetylstrophanthidin increased regurgitant flow per minute 28+/-14% (P < 0.001) without change in effective cardiac output. The increase in cardiac contractility, tachycardia, and systemic vasodilatation induced by glucagon preferentially enhanced forward blood flow, which led to reduction in left ventricular volume overload, while it increased cardiac output. Contrarily, acetylstrophanthidin increased aortic regurgitation and, despite its inotropic effect, increased left ventricular volume overload without an increase in cardiac output.

A study of the contractures induced in frog atrial trabeculae by a reduction of the bathing sodium concentration

1. The relationship between the [Ca](o), the [Na](o) and the strength of the contracture evoked when the [Na](o) is reduced has been investigated in isolated frog atrial trabeculae.2. The strength of the contracture varies by the [Ca](o) (2) and by 4 radical([Na](o)) over the lower tension range.3. The contracture induced by reduction of [Na](o) is not sustained, but relaxes spontaneously. The rate of this relaxation is only dependent on the [Na](o) is the presence of strophanthidin.4. After the spontaneous relaxation of an Na-free contracture, the ability of the trabecula to develop tension upon a second challenge with Na-free solution returns in about 3 min if the muscle is perfused with Na-containing fluid. This recovery process is slowed if the [Na](o) is low during the recovery period, but the recovery is hastened by electrical stimulation of the preparation or by perfusion with K-free or strophanthidin containing sodium-Ringer.5. It is suggested that the influx of Ca(2+) which induces the Na-free contracture depends on the presence of Na(+) inside the cells. When the intracellular Na concentration falls, the Ca influx falls, and the muscle relaxes as a result of the activity of an intracellular relaxing structure.

Interaction of external K, Na, and cardioactive steroids with the Na-K pump of the human red blood cell

The interaction of extracellular Na (Na(o)), K (K(o)), and strophanthidin with the Na-K pump of the human red blood cell has been investigated. Inhibition by submaximal concentrations of strophanthidin rapidly reaches a level which does not increase further over a relatively long period of time. Under these circumstances, it is possible to apply a steady-state kinetic analysis to the interaction of Na(o), K(o), and strophanthidin with the pump. In Na-free solutions, strophanthidin increases the apparent K(1/2) of the pump for K(o), but does not change the form of the relation between the reciprocal of the active K influx ((i)M(K) (P-1)) and the reciprocal of [K(o)] ([K(o)](-1)); the relation both in the presence and absence of strophanthidin is adequately described by a straight line. In solutions containing Na, strophanthidin changes the form of the curve describing the relation between (i)M(K) (P-1) vs. [K(o)](-1); the curve becomes more parabolic in solutions containing strophanthidin. The rate of ouabain binding to K-free cells has also been measured; in the absence of K, the rate of binding is unaffected by Na(o). The data are considered in terms of a simple kinetic model. The findings can be explained if it is supposed that at low external K the form of the pump combined with one Na(o) is more likely to combine with strophanthidin than is the uncombined form of the pump. The uncombined form of the pump is more likely to combine with K even at very low K(o) than with strophanthidin.

Effect of calcium on acetylstrophanthidin-induced transient depolarizations in canine Purkinje tissue

The role of calcium ions (Ca 2+ ) in the generation of transient depolarizations (TDs) by acetylstrophanthidin was examined. Transmembrane activity was recorded from isolated canine false tendons exposed to acetylstrophanthidin; concentrations from 7.5 x 10 -8 to 2 x 10 -7 g/ml caused TDs coupled to driven action potentials and depressed slow diastolic depolarization. TDs could reach threshold and induce extrasystoles. Elevation of the Ca 2+ concentration increased the amplitude of TDs induced by acetylstrophanthidin. High Ca 2+ concentration (12.5 mM) caused TDs and depression of slow diastolic depolarization in the absence of acetylstrophanthidin. Elevation of potassium (K + ) concentration depressed and reduction of K + concentration potentiated TDs caused by either acetylstrophanthidin or high Ca 2+ concentration. The production of TDs and the depression of slow diastolic depolarization by acetylstrophanthidin were reversed by reduction of the Ca 2+ concentration or addition of manganese (2 mM) to the superfusing Tyrode's solution. The results suggest that TDs and arrhythmias produced by acetylstrophanthidin may be caused by a transient Ca 2+ influx.