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 microg/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.

Calcium-dependent depolarizations and contractions in ventricular myocardial fibers: effects of adrenaline and cardiac glycosides.

Intracellular membrane potentials and contractions were recorded in ventricular trabeculae from sheep and calf hearts. Using a sucrose gap and constant current pulses of 500 msec duration, Ca-dependent depolarizations were elicited in Ca-containing Tyrode's solution + tetrodotoxin (TTX) (1-2 X 10(-5) gm/ml; [Na]e 149.2 mM) or in Na-free solution with sucrose as sodium substitute. Adrenaline (10(-8) -5 X 10(-7) gm/ml), which has been shown to enhance Ca inward current, increased (dV/dt)max and contractile force both in Na-free and in Na-containing solutions + TTX. Digitoxigenin (10(-7) -3 X 10(-7) gm/ml), however, did not change (dV/dt)max and contractions in the absence of Na. In the presence of Na (+TTX), digitoxigenin increased contractile force but decreased (dV/dt)max. Under the assumption that the Ca-dependent depolarizations are carried by Ca it is concluded 1) that in contrast to adrenaline, digitoxigenin does not increase the permeability of the myocardial cell membrane to Ca and 2) that the positive inotropic effect of digitoxigenin is not due to an enhancement in Ca inward current during depolarization.