Dihydroouabain is an antagonist of ouabain inotropic action

Ouabain-digoxin antagonism in rat arteries and neurones

'Classic' cardiotonic steroids (CTSs) such as digoxin and ouabain selectively inhibit Na+, K+ -ATPase (the Na+ pump) and, via Na+ / Ca2+ exchange (NCX), exert cardiotonic and vasotonic effects. CTS action is more complex than previously thought: prolonged subcutaneous administration of ouabain, but not digoxin, induces hypertension, and digoxin antagonizes ouabain's hypertensinogenic effect. We studied the acute interactions between CTSs in two indirect assays of Na+ pump function: myogenic tone (MT) in isolated, pressurized rat mesenteric small arteries, and Ca2+ signalling in primary cultured rat hippocampal neurones. The 'classic' CTSs (0.3-10 nm) behaved as 'agonists': all increased MT70 (MT at 70 mmHg) and augmented glutamate-evoked Ca2+ (Fura-2) signals. We then tested one CTS in the presence of another. Most CTSs could be divided into ouabain-like (ouabagenin, dihydroouabain (DHO), strophanthidin) or digoxin-like CTS (digoxigenin, digitoxin, bufalin). Within each group, the CTSs were synergistic, but ouabain-like and digoxin-like CTSs antagonized one another in both assays: For example, the ouabain-evoked (3 nm) increases in MT70 and neuronal Ca2+ signals were both greatly attenuated by the addition of 10 nm digoxin or 10 nm bufalin, and vice versa. Rostafuroxin (PST2238), a digoxigenin derivative that displaces 3H-ouabain from Na+, K+ -ATPase, and attenuates some forms of hypertension, antagonized the effects of ouabain, but not digoxin. SEA0400, a Na+ / Ca2+ exchanger (NCX) blocker, antagonized the effects of both ouabain and digoxin. CTSs bind to the α subunit of pump αβ protomers. Analysis of potential models suggests that, in vivo, Na+ pumps function as tetraprotomers ((αβ)4) in which the binding of a single CTS to one protomer blocks all pumping activity. The paradoxical ability of digoxin-like CTSs to reactivate the ouabain-inhibited complex can be explained by de-oligomerization of the tetrameric state. The interactions between these common CTSs may be of considerable therapeutic relevance.

Dual effect of initial [K] on vascular tone in rat mesenteric arteries

A slight increase in extracellular concentration of potassium ([K(+)](o)) can act as a vasodilator in rat mesenteric vascular bed. However, in recent years, several groups have failed to consistently observe relaxation of rat mesenteric arteries in these conditions. The aim of the present study was to provide a mechanistic understanding of this discrepancy. In rat small mesenteric arteries, 37 of 40 arteries mounted for measurement of isometric force and pre-contracted with phenylephrine (PE) did not relax when ([K(+)](o) was raised from 5.9 mM (control ([K(+)](o) to 11.2 or 21.2 mM. However, when ([K(+)](o) was briefly lowered to 1.2 mM, increasing ([K(+)](o) to between 5.9 and 41.2 mM evoked relaxation. This relaxation was not reduced by barium or by removal of the endothelium, but was abolished by 0.1 mM ouabain. Raising ([K(+)](o) from concentrations between 0 and 5.9 mM to 13.8 mM elicited a relaxation of PE-induced tone that was inversely proportional to initial ([K(+)](o). Relaxation was associated with a ouabain-sensitive hyperpolarization of smooth muscle cells. In arteries exposed to dihydroouabain (DHO), raising ([K(+)](o) from 5.9 to 13.8 mM and simultaneously washing out DHO resulted in relaxation of PE-induced force. These results suggest that only when the initial ([K(+)](o) is less than approximately 5 mM do small elevations in ([K(+)](o) evoke smooth muscle hyperpolarization and relaxation via activation of Na,K-ATPase, and not inwardly rectifying K(+) channels. Therefore, small differences in the initial ([K(+)](o) (4.6 vs 5.9 mM) can strongly influence the variations of vascular tone to increases in ([K(+)](o).

Two biologically active isomers of dihydroouabain isolated from a commercial preparation

Ouabain is a plant-derived cardiac glycoside that inhibits the catalytic activity of Na(+),K(+)-ATPase (sodium pump; NKA). Dihydroouabain, a derivative of ouabain with a reduced lactone ring, is commonly used as a sodium pump antagonist. It has been assumed that commercially available dihydroouabain is homogeneous. We now report that preparations of dihydroouabain contain two components each with a different potency for inhibition of sodium pump activity. We used reverse-phase HPLC chromatography, UV spectrophotometry, electrospray ionization-mass spectrometry (ESI-MS), nuclear magnetic resonance (NMR) spectroscopy and two independent bioassays to characterize these compounds. The two dihydroouabain fractions (Dho-A and Dho-B) resolved by 3 min chromatographically, had UV absorbance maxima at 196 nm, and comprised 37% and 63% of the stock dihydroouabain, respectively. The molar potency of each component for inhibition of NKA from porcine cerebral cortex differed by 4. 4-fold (Dho-A, IC(50) = 7.13 +/- 0.8 microM; Dho-B, IC(50) = 1.63 +/- 0.12 microM). The relative potencies were 9% and 40% of those of ouabain, respectively. A similar pattern for phosphorylation of NKA was observed. Mass spectrometry (ESI-MS) and fragmentation patterns are consistent with Dho-A and Dho-B being isomers of identical molecular mass (587 Da) and each with six hydroxyl groups, a deoxyhexose sugar moiety and a lactone ring. Furthermore, NMR spectroscopy revealed structural differences between Dho-A and Dho-B by displaying noticeably different chemical shifts at only two groups of proton resonances assigned to H-21 and H-22. The ESI-MS and NMR results confirm the presence of the isomerism at C20 of the lactone ring. Our results demonstrate the existence of two molecular forms of dihydroouabain, each with a different biological potency. These findings underscore the importance of characterizing the purity of dihydroouabain commercial preparations. It also provides possible molecular models for investigating the metabolism of endogenous ouabain-like factors recently reported in mammals.

Ca2+ flux through promiscuous cardiac Na+ channels: slip-mode conductance

The tetrodotoxin-sensitive sodium ion (Na+) channel is opened by cellular depolarization and favors the passage of Na+ over other ions. Activation of the beta-adrenergic receptor or protein kinase A in rat heart cells transformed this Na+ channel into one that is promiscuous with respect to ion selectivity, permitting calcium ions (Ca2+) to permeate as readily as Na+. Similarly, nanomolar concentrations of cardiotonic steroids such as ouabain and digoxin switched the ion selectivity of the Na+ channel to this state of promiscuous permeability called slip-mode conductance. Slip-mode conductance of the Na+ channel can contribute significantly to local and global cardiac Ca2+ signaling and may be a general signaling mechanism in excitable cells.

Comparison of the inhibitory effect of ouabain and dihydroouabain on the Na(+)-K+ ATPase from frog skin

1. The effects of ouabain and dihydroouabain (DHO) on Na(+)-K+ ATPase from frog skin were investigated. 2. Ouabain seemed to be a more effective inhibitor, reaching near maximum inhibition at 10(-5) M while DHO reached near maximum at 10(-3) M. 3. Both inhibitors acted within the first few minutes, but ouabain inhibited more strongly over virtually the whole 2 hr considered. 4. Pre-incubation of the enzyme with ATP or Na+ reduced the level of inhibition due to subsequent ouabain or DHO, but preincubation with K+ did not.

In vivo assessment of the inotropic and toxic effects of oxidized ouabain

Oxidized ouabain, a product of the oxidative cleavage of the rhamnose ring in ouabain has been found to have a higher inotropic toxic ratio in cultured cardiac myocytes. The purpose of our study was to evaluate the efficacy and toxicity of oxidized ouabain in comparison with ouabain in intact animals. Drugs were infused to healthy cats; the positive inotropic effect, and the time-course of development of arrhythmia were followed and recorded until death. Oxidized ouabain was associated with a higher increase in arterial blood pressure, a mean increase of 41 +/- 19% as compared with 21 +/- 8% in the ouabain group (p < 0.10). There were no significant differences in maximal increases of dP/dt or dP/dt/P (65 +/- 29%, 28 +/- 10% for oxidized ouabain and 49 +/- 16%, 27 +/- 11% for ouabain, respectively). The mean doses causing persistent arrhythmia (toxic dose) were 93 +/- 23 micrograms/kg of oxidized ouabain vs 39 +/- 14 micrograms/kg of ouabain. Lethal arrhythmias were produced by 215 +/- 46 micrograms/kg of oxidized ouabain and 62 +/- 16 micrograms/kg of ouabain. The ratio of toxic to lethal doses was 0.62 +/- 0.11 for ouabain vs 0.45 +/- 0.09 for oxidized ouabain (p < 0.05), but the inotropic to toxic dose ratios were not different. We conclude that oxidized ouabain acts similarly to the known cardiac glycosides in doses which produce inotropic effects in cats, has a lower potency as compared to ouabain, and appears to have a more benign course of intoxication.

Selective inhibition by ethylisopropylamiloride of the positive inotropic effect evoked by low concentrations of ouabain in rat isolated ventricles

The biphasic positive inotropic effect of ouabain has been studied in rat ventricular strips in the presence of ethylisopropylamiloride (EIPA) an inhibitor of Na+/H+ exchange. EIPA (10-20 microM) depressed dose-dependently the high affinity component of the ouabain inotropic effect, whereas it did not significantly modify the low affinity inotropic effect related to high concentrations of ouabain. At the EIPA concentrations studied, there was no observable modification of the inotropic effect of Bay K 8644 (10 nM, 0.3 microM) or of isoprenaline (10 nM, 1 microM). These results indicate that the inotropic effect of ouabain resulting from its interaction with high affinity sites is selectively sensitive to EIPA.

Schistosoma mansoni: preparation, characterization of (Na+ + K+)ATPase from tegument and carcass

The preparation and some biochemical properties of a (Na+ + K+)ATPase from male adult Schistosoma mansoni are described. After incubation in a membrane disruption medium, the tegument and carcass of the worms were separated and treated to obtain fractions enriched in (Na+ + K+)ATPase. The activity of the tegumental ouabain sensitive (Na+ + K+)ATPase at 37 C was 20.3 mumole Pi X mg-1 protein X hr-1 and represented 32% of the total ATPase activity. The (Na+ + K+)ATPase prepared from the carcass had a lower specific activity (3.7 mumole Pi X mg-1 protein X hr-1) but a higher relative activity (55%). Similar concentrations of Na+ and K+ activated the enzymes from both sources, and both enzymes were inhibited by similar concentrations of calcium. However, the enzyme from carcass was ten times more sensitive to ouabain than the enzyme from tegument. Comparison with results obtained on the (Na+ + K+)ATPase of human heart showed that the enzymes from the worms were more resistant to ouabain. The half maximal inhibitory concentration of dihydroouabain compared to that of ouabain was also different in the enzymes from human and worm. We conclude that (1) there exists at least one structural difference between the (Na+ + K+)ATPase of S. mansoni and that of the human host, and (2) it is useful to separately study the enzymes from tegument and carcass because they differ in sensitivity to cardiac glycosides.

Evidence that mammalian lignans show endogenous digitalis-like activities

Enterolactone, a lignan that has been identified in biological samples from man and several mammals, shares with ascorbic acid and cardiac glycosides a gamma-butyrolactone. It displaces 3H-ouabain from its binding sites on cardiac digitalis receptor and inhibits, dose dependently, the Na+, K+-ATPase activity of human and guinea-pig heart. The time dependence of this inhibition resembles that of dihydroouabain, a cardiac glycoside in which the lactone ring does not contain conjugated double bonds. The active concentrations of enterolactone as inhibitor of Na+,K+-ATPase are in the 10(-4) M range and, at those concentrations, the cross-reactivity with antidigoxin antibodies is low. Lignans may contribute to the putative digitalis-like activity found in tissues, blood and urine of several mammals including man.

Cardiac glycoside receptors in cultured heart cells--II. Characterization of a high affinity and a low affinity binding site in heart muscle cells from neonatal rats

The binding of [3H]ouabain has been studied in (Na+ + K+)-ATPase enriched cardiac cell membranes, as well as in cardiac muscle and non-muscle cells in culture--all obtained from hearts of neonatal rats. The binding has been correlated with ouabain-induced inhibition of (Na+ + K+)-ATPase (cardiac cell membranes) and the inhibition of active (86Rb+ + K+)-influx (cardiac muscle and non-muscle cells in culture). Furthermore, the effect of ouabain on the amplitude of cell-wall motion and contraction velocity has been studied in electrically driven cardiac muscle cells. In muscle and non-muscle cells, two classes of ouabain binding sites have been identified. In rat heart muscle cells, the high affinity binding site has a dissociation constant (KD) of 3.2 X 10(-8) M and a binding capacity (B) of 0.2 pmole/mg protein (80,000 sites/cell); the values for the low affinity binding site are: KD = 7.1 X 10(-6) M; B = 2.6 pmole/mg protein (10(6) sites/cell). The binding to both types of binding sites is depressed by K+ and abolished after heat denaturation of the cells. The kinetics of [3H]ouabain binding to rat heart muscle cells (association and dissociation rate constants, K+- and temperature-dependence of association and dissociation processes) have been characterized. In rat heart muscle and non-muscle cells, the binding of [3H]ouabain to the low affinity site results in inhibition of the (86Rb+ + K+)-influx (EC50 = 1.3 and 1.5 X 10(-5) M ouabain), a decrease in cell-K+ (EC50 = 1.9 and 1.4 X 10(-5) M) and an increase in cell-Na+ (10(-5)-10(-4) M). The ouabain-induced positive inotropic effect (increase in amplitude of cell-wall motion, increase in contraction velocity) in cardiac muscle cells is observed only at ouabain concentrations greater than or equal to 5 X 10(-6) M, and it is therefore probably attributed to occupation of the low affinity binding site. Coupling of occupation of the low affinity site by ouabain with drug-induced inhibition of the sodium pump and with drug-induced positive inotropic action is further substantiated by kinetic measurements. In contrast, occupation of the high affinity binding site does not produce any measurable inhibition of the sodium pump activity or positive inotropy.(ABSTRACT TRUNCATED AT 400 WORDS)

Digitalis receptors affinity labelling and relation with positive inotropic and cardiotoxic effects

Affinity labelling of the digitalis receptor has indicated that it is situated on the N-terminal part of the alpha-subunit of the (Na+,K+)ATPase. Biochemical and pharmacological properties of the (Na+,K+)ATPase studied on intact chick embryonic hearts and under heart cell culture conditions have indicated the existence of two families of ouabain binding sites i.e.: a low affinity binding sites with a dissociation constant (Kd) of 2-6 microM for the ouabain-receptor complex and a high affinity binding site with a Kd of 26-48 nM. High and low affinity sites also are present at all embryonic stages studied. Inhibition of 86Rb+ uptake in cultured cardiac cells and increase in intracellular Na+ concentration, due to (Na+,K+)ATPase blockade, occur in an ouabain concentration range corresponding to the saturation of the low affinity ouabain site. Ouabain stimulated 45Ca2+ uptake increases in parallel with the increase in the intracellular Na+ concentration. It is suppressed in Na+ free medium or when Na+ is replaced by Li+ suggesting that the increase is due to the indirect activation of the Na+/Ca2+ exchange system in the plasma membrane. Dose-response curves for the inotropic effects of ouabain on papillary muscle and on ventricular cells in culture indicate the development of the cardiotonic properties is parallel to the saturation of the low affinity binding site for ouabain. Therefore, inhibition of the cardiac (Na+,K+)ATPase corresponding to low affinity ouabain binding sites seems to be responsible for both the cardiotonic and cardiotoxic effects of the drug.

The inotropic effect of ouabain and its antagonism by dihydroouabain in rat isolated atria and ventricles in relation to specific binding sites

The inotropic effect of ouabain has been studied in rat ventricles and atria. The concentration-effect curve of ouabain may be fitted by a model assuming the existence of two saturable components. The component with the higher sensitivity to ouabain accounted for 30% of the maximal increase in systolic tension in ventricles and for only 5% in atria. Increase in diastolic tension was only apparent at ouabain concentrations required to observe the low sensitivity component. [3H]-ouabain binding has been examined in microsomes prepared from atria and ventricles. High and low affinity binding sites have been observed. The ratio of high and low affinity ouabain binding sites was 4 fold lower in microsomes from rat atria than from rat ventricles. This could account for the difference in the response of these two tissues to the inotropic action of ouabain. In ventricular strips the high sensitivity component was much less apparent in the presence of dihydroouabain than with ouabain. When ventricular strips were preincubated in the presence of dihydroouabain 3 microM, the increase in systolic tension evoked by ouabain 1 microM was significantly reduced. Cumulative concentration-effect curve studies showed dihydroouabain antagonism to the high sensitivity component.