Quantitative correlation of cardiac glycoside binding to its receptor and inhibition of the sodium pump in chicken heart cells in culture

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.

The red blood cell: a model for ouabain receptor regulation in the heart?

The assumption that the red blood cell can be used as a model for ouabain receptor regulation in heart muscle has been tested using isolated tissues from humans, guinea pigs, and chickens. The following results were obtained: The affinity of the ouabain receptor was similar in both human erythrocytes and right atrial appendage, but the density of binding sites was much lower on the erythrocytes. There was no correlation between the binding capacity in both tissues. Ouabain receptor occupation was closely correlated with inhibition of Na+/K+-transport in human erythrocytes and chick heart nonmuscle cells in culture. In contrast, in chick heart muscle cells, an occupation of 40% of the receptors decreased the Na+/K+-transport rate by only 10%. In hypokalemia, the ouabain binding capacity was increased in human and guinea pig erythrocytes but not in guinea pig heart muscle. Such increases were seen in chick heart nonmuscle cells in moderate hypokalemia but in heart muscle cells only after severe hypokalemia. Incubation of chick heart muscle cells in toxic but not in "therapeutic" ouabain concentrations increased the number of ouabain receptors. Increases in receptor number attenuated the positive inotropic and toxic actions of ouabain. These variations between ouabain receptor regulation in red blood cells and heart muscle of several species may be attributable to the lack of a "sodium pump reserve" in erythrocytes and heart nonmuscle cells. Such variations indicate that the human erythrocyte is not a suitable model for the ouabain receptor in the human heart.

Cardiac glycoside tolerance in cultured chicken heart muscle cells--a dose-dependent phenomenon

In cultured heart muscle cells from 10-13 day-old chicken embryos, the effects of acute (4 h) and chronic (3 days) exposure of the cells to varying concentrations of ouabain have been studied. In these cells, the cardiac glycoside ouabain binds to a specific cardiac glycoside receptor (KD = 4 X 10(-7) M; 750,000 receptors/cell). Binding to this receptor results in inhibition of active Na+/K+-transport [EC50 for active (86Rb+ + K+)-influx = 4 X 10(-6) M], and in an increase in beating velocity ("positive inotropic effect"; EC50 = 4 X 10(-7) M); toxic signs (arrhythmias) appear at concentrations greater than or equal to 6 X 10(-7) M. During exposure of the cells to 3 X 10(-6) M ouabain for 3 days, tolerance develops with respect to both the positive inotropic and the toxic effect. The mechanism underlying this tolerance is identified as an increase in the number of active sodium pump molecules per cell, while the binding properties of the cardiac glycoside receptor remain unchanged. The development of cardiac glycoside tolerance is only observed in the presence of severe impairment of Na+/K+-homeostasis, due to cardiac glycoside-induced inhibition of active Na+/K+-transport. This, however, only occurs in the presence of toxic (receptor occupation greater than or equal to 60%), but not in the presence of positive inotropic, non-toxic (receptor occupation 20-60%), ouabain concentrations. We conclude that the development of cardiac glycoside tolerance during long-term treatment in patients with heart failure should not occur with submaximal dose regimens, when toxic signs (arrhythmias) are absent.

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)

Action of ouabain on rat heart: comparison with its effect on guinea-pig heart

The inotropic dose-response curve of ouabain in rat cardiac ventricular strips exceeded a concentration range of two decades (1 X 10(-7) M to 3 X 10(-5) M) displaying an intermediate plateau phase. In guinea-pig ventricular strips the inotropic ouabain concentrations spanned only one decade (1 X 10(-7) M-1 X 10(-6) M). Ouabain-intoxication in guinea-pig ventricular strips occurring at 3 X 10(-6) M consisted of arrhythmia and contracture, while in rat ventricular strips at the toxic concentration of 1 X 10(-4) M only a progressive increase in diastolic tension was observed. By means of atomic absorption spectroscopy the ouabain-induced loss of cellular potassium and gain of sodium in rat ventricular strips was detected only at concentrations of ouabain higher than 10(-4) M. Ouabain reduced the activity of Na/K-ATPase prepared from rat and guinea-pig cardiac ventricles to half of its maximum at 6.5 X 10(-5) M in rat and 1.0 X 10(-6) M in guinea-pig, rat heart Na/K-ATPase thus being about 60 fold less sensitive towards ouabain. Specific [3H]-ouabain binding to membrane suspensions prepared from rat and guinea-pig ventricles was characterized by a similar affinity in rat (KD = 4 X 10(-8) M) and guinea-pig (KD = 13 X 10(-8) M). The number of ouabain binding sites in rat membranes was only about 10% of the number found in guinea-pig membranes. In rat the presence of additional ouabain-binding with low affinity and high capacity seemed possible, but could not be verified for methodological reasons. In the light of the biochemical results and binding data, the wider range of ouabain concentration exerting a positive inotropic effect in the rat may be attributed to the existence in the latter of two populations of receptors with different affinities for ouabain and different capacities. In contrast, in the guinea-pig, there is a single population. Nevertheless it is probable that all the receptors in both species are part of the Na/K-ATPase complex and mediate a positive inotropic effect after ouabain-binding in an identical manner.

Two binding sites for ouabain in cardiac cell membranes

Cardiac glycoside receptors were defined by simultaneous measurement of 3H-ouabain binding and its effects on cardiac cell membranes, contracting cardiac muscle and cultured cardiac cells. These measurements show that: Rat and guinea pig cardiac cell membranes have two specific ouabain binding sites. In both species, ouabain binding to the high affinity site on cell membranes correlates with the positive inotropic effect in contracting cardiac muscle. Inhibition of (Na+ + K+)-ATPase activity corresponds to binding to the low affinity site. This questions the hypothesis that (Na+ + K+)-ATPase inhibition is necessary for ouabain-induced positive inotropy. K+ may induce an heterogeneity in the ouabain binding sites of the digitalis-sensitive cat and human heart.

Cardiac glycoside receptors in cultured heart cells--I. Characterization of one single class of high affinity receptors in heart muscle cells from chick embryos

Binding of (3H)-ouabain and ouabain-induced inhibition of the sodium pump and of the (Na+ + K+)-ATPase have been characterized in cultured cardiac muscle and non muscle cells, as well as in cardiac cell membranes--all obtained from chick embryos. In both cell types, ouabain binds to a single type of binding sites in a temperature-dependent manner. The association rate but not the dissociation rate, is lowered by K+; specific binding is lost after heat-denaturation of the cells. Binding parameters (association and dissociation rate constants, activation energies for association and dissociation) are similar in muscle and non muscle cells. The dissociation constant of specific ouabain binding is 1.5 X 10(-7)M in cardiac muscle cells, and 1.9 X 10(-7)M in cardiac non muscle cells, the binding capacity being 2.6 and 2.1 pmoles/mg protein respectively. Specific binding of ouabain to the cells is coupled to inhibition of the sodium pump, as can be seen from ouabain-induced inhibition of active (86Rb+ + K+)-uptake, decrease in cellular K+, and increase in cellular Na+ (EC50 = 10(-7)-10(-6)M). The data obtained with cardiac cells are in good agreement with results found for ouabain binding (dissociation constant 4.3 X 10(-7)M) and (Na+ + K+)-ATPase inhibition (EC50 = 1.4 X 10(-6)M) in cardiac cell membranes prepared from the same tissue. Due to the experimental evidence it is concluded that the binding site for ouabain is identical with the cardiac glycoside receptor of these cells. In cardiac non muscle cells, binding of ouabain to its receptor is strictly coupled to inhibition of active K+-transport in a stoichiometric manner. In cardiac muscle cells, however, active K+-transport is inhibited by less than 10% when up to 40% of cardiac glycoside receptors have bound ouabain. It is assumed that this non-stoichiometric coupling of receptor occupancy and sodium pump inhibition in cardiac muscle cells may prevent substantial changes of Na+- and K+-contents in the heart in the presence of therapeutic levels of cardiac glycosides.

Cardiac glycoside binding sites in cultured heart muscle cells

Binding of (3H)-ouabain to cultured cardiac muscle and non muscle cells from chicken embryos and neonatal rats has been characterized and correlated with ouabain-induced inhibition of the sodium pump, as well as with the positive inotropic action of the drug. Cardiac muscle and non muscle cells from 10-12 day-old chicken embryos are characterized by a single class of ouabain binding sites (muscle cells: dissociation constant KD = 1.5 X 10(-7) M; binding capacity B = 2.6 pmoles/mg cell protein). Two classes of ouabain binding sites, however, have been found in cardiac muscle and non muscle cells from 1-3 day-old, neonatal rats (muscle cells: high affinity, low capacity sites: KD = 3.2 X 10(-8) M, B = 0.2 pmoles/mg protein; low affinity, high capacity sites: KD = 1.7 X 10(-6) M, B = 2.6 pmoles/mg protein). Half maximal inhibition of active (86Rb+ + K+)-influx occurs at 5.8 X 10(-7)M ouabain in chicken heart muscle cells and at 1.3 X 10(-5)M in rat heart muscle cells [( K+] = 0,75 mM). Decreases in cell-K+ (EC50 = 6.7 X 10(-7)M and 1.9 X 10(-5)M) and increases in cell-Na+ (7.4 X 10(-7) and 10(-5) - 10(-4)M) parallel ouabain-induced inhibition of the sodium pump. Up to 10(-6)M, ouabain does not affect velocity of cell wall motion in cultured rat heart muscle cells. A concentration-dependent increase in cell wall motion is observed at concentrations between 5 X 10(-6) and 5 X 10(-5)M, being indicative of a positive inotropic effect. At 10(-4)M ouabain, arrhythmias are present. Our data demonstrate the existence of one single class of cardiac glycoside receptors in cultured cardiac muscle cells from chicken embryos.(ABSTRACT TRUNCATED AT 250 WORDS)