Ouabain receptor in isolated adult dog heart myocytes

Apparent cooperativity of [3H]ouabain binding to myocytes obtained from guinea-pig heart

Kinetics of [3H]ouabain binding to intact cardiac cells were examined using myocytes obtained from guinea-pig heart. In intact cells, the use of excess unlabeled ouabain results in an under-estimation of nonspecific binding, presumably due to cytotoxic effects of the unlabeled glycoside; estimation of the specific binding, as that to rapidly releasing sites yields more accurate results. Specific [3H]ouabain binding to myocytes is promoted by an increase in Na+ influx, indicating that normal intracellular Na+ concentration is insufficient to fully stimulate glycoside binding. High concentrations of [3H]ouabain seem to increase the apparent affinity of binding sites for the glycoside via increases in intracellular Na+ concentration resulting from sodium-pump inhibition; hence the binding reaction may be regarded as having a novel type of cooperativity. This cooperativity has kinetics different from those of classical positive cooperativity based on binding-site interactions, and is apparent with toxic concentrations of the glycoside that cause marked increases in intracellular Na+ concentrations.

Inhibition of sarcolemmal Na, K, Mg ATPase from the guinea pig heart is not compatible with a homogeneous population of non-interacting ouabain receptors

The inhibition of sarcolemmal Na, K, Mg ATPase from the guinea pig heart by ouabain was evaluated with a coupled enzyme assay. Models of negative cooperativity and of two independent receptors fitted the inhibition data equally well. The analysis was not compatible with a homogeneous population of non-interacting ouabain receptors.

Characterization of digoxin binding and daunorubicin uptake by isolated mature rat cardiac myocytes

Myocytes isolated from ventricular muscle of mature rat heart have been used for characterization of digoxin binding and to establish whether a relationship exists between digoxin binding and uptake of daunorubicin. High- and low-affinity digoxin binding sites have been identified; respectively, 0.9 +/- 0.1 X 10(7) sites/myocyte, Kd 70-77 nM and 7 +/- 2 X 10(7) sites/myocyte, Kd 1.4-1.7 microM. Myocytes accumulate daunorubicin to an intracellular concentration 30-40 times that in the medium. We find no evidence that saturation of digoxin binding sites alters daunorubicin uptake or that daunorubicin influences binding of digoxin. Alteration of sarcolemmal membrane properties is demonstrated by inhibition of amino acid transport reflected in protein synthesis rates. Calmodulin activation of phosphodiesterase appears insensitive to daunorubicin.

Activatory effect of two cardioglycosides on Cavia cobaya kidney Na+/K+-ATPase activity

Ouabain and K-strophanthoside promote an enhancement of Na+/K+-ATPase activity in a range of cardioglycoside concentrations from 100 nM to 100 pM, with a maximum (+30%) between 10 and 4 nM. Binding experiments with [3H]ouabain show upward-curved Scatchard plots and evidence two intrinsic affinity constants for the ligand: (a) High-affinity constant: 350 nM (microsomes) and 15 nM (purified enzyme). (b) Low-affinity constant: 2100 nM (microsomes) and 890 nM (purified enzyme). The reaction velocity trend indicates that at ouabain concentrations higher than 20 nM but lower than the minimal inhibiting level, the enhanced reaction velocity is tending towards the control values.

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)

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.

Heterogeneity of ouabain specific binding sites and (Na+ + K+)-ATPase inhibition in microsomes from rat heart

Cardiac glycoside binding to microsomes prepared from rat heart ventricles and enriched in (Na+ + K+)-ATPase was measured by a rapid filtration technique. The relation between ouabain binding to microsomes and (Na+ + K+)-ATPase activity has also been examined. Data were statistically analysed by means of two different non linear regression methods. The experimental results were fitted the most closely by a model describing that ouabain specific binding occurred at two classes of independent sites. High affinity sites were characterized by a dissociation constant of 0.21 +/- 0.01 microM and a low capacity (9.4 +/- 1.4 pmoles/enzymatic unit). Low affinity sites were characterized by a dissociation constant equal to 13 +/- 3 microM and a capacity equal to 87 +/- 15 pmoles/enzymatic unit. Similar results were obtained with the more lipophilic glycoside digoxin. It was also observed that dihydroouabain, a ouabain derivative with a saturated lactone ring, competes with 3H-ouabain for the binding to the two classes of sites. Binding to these two classes of sites appeared to be associated with a corresponding inhibition of (Na+ + K+)-ATPase activity.

Isolation of calcium tolerant myocytes from adult rat hearts: review of the literature and description of a method

Myocytes have been isolated from adult rat hearts since 1969. The early preparations exhibited the Ca2+ paradox. Over the ensuing years, numerous groups have reported the isolation of Ca2+ tolerant cardiac myocytes. In the present review, detailed comparisons have been made of the yields, viability, and relative Ca2+ tolerance of these different myocyte preparations. The factors to which these investigators attributed the increased Ca2+ tolerance are considered, and the current information regarding the mechanism of the Ca2+ paradox is reviewed. A method is given which incorporates several of the modifications described. By this method 40-60% of the ventricular weight was disaggregated into single myocytes within 45 min after the sacrifice of the rats. Viability without further purification was 82 +/- 0.7% (n = 35) and Nai+/Ki+ ratios were normal. Upon incubation with 2 mM Ca2+ for 1 hr at 37 degrees C, viability decreased by 6% and ATP and creatine phosphate remained at physiological levels. The preparation is very stable since upon incubation in culture medium containing fetal bovine serum and 1.25 mM free Ca2+ at 25 degrees C for 20 hr, viability decreased only 13% (rod-shaped and trypan blue criteria). The factors which contribute to the quality and Ca2+ tolerance of this preparation are discussed.

Biochemical regulators in cardiac hypertrophy

In recent years research has shown that muscle is capable of reacting to mechanical stimuli by altering biochemical processes. Myocardium is probably the source of a biochemical factor, or factors which activate myocardial protein synthesis. In experimentally induced cardiac hypertrophy adaptive alterations have been shown to occur not only in the adrenal medulla but also in the adrenal cortex. Finally, detection of cross reactivity between digitalis glycosides and a number of steroid hormones has succeeded. We assume that such cross reactivity indicates the existence of an endogenic factor of steroid character, which is produced in the adrenal gland and functions as an endogenic cardiotonic agent. During experimental cardiac hypertrophy its synthesis is possibly increased. We propose the term "endocardin" or "endocardiotonin" for this agent.