Digitalis receptor desensitization in rat ventricle: ouabain produces two inotropic effects

Inotropic effect of ouabain in hypertrophied rat heart

The contribution of the heterogeneous digitalis receptors to the inotropic effect of ouabain was studied in hypertrophied rat hearts (aortic stenosis) by using isolated Langendorff heart preparations. Development and washout of the biological effects as well as the dose/response curves revealed two inotropic components of high and low drug sensitivity. Maximal inotropy was observed with 100 microM ouabain in both control and hypertrophied rat hearts. The high-sensitivity component accounted for only one-third of the response in control hearts but for two-thirds in hypertrophied hearts. The respective apparent affinities (10-20 nM and 10-20 microM) of the two inotropic components found in isolated hearts were similar to those of the high- and low-affinity Na+,K(+)-ATPase activities detected in isolated cardiac sarcolemma. Onset and reversion of the pharmacological effects of ouabain occurred at respective rates that were similar to those of the association and dissociation of ouabain to the Na+,K(+)-ATPase level. In hypertrophied heart, the high- and low-sensitivity components (or receptors) reacted seven- and threefold, respectively, more slowly than the corresponding sites in normal hearts. These alterations in inotropic responsiveness and properties of the digitalis receptors in cardiac hypertrophy suggest that new regulations should be taken into account in the adaptation to pressure overload.

Two active Na+/K+-ATPases of high affinity for ouabain in adult rat brain membranes

The degree of heterogeneity of active Na+/K(+)-ATPases has been investigated in terms of ouabain sensitivity. A mathematical analysis of the dose-response curves (inhibition of Na+/K(+)-ATPase) at equilibrium is consistent with the putative existence of three inhibitory states for ouabain two of high (very high plus high) and one of low affinity. The computed IC50 values are: 23.0 +/- 0.15 nM, 460 +/- 4.0 nM and 320 +/- 4.6 microM, respectively. The relative abundance of the three inhibitory states was estimated as: 39%, 36% and 20%, respectively. Direct measurements of [3H]ouabain-binding at equilibrium carried out on membrane preparations with ATP, Mg2+ and Na+ also revealed two distinct high affinity-binding sites, the apparent Kd values of which were 17.0 +/- 0.2 nM (very high) and 80 +/- 1 nM (high), respectively. Dissociation processes were studied at different ouabain concentrations according to both reversal of enzyme inhibition and [3H]ouabain release. The reversal of enzyme inhibition occurred at three different rates, depending upon the ouabain doses used (10 nM, 2 and 100 microM). When the high-affinity sites were involved (ouabain doses lower than 2 microM) the dissociation process was biphasic. A similar biphasic pattern was also detected by [3H]ouabain-release. The time-course of [3H]ouabain dissociation (0.1 microM) was also biphasic. These data indicate that the three catalytic subunits of rat brain Na+/K(+)-ATPase alpha 1, alpha 2 and alpha 3 (Hsu, Y.-M. and Guidotti, G. (1989) Biochemistry 28, 569-573) are able to hydrolyse ATP and exhibit different affinities for cardiac glycosides.

Comparison of the effects of neuropeptide Y (NPY) and 4-norleucine-NPY on isolated perfused rat hearts; effects of NPY on atrial and ventricular strips of rat heart and on rabbit heart mitochondria

Isolated perfused rat hearts were used to compare the effects of the synthetic neuropeptide Y (NPY) and 4-norleucine-NPY on cardiac function. Each peptide exhibited both negative inotropic and chronotropic effects, and also caused coronary vasoconstriction leading to a reduction in coronary flow. A comparison of the IC50 values from dose-response curves using 10(-14) to 10(-7) M peptides (IC50 is the peptide concentration that produced a 50% decrease of the maximal effect) indicated that NPY was more potent as inhibitor of contractility and less potently inhibited coronary flow and heart rate, whereas 4-norleucine-NPY had more inhibitory influence on coronary flow and heart rate and less on cardiac contractility. This difference in potencies suggests that the inhibitory effects of NPY on contractility, coronary flow and heart rate may be independent of each other. Since NPY also decreased the contractile force of isolated left atrial and right ventricular strips of the rat heart, the coronary flow decrease cannot be the cause of the negative inotropy of isolated heart. Pretreatment of atrial and ventricular strips with NPY did not influence the positive inotropic effect produced by the cardiac glycoside ouabain indicating that sarcolemmal Na+, K+-ATPase was not involved in the inhibitory inotropic effect of NPY. Further studies towards elucidating the mechanism of the negative inotropy of cardiac muscles using isolated heart mitochondria revealed that NPY uncoupled oxidative phosphorylation and blocked mitochondrial calcium uptake; the former event fosters negative inotropy. Since these effects on mitochondria occurred at concentrations 100-fold higher than those required for negative inotropy, the two effects of NPY may not be related.

Postnatal changes in the inotropic effect of ouabain on the rat heart ventricle

The concentration-dependent effects of ouabain on the contractility of postnatally developing rat heart ventricles were studied. Ouabain caused a positive inotropic effect in right ventricular strips of neonatal rats up to the age of about 30 days but a negative inotropic effect in the adult cardiac tissue. When extracellular Ca concentration was lowered from 2.5 to 1.0 mmol/l and the rate of stimulation was simultaneously elevated from 0.2 to 1.0 Hz a clear positive inotropic effect was also generated in the adult rat heart. The positive inotropic effect of ouabain showed a biphasic developmental pattern: the contractile force first grew from birth to about 15 days of age but steeply declined near to the adult level during the 3rd postnatal week. The force response to ouabain occurred within two distinct dose-ranges. In the newborn only the high-dose (above 3 X 10(-6) mol/l) effect was seen but in rats older than 5 days a mixed low-dose (below 3 X 10(-6) mole/l)/high-dose effect was apparent. In both ranges the positive inotropic effect of ouabain seemed to be dependent on caffeine sensitive Ca store, perhaps the sarcoplasmic reticulum. It is suggested that during the 3rd postnatal week a transition from extracellular to intracellular Ca stores occurs in the rat heart, which is reflected as a changing inotropic effect of ouabain on the developing cardiac tissue.

High affinity and low affinity ouabain binding sites in the rat heart

Ventricular muscle of rat heart has two classes of receptors which are responsible for the positive inotropic effect of ouabain. Low affinity receptors are apparently related to Na+, K+-ATPase. To determine if high affinity receptors are also sarcolemmal Na+, K+-ATPase of muscle cells, their characteristics were examined. Binding of [3H]ouabain to the high affinity binding site required ATP in the presence of Mg2+ and Na+, was stimulated by Na+ in the presence of Mg2+ and ATP, and was inhibited by K+. Digoxin, digitoxin and cassaine all inhibited [3H]ouabain binding to the high affinity site. Cassaine was about an order of magnitude less potent than the glycosides. These results indicate similarities in high affinity ouabain binding sites in ventricular muscle of rat heart and Na+, K+-ATPase obtained from other sources. Destruction of sympathetic nerve terminals with 6-hydroxydopamine failed to affect the high affinity ouabain binding sites indicating that high affinity sites do not represent the Na+, K+-ATPase in sympathetic nerve terminals. Labeling of Na+, K+-ATPase from [gamma-32P]ATP indicates that high affinity ouabain binding sites account for 25% of the total enzyme molecules present in ventricular muscle of rat heart.

Respective involvements of high- and low-affinity digitalis receptors in the inotropic response of isolated rat heart to ouabain

High- and low-affinity digitalis receptors coexist in rat cardiac sarcolemma. In this study, their relative involvement in the inotropic effect of ouabain was evaluated on an isolated Langendorff rat heart preparation working under isovolumic conditions at a low external calcium concentration (0.25 mM). This involvement was estimated according to both the development of the inotropic response to ouabain (10(-8)-10(-4)M) and the time course of the washing out of the biological effect. In each phenomenon considered, and whatever the index of inotropy chosen, the high-affinity digitalis receptor (EC50: 1-2 X 10(-8) M) contributed to 25-40% of the maximal inotropy (evoked by 10(-4) M ouabain). Low-affinity receptors (EC50: 1-2 X 10(-5) M) accounted for 60-75%. These apparent affinities were identical to those previously determined in sarcolemma isolated from rat heart perfused with 0.25 mM Ca2+. The biphasic effect of ouabain was related to both the inhibition of high- and low-sensitivity Na+, K+-ATPase forms and the corresponding number of ouabain-binding sites occupied. These results support the concept that the Na+, K+-ATPase highly sensitive to ouabain as revealed by lowering calcium is the in vivo manifestation of the high-sensitivity inotropic component.

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 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)

Binding sites for ouabain in human and rat erythrocytes and in rat heart cells

The presence and properties of two classes of binding sites for digitalis in erythrocytes and in heart cells are reviewed. Methods to distinguish between these two binding sites are summarized for intact cells and for their membranes. Our biochemical data do not suggest a physiological role for each class of sites. The membrane sites were modified by varying (a) the content of cholesterol or (b) the distribution of fatty acids leading to changes in the microviscosity thus affecting the degree of binding of ouabain to the two classes of sites. Oxidation of ouabain with periodate forms a di-aldehyde which could bind via Schiff base to the digitalis sites but could also attach covalently to these sites. The role of the sugar moiety in the process of ouabain binding becomes of increasing importance. A mild, controlled periodate oxidation of ouabain, especially in presence of phosphate cleaves only the bond between C-2 and C-3 of the rhamnose without affecting the steroid moiety. The periodate oxidation provided additional information for assigning a distorted chair conformation or a transient boat conformation for rhamnose in ouabain. It was also established by 1H NMR spectroscopy that this chair conformation is a 1C4 pyranose ring.

Chronic diabetes decreases the ouabain inotropic response in rat left atria and papillary muscles

The inotropic response to ouabain in rat left atria were markedly depressed in animals made diabetic for 6 months. Similarly, ouabain inotropy was also decreased in papillary muscles obtained from 70-day diabetic rats. The effect of isoproterenol was not reduced in the above papillary muscle experiments and was not tested in the atria experiments. Ouabain produced a monophasic dose-response curve in atria but a biphasic curve in papillary muscles. Timolol had no effect on the ouabain response. Left atria and papillary muscles from animals made diabetic for 7 days responded in a normal fashion to both ouabain and isoproterenol.

Impact of neonatal treatment with cardioactive glycosides (digoxin, ouabain) on receptor binding capacity, blood level and cardiac function in the adult rat. Extension of the imprinting theory

A single neonatal treatment with a cardioactive glycoside (ouabain, digoxin) altered the response of the adult rat to digitaloid treatment. As demonstrated by RIA, re-exposure to digoxin at 2 months of age was followed within 30 min by a more than twofold increase in serum digoxin over the not pretreated control and, although a steady concentration decrease followed, the experimental rats still had a higher serum digoxin level than the controls at 4 h. In the not presensitized control group the serum digoxin peak appeared at 60 min at a considerably lower level than the 30-min maximum of the experimental rats. Neonatal pretreatment with ouabain depressed myocardial ouabain binding, but enhanced the Na+ K+ -ATPase activity. The above differences were conspicuous in the functional response, yet a greater atrial response to the positive inotropic action of K-strophanthoside and a greater ventricular response to beta adrenergic excitation were readily seen in the experimental group. It follows that not only hormones, but also other ligands acting at receptor level can be regarded as potential inducers of imprinting.