Kinetics studies on the interaction between ouabain and (Na+,K+)-ATPase

Advances in understanding the role of cardiac glycosides in control of sodium transport in renal tubules

Cardiotonic steroids have been used for the past 200 years in the treatment of congestive heart failure. As specific inhibitors of membrane-bound Na(+)/K(+) ATPase, they enhance cardiac contractility through increasing myocardial cell calcium concentration in response to the resulting increase in intracellular Na concentration. The half-minimal concentrations of cardiotonic steroids required to inhibit Na(+)/K(+) ATPase range from nanomolar to micromolar concentrations. In contrast, the circulating levels of cardiotonic steroids under physiological conditions are in the low picomolar concentration range in healthy subjects, increasing to high picomolar levels under pathophysiological conditions including chronic kidney disease and heart failure. Little is known about the physiological function of low picomolar concentrations of cardiotonic steroids. Recent studies have indicated that physiological concentrations of cardiotonic steroids acutely stimulate the activity of Na(+)/K(+) ATPase and activate an intracellular signaling pathway that regulates a variety of intracellular functions including cell growth and hypertrophy. The effects of circulating cardiotonic steroids on renal salt handling and total body sodium homeostasis are unknown. This review will focus on the role of low picomolar concentrations of cardiotonic steroids in renal Na(+)/K(+) ATPase activity, cell signaling, and blood pressure regulation.

Brain Na+,K+-ATPase isozyme activity and protein expression in ouabain-induced hypertension

In normotensive rats, chronic infusion of exogenous ouabain causes hypertension involving central mechanisms. To determine whether ouabain-induced hypertension is associated with specific changes in brain Na+,K+-ATPase activity and expression, we assessed brain Na+,K+-ATPase isozyme activity and protein expression in rats treated with ouabain (50 microg/day s.c. or 10 microg/day i.c.v. for 14 days). Resting mean arterial pressure (MAP) was higher in s.c.- and i.c.v.-ouabain-treated animals vs. control (124+/-2 vs. 105+/-2 and 130+/-2 vs. 109+/-2, respectively, p<0.01). Ouabain infused s.c. or i.c.v. for 14 days had no effect on Na+,K+-ATPase isozyme activity in hypothalamic, pontine/medullary or cortical microsomes. However, the percent increase in total Na+,K+-ATPase activity produced in vitro by antibody Fab fragments that bind ouabain with high affinity (Digibind) was two-fold greater in s.c.- and i.c.v.-ouabain-treated rats vs. control, but only in hypothalamic microsomes. Thus, ouabain infused s.c. or i.c.v. does appear to directly inhibit Na+,K+-ATPase activity in the hypothalamus. On the other hand, in the hypothalamus, s.c.- and i.c.v.-ouabain infusions tended to increase alpha3 (by 30-44%), but had no effect on alpha1 or alpha2 Na+,K+-ATPase isozyme protein expression. In addition, ouabain was found to partially dissociate from the Na+,K+-ATPase enzyme following sample processing. Thus, the inability to detect a decrease in enzyme activity in the hypothalamus in response to ouabain may be due, in part, to an increase in enzyme expression and the dissociation of ouabain during sample processing.

Forskolin-induced down-regulation of Na+,K(+)-ATPase activity is not associated with internalization of the enzyme

Activation by protein kinase A by forskolin phosphorylates and inactivates Na+,K(+)-ATPase in COS-7 cells (Cheng et al. 1997b). In this study we show, using [3H]ouabain binding, that forskolin-induced inhibition of Na+,K(+)-ATPase activity is not because of internalization of the enzyme. The effect of forskolin on Na+,K(+)-ATPase activity was examined by two independent methods, ouabain-sensitive 86Rb+ uptake in intact cells and ATP hydrolysis in microsomal preparations from cells. The change in number of functional pumps on cell surface before and after protein kinase A activation was assessed by [3H]ouabain binding measured under equilibrium conditions. Cells, which had been ATP-depleted by antimycin A and 2-deoxyglucose treatment, served as a positive control for the internalization of Na+,K(+)-ATPase. Activation of protein kinase A with forskolin in combination with the phosphodiesterase inhibitor 3-isobutyl-1-methyl xanthine, inhibited Na+,K(+)-ATPase activity, but this treatment had no effect on specific ouabain binding. No change in ouabain binding was found following activation of protein kinase C by phorbol ester or diacyl glycerol analogue treatment in cells. These data suggest that protein kinase A phosphorylation and inhibition of Na+,K(+)-ATPase activity does not lead to any internalization of the enzyme in COS-7 cells.

Influence of extracellular K+ concentration on the time-course of Na+/K+-ATPase inhibition by cardiac glycosides with fast and low binding kinetics

The magnitude of the K+ antagonism of cardiac glycoside binding to Na+/K+-ATPase prepared from porcine heart, was estimated from the enzyme activities determined in the presence of different concentrations of K+ ([K+]), ouabain, and alpha-methyl-digitoxigenin-glucoside, the latter showing a 30 fold greater dissociation rate than ouabain. An increase of [K+] (3-20 mmol/l) prolonged the half-lives of Na+/K+-ATPase inhibition and caused a rightward shift of the cardiac glycoside's dose-response curves by the same factor, almost maximal (4 fold) at 14 mmol/l K+. These data could be verified from the cardiac glycoside-elevated intravesicular Na+ concentrations of rat brain vesicles. These concentrations declined rapidly in brain vesicles treated with alpha-methyl-digitoxigenin-glucoside but not with ouabain after K+ was increased from 3.5 to 14 mM. The results suggest that the magnitude of the K+ antagonism under physiological conditions is only limited by the lifespan of the cardiac glycoside-binding E2P enzyme conformation reduced by K+.

Kinetic modeling of ouabain tissue distribution based on slow and saturable binding to Na,K-ATPase

The significance of the binding to Na,K-ATPase in the tissue distribution of ouabain was previously documented (Harashima et al., Pharm. Res. 9:474-479, 1992). The purpose of this study was to obtain a kinetic model of ouabain tissue distribution. In most tissues, the ouabain concentration continued to rise after the termination of infusion (5 min), with the peak tissue concentration at approximately 20 min. This delay could not be explained by the rapid equilibrium model (RE model), nor could the kinetics of ouabain be explained by an RE model modified for saturable binding. Since ouabain binding to Na,K-ATPase is slow, the association and dissociation processes were incorporated into a model that can accurately fit the observed time courses of ouabain. The obtained binding parameters corresponded well with the observed values in the in vitro binding experiments, except for muscle. These results quantitatively support the role of the slow and saturable binding of ouabain to Na,K-ATPase in its tissue distribution.

Effects of ouabain on contractile response to norepinephrine in isolated rat aorta

1. Inhibition of sodium(Na+), potassium(K+)-ATPase activity was dependent on the concentration of ouabain and inverse to the concentration of potassium ([K+]) in the reaction mixture. 2. Contractility of isolated rat aorta preparation in response to norepinephrine was augmented by ouabain and low [K+]. 3. Results suggest that inhibition of Na+,K(+)-ATPase activity will be correlated with augmentation of vascular contractility maybe through activation of Na(+)-calcium exchange system.

Enhancement of vascular contractility by plasma substances obtained from pregnancy-induced hypertension

1. Augmentation of norepinephrine (NE)-induced contraction of isolated dog mesenteric arteries by ouabain was significantly enhanced under 4 mM potassium ([K+]) medium condition compared with usual medium condition at 6 mM [K+]. 2. Vascular contractility to NE was significantly enhanced by plasma substances obtained from pregnancy-induced hypertension (PIH) patients compared with that from normotensive pregnant women under 4 mM [K+] medium condition despite of no difference between them under 6 mM [K+] medium condition. 3. These results suggest the possible involvement of the ouabain-like substance in development of hypertension in PIH patients.

Localization and characterization of binding sites with high affinity for [3H]ouabain in cerebral cortex of rabbit brain using quantitative autoradiography

[3H]Ouabain binding was studied in sections of rabbit somatosensory cortex by quantitative autoradiography and in rabbit brain microsomal membranes using a conventional filtration assay. KD values of 8-12 nM for specific high-affinity binding of [3H]ouabain were found by both methods. High-affinity binding was not uniformly distributed in somatosensory cortex and was localized predominantly to laminae 1, 3, and 4. [3H]Ouabain binding in tissue sections was stimulated by the ligands Mg2+/Pi or Mg2+/ATP/Na+ and was inhibited by K+ (IC50 = 0.7-0.9 mM), N-ethylmaleimide, 5,5'-dithiobis(2-nitrobenzoic acid), and erythrosin B. We conclude that [3H]ouabain is reversibly and specifically bound with high affinity in rabbit brain tissue sections under conditions that favor phosphorylation of Na+,K+-ATPase. Quantitative autoradiography is a powerful tool for assessing the affinity and number of specific ouabain binding sites in brain tissue.

Multiple interactions of unsaturated fatty acids with opiate and ouabain binding sites and beta-adrenergic sensitive adenylate cyclase system

The unsaturated fatty acids oleic, linoleic and arachidonic inhibited binding of ligands to the ouabain, opiate, and beta-adrenergic plasma membrane receptors. Low concentrations of fatty acids slightly increased the binding of ouabain to its binding sites. The effect of these fatty acids on beta-adrenergic sensitive adenylate cyclase was more complex. 0.2-0.3 mM fatty acids increased adenylate cyclase activity, while higher concentrations of arachidonic and linoleic acids, but not oleic acid, inhibited basal, beta-agonist- and NaF-stimulated activities in membranes of A431 and C6 cells. To evaluate which aspects of the unsaturated fatty acid molecules might be responsible for the observed effects, myristic acid, monoolein and taurodeoxycholic acid were studied. They also inhibited binding to the opiate receptor. Myristic acid, did not inhibit ouabain binding, binding to beta-receptor, nor adenylate cyclase activity. Monoolein, had no inhibitory effect on ouabain binding but behaved similar to oleic acid in the beta-receptor/adenylate cyclase system. Taurodeoxycholic acid inhibited binding to all three receptors as well as adenylate cyclase activity. We conclude that the effects of unsaturated fatty acids on ligand binding and adenylate cyclase activity are the result of their multiple interactions with various molecular processes rather than any unique property of long chain unsaturated fatty acids, per se.

Effects of ethanol on ouabain inhibition of mouse brain (Na+,K+)ATPase activity

Plots of ouabain inhibition of mouse cerebral cortical (Na+,K+)ATPase activity fitted a two-site model significantly better than a one-site model, consistent with the presence of two forms of the enzyme with different affinities for ouabain. The fraction of enzyme activity with high affinity for ouabain (HAO: Ki = 500 nM), suggested to be localized neuronally, constituted the major portion (60-70%) of activity. Ouabain inhibition of both components of enzyme activity was reduced as KCl concentrations were increased. In vitro, only high concentrations of ethanol affected (Na+,K+)ATPase activity and ouabain inhibition of activity. Ethanol (500 mM) selectively reduced the activity, and increased the sensitivity to ouabain inhibition, of the HAO component, with no significant effect on the low-affinity (LAO) component. On the other hand, following chronic treatment of mice with ethanol in vivo, in a paradigm that produced tolerance and physical dependence, the sensitivity to ouabain of the HAO form of the enzyme was selectively increased. The relative proportions, and the activities of the HAO and LAO components, were not altered. The effects of ethanol, added in vitro, on the HAO component were decreased in ethanol-tolerant animals. The selective effect of chronic ethanol ingestion on (Na+,K+)ATPase activity indicates the specificity of action of ethanol in the CNS.

Demonstration and characterization of two classes of cardiac glycoside binding sites to rat heart membrane preparations using quantitative computer modeling

Cardiac glycoside binding to rat heart membrane preparations was measured by rapid filtration technique. The binding data were analyzed using quantitative computer analysis. The experimental results using [3H]-ouabain as the labeled ligand were consistent with a model in which cardiac glycoside specific binding occurs at two independent classes of sites. The high affinity sites were characterized by a dissociation constants of 40 nM, 50 nM, and 61 nM for ouabain, digoxin and digitoxin, respectively, with a binding capacity of 1.3 pmoles/mg protein. The lower affinity sites for ouabain were characterized by dissociation constants of 2.3 microM, 67 nM and 71 nM for ouabain, digoxin and digitoxin, respectively, with a binding capacity of 3 pmoles/mg protein. Potassium ions inhibit [3H]-ouabain binding in a dose dependent manner with an IC50 of 500 microM. Quantitative computer modelling indicated that potassium inhibits ouabain binding at both binding sites.

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.

Cyclic AMP-dependent stimulation of Na,K-ATPase in shark rectal gland

Scatchard analysis of 3H ouabain bound to isolated rectal gland cells as a function of increasing ouabain concentrations produced a concave curvilinear plot that was resolved into two specific sites with either a high (I) or low (II) affinity for ouabain. Cyclic cAMP/theophylline (+/- furosemide, 10(-4) M) increased the amount of 3H ouabain bound to the high-affinity site I. Vanadate, a phosphate congener which promotes formation of the ouabain-binding state of the enzyme, mimicked the effects of cAMP/theophylline at low concentrations of ouabain, suggesting that cAMP/theophylline increases binding to site I by enhancing the rate of turnover of resident enzyme. Enhanced 86Rb uptake seen following cAMP/theophylline administration was primarily associated with increased flux through the high-affinity ouabain site, and this stimulation was not obliterated by the co-administration of furosemide. A model was presented which suggested the presence of two noninteracting pools of enzyme or isozymes which exhibit either a high or low affinity for ouabain. Cyclic AMP both stimulated turnover via site I, and modified the kinetics of binding of 3H ouabain to site II. The (ave) Kd of 3H ouabain for site II was increased from 3.6 microM (controls) to 0.5 microM (cAMP/theophylline) and the Hill coefficient was modified from 0.45 (controls) to 1.12 (cAMP/theophylline), suggesting a transition from a negative- to a noncooperative binding state. While furosemide reversed the effects of cAMP/theophylline on site II kinetics, it did not obliterate cAMP/theophylline effects on site I. This suggests that cAMP may alter the intrinsic turnover rate of this particular pool of Na,K-ATPase in shark rectal gland.

Effects of K+ on the interaction between cardiac glycosides and Na,K-ATPase

Inhibition of Na,K-ATPase by cardiac glycosides is at least partially antagonized by K+. The kinetics of the antagonism, however, appear complicated because K+ is capable of reducing both association and dissociation rate constants for the glycoside-enzyme interaction. In order to better understand the effect of K+, inhibition of partially purified Na,K-ATPase obtained from rat brain, guinea-pig heart and rat heart by ouabain, digoxin, digoxigenin, dihydrodigoxin and cassaine were compared in the presence of 1, 3 or 10 mM K+. Higher concentrations of K+ caused a parallel shift to the right in the concentration-inhibition curves for these compounds. For ouabain or digoxin, the extent of the shift was minimal with rat brain enzyme, intermediate with guinea-pig heart enzyme and more substantial with rat heart enzyme. For digoxigenin, dihydrodigoxin or cassaine, the extent of the shift was substantial in all enzyme preparations. These results could not be explained from either the affinity of the enzyme for the compound or its lipid solubility alone. The concentrations of these compounds required to cause a 50 percent inhibition of enzyme activity were markedly different with rat brain enzyme, but relatively similar with rat heart enzyme. The effects of K+, which depend on the source of the enzyme and chemical structures of the compounds, have to be considered in studies on comparative effects of various compounds on Na,K-ATPase, [3H]ouabain binding, sodium pumping and the force of myocardial contraction.

Interaction of cardiac glycosides and Na,K-ATPase is regulated by effector-controlled equilibrium between two limit enzyme conformers

The paper describes the dissociation parameters of the complexes between [3H]-digitoxin and Na,K-ATPase (Na+ + K+-activated, Mg2+-dependent ATP phosphohydrolase, E.C. 3.6.1.3) from pig cardiac muscle and brain cortex formed and dissociated in the presence of different combinations and concentrations of the enzyme effectors ATP, Mg2+, Na+ and K+. Systematic variation of effector-ligation of Na,K-ATPase allowed production of glycoside complexes with two enzyme conformers only, which showed either rapid or slow dissociation kinetics. Appropriate changes of enzyme ligation allowed the interconversion of the two conformer types. Biphasic, rapid and slow glycoside release was not bound with the presence of two Na,K-ATPase isozymes, but caused by the enzyme ligation-determined coexistence of the two conformers of Na,K-ATPase. The rate constants for the rapid and slow glycoside release were within the complexes of each dissociation type much alike indicating uniform isomerization kinetics of the two conformers even when differently liganded. Taken together, the observations indicated the effector-controlled isomerizations of two conformers of Na,K-ATPase possessing different geometries of the glycoside binding domain. Present findings and relevant literature data were integrated in a circular, consecutive and simultaneous model for induced conformation changes that accounted for the regulation of the interaction of cardiac glycosides and Na,K-ATPase through an effector-controlled equilibrium between two limit enzyme conformers.

Partial purification and properties of the inhibitors of Na, K-ATPase and ouabain-binding in bovine central nervous system

Endogenous inhibitors of Na,K-ATPase and ouabain-binding were partially purified from bovine central nervous system, and some of their properties were studied. They were eluted as low-molecular-weight fractions by gel filtration. They could be adsorbed by both Amberlite IR 120 and Amberlite IRA 400 at acidic and basic pH, respectively, indicating that they could act as both anions and cations at different pH. These inhibitors of ouabain-binding appeared to affect specific binding of ouabin, and Scatchard plot analysis showed that the inhibition was competitive, suggesting that they could bind to the same site as ouabain, presumably to Na,K-ATPase itself. The inhibitory activities were heat stable, but charring inactivated them completely.

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.

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.

[The problem of the cellular receptor for cardiac glycosides (author's transl)]

This review concerns the Na+, K+ -ATPase as well as the Na+, K+ -pump in the intact membrane and the highly specific inhibition of this transport system by cardiac glycosides. The interaction between glycoside and enzyme and the regulation of the kinetics of glycoside binding by ATP, K+, Na+, Mg2+ and Ca2+ are described. Emphasis is placed on the significance of the Na+, K+ -pump as the pharmacological receptor for cardiac glycosides. The problem encountered and progress made in attempting to correlate the inotropic action of cardiac glycosides with the binding of these drugs to the heart muscle and with the inhibition of the Na+, K+ -pump are reported. Recent results concerning increases of the intracellular Na+ concentration which are obtained by a partial inhibition of the Na+, K+ -pump and which are followed by an elevation of the intracellular Ca2+ -activity are reviewed. The discovery of a digitalis-like endogenous activity corresponds to the high specificity of the receptor for cardiac glycosides.

Preferential sensitivity of the left canine purkinje system to cardiac glycosides

Previous studies have shown that the toxic effects of cardiac glycosides are not manifested uniformly throughout the myocardium. The purpose of our study was to determine whether cardiac glycosides exert different effects on the right vs. left peripheral Purkinje systems and to ascertain mechanisms involved. Control in vitro measurements of paired right and left canine Purkinje fibers showed higher spontaneous rates in left (24.2 +/- 1.75 beats/min) than in right (11.6 +/- 1.55 beats/min, P less than 0.01, n = 81) Purkinje fiber bundles. Following overdrive stimulation, left Purkinje fiber bundles also showed earlier escape beats. After ouabain exposure (2 X 10-7 M), left Purkinje fiber bundles showed earlier signs of toxicity in 20 of 28 experiments, as determined by changes in the maximum diastolic potential, the degree of diastolic depolarization, spontaneous escape intervals, and the magnitude of delayed after-depolarizations. The enhanced sensitivity of left Purkinje fiber bundles was independent of the extracellular potassium concentration and glycoside polarity, and was also observed in situ. We conclude that distal Purkinje fibers are functionally dissimilar and that the left Purkinje system shows greater sensitivity to cardiac glycosides than the right Purkinje system. These data also support the observation that digitalis-induced dysrhythmias arise in the left ventricle.

Inotropic effects and Na+,K+-ATPase inhibition of ouabain in isolated guinea-pig atria and diaphragm

Effects of ouabain on force of contraction were compared in electrically driven isolated tissue preparations of guinea-pig left atria and diaphragm. A distinct and steady positive inotropic effect of ouabain was observed in atrial preparations, whereas in diaphragm preparations, ouabain produced only a slight and transient positive inotropic effect, followed by the negative inotropic phase. The transient positive inotropic effect of ouabain was observed even in the absence of extracellular calcium, but was markedly dependent on the extracellular sodium concentration. In vitro [3H]ouabain binding studies revealed that the affinity of Na+,K+-ATPase for ouabain was about eight times higher and tissue concentration of the enzyme was significantly lower in diaphragm than in cardiac tissue. The Ki value for ouabain inhibition of the cardiac Na+,K+-ATPase was also approximately ten times higher than for the diaphragm enzyme. Ouabain-sensitive 86Rb uptake, an estimate of sodium pump activity, was inhibited by ouabain at a time when it produced its transient positive inotropic effect in diaphragm preparations. These results indicate that the lack of a distinct and steady positive inotropic effect of ouabain in diaphragm was due neither to the difference in the ouabain-Na+,K+-ATPase interaction between diaphragm and cardiac tissues nor the failure of sodium pump inhibition by ouabain in diaphragm.

The influence of reduced serum potassium level on the toxicity of some cardenolides in guinea pigs

Recent experiments on the pharmacological properties of the semisynthetic cardiotonic steroid strophanthidin-3-bromoacetate (SBA) have challenged the well-known potassium digitalis antagonism in isolated heart muscle preparations. In order to establish these results in vivo, the minimum lethal doses (LDmin) of ouabain (OUA), digoxin (DO), digotoxin (DT), k-strophanthidin (STR) and SBA were determined by the infusion toxicity method in guinea pigs at normokalemia and hypokalemia. The experimentally induced decrease of the serum potassium concentration (5.0 mmoles/l vs. 3.3 mmoles/l) significantly reduced the LDmin of DO (1.42 vs. 1.05 mumoles/kg), DT (1.78 vs. 1.24 mumoles/kg) and STR (20.16 vs. 15.98 mumoles/kg), whereas the LDmin of OUA (0.37 vs. 0.34 mumoles/kg) was not altered. Contrary, the LDmin of SBA was even slightly, but not significantly increased during hypokalemia (16.77 vs. 19.04 mumoles/kg). In addition, from the experimental data an optimum time of infusion (Topt), corresponding to the LDmin, can be derived, which is equivalent to the time for optimum "utilization" of the drug. The obtained sequence: STR less than OUA less than DO less than DT less than SBA represents the well-known differences in the onset of the pharmacological action in man resp. animal. Hypokalemia in general resulted in a shortening of the Topt, thus indicating a more rapid "utilization" of the drug tested. The above differences of the cardenolide action at reduced serum potassium concentration may be dependent on the recently reported divergent influence of potassium on the association- resp. dissociation rate constants for the interaction of these drugs with their binding site at the Na+-K+-ATPase.

Inhibition by digoxin and SC4453 of (Na+ + K+)-ATPase prepared from human heart, guinea-pig heart and guinea-pig brain

SC4453 is a digoxin analogue with a pyridazine instead of a lactone ring on C17 beta. SC4453 was compared with digoxin with respect to inhibition of (Na+ + K+)-ATPase prepared from human heart, guinea-pig heart and guinea-pig brain. SC4453 was slightly less potent than digoxin but showed a similar sensitivity to K+. As for cardenolides, species differences in sensitivity to SC4453 were accounted for by differences in the rate of dissociation from the receptors. These observations confirm that the human heart is one of the tissues most sensitive to cardiac glycosides.

Ouabain receptor binding of hydroxyprogesterone derivatives

1 A specific and sensitive radioreceptor assay ahs been devised which is based on high affinity, saturable binding of 9 nM [3H]-ouabain to the total particulate fraction isolated from dog heart. Ouabain and other cardiac glycosides, including the aglycones, were about equipotent in their ability to displace [3H]-ouabain from its receptor, the IC50s ranging from 10 to 30 nM. 2 The only other substances found to compete significantly in the assay were derivatives of hydroxyprogesterone having a 17 alpha-acetate substituent: chlormadinone acetate, megestrol acetate, cyproterone acetate and medroxyprogesterone acetate, with IC50s of 2, 7.4, 9 and 21 microM, respectively. Prednisolone-3,20-bisguanyl-hydrazone, reported to have inotropic activity, gave an IC50 of 6.4 microM. Cyproterone-17 alpha-OH was less active (IC50 90 microM) than cyproterone-17 alpha-acetate. 3 A large number of peptide and protein hormones, steroid hormones and their metabolites, amines, and drugs were inactive.

Differential effect of potassium on the action of digoxin and digoxigenin in guinea-pig heart

The effect of potassium on the binding of digoxin or digoxigenin to isolated Na+, K+-ATPase was compared with that of potassium on the positive inotropic action of the agents in guinea-pig hearts. The binding of digoxigenin to the enzyme in vitro was reduced to a greater extent by potassium than was the binding of digotoxin. The digoxigenin-induced increase in the force of contraction of left atrial preparations estimated at steady state was reduced at higher potassium concentrations. Potassium had a lesser effect when digoxin was used as the inotropic agent. In contrast, potassium concentrations. Potassium had a lesser effect when digoxin was used as the ininotropic agent. In contrast, potassium reduced the rate of development and also the rate of loss of the positive inotropic action of digoxin observed with left atrial and Langendorff preparations, respectively, to a greater extent than those of digoxigenin. The loss of the positive inotropic effect was more rapid with digoxigenin than with digoxin at each KCl concentration. These data support the contention that the extent of the interaction of digitalis with Na+,K+-ATPase determines the degree of the positive inotropic effect.

Reduction of the equilibrium binding of cardiac glycosides and related compounds to Na+,K+-ATPase as a possible mechanism for the potassium-induced reversal of their toxicity

The influence of potassium ions on the equilibrium state of the binding of cardiac glycosides and their derivatives to partially purified dog heart and rat brain enzyme preparations was studied in vitro. The addition of potassium to the incubation mixture containing enzyme preparation, 3H-ouabain, Na+, Mg2+ and ATP, at the time when the binding reaction is close to equilibrium, caused an immediate reduction of the bound drug concentration; the concentration apparently shifting toward a lower equilibrium state. The degree of the potassium-induced reduction in bound drug concentration was dependent on the potassium concentration and on the chemical structure of the compound. The binding of aglycones, pentacetyl-gitoxin and cassaine was affected to a greater extent than that of the glycosides. These data suggest that one of the mechanisms by which potassium antagonizes the toxic actions of digitalis on the heart is to reduce the drug binding to cardiac Na+,K+-ATPase.

Prednisolone-3,20-bisguanylhydrazone: the mode of interaction with rat brain sodium and potassium-activated adenosine triphosphatase

The mechanism of interaction between prednisolone-3,20-bisguanylhydrazone (PBGH) and Na+,K+-ATPase (ATP phosphohydrolase, EC 3.6.1.3) was studied using partially purified rat brain enzyme preparations. PBGH inhibited Na+,K+-ATPase rapidly and reversibly. The enzyme-inhibiting action of PBGH was competitively antagonized by potassium. PBGH inhibited Na+,Mg2+ and ATP-supported binding of (3H)-ouabain to the enzyme. When PBGH was added to the incubation mixture at the time when the (3H)-ouabain binding was close to its equilibrium state, the concentration of (3H)-ouabain complex was rapidly reduced and shifted to a lower equilibrium state. A double reciprocal plot analysis of the (3H)-ouabain binding data indicates that the inhibition of ouabain binding by PBGH is apparently competitive. Binding of (3H)-ouabain in the presence of Tris-phosphate and Mg2+ was also inhibited by unlabeled PBGH. Thus, it appears that the binding of PBGH precludes the binding of ouabain to Na+,K+-ATPase.

Membrane (Na+ + K+)-ATPase of canine brain, heart and kidney. Tissue-dependent differences in kinetic properties and the influence of purification procedures

Effects of commonly used purification procedures on the yield and specific activity of (Na+ + K+)-ATPase (Mg2+-dependent, Na+ + K+-activated ATP phosphohydrolase, EC 3.6.1.3), the turnover number of the enzyme, and the kinetic parameters for the ATP-dependent ouabain-enzyme interaction were compared in canine brain, heart and kidney. Kinetic parameters were estimated using a graphical analysis of non-steady state kinetics. The protein recovery and the degree of increase in specific activity of (Na+ + K+)-ATPase and the ratio between (Na+ + K+)-ATPase and Mg2+-ATPase activities during the successive treatments with deoxycholate, sodium iodide and glycerol were dependent on the source of the enzyme. A method which yields highly active (Na+ + K+)-ATPase preparations from the cardiac tissue was not suitable for obtaining highly active enzyme preparations from other tissues. Apparent turnover numbers of the brain (Na+ + K+)-ATPase preparations were not significantly affected by the sodium iodide treatment, but markedly decreased by deoxycholate or glycerol treatments. Similar glycerol treatment, however, failed to affect the apparent turnover number of cardiac enzymes preparations. Cerebral and cardiac enzyme preparations obtained by deoxycholate, sodium iodide and glycerol treatments had lower affinity for ouabain than renal enzyme preparations, primarily due to higher dissociation rate constants for the ouabain.enzyme complex. This tissue-dependent difference in ouabain sensitivity seems to be an artifact of the purification procedure, since less purified cerebral or cardiac preparations had lower dissociation rate constants. Changes in apparent association rate constants were minimal during the purfication procedure. These results indicate that the presentyl used purification procedures may alter the properties of membrane (Na+ + K+)-ATPase and affect the interaction between cardiac glycosides and the enzyme. The effect of a given treatment depends on the source of the enzyme. For the in vitro studies involving purified (Na+ + K+)-ATPase preparations, the influence of the methods used to obtain the enzyme preparation should be carefully evaluated.

Membrane adenosinetriphosphatase: a digitalis receptor?

The enzyme Na+,K+-ATPase is a good model for receptor studies because of its known functional correlates. The binding of digitalis to the enzyme observed in vitro satisfied the criteria for receptor binding. Studies of the relationship between the digitalis binding and the drug action reveal an impressive correlation between these events but fail to provide proof of a causal relationship. Studies with other Na+,K+-ATPase inhibitors and agents that affect transmembrane Na+ movements (steps that would follow Na+,K+-ATPase inhibition) provide further supportive evidence that sodium pump inhibition and the resulting enhancement of intracellular Na+ transients cause the inotropic action of digitalis.