Ouabain-receptor interactions in (Na+ + K+)-ATPase preparations. A contribution to the problem of nonlinear Scatchard plots

Na,K pump stimulation by intracellular Na in isolated, intact sheep cardiac Purkinje fibers

Regulation of the Na,K pump in intact cells is strongly associated with the level of intracellular Na+. Experiments were carried out on intact, isolated sheep Purkinje strands at 37 degrees C. Membrane potential (Vm) was measured by an open-tipped glass electrode and intracellular Na+ activity (aNai) was calculated from the voltage difference between an Na+-selective microelectrode (ETH 227) and Vm. In some experiments, intracellular potassium (aiK) or chloride (aCli) was measured by a third separate microelectrode. Strands were loaded by Na,K pump inhibition produced by K+ removal and by increasing Na+ leak by removing Mg++ and lowering free Ca++ to 10(-8) M. Equilibrium with outside levels of Na+ was reached within 30-60 min. During sequential addition of 6 mM Mg++ and reduction of Na+ to 2.4 mM, the cells maintained a stable aNai ranging between 25 and 90 mM and Vm was -30.8 +/- 2.2 mV. The Na,K pump was reactivated with 30 mM Rb+ or K+. Vm increased over 50-60 s to -77.4 +/- 5.9 mV with Rb+ activation and to -66.0 +/- 7.7 mV with K+ activation. aiNa decreased in both cases to 0.5 +/- 0.2 mM in 5-15 min. The maximum rate of aiNa decline (maximum delta aNai/delta t) was the same with K+ and Rb+ at concentrations greater than 20 mM. The response was abolished by 10(-5) M acetylstrophantidin. Maximum delta aNai/delta t was independent of outside Na+, while aKi was negatively correlated with aNai (aKi = 88.4 - 0.86.aNai). aCli decreased by at most 3 mM during reactivation, which indicates that volume changes did not seriously affect aNai. This model provided a functional isolation of the Na,K pump, so that the relation between the pump rate (delta aNai/delta t) and aiNa could be examined. A Hill plot allowed calculation of Vmax ranging from 5.5 to 27 mM/min, which on average is equal to 25 pmol.cm-2.s-1.K 0.5 was 10.5 +/- 0.6 mM (the aNai that gives delta aNai/delta t = Vmax/2) and n equaled 1.94 +/- 0.13 (the Hill coefficient). These values were not different with K+ or Rb+ as an external activator. The number of ouabain-binding sites equaled 400 pmol.g-1, giving a maximum Na+ turnover of 300 s-1. The Na,K pump in intact Purkinje strands exhibited typical sigmoidal saturation kinetics with regard to aNai as described by the equation upsilon/Vmax = aNai(1.94)/(95.2 + aNai(1.94)). The maximum sensitivity of the Na,K pump to aiNa occurred at approximately 6 mM.

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.

Ouabain binding and Na+ content in resistance vessels and skeletal muscles of spontaneously hypertensive rats and K+-depleted rats

The possible role of Na+ in the development of hypertension in rats was explored in measurements of intracellular Na+, 22Na efflux, and 3H-ouabain binding sites in resistance vessels and skeletal muscles. In resistance vessels obtained from 13-week-old spontaneously hypertensive rats (SHR) or age-matched Wistar-Kyoto rats (WKY), (Na)i, total or ouabain-resistant 22Na efflux, and the concentration of 3H-ouabain binding sites showed no significant differences. Soleus muscles obtained from 6-week-old and 13-week-old SHR contained 5 to 11% more 3H-ouabain binding sites than those of WKY. The small difference in ouabain binding probably was related more to variations in growth rate and strain than to the hypertension. In SHR and WKY the Na+ and K+ contents of gastrocnemius muscles were almost identical at 6 and 13 weeks of age. By contrast, in Wistar rats in which the (Na)i of skeletal muscle was increased sixfold by K+ depletion, the systolic blood pressure was decreased by 10%. The K+ depletion was associated with a 35 to 55% decrease in the concentration of 3H-ouabain binding sites in both resistance vessels and skeletal muscles. The results provide no support for any simple cause-effect relationships between either elevated (Na)i or altered concentration of 3H-ouabain binding sites and hypertension in SHR.

A method for the determination of the total number of 3H-ouabain binding sites in biopsies of human skeletal muscle

A new method based on vanadate facilitated binding of 3H-ouabain has been applied for the quantitative determination of the number of 3H-ouabain binding sites (Na-K-pumps) in needle biopsies of human skeletal muscle. Samples of the vastus lateralis muscle weighing 2-8 mg showed specific and saturable binding of 3H-ouabain with an apparent KD of 1.9 X 10(-8) mol/l. In 20 healthy human subjects in the age range 25-80 years, the number of 3H-ouabain binding sites was 278 +/- 15 pmol/g wet weight with no relation to age or sex. In samples of the intercostal and rectus abdominis muscles, the number of 3H-ouabain binding sites varied from 225 to 280 pmol/g wet weight. These values are at least 2 times higher than those previously reported for human skeletal muscle. The number of 3H-ouabain and 3H-digoxin binding sites were identical, and ouabain (10(-3) mol/l) completely displaced specifically bound 3H-digoxin. When biopsies were frozen in liquid N2 immediately after withdrawal, storage at -20 degrees C for up to 11 weeks caused no significant change in the number of 3H-ouabain binding sites. The method allows quantitative determination of the number of 3H-ouabain binding sites in standard biopsies of human skeletal muscle to be performed by simple procedures within a few hours. This can be used for the study of conditions where the number of Na-K-pumps is known to undergo fluctuations.

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.

Brain NaK-ATPases in mice differentially sensitive to alcohols

The isozymes of NaK-ATPase were studied in the particulate fraction of homogenates prepared from cortex of LS and SS mice, two lines of mice that have been selectively bred for differential response to ethanol. In addition to a ouabain-insensitive Mg-ATPase, ouabain dose-response curves have suggested the presence, in brain, of two NaK-ATPase activities with different sensitivities to ouabain. These are designated low Ki (4 X 10(-7) M) and high Ki (2 X 10(-4) M). Ethanol differentially inhibited the ATPases: The ouabain-insensitive activity was less sensitive to ethanol inhibition than were the two ouabain-inhibitable activities. The low Ki (brain specific) NaK-ATPase was more sensitive than was the high Ki activity. However, ethanol inhibited all three components of the ATPase activity in an identical fashion in the two mouse lines. The low Ki activity was also more labile to thermal and p-hydroxymercurobenzoate denaturation than was the high Ki activity. These measures did not differ between the LS and SS lines. SDS-polyacrylamide electrophoresis of particulate fraction of cortical homogenates obtained from LS and SS mice revealed the presence of two protein bands with similar 32P labeling in both lines. Given that electrophoretic pattern and heat or p-hydroxymercurobenzoate inhibition were identical, it seems unlikely that differences in ATPase activity, or inhibition by ethanol, are responsible for the different response of LS and SS mice to ethanol.

Identification of putative calcium channels in skeletal muscle microsomes

Saturable binding sites for the labelled calcium antagonist (+/-)[3H]nimodipine were found in guinea-pig hind limb skeletal muscle homogenates. Binding sites were enriched in a microsomal pellet by differential centrifugation of the homogenate. [3H]Nimodipine binding (Kd = 1.5 +/- 0.03 nM, Bmax = 2.1 +/- 0.25 pmol/protein, at 37 degrees C) copurified (6-fold) in this fraction with [3H]ouabain binding (6.6-fold) and 125I-alpha-bungarotoxin binding (5-fold). d-cis-Diltiazem (but not 1-cis-diltiazem) stimulated (+/-) [3H]nimodipine binding (ED50 1 microM) by increasing the Bmax. Binding sites discriminated between the optical enantiomers of 1.4-dihydropyridine calcium antagonists and the optically pure enantiomers of D-600. The data confirm, with biochemical techniques, the presence of 1,4-dihydropyridine and (+/-) D-600 inhibitable calcium channels in skeletal muscle, previously found with electrophysiological techniques.

Interactions of erythrosin B (U.S. F, D & C Red 3) with rat cortical membranes

Erythrosin B inhibits Na,K-ATPase in rat brain tissue as demonstrated by studying glycoside binding, ATPase activity and ion fluxes. The potency of the noncompetitive inhibition of [3H]-ouabain binding by erythrosin B is influenced by glycoside concentration, monovalent cation concentration, and incubation time. [14C]-Erythrosin B binds to synaptic membranes prepared from rat cortex. Erythrosin B and some of its structural analogs inhibit both [3H]-ouabain and [14C]-erythrosin B binding, but ouabain and other glycosides do not inhibit the binding of [14C]-erythrosin B. Subcellular distributions of [3H]-ouabain and [14C]-erythrosin B binding in fractionated cortical tissue preparations are equivalent and parallel ATPase activity. The dissimilar response of [3H]-ouabain binding and [14C]-erythrosin B binding to changes in tissue preparation, incubation temperature, and partial solubilization of binding sites by deoxycholate (DOC) suggests two separate binding sites for erythrosin and ouabain to rat cortical membranes.

Effect of age, potassium depletion and denervation on specific displaceable [3H]ouabain binding in rat skeletal muscle in vivo

1. Following intraperitoneal injection of [(3)H]ouabain in rats, the isotope is rapidly distributed in blood plasma available for binding to the Na-K-ATPase in the plasma membranes of most tissues. In skeletal muscle tissue excised and washed 4 x 30 min in ice-cold buffer, 95% of the (3)H activity retained was shown to be [(3)H]ouabain using a specific binding assay.2. The [(3)H]ouabain bound to soleus and extensor digitorum longus (e.d.l.) muscles in vivo and retained following wash-out in the cold showed the same saturation characteristics as those determined when binding took place in vitro.3. In soleus and e.d.l. muscles obtained from 28-day-old rats, the number of [(3)H]ouabain binding sites measured in vivo was 583+/-19 and 720+/-22 pmol/g wet wt., respectively, i.e. in good agreement with previous and present results obtained in vitro.4. In vivo measurements showed that 7 days after denervation, the number of [(3)H]ouabain binding sites in soleus and e.d.l. muscles was reduced by 22 and 13%, respectively.5. In the age interval from 28 to 85 days, the number of [(3)H]ouabain binding sites in soleus was found to decrease by 58%. Following I.P. injection of [(3)H]ouabain, the 85-day-old rats showed a more pronounced and sustained rise in plasma (3)H activity, which in part can be due to the reduced capacity for [(3)H]ouabain binding in skeletal muscle.6. K depletion induced by the administration of K-deficient diet for 3 weeks reduced [(3)H]ouabain binding by 63% in soleus muscles. In the K-depleted animals, the plasma (3)H activity measured 15 min after I.P. injection of [(3)H]ouabain was 77% higher than in controls receiving the same dose per kg body weight.7. The present in vivo results provide further support for the idea that increased digitalis toxicity due to increasing age or K depletion is related to reduced binding capacity for digitalis glycosides in skeletal muscle.

Sympathetic nerve terminal destruction has no effect on specific [3H]ouabain binding to intact mouse and rat skeletal muscle

Following pretreatment with 6-OH-dopamine, the sympathetic nerve endings of the iris and skeletal muscle of mice and rats were not longer detectable by cytochemistry for noradrenaline. In the same animals, the specific binding of [3H] ouabain to intact soleus muscles was not significantly different from that measured in controls, Neither ouabain-suppressible 42K uptake nor Na-K contents were affected by chemical sympathectomy.

Isolation and partial characterization of an acetylcholine receptor-enriched membrane fraction from skeletal muscle

A procedure for purification of the bungarotoxin-binding fraction of sarcolemma from rabbit skeletal muscle is described. Muscle is homogenized in 0.25M sucrose without high salt extraction and membrane fractions separated initially by differential centrifugation procedures. An ultracentrifugation pellet enriched in cell surface and sarcoplasmic reticulum markers is further fractionated on a dextran gradient (density = 1.0 to 1.09). Two fractions are identified as sarcolemma according to high specific activities for lactoperoxidase-iodination, Na+, K+-ATPase and alpha-bungarotoxin-binding. No Ca++, Mg++-ATPase activity is found in these fractions. A third fraction, the dextran gradient pellet, is enriched in Ca++, Mg++-ATPase activity and lactoperoxidase iodinatable material and characterized by low bungarotoxin binding. This fraction represents a mixture of sarcoplasmic reticulum and transverse tubules with some sarcolemma contamination.

Molecular interactions in biomedicine: modulation of regulatory behavior by cross reactivity: relevance to analysis of receptor function

The influence of cross reactivity on biomolecular regulation is not frequently incorporated into theoretical analysis and is often eliminated by experimental design. Nonetheless, cross reactive molecules produce effects on binding which may considerably modulate their behavior in quite diverse systems, i.e., drug-tissue receptor, antigen-antibody, substrate-enzyme, protein-DNA and hormone-tumor receptor interactions. In particular, certain effects vary at different concentrations suggesting a simple control mechanism.

Phenotypic characterization of ouabain-resistant Aedes albopictus cells

The phenotype of a ouabain-resistant Aedes albopictus cell line has been partially characterized. Treatment of ouabain-sensitive cells with 0.005-1.0 mM ouabain resulted in an 80% reduction in the uptake of 86rubidium (86Rb+), an ion with an affinity for the K+ pump binding site; ouabain-resistant cells showed only a 40% reduction with 1.0 mM ouabain. When ouabain-sensitive cells were incubated in the presence of ouabain (0.1 mM) for one and one-half to three hours, the molar ratio of intracellular Na+/K+ rose from 0.2 to 4.2. In ouabain-resistant cells, a similar treatment had very little effect. Based on [3H] ouabain-binding studies, ouabain-resistant cells were estimated to have 60% fewer binding sites per cell than ouabain-sensitive cells. The spontaneous mutation rate from ouabain sensitivity to ouabain resistance was calculated to be 1-6 x 10(-8) mutations/cell/generation, a value similar to that reported for mammalian cells at the analogous locus.

Dehydro-digitoxosides of digitoxigenin and digoxigenin: binding to beef heart (Na+ + K+)-ATPase in relation to unchanged digitoxosides

Dehydro-digitoxosides are metabolites of digitalis glycosides. In order to study their possible biological activity their affinity to (Na+ + K+)-activated ATPase was determined and compared with unchanged glycosides. Based on the dissociation constants of glycoside-enzyme-complexes, the affinity of the dehydro-digitoxosides ranged in the same order of magnitude as that of the native glycosides. Comparing mono-, bis-, and tris-digitoxosides of digitoxigenin (dt-1, dt-2, dt-3) and of digoxin (dg-1, dg-2, dg-3) with the corresponding dehydrodigitoxosides (3'-dehydro-dt-1, 9'-dehydro-dt-2, 15'-dehydro-dt3, 3'-dehydro-dg-1 and 9'-dehydro-dg-2, respectively) the dehydro-digitoxosides had lower affinities to the enzyme. The highest dissociation constants (KD) were found for 3'-dehydro-dt-1 and 3'-dehydro-dg-1. The half maximal inhibition of (Na+ + K+)-ATPase activity (I50) coresponded to affinity measurements in all but two cases: dehydro-dt-3 and dehydro-dt-2 showed very low I50 values.

Subcellular mechanisms of lead neurotoxicity

The neurotoxic effects of inorganic lead (Pb) involve inhibition of calcium-dependent acetylcholine release and increases in calcium-dependent dopamine release. These apparently differential effects of Pb are associated with differing Pb-calcium (Ca) interactions: Pb blocks 45Ca binding to peripheral cholinergic ganglia and increases 45Ca binding to synaptosomes prepared from caudate nucleus (CN). Pb-induced increases in CN 45Ca binding did not result from nonspecific disruption of selective ion permeability of the membrane. Also, the Na-K ATPase-linked Ca extrusion system of synpatosomes was not affected by Pb. A Pb-sodium (Na) interaction was found such that elevation of intrasynaptosomal Na reversed effects of Pb on 45Ca binding. The intracellular localization of this effect appeared to be primarily at the mitochondrial level. Pb inhibited Na-induced release of 45Ca from preloaded mitochondria. This action may be translated into increased transmembrane flux of exogenous Ca, and thence into increased exocytotic events at the synapse. The apparently neurotransmitter-specific effects of Pb, cholinergic inhibition and dopaminergic augmentation, are hypothesized to result from different Pb-Ca interactions which are determined by the specific localization of Pb within nerve endings.

Active Na-K transport and the rate of ouabain binding. The effect of insulin and other stimuli on skeletal muscle and adipocytes

1. The effect of stimulation or inhibition of active Na-K transport on [(3)H]ouabain binding has been investigated in isolated soleus muscles and adipocytes.2. In rat soleus muscle, the ouabain-sensitive component of (42)K influx was stimulated by insulin (100 m-u/ml.), adrenaline (6 x 10(-6)M), and by pre-incubation with veratrine (10(-5)M) or in a K-free buffer. In all of these instances, the rate of ouabain binding was increased by 41-113%. Conversely, pre-treatment with tetracaine (0.2 mM) decreased the (42)K-influx and diminished the rate of [(3)H]ouabain binding by 36%.3. Neither insulin, adrenaline or tetracaine produced any detectable change in the total number of ouabain-binding sites (as measured under equilibrium conditions) in rat soleus muscle.4. In mouse and guinea-pig soleus muscle and in fat cells isolated from rats, insulin also increased the rate of [(3)H]ouabain binding without producing any significant change in the total number of ouabain-binding sites.5. Both in soleus muscle and the epididymal fat pad of the rat, there was a linear correlation between (42)K influx and the initial rate of [(3)H]ouabain binding.6. It is concluded that the rate of ouabain binding is determined significantly by the rate of active Na-K transport, but within the time intervals studied (4-6 hr) stimulation or inhibition of the Na pump does not lead to any appreciable change in the total number of Na pumps. It seems unlikely that the stimulation of active Na-K transport by insulin or adrenaline is due to unmasking or de novo synthesis of Na pumps.