Multiplicity of cardiac glycoside receptors in the heart

Testis-Specific Isoform of Na+-K+ ATPase and Regulation of Bull Fertility

An advanced understanding of sperm function is relevant for evidence-based male fertility prediction and addressing male infertility. A standard breeding soundness evaluation (BSE) merely identifies gross abnormalities in bulls, whereas selection based on single nucleotide polymorphisms and genomic estimated breeding values overlooks sub-microscopic differences in sperm. Molecular tools are important for validating genomic selection and advancing knowledge on the regulation of male fertility at an interdisciplinary level. Therefore, research in this field is now focused on developing a combination of in vitro sperm function tests and identifying biomarkers such as sperm proteins with critical roles in fertility. The Na+-K+ ATPase is a ubiquitous transmembrane protein and its α4 isoform (ATP1A4) is exclusively expressed in germ cells and sperm. Furthermore, ATP1A4 is essential for male fertility, as it interacts with signaling molecules in both raft and non-raft fractions of the sperm plasma membrane to regulate capacitation-associated signaling, hyperactivation, sperm-oocyte interactions, and activation. Interestingly, ATP1A4 activity and expression increase during capacitation, challenging the widely accepted dogma of sperm translational quiescence. This review discusses the literature on the role of ATP1A4 during capacitation and fertilization events and its prospective use in improving male fertility prediction.

Total synthesis, chemical modification and structure-activity relationship of bufadienolides

Bufadienolides are a type of natural cardiac steroids and originally isolated from the Traditional Chinese Medicine Chan'Su, they have been used for the treatment of heart disease in traditional remedies as well as in modern medicinal therapy with potent anti-tumor activities. Due to their unique molecular structures with unsaturated six-membered lactones attached to the steroid core, bufadienolides have received great attention in the synthetic organic community. This review presents total synthetic efforts to some representative bufadienolides, chemical modification of bufadienolides will also be given to discuss their structure-activity relationship in anti-tumor.

Multiple digitalis receptors A molecular perspective

The Na,K-ATPase is the only established receptor for cardiac glycosides like digoxin or ouabain. There are now known to be three different isoforms of its principal subunit. These isoforms can differ from one another in their intrinsic affinity for cardiac glycosides. Recent work examines the molecular structure of the binding site. The relative level of expression of the isoforms in cardiac tissue is modified in several developmental, hormonal, and pathological states, contributing to alterations in the digitalis sensitivity of the tissue.

Extrusion of intracellular calcium ion after in vitro ischemia in the rat hippocampal CA1 region

Simultaneous recordings of intracellular Ca(2+) ([Ca(2+)](i)) signal and extracellular DC potential were obtained from the CA1 region in 1-[6-amino-2-(5-carboxy-2-oxazolyl)-5-benzofuranyloxy]-2-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid penta-acetoxymethyl ester (Fura-2/AM)-loaded rat hippocampal slices. Superfusion with oxygen- and glucose-deprived medium (in vitro ischemia) for 5-6 min produced a rapid rise of the [Ca(2+)](i) level in the stratum radiatum (rising phase of the [Ca(2+)](i) signal), which occurred simultaneously with a rapid negative DC potential (rapid negative potential). When oxygen and glucose were reintroduced, the increased [Ca(2+)](i) signal diminished rapidly (falling phase of the [Ca(2+)](i) signal) during the generation of a slow negative DC potential (slow negative potential), which occurred within 1 min from the onset of the reintroduction. Thereafter, the [Ca(2+)](i) signal partially and the slow negative potential completely returned to the preexposure level approximately 6 min after the reintroduction. The changes in [Ca(2+)](i) signal during and after in vitro ischemia were very similar to the changes in the membrane potential of glial cells. The rising and falling phases of [Ca(2+)](i) signal corresponded to the rapid depolarization and a depolarizing hump, respectively, in the repolarizing phase of glial cells. A prolonged application of in vitro ischemia or a reintroduction of either glucose or oxygen suppressed the falling phase after ischemic exposure. The application of ouabain (30 microM) generated both a rapid negative potential and a rapid elevation of [Ca(2+)](i), but no slow negative potential or rapid reduction in [Ca(2+)](i) were observed. When oxygen and glucose were reintroduced to slices in the Na(+)-free or ouabain- or Ni(2+)-containing medium, the falling phase was suppressed. The falling phase was significantly accelerated in Ca(2+)- and Mg(2+)-free with EGTA-containing medium. In contrast, the falling phase was significantly slower in the Ca(2+)-free with high Mg(2+)- and EGTA-containing medium. The falling phase of the [Ca(2+)](i) signal after ischemic exposure is thus considered to be primarily dependent on the reactivation of Na(+), K(+)-ATPases, while the extrusion of cytosolic Ca(2+) via the forward-mode operation of Na(+)/Ca(2+) exchangers in glial cells is thought to be directly involved in the rapid reduction of [Ca(2+)](i) after ischemic exposure.

The alpha(1)- and alpha(2)-isoforms of Na-K-ATPase play different roles in skeletal muscle contractility

The Na-K-ATPase, which maintains the Na(+) and K(+) gradients across the plasma membrane, can play a major role in modulation of skeletal muscle contractility. Although both alpha(1)- and alpha(2)-isoforms of the Na-K-ATPase are expressed in skeletal muscle, the physiological significance of these isoforms in contractility is not known. Evaluation of the contractile parameters of mouse extensor digitorum longus (EDL) was carried out using gene-targeted mice lacking one copy of either the alpha(1)- or alpha(2)-isoform gene of the Na-K-ATPase. The EDL muscles from heterozygous mice contain approximately one-half of the alpha(1)- or alpha(2)-isoform, respectively, which permits differentiation of the functional roles of these isoforms. EDL from the alpha(1)(+/-) mouse shows lower force compared with wild type, whereas that from the alpha(2)(+/-) mouse shows greater force. The different functional roles of these two isoforms are further demonstrated because inhibition of the alpha(2)-isoform with ouabain increases contractility of alpha(1)(+/-) EDL. These results demonstrate that the Na-K-ATPase alpha(1)- and alpha(2)-isoforms may play different roles in skeletal muscle contraction.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.

Affinity and dose-dependent digoxin Na+K+ATPase dissociation by monoclonal digoxin-specific antibodies

The effect of three monoclonal digoxin-specific antibodies and of polyclonal Digidot as reference on digoxin dissociation from rat brain Na+K+ATPase microsomes was studied to determine the role of the affinity constant (Ka) and dose of the antibody on the rate of digoxin dissociation from Na+K+ATPase. Stoichiometrical doses of 1C10, 6C9, 9F5 IgG, and Digidot (Ka = 6 10(9), 3.1 10(8), 2.5 10(7), and 8.5 10(9) M-1, respectively) resulted in digoxin dissociation related to Ka. When the IgG:digoxin molar ratio increased from 0.25 to 10, digoxin dissociation from Na+K+ATPase sites also increased according to the Hill equation, allowing comparative parameters among the three antibodies to be determined. 1C10 IgG was 2- and 10-fold more efficacious than 6C9 and 9F5, respectively. This in vitro model appears to be a useful predictive screening assay before in vivo experimentation.

Immunologic identification of Na+,K(+)-ATPase isoforms in myocardium. Isoform change in deoxycorticosterone acetate-salt hypertension

There are three isoforms of the catalytic (alpha) subunit of the Na+,K(+)-ATPase, each derived from a different gene, that differ in their sensitivity to inhibition by cardiac glycosides. Antibodies specific for the three isoforms were used to study Na+,K(+)-ATPase isoform expression in ventricular myocardium, where an understanding of digitalis receptor diversity is most important. In the rat heart, there is simultaneous expression of two isoforms in adult ventricle, and immunofluorescence studies demonstrated that both isoforms are expressed uniformly in cardiomyocytes. Hypertension and hypertrophy have been reported to selectively depress alpha 2 isoform mRNA levels, and we show in the present study that alpha 2 protein levels were correspondingly depressed in rats made hypertensive by uninephrectomy and treatment with deoxycorticosterone acetate and a high-salt diet. In the human heart, where mRNA for all three alpha isoforms has been reported, we detected all three isoform proteins (alpha 1, alpha 2, and alpha 3). Two isoforms (alpha 1 and alpha 3) predominated in the macaque heart; dissection of the heart showed uniformity of isoform expression in different ventricular regions but markedly less alpha 3 in the atrium. Finally, isoform-specific antibodies were used to detect which alpha isoforms were expressed in the ventricles of several commonly used experimental animals to test the correlation of isoform expression with cardiac glycoside-response heterogeneity. Two isoforms (alpha 1 and alpha 3) were found in canine myocardium, whereas only one (alpha 1) was found in sheep and guinea pig. Expression of Na+,K(+)-ATPase isoforms can thus be readily followed and related to the physiology of the digitalis receptor.

A comparison of the affinities of rat (Na+ + K+)-ATPase isozymes for cardioactive steroids, role of lactone ring, sugar moiety and KCl concentration

Binding experiments at equilibrium were performed to study pharmacological properties of isozymes of (Na+ + K+)-ATPase from rat tissues. Experiments were performed on brain (alpha 3 isozyme), kidney (alpha 1 isozyme) and heart microsomes (alpha 1 and alpha 2 isozymes). Affinity of series of ouabain and digoxin derivatives was studied in competition experiments. It was observed that: (i) ouabain and digoxin had higher affinity (P less than 0.01) for alpha 3 isozyme (Kd of 0.071 +/- 0.004 and 0.066 +/- 0.001 microM, respectively) than for alpha 1 isozyme (Kd of 15.9 +/- 0.8 and 1.78 +/- 0.46 microM, respectively) and alpha 2 isozyme (Kd of 0.26 +/- 0.04 and 0.15 +/- 0.06 microM, respectively); (ii) saturation of the C20-C22 bond on the C17 beta lactone ring present in ouabain and digoxin markedly decreased the drug affinity for all isozymes (P less than 0.01); and (iii) suppression of the C3 beta osidic chain decreased the affinity of ouabain and digoxin to a higher extent for alpha 2 and alpha 3 than for alpha 1 (P less than 0.01). The presence of 10 mM KCl in the incubation medium decreased ouabain affinity for the alpha 1 isozyme to a much higher extent (Kd increase of about 20-fold) than for the other isozymes (Kd increase of about 2-fold). The results show that the isozymes of (Na+ + K+)-ATPase from rat tissue are differently sensitive to changes in the substituents of the cardioactive steroids and to the presence of 10 mM KCl.

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.

Interactions of cardiac glycosides with cells and membranes. IV. Effects of ouabain and bumetanide on 86Rb+ influx in cultured cardiac myocytes from neonatal rats

Ouabain at nanomolar concentrations stimulates total Rb+ influx by 20 +/- 2% in monolayer cultures of myocytes which were either in physiologic ionic steady-state conditions ('control') or 'loaded with Na+' following exposure to K+-free medium. The ouabain-stimulated Rb+ influx was completely abolished by 0.1 mM bumetanide both in 'control' and in 'Na+-loaded' myocytes. Thus, addition of nanomolar concentrations of ouabain to myocytes markedly stimulate the bumetanide-sensitive Rb+ influx. This influx was increased up to 3- and 4-fold in 'control' and 'Na+-loaded' myocytes, respectively. Ouabain at nanomolar concentrations had no significant effect on the component of 86Rb+ influx which is inhibited by millimolar concentrations of ouabain (the so called 'ouabain-sensitive' or 'pump-mediated' Rb+ influx) in 'control' and 'Na+-loaded' cells. It is proposed that the increased rates of bumetanide-sensitive Rb+ influx are accompanied by an increased bumetanide-sensitive Na+ influx through the Na+/K+ cotransporter and thus to a transient increase in intracellular Na+ concentrations [Na+]i. The increase in [Na+]i, subsequently causes a transient elevation in [Ca2+]i via the Na+/Ca2+ exchanger and may be involved in the regulation of cardiac cells' contractility.

High sensitivity of the Na+, K+-pump of human red blood cells to genins of cardiac glycosides

1. Four different cardiac glycosides (ouabain, digitoxin, digoxin and gitoxin) and their corresponding genins were tested on Na+, K+-pump fluxes measured under steady-state and initial rate conditions (non equilibrium conditions) in human and rat erythrocytes and in mouse macrophages. 2. In human red cells, Na+, K+-pump fluxes exhibited up to 8 fold higher sensitivity to genins than to glycosides. In addition genins, but not the corresponding glycosides, exhibited double reactivity with regard to the erythrocyte Na+, K+-pump (with the exception of gitoxigenin). A weak reactivity component was similar to the one of the corresponding glycosides (IC50 of about 10(-6) M) and a high reactivity component exhibited IC50 values varying from 0.1 to 0.5 X 10(-6) M for digitoxigenin and ouabagenin respectively. 3. In contrast with human red cells, the initial rate of Na+, K+-pump fluxes in rat erythrocytes and mouse macrophages was less sensitive to genins than to the corresponding cardiac glycosides. 4. Dihydroouabain was 3, 10 and 75 times less active than ouabain in inhibiting the initial rate of Na+, K+-pump fluxes in human and rat erythrocytes and in mouse macrophages respectively. 5. In conclusion, Na+, K+-pump fluxes measured under initial rate conditions in human erythrocytes exhibit an unusually high sensitivity to genins of cardiac glycosides. This property probably results from the fast binding rate constants of genins and the slow association rates of glycosides to human red cells.

Comparison of ouabain receptors in sheep myocardium and Purkinje fibres

The conducting system of the heart has been reported to be more sensitive to the toxic effects of digitalis than the working myocardium. To investigate the molecular basis of these observations, we have characterized the ouabain receptor in Purkinje fibres and ventricular muscle of the digitalis-sensitive sheep heart using cell membrane preparations, crude homogenates and contracting heart tissues. [3H]-Ouabain binding has the following characteristics: in sheep left ventricular cell membranes, specific binding was of high affinity (KD 1.9 X 10(-9) M at 37 degrees); was co-incident with an inhibition of (Na+ + K+)-ATPase activity; and was inhibited by K+ and unlabelled cardiotonic steroids; in crude homogenates, the maximal binding capacity but not the affinity for ouabain varied in different parts of the sheep heart with Purkinje fibres containing markedly fewer binding sites (0.33 X 10(14)/g wet weight; left ventricle, 1.3 X 10(14)/g wet weight) and in isolated, contracting Purkinje fibres and right ventricular moderator band strips, concentration-response curves for [3H]-ouabain binding, increase in force of contraction and inhibition of [86Rb+]-uptake were co-incident. In both contracting tissues, a ouabain concentration of 3 X 10(-7) M occupied about 50% of the specific binding sites, gave the maximal inotropic effect without toxicity and inhibited [86Rb+]-uptake by about 50%. The maximal binding capacity was lower in contracting Purkinje fibres (2 X 10(14) binding sites/g wet weight) than in contracting moderator band strips (3.9 X 10(14) binding sites/g wet weight). The maximal inotropic effects were reached slightly faster in Purkinje fibres but toxicity also occurred faster in these fibers. We conclude that the specific ouabain binding site is the receptor mediating positive inotropy and inhibition of (Na+ + K+)-ATPase in the sheep heart. Further, this receptor is identical in both the conducting system and working myocardium but the conducting system contains many fewer receptors. This change in receptor number, rather than affinity, may underlie the increased ouabain toxicity observed in Purkinje fibres.