Isoform-specific stimulation of cardiac Na/K pumps by nanomolar concentrations of glycosides

Glutamate transporter coupling to Na,K-ATPase

Deactivation of glutamatergic signaling in the brain is mediated by glutamate uptake into glia and neurons by glutamate transporters. Glutamate transporters are sodium-dependent proteins that putatively rely indirectly on Na,K-ATPases to generate ion gradients that drive transmitter uptake. Based on anatomical colocalization, mutual sodium dependency, and the inhibitory effects of the Na,K-ATPase inhibitor ouabain on glutamate transporter activity, we postulated that glutamate transporters are directly coupled to Na,K-ATPase and that Na,K-ATPase is an essential modulator of glutamate uptake. Na,K-ATPase was purified from rat cerebellum by tandem anion exchange and ouabain affinity chromatography, and the cohort of associated proteins was characterized by mass spectrometry. The alpha1-alpha 3 subunits of Na,K-ATPase were detected, as were the glutamate transporters GLAST and GLT-1, demonstrating that glutamate transporters copurify with Na,K-ATPases. The link between glutamate transporters and Na,K-ATPase was further established by coimmunoprecipitation and colocalization. Analysis of the regulation of glutamate transporter and Na,K-ATPase activities was assessed using [(3)H]D-aspartate, [(3)H]L-glutamate, and rubidium-86 uptake into synaptosomes and cultured astrocytes. In synaptosomes, ouabain produced a dose-dependent inhibition of glutamate transporter and Na,K-ATPase activities, whereas in astrocytes, ouabain showed a bimodal effect whereby glutamate transporter activity was stimulated at 1 microm ouabain and inhibited at higher concentrations. The effects of protein kinase inhibitors on [(3)H]D-aspartate uptake indicated the selective involvement of Src kinases, which are probably a component of the Na,K-ATPase/glutamate transporter complex. These findings demonstrate that glutamate transporters and Na,K-ATPases are part of the same macromolecular complexes and operate as a functional unit to regulate glutamatergic neurotransmission.

Isoform-specific regulation of the Na+ -K+ pump by adenosine in guinea pig ventricular myocytes

AIM

The present study investigated the effect of adenosine on Na(+)-K(+) pumps in acutely isolated guinea pig (Cavia sp.) ventricular myocytes.

METHODS

The whole-cell, patch-clamp technique was used to record the Na(+)-K(+) pump current (I(p)) in acutely isolated guinea pig ventricular myocytes.

RESULTS

Adenosine inhibited the high DHO-affinity pump current (I(h)) in a concentration-dependent manner, which was blocked by the selective adenosine A(1) receptor antagonist DPCPX and the general protein kinase C (PKC) antagonists staurosporine, GF 109203X or the specific delta isoform antagonist rottlerin. In addition, the inhibitory action of adenosine was mimicked by a selective A(1) receptor agonist CCPA and a specific activator peptide of PKC-delta, PP114. In contrast, the selective A(2A) receptor agonist CGS21680 and A(3) receptor agonist Cl-IB-MECA did not affect I(h). Application of the selective A(2A) receptor antagonist SCH58261 and A(3) receptor antagonist MRS1191 also failed to block the effect of adenosine. Furthermore, H89, a selective protein kinase A (PKA) antagonist, did not exert any effect on adenosine-induced I(h) inhibition.

CONCLUSION

The present study provides the electrophysiological evidence that adenosine can induce significant inhibition of I(h) via adenosine A(1) receptors and the PKC-delta isoform.

Effects of strictosamide on mouse brain and kidney Na+, K+-ATPase and Mg2+-ATPase activities

Present study reports on the general bioactivity of strictosamide and on its effects on Na(+),K(+)-ATPase and Mg(2+)-ATPase activities of Charles River male mouse. Strictosamide is the main glycoalkaloid of Sarcocephalus latifolius (Rubiaceae) leaves and roots, used as medicinal plant in folk medicine. In this work, we studied the in vitro effects of various concentrations of strictosamide (0.25, 0.5, 1 or 2 mg/mL) and the in vivo effects of single doses (50, 100 or 200 mg/kg, i.p.) of this compound on kidney and brain Na(+),K(+)-ATPase and Mg(2+)-ATPase activities. Results of general study showed that strictosamide is slightly toxic to Charles River mouse (LD(50)=723.17 mg/kg), producing CNS depression and kidney toxicity, but the exact mechanism of these effects could not be defined. Strictosamide inhibited the in vitro and in vivo Mg(2+)-ATPase activity on kidney but had nonsignificant effect on brain. Furthermore, strictosamide had nonsignificant in vitro and in vivo effect on kidney Na(+),K(+)-ATPase activity but produced an in vivo increase of Na(+),K(+)-ATPase activity of brain, these findings suggesting that strictosamine may be related to the induction of alpha(2) isoform of Na(+),K(+)-ATPase and may account for the folk use of Sarcocephalus latifolius root infusion on hypertension.

Ca2+ influx mechanisms in caveolae vesicles of pulmonary smooth muscle plasma membrane under inhibition of alpha2beta1 isozyme of Na+/K+-ATPase by ouabain

AIMS

We sought to determine the mechanisms of an increase in Ca(2+) level in caveolae vesicles in pulmonary smooth muscle plasma membrane during Na(+)/K(+)-ATPase inhibition by ouabain.

MAIN METHODS

The caveolae vesicles isolated by density gradient centrifugation were characterized by electron microscopic and immunologic studies and determined ouabain induced increase in Na(+) and Ca(2+) levels in the vesicles with fluorescent probes, SBFI-AM and Fura2-AM, respectively.

KEY FINDINGS

We identified the alpha(2)beta(1) and alpha(1)beta(1) isozymes of Na(+)/K(+)-ATPase in caveolae vesicles, and only the alpha(1)beta(1) isozyme in noncaveolae fraction of the plasma membrane. The alpha(2)-isoform contributes solely to the enzyme inhibition in the caveolae vesicles at 40 nM ouabain. Methylisobutylamiloride (Na(+)/H(+)-exchange inhibitor) and tetrodotoxin (voltage-gated Na(+)-channel inhibitor) pretreatment prevented ouabain induced increase in Na(+) and Ca(2+) levels. Ouabain induced increase in Ca(2+) level was markedly, but not completely, inhibited by KB-R7943 (reverse-mode Na(+)/Ca(2+)-exchange inhibitor) and verapamil (L-type Ca(2+)-channel inhibitor). However, pretreatment with tetrodotoxin in conjunction with KB-R7943 and verapamil blunted ouabain induced increase in Ca(2+) level in the caveolae vesicles, indicating that apart from Na(+)/Ca(+)-exchanger and L-type Ca(2+)-channels, "slip-mode conductance" of Na(+) channels could also be involved in this scenario.

SIGNIFICANCE

Inhibition of alpha(2) isoform of Na(+)/K(+)-ATPase by ouabain plays a crucial role in modulating the Ca(2+) influx regulatory components in the caveolae microdomain for marked increase in (Ca(2+))(i) in the smooth muscle, which could be important for the manifestation of pulmonary hypertension.

Hormesis and medicine

Evidence is presented which supports the conclusion that the hormetic dose-response model is the most common and fundamental in the biological and biomedical sciences, being highly generalizable across biological model, endpoint measured and chemical class and physical agent. The paper provides a broad spectrum of applications of the hormesis concept for clinical medicine including anxiety, seizure, memory, stroke, cancer chemotherapy, dermatological processes such as hair growth, osteoporosis, ocular diseases, including retinal detachment, statin effects on cardiovascular function and tumour development, benign prostate enlargement, male sexual behaviours/dysfunctions, and prion diseases.

Dual activity of the H1-H2 domain of the (Na(+)+K+)-ATPase

(Na(+)+K(+))-ATPase is a target receptor of digitalis (cardiac glycoside) drugs. It has been demonstrated that the H1-H2 domain of the alpha-subunit of the (Na(+)+K(+))-ATPase is one of the digitalis drug interaction sites of the enzyme. Despite the extensive studies of the inhibitory effect of digitalis on the (Na(+)+K(+))-ATPase, the functional property of the H1-H2 domain of the enzyme and its role in regulating enzyme activity is not completely understood. Here we report a surprise finding: instead of inhibiting the enzyme, binding of a specific monoclonal antibody SSA78 to the H1-H2 domain of the (Na(+)+K(+))-ATPase elevates the catalytic activity of the enzyme. In the presence of low concentration of ouabain, monoclonal antibody SSA78 significantly protects enzyme function against ouabain-induced inhibition. However, higher concentration of ouabain completely inactivates the (Na(+)+K(+))-ATPase even in the presence of SSA78. These results suggest that the H1-H2 domain of the (Na(+)+K(+))-ATPase is capable of regulating enzyme function in two distinct ways for both ouabain-sensitive and -resistant forms of the enzyme: it increases the activity of the (Na(+)+K(+))-ATPase during its interaction with an activator; it also participates in the mechanism of digitalis or ouabain-induced inhibition of the enzyme. Understanding the dual activity of the H1-H2 domain will help better understand the structure-function relationships of the (Na(+)+K(+))-ATPase and the biological processes mediated by the enzyme.

Relationships among endogenous ouabain, alpha-adducin polymorphisms and renal sodium handling in primary hypertension

OBJECTIVE

The basolateral Na pump drives renotubular reabsorption. In cultured renal cells, mutant adducins, as well as sub-nanomolar ouabain concentrations, stimulate the Na-K pump.

METHODS

To determine whether these factors interact and affect Na handling and blood pressure (BP) in vivo, we studied 155 untreated hypertensive patients subdivided on the basis of their plasma endogenous ouabain or alpha-adducin genotype (ADD1 Gly460Trp-rs4961).

RESULTS

Under basal conditions, proximal tubular reabsorption and plasma Na were higher in patients with mutated Trp ADD1 or increased endogenous ouabain (P = 0.002 and 0.05, respectively). BPs were higher in the high plasma endogenous ouabain group (P = 0.001). Following volume loading, the increment in BP (7.73 vs. 4.81 mmHg) and the slopes of the relationship between BP and Na excretion were greater [0.017 +/- 0.002 vs. 0.009 +/- 0.003 mmHg/(muEq min)] in ADD1 Trp vs. ADD1 Gly carriers (P < 0.05). BP changes were similar, whereas the slopes of the relationship between BP and Na excretion were lower [0.016 +/- 0.003 vs. 0.008 +/- 0.002 mmHg/(muEq min)] in patients with low vs. high endogenous ouabain (P < 0.05). In patients with high endogenous ouabain, volume loading increased the BP in the ADD1 Trp group but not in the Gly group (P < 0.05). Thus, patients with ADD1 Trp alleles are sensitive to salt and tubular Na reabsorption remains elevated after volume expansion.

CONCLUSION

With saline loading, BP changes are similar in high and low endogenous ouabain patients, whereas tubular Na reabsorption increases in the high endogenous ouabain group. Saline loading unmasks differences in renal Na handling in patients with mutant adducin or high endogenous ouabain and exposes an interaction of endogenous ouabain and Trp alleles on BP.

Diverse biological responses to different cardiotonic steroids

Cardiotonic steroids (CS) such as ouabain, digoxin and bufalin, are steroidal drugs prepared from the seeds and dried leaves of the genus Digitalis, and the skin and parotid gland of amphibians, are used as a cardiac stimulant. Steroids similar or identical to the cardiotonic steroids were identified in human tissues. The available literature unequivocally supports the notion that these endogenous CS function as hormones in mammals. Recent studies show that although similar in structure, the different CS exhibit diverse biological responses. This was shown at the molecular, cellular, tissue and whole animal levels. This review summarizes these diversities, raises a possible explanation for their presence and discusses their implication on the physiological role of the different steroids.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Ouabain stimulates protein kinase B (Akt) phosphorylation in opossum kidney proximal tubule cells through an ERK-dependent pathway

Endogenous cardiotonic glycosides bind to the inhibitory binding site of the plasma membrane sodium pump (Na(+)/K(+)-ATPase). Plasma levels of endogenous cardiotonic glycosides increase in several disease states, such as essential hypertension and uremia. Low concentrations of ouabain, which do not inhibit Na(+)/K(+)-ATPase, induce cell proliferation. The mechanisms of ouabain-mediated response remain unclear. Recently, we demonstrated that in opossum kidney (OK) proximal tubular cells, low concentrations of ouabain induce cell proliferation through phosphorylation of protein kinase B (Akt) in a calcium-dependent manner. In the present study, we identified ERK as an upstream kinase regulating Akt activation in ouabain-stimulated cells. Furthermore, we provide evidence that low concentrations of ouabain stimulate Na(+)/K(+)-ATPase-mediated (86)Rb uptake in an Akt-, ERK-, and Src kinase-dependent manner. Ouabain-mediated ERK phosphorylation was inhibited by blockade of intracellular calcium release, calcium entry, tyrosine kinases, and phospholipase C. Pharmacological inhibition of phosphoinositide-3 kinase and Akt failed to inhibit ouabain-stimulated ERK phosphorylation. Ouabain-mediated Akt phosphorylation was inhibited by U0126, a MEK/ERK inhibitor, suggesting that ouabain-mediated Akt phosphorylation is dependent on ERK. In an in vitro kinase assay, active recombinant ERK phosphorylated recombinant Akt on Ser(473). Moreover, transient transfection with constitutively active MEK1, an upstream regulator of ERK, increased Akt phosphorylation and activation, whereas overexpression of constitutively active Akt failed to stimulate ERK phosphorylation. Ouabain at low concentrations also promoted cell proliferation in an ERK-dependent manner. These findings suggest that ouabain-stimulated ERK phosphorylation is required for Akt phosphorylation on Ser(473), cell proliferation, and stimulation of Na(+)/K(+)-ATPase-mediated (86)Rb uptake in OK cells.

Cardiotonic steroids on the road to anti-cancer therapy

The sodium pump, Na(+)/K(+)-ATPase, could be an important target for the development of anti-cancer drugs as it serves as a versatile signal transducer, it is a key player in cell adhesion and its aberrant expression and activity are implicated in the development and progression of different cancers. Cardiotonic steroids, known ligands of the sodium pump have been widely used for the treatment of heart failure. However, early epidemiological evaluations and subsequent demonstration of anti-cancer activity in vitro and in vivo have indicated the possibility of developing this class of compound as chemotherapeutic agents in oncology. Their development to date as anti-cancer agents has however been impaired by a narrow therapeutic margin resulting from their potential to induce cardiovascular side-effects. The review will thus discuss (i) sodium pump structure, function, expression in diverse cancers and its chemical targeting and that of its sub-units, (ii) reported in vitro and in vivo anti-cancer activity of cardiotonic steroids, (iii) managing the toxicity of these compounds and the limitations of existing preclinical models to adequately predict the cardiotoxic potential of new molecules in man and (iv) the potential of chemical modification to reduce the cardiovascular side-effects and improve the anti-cancer activity of new molecules.

Endogenous and exogenous cardiac glycosides: their roles in hypertension, salt metabolism, and cell growth

Cardiotonic steroids (CTS), long used to treat heart failure, are endogenously produced in mammals. Among them are the hydrophilic cardenolide ouabain and the more hydrophobic cardenolide digoxin, as well as the bufadienolides marinobufagenin and telecinobufagin. The physiological effects of endogenous ouabain on blood pressure and cardiac activity are consistent with the "Na(+)-lag" hypothesis. This hypothesis assumes that, in cardiac and arterial myocytes, a CTS-induced local increase of Na(+) concentration due to inhibition of Na(+)/K(+)-ATPase leads to an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) via a backward-running Na(+)/Ca(2+) exchanger. The increase in [Ca(2+)](i) then activates muscle contraction. The Na(+)-lag hypothesis may best explain short-term and inotropic actions of CTS. Yet all data on the CTS-induced alteration of gene expression are consistent with another hypothesis, based on the Na(+)/K(+)-ATPase "signalosome," that describes the interaction of cardiac glycosides with the Na(+) pump as machinery activating various signaling pathways via intramembrane and cytosolic protein-protein interactions. These pathways, which may be activated simultaneously or selectively, elevate [Ca(2+)](i), activate Src and the ERK1/2 kinase pathways, and activate phosphoinositide 3-kinase and protein kinase B (Akt), NF-kappaB, and reactive oxygen species. A recent development indicates that new pharmaceuticals with antihypertensive and anticancer activities may be found among CTS and their derivatives: the antihypertensive rostafuroxin suppresses Na(+) resorption and the Src-epidermal growth factor receptor-ERK pathway in kidney tubule cells. It may be the parent compound of a new principle of antihypertensive therapy. Bufalin and oleandrin or the cardenolide analog UNBS-1450 block tumor cell proliferation and induce apoptosis at low concentrations in tumors with constitutive activation of NF-kappaB.

Novel environmental toxins: steryl glycosides as a potential etiological factor for age-related neurodegenerative diseases

Amyotrophic lateral sclerosis-parkinsonism dementia complex (ALS-PDC) is a unique neurodegenerative disease found on the island of Guam. This disease presents as a spectrum of neurological disorders characterized by features of ALS, parkinsonism, dementia, or a combination. The strongest epidemiological link has been to the consumption of the seeds from the cycad plant that purportedly contained a neurotoxin. Mice fed washed cycad flour show signs that mimic ALS-PDC, which include progressive deficits in motor, cognitive, and olfactory functions associated with neuron loss in the spinal cord, nigrostriatal system, cortex, hippocampus, and olfactory bulb. Through a series of chemical extractions of washed cycad flour, we identified steryl glycoside molecules as bioactive molecules that are neurotoxic in culture and in mice. A detailed review of this class of molecule revealed that the molecules are abundant in the environment, particularly in plants and bacteria. Lipid analysis showed that some bacteria that are associated with some forms of neurodegenerative disorders have the capacity to synthesize steryl glycosides. Furthermore, certain steryl glycosides have been found to be a cell stress mediator and may have some immunomodulary effects. We hypothesize that steryl glycosides are putative neurotoxins involved in the etiopathogenesis of several age-related neurodegenerative disorders.

Erythrocyte sodium pump stimulation by ouabain and an endogenous ouabain-like factor

Cardiac glycosides inhibit the sodium pump. However, some studies suggest that nanomolar ouabain concentrations can stimulate the activity of the sodium pump. In this study, using the Na(+)/K(+)-ATPase of human erythrocytes, we compared the effect of digoxin, ouabain and an ouabain like-factor (OLF), on (86)Rb uptake. Ouabain concentrations below 10(-9) M significantly stimulate Rb(+) uptake, and the maximal increase above base-line values is 18 +/- 5% at 10(-10) M ouabain. No stimulation is observed in the same conditions by digoxin. OLF behaved like ouabain, producing an activation of Rb(+) flux at concentrations lower than 10(-9) M ouabain equivalents (14 +/- 3% at 10(-10) M). Western blot analysis revealed the presence of both alpha(1) and alpha(3) pump isoforms in human erythrocytes. Our data confirm the analogies between OLF and ouabain and suggest that Na(+)/K(+)-ATPase activation may be related to the alpha(3) isoform. In addition, we investigated whether ouabain at different concentrations was effective in altering the intracellular calcium concentration of erythrocytes. We found that ouabain at concentration lower than 10(-9) M did not affect this homeostasis.

Endogenous and exogenous cardiac glycosides and their mechanisms of action

Cardiac glycosides have been used for decades to treat congestive heart failure. The recent identification of cardiotonic steroids such as ouabain, digoxin, marinobufagenin, and telocinobufagin in blood plasma, adrenal glands, and hypothalamus of mammals led to exciting new perspectives in the pathology of heart failure and arterial hypertension. Biosynthesis of ouabain and digoxin occurs in adrenal glands and is under the control of angiotensin II, endothelin, and epinephrine released from cells of the midbrain upon stimulation of brain areas sensing cerebrospinal Na(+) concentration and, apparently, the body's K(+) content. Rapid changes of endogenous ouabain upon physical exercise may favor the economy of the heart by a rise of intracellular Ca(2)(+) levels in cardiac and atrial muscle cells. According to the sodium pump lag hypothesis, this may be accomplished by partial inhibition of the sodium pump and Ca(2+) influx via the Na(+)/Ca(2+) exchanger working in reverse mode or via activation of the Na(+)/K(+)-ATPase signalosome complex, generating intracellular calcium oscillations, reactive oxygen species, and gene activation via nuclear factor-kappaB or extracellular signal-regulated kinases 1 and 2. Elevated concentrations of endogenous ouabain and marinobufagenin in the subnanomolar concentration range were found to stimulate proliferation and differentiation of cardiac and smooth muscle cells. They may have a primary role in the development of cardiac dysfunction and failure because (i) offspring of hypertensive patients evidently inherit elevated plasma concentrations of endogenous ouabain; (ii) such elevated concentrations correlate positively with cardiac dysfunction, hypertrophy, and arterial hypertension; (iii) about 40% of Europeans with uncomplicated essential hypertension show increased concentrations of endogenous ouabain associated with reduced heart rate and cardiac hypertrophy; (iv) in patients with advanced arterial hypertension, circulating levels of endogenous ouabain correlate with BP and total peripheral resistance; (v) among patients with idiopathic dilated cardiomyopathy, high circulating levels of endogenous ouabain and marinobufagenin identify those individuals who are predisposed to progressing more rapidly to heart failure, suggesting that endogenous ouabain (and marinobufagenin) may contribute to toxicity upon digoxin therapy. In contrast to endogenous ouabain, endogenous marinobufagenin may act as a natriuretic substance as well. It shows a higher affinity for the ouabain-insensitive alpha(1) isoform of Na(+)/K(+)-ATPase of rat kidney tubular cells and its levels are increased in volume expansion and pre-eclampsia. Digoxin, which is synthesized in adrenal glands, seems to counteract the hypertensinogenic action of ouabain in rats, as do antibodies against ouabain, for example, (Digibind) and rostafuroxin (PST 2238), a selective ouabain antagonist. It lowers BP in ouabain- and adducin-dependent hypertension in rats and is a promising new class of antihypertensive medication in humans.

Mimicking human bipolar ion dysregulation models mania in rats

Psychiatric diseases in general, and bipolar illness in particular, are difficult to model in animals since the subjective nature of the core symptoms appears to preclude objective observation of behavioral changes. An adequate animal model of a psychiatric condition must fulfill three core criteria: share pathophysiological characteristics of the human condition (face validity), have similar behavioral manifestations as the human disease (construct validity), and improve with medications that improve the symptoms seen in afflicted humans (predictive validity). The ouabain model for bipolar illness mimics a widely reproduced biologic abnormality in mania: reduced sodium pump activity. An intracerebroventricular (ICV) administration of 5microL 10(-3)M ouabain induces motoric hyperactivity preventable by lithium, carbamazepine, and haloperidol. ICV ouabain may also produce environmentally dependent hypoactivity. The model, however, has not yet been examined for other potential manic behavior in rats such as reduced need for sleep, increased sexual activity, or increased irritability. While additional characterization of the model is required, the ouabain model for bipolar illness is the only available animal model that fulfills the three criteria for an adequate animal model for bipolar illness.

Ouabain induces cell proliferation through calcium-dependent phosphorylation of Akt (protein kinase B) in opossum kidney proximal tubule cells

Cardiotonic glycosides, like ouabain, inhibit Na+-K+-ATPase. Recent evidence suggests that low molar concentrations of ouabain alter cell growth. Studies were conducted to examine the effect of ouabain on Akt phosphorylation and rate of cell proliferation in opossum kidney (OK) proximal tubule cells. Cells exposed to 10 nM ouabain displayed increased Akt Ser473phosphorylation, as evidenced by an increase in phospho-Akt Ser473band density. Ouabain-stimulated Akt Ser473phosphorylation was inhibited by pretreatment with phosphatidylinositol 3-kinase (PI3K) inhibitors (LY294002 and wortmannin), a PLC inhibitor (edelfosine), and an Akt inhibitor. Moreover, ouabain-mediated Akt Ser473phosphorylation was suppressed by reduction of extracellular calcium (EGTA) or when intracellular calcium was buffered by BAPTA-AM. An inhibitor of calcium store release (TMB-8) and an inhibitor of calcium entry via store-operated calcium channels ( SKF96365 ) also suppressed ouabain-mediated Akt Ser473phosphorylation. In fura-2 AM-loaded cells, 10 nM ouabain increased capacitative calcium entry (CCE). Ouabain at 10 nM did not significantly alter baseline cytoplasmic calcium concentration in control cells. However, treatment with 10 nM ouabain caused a significantly higher ATP-mediated calcium store release. After 24 h, 10 nM ouabain increased the rate of cell proliferation. The Akt inhibitor, BAPTA-AM, SKF96365 , and cyclopiazonic acid suppressed the increase in the rate of cell proliferation caused by 10 nM ouabain. Ouabain at 10 nM caused a detectable increase in86Rb uptake but did not significantly alter Na+-K+-ATPase (ouabain-sensitive pNPPase) activity in crude membranes or cell sodium content. Taken together, the results point to a role for CCE and Akt phosphorylation, in response to low concentrations of ouabain, that increase the rate of cell proliferation without inhibiting Na+-K+-ATPase-mediated ion transport.

Ouabain protects rat hearts against ischemia-reperfusion injury via pathway involving src kinase, mitoKATP, and ROS

We showed recently that mitochondrial ATP-dependent K(+) channel (mitoK(ATP)) opening is required for the inotropic response to ouabain. Because mitoK(ATP) opening is also required for most forms of cardioprotection, we investigated whether exposure to ouabain was cardioprotective. We also began to map the signaling pathways linking ouabain binding to Na(+)-K(+)-ATPase with the opening of mitoK(ATP). In Langendorff-perfused rat hearts, 10-80 microM ouabain given before the onset of ischemia resulted in cardioprotection against ischemia-reperfusion injury, as documented by an improved recovery of contractile function and a reduction of infarct size. In skinned cardiac fibers, a ouabain-induced protection of mitochondrial outer membrane integrity, adenine nucleotide compartmentation, and energy transfer efficiency was evidenced by a decreased release of cytochrome c and preserved half-saturation constant of respiration for ADP and adenine nucleotide translocase-mitochondrial creatine kinase coupling, respectively. Ouabain-induced positive inotropy was dose dependent over the range studied, whereas ouabain-induced cardioprotection was maximal at the lowest dose tested. Compared with bradykinin (BK)-induced preconditioning, ouabain was equally efficient. However, the two ligands clearly diverge in the intracellular steps leading to mitoK(ATP) opening from their respective receptors. Thus BK-induced cardioprotection was blocked by inhibitors of cGMP-dependent protein kinase (PKG) or guanylyl cyclase (GC), whereas ouabain-induced protection was not blocked by either agent. Interestingly, however, ouabain-induced inotropy appears to require PKG and GC. Thus 5-hydroxydecanoate (a selective mitoK(ATP) inhibitor), N-(2-mercaptopropionyl)glycine (MPG; a reactive oxygen species scavenger), ODQ (a GC inhibitor), PP2 (a src kinase inhibitor), and KT-5823 (a PKG inhibitor) abolished preconditioning by BK and blocked the inotropic response to ouabain. However, only PP2, 5-HD, and MPG blocked ouabain-induced cardioprotection.

Ionic mechanisms of cardiac cell swelling induced by blocking Na+/K+ pump as revealed by experiments and simulation

Although the Na(+)/K(+) pump is one of the key mechanisms responsible for maintaining cell volume, we have observed experimentally that cell volume remained almost constant during 90 min exposure of guinea pig ventricular myocytes to ouabain. Simulation of this finding using a comprehensive cardiac cell model (Kyoto model incorporating Cl(-) and water fluxes) predicted roles for the plasma membrane Ca(2+)-ATPase (PMCA) and Na(+)/Ca(2+) exchanger, in addition to low membrane permeabilities for Na(+) and Cl(-), in maintaining cell volume. PMCA might help maintain the [Ca(2+)] gradient across the membrane though compromised, and thereby promote reverse Na(+)/Ca(2+) exchange stimulated by the increased [Na(+)](i) as well as the membrane depolarization. Na(+) extrusion via Na(+)/Ca(2+) exchange delayed cell swelling during Na(+)/K(+) pump block. Supporting these model predictions, we observed ventricular cell swelling after blocking Na(+)/Ca(2+) exchange with KB-R7943 or SEA0400 in the presence of ouabain. When Cl(-) conductance via the cystic fibrosis transmembrane conductance regulator (CFTR) was activated with isoproterenol during the ouabain treatment, cells showed an initial shrinkage to 94.2 +/- 0.5%, followed by a marked swelling 52.0 +/- 4.9 min after drug application. Concomitantly with the onset of swelling, a rapid jump of membrane potential was observed. These experimental observations could be reproduced well by the model simulations. Namely, the Cl(-) efflux via CFTR accompanied by a concomitant cation efflux caused the initial volume decrease. Then, the gradual membrane depolarization induced by the Na(+)/K(+) pump block activated the window current of the L-type Ca(2+) current, which increased [Ca(2+)](i). Finally, the activation of Ca(2+)-dependent cation conductance induced the jump of membrane potential, and the rapid accumulation of intracellular Na(+) accompanied by the Cl(-) influx via CFTR, resulting in the cell swelling. The pivotal role of L-type Ca(2+) channels predicted in the simulation was demonstrated in experiments, where blocking Ca(2+) channels resulted in a much delayed cell swelling.

Involvement of Na(+), K(+)-ATPase and endogenous digitalis-like compounds in depressive disorders

BACKGROUND

Sodium and potassium-activated adenosine triphosphatase (Na(+), K(+)-ATPase) and endogenous digitalis-like compounds (DLC) in the brain have been implicated in the pathogenesis of mood disorders. This hypothesis was examined by the determination of Na(+), K(+)-ATPase/DLC system in parietal cortex of patients with different mood disorders and two animal models of depression.

METHODS

Na(+), K(+)-ATPase concentrations in human brain synaptosomal fractions, from patients with mood disorders, schizophrenia, and normal individuals, were determined by (3)H-ouabain binding assay. Alpha isoforms were quantified by Western blotting. Brain DLC were measured using sensitive enzyme linked immunosorbant assay (ELISA). The effects of ouabain and ouabain-antibodies on behavior were determined in two animal models of depression.

RESULTS

(3)H-ouabain binding in bipolar patients was significantly lower than in major depressed and schizophrenic patients. Na(+), K(+)-ATPase alpha isoforms in synaptosomal fractions were not different among the groups. DLC levels in the parietal cortex of bipolar patients were significantly higher than in normal individuals and depressed patients. Injection of lipopolysaccharide (intraperitoneally) to rats elicited depression-like symptoms, which were significantly attenuated by pre-injection of ouabain-antibodies. Injection of ouabain and ouabain-antibodies (intracerebroventricular) reduced depression-like symptoms in the forced swimming test in rats.

CONCLUSIONS

The results support the possibility that Na(+), K(+)-ATPase and endogenous DLC participate in the pathogenesis of depressive disorders.

Repeated electroconvulsive shock induces changes in high-affinity [3H]-ouabain binding to rat striatal membranes

Repeated electroconvulsive shock is an effective treatment for affective disorders. Striatum, hippocampus and brainstem are involved in affective disorders. Sodium-potassium/ATPase is of paramount importance for the proper functioning of the brain and its involvement in the affective disorders has been claimed for a long time. Sodium-potassium/ATPase has an extracellular regulatory binding site to which cardiotonic glycosides, such as ouabain, bind to, thus regulating the activity of the enzyme. Endogenous "ouabain-like" substances exist in the brain and their actions on the sodium-potassium/ATPase resemble ouabain biological properties. The aim of this work was to determine if electroconvulsive shock (ECS) would induce changes in the high-affinity binding of ouabain to the sodium-potassium/ATPase from rat brain regions. Adult, male Wistar rats received one (ECSx1 group) or seven electroshocks (ECSx7 group) delivered daily through ear-clips electrodes. Control rats received the same manipulations; however, no current was delivered through the electrodes (SHAMx1 and SHAMx7 groups). All groups were sacrificed 24 h after the last ECS session. The B (max) and K (D) of high-affinity [(3)H]-ouabain binding were determined in crude membrane preparations from the striatum, hippocampus and brainstem. The results obtained showed a statistically significant increase in the affinity of [(3)H]-ouabain (lower K (D)) to striatal membranes in those rats receiving seven ECS. In the striatum there was no change in the K (D) after one ECS; as well as there was no change in the B (max) after a single or seven ECS. High-affinity [(3)H]-ouabain binding to hippocampus and brainstem did not reveal any significant differences either in K (D) or B (max) after one or seven ECS. The increased affinity of ouabain to the striatal sodium-potassium/ATPase induced by repeated ECS suggests an increased interaction in vivo of the endogenous "ouabain-like" substances with the enzyme and the involvement of the extracellular regulatory allosteric ouabain binding site in the striatal sodium-potassium/ATPase in the effects of electroconvulsive shock.

Ouabain-induced hypertension enhances left ventricular contractility in rats

Chronic ouabain treatment produces hypertension acting on the central nervous system and at vascular levels. However, cardiac effects in this model of hypertension are still poorly understood. Hence, the effects of hypertension induced by chronic ouabain administration ( approximately 8 microg day(-1), s.c.) for 5 weeks on the cardiac function were studied in Wistar rats. Ouabain induces hypertension but not myocardial hypertrophy. Awake ouabain-treated rats present an increment of the left ventricular systolic pressure and of the maximum positive and negative dP/dt. Isolated papillary muscles from ouabain-treated rats present an increment in isometric force, and this effect was present even when inotropic interventions (external Ca(2+) increment and increased heart rate) were performed. However, the sarcoplasmic reticulum activity and the SERCA-2 protein expression did not change. On the other hand, the activity of myosin ATPase increased without changes in myosin heavy chain protein expression. In addition, the expression of alpha(1) and alpha(2) isoforms of Na(+), K(+)-ATPase also increased in the left ventricle from ouabain-hypertensive rats. The present results showed positive inotropic and lusitropic effects in hearts from awake ouabain-treated rats, which are associated with an increment of the isometric force development and of the activity of myosin ATPase and expression of catalytic subunits of the Na(+), K(+)-ATPase.

How does salt retention raise blood pressure?

A critical question in hypertension research is: How is long-term blood pressure controlled? Excessive NaCl ingestion or NaCl retention by the kidneys and the consequent tendency toward plasma volume expansion lead to hypertension. Nevertheless, the precise mechanisms linking salt to high blood pressure are unresolved. The discovery of endogenous ouabain, an adrenocortical hormone, provided an important clue. Ouabain, a selective Na+ pump inhibitor, has cardiotonic and vasotonic effects. Plasma endogenous ouabain levels are significantly elevated in approximately 40% of patients with essential hypertension and in animals with several forms of salt-dependent hypertension. Also, prolonged ouabain administration induces hypertension in rodents. Mice with mutant Na+ pumps or Na/Ca exchangers (NCX) and studies with a ouabain antagonist and an NCX blocker are revealing the missing molecular mechanisms. These data demonstrate that alpha2 Na+ pumps and NCX1 participate in long-term regulation of vascular tone and blood pressure. Pharmacological agents or mutations in the alpha2 Na+ pump that interfere with the action of ouabain on the pump, and reduced NCX1 expression or agents that block NCX all impede the development of salt-dependent or ouabain-induced hypertension. Conversely, nanomolar ouabain, reduced alpha2 Na+ pump expression, and smooth muscle-specific overexpression of NCX1 all induce hypertension. Furthermore, ouabain and reduced alpha2 Na+ pump expression increase myogenic tone in isolated mesenteric small arteries in vitro, thereby tying these effects directly to the elevation of blood pressure. Thus, endogenous ouabain, and vascular alpha2 Na+ pumps and NCX1, are critical links between salt and hypertension. New pharmacological agents that act on these molecular links have potential in the clinical management of hypertension.

Transmural gradients in Na/K pump activity and [Na+]I in canine ventricle

There are well-documented differences in ion channel activity and action potential shape between epicardial (EPI), midmyocardial (MID), and endocardial (ENDO) ventricular myocytes. The purpose of this study was to determine if differences exist in Na/K pump activity. The whole cell patch-clamp was used to measure Na/K pump current (I(P)) and inward background Na(+)-current (I(inb)) in cells isolated from canine left ventricle. All currents were normalized to membrane capacitance. I(P) was measured as the current blocked by a saturating concentration of dihydro-ouabain. [Na(+)](i) was measured using SBFI-AM. I(P)(ENDO) (0.34 +/- 0.04 pA/pF, n = 17) was smaller than I(P)(EPI) (0.68 +/- 0.09 pA/pF, n = 38); the ratio was 0.50 with I(P)(MID) being intermediate (0.53 +/- 0.13 pA/pF, n = 19). The dependence of I(P) on [Na(+)](i) or voltage was essentially identical in EPI and ENDO (half-maximal activation at 9-10 mM [Na(+)](i) or approximately -90 mV). Increasing [K(+)](o) from 5.4 to 15 mM caused both I(P)(ENDO) and I(P)(EPI) to increase, but the ratio remained approximately 0.5. I(inb) in EPI and ENDO were nearly identical ( approximately 0.6 pA/pF). Physiological [Na(+)](i) was lower in EPI (7 +/- 2 mM, n = 31) than ENDO (12 +/- 3 mM, n = 29), with MID being intermediate (9 +/- 3 mM, n = 22). When cells were paced at 2 Hz, [Na(+)](i) increased but the differences persisted (ENDO 14 +/- 3 mM, n = 10; EPI 9 +/- 2 mM, n = 10; and MID intermediate, 11 +/- 2 mM, n = 9). Based on these results, the larger I(P) in EPI appears to reflect a higher maximum turnover rate, which implies either a larger number of active pumps or a higher turnover rate per pump protein. The transmural gradient in [Na(+)](i) means physiological I(P) is approximately uniform across the ventricular wall, whereas transporters that utilize the transmembrane electrochemical gradient for Na(+), such as Na/Ca exchange, have a larger driving force in EPI than ENDO.

Rapid eye movement sleep deprivation induces changes in the high-affinity binding of [3H]-ouabain to the rat cortical membranes

Rapid eye movement sleep (REMS) suppresses seizures. On the other hand, REMS deprivation (REMSD) increases brain susceptibility to seizures. Sodium-potassium/ATPase is involved in the control of brain excitability. Ouabain, a cardiotonic glycoside, binds to a regulatory extracellular allosteric site in the sodium-potassium/ATPase inhibiting/stimulating its activity depending on its concentration. Endogenous ouabain-like substances exist in the brain; therefore, changes in the ouabain binding site may be involved in the increased brain excitability induced by REMSD. Adult, Wistar male rats were deprived of REMS for 96 hours by the flower-pot method (REMSD). A stress control group was kept in the same environment on a larger platform (LP). A third group of rats was kept in the same room in their home-cages (CONTROL). After REMSD all rats were sacrificed by decapitation and their cerebral cortex dissected. High-affinity [3H]-ouabain binding was carried out in cortical crude membrane preparation using 8 concentrations of [3H]-ouabain (1-24 nM). The results show a statistically significant increase of KD in the REMSD rats compared to both CONTROL and LP groups. There were no statistically significant differences in the Bmax among the experimental groups. There was also no change either in cortical activity of K+ stimulated p-nitrophenylphosphatase, the dephosphorylation reaction of phosphorylated sodium-potassium/ATPase or in Mg2+-stimulated p-nitrophenylphosphatase. An increase in the KD of [3H]-ouabain binding to the sodium-potassium/ATPase in REMSD rats indicates a lower affinity to the endogenous inhibitors/stimulators of the enzyme. Therefore, this decreased affinity of the endogenous ouabain-like substances may be involved in the increased excitability induced by REMSD.

Digitalis: new actions for an old drug

The mechanisms by which digitalis causes its therapeutic and toxic actions have been studied for nearly a half century, revealing a great deal about cardiac cell regulation of intracellular ions via the Na-K-ATPase (NKA) and how it is altered by cardiac glycosides. However, recent observations suggest that digitalis may have additional effects on cardiac cell function in both the short and long term that include intracellular effects, interactions with specific NKA isoforms in different cellular locations, effects on intracellular (including nuclear) signaling, and long-term regulation of intracellular ionic balances through circulating ouabain-like compounds. The purpose of this review is to examine the current status of a number of the newest and most interesting developments in the study of digitalis with a particular focus on cardiac function, although we will also discuss some of the new advances in other relevant cardiovascular effects. This new information has important implications for both our understanding of ionic regulation in normal and diseased hearts as well as for potential avenues for the development of future therapeutic interventions for the treatment of heart failure.

The highly conserved cardiac glycoside binding site of Na,K-ATPase plays a role in blood pressure regulation

The Na,K-ATPase contains a binding site for cardiac glycosides, such as ouabain, digoxin, and digitoxin, which is highly conserved among species ranging from Drosophila to humans. Although advantage has been taken of this site to treat congestive heart failure with drugs such as digoxin, it is unknown whether this site has a natural function in vivo. Here we show that this site plays an important role in the regulation of blood pressure, and it specifically mediates adrenocorticotropic hormone (ACTH)-induced hypertension in mice. We used genetically engineered mice in which the Na,K-ATPase alpha2 isoform, which is normally sensitive to cardiac glycosides, was made resistant to these compounds. Chronic administration of ACTH caused hypertension in WT mice but not in mice with an ouabain-resistant alpha2 isoform of Na,K-ATPase. This finding demonstrates that the cardiac glycoside binding site of the Na,K-ATPase plays an important role in blood pressure regulation, most likely by responding to a naturally occurring ligand. Because the alpha1 isoform is sensitive to cardiac glycosides in humans, we developed mice in which the naturally occurring ouabain-resistant alpha1 isoform was made ouabain-sensitive. Mice with the ouabain-sensitive "human-like" alpha1 isoform and an ouabain-resistant alpha2 isoform developed ACTH-induced hypertension to greater extent than WT animals. This result indicates that the cardiac glycoside binding site of the alpha1 isoform can also mediate ACTH-induced hypertension. Taken together these results demonstrate that the cardiac glycoside binding site of the alpha isoforms of the Na,K-ATPase have a physiological function and supports the hypothesis for a role of the endogenous cardiac glycosides.

Physiological concentrations of ouabain rapidly inhibit prolactin release from the tilapia pituitary

Ouabain, a cardiac glycoside and inhibitor of Na(+), K(+)-ATPase, is now believed to be a steroid hormone in mammals. We have recently identified ouabain immunoreactivity in the plasma of the tilapia, a euryhaline teleost. Changes in plasma concentrations of immunoreactive ouabain (20-40 pM) in response to salinity change were well correlated with the changes in plasma osmolality and cortisol. Our previous studies have shown that cortisol rapidly inhibits prolactin (PRL) release from the tilapia pituitary by suppressing intracellular Ca(2+) ([Ca(2+)]i) and cAMP. In the present study, low doses of ouabain (10-1000 pM) inhibited PRL release dose-dependently during 2-24 h of incubation. There was no effect on growth hormone (GH) release, except for a significant increase at 1000 pM during 8-24 h of incubation. Significant dose-related increases in PRL release were observed at higher doses of ouabain (100-1000 nM), whereas significant inhibition was seen in GH release at 1000 nM during 2-24h of incubation. Ouabain at 1-100 pM had no effect on Na(+), K(+)-ATPase activity of the pituitary homogenate. The enzyme activity was inhibited by higher concentrations of ouabain, 10% at 1 nM, 15% at 10 nM, 28% at 100 nM, and 45% at 1000 nM. Ouabain also attenuated stimulation of PRL release by the Ca(2+) ionophore, A23187, and by a combination of dibutyryl cAMP and a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthin. Intracellular Ca(2+) concentrations were monitored in the dispersed PRL cells with the Ca(2+)-sensitive dye, fura-2. Ouabain at 1 nM reversibly reduced [Ca(2+)]i within seconds, whereas 1 microM ouabain increased [Ca(2+)]i. A rapid reduction in [Ca(2+)]i was also observed when PRL cells were exposed to 1 microM cortisol, whereas there was no consistent effect at 1 nM. These results suggest that ouabain at physiological concentrations rapidly inhibits PRL release from the tilapia pituitary by suppressing intracellular Ca(2+) and cAMP metabolism. The stimulation of PRL release by high concentrations of ouabain (100-1000 nM) may result from an increase in [Ca(2+)]i, and subsequent depolarization due to the inhibition of Na(+), K(+)-ATPase activity.

Sodium pump alpha2 subunits control myogenic tone and blood pressure in mice

A key question in hypertension is: How is long-term blood pressure controlled? A clue is that chronic salt retention elevates an endogenous ouabain-like compound (EOLC) and induces salt-dependent hypertension mediated by Na(+)/Ca(2)(+) exchange (NCX). The precise mechanism, however, is unresolved. Here we study blood pressure and isolated small arteries of mice with reduced expression of Na(+) pump alpha1 (alpha1(+/-)) or alpha2 (alpha2(+/-)) catalytic subunits. Both low-dose ouabain (1-100 nm; inhibits only alpha2) and high-dose ouabain (> or =1 microm; inhibits alpha1) elevate myocyte Ca(2)(+) and constrict arteries from alpha1(+/-), as well as alpha2(+/-) and wild-type mice. Nevertheless, only mice with reduced alpha2 Na(+) pump activity (alpha2(+/-)), and not alpha1 (alpha1(+/-)), have elevated blood pressure. Also, isolated, pressurized arteries from alpha2(+/-), but not alpha1(+/-), have increased myogenic tone. Ouabain antagonists (PST 2238 and canrenone) and NCX blockers (SEA0400 and KB-R7943) normalize myogenic tone in ouabain-treated arteries. Only the NCX blockers normalize the elevated myogenic tone in alpha2(+/-) arteries because this tone is ouabain independent. All four agents are known to lower blood pressure in salt-dependent and ouabain-induced hypertension. Thus, chronically reduced alpha2 activity (alpha2(+/-) or chronic ouabain) apparently regulates myogenic tone and long-term blood pressure whereas reduced alpha1 activity (alpha1(+/-)) plays no persistent role: the in vivo changes in blood pressure reflect the in vitro changes in myogenic tone. Accordingly, in salt-dependent hypertension, EOLC probably increases vascular resistance and blood pressure by reducing alpha2 Na(+) pump activity and promoting Ca(2)(+) entry via NCX in myocytes.

Basis of chloride transport in ciliary epithelium

The aqueous humor is formed by the bilayered ciliary epithelium. The pigmented ciliary epithelium (PE) faces the stroma and the nonpigmented ciliary epithelium (NPE) contacts the aqueous humor. Cl(-) secretion likely limits the rate of aqueous humor formation. Many transport components underlying Cl(-) secretion are known. Cl(-) is taken up from the stroma into PE cells by electroneutral transporters, diffuses to the NPE cells through gap junctions and is released largely through Cl(-) channels. Recent work suggests that significant Cl(-) recycling occurs at both surfaces of the ciliary epithelium, providing the basis for modulation of net secretion. The PE-NPE cell couplet likely forms the fundamental unit of secretion; gap junctions within the PE and NPE cell layers are inadequate to maintain constancy of ionic composition throughout the epithelium under certain conditions. Although many hormones, drugs and signaling cascades are known to have effects, a persuasive model of the regulation of aqueous humor formation has not yet been developed. cAMP likely plays a central role, potentially both enhancing and reducing secretion by actions at both surfaces of the ciliary epithelium. Among other hormone receptors, A(3) adenosine receptors likely alter intraocular pressure by regulating NPE-cell Cl(-) channel activity. Recently, functional evidence for the regional variation in ciliary epithelial secretion has been demonstrated; the physiologic and pathophysiologic implications of this regional variation remain to be addressed.

Effects of nanomolar concentration dihydroouabain on calcium current and intracellular calcium in guinea pig ventricular myocytes

The effects of nanomolar concentration of dihydroouabain (DHO) on L-type calcium current (ICa-L), TTX-sensitive calcium current (ICa(TTX)), and intracellular calcium concentration ([Ca2+]i) were investigated in guinea pig ventricular myocytes. The whole-cell patch-clamp technique was used to record ICa-L and ICa(TTX); [Ca2+]i was detected and recorded with the confocal microscopy. The nanomolar concentration of DHO increased the ICa-L, ICa(TTX), and [Ca2+]i, which could be partially inhibited by nisoldipine or TTX, but still appeared in the absence of extracellular K+ and Na+. These data suggest that DHO could increase [Ca2+]i in non-beating myocytes via stimulating the ICa-L and ICa(TTX), or perhaps triggering directly a release of intracellular calcium.

Cardiac Na+-Ca2+ exchanger current induced by tyrphostin tyrosine kinase inhibitors

Tyrosine kinase (TK) inhibitors genistein and tyrphostin A23 (A23) inhibited Ca(2+) currents in guinea-pig ventricular myocytes investigated under standard whole-cell conditions (K(+)-free Tyrode's superfusate; EGTA-buffered (pCa-10.5) Cs(+) dialysate). However, the inhibitors (100 microM) also induced membrane currents that reversed between -40 and 0 mV, and the objective of the present study was to characterize these currents. Genistein-induced current behaved like Cl(-) current, and was unaffected by either the addition of divalent cations (0.5 mM Cd(2+); 3 mM Ni(2+)) that block the Na(+)-Ca(2+) exchanger (NCX), or the removal of external Na(+) and Ca(2+). A23-induced current was independent of Cl(-) driving force, and strongly suppressed by addition of Cd(2+) and Ni(2+), and by removal of either external Na(+) or Ca(2+). These and other results suggested that A23 activated an NCX current driven by submembrane Na(+) and Ca(2+) concentrations higher than those in the bulk cytoplasm. Improved control of intracellular Na(+) and Ca(2+) concentrations was obtained by suppressing cation influx (10 microM verapamil) and raising dialysate Na(+) to 7 mM and dialysate pCa to 7. Under these conditions, stimulation by A23 was described by the Hill equation with EC(50) 68 +/- 4 microM and coefficient 1.1, tyrphostin A25 was as effective as A23, and TK-inactive tyrphostin A1 was ineffective. Phosphotyrosyl phosphatase inhibitor orthovanadate (1 mM) antagonized the action of 100 microM A23. The results suggest that activation of cardiac NCX by A23 is due to inhibition of genistein-insensitive TK.

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.

Ouabain at Nanomolar Concentration Promotes Synthesis and Release of Angiotensin II from the Endothelium of the Tail Vascular Bed of Spontaneously Hypertensive Rats

The effects of 1 nM ouabain (OUA) on the contractile actions of phenylephrine (PHE, 0.001-100 microg) and functional activity of the sodium pump (NKA) in isolated-perfused tail vascular beds from WKY and SHR were investigated. In preparations from SHR, perfusion with OUA in the presence of endothelium (E+) increased the sensitivity (pED50) of PHE (before: 2.14 +/- 0.06 versus after: 2.47 +/- 0.07; P < 0.05) without altering the maximal response (Emax). After endothelial damage, OUA reduced the Emax of PHE in SHR (before: 350 +/- 29 versus after: 293 +/- 25 mm Hg; P < 0.05). In SHR/E+, pretreatment with losartan (10 microM) or enalaprilat (1 microM) prevented the increased sensitivity to PHE induced by OUA. OUA increased NKA activity in SHR/E+ (before: 45 +/- 6 versus after: 58 +/- 5%, P < 0.05). Losartan (10 mg/Kg, i.v.) also abolished the increment in systolic and diastolic blood pressure induced by OUA (0.18 microg/Kg, i.v.) in anesthetized SHR. OUA did not alter the actions of PHE in either anesthetized WKY rats or vascular preparations. Results suggest that 1 nM OUA increased the vascular reactivity to PHE only in SHR/E+. This effect is mediated by OUA-induced activation of endothelial angiotensin converting enzyme that promotes the local formation of angiotensin II, which sensitizes the vascular smooth muscle to the actions of PHE.

Ouabain increases myofibrillar Ca 2+ sensitivity but does not influence the Ca 2+ release in human skinned fibres

The present study investigated the impact of the Na(+) pump inhibitor ouabain (g-strophanthin) on Ca(2+) sensitivity and Ca(2+) release in human right auricular trabeculae (coronary bypass) and in skinned muscle fibres from left ventricular myocardium (cardiac transplantation, dilated cardiomyopathy). A time-dependent increase in force of contraction was observed in right auricular trabeculae in response to ouabain (100 nM) before the intracellular Ca(2+) transient (fura-2) increased (n=6). In triton X-skinned fibres (no sarcoplasmic reticulum) of human failing myocardium, ouabain (1-100 nM) concentration-dependently increased tension at a free extracellular Ca(2+) concentration of 1 microM and the Hill coefficient of the Ca(2+)-dependent tension development. Ouabain (1-100 nM) did not directly induce a Ca(2+) release out of the sarcoplasmic reticulum, nor did it alter the caffeine (10 mM) induced sarcoplasmic reticulum Ca(2+) release in saponin-skinned fibre preparations in which the sarcoplasmic reticulum had been Ca(2+)-loaded. In conclusion, ouabain increases myofibrillar Ca(2+) sensitivity possibly due to an increase in the cooperativity of the thick and thin myofilaments. This mechanism may additionally contribute to the positive inotropic effect of ouabain.

Ouabain stimulates endothelin release and expression in human endothelial cells without inhibiting the sodium pump

Ouabain, a sodium pump (Na+/ K+-ATPase) inhibitor, has been shown to act as a hormone and is possibly involved in the pathogenesis of hypertension. The mechanism by which ouabain may act was investigated using primary cultures of human umbilical artery endothelial cells (HUAECs), which are known to express and release the vasoconstrictive hormone endothelin (ET-1). Five minutes after application, low concentrations of ouabain induced Ca2+ oscillations and stimulated ET-1 release from endothelial cells into the medium. To investigate whether the observed effects were due to inhibition of the sodium pump, the effects of ouabain on the uptake of 86Rb+ by HUAECs were examined. Unexpectedly, ouabain concentrations below 10 nm stimulated 86Rb+ uptake by 15-20%, and in some experiments by 50%, results that are consistent with a stimulation of the pump. Within the concentration range 1-10 nm, ouabain induced a 2.5-fold stimulation (phosphorylation) of mitogen-activated protein kinase (MAP kinase). After incubation of HUAECs with ouabain for 12 h, the glycoside stimulated cell growth by 49 +/- 4%, as measured by cell number, with a maximum response at 5 nm. At similar concentrations, ouabain also increased ET-1 mRNA abundance by 19.5 +/- 3.1%. The results indicate that, by influencing ET-1 expression and release, ouabain may contribute to the regulation of vascular tone. The data also confirm that it is not a global inhibition of the sodium pump that is involved in the mechanism of action of this cardiac glycoside.

Myocardial Na,K-ATPase and digoxin therapy in human heart failure

The specific binding of digitalis glycosides to the Na,K-ATPase is used as a tool for Na,K-ATPase quantification with high accuracy and precision. In myocardial biopsies from patients with heart failure, total Na,K-ATPase concentration is decreased, and the decrease in Na,K-ATPase concentration correlates with a decrease in heart function. During digitalization, a fraction of remaining pumps are occupied by digoxin. No evidence for an endogenous digitalis-like factor of any clinical importance was obtained. It is recommended that digoxin be administered to heart failure patients who still have dyspnea after institution of mortality-reducing therapy.