Action of ouabain on rat heart: comparison with its effect on guinea-pig heart

Disrupting Na+ ion homeostasis and Na+/K+ ATPase activity in breast cancer cells directly modulates glycolysis in vitro and in vivo

BACKGROUND

Glycolytic flux is regulated by the energy demands of the cell. Upregulated glycolysis in cancer cells may therefore result from increased demand for adenosine triphosphate (ATP), however it is unknown what this extra ATP turnover is used for. We hypothesise that an important contribution to the increased glycolytic flux in cancer cells results from the ATP demand of Na+/K+-ATPase (NKA) due to altered sodium ion homeostasis in cancer cells.

METHODS

Live whole-cell measurements of intracellular sodium [Na+]i were performed in three human breast cancer cells (MDA-MB-231, HCC1954, MCF-7), in murine breast cancer cells (4T1), and control human epithelial cells MCF-10A using triple quantum filtered 23Na nuclear magnetic resonance (NMR) spectroscopy. Glycolytic flux was measured by 2H NMR to monitor conversion of [6,6-2H2]D-glucose to [2H]-labelled L-lactate at baseline and in response to NKA inhibition with ouabain. Intracellular [Na+]i was titrated using isotonic buffers with varying [Na+] and [K+] and introducing an artificial Na+ plasma membrane leak using the ionophore gramicidin-A. Experiments were carried out in parallel with cell viability assays, 1H NMR metabolomics of intracellular and extracellular metabolites, extracellular flux analyses and in vivo measurements in a MDA-MB-231 human-xenograft mouse model using 2-deoxy-2-[18F]fluoroglucose (18F-FDG) positron emission tomography (PET).

RESULTS

Intracellular [Na+]i was elevated in human and murine breast cancer cells compared to control MCF-10A cells. Acute inhibition of NKA by ouabain resulted in elevated [Na+]i and inhibition of glycolytic flux in all three human cancer cells which are ouabain sensitive, but not in the murine cells which are ouabain resistant. Permeabilization of cell membranes with gramicidin-A led to a titratable increase of [Na+]i in MDA-MB-231 and 4T1 cells and a Na+-dependent increase in glycolytic flux. This was attenuated with ouabain in the human cells but not in the murine cells. 18FDG PET imaging in an MDA-MB-231 human-xenograft mouse model recorded lower 18FDG tumour uptake when treated with ouabain while murine tissue uptake was unaffected.

CONCLUSIONS

Glycolytic flux correlates with Na+-driven NKA activity in breast cancer cells, providing evidence for the 'centrality of the [Na+]i-NKA nexus' in the mechanistic basis of the Warburg effect.

Effects of exposure to sediment-associated fipronil on cardiac function of Neotropical armored catfish Hypostomus regani

Fipronil is widely used as a broad-spectrum insecticide in agriculture, urban environments, and veterinary medicine. Fipronil can enter aquatic ecosystems and spread to sediment and organic matter, representing a risk to non-target species. This study aimed to evaluate the effects of short-term (96 h) exposure to a low and realistic concentration of sediment-associated fipronil (4.2 µg.kg^-1 of Regent® 800 WG) on myocardial contractility of armored catfish Hypostomus regain, a benthic fish species. Fipronil exposure induced increased inotropism and acceleration of contractile kinetics, although no alterations in the relative ventricular mass were observed. This better cardiac function was associated with an elevated expression and/or function of the Na⁺/Ca²⁺ exchanger and its marked contribution to contraction and relaxation, probably due to a stress-induced adrenergic stimulation. Ventricle strips of exposed fish also exhibited a faster relaxation and a higher cardiac pumping capacity, indicating that armored catfish were able to perform cardiac adjustments to face the exposure. However, a high energetic cost to maintain an increased cardiac performance can make fish more susceptible to other stressors, impairing developmental processes and/or survival. These findings highlight the need for regulations of emerging contaminants, such as fipronil, to ensure adequate protection of the aquatic system.

Much More than a Cardiotonic Steroid: Modulation of Inflammation by Ouabain

Since the discovery of ouabain as a cardiotonic steroid hormone present in higher mammals, research about it has progressed rapidly and several of its physiological and pharmacological effects have been described. Ouabain can behave as a stress hormone and adrenal cortex is its main source. Direct effects of ouabain are originated due to the binding to its receptor, the Na⁺/K⁺-ATPase, on target cells. This interaction can promote Na⁺ transport blockade or even activation of signaling transduction pathways (e.g., EGFR/Src-Ras-ERK pathway activation), independent of ion transport. Besides the well-known effect of ouabain on the cardiovascular system and blood pressure control, compelling evidence indicates that ouabain regulates a number of immune functions. Inflammation is a tightly coordinated immunological function that is also affected by ouabain. Indeed, this hormone can modulate many inflammatory events such as cell migration, vascular permeability, and cytokine production. Moreover, ouabain also interferes on neuroinflammation. However, it is not clear how ouabain controls these events. In this brief review, we summarize the updates of ouabain effect on several aspects of peripheral and central inflammation, bringing new insights into ouabain functions on the immune system.

Profound regulation of Na/K pump activity by transient elevations of cytoplasmic calcium in murine cardiac myocytes

Small changes of Na/K pump activity regulate internal Ca release in cardiac myocytes via Na/Ca exchange. We now show conversely that transient elevations of cytoplasmic Ca strongly regulate cardiac Na/K pumps. When cytoplasmic Na is submaximal, Na/K pump currents decay rapidly during extracellular K application and multiple results suggest that an inactivation mechanism is involved. Brief activation of Ca influx by reverse Na/Ca exchange enhances pump currents and attenuates current decay, while repeated Ca elevations suppress pump currents. Pump current enhancement reverses over 3 min, and results are similar in myocytes lacking the regulatory protein, phospholemman. Classical signaling mechanisms, including Ca-activated protein kinases and reactive oxygen, are evidently not involved. Electrogenic signals mediated by intramembrane movement of hydrophobic ions, such as hexyltriphenylphosphonium (C6TPP), increase and decrease in parallel with pump currents. Thus, transient Ca elevation and Na/K pump inactivation cause opposing sarcolemma changes that may affect diverse membrane processes.

Positive inotropic effect of ouabain on isolated heart is accompanied by activation of signal pathways that link Na+/K+-ATPase to ERK1/2

Exposure of cultured rat cardiac myocytes to ouabain is known to cause the interaction of Na+/K+-ATPase with adjacent proteins, leading to activation of multiple signal transduction pathways, regulation of growth-related genes, and hypertrophy. The aim of this work was to determine if the proximal signaling events identified in cultured myocytes also occur in isolated intact hearts of rat and guinea pig in response to positive inotropic doses of ouabain. Langendorff rat heart preparations were exposed to 50 microM ouabain to produce positive inotropy without toxicity, and assayed for Src kinase, protein kinase C, and extracellular signal-regulated kinases 1 and 2 (ERK(1/2)). These kinases were rapidly activated by ouabain as in cultured cells. In isolated guinea pig hearts, 1 microM ouabain caused ERK(1/2) activation comparable to the effect of 50 microM ouabain in rat heart and consistent with the higher ouabain sensitivity of the contractility of guinea pig heart. These data show that the proximal ouabain-induced signal pathways previously noted in cultured cells are not artifacts of dispersion/culturing of myocytes, and are not the peculiar properties of the rat heart with its relatively low ouabain sensitivity. They also suggest that treatment with positive inotropic doses of cardiac glycosides is likely to be associated with changes in the cardiac phenotype.

PACAP induces bradycardia in guinea-pig heart by stimulation of atrial cholinergic neurones

Based on previous studies which indicated that pituitary adenylate cyclase activating peptide (PACAP) acts as a positive inotropic and chronotropic substance in different species via the cAMP signal transduction pathway, the objective of the present work was to investigate cAMP-regulated myocardial key proteins in response to PACAP in isolated ventricular cells of the guinea pig. Surprisingly, the two molecular forms of PACAP, PACAP(1-27) and PACAP(1-38), showed no effect on intracellular cAMP-levels, L-type Ca2+ channel current or phosphorylation of troponin inhibitor (TnI) and phospholamban (PLB). Additionally, inotropy of isolated guinea-pig ventricular strips was not affected by the neuropeptide. However, in isolated spontaneously beating guinea-pig atria, PACAP(1-27) and PACAP(1-38), but not VIP induced severe bradycardia in a dose-dependent manner. This effect could be prevented by preincubation with the PACAP receptor antagonist PACAP(6-38), by atropine and by omega-conotoxin, a blocker of neuronal N-type Ca2+ channels. PACAP stimulates release of [3H]-labelled acetylcholine. Only preparations showing an increase in [3H]acetylcholine release developed bradycardia, indicating a causal relationship between both phenomena. It was concluded that PACAP exerts no influence on guinea-pig ventricular tissue, but induces negative chronotropic effects in isolated guinea-pig atria by stimulation of acetylcholine release from parasympathetic neurons via PACAP type 1 receptors.

Differential effect of R 56865 on ouabain binding to isolated sarcolemma and intact atrial tissue of guinea-pig

1. R 56865 (N-[1-[4-(4-fluorophenoxy)-butyl]-4-piperidinyl]-N-methyl- 2-benzothiazolamine) is a compound known to antagonize cardiac glycoside intoxication. Therefore, the effect of the compound on ouabain binding to intact cardiac tissue as well as cardiac membrane preparations was investigated. 2. The binding of ouabain to highly purified sarcolemmal membranes was not influenced by R 56865 1 x 10(-6) mol l-1 (ouabain: KD = 1.3 x 10(-7) mol l-1, Bmax = 160 pmol mg-1; ouabain + R 56865: KD = 1.4 x 10(-7) mol l-1, Bmax = 168 pmol mg-1). 3. In contrast to the results in purified membranes, the binding of ouabain (10(-8) mol l-1 to 5 x 10(-7) mol l-1) to intact atria was significantly reduced. 4. Ouabain, 5 x 10(-7) mol l-1, led to a transient positive inotropic effect of about 220% followed by a developing negative inotropic effect after 3 h. R 56865, 10(-7) mol l-1, led to a maximal positive inotropic effect of about 290% also followed by a delayed decline of contractile force. A tenfold higher concentration of R 56865 led to sustained positive inotropic effect of about 250% in the same time interval. 5. The different effects of R 56865 on ouabain binding in subcellular preparations and intact tissue do not support the view that R 56865 interferes directly with the action of ouabain on Na/K-ATPase. An indirect effect, which may be mediated by a lowered intracellular sodium load is discussed.

Two functional Na+/K(+)-ATPase isoforms in the left ventricle of guinea pig heart

Guinea pig left ventricular muscle contains two distinct molecular forms of the Na+/K(+)-ATPase catalytic alpha subunit. Sarcolemmal vesicles highly enriched in Na+/K(+)-ATPase were isolated by a new procedure that yielded specific activities of 60-100 mumol Pi.h-1.mg-1. SDS/PAGE of isolated sarcolemma after reduction and alkylation of the sulfhydryl groups and identification on immunoblots with specific anti-(alpha subunit) antibodies indicated the presence of two major polypeptides of 100 kDa and 103 kDa, respectively. The two alpha subunits were functional: the dose/response curves of Na+/K(+)-ATPase activity with ouabain, dihydroouabain and digitoxigenin were biphasic, revealing the presence of high-affinity [concentration of drug causing 50% inhibition (IC50) = 10 nM] and low-affinity (IC50 = 2 microM) forms with proportional contributions of 55% and 45%, respectively. The involvement of the high-affinity form in the positive inotropic effect of digitalis and of the low-affinity sites in both inotropy and toxicity are consistent with the literature data on rodents.

Diastolic tension and contraction amplitude in calcium-loaded rat ventricular myocardium are differently affected by drugs

1. To obtain a measure of drug effects on myocardial function during diastole, the following experimental protocol was designed: rapid electrical stimulation (5 Hz) at high Ca02+ caused an elevated diastolic tension, which could be subjected to drug-induced alterations. 2. Antiarrhythmic drugs (quinidine, propafenone, procainamide, mexiletine) were able to lower diastolic force without appreciably decreasing contraction amplitude. Calcium antagonists (nifedipine, verapamil) lowered both parameters in parallel. 3. Veratridine and Bay K 8644 both enhanced diastolic tension, but only Bay K 8644 concomitantly elevated contraction amplitude. 4. These findings may be explained when taking into account differential actions of sodium- and calcium channel modulating drugs, respectively, on cellular Ca2+ movements. In quantitative terms, the non-linear dependence of myocardial force on Cai2+ also had to be considered.

Inotropic effect of ouabain in hypertrophied rat heart

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

Intracellular sodium affects ouabain interaction with the Na/K pump in cultured chick cardiac myocytes

Whether a given dose of ouabain will produce inotropic or toxic effects depends on factors that affect the apparent affinity (K0.5) of the Na/K pump for ouabain. To accurately resolve these factors, especially the effect of intracellular Na concentration (Nai), we have applied three complementary techniques for measuring the K0.5 for ouabain in cultured embryonic chick cardiac myocytes. Under control conditions with 5.4 mM Ko, the value of the K0.5 for ouabain was 20.6 +/- 1.2, 12.3 +/- 1.7, and 6.6 +/- 0.4 microM, measured by voltage-clamp, Na-selective microelectrode, and equilibrium [3H]ouabain-binding techniques, respectively. A significant difference in the three techniques was the time of exposure to ouabain (30 s-30 min). Since increased duration of exposure to ouabain would increase Nai, monensin was used to raise Nai to investigate what effect Nai might have on the apparent affinity of block by ouabain. Monensin enhanced the rise in Na content induced by 1 microM ouabain. In the presence of 1 microM [3H]ouabain, total binding was found to be a saturating function of Na content. Using the voltage-clamp method, we found that the value of the K0.5 for ouabain was lowered by nearly an order of magnitude in the presence of 3 microM monensin to 2.4 +/- 0.2 microM and the magnitude of the Na/K pump current was increased about threefold. Modeling the Na/K pump as a cyclic sequence of states with a single state having high affinity for ouabain shows that changes in Nai alone are sufficient to cause a 10-fold change in K0.5. These results suggest that Nai reduces the value of the apparent affinity of the Na/K pump for ouabain in 5.4 mM Ko by increasing its turnover rate, thus increasing the availability of the conformation of the Na/K pump that binds ouabain with high affinity.

The protective action of R56865 against ouabain-induced intoxication in rat heart isolated atria and ventricles

The action of R56865 has been examined on the contractile effects produced by ouabain concentrations interacting with high (3 microM) and low (300 microM) affinity digitalis receptors on electrically stimulated ventricular strips isolated from rat. R56865 1 microM reduced the increase in resting tension produced by ouabain 300 microM and left unalterated the inotropic effect evoked by ouabain 3 and 300 microM that was reduced by higher concentrations (3 and 6 microM) of R56865. The action of R56865 was also studied on ouabain-induced intoxication in electrically stimulated and spontaneously beating atria of rat. On electrically stimulated (3 Hz) whole atria, R56865 0.3 microM reduced the maximal increase in resting tension produced by ouabain 300 microM and delayed the time to onset of the ouabain-induced arrhythmias but did not affect ouabain's inotropic effect. Higher concentrations of R56865 were required to reduce the inotropic effect of ouabain. The protective action of R56865 against ouabain-induced intoxication was most pronounced on spontaneously beating atria where it reduced spontaneous rate of beats. Experiments in electrically driven left atria indicated that only a part of the protective effect of R56865 could be related to its bradycardic action. The effect of R56865 was also examined on ouabain-induced inhibition of sodium pump in human red blood cells. R56865 6 microM did not modify the inhibition produced by ouabain (from 0.3 to 10 nM), this indicates that the protective action of R56865 against ouabain-induced intoxication is not due to an interaction with the inhibitory effect of ouabain on sodium pump.

Interpretation of [3H]ouabain binding in guinea-pig ventricular myocardium in relation to sodium pump activity

1. The attempt was made to analyse the complex [3H]ouabain binding curves obtained in intact cardiac ventricular preparations electrically stimulated at different frequencies. The result of this analysis was used to draw conclusions from the binding curves on the frequency dependence of sodium pump activity. 2. [3H]Ouabain binding to isolated, electrically stimulated (1.5 Hz) ventricular strips of guinea-pig hearts was investigated. The positive inotropic effects were studied in separate experiments. Specific [3H]ouabain binding barely reached an equilibrium within 3 h of incubation. A binding curve was constructed using the equilibrium values of specific [3H]ouabain binding obtained at different ouabain concentrations. This binding curve revealed a concentration-proportional component at positive inotropic concentrations and a saturating component at high, toxic concentrations. At very low, inotropically ineffective ouabain concentrations, however, binding values were higher than expected from a linear relationship between ouabain concentration and binding. 3. The peculiar shape of the binding curve could be largely accounted for by a mathematical model, which takes into consideration biochemical properties and physiological regulation of the sodium pump. The model predicts a concentration-proportional pattern of binding which takes place in the non-toxic ouabain concentration range. The slope of the concentration-proportional component of the binding curve should represent a measure of sodium pump activity. 4. Investigation of binding curves at various stimulation frequencies revealed that, as predicted by the model, the slope of the concentration-proportional component of the binding curves was increased and the maximum non-toxic equilibrium binding was decreased with increasing beat frequencies. 5. Quantitative evaluation of the binding curves led to the conclusion that sodium pump activity is a linear function of stimulation frequency in guinea-pig ventricular preparations, the activity in resting preparations amounting to about 15% of the maximum activity. Comparison of the present results with former studies on sodium pump function suggests that [3H]ouabain binding reflects steady-state sodium pump activity. If the complex pattern of binding curves is taken into consideration, [3H]ouabain binding measurements may serve as a means of studying sodium pump function.

Rat cardiac ventricle has two Na+,K+-ATPases with different affinities for ouabain: developmental changes in immunologically different catalytic subunits

The mechanism of the inotropic effect of cardiac glycosides on the heart has long been controversial. Inotropic effects at low concentrations of cardiac glycosides indicate more than one class of receptor or more than one cellular mechanism. In the brain of the rat, high- and low-affinity cardiac glycoside receptors have been shown to be associated with two structurally different isoforms of the catalytic subunit of the Na+,K+-ATPase, termed alpha and alpha(+). Evidence is presented here that the high- and low-affinity sites in rat cardiac ventricle are associated with Na+,K+-ATPase catalytic subunit forms similar to the alpha(+) and alpha forms in the brain. Membranes from the rat ventricle contained polypeptides with the electrophoretic mobilities of alpha and alpha(+), which could be stained by isoform-specific anti-Na+,K+-ATPase antibodies on electrophoretic blots. Both polypeptides also displayed Na+-stimulated phosphorylation with [gamma-32P]ATP. Inhibition of Na+,K+-ATPase activity by ouabain demonstrated the presence of both high- and low-affinity ATPases proportional to the presence of the alpha(+) and alpha polypeptides. The ratios of the two isoforms changed with postnatal maturation, paralleling known changes in cardiac physiology and cardiac glycoside sensitivity. Cardiac glycoside sensitivity can evidently be regulated at the level of gene expression by developmental signals.

Acrihellin, a cardioactive steroid escaping from the organ-bath

The concentration of acrihellin rapidly declines in oxygenated Tyrode-solution, because the compound escapes from the organ-bath being enriched in droplets sprayed from the surface of the bubbled solution. As checked by radiochromatography, acrihellin remains chemically unaltered during this process. Hellebrin and hellebrigenin persist in gassed Tyrode-solution, suggesting that the 3 beta-substituent dimethylacrylic acid endows acrihellin with amphiphilic properties, thus promoting its enrichment at gas-water interphases. Measurements of the inotropic effects in guinea pig left atria performed at concentrations of acrihellin kept constant yielded a dose-response curve, which closely resembles that of the conventional cardioactive steroid ouabain.

Selective inhibition by ethylisopropylamiloride of the positive inotropic effect evoked by low concentrations of ouabain in rat isolated ventricles

The biphasic positive inotropic effect of ouabain has been studied in rat ventricular strips in the presence of ethylisopropylamiloride (EIPA) an inhibitor of Na+/H+ exchange. EIPA (10-20 microM) depressed dose-dependently the high affinity component of the ouabain inotropic effect, whereas it did not significantly modify the low affinity inotropic effect related to high concentrations of ouabain. At the EIPA concentrations studied, there was no observable modification of the inotropic effect of Bay K 8644 (10 nM, 0.3 microM) or of isoprenaline (10 nM, 1 microM). These results indicate that the inotropic effect of ouabain resulting from its interaction with high affinity sites is selectively sensitive to EIPA.

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

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

Sodium load and high affinity ouabain binding in rat and guinea-pig cardiac tissue

An estimation of the actual Na/K-ATPase transport activity in intact cardiac cells was made by measuring the binding of [3H]-ouabain to rat and guinea-pig ventricular strips. At the low [3H]-ouabain concentration of 1 nM equilibrium binding was hardly obtained after an incubation time of five hours. Different procedures known to alter the sodium load of the cardiac preparations influenced [3H]-ouabain binding: the sodium ionophore monensin enhanced [3H]-ouabain binding, the local anaesthetic dibucaine and a reduction of external sodium ion concentration diminished [3H]-ouabain binding; [3H]-ouabain binding was similarly affected by these procedures in the rat and guinea-pig. Since [3H]-ouabain binding occurred predominantly at the high-affinity binding sites of rat myocardium under the applied experimental conditions, it was concluded that these binding sites represent Na/K-ATPase molecules involved in sodium ion transport.