Stimulatory (insulin-mimetic) and inhibitory (ouabain-like) action of vanadate on potassium uptake and cellular sodium and potassium in heart cells in culture

Genotoxicity and heating Performance of VxFe3-xO4 nanoparticles in Health applications

The applications of magnetic nanoparticles (MNPs) as biocatalysts in different biomedical areas have been evolved very recently. One of the main challenges in this field is to design affective MNPs surfaces with catalytically active atomic centres, while producing minimal toxicological side effects on the hosting cell or tissues. MNPs of vanadium spinel ferrite (VFe2O4) are a promising material for mimicking the action of natural enzymes in degrading harmful substrates due to the presence of active V5+ centres. However, the toxicity of this material has not been yet studied in detail enough to grant biomedical safety. In this work, we have extensively measured the structural, compositional, and magnetic properties of a series of VxFe3-xO4 spinel ferrite MNPs to assess the surface composition and oxidation state of V atoms, and also performed systematic and extensive in vitro cytotoxicity and genotoxicity testing required to assess their safety in potential clinical applications. We could establish the presence of V5+ at the particle surface even in water-based colloidal samples at pH 7, as well as different amounts of V2+ and V3+ substitution at the A and B sites of the spinel structure. All samples showed large heating efficiency with Specific Loss Power values up to 400 W/g (H0 = 30 kA/m; f = 700 kHz). Samples analysed for safety in human hepatocellular carcinoma (HepG2) cell line with up to 24h of exposure showed that these MNPs did not induce major genomic abnormalities such as micronuclei, nuclear buds, or nucleoplasmic bridges (MNIs, NBUDs, and NPBs), nor did they cause DNA double-strand breaks (DSBs) or aneugenic effects-types of damage considered most harmful to cellular genetic material. The present study is an essential step towards the use of these type of nanomaterials in any biomedical or clinical application.

Myasthenia gravis sera have no effect on cardiomyocytes in vitro

Myasthenia gravis (MG) is an autoimmune disorder primarily caused by circulating autoantibodies targeting the nicotinic acetylcholine receptor. Several studies have suggested a link between MG and heart disease. Girardi heart cells were treated with MG sera, measuring cytotoxic effects using flow cytometry, adenylate kinase (AK) release and evaluating morphology. MG sera did not induce morphological changes in the cells. AK release from cells treated with MG sera did not exceed controls and flow cytometric examination did not reveal any increase in dead or apoptotic cells. We conclude that MG sera have no cytotoxic effect in our heart cell culture system.

Mechanisms of vanadium action: insulin-mimetic or insulin-enhancing agent?

The demonstration that the trace element vanadium has insulin-like properties in isolated cells and tissues and in vivo has generated considerable enthusiasm for its potential therapeutic value in human diabetes. However, the mechanisms by which vanadium induces its metabolic effects in vivo remain poorly understood, and whether vanadium directly mimics or rather enhances insulin effects is considered in this review. It is clear that vanadium treatment results in the correction of several diabetes-related abnormalities in carbohydrate and lipid metabolism, and in gene expression. However, many of these in vivo insulin-like effects can be ascribed to the reversal of defects that are secondary to hyperglycemia. The observations that the glucose-lowering effect of vanadium depends on the presence of endogenous insulin whereas metabolic homeostasis in control animals appears not to be affected, suggest that vanadium does not act completely independently in vivo, but augments tissue sensitivity to low levels of plasma insulin. Another crucial consideration is one of dose-dependency in that insulin-like effects of vanadium in isolated cells are often demonstrated at high concentrations that are not normally achieved by chronic treatment in vivo and may induce toxic side effects. In addition, vanadium appears to be selective for specific actions of insulin in some tissues while failing to influence others. As the intracellular active forms of vanadium are not precisely defined, the site(s) of action of vanadium in metabolic and signal transduction pathways is still unknown. In this review, we therefore examine the evidence for and against the concept that vanadium is truly an insulin-mimetic agent at low concentrations in vivo. In considering the effects of vanadium on carbohydrate and lipid metabolism, we conclude that vanadium acts not globally, but selectively and by enhancing, rather than by mimicking the effects of insulin in vivo.

Voltage-dependent stimulation of the Na(+)-K(+) pump by insulin in rabbit cardiac myocytes

Insulin enhances Na(+)-K(+) pump activity in various noncardiac tissues. We examined whether insulin exposure in vitro regulates Na(+)-K(+) pump function in rabbit ventricular myocytes. Pump current (I(p)) was measured using the whole-cell patch-clamp technique at test potentials (V(m)s) from -100 to +60 mV. When the Na(+) concentration in the patch pipette ([Na](pip)) was 10 mM, insulin caused a V(m)-dependent increase in I(p). The increase was approximately 70% when V(m) was at near physiological diastolic potentials. This effect persisted after elimination of extracellular voltage-dependent steps and when K(+) and K(+)-congeners were excluded from the patch pipettes. When [Na](pip) was 80 mM, causing near-maximal pump stimulation, insulin had no effect, suggesting that it did not cause an increase in membrane pump density. Effects of tyrphostin A25, wortmannin, okadaic acid, or bisindolylmaleimide I in pipette solutions suggested that the insulin-induced increase in I(p) involved activation of tyrosine kinase, phosphatidylinositol 3-kinase, and protein phosphatase 1, whereas protein phosphatase 2A and protein kinase C were not involved.

Metabolic effects of vanadyl sulfate in humans with non-insulin-dependent diabetes mellitus: in vivo and in vitro studies

To investigate the efficacy and mechanism of action of vanadium salts as oral hypoglycemic agents, 16 type 2 diabetic patients were studied before and after 6 weeks of vanadyl sulfate (VOSO4) treatment at three doses. Glucose metabolism during a euglycemic insulin clamp did not increase at 75 mg/d, but improved in 3 of 5 subjects receiving 150 mg VOSO4 and 4 of 8 subjects receiving 300 mg VOSO4. Basal hepatic glucose production (HGP) and suppression of HGP by insulin were unchanged at all doses. Fasting glucose and hemoglobin A1c (HbA1c) decreased significantly in the 150- and 300-mg VOSO4 groups. At the highest dose, total cholesterol decreased, associated with a decrease in high-density lipoprotein (HDL). There was no change in systolic, diastolic, or mean arterial blood pressure on 24-hour ambulatory monitors at any dose. There was no apparent correlation between the clinical response and peak serum level of vanadium. The 150- and 300-mg vanadyl doses caused some gastrointestinal intolerance but did not increase tissue oxidative stress as assessed by thiobarbituric acid-reactive substances (TBARS). In muscle obtained during clamp studies prior to vanadium therapy, insulin stimulated the tyrosine phosphorylation of the insulin receptor, insulin receptor substrate-1 (IRS-1), and Shc proteins by 2- to 3-fold, while phosphatidylinositol 3-kinase (PI 3-kinase) activity associated with IRS-1 increased 4.7-fold during insulin stimulation (P = .02). Following vanadium, there was a consistent trend for increased basal levels of insulin receptor, Shc, and IRS-1 protein tyrosine phosphorylation and IRS-1-associated PI 3-kinase, but no further increase with insulin. There was no discernible correlation between tyrosine phosphorylation patterns and glucose disposal responses to vanadyl. While glycogen synthase fractional activity increased 1.5-fold following insulin infusion, there was no change in basal or insulin-stimulated activity after vanadyl. There was no increase in the protein phosphatase activity of muscle homogenates to exogenous substrate after vanadyl. Vanadyl sulfate appears safe at these doses for 6 weeks, but at the tolerated doses, it does not dramatically improve insulin sensitivity or glycemic control. Vanadyl modifies proteins in human skeletal muscle involved in early insulin signaling, including basal insulin receptor and substrate tyrosine phosphorylation and activation of PI 3-kinase, and is not additive or synergistic with insulin at these steps. Vanadyl sulfate does not modify the action of insulin to stimulate glycogen synthesis. Since glucose utilization is improved in some patients, vanadyl must also act at other steps of insulin action.

Vanadium: a review of its potential role in the fight against diabetes

The potential role of vanadium in human health is described as a building material of bones and teeth. However, another very interesting and promising application for vanadium in human health emerges from recent studies that evaluated the role of vanadium in the management of diabetes. Vanadium is present in a variety of foods that we commonly eat. Skim milk, lobster, vegetable oils, many vegetables, grains and cereals are rich source of vanadium (>1 ppm). Fruits, meats, fish, butter, cheese, and beverages are relatively poor sources of vanadium. The daily dietary intake in humans has been estimated to vary from 10 microg to 2 mg of elemental vanadium, depending on the environmental sources of this mineral in the air, water, and food of the particular region tested. In animals, vanadium has been shown essential (1-10 microg vanadium per gram of diet). There is only circumstantial evidence that vanadium is essential for humans. However, in doses ranging from 0.083 mmol/d to 0.42 mmol/d, vanadium has shown therapeutic potential in clinical studies with patients of both insulin-dependent diabetes mellitus (IDDM) and noninsulin-dependent diabetes mellitus (NIDDM) type. Although vanadium has a significant biological potential, it has a poor therapeutic index, and attempts have been made to reduce the dose of vanadium required for therapeutic effectiveness. Organic forms of vanadium, as opposed to the inorganic sulfate salt of vanadium, are recognized as safer, more absorbable, and able to deliver a therapeutic effect up to 50% greater than the inorganic forms. The goal is to provide vanadium with better gastrointestinal absorption, and in a form that is best able to produce the desired biological effects. As a result, numerous organic complexes of vanadium have been developed including bis(maltolato)oxovanadium (BMOV), bis(cysteinamide N-octyl)oxovanadium known as Naglivan, bis(pyrrolidine-N-carbodithioato)oxovanadium, vanadyl-cysteine methyl ester, and bis-glycinato oxovanadium (BGOV). The health benefits of vanadium and the safety and efficacy of the available vanadium supplements are discussed in this review.

In vivo and in vitro studies of vanadate in human and rodent diabetes mellitus

In vivo vanadate and vanadyl have been shown to mimic the action of insulin and to be effective treatment for animal models of both Type I and Type II diabetes. The molecular mechanism of action of the vanadium salts on insulin sensitivity remains uncertain, and several potential sites proposed for the insulin-like effects are reviewed. In human trials, insulin sensitivity improved in patients with NIDDM, as well as in some patients with IDDM after two weeks of treatment with sodium metavanadate. This increase in insulin sensitivity was primarily due to an increase in non-oxidative glucose disposal, whereas oxidative glucose disposal and both basal and insulin stimulated suppression of hepatic glucose output (HGP) were unchanged. Clinically, oral vanadate was associated with a small decrease in insulin requirements in IDDM subjects. Of additional benefit, there was a decrease in total cholesterol levels in both IDDM and NIDDM subjects. Furthermore, there was an increase in the basal activities of MAP and S6 kinases to levels similar to the insulin-stimulated levels in controls, but there was little or no further stimulation with insulin was seen. Further understanding of the mechanism of vanadium action may ultimately be useful in the design of drugs that improve glucose tolerance.

Cultured human myocardial fibroblasts of pediatric origin: natural human interferon-alpha is more effective than recombinant interferon-alpha 2a in carrier-state coxsackievirus B3 replication

Cultured human myocardial fibroblasts of pediatric origin seem to be a useful species-specific model for studying various heart diseases which involve the myocardial interstitium, for example enterovirus heart disease. Cells were propagated from small samples of human ventricular tissues (0.2 g) obtained from standard surgical procedure for the correction of Fallot-tetralogy. Cultured cells exhibited typical fibroblastoid morphology over a period of 4 months and were uniformly immunoreactive with a monoclonal antibody directed against prolyl-4-hydroxylase, a marker enzyme of fibroblasts. Infection of cell cultures with coxsackievirus B3, a cardiotropic enterovirus, resulted in a typical carrier-state type of virus persistence. Average virus titers of 2.3 x 10(5) plaque-forming units/ml (SD = 9.9 x 10(4)) were maintained over a period of up to 10 weeks by productive infection of about 8-10% of the cell population. Coxsackievirus B3 carrier cultures of human myocardial fibroblasts were used to evaluate in vitro the long-term antiviral effects of recombinant interferon alpha-2a and natural human interferon-alpha. Recombinant interferon-alpha reduced virus yields by 90% with a concentration of 423 IU/ml, whereas with natural interferon-alpha a 90% reduction of virus yields was achieved with concentrations as low as 21 IU/ml. Antiviral effects of both recombinant and natural interferon-alpha were highly specific and not related to inhibition of cell-proliferation (< 50% with interferon-alpha concentrations as high as 6250 IU/ml). Since effective concentrations of interferon-alpha can be easily attained in vivo with subcutaneous application, interferon-alpha (in particular: natural interferon-alpha) may become useful in the treatment of patients with enterovirus myocarditis and enterovirus induced dilated cardiomyopathy.

The use of post-binding agents in studying insulin action and its relation to experimental diabetes

This review includes data related to two substances that modulate insulin mechanisms, both in vitro and in the whole animal model. It seems to us that these agents (vanadate and PMXB) will be of potential use in the next decade for basic and applied research. They may assist in characterizing the essential post-binding events involved in insulin action, which cannot presently be identified. As vanadate and PMXB modulate the effects of insulin both in vivo and in vitro, they may be of use in clinical and pathophysiological research as well. VO3- is a low molecular weight substance which permeates the intestinal tract and mimics the actions of insulin in target tissues. Studies that were summarized here may even suggest that VO3- is superior to insulin in stimulating its effects in tissues that are down-regulated or desensitized to the hormone itself. Both VO3- and PMXB may be useful in the treatment of diabetes in the future if long-range toxicity studies prove these agents to be clinically safe. PMXB has already been in use in medicine for several decades now.

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.

Effects of various vanadium compounds on cochlear potentials

The effects of vanadium compounds, sodium vanadate, ammonium vanadate, potassium vanadate, vanadium oxysulfate, vanadium acetylacetonate, and vanadium trichloride, on endocochlear potential (EP) and cochlear microphonic potential (CM) were examined in the guinea pig cochlea. The perilymphatic space was perfused for 30 min with 1 mM solution of each compound and changes of EP, CM, and negative EP were observed. Upon perfusion with pentavalent vanadium solutions, such as sodium vanadate, ammonium vanadate, and potassium vanadate, the EP showed an overshoot at the beginning of perfusion and then a gradual decrease, while the CM showed only a gradual decrease. The other compounds had no effects on EP and CM. Since the negative EP showed no differences due to perfusion of any compound, it is concluded that the vanadate compounds have inhibitory effects on EP primarily, and on CM only secondarily. The chemical mechanism of the effects of vanadates was discussed concerning the function of the stria vascularis and also its participation in acute hearing loss.

Vanadate ions: central nervous system action on glucoregulation

Vanadate (VO-3), an essential trace element with insulin-mimetic actions, produces systemic hyperglycemia following central administration in mice. The hyperglycemic effect is due to specific action of vanadate or its reduced form, vanadyl (VO2+); other ions of similar atomic weight (Cr, Mn) or structure (phosphate) have no effect. The effect of central vanadate to raise circulating glucose is blocked by coadministration of 3-O-methylglucose or polymyxin B, which prevent insulin- and vanadate-stimulated glucose transport. Finally, the central hyperglycemic effect is prevented by treatments which block sympathetic outflow from the CNS or diminish the levels of circulating epinephrine. These results show that vanadate is able to influence peripheral glucoregulation by increasing sympathetic outflow from the CNS. Moreover, they suggest that this effect is linked to action of VO-3 or its reduced form, VO2+, to stimulate glucose transport into neuronal cells.

The role of endogenous noradrenaline in the beta-blocker withdrawal phenomenon--studies with cultured heart cells

An in vitro model to evaluate the role of endogenous noradrenaline in the beta-blocker withdrawal phenomenon is described: Beating chicken heart muscle cells (5000 beta 1-adrenoceptors/cell) and heart nonmuscle cells (3000 beta 2-adrenoceptors/cell) were cultured in serum-free, hormone-supplemented medium. Basal state, subtype selective down-regulation of beta-adrenoceptors by endogenous noradrenaline (decrease in receptor number, beta 1 more than beta 2) was simulated by addition of noradrenaline to the culture medium; chronic beta-blockade was simulated by exposure of the cells for 3 days to various beta-blockers (propranolol, no ISA; timolol, slight ISA; pindolol, strong ISA). Beta-blocker withdrawal phenomenon--increased response in isoproterenol-induced cAMP production and positive inotropy--is correlated with the increase in the number of beta-adrenoceptors after withdrawal of the drugs. Propranolol induces a withdrawal phenomenon at every degree of noradrenaline-induced basal state down-regulation of beta-adrenoceptors; in contrast, a withdrawal phenomenon by pindolol is only seen at a higher degree of beta-adrenoceptor down-regulation. In the presence of physiological noradrenaline concentrations pindolol affects beta-adrenoceptor subtypes in a qualitatively different manner: the number of beta 1-adrenoceptors is increased, the number of beta 2-adrenoceptors is decreased. This finding demonstrates that the intrinsic sympathomimetic activity of nonselective beta-blockers can manifest itself only if the receptors are not strongly down-regulated. As beta 2-adrenoceptors are present in a much less down-regulated state than beta 1, ISA mainly acts on beta 2-adrenoceptor subtype, thus, presenting a beta 2-"pseudo-selectivity" of ISA.

Effect of vanadate of PHA-induced proliferation of human lymphocytes from young and old subjects

The effect of sodium orthovanadate on mitogen-induced proliferation of lymphocytes from young and old human subjects is reported. We found that vanadate is not mitogenic per se; it has an inhibitory effect during the first 3 days of culture, when both differentiation and proliferation take place; it enhances DNA synthesis, acting as a co-mitogen, in the following days of culture, when proliferation prevails. In spite of the fact that lymphocytes from the two groups differ in their responsiveness to PHA and in the activity of (Na+,K+)ATPase, no difference was found as for the effects of vanadate.

Vanadium metabolism in wild type and respiratory-deficient strains of S. cerevisiae

Vanadium metabolism was studied in a wild type and respiratory-deficient strain of S. cerevisiae. Inhibition of growth by vanadate [V(+5)], vanadate accumulation, and conversion of medium vanadate [V(+5)] to both cell-associated and medium vanadyl [V(+4)] and vanadate [V(+5)] were compared. The growth of both the parental and respiratory-deficient strains was inhibited by vanadate at concentrations greater than or equal to 1 mM. Both parental and respiratory-deficient strains accumulated vanadate and converted medium vanadate to cellular vanadyl as detected using electron spin resonance (ESR). The accumulation of cell-associated vanadyl was correlated with the loss of medium vanadate in both strains using a chemical assay. In contrast, the respiratory-deficient strain showed a greater amount of a cell-associated vanadate compound, as detected with vanadium-51 nuclear magnetic resonance (51V-NMR), than the wild type strain or a representative respiratory-competent vanadate-resistant mutant. These data imply that mitochondrial function may be directly involved in vanadium metabolism.

The effect of vanadate on Na+,K+-ATPase activity of mouse cerebral cortex during bicuculline-induced seizures

The effect of bicuculline-induced seizures on Na+,K+-ATPase activity of mouse cerebral cortex homogenates, using two different procedures of sample preparation (freezing in situ or decapitation of animals without freezing) is described. Regardless of tissue treatment Na+,K+-ATPase activities during bicuculline-induced seizures did not differ significantly from the appropriate controls when vanadate-free ATP was used as substrate. The response of Na+,K+-ATPase to K+ activation was also similar; the increase in potassium concentration from 2 to 20 mM caused a 33.0 and 32.3% increase of enzyme activity in cortical homogenates from control and convulsing mice, respectively. Vanadate added to the assay medium inhibited Na+,K+-ATPase activity in a dose-dependent manner; with both types of tissue treatment there was, however, a tendency towards lesser inhibition of the enzyme from convulsing mice and at 1 X 10(-7) M vanadate this difference, though slight, was statistically significant: -22.59 vs -27.55% (freezing) and -28.73 vs -38.42% (decapitation) for seizures vs controls, respectively. The reduced sensitivity of Na+,K+-ATPase towards vanadate inhibition in cortical homogenates prepared from mice with convulsions suggests that vanadate might play a role in the modulation of enzyme activity during seizures in vivo.

Cardiac glycoside tolerance in cultured chicken heart muscle cells--a dose-dependent phenomenon

In cultured heart muscle cells from 10-13 day-old chicken embryos, the effects of acute (4 h) and chronic (3 days) exposure of the cells to varying concentrations of ouabain have been studied. In these cells, the cardiac glycoside ouabain binds to a specific cardiac glycoside receptor (KD = 4 X 10(-7) M; 750,000 receptors/cell). Binding to this receptor results in inhibition of active Na+/K+-transport [EC50 for active (86Rb+ + K+)-influx = 4 X 10(-6) M], and in an increase in beating velocity ("positive inotropic effect"; EC50 = 4 X 10(-7) M); toxic signs (arrhythmias) appear at concentrations greater than or equal to 6 X 10(-7) M. During exposure of the cells to 3 X 10(-6) M ouabain for 3 days, tolerance develops with respect to both the positive inotropic and the toxic effect. The mechanism underlying this tolerance is identified as an increase in the number of active sodium pump molecules per cell, while the binding properties of the cardiac glycoside receptor remain unchanged. The development of cardiac glycoside tolerance is only observed in the presence of severe impairment of Na+/K+-homeostasis, due to cardiac glycoside-induced inhibition of active Na+/K+-transport. This, however, only occurs in the presence of toxic (receptor occupation greater than or equal to 60%), but not in the presence of positive inotropic, non-toxic (receptor occupation 20-60%), ouabain concentrations. We conclude that the development of cardiac glycoside tolerance during long-term treatment in patients with heart failure should not occur with submaximal dose regimens, when toxic signs (arrhythmias) are absent.

The vasopressor response to centrally administered ouabain

Ouabain (80 micrograms/kg) injected into the lateral cerebroventricles (ICV) of rats produced a prompt and sustained increase in arterial blood pressure. A diastolic blood pressure increase of about 40 mm Hg began within 10 minutes of injection and lasted at lest 1 hour. This dose of ouabain had no effect on arterial pressure when given intravenously. The vasopressor response to intracerebroventricularly administered ouabain was not blocked by prior intravenous administration of phentolamine (1 mg/kg) or hexamethonium (3 mg/kg). However, continuous intravenous infusion of saralasin (2 micrograms/kg per min) prevented the pressor response to intracerebroventricularly administered ouabain. In addition, bilateral nephrectomy, adrenalectomy, pretreatment with intravenously administered propranolol (2 mg/kg) or captopril (10 mg/kg) abolished the increase in blood pressure evoked by intracerebroventricularly administered ouabain. Plasma renin and epinephrine levels at the peak of the pressor response to intracerebroventricularly administered ouabain were respectively, about 2.5- and 2-fold higher than in control rats. Our data indicate that ouabain administered into the central nervous system produces a hypertensive effect which does not primarily involve peripheral alpha-adrenergic receptors, but appears to be due to angiotensin II produced by renin of renal origin. These data suggest that digitalis agents can interact with sites in the central nervous system to induce a release of renin from the kidney; this release appears to involve activation of beta-adrenergic receptors by catecholamines from the adrenal medulla, perhaps through a direct adrenal-kidney vascular network.

Exogenous factors influencing the human erythrocyte sodium-lithium countertransport system

It has recently been found that the Na+-Li+ countertransport across the human erythrocyte membrane is increased in patients with essential hypertension. We investigated the influence of hypokalaemia, oral contraceptives, diabetes mellitus and essential hypertension on the activity of this transport system. Normal values for the maximal Na+-Li+ transport rate were 0.25 +/- 0.08 mmol l-1 h-1 (males, n = 18) and 0.23 +/- 0.06 (females, n = 14). We found elevated values in women taking oral contraceptives (0.34 +/- 0.07, n = 10, P less than 0.001), in patients with chronic hypokalaemia due to diuretic or laxative abuse (0.41 +/- 0.16, n = 13, P less than 0.005) and in those with essential hypertension (0.32 +/- 0.08, n = 24, P less than 0.001) (all data mean +/- SD). Thus our results with hypertensive patients support the findings of other investigators. However, oral contraceptives and drug-induced hypokalaemia greatly modify this system, indicating a regulation of the Na+-Li+ countertransport by hormones. Thus the transport rate does not seem to be an appropriate test for the diagnosis of essential hypertension.

Cardiac glycoside receptors in cultured heart cells--II. Characterization of a high affinity and a low affinity binding site in heart muscle cells from neonatal rats

The binding of [3H]ouabain has been studied in (Na+ + K+)-ATPase enriched cardiac cell membranes, as well as in cardiac muscle and non-muscle cells in culture--all obtained from hearts of neonatal rats. The binding has been correlated with ouabain-induced inhibition of (Na+ + K+)-ATPase (cardiac cell membranes) and the inhibition of active (86Rb+ + K+)-influx (cardiac muscle and non-muscle cells in culture). Furthermore, the effect of ouabain on the amplitude of cell-wall motion and contraction velocity has been studied in electrically driven cardiac muscle cells. In muscle and non-muscle cells, two classes of ouabain binding sites have been identified. In rat heart muscle cells, the high affinity binding site has a dissociation constant (KD) of 3.2 X 10(-8) M and a binding capacity (B) of 0.2 pmole/mg protein (80,000 sites/cell); the values for the low affinity binding site are: KD = 7.1 X 10(-6) M; B = 2.6 pmole/mg protein (10(6) sites/cell). The binding to both types of binding sites is depressed by K+ and abolished after heat denaturation of the cells. The kinetics of [3H]ouabain binding to rat heart muscle cells (association and dissociation rate constants, K+- and temperature-dependence of association and dissociation processes) have been characterized. In rat heart muscle and non-muscle cells, the binding of [3H]ouabain to the low affinity site results in inhibition of the (86Rb+ + K+)-influx (EC50 = 1.3 and 1.5 X 10(-5) M ouabain), a decrease in cell-K+ (EC50 = 1.9 and 1.4 X 10(-5) M) and an increase in cell-Na+ (10(-5)-10(-4) M). The ouabain-induced positive inotropic effect (increase in amplitude of cell-wall motion, increase in contraction velocity) in cardiac muscle cells is observed only at ouabain concentrations greater than or equal to 5 X 10(-6) M, and it is therefore probably attributed to occupation of the low affinity binding site. Coupling of occupation of the low affinity site by ouabain with drug-induced inhibition of the sodium pump and with drug-induced positive inotropic action is further substantiated by kinetic measurements. In contrast, occupation of the high affinity binding site does not produce any measurable inhibition of the sodium pump activity or positive inotropy.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiac glycoside receptors in cultured heart cells--I. Characterization of one single class of high affinity receptors in heart muscle cells from chick embryos

Binding of (3H)-ouabain and ouabain-induced inhibition of the sodium pump and of the (Na+ + K+)-ATPase have been characterized in cultured cardiac muscle and non muscle cells, as well as in cardiac cell membranes--all obtained from chick embryos. In both cell types, ouabain binds to a single type of binding sites in a temperature-dependent manner. The association rate but not the dissociation rate, is lowered by K+; specific binding is lost after heat-denaturation of the cells. Binding parameters (association and dissociation rate constants, activation energies for association and dissociation) are similar in muscle and non muscle cells. The dissociation constant of specific ouabain binding is 1.5 X 10(-7)M in cardiac muscle cells, and 1.9 X 10(-7)M in cardiac non muscle cells, the binding capacity being 2.6 and 2.1 pmoles/mg protein respectively. Specific binding of ouabain to the cells is coupled to inhibition of the sodium pump, as can be seen from ouabain-induced inhibition of active (86Rb+ + K+)-uptake, decrease in cellular K+, and increase in cellular Na+ (EC50 = 10(-7)-10(-6)M). The data obtained with cardiac cells are in good agreement with results found for ouabain binding (dissociation constant 4.3 X 10(-7)M) and (Na+ + K+)-ATPase inhibition (EC50 = 1.4 X 10(-6)M) in cardiac cell membranes prepared from the same tissue. Due to the experimental evidence it is concluded that the binding site for ouabain is identical with the cardiac glycoside receptor of these cells. In cardiac non muscle cells, binding of ouabain to its receptor is strictly coupled to inhibition of active K+-transport in a stoichiometric manner. In cardiac muscle cells, however, active K+-transport is inhibited by less than 10% when up to 40% of cardiac glycoside receptors have bound ouabain. It is assumed that this non-stoichiometric coupling of receptor occupancy and sodium pump inhibition in cardiac muscle cells may prevent substantial changes of Na+- and K+-contents in the heart in the presence of therapeutic levels of cardiac glycosides.

Cardiac glycoside binding sites in cultured heart muscle cells

Binding of (3H)-ouabain to cultured cardiac muscle and non muscle cells from chicken embryos and neonatal rats has been characterized and correlated with ouabain-induced inhibition of the sodium pump, as well as with the positive inotropic action of the drug. Cardiac muscle and non muscle cells from 10-12 day-old chicken embryos are characterized by a single class of ouabain binding sites (muscle cells: dissociation constant KD = 1.5 X 10(-7) M; binding capacity B = 2.6 pmoles/mg cell protein). Two classes of ouabain binding sites, however, have been found in cardiac muscle and non muscle cells from 1-3 day-old, neonatal rats (muscle cells: high affinity, low capacity sites: KD = 3.2 X 10(-8) M, B = 0.2 pmoles/mg protein; low affinity, high capacity sites: KD = 1.7 X 10(-6) M, B = 2.6 pmoles/mg protein). Half maximal inhibition of active (86Rb+ + K+)-influx occurs at 5.8 X 10(-7)M ouabain in chicken heart muscle cells and at 1.3 X 10(-5)M in rat heart muscle cells [( K+] = 0,75 mM). Decreases in cell-K+ (EC50 = 6.7 X 10(-7)M and 1.9 X 10(-5)M) and increases in cell-Na+ (7.4 X 10(-7) and 10(-5) - 10(-4)M) parallel ouabain-induced inhibition of the sodium pump. Up to 10(-6)M, ouabain does not affect velocity of cell wall motion in cultured rat heart muscle cells. A concentration-dependent increase in cell wall motion is observed at concentrations between 5 X 10(-6) and 5 X 10(-5)M, being indicative of a positive inotropic effect. At 10(-4)M ouabain, arrhythmias are present. Our data demonstrate the existence of one single class of cardiac glycoside receptors in cultured cardiac muscle cells from chicken embryos.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin action on cardiac glucose transport. Studies on the role of the Na+/K+ pump

Isolated muscle cells from adult rat heart have been used to study the relationship between myocardial glucose transport and the activity of the Na+/K+ pump. 86Rb+-uptake by cardiac cells was found to be linear up to 2 min with a steady-state reached by 40-60 min, and was used to monitor the activity of the Na+/K+ pump. Ouabain (10(-3) mol/l) inhibited the steady-state uptake of 86Rb+ by more than 90%. Both, the ouabain-sensitive and ouabain-insensitive 86Rb+-uptake by cardiac cells were found to be unaffected by insulin treatment under conditions where a significant stimulation of 3-O-methylglucose transport occurred. 86Rb+-uptake was markedly reduced by the presence of calcium and/or magnesium, but remained unresponsive towards insulin treatment. Inhibition of the Na+/K+ pump activity by ouabain and a concomitant shift in the intracellular Na+ :K+ ratio did not affect basal or insulin stimulated rates of 3-O-methylglucose transport in cardiac myocytes. The data argue against a functional relationship between the myocardial Na+/K+ pump and the glucose transport system.

Vanadium stimulates the (Na+,K+) pump in friend erythroleukemia cells and blocks erythropoiesis

Friend murine erythroleukemia cells underwent apparently normal erythropoiesis when treated with dimethyl sulfoxide. One of the earliest events associated with this induction was a decrease in ouabain sensitive 86Rb+ uptake, an assay of the plasma membrane Na,K(ATPase). Ammonium vanadate (10 microM) blocked differentiation of these cells without affecting cell viability. Vanadium was taken up by Friend cells and prevented the dimethyl sulfoxide-induced decrease in ouabain sensitive 86Rb+ uptake. Vanadate reactivated 86Rb+ transport previously inhibited by dimethyl sulfoxide treatment but had no affect on 86Rb+ transport in untreated cells. These results suggest an essential role for the (Na,K)ATPase in cell differentiation.