The acute changes seen in cardiac glycoside receptor sites, 86rubidium uptake and intracellular sodium concentrations in the erythrocytes of patients during the early phases of digoxin therapy are not found during chronic therapy: pharmacological and therapeutic implications for chronic digoxin therapy

Ouabain, endogenous ouabain and ouabain-like factors: The Na+ pump/ouabain receptor, its linkage to NCX, and its myriad functions

In this brief review we discuss some aspects of the Na⁺ pump and its roles in mediating the effects of ouabain and endogenous ouabain (EO): i) in regulating the cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) via Na/Ca exchange (NCX), and ii) in activating a number of protein kinase (PK) signaling cascades that control a myriad of cell functions. Importantly, [Ca²⁺]_CYT and the other signaling pathways intersect at numerous points because of the influence of Ca²⁺ and calmodulin in modulating some steps in those other pathways. While both mechanisms operate in virtually all cells and tissues, this article focuses primarily on their functions in the cardiovascular system, the central nervous system (CNS) and the kidneys.

The diagnostic value of determination of intraerythrocytic sodium and potassium concentrations versus plasma digoxin concentration in digoxin intoxication

Plasma digoxin measurements have proved unserviceable as a means of differentiating between toxic and non-toxic patients. In order to assess the value of a biological effect of digoxin in this discrimination, intraerythrocytic sodium and potassium concentrations were determined in 55 chronically digitalized patients of whom 10 were digoxin-intoxicated according to ECG criteria. Digitoxicity was associated with elevated intraerythrocytic sodium concentration (mean +/- SEM 19.3 +/- 1.2 versus 11.3 +/- 0.3 mmol/l, p less than 0.001) and reduced intraerythrocytic potassium concentration (94.6 +/- 2.3 versus 100.0 +/- 0.6 mmol/l, p less than 0.001) compared to non-toxic patients. Mean (+/- SEM) plasma digoxin concentrations in the two groups were 3.14 +/- 0.41 and 1.57 +/- 0.09 nmol/l, respectively (p less than 0.001). When diagnosing toxicity in chronically digitalized patients, plasma digoxin and intraerythrocytic sodium determinations showed sensitivities of 60 and 100%, respectively. The predictive values of a positive test were 75% for plasma digoxin and 83% for intraerythrocytic sodium.

Divergent signaling pathways mediate induction of Na,K-ATPase α1 and β1 subunit gene transcription by low potassium

Prolonged inhibition of Na,K-ATPase enzymatic activity by exposure of a variety of mammalian cells to low external K+ yields a subsequent adaptive up-regulation of Na,K-ATPase expression. The aim of this study was to examine the intracellular signal transduction system that is responsible for mediating increased Na,K-ATPase subunit gene expression in primary cultures of neonatal rat cardiac myocytes. In this work, we show long-term inhibition of Na,K-ATPase function with 0.6 mM K+ resulted in hypertrophy of cardiac myocytes and augmentation of Na,K-ATPase alpha1 and beta1 subunit gene expression. Transient transfection experiments in neonatal rat cardiac myocytes demonstrated that low K+ induction of alpha1 and beta1 gene transcription was dependent on intracellular Ca2+ and activation of calcineurin. Based on effects of pharmacological inhibitors, protein kinase A (PKA), extracellular signal-regulated kinase 1/2 (ERK1/2) and histone deacetylase were found to be unique downstream components in the low K+ signal transduction pathway leading to increased alpha1 subunit promoter activity. Similarly, low K+-induced beta1 subunit gene transcription was dependent on activation of protein kinase C (PKC), c-Jun-N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). These findings indicate that persistent inhibition of Na,K-ATPase activity with low external K+ activates overlapping and Na,K-ATPase subunit gene-specific signaling pathways in cardiac myocytes.

Regulation of Na,K-ATPase beta 1 subunit gene transcription by low external potassium in cardiac myocytes. Role of Sp1 AND Sp3

Expression of Na,K-ATPase activity is up-regulated in cells incubated for extended intervals in the presence of low external K(+). Our previous data showed that exposure of cardiac myocytes to low K(+) increased the steady-state abundance of Na,K-ATPase beta1 subunit mRNA. In the present study we determined that incubation of primary cultures of neonatal rat cardiac myocytes with low K(+) augmented Na,K-ATPase beta1 gene expression at a transcriptional level and that this effect required extracellular Ca(2+). The stimulatory effect of low K(+) on Na,K-ATPase beta1 gene transcription was not dependent on increased contractile activity of cardiac myocytes. Na,K-ATPase beta1 5'-flanking region deletion plasmids used in transient transfection analysis demonstrated that the region between nucleotides -62 to -42 of the beta1 promoter contained a low K(+) response element. Site-directed mutagenesis of a potential GC box core motif GCG in the -58/-56 region of the beta1 promoter decreased basal and low K(+)-mediated transcription. Mutation of the core sequence of a putative GC box element located between nucleotides -101 and -99 further decreased the low K(+) effect on beta1 gene transcription. Electrophoretic mobility shift assays using oligonucleotides spanning the proximal and distal GC box elements of the beta1 promoter showed enhanced binding of two complexes in response to low K(+). The inclusion of a consensus GC box sequence as a competitor in gel shift analysis reduced factor binding to the low K(+) response elements. Antibodies to transcription factors Sp1 and Sp3 interacted with components of both DNA-binding complexes and binding of nuclear factors was abolished in gel shift studies using GC box mutants. Together these data indicate that enhanced binding of Sp1 and Sp3 to two GC box elements in the rat Na,K-ATPase beta1 subunit gene promoter mediates beta1 gene transcription up-regulation in neonatal rat cardiac myocytes exposed to low external K(+).

Effect of chronic ouabain infusion on food, water, and NaCl intake, body composition, and plasma hormones of Sprague-Dawley rats

To examine whether the sodium-potassium pump (Na+,K(+)-ATPase) mediates food or NaCl intake, male Sprague-Dawley rats with ad lib access to food, water, and 300 mM NaCl solution were infused for 27 days with the Na+,K(+)-ATPase inhibitor, ouabain (4, 8, 16, 32, or 64 micrograms/h, SC). Ouabain significantly decreased NaCl preference and increased body weight but had no effect on food or NaCl intake, carcass fat, protein, or ash content. Ouabain's effect on NaCl preference was apparently due to a nonsignificant increase in water intake, and its effect on body weight was due to a significant increase in carcass water content. During the first 5 days of treatment, 4-32 micrograms/h ouabain decreased and 64 micrograms/h ouabain increased plasma corticosterone levels relative to controls. At the end of the experiment, all the ouabain-treated rats had significantly elevated plasma ouabain levels but normal plasma osmolarity, solids, pH, sodium, calcium, glucose, insulin, aldosterone, and corticosterone levels. The groups receiving 4-32 micrograms/h ouabain also had normal plasma concentrations of potassium, ACTH, and renin activity. The group receiving 64 micrograms/h ouabain had elevated ACTH and potassium levels and reduced plasma renin activity. These results suggest that chronic administration of low doses of ouabain specifically increases water retention. The hypotheses that the sodium-potassium pump mediates food and NaCl intake are neither supported nor refuted.

The effect of high salt intake on Na+, K(+)-ATPase activity of tissues in the rat

1. The effect of chronic feeding of high salt diet on Na+, K(+)-ATPase activity of heart, liver, skeletal muscle, kidney and aorta was studied in the rat. 2. Groups of rats were either given tap water or 18 g/L saline to drink. After 7 days, 3 months or 12 months, the control group and salt loaded groups were sacrificed and Na+, K(+)-ATPase activity of heart, liver, skeletal muscle and kidney was determined by a coupled enzyme assay and that of the aorta by the K(+)-stimulated hydrolysis of 3-0-methylfluorescein phosphate. 3. Na+, K(+)-ATPase activity of heart, liver, skeletal muscle and aorta were not different between the experimental and control groups at 7 days. After 3 months, Na+, K(+)-ATPase activity of liver in salt-loaded group was higher than the control group. After 12 months of salt loading all tissues examined showed higher Na+, K(+)-ATPase activity compared to control groups. The activity of renal medulla of salt-loaded group was higher than that of control group as early as 7 days. 4. We conclude that long term salt loading causes an increase in the activity of Na+, K(+)-ATPase of kidney, heart, liver, muscle and aorta.

The chronic effects of long-term digoxin administration on Na+/K(+)-ATPase activity in rat tissues

We have studied the effects of digoxin administration on Na+/K(+)-ATPase activity in heart, liver, muscle, renal medulla and aorta in the rat. Adult male rats were either treated with digoxin for 3 days, 7 days (5 mg/kg per day) or 3 months (3 mg/kg per day). Another group of rats were treated with the vehicle as controls. At the end of the experimental period, blood samples were taken for digoxin measurements, the animals were sacrificed, and the heart, liver, kidney, skeletal muscle and aorta were removed, homogenised and assayed for Na+/K(+)-ATPase activity. In all tissues except the aorta Na+/K(+)-ATPase activity was measured by an enzyme coupled reaction. Na+/K(+)-ATPase activity in the aorta was measured by a fluorometric potassium dependent 3-O-methyl fluorescein phosphatase activity. Plasma digoxin concentration in the digoxin group was 5.34 nmol/l (S.E.M., 0.09) in the 3-day group and 4.38 (0.68) and 4.89 (0.73) nmol/l in the 7-day and 3-month groups, respectively. After treatment for 3 days and 7 days, the Na+/K(+)-ATPase activity in all tissues was significantly lower in the digoxin group (the decrease in activity ranging from 13.4% in muscle to 46.9% in the renal medulla). After 3 months of treatment, Na+/K(+)-ATPase activity in all the tissues except the aorta was similar in the digoxin and control groups. In the aorta the activity remained low. We conclude that in rats digoxin administration causes upregulation of the Na+/K(+)-ATPase in most tissues.

Oral magnesium supplementation restores the concentrations of magnesium, potassium and sodium-potassium pumps in skeletal muscle of patients receiving diuretic treatment

In 76 consecutive patients who had received diuretics for 1-17 years for arterial hypertension or congestive heart failure, muscle concentrations of magnesium, potassium, and sodium-potassium pumps were significantly reduced compared to 31 age- and sex-matched controls. Thirty-six patients with muscle magnesium and/or potassium below the control level received oral magnesium hydroxide supplement for 2-12 weeks (n = 20) or 26 weeks (n = 16). After short-term (2-12 weeks) magnesium supplementation muscle parameters were increased, but far from normalized. After magnesium supplementation for 26 weeks, the muscle concentrations of magnesium, potassium and sodium-potassium pumps were normalized in most cases. Oral magnesium supplementation may restore diuretic-induced disturbances in the concentrations of magnesium, potassium and sodium potassium pumps in skeletal muscle. A supplemental period of at least 6 months seems to be required before complete normalization can be expected.

Digitalis and heart failure: does digitalis really produce beneficial effects through a positive inotropic action?

Although digitalis was introduced to medicine long ago, the drug is still extensively used in clinical practice today. Opinions on its mechanism of action have undergone much change in the course of time, and the way in which cardiovascular effects are produced is still not completely clear. Limitations and contraindications for the use of digitalis substances are reported, especially in the treatment of ischemic heart disease. Preliminary data regarding the effects of digitalis on the diastolic phase are unfavorable, although the relationship between digitalis and diastolic function ought to be studied in greater depth in various clinical conditions. In spite of many recent trials, the old question of the usefulness of digitalis in the chronic treatment of patients in sinus rhythm and heart failure is still debated. An important clinical benefit in the chronic use of digitalis appears restricted to a relatively small proportion of patients with severe congestive heart failure, while in the majority of chronically treated subjects the effects of the drug are scanty or insignificant. The beneficial effect of digitalis used chronically is essentially believed to be due to its positive inotropic action. Since the vagomimetic and the antiadrenergic effects of digitalis have been demonstrated to be independent from its inotropic action, they could be considered determinants of the clinical benefits of digitalis. These indirect effects may be useful in the control of the negative neuroendocrine response developing during congestive heart failure. Thus the statement that digitalis is essentially an inotropic agent seems restrictive; its definition should reflect the favorable effects obtained in some cases of congestive heart failure rather than its various and contrasting underlying mechanisms of action.

Influence of physical exercise on serum digoxin concentration and heart rate in patients with atrial fibrillation

Heart rate and serum digoxin concentration in eight patients with atrial fibrillation were studied at rest and during exercise when initial serum digoxin concentrations were zero and at low and high therapeutic values. Eight patients with ischemic heart disease and in sinus rhythm were studied for comparison. Though the serum digoxin concentration decreased significantly during exercise, the absolute reduction in heart rate was the same at rest and during exercise in patients with atrial fibrillation. Compared with the control patients in sinus rhythm, the heart rate in patients with atrial fibrillation was not adequately controlled during exercise by any serum digoxin concentration tested despite a reduction in heart rate with increasing digoxin concentration. The effects of digoxin on heart rate regulation in atrial fibrillation are complex and include direct effects on the myocardium as well as indirect effects mediated by modulation of the autonomic nervous system; the present results indicate that the drug is not displaced from the target organs by decreasing serum concentrations during exercise. In atrial fibrillation, because the demands on the filter function of the atrioventricular node are highly unphysiological, the effect of digoxin on heart rate during exercise is not adequate.

Verapamil-digoxin interaction in chronic hemodialysis patients

Verapamil increases the serum-digoxin concentration (SDC) in digoxin treated normals due to a compromised renal and extrarenal clearance. In chronic hemodialysis patients (CHD-patients) treated with digoxin where the renal elimination is diminished, verapamil has been shown to cause substantial increases of SDC with risk of digoxin intoxication. The effect of verapamil treatment on SDC in 8 nearly anephric (Uvol less than 1 l/d) CHD-patients on digoxin treatment was assessed. The patients were continuously treated with verapamil for two periods of two weeks at two dosage levels, 120 mg/d and 240 mg/d, whereafter verapamil was withdrawn. SDC and serum-verapamil were measured weekly. The SDC increased from 1.1 mmol/l to 1.7 mmol/l (p less than 0.05, N = 7) during the first two weeks. Increasing the dose of verapamil to 240 mg/d did not cause a further increment in SDC; on the contrary, the mean SDC decreased. The SDC increments varied between 0 and 200% of baseline values. We conclude that verapamil treatment decreases digoxin clearance in CHD-patients and that the influence of verapamil on SDC in CHD-patients shows great interindividual variation with no close dose dependency and decreases to pretreatment level in 2-3 weeks.

Clinical application of sodium-23 nuclear magnetic resonance for measurement of red cell sodium concentrations

Red cell sodium (RBC-Na+) concentrations were measured using 23Na nuclear magnetic resonance (NMR), without the destruction of erythrocyte membranes. Subjects were categorized into four groups: 20 normotensive subjects (NT group), 20 age-matched essential hypertensive patients (EHT group), 10 patients with primary aldosteronism (PA group), and 18 patients treated with digoxin (DIG group). Although RBC-Na+ concentrations were similar between the NT group (6.14 +/- 0.80 (Mean +/- SD) mmol/l) and the EHT group (5.92 +/- 0.99), they were significantly higher in both the PA group (7.55 +/- 0.88, p less than 0.001) and the DIG group (8.43 +/- 3.81, p less than 0.02). In the PA group, RBC-Na+ concentrations decreased significantly after resection of the adenoma, and there was an inverse relationship between serum potassium and RBC-Na+ concentrations (r = -0.65, p less than 0.01). In the DIG group, RBC-Na+ concentrations tended to increase in proportion to serum digoxin levels (r = 0.53, p less than 0.05). These results support the view that RBC-Na+ concentrations are determined primarily by Na+/K+-pump activity of red cell membranes. This study showed also that Na+ NMR is an useful method determining intracellular Na+ concentrations.

Digoxin--a redundant drug in congestive cardiac failure

Controversy continues concerning the use of digoxin as a positive inotropic agent in the treatment of heart failure in patients in sinus rhythm. Digoxin is properly used to control the heart rate in patients in atrial fibrillation. The findings from 14 uncontrolled and 6 controlled clinical trials have been examined. Digoxin does exert a small chronic positive inotropic effect. Although some individual patients, particularly those with fluid overload, appear to benefit from digoxin, controlled clinical trials in patients, most of whom have been treated with diuretics, have failed to demonstrate an increase of exercise capacity. No mortality trial has been attempted. Digoxin has the potential to be harmful in patients with ischemic heart disease. Alternative and safer therapies have been shown to be equal or superior to digoxin.

Erythrocyte 3H-ouabain binding and digitalis treatment in ethanol addicted patients

The binding of 3H-ouabain to human erythrocytes was analyzed in a population of hospitalized male ethanol addicted patients under long term digitalis treatment. In the non-alcoholic patient group the long term digitalis treatment induced an increase in Bmax and Kd values; such modification was not observed in the alcoholic patients. Chronic alcohol intake itself induced an increase in 3H-ouabain kinetic parameters. These observations confirm that ouabain binding to human erythrocytes is subject to pharmacological and toxicological regulation and that adaptive changes in peripheral tissues can be useful in predicting possible parallel modifications in other less accessible tissues.

Combined use of an automated sample processor and a polymer-based high-performance liquid chromatographic column to determine the pharmacokinetics of labetalol in man

An improved high-performance liquid chromatographic (HPLC) assay has been developed for the analysis of labetalol in human plasma. The method is based on the combined use of an automated sample processor, reversed-phase analysis on a microparticulate polymer-based HPLC column and fluorescence detection. The pH stability of the polymeric column packing material allowed the use of a mobile phase adjusted to pH 9.5, which was optimal for the fluorescence of labetalol. Assay validation was undertaken over the labetalol concentration range 2-100 ng/ml. Calibration curves were essentially linear, and the mean coefficient of variation was 5.3%. The assay has been used for the analysis of clinical samples in support of pharmacokinetic studies.

Red blood cell Na+,K+-ATPase in men with newly diagnosed or previously treated essential hypertension

Alterations of cellular function of Na+,K+-adenosine triphosphatase (ATPase; Na+-K+ pump) have been implicated in the pathophysiology of essential hypertension. Therefore, this aspect of red blood cell (RBC) Na metabolism was studied in black men with newly diagnosed, untreated essential hypertension (NEH) and a normotensive control group. RBC Na content, Na+-K+ pump number (ouabain binding sites), and pump activity were measured. No statistically significant differences were found between the two groups for any of these three parameters. However, a group of previously treated essential hypertensive subjects (PEH) who had been withdrawn from therapy in the preceding 6 weeks were also studied. This group differed significantly from the NEH subjects in regard to all RBC Na+-K+ pump parameters. Their RBC Na content (10.27 +/- 3.23 vs 7.77 +/- 2.52 mmol Na/LRBC; p = 0.006) was higher, and their Na+-K+ pump activity (166 +/- 50 vs 221 +/- 87 nmol inorganic phosphate/mg membrane protein/hr; p = 0.03) and Na+-K+ pump number (213 +/- 40 vs 284 +/- 85 binding sites/RBC; p = 0.001) were lower compared with those in NEH subjects. Although the PEH subjects were older and somewhat less hypertensive than their NEH counterparts, these factors were not found to influence the Na+-K+ pump parameters. These results indicate that chronic diuretic therapy of patients with essential hypertension is associated with a reduced number of RBC Na+-K+ pumps. Since RBCs are not considered target cells for diuretics, the effects of these drugs on RBC electrolyte metabolism may occur at the time of erythropoiesis by the production of RBCs with fewer Na+-K+ pumps.(ABSTRACT TRUNCATED AT 250 WORDS)

The red blood cell: a model for ouabain receptor regulation in the heart?

The assumption that the red blood cell can be used as a model for ouabain receptor regulation in heart muscle has been tested using isolated tissues from humans, guinea pigs, and chickens. The following results were obtained: The affinity of the ouabain receptor was similar in both human erythrocytes and right atrial appendage, but the density of binding sites was much lower on the erythrocytes. There was no correlation between the binding capacity in both tissues. Ouabain receptor occupation was closely correlated with inhibition of Na+/K+-transport in human erythrocytes and chick heart nonmuscle cells in culture. In contrast, in chick heart muscle cells, an occupation of 40% of the receptors decreased the Na+/K+-transport rate by only 10%. In hypokalemia, the ouabain binding capacity was increased in human and guinea pig erythrocytes but not in guinea pig heart muscle. Such increases were seen in chick heart nonmuscle cells in moderate hypokalemia but in heart muscle cells only after severe hypokalemia. Incubation of chick heart muscle cells in toxic but not in "therapeutic" ouabain concentrations increased the number of ouabain receptors. Increases in receptor number attenuated the positive inotropic and toxic actions of ouabain. These variations between ouabain receptor regulation in red blood cells and heart muscle of several species may be attributable to the lack of a "sodium pump reserve" in erythrocytes and heart nonmuscle cells. Such variations indicate that the human erythrocyte is not a suitable model for the ouabain receptor in the human heart.

The effects of serum, lithium, ethacrynic acid, and a low external concentration of potassium on specific [3H]-ouabain binding to human lymphocytes after incubation for 3 days

We have quantified specific [3H]-ouabain binding sites in normal human lymphocytes, and have measured the changes in the numbers of those sites which occur in response to various stimuli. We have confirmed previous findings that incubation for 72 h in the presence of fetal calf serum causes an increase in [3H]-ouabain binding, and that this does not occur if the cells are incubated in fetal calf serum which has first been dialysed. During incubation of the lymphocytes for 3 days in the presence of dialysed fetal calf serum each of the following stimuli caused an increase in specific [3H]-ouabain binding: addition of ethacrynic acid (1 mumol l-1), addition of lithium (1 mmol l-1), and reduction of the external potassium concentration (to 0.75 mmol l-1). By analogy with the similar results in HeLa cells reported by others, we suggest that the increase in [3H]-ouabain binding may, in the case of ethacrynic acid and the reduction of the external potassium concentration, be initiated by an increase in the intracellular sodium concentration. The mechanisms whereby fetal calf serum and lithium cause an increase in [3H]-ouabain binding are not clear.

In vivo cation transport during short-term and long-term digoxin therapy

We have studied the effects of digoxin on cation transport in vivo by measuring the changes in plasma and red cell rubidium concentrations following an oral load of rubidium chloride. In eight patients who had been taking digoxin for 7 to 10 days (mean plasma digoxin concentration 1.3 ng ml-1) the rise in plasma rubidium concentrations was enhanced and the rise in red cell rubidium concentrations was attenuated following the oral load of rubidium chloride, by comparison with the changes in well-matched controls. In contrast, the disposition of rubidium was not altered in 12 patients who had been taking digoxin for more than 3 months (mean plasma digoxin concentration 1.1 ng ml-1). These results suggest that the inhibitory effects of digoxin on cation transport are detectable in vivo during short-term therapy, but not during long-term therapy, and confirm our previous in vitro findings.

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.

Digitalis receptors and digoxin sensitivity in renal failure

The number and the in vitro and in vivo affinity of digitalis receptors for digoxin were measured in patients with normal renal function and in uraemics. In uraemic patients the number of receptors was decreased. Although their in vitro affinity for digoxin was unchanged the in vivo affinity was decreased, probably due to hyperkalaemia. These findings provide a possible pharmacodynamic explanation for decreased activity of cardiac glycosides in chronic renal failure.

William Withering and digitalis, 1785 to 1985

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Erythrocytic cation transport receptor numbers and activity in pregnancies complicated by essential hypertension and pre-eclampsia

Various functions of erythrocytic cation transport were studied in normotensive and hypertensive pregnancy (women with pre-eclampsia and essential hypertension). The results showed that in pregnancy there is an increase in the number of erythrocytic glycoside binding sites accompanied by a proportional increase in the active inward transport of rubidium (used as a substitute for potassium). There was no evidence of an effect of pregnancy on intraerythrocytic sodium concentrations. These changes were apparently entirely attributable to pregnancy and not affected by pre-eclampsia or essential hypertension. It is suggested that these alterations indicate an adaptive increase in sodium pump numbers and activity secondary to a tendency for the intraerythrocytic sodium concentration to rise during pregnancy and compensating for that tendency.

Digitalis 'receptors' during chronic digoxin treatment

The effect of digoxin treatment on Mg-dependent [Na+-K+]-ATPase (the receptor for cardiac glycosides) was assessed by comparison of intracellular Na+-K+, 86Rb uptake and number of digoxin binding sites in the erythrocytes of 138 patients on long term digoxin and of 133 control subjects. The parameters were also assessed in thirty-two patients followed longitudinally for 1 y. The results indicate that the cells adapt to chronic exposure to 'therapeutic' dosage of digoxin with an overcompensatory synthesis of new receptors, a possible mechanism through which the normal intraerythrocytic ionic equilibrium is re-established. The process of synthesis of new receptors appears to be completed in erythrocytes in a period of 4 months after the start of digoxin treatment.

Influence of digitalis and diuretics on ouabain binding sites on human erythrocytes

It has been reported that during chronic treatment with digitalis, the number of digitalis binding sites is increased in human erythrocytes [22]. From this finding a tachyphylaxis for cardiac glycosides has been postulated. We reinvestigated this problem in several groups of patients. The number of 3H-ouabain binding sites per erythrocyte in control persons (group I) was 214 +/- 60, n = 43 (means +/- SD). The dissociation constant (KD) was 1.8 +/- 0.5 nM. Thirteen patients (group II) taking cardiac glycosides only, for at least 6 months, had 281 +/- 99 (p less than 0.05) ouabain binding sites per single red cell, KD = 1.8 +/- 0.7 nM. Group III (34 patients) took digitalis for more than 6 months and diuretics for at least 3 months (352 +/- 126 (p less than 0.001), KD = 1.6 +/- 0.6). Twenty-three of these (group IV) were taking a combination with "K+-saving" diuretics (336 +/- 194 (p less than 0.01), KD = 1.6 +/- 0.5) and (group V, 11 patients) a combination with "K+-losing" diuretics (462 +/- 133 (p less than 0.001), KD = 1.4 +/- 0.4). Nine patients (group VI) had a chronic hypokalemia, mainly due to taking furosemide (437 +/- 98 (p less than 0.001), KD = 1.5 +/- 0.4). Four control persons took 50 mg hydrochlorothiazide daily for more than 4 months without measurable K+-losses and without changes in ouabain binding sites. It is concluded from these findings that diuretic treatment with chronic hypokalemia in addition to digitalis is accompanied by a significant increase in ouabain binding sites in human red cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac glycosides and sodium/potassium-ATPase

The sodium/potassium-ATPase complex is, according to modern research, the binding site for cardiac glycosides on the outer surface of the cell membrane and their receptor. Inhibition of this enzyme by cardiac glycosides leads for instance in the heart to a decrease or a delay in membrane sodium/potassium-ion transport, and indirectly to an increase in the intracellular ionized calcium-concentration and an increase in cardiac contractile force. According to recent observations the activity of the sodium/potassium-ATPase or its concentration, and therefore the concentration of binding sites can increase in some tissues after long term treatment with cardiac glycosides. This might explain the occasionally observed tolerance to digitalis glycosides.

The role of digitalis in the future

Cardiac glycosides exert an acute positive inotropic effect on the normal and failing heart. Recent evidence establishes that the positive inotropic effect is maintained over several months in many patients. The effectiveness of long-term treatment with cardiac glycosides in relieving symptoms is less certain; only a small subset of patients benefits. An effect on mortality is not established. The use of digoxin in the treatment of mild heart failure is questionable since the drug has serious side-effects and the efficacy in patients already taking diuretics has not been established. The use of glycosides in the treatment of severe chronic heart failure is being challenged because of the availability of powerful diuretics, new vasodilators and alternative positive inotropes.

Changes in red blood cell electrolyte concentrations in digitalis intoxication

The value of red blood cell (RBC) sodium/potassium ratio in diagnosing digitalis toxicity was studied in 60 adult patients. The normal ratio was established in 34 healthy volunteers and in 10 patients with heart disease not receiving digoxin (group I). During chronic digoxin therapy, RBC sodium/potassium (Na/K) ratio and plasma digoxin were measured in 50 nontoxic patients (group II), in 10 toxic patients (group III), and in 9 of these 10 toxic patients after resolution of digoxin toxicity. Red cell sodium and RBC Na/K ratio in nontoxic patients were significantly greater than in the control group. Red cell sodium and the RBC Na/K ratio in toxic patients were significantly greater than in nontoxic patients. Despite significant group differences in these variables, however, there was considerable overlap among the subjects studied. After resolution of toxicity, red cell sodium and the RBC Na/K ratio fell to values indistinguishable from those in the nontoxic group. Although there was a statistically significant positive correlation between RBC Na/K ratio and plasma digoxin level, the RBC Na/K ratio had weak predictive value (33%) in determining digoxin toxicity. Plasma digoxin was a better predictor of digoxin toxicity (60%). The combination of plasma digoxin concentration and of RBC Na/K ratio had a higher calculated predictive value (75%), and may be more useful as an indicator of digoxin intoxication.

Changes in erythrocyte sodium, sodium transport and 3H ouabain binding capacity during digoxin administration in the pig

Time course of the changes in erythrocyte sodium content, sodium transport, 3H ouabain binding capacity and Na+, K+-ATPase activity were measured for 14 weeks, in 6 young pigs treated with digoxin and in 6 control pigs. After one week of treatment the erythrocyte sodium content increased from 5.4 mmol/kg cells to 6.9 mmol/kg cells and the efflux rate constant of sodium decreased. With prolonged treatment the erythrocyte sodium content returned to normal and the 3H ouabain binding capacity increased by week 5. The plasma digoxin concentration decreased from 1.1 ng/ml at week 5 to 0.6 ng/ml at week 8 probably due to the decline in dose (microgram/kg) of digoxin with age. The efflux rate constant of sodium and Na+, K+-ATPase activity were higher towards the end of treatment. It is concluded that with prolonged administration of digoxin there is an increase in erythrocyte sodium pump units.

Erythrocyte Na+, K+-ATPase and serum digoxin concentrations

Digoxin therapy has been made more rational by the measurement of serum digoxin concentrations. However, difficulties remain because of the overlap between "therapeutic" and "toxic" serum concentrations and the lack of an obvious therapeutic endpoint in many patients. An assay which measures the degree of interaction between digoxin and its putative receptor, the membrane Na+, K+-ATPase, might be capable of resolving some of these difficulties. Therefore, as a first approach in this direction we evaluated the relationship between serum digoxin concentration and the degree of inhibition of RBC ghost Na+, K+-ATPase activity in patients receiving digoxin therapy. Utilizing an improved micro-assay technique, Na+, K+-ATPase activity was determined in aliquots of RBC ghosts before and after removal of bound digoxin. In 27 patients a significant relationship was present between serum digoxin concentration and the degree of RBC ghost Na+, K+-ATPase inhibition. However, at any serum digoxin concentration, there was a variation in the magnitude of enzyme inhibition from patient to patient. This study confirms the feasibility of determining the degree of in vivo RBC Na+, K+-ATPase inhibition in man and demonstrates, for the first time, a highly significant but somewhat variable relationship between serum digoxin concentrations and the magnitude of RBC digoxin receptor inactivation. This quantitative, functional, individualized assay of digoxin effects may prove to be of clinical value in the future.

The long-term effect of verapamil on plasma digoxin concentration and renal digoxin clearance in healthy subjects

Single-dose investigations in healthy subjects have demonstrated substantial impairment of renal and extrarenal clearance of digoxin during coadministration of verapamil. A longitudinal study has been performed to assess the changes in digoxin disposition during long-term verapamil therapy. After one week of verapamil 240 mg/d mean plasma digoxin had risen from 0.21 +/- 0.01 ng/ml (SE) to 0.34 +/- 0.01 ng/ml(p less than 0.01), and renal digoxin clearance had fallen from 197.57 +/- 17.37 ml/min to 128.20 +/- 10.33 ml/min (p less than 0.001). These changes gradually subsided, and after six weeks, renal digoxin clearance had normalized and plasma digoxin had declined to 0.27 +/0 0.02 ng/ml (NS). The 24-h urinary recovery of digoxin increased from 46.46 +/- 3.23% before to 69.78 +/- 3.69% (p less than 0.001) after six weeks of verapamil co-administration, and this elevation persisted throughout the study. The verapamil-induced suppression of renal digoxin elimination disappears over a few weeks of drug exposure, whereas the inhibition of the extrarenal clearance of digoxin seems to persist.

Techniques for studying the pharmacodynamic effects of cardiac glycosides on patients' own erythrocytes during glycoside therapy

We have measured the effects of digoxin on the cation transport mechanisms of patients' erythrocytes during treatment with digoxin for atrial fibrillation and cardiac failure in sinus rhythm. The results show that during short-term treatment with digoxin there is occupation of erythrocytic cardiac glycoside receptors by digoxin with resultant inhibition of active cation transport. These effects correlate well with the patients' clinical responses to treatment. During long-term treatment, however, these effects are not seen, suggesting that there is pharmacological tolerance to the effects of digoxin. The clinical implications of these results are discussed.

The early and late effects of digoxin treatment on the sodium transport, sodium content and Na+K+- ATPase or erythrocytes

1 Erythrocyte sodium content, sodium transport (ouabain sensitive sodium flux Eos, and ouabain sensitive efflux rate constant ERCos) sodium, potassium activated ouabain sensitive adenosine triphosphatase (Na+K+ATPase) and plasma digoxin were measured in patients during acute digitalisation and in patients who were on long-term digoxin treatment. 2 In the six patients who were studied during digitalisation, the ERCos and Na+K+ATPase activity decreased and erythrocyte sodium content increased during days 2-4 treatment, but there was no change in Eos. 3 In 39 patients on long term digoxin therapy (2-119 months) the erythrocyte sodium content was normal, but the erythrocyte Na+K+ATPase activity was higher than the control group. When the results from these 39 patients were divided according to the duration of treatment it was found that the erythrocyte sodium content was higher in patients treated for 2-4 months than in patients treated for longer periods and the erythrocyte Na+K+ATPase activity increased with duration of treatment. In eight patients (duration of treatment greater than 29 months) in whom ERCos and Eos were measured, ERCos and Eos were higher than the control group. 4 The results suggest that the effects of digoxin on erythrocytes which occur during acute digoxin treatment do not persist in the long term. 5 The possible explanation for the higher ERCos, Eos and Na+K+ATPase activity in patients treated with digoxin for more than 2 months is discussed.

The characteristics of the binding of 12-alpha-[3H]-digoxin to the membranes of intact human erythrocytes: relevance to digoxin therapy

1 The characteristics of the binding of 12-alpha-[3H]-digoxin to the membranes of intact human erythrocytes are described. 2 Only one class of binding site can be demonstrated. Binding is time- and temperature-dependent, saturable and slowly reversible; it is inhibited by other cardiac glycosides and by potassium. 3 Pre-incubation with unlabelled digoxin reduces the subsequent binding of 12-alpha-[3H]-digoxin in stoichiometric fashion. 4 The possible application of the measurement of the binding of 12-alpha-[3H]-digoxin to the study of biochemical pharmacological events occurring during digoxin therapy is discussed.