The erythrocyte sodium and potassium in patients treated with digoxin

Short-Term Supplementation of Sodium Nitrate vs. Sodium Chloride Increases Homoarginine Synthesis in Young Men Independent of Exercise

The aim of the study was to investigate the effects of short-term oral administration of inorganic nitrate (NaNO3; n = 8) or placebo (NaCl; n = 9) (each 0.1 mmol/kg body weight/d for 9 days) on plasma amino acids, creatinine, and oxidative stress in healthy young men. At baseline, the plasma concentrations of amino acids did not differ between the groups. At the end of the study, the plasma concentrations of homoarginine (hArg; by 24%, p = 0.0001), citrulline and ornithine (Cit/Orn; by 16%, p = 0.015), and glutamine/glutamate (Gln/Glu; by 6%, p = 0.0003) were higher in the NaNO3 group compared to the NaCl group. The plasma concentrations of sarcosine (Sarc; by 28%, p < 0.0001), tyrosine (by 14%, p = 0.0051), phenylalanine (by 8%, p = 0.0026), and tryptophan (by 8%, p = 0.0047) were lower in the NaNO3 group compared to the NaCl group. These results suggest that nitrate administration affects amino-acid metabolism. The arginine/glycine amidinotransferase (AGAT) catalyzes two reactions: (1) the formation of l-homoarginine (hArg) and l-ornithine (Orn) from l-arginine (Arg) and l-lysine (Lys): Arg + Lys <−> hArg + Orn, with equilibrium constant Kharg; (2) the formation of guanidinoacetate (GAA) and Orn from Arg and glycine (Gly): Arg + Gly <−> GAA + Orn, with equilibrium constant Kgaa. The plasma Kgaa/KhArg ratio was lower in the NaNO3 group compared to the NaCl group (1.57 vs. 2.02, p = 0.0034). Our study suggests that supplementation of inorganic nitrate increases the AGAT-catalyzed synthesis of hArg and decreases the N-methyltransferase-catalyzed synthesis of GAA, the precursor of creatine. To our knowledge, this is the first study to demonstrate elevation of hArg synthesis by inorganic nitrate supplementation. Remarkably, an increase of 24% corresponds to the synthesis capacity of one kidney in healthy humans. Differences in the association between plasma concentrations of amino acids in the NaNO3 and NaCl groups suggest changes in amino-acid homeostasis. Plasma concentrations of the oxidative stress marker malondialdehyde (MDA) did not change after supplementation of NaNO3 or NaCl over the whole exercise time range. Plasma nitrite concentration turned out to be a more discriminant marker of NaNO3 ingestion than plasma nitrate (area under the receiver operating characteristic curve: 0.951 vs. 0.866, p < 0.0001 each).

Intraerythrocytic sodium and potassium concentrations during acute and chronic digitalization

To assess the cellular effects of digoxin, intraerythrocytic sodium and potassium concentrations were measured in 17 patients during the early phase of digitalization, in 45 patients on long-term therapy and in 64 non-digitalized control patients. Acute digitalization raised intraerythrocytic sodium from 11.6 +/- 0.4 to 16.7 +/- 1.0 mmol/l (mean +/- SEM) (p less than 0.01) and reduced intraerythrocytic potassium from 100.1 +/- 1.3 to 95.9 +/- 1.8 mmol/l (p less than 0.01). These changes were strongly correlated with the steady-state plasma digoxin concentration. During a few weeks of digoxin therapy, the intraerythrocytic cation composition normalized gradually. In patients on chronic treatment, neither intraerythrocytic sodium (11.3 +/- 0.3 mmol/l) nor potassium concentrations (100.0 +/- 0.6 mmol/l) differed significantly from the values of the control group (11.4 +/- 0.2 and 99.9 +/- 0.5 mmol/l, respectively). The changes in intraerythrocytic cation concentrations, induced by acute digitalization, seem to disappear during chronic administration of the drug.

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.

Reduction of erythrocyte (Na+-K+)ATPase activity in type 1 (insulin-dependent) diabetic subjects and its activation by homologous plasma

The (Na+-K+)ATPase and (Mg2+)ATPase activities of erythrocyte membranes of Type 1 (insulin-dependent) diabetic patients were found to be significantly reduced compared to matched controls (p less than 0.005). On the contrary, erythrocyte Na+ and K+ contents were similar in diabetic patients and in normal subjects. When erythrocyte membranes from diabetic patients were incubated with their own plasma, a significant increase was observed in sodium-potassium ATPase activity (p less than 0.005), whereas (Mg2+)ATPase activity was not affected. The plasma stimulatory effect showed saturation kinetics. Maximum average stimulation was 96% (+/- 21.3). A similar stimulation pattern, although more limited in extent (maximum 48.3% +/- 12.2), was found when erythrocyte membranes from normal subjects were incubated with diabetic plasma. Normal plasma exhibited a modest stimulatory effect on erythrocyte (Na+-K+)ATPase activity. Similar stimulatory effects by diabetic plasma were observed on a (Na+-K+) ATPase preparation from beef heart. It is proposed that diabetic plasma contains a specific (Na+-K+)ATPase activator in a higher concentration than normal plasma. This may explain why a normal cellular electrolyte content was found in diabetic erythrocytes in spite of a reduced Na+-K+ pump activity. Purification experiments indicate that the plasma activator is a protein with a molecular weight greater than 50,000. Both the (Na+-K+)ATPase activity and the stimulatory effect of diabetic plasma were not influenced by the metabolic control, since they did not correlate significantly with fasting blood glucose and daily insulin dosage. Moreover, no correlation was found with duration of diabetes or age at diagnosis of diabetes.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of oral digoxin on sodium excretion, renin-angiotensin-aldosterone system and blood pressure in normotensive subjects

The effect of digoxin (0.25 mg t.d.s.) given orally for 5 days, on sodium excretion, plasma renin activity, plasma aldosterone and blood pressure has been studied in 6 normotensive subjects, on a constant sodium intake. Average serum digoxin level measured in 5 subjects on the 5th day of treatment was in the upper range of the therapeutic level (1.92 +/- 0.3 nmol/l). Red cell sodium content increased from 9.75 +/- 1.2 mmol/l of cells in the control period to 15.3 +/- 1.7 mmol/l of cells on the 5th day of digoxin treatment when measured in 4 of the 6 subjects. In spite of this evidence of inhibition of the red cell sodium pump, there was no detectable change in sodium excretion, blood pressure, plasma renin activity and aldosterone.

A study of two diuretic/potassium combinations in heart failure

The effect of potassium supplements was studied in 28 patients taking long term frusemide (40-80 mg daily). Plasma potassium fell when supplements were stopped, and rose towards prior values on the potassium/frusemide combination, Diumide K. In a crossover study in 14 of these patients comparing equivalent doses of frusemide, Diumide K (frusemide 40 mg, potassium 8 mmol), bumetanide, and Burinex K (bumetanide 0.5 mg, potassium 7.7 mmol) plasma potassium was lower on frusemide than on bumetanide. On Diumide K and Burinex K plasma potassium rose significantly but did not reach the levels on prior therapy. Small doses of potassium in combined formulations seem to be effective in countering the mild hypokalaemia caused by loop diuretics.

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.

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.