Skeletal muscle digoxin concentration and its relation to serum digoxin concentration and cardiac effect in healthy man

Acute oral digoxin in healthy adults hastens fatigue and increases plasma K+ during intense exercise, despite preserved skeletal muscle Na+,K+-ATPase

We investigated acute effects of the Na+,K+-ATPase (NKA) inhibitor, digoxin, on muscle NKA content and isoforms, arterial plasma [K+] ([K+]a) and fatigue with intense exercise. In a randomised, crossover, double-blind design, 10 healthy adults ingested 0.50 mg digoxin (DIG) or placebo (CON) 60 min before cycling for 1 min at 60%V ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}{\mathrm{peak}}}}$ then at 95%V ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_{\mathrm{2}}}{\mathrm{peak}}}}$ until fatigue. Pre- and post-exercise muscle biopsies were analysed for [3H]-ouabain binding site content without (OB-Fab) and after incubation in digoxin antibody (OB+Fab) and NKA α1-2 and β1-2 isoform proteins. In DIG, pre-exercise serum [digoxin] reached 3.36 (0.80) nM [mean (SD)] and muscle NKA-digoxin occupancy was 8.2%. Muscle OB-Fab did not differ between trials, whereas OB+Fab was higher in DIG than CON (8.1%, treatment main effect, P = 0.001), whilst muscle NKA α1-2 and β1-2 abundances were unchanged by digoxin. Fatigue occurred earlier in DIG than CON [-7.7%, 2.90 (0.77) vs. 3.14 (0.86) min, respectively; P = 0.037]. [K+]a increased during exercise until 1 min post-exercise (P = 0.001), and fell below baseline at 3-10 (P = 0.001) and 20 min post-exercise (P = 0.022, time main effect). In DIG, [K+]a (P = 0.035, treatment effect) and [K+]a rise pre-fatigue were greater [1.64 (0.73) vs. 1.55 (0.73), P = 0.016], with lesser post-exercise [K+]a decline than CON [-2.55 (0.71) vs. -2.74 (0.62) mM, respectively, P = 0.003]. Preserved muscle OB-Fab with digoxin, yet increased OB+Fab with unchanged NKA isoforms, suggests a rapid regulatory assembly of existing NKA α and β subunits exists to preserve muscle NKA capacity. Nonetheless, functional protection against digoxin was incomplete, with earlier fatigue and perturbed [K+]a with exercise. KEY POINTS: Intense exercise causes marked potassium (K+) shifts out of contracting muscle cells, which may contribute to muscle fatigue. Muscle and systemic K+ perturbations with exercise are largely regulated by increased activity of Na+,K+-ATPase in muscle, which can be specifically inhibited by the cardiac glycoside, digoxin. We found that acute oral digoxin in healthy adults reduced time to fatigue during intense exercise, elevated the rise in arterial plasma K+ concentration during exercise and slowed K+ concentration decline post-exercise. Muscle functional Na+,K+-ATPase content was not reduced by acute digoxin, despite an 8.2% digoxin occupancy, and was unchanged at fatigue. Muscle Na+,K+-ATPase isoform protein abundances were unchanged by digoxin or fatigue. These suggest possible rapid assembly of existing subunits into functional pumps. Thus, acute digoxin impaired performance and exacerbated plasma K+ disturbances with intense, fatiguing exercise in healthy participants. These occurred despite the preservation of functional Na+,K+-ATPase in muscle.

Digoxin and exercise effects on skeletal muscle Na+,K+-ATPase isoform gene expression in healthy humans

In muscle, digoxin inhibits Na+,K+-ATPase (NKA) whereas acute exercise can increase NKA gene expression, consistent with training-induced increased NKA content. We investigated whether oral digoxin increased NKA isoform mRNA expression (qPCR) in muscle at rest, during and post-exercise in 10 healthy adults, who received digoxin (DIG, 0.25 mg per day) or placebo (CON) for 14 days, in a randomised, double-blind and cross-over design. Muscle was biopsied at rest, after cycling 20 min (10 min each at 33%, then 67%V ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_2}{\mathrm{peak}}}}$ ), then to fatigue at 90%V ̇ O 2 peak ${{\dot{V}}_{{{{\mathrm{O}}}_2}{\mathrm{peak}}}}$ and 3 h post-exercise. No differences were found between DIG and CON for NKA α1-3 or β1-3 isoform mRNA. Both α1 (354%, P = 0.001) and β3 mRNA (P = 0.008) were increased 3 h post-exercise, with α2 and β1-2 mRNA unchanged, whilst α3 mRNA declined at fatigue (-43%, P = 0.045). In resting muscle, total β mRNA (∑(β1+β2+β3)) increased in DIG (60%, P = 0.025) and also when transcripts for each isoform were normalised to CON then either summed (P = 0.030) or pooled (n = 30, P = 0.034). In contrast, total α mRNA (∑(α1+α2+α3), P = 0.348), normalised then summed (P = 0.332), or pooled transcripts (n = 30, P = 0.717) did not differ with DIG. At rest, NKA α1-2 and β1-2 protein abundances were unchanged by DIG. Post-exercise, α1 and β1-2 proteins were unchanged, but α2 declined at 3 h (19%, P = 0.020). In conclusion, digoxin did not modify gene expression of individual NKA isoforms at rest or with exercise, indicating NKA gene expression was maintained consistent with protein abundances. However, elevated resting muscle total β mRNA with digoxin suggests a possible underlying β gene-stimulatory effect. HIGHLIGHTS: What is the central question of this study? Na+,K+-ATPase (NKA) in muscle is important for Na+/K+ homeostasis. We investigated whether the NKA-inhibitor digoxin stimulates increased NKA gene expression in muscle and exacerbates NKA gene responses to exercise in healthy adults. What is the main finding and its importance? Digoxin did not modify exercise effects on muscle NKA α1-3 and β1-3 gene transcripts, which comprised increased post-exercise α1 and β3 mRNA and reduced α3 mRNA during exercise. However, in resting muscle, digoxin increased NKA total β isoform mRNA expression. Despite inhibitory-digoxin or acute exercise stressors, NKA gene regulation in muscle is consistent with the maintenance of NKA protein contents.

Oral digoxin effects on exercise performance, K+ regulation and skeletal muscle Na+ ,K+ -ATPase in healthy humans

Abstract

We investigated whether digoxin lowered muscle Na⁺,K⁺‐ATPase (NKA), impaired muscle performance and exacerbated exercise K⁺ disturbances. Ten healthy adults ingested digoxin (0.25 mg; DIG) or placebo (CON) for 14 days and performed quadriceps strength and fatiguability, finger flexion (FF, 105%_{peak‐workrate}, 3 × 1 min, fourth bout to fatigue) and leg cycling (LC, 10 min at 33% VO2peak and 67% VO2peak, 90% VO2peak to fatigue) trials using a double‐blind, crossover, randomised, counter‐balanced design. Arterial (a) and antecubital venous (v) blood was sampled (FF, LC) and muscle biopsied (LC, rest, 67% VO2peak, fatigue, 3 h after exercise). In DIG, in resting muscle, [³H]‐ouabain binding site content (OB‐F_ab) was unchanged; however, bound‐digoxin removal with Digibind revealed total ouabain binding (OB+F_ab) increased (8.2%, P = 0.047), indicating 7.6% NKA–digoxin occupancy. Quadriceps muscle strength declined in DIG (−4.3%, P = 0.010) but fatiguability was unchanged. During LC, in DIG (main effects), time to fatigue and [K⁺]_a were unchanged, whilst [K⁺]_v was lower (P = 0.042) and [K⁺]_a‐v greater (P = 0.004) than in CON; with exercise (main effects), muscle OB‐F_ab was increased at 67% VO2peak (per wet‐weight, P = 0.005; per protein P = 0.001) and at fatigue (per protein, P = 0.003), whilst [K⁺]_a, [K⁺]_v and [K⁺]_a‐v were each increased at fatigue (P = 0.001). During FF, in DIG (main effects), time to fatigue, [K⁺]_a, [K⁺]_v and [K⁺]_a‐v were unchanged; with exercise (main effects), plasma [K⁺]_a, [K⁺]_v, [K⁺]_a‐v and muscle K⁺ efflux were all increased at fatigue (P = 0.001). Thus, muscle strength declined, but functional muscle NKA content was preserved during DIG, despite elevated plasma digoxin and muscle NKA–digoxin occupancy, with K⁺ disturbances and fatiguability unchanged.

Key points

The Na⁺,K⁺‐ATPase (NKA) is vital in regulating skeletal muscle extracellular potassium concentration ([K⁺]), excitability and plasma [K⁺] and thereby also in modulating fatigue during intense contractions.

NKA is inhibited by digoxin, which in cardiac patients lowers muscle functional NKA content ([³H]‐ouabain binding) and exacerbates K⁺ disturbances during exercise.

In healthy adults, we found that digoxin at clinical levels surprisingly did not reduce functional muscle NKA content, whilst digoxin removal by Digibind antibody revealed an ∼8% increased muscle total NKA content.

Accordingly, digoxin did not exacerbate arterial plasma [K⁺] disturbances or worsen fatigue during intense exercise, although quadriceps muscle strength was reduced.

Thus, digoxin treatment in healthy participants elevated serum digoxin, but muscle functional NKA content was preserved, whilst K⁺ disturbances and fatigue with intense exercise were unchanged. This resilience to digoxin NKA inhibition is consistent with the importance of NKA in preserving K⁺ regulation and muscle function.

A population pharmacokinetic analysis of the influence of nutritional status of digoxin in hospitalized Korean patients

BACKGROUND

Safe and effective use of digoxin in hospitalized populations requires information about the drug's pharmacokinetics and the influence of various factors on drug disposition. However, no attempts have been made to link an individual's digoxin requirements with nutritional status.

OBJECTIVES

The main goal of this study was to estimate the population pharmacokinetics of digoxin and to identify the nutritional status that explains pharmacokinetic variability in hospitalized Korean patients.

METHODS

Routine therapeutic drug-monitoring data from 106 patients who received oral digoxin at Seoul National University Bundang Hospital were retrospectively collected. The pharmacokinetics of digoxin were analyzed with a 1-compartment, open-label pharmacokinetic model by using a nonlinear mixed-effects modeling tool (NONMEM) and a multiple trough screening approach.

RESULTS

The effect of demographic characteristics and biochemical and nutritional indices were explored. Estimates generated by using NONMEM indicated that the CL/F of digoxin was influenced by renal function, serum potassium, age, and percentage of ideal body weight (PIBW). These influences could be modeled by following the equation CL/F (L/h) = 1.36 × (creatinine clearance/50)(1.580) × K(0.835) × 0.055 × (age/65) × (PIBW/100)(0.403). The interindividual %CV for CL/F was 34.3%, and the residual variability (SD) between observed and predicted concentrations was 0.225 μg/L. The median estimates from a bootstrap procedure were comparable and within 5% of the estimates from NONMEM. Correlation analysis with the validation group showed a linear correlation between observed and predicted values.

CONCLUSIONS

The use of this model in routine therapeutic drug monitoring requires that certain conditions be met which are consistent with the conditions of the subpopulations in the present study. Therefore, further studies are needed to clarify the effects of nutritional status on digoxin pharmacokinetics. The present study established important sources of variability in digoxin pharmacokinetics and highlighted the relationship between pharmacokinetic parameters and nutritional status in hospitalized Korean patients.

Effect of autonomic blockade on digoxin-induced ECG changes at rest and during exercise in healthy subjects

The effect of pharmacological autonomic blockade on digoxin-induced electrocardiographic changes at rest and during exercise was studied in nine healthy men. The subjects performed bicycle exercise tests on four occasions after: (1) intravenous injection of sodium chloride, (2) intravenous injection of propranolol and atropine, (3) oral ingestion of digoxin 0.5 mg daily for 2 weeks and intravenous injection of sodium chloride, and (4) oral ingestion of digoxin 0.5 mg daily for 2 weeks and intravenous injection of propranolol and atropine. The ST-T segment was significantly depressed after digoxin plus sodium chloride and digoxin plus propranolol and atropine in comparison with sodium chloride only and propranolol and atropine only. The digoxin-induced ST-T changes after sodium chloride and propranolol-atropine injections were not significantly different. Thus, judging from the results of this study, an effect mediated via the autonomic nervous system is an unlikely explanation of digitalis-induced ST-T changes.

Effect of salbutamol on digoxin pharmacokinetics

A single dose of the beta 2-adrenoceptor agonist salbutamol has previously been shown to decrease serum digoxin concentration in healthy volunteers. A possible explanation of the phenomenon is a beta 2-adrenoceptor-mediated increase in the specific binding of digoxin to skeletal muscle. The present study was undertaken to further elucidate the effect of salbutamol on the pharmacokinetics of digoxin in man. Nine volunteers were studied on two occasions during salbutamol or placebo treatment. On test days salbutamol, 4 micrograms.kg-1.h-1 or saline was infused for 10 h, preceded and followed by four and three days, respectively, of oral administration. A single i.v. injection of digoxin 15 micrograms.kg-1, was given 20 min after starting the infusion. At the end of the infusion a muscle biopsy was taken from the vastus lateralis. Blood samples for the analysis of serum digoxin and potassium were repeatedly taken over 72 h. Urine was collected over a period of 24 h for determination of the renal excretion of digoxin and potassium. The serum digoxin concentration, expressed as the AUC 0-6 h was 15% lower during salbutamol infusion than during saline infusion. Salbutamol caused significantly faster elimination of digoxin from the central volume of distribution to deeper compartments. Salbutamol had no effect on the renal clearance of digoxin. The skeletal muscle digoxin concentration tended to be higher (48%) during salbutamol compared to placebo treatment. The serum potassium concentration was significantly lower after salbutamol compared to placebo, as was the rate of renal excretion of potassium.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle binding and renal excretion of digoxin in man

Ten healthy subjects were treated with three or four different doses of digoxin, 0.25 to 0.88 mg daily, in random order. Digoxin concentrations in serum (SDC) and skeletal muscle (SMDC) were determined as well as its renal clearance after 2 weeks of treatment with each dose. The mean ratio SMDC/SDC decreased non-significantly by about 20% from 33 +/- 15 at the lowest SDC interval (0.5-0.9 nmol/l) to 28 +/- 7 at the highest concentration interval (2.0-2.4 nmol/l). This is in accordance with findings from previous studies. No significant change was observed in the renal clearance of digoxin with increasing digoxin concentration, supporting the concept of first-order renal elimination of digoxin.

Binding of digoxin to slow- and fast-twitch skeletal muscle fibres

We have found previously great interindividual variations in the binding of digoxin to skeletal muscle even after standardized rest. The present study was performed in order to find out if there is a difference in the binding of digoxin to slow- and fast-twitch fibres in man at rest and after moderate exercise. Seven healthy digitalized subjects (digoxin 0.50 mg/day) were investigated after 90 min of supine rest and after a 1 h moderate bicycle exercise. Muscle biopsy specimens were taken immediately before and 5 min after exercise and dissected under a microscope to single fibres. After histochemical typing of all fibres the digoxin content in slow- and fast-twitch fibres was measured separately. At rest, digoxin binding to slow-twitch fibres was 33% higher than to fast-twitch fibres (P less than 0.01). During exercise the digoxin binding increased by 28% in slow-twitch fibres but was unchanged in fast-twitch fibres. The difference in digoxin binding to the two fibre types may explain, at least partly, the interindividual variations in the binding of digoxin to skeletal muscle.

Glucose uptake and binding of digoxin to skeletal muscle

Physical exercise induces increased uptake of both digoxin and glucose in exercising skeletal muscle. Glucose uptake could be a regulatory factor for the digoxin binding to skeletal muscle, since in dogs, insulin and glucose infusion have been reported to increase the uptake of digoxin in muscle. In the present study on eight healthy digitalized subjects (0.5 mg digoxin daily) the uptake of glucose in skeletal muscle was achieved by infusion of 6 mg/kg body weight/min glucose, 0.004 IE/kg body weight/min insulin and 300 micrograms/h somatostatin. Serum and skeletal muscle digoxin levels were analysed before and during the infusion. We found no changes in the digoxin levels in serum and skeletal muscle in spite of an increased uptake of glucose in the muscle. Thus, glucose uptake in skeletal muscle is probably not an important regulatory factor for the change in muscle digoxin binding induced by exercise.

Beta-blockade and binding of digoxin to skeletal muscle

The effect of beta-blockade and a 1-h bicycle exercise test on the digoxin concentration in skeletal muscle (thigh) and serum was studied in 10 healthy men, who had ingested 0.5 mg digoxin daily for 2 weeks. Each subject performed two exercise tests at 100-140 W during maintenance digoxin treatment and 24 h after the latest dose. They rested in the supine position for 2.5 h before the exercise. Sixty minutes before the start of the exercise 0.25 mg/kg b.w. propranolol or saline (control) were injected (single-blind). At the end of the exercise the mean heart rate was 30% lower with beta-blockade (P less than 0.001). During exercise the mean skeletal muscle digoxin concentration increased by 29% (P less than 0.01) in the control situation and by 12% (NS) with beta-blockade. The results indicate that propranolol partly inhibits the exercise-induced increase in skeletal muscle digoxin binding. This might be due to inhibition of a catecholamine-induced stimulation of Na+-K+ATPase during exercise.

Effect of digoxin on the electrocardiogram at rest and during exercise in healthy subjects

The effect of digoxin on the electrocardiogram at rest and during and after exercise was studied in 11 healthy subjects. Exercise was performed on a heart rate-controlled bicycle ergometer with stepwise increased loads up to a heart rate of 170 beats/min. The subjects were studied after peroral intake of digoxin at 2 dose levels and after withdrawal of digoxin. Administration of digoxin induced significant ST-T depression at rest and during exercise even at the small dose (2.4 +/- 0.8 microgram/kg body weight, mean +/- standard deviation). The ST-T changes were numerically small and dose-dependent. The most pronounced ST and T depression occurred at a heart rate of 110 to 130 beats/min. At higher heart rates the ST depression was less pronounced but still statistically significant. During the first minutes after exercise no significant digitalis-induced ST-T depression was seen. This reaction is not of the type usually seen in myocardial ischemia. Fourteen days after withdrawal of the drug there were no significant digitalis-induced ST-T changes at rest or during or after exercise.

Tissue-serum correlates of digoxin-amiodarone pharmacokinetic interaction in rats: evidence for selective tissue accumulation and reduced tissue binding

The pharmacokinetic interaction between digoxin (1) and amiodarone (2) has drawn increasing attention during recent years, but the tissue correlates of such an interaction are not known. This issue was therefore investigated in three groups of Sprague-Dawley rats. When 1 alone was given (250 micrograms/d), the serum concentration of 1 was 0.88 +/- 0.36 ng/mL; when 1 was combined with 2 (66 mg/kg/d), the level of 1 was 2.62 +/- 1.23 ng/mL (p less than 0.05) and was 5.49 +/- 1.07 ng/mL (p less than 0.05) when the dose of 2 was 132 mg/kg/d. These increases correlated with the serum levels of 2 and the deethyl metabolite 3. The myocardial level of 1 was 29.40 +/- 1.34 ng/g without 2; after a low dose of 2, it was 35.80 +/- 7.52 ng/g (nonsignificant) and, after a high dose, it was 42.80 +/- 7.20 (p less than 0.05). In skeletal muscle, the level of 1 was 22.50 +/- 14.7 ng/g without 2, 41.00 +/- 2.45 ng/g (p less than 0.05) after a low dose, and 77.60 +/- 17.45 ng/g (p less than 0.05) after a high dose. The corresponding values for the brain were 32.20 +/- 5.60 ng/g, 48.20 +/- 7.60 ng/g (p less than 0.05), and 60.90 +/- 11.00 ng/g (p less than 0.05). The tissue-serum ratios for 1 in all three tissues were reduced by 2, suggesting a decrease in the tissue binding of the glycoside. There was less uptake of 1 in the myocardium compared with uptake in the skeletal muscle and brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Quinidine-induced changes in serum and skeletal muscle digoxin concentration; evidence of saturable binding of digoxin to skeletal muscle

Eleven patients with atrial fibrillation on maintenance digoxin therapy were investigated by analysis of serum (SDC) and skeletal muscle (SMDC) digoxin concentrations before and 24 h and 2 weeks after starting quinidine treatment. After cardioversion the maintenance dose of digoxin was reduced in order to obtain the same steady-state SDC after 2 weeks, as before quinidine. SDC was increased by quinidine therapy from 1.56 to 2.40 nmol/l after 24 h. With the reduced digoxin dose SDC was 1.68 nmol/l after 2 weeks. The ratio SMDC/SDC decreased after 24 h of quinidine treatment from 35.4 to 29.0 (p less than 0.01). After 2 weeks of quinidine treatment with the reduced digoxin dose, the ratio had risen to 38.1, which did not differ significantly from the initial ratio. The present data suggest that the reduced skeletal muscle binding of digoxin during quinidine therapy is due to saturation of digoxin binding sites secondary to the increase in the total body load of digoxin at steady-state, and not to direct interference by quinidine with digoxin binding sites.

Physical exercise and binding of digoxin to skeletal muscle--effect of muscle activation frequency

Ten healthy subjects who had ingested 0.5 mg digoxin daily for at least 10 days, performed a 1-hour bicycle exercise test on two occasions, 24 h after the latest dose, with the same work load but at two different pedalling rates, 40 and 80 rpm. During exercise the mean digoxin concentration in the thigh muscle increased by 8% at 40 rpm (n.s.) and by 29% at 80 rpm (p less than 0.01). The serum digoxin concentration decreased by 39% at both pedalling rates (p less than 0.001). The results suggest that the increase in skeletal muscle digoxin concentration during exercise is related to the neuromuscular activation frequency. The digoxin concentration in erythrocytes was measured in 16 healthy subjects before and 1 minute after a 1-hour bicycle exercise test. The erythrocyte digoxin concentration decreased by 12% (p less than 0.01) during the exercise indicating that the increased uptake of digoxin in skeletal muscle during exercise influences the digoxin concentration in other tissues.

Skeletal muscle digoxin binding in patients with renal failure

For digoxin analyses blood and skeletal muscle samples were taken from seven digoxin-treated patients with chronic renal failure. The ratio between skeletal muscle and serum digoxin concentration in the patients with renal failure was not significantly different from the ratios in two control groups consisting of subjects with normal renal function. In the group of patients with renal failure there was no relationship between the glomerular filtration rate and muscle digoxin binding (specific plus unspecific). The present study does not indicate reduced skeletal muscle digoxin binding in patients with chronic renal failure.

Cardiac effects of treatment with quinidine and digoxin, alone and in combination

Systolic time intervals (QS2-I and LVET-I) and echocardiographically determined ejection fraction and velocity of circumferential fiber shortening were recorded in 10 healthy volunteers as measures of inotropic effect during maintenance treatment with 4 consecutive drug regimens: (1) quinidine, 1,200 mg/day; (2) digoxin, average dose 0.31 mg/day; (3) the combination of (1) and (2); and (4) digoxin alone (average dose 0.65 mg/day) to provide the same steady-state serum concentration of digoxin as during the period with combination of digoxin and quinidine. The steady-state serum concentration of digoxin during the low-dose regimen increased from 0.72 +/- 0.15 (mean +/- standard deviation [SD]) to 1.63 +/- 0.28 nmol/liter when quinidine was added. With the high dose of digoxin alone, the serum digoxin level reached 1.68 +/- 0.50 nmol/liter. Skeletal muscle digoxin concentrations during these periods were 27.7 +/- 8.3, 48.7 +/- 16.2, and 51.6 +/- 23.6 nmol/kg of dry weight, respectively. The skeletal muscle to serum concentration ratio of digoxin decreased significantly during quinidine treatment. Systolic time intervals were significantly prolonged by quinidine alone and shortened by digoxin alone, the latter effect being dose-dependent. Subtracting the effect of quinidine itself, the induced increase in digoxin level caused a significant increase in inotropic effect. When these corrected values were compared with those attained during the period with the same steady-state digoxin concentration but in the absence of quinidine, no significant differences were found. Echocardiographically measured ejection fraction and velocity of circumferential fiber shortening showed trends for similar drug effects, as did the systolic time intervals. This study, performed under steady-state conditions, demonstrates that the quinidine-induced increase in steady-state serum digoxin concentration will, with due consideration to quinidine's own pharmacodynamic properties, be accompanied by increased cardiac effects. This indicates that quinidine is not interfering with active receptor sites in the heart for digoxin.

Physical exercise and digoxin binding to skeletal muscle: relation to exercise intensity

The effect of a 1 h bicycle exercise test on digoxin concentration in skeletal muscle (thigh) and serum was studied in 10 healthy men, who had ingested digoxin 0.5 mg daily for 2 weeks. During maintenance digoxin treatment each subject performed 2 exercise tests, at 70-90 W and 140-180 W both 24 h after the last dose, at a 2-7 day interval. During exercise at the lower work load the mean skeletal muscle digoxin concentration increased by 9% (n.s.) and the mean serum digoxin concentration decreased by 26% (p less than 0.001). The high work load induced a mean increase in skeletal muscle digoxin of 20% (p less than 0.05) and a mean decrease in serum digoxin of 40% (p less than 0.001). The results indicate that the increased uptake of digoxin into exercised skeletal muscle and the decrease in serum digoxin during exercise is related to the intensity of the exercise.

Effect of digoxin upon intracellular potassium in man

The effect of digoxin upon intramuscular potassium was studied by use of whole body counting and biopsy technique. Twelve healthy subjects and twelve outpatients with mild cardiac insufficiency or atrial arrhythmia were digitalised. Before and after digitalization total body potassium (TBK) was measured. Potassium concentration in muscle specimens (MK) was analysed by the neutron activation technique. Digoxin was measured in serum and in skeletal muscle tissue by radioimmunoassay, and QS2-index as a measure of the electromechanical systole. In both groups a significant decrease in TBK (P less than 0.05) and MK (P less than 0.01) was demonstrated in connection to digitalization. There was no correlation between the decrease in TBK and MK, or between the concentrations of digoxin in serum or muscle and the decrease in potassium concentration. The digoxin in serum in healthy subjects was 0.9 +/- 0.33 nmol/l and in patients 1.2 +/- 0.41 nmol/l. The digoxin in muscle was 39 +/- 10.9 nmol/kg dry weight in seven of the healthy individuals and 37 +/- 9.5 nmol/kg dry weight in nine patients. After digitalization a decrease of QS2-index was found in both groups (P less than 0.01).

Skeletal muscle digoxin concentration during digitalization and during withdrawal of digoxin treatment

Blood samples and skeletal muscle biopsies (m. quadriceps femoris, vastus lateralis) were taken from 15 patients during digitalization or during withdrawal of digoxin treatment for analysis of serum and skeletal muscle digoxin concentrations. A percutaneous needle biopsy technique was used for muscle sampling and digoxin was analysed by radioimmunoassay. During "slow" digitalization with 0.25 mg digoxin daily the skeletal muscle digoxin concentrations after 2 and 4 days were 45% (range 19%--62%; n = 3) and 78% (range 56%--92%; n= 3) respectively, of the steady state concentration (defined as the digoxin concentration after 25--40 days of treatment). After 9 and 11 days of treatment the skeletal muscle digoxin concentrations were 106% (range 84%--133%; n = 5) and 116% (range 72%--164%; n = 3) respectively, of the steady state concentration. A doubling of the digoxin dose gave a proportional increase in skeletal muscle digoxin concentration (three patients). The magnitude of the estimated half-life of skeletal muscle digoxin was the same as previously reportedly in healthy subjects. No significant correlations were found between changes in systolic time intervals and steady state serum or skeletal muscle digoxin concentrations.