Beta-blockade and binding of digoxin to skeletal muscle

Quantitatively Predicting Effects of Exercise on Pharmacokinetics of Drugs Using a Physiologically Based Pharmacokinetic Model

Exercise significantly alters human physiological functions, such as increasing cardiac output and muscle blood flow and decreasing glomerular filtration rate (GFR) and liver blood flow, thereby altering the absorption, distribution, metabolism, and excretion of drugs. In this study, we aimed to establish a database of human physiological parameters during exercise and to construct equations for the relationship between changes in each physiological parameter and exercise intensity, including cardiac output, organ blood flow (e.g., muscle blood flow and kidney blood flow), oxygen uptake, plasma pH and GFR, etc. The polynomial equation P = ΣaiHRi was used for illustrating the relationship between the physiological parameters (P) and heart rate (HR), which served as an index of exercise intensity. The pharmacokinetics of midazolam, quinidine, digoxin, and lidocaine during exercise were predicted by a whole-body physiologically based pharmacokinetic (WB-PBPK) model and the developed database of physiological parameters following administration to 100 virtual subjects. The WB-PBPK model simulation results showed that most of the observed plasma drug concentrations fell within the 5th-95th percentiles of the simulations, and the estimated peak concentrations (Cmax) and area under the curve (AUC) of drugs were also within 0.5-2.0 folds of observations. Sensitivity analysis showed that exercise intensity, exercise duration, medication time, and alterations in physiological parameters significantly affected drug pharmacokinetics and the net effect depending on drug characteristics and exercise conditions. In conclusion, the pharmacokinetics of drugs during exercise could be quantitatively predicted using the developed WB-PBPK model and database of physiological parameters. SIGNIFICANCE STATEMENT: This study simulated real-time changes of human physiological parameters during exercise in the WB-PBPK model and comprehensively investigated pharmacokinetic changes during exercise following oral and intravenous administration. Furthermore, the factors affecting pharmacokinetics during exercise were also revealed.

Oral salbutamol decreases serum digoxin concentration

The effect of a therapeutic dose of oral salbutamol on serum and skeletal muscle digoxin concentrations has been studied in volunteers digitalised with digoxin. On one occasion a biopsy was taken from the quadriceps after 2 h of supine rest and then 3-4 mg salbutamol was given orally. Blood samples were taken before and after that dose and another muscle biopsy specimen was taken from the same thigh 180 min after the medication. On another occasion control blood sampling, ECG and blood pressure recordings were made but without muscle biopsies or salbutamol administration. Compared to the control measurements, salbutamol decreased the serum digoxin concentration (0.30 nmol.l-1). It also reduced the serum potassium concentration (0.58 mmol.l-1). The digoxin concentration in skeletal muscle did not change significantly after the intake of salbutamol. Thus, even a therapeutic oral dose of salbutamol reduces the serum digoxin concentration in man.

Clinical value of serum digoxin assays in outpatients: improvement by the standardization of blood sampling

Everyday physical activity previously has been shown to affect serum digoxin concentrations. Standardized rest in the supine position increases outpatient serum digoxin levels 0% to 75%. The present study comprising 56 outpatients treated with digoxin was undertaken to elucidate the clinical importance of a standardized period of rest before collection of the blood sample. Blood samples were taken about 24 hours after the latest dose, before and after 2 hours of rest in the supine position. A careful clinical examination, including electrocardiogram (ECG) findings, systolic time intervals, and chest x-ray studies, was performed to identify adverse effects/intoxication or failure of digitalis treatment. Signs of failure of digitalis treatment occurred in 12% of the patients, with a serum digoxin concentration of 0.68 +/- 0.15 (mean +/- SD) nmol/L before rest and 0.85 +/- 0.22 nmol/L after rest. Eleven percent showed signs of adverse effects/intoxication, with serum digoxin concentrations of 1.70 +/- 0.70 nmol/L before rest and 2.08 +/- 0.80 nmol/L after rest. The serum digoxin concentrations of the adequately treated patients (77%) were 1.02 +/- 0.35 nmol/L before rest and 1.28 +/- 0.41 nmol/L after rest. The importance of standardized rest before blood sampling is illustrated by the fact that only one third of the patients without signs of adverse effects/intoxication or failure of digitalis treatment had serum digoxin concentrations within the therapeutic range most commonly used (1.2 to 2.6 nmol/L) without supine rest. If allowed to rest in the supine position before blood sampling, approximately 60% of the adequately treated patients had serum digoxin concentrations within this range.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of salbutamol on digoxin concentration in serum and skeletal muscle

Beta 2-receptor stimulation has been reported to increase the concentration of 3H-ouabain in rat skeletal muscle. The present study was undertaken to see if beta 2-adrenoceptor stimulation by i.v. injection of salbutamol had any influence on the pharmacokinetics of digoxin in man. Ten volunteers were digitalized with digoxin and were investigated on two occasions. On each occasion a muscle biopsy was taken from the quadriceps after 2 h of supine rest. An injection of salbutamol 4 micrograms.kg-1 b.wt. or saline was then given intravenously. Blood samples were taken before and after the injection and further muscle biopsy was taken from the same thigh 120 min after the injection. Compared to the injection of saline, salbutamol caused a decrease in the serum digoxin and potassium concentrations. The change in serum potassium was significantly correlated with that in digoxin. The digoxin concentration in skeletal muscle was not significantly changed by either the salbutamol or saline injections.

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.