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.