BLOOD MAGNESIUM AND POTASSIUM ALTERATIONS WITH MAXIMAL TREADMILL EXERCISE TESTING - EFFECTS OF BETA-ADRENERGIC-BLOCKADE

To test alterations in plasma potassium and magnesium levels with maximal exercise, 15 sedentary, healthy men (mean age 29 years) participated in a double-blind crossover study for 11 weeks with propranolol, atenolol, and placebo. Maximal exercise tests were done at baseline and after placebo and beta-blockade phases. Blood for analysis was collected via indwelling brachial vein angiocatheters at baseline and during and after testing. Plasma potassium and magnesium levels increased at peak exercise with atenolol, propranolol, and placebo. There was no difference among groups in baseline recovery for magnesium (mean 28 minutes, range 24 to 30 minutes). Potassium levels returned to baseline more rapidly (compared with magnesium) in the placebo and atenolol groups (mean 10 minutes); however, recovery time was prolonged with propranolol (26 minutes) compared with placebo and atenolol (p < 0.01). In conclusion, plasma magnesium and potassium levels increase significantly with maximal exercise and are unaffected by atenolol or propranolol beta-blockade. Propranolol, however (compared with atenolol and placebo), prolongs the time of return to baseline of plasma potassium after exercise.