The effect of beta-blockade on plasma potassium concentrations and muscle excitability following static exercise

The effects of beta-blockade on plasma [K+], muscle excitability and force during fatiguing exercise were examined. Nine healthy males (mean age 22.3+/-1.7 yr) performed a 3-min fatigue protocol that consisted of a sustained submaximal contraction (30% of the maximal voluntary contraction, MVC) of the right quadriceps muscle. Subjects performed the exercise after treatment with either placebo, beta1-selective (metoprolol, 100 mg) or an equipotent dose of non-selective beta1,2-blockade (propranolol, 80 mg, n=6; 100 mg, n=2; 120 mg, n=1) twice daily for 3 days before testing according to a randomized double-blind design. Brachial arterial and femoral venous blood samples were drawn before, during, and for 15 min following the contraction, together with maximal stimulation of the right femoral nerve to evoke a twitch and a compound muscle action potential (M-wave); the M-wave amplitude being used as an index of sarcolemmal excitability. The exercise-induced rise in plasma [K+] did not differ between treatments, but K+ re-uptake during recovery was slower following propranolol. The recovery of the twitch was significantly related to the recovery of plasma [K+] in all trials, but the evoked M-waves were unaffected by either the contraction or the drug treatment. Propranolol resulted in a significantly (P<0.05) greater reduction (51.9+/-7.3%) in MVC following the 3-min contraction compared with metoprolol (40.7+/-3.6%) or placebo (38. 9+/-3.6%). These results suggest that while beta1,2-blockade may significantly affect the recovery of muscle force and K+ homeostasis after fatiguing exercise (presumably through an inhibition of the Na+,K+-ATPase), it does not appear to affect surface membrane excitability.