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 f orce 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 co ntraction, MVC) of the right quadriceps muscle. Subjects performed the exercise after treatment with either placebo, beta(1)-selective (meto prolol, 100 mg) or an equipotent dose of non-selective beta(1,2)- bloc kade (propranolol, 80 mg, n=6; 100 mg, n=2; 120 mg, n=1) twice daily f or 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 maxi mal stimulation of the right femoral nerve to evoke a twitch and a com pound muscle action potential (M-wave); the M-wave amplitude being use d as an index of sarcolemmal excitability. The exercise-induced rise i n plasma [K+] did not differ between treatments, but K+ re-uptake duri ng recovery was slower following propranolol. The recovery of the twit ch was significantly related to the recovery of plasma [K+] in all tri als, 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 contracti on compared with metoprolol (40.7+/-3.6%) or placebo (38.9+/-3.6%). Th ese results suggest that while beta(1,2)-blockade may significantly af fect the recovery of muscle force and K+ homeostasis after fatiguing e xercise (presumably through an inhibition of the Na+,K+-ATPase), it do es not appear to affect surface membrane excitability.