K+ SHIFTS OF SKELETAL-MUSCLE DURING STEPWISE BICYCLE EXERCISE WITH AND WITHOUT BETA-ADRENOCEPTOR BLOCKADE

1. K+ efflux rate and control of K+ reuptake rate in exercising muscle cells was examined in six healthy female volunteers. 2. A K+-selective electrode in the femoral vein continuously monitored K+ concentration ([K+](fv)) during bicycling. Power was increased stepwise 5-6 times by 30-40 W every fourth minute until exhaustion before and after I.V. administration of propranolol. Leg blood flow was measured by bolus injections of Cardiogreen. 3. [K+](fv) increased from about 4.3 to 6.8 mmol l(-1) at exhaustion both before and after propranolol administration, but after drug infusion endurance was reduced from 22.2+/-0.6 to 19.7+/-1.1 min, so [K+](fv) rose more rapidly. 4. The exercise-induced efflux rate of K+ from the muscle cells was estimated to be about 11 mu mol kg(-1) s(-)1 at exhaustion both before and after propranolol administration. 5. As an indicator of rate of net loss of K+ from the leg, veno-arterial concentration differences ([K+](fv-a)) during first, fourth and fifth power increments were high after 15 and 40 s, but declined toward the end of each power step. Propranolol accentuated [K+](fv-a) only after 15 and 40 s of the first and fourth increments. 6. The exercise-induced increase in reuptake rate of K+ in the muscle, estimated at exhaustion, was not significantly changed by propranolol and was about 10 mu mol kg(-1) s(-1), corresponding to about 15 % of maximum Na+-K+ pump capacity in man. 7. Extracellular accumulation and loss of K+ from muscle during bicycle exercise is due to Na+-K+ pump lag. The higher [K+](fv) during propranolol is mainly due to impaired redistribution outside the exercising muscles. In addition at low powers, beta-adrenoceptor blockade caused a transiently increased net loss due to an accentuated Na+-K+ pump lag.