This study examines lactate and K+ fluxes from muscle to blood during and after intense exercise. Ten men performed exhaustive dynamic exercise (mean load 65 W, mean duration 3.18 min) with the knee extensors of one leg. The mean lactate efflux was 15.5 (range 8.9–24.0) mmol min-1 at exhaustion, and it was linearly related to the lactate gradient. A linear relationship was also obtained if the H+ gradient was taken into account. Muscle pH decreased from 7.14 at rest to 6.71 (range 6.50–6.87) at exhaustion. At rest and during late recovery blood lactate was distributed across the erythrocyte membrane according to the membrane potential (intra-/extracellular ratio of 0.5), but during rapid lactate release this ratio decreased to 0.2. In-vitro experiments demonstrated a time constant of 1.2 min for lactate efflux from the erythrocytes.
Approximately 70% of the K+ ions released from the muscle to the blood accumulated in the plasma; the rest were taken up by other tissues. However, erythrocytes were not involved as a dilution space. The small change in erythrocyte K+ concentration was due to cellular volume changes. During recovery the kinetics of K+ reuptake by the muscle were described by a very fast (< 1 min) and a slow component (> 1 min): the magnitude of the former was equivalent to what had accumulated in the plasma. Individuals displayed a wide range of intramuscular lactate concentrations and pH values at exhaustion. Further, the pH changes were not as extreme as previously reported, suggesting that pH may not be the only factor involved in the fatigue process. A possible role for the potassium shifts as a limiting factor for muscle function is discussed.