Extracellular K+ concentration and K+ balance of the gastrocnemius muscle of the dog during exercise.

Exercise-induced changes of the extracellular and the venous K+ concentrations ([K+]e, [K+]ven) were measured continuously in autoperfused dog gastrocnemii using PVC liquid membrane electrodes. O2 consumption (VO2), blood flow and performance of the muscles were also determined. The muscles were stimulated indirectly and isotonic tetanic contractions (0.2 s) were produced every 0.7 s for more than 45 min. 1. [K+]e started to increase immediately after the onset of exercise and reached a maximal value of 7.4 meq/l after 4 min of exercise, thereafter it decreased, reaching 5.3 meq/1 at the end of the 12 min exercise. After 6 min of recovery [K+]e became lower than the control level. [K+]ven reached maximal values of 5.0 meq/l after 30 s of exercise and thereafter decreased slowly to 3.8 meq/l at the end of exercise. Thus, a K+ gradient of up to 3 meq/l was observed between the interstitial space and the venous blood. About 30 s after the end of exercise a net K+ uptake of up to 0.3 mu eq per min per g was observed. 2. The K+ loss of the muscle fibres reached maximal values of 1.5 mu eq per min per g at 1.5 min after the onset of exercise. Total K+ loss was 7% during 12 min of exercise and 12.5% of intracellular K+ during 45 min of exercise. 3. The changes of [K+]e correlated closely with performance and VO2 during exercise but not during recovery. On the other hand, blood flow correlated with the changes of [K+]e during both exercise and recovery. These results support the hypothesis that changes of [K+]e of the working muscle might be an important factor regulating exercise hyperemia.