Interstitial and arterial-venous [K+] in human calf muscle during dynamic exercise: effect of ischaemia and relation to muscle pain

1. Changes in the concentration of interstitial K+ surrounding skeletal mus cle fibres ([K+](I)) probably play some role in the regulation of cardiovas cular adjustments to muscular activity, as well as in the aetiology of musc le pain and fatigue during high-intensity exercise. However, there is very little information on the response of [K+](I) to exercise in human skeletal muscle. 2. Five young healthy subjects performed planter flexion exercise for four 5 min periods at increasing power outputs (similar to1-6 W) with 10 min int ervening recovery periods, as well as for two 5 min periods with ischaemia at similar to1 and similar to3 W. Microdialysis probes were inserted into t he gastrocnemius medialis muscle of the right leg to measure [K+](I), and K + release from the plantar flexors during and after incremental exercise wa s calculated from plasma flow and arterial-venous differences for K+. Calf muscle pain was assessed using a visual analogue scale. 3. On average, [K+](I) was 4.4 mmol l(-1) at rest and increased during minu tes 3-5 of incremental exercise by similar to1-7 mmol l(-1) as a positive f unction of power output. K+ release also increased as a function of exercis e intensity, although there was a progressive increase by similar to1-6 mmo l l(-1) in the [K+] gradient between the interstitium and arterial-venous p lasma. 4. [K+](I) was lower during ischaemic exercise than control exercise. In co ntrast to this effect of ischaemia on [K+](I), muscle pain was relatively h igher during ischaemic exercise, which demonstrates that factors other than changes in [K+](I) are responsible for ischaemic muscle pain. 5. In conclusion, this study has demonstrated that during 5 min of dynamic exercise, [K+](I) increases during the later period of exercise as a positi ve function of exercise intensity, ischaemia reduces [K+](I) during rest an d exercise, and the increase in [K+](I) is not responsible for muscle pain during ischaemic exercise.