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 muscle fibres ([K⁺]_I) probably play some role in the regulation of cardiovascular adjustments to muscular activity, as well as in the aetiology of muscle 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 plantar flexion exercise for four 5 min periods at increasing power outputs (∼1-6 W) with 10 min intervening recovery periods, as well as for two 5 min periods with ischaemia at ∼1 and ∼3 W. Microdialysis probes were inserted into the gastrocnemius medialis muscle of the right leg to measure [K⁺]_I, and K⁺ release from the plantar flexors during and after incremental exercise was 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 minutes 3-5 of incremental exercise by ∼1-7 mmol l^-1 as a positive function of power output. K⁺ release also increased as a function of exercise intensity, although there was a progressive increase by ∼1-6 mmol l^-1 in the [K⁺] gradient between the interstitium and arterial-venous plasma. 4. [K⁺]_I was lower during ischaemic exercise than control exercise. In contrast to this effect of ischaemia on [K⁺]_I, muscle pain was relatively higher 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 positive function of exercise intensity, ischaemia reduces [K⁺]_I during rest and exercise, and the increase in [K⁺]_I is not responsible for muscle pain during ischaemic exercise.