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

Abstract

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. Five young healthy subjects performed plantar flexion exercise for four 5 min periods at increasing power outputs ( approximately 1-6 W) with 10 min intervening recovery periods, as well as for two 5 min periods with ischaemia at approximately 1 and approximately 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. On average, [K+]I was 4.4 mmol l(-1) at rest and increased during minutes 3-5 of incremental exercise by approximately 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 approximately 1-6 mmol l-1 in the [K+] gradient between the interstitium and arterial-venous plasma. [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. 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.

Figure 1

An illustration of the experimental protocol.

Figure 4. Perception of pain during rest and exercise

The subjective assessment of calf muscle pain, as represented by the visual analogue score, during rest and dynamic plantar flexion exercise of increasing intensity (Expt 1).

Figure 2

Interstitial K⁺ response (mean ± s.e.m.) in the human gastrocnemius medialis muscle (n = 5) during rest and dynamic plantar flexion exercise of increasing intensity (Expt 1).

Figure 3

The relationship between Δ[K⁺]_I and power output during dynamic plantar flexion exercise for five subjects. See Results for explanation of Δ[K⁺]_I.

Figure 5. Effect of ischaemia on [K⁺]_I during rest and exercise

Interstitial K⁺ response (mean ± s.e.m.) in the human gastrocnemius medialis muscle (n = 5) during rest and dynamic plantar flexion exercise with and without ischaemia (Expt 2). The dashed, horizontal line is set at the mean of the pre-exercise values obtained during the initial 15 min period of Expt 1 for comparison. The dashed vertical lines define the periods of exercise, whereas the dotted vertical lines define the period of cuff inflation (i.e. ischaemia) which, in the case of the periods labelled ‘cuff & 0.9 W’ and ‘cuff & 2.6 W’, begins at the start of exercise (i.e. minutes 35 and 50).

Figure 6. Effect of ischaemia on perception of pain during rest and exercise

The subjective assessment of calf muscle pain, as represented by the visual analogue score, during rest and dynamic plantar flexion exercise with and without ischaemia (Expt 2). The dashed vertical lines define the periods of exercise, whereas the dotted vertical lines define the period of cuff inflation (i.e. ischaemia) which, in the case of the periods labelled ‘cuff & 0.9 W’ and ‘cuff & 2.6 W’, begins at the start of exercise (i.e. minutes 35 and 50).

Figure 7. Pain-[K⁺]_I relationship during ischaemic nad non-ischaemic exercise

The relationship between mean values of calf muscle pain (i.e. visual analogue score) and muscle [K⁺]_I during dynamic plantar flexion exercise at 0.9 and 2.6 W with and without ischaemia (i.e. Expts 1 and 2).