Interstitial K+ in human skeletal muscle during and after dynamic graded exercise determined by microdialysis

Interstitial K+ concentrations were measured during one-legged knee-extensor exercise by use of microdialysis with probes inserted in the vastus lateralis muscle of the subjects. K+ in the dialysate was measured either by flame photometry or a K+-sensitive electrode placed in the perfusion outlet. The correction for fractional K+ recovery was based on the assumption of identical fractional thallium loss. The interstitial K+ was 4.19 +/- 0.09 mM at rest and increased to 6.17 +/- 0.19, 7.48 +/- 1.18, and 9.04 +/- 0.74 mM at 10, 30, and 50 W exercise, respectively. The individual probes demonstrated large variations in interstitial K+, and values >10 mM were obtained. The observed interstitial K+ was markedly higher than previously found for venous K+ concentrations at similar work intensities. The present data support a potential role for interstitial K+ in regulation of blood flow and development of fatigue.