Microdialysis and the measurement of muscle interstitial K⁺ during rest and exercise in humans

The purpose of this study was to examine whether microdialysis and the internal reference thallium-201 (²⁰¹Tl) could accurately measure muscle interstitial K⁺ (K⁺) before, during, and after exercise. The relative loss of ²⁰¹Tl and simultaneous relative recovery of K+ were measured in vitro for 12 microdialysis probes that were bathed in Ringer acetate medium and perfused at various flows (3-10 μl/min), ²⁰¹Tl loss was linearly related to K⁺ recovery, and their level of agreement was not different from zero. Microdialysis and ²⁰¹Tl were then used to measure K(i)⁺ in the gastrocnemius medialis muscle of four humans during rest and static plantar flexion exercise. At rest, K(i)⁺ was 3.9-4.3 mmol/l when the perfusate flow was 2 or 5 μl/min. During exercise, K(i)⁺ increased from 6.9 ± 0.4 to 7.5 ± 0.3 mmol/l at low to high intensity and declined to 5.2 ± 0.3 mmol/l after exercise. These results suggest that large changes in K(i)⁺ in human skeletal muscle can be accurately measured by using microdialysis and ²⁰¹Tl.