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⁺($K_i^{+}$ ) before, during, and after exercise. The relative loss of²⁰¹Tl 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 and²⁰¹Tl 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 and²⁰¹Tl.