POTASSIUM-LOSS FROM SKELETAL-MUSCLE DURING EXERCISE IN MAN - A RADIOISOTOPE STUDY

Muscle potassium (K+) content decreases during exercise. Previous stud ies, in humans, have used measurements of arteriovenous plasma potassi um concentration differences (AV A[K]) and/or muscle biopsy to measure the loss of muscle K+ during exercise. In the current study a non-inv asive method was developed to measure skeletal muscle K+ before and af ter exercise, using an isotope of K+, potassium-43 (K-43+). Twelve sub jects performed single-leg extension exercise for 2 h at 50% of their maximum predicted heart rate. The level of radioactivity from the quad riceps femoris was determined before exercise and during two periods p ost-exercise. After correction for counts arising outside the exercise d muscle, we estimate a decrease in muscle K+ content of 3.2 +/- 1.55% (mean +/- S.E.M.) following exercise. The muscle K+ was not restored following 75 min of recovery. The decrease in muscle K+ following exer cise in our study is considerably less than that suggested by previous studies using AV DELTA[K+] measurements but not so dissimilar from re sults obtained using muscle biopsy. We conclude that a small but signi ficant loss of K+ occurs following prolonged dynamic exercise, and tha t complete recovery of muscle K+ is slow.