Loss of potassium from muscle during moderate exercise in humans: a result of insufficient activation of the Na+-K+-pump?
In this study, we have investigated whether the muscle net potassium (K+) loss, observed during two-legged intermittent static knee-extensions at 30% MVC (n = 9), is caused by an insufficient activation of the Na+-K+-pumps. Furthermore, we have investigated whether the changes in the K+ homeostasis can be causally related to fatigue. K+ loss was calculated from the arterio-venous concentration difference and plasma flow. In three subjects, femoral venous K+ concentration was measured continuously with a K+ selective electrode. Na+-K+-pump activity was estimated from the rate of removal of K+ from the blood during 30-s pauses inserted into the exercise protocol. A large net K+ loss took place during the first minutes of exercise, but diminished quickly and disappeared after 20 min. An increasing net K+ loss reappeared after 30 min. Only 10% of the lost K+ had been regained after the 20-min recovery. A lag in the activation of the Na+-K+-pumps may explain the K+ loss at the beginning of exercise, but gradual pump activation prevented a net K+ loss after 20 min of exercise. The reappearance of the net K+ loss in the later stage of exercise and the subsequent slow recovery of intracellular K+ seemed to be caused by an insufficient further activation of the pumps, rather than by the capacity of the pumps being surpassed. Fatigue was not related to the accumulation of K+ in the interstitium. However, during exercise, the decrease in intracellular K+ content was linearly related to the fall of maximal force. We conclude that during repeated isometric contractions, insufficient activation of the Na+-K+-pumps causes a continuous muscle K+ loss which was associated with fatigue.
Verburg
E
E
National Institute of Occupational Health, Oslo, Norway.
Hallén
J
J
Sejersted
O M
OM
Vøllestad
N K
NK
D000328:Adult; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000502:physiology; D011188:Potassium / Q000097:blood / Q000378:metabolism*; D015221:Potassium Channels / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D015221:Potassium Channels; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium