Muscle Na,K-pump dysfunction may expose the heart to dangerous K levels during exercise.

During exercise, potassium leaks out of muscle cells and interstitial potassium concentration may rise to 15 mmol/l. Potassium is then in part transported back to the cells by the Na,K-pump. The leak of potassium from working human muscles can be calculated to 15 mumol/g/min. Human muscles contain 300 pmol/g wet weight Na,K-pumps and have a maximum capacity for potassium uptake of 5 mumol/g/min. At rest only a small percentage is used, but during maximum stimulation the entire capacity can probably be used. During exercise, total potassium leakage from the human muscle pool is 400 mmol/min and maximum active reuptake capacity is 100 mmol/min. ECV contains only 50 mmol potassium. During heavy exercise the capacity for active potassium uptake in muscle pool may be exceeded and interstitial potassium equilibrates with plasma potassium, which then may rise to 10 mmol/l. Training can decrease the rise in plasma potassium during exercise by 0.2-0.5 mmol/l. Variation in muscle Na,K-pump concentration may be associated with changed plasma potassium homeostasis during exercise, which if disadvantageous may be dangerous to the heart.