Commentaries on viewpoint: maximal Na⁺-K⁺-ATPase activity is upregulated in association with muscle activity.

TO THE EDITOR: Since the active reuptake of fiber K lost during action potentials in contracting muscles is limited by the maximal transport capacity of fiber Na -K pumps, substantial effort has been made to determine how this capacity is regulated during exercise. The methods employed includes ouabain-based determination of muscle Na -K pump content, 3-Omethylfluorescein phosphate-based determination of maximal enzyme activity and, lately, K and Na -stimulated 33P-ATP hydrolysis in fractionated muscle membranes (2,4). However, other studies show that muscle activity can induce far larger changes in Na -K pump activity than can be explained by reported changes in maximal Na -K pump transport capacity. This effect seems related to an exerciseinduced release of compounds such as catecholamines and ATP that causes an increase in the Na affinity of the Na -K pumps (1). Combined with an increase in intracellular Na concentration this may, depending on the intensity of muscle excitation, lead to more than eightfold increase in Na -K pump activity (5). Therefore, detailed evaluation of the regulation of Na -K pump activity in muscles necessitates methods that allow for examinations of not only the maximal transport capacity of the protein but also of its affinity for Na . In this context, the method to determine Na stimulated P-ATP hydrolysis in muscle membranes presented by Juel (3) may represent a useful tool that by allowing the examination of both the maximal transport capacity and the Na affinity of Na -K pumps in muscles can take us one step further in the unveiling of K homeostasis in active skeletal muscles.