Exercise-induced increase in maximal in vitro Na-K-ATPase activity in human skeletal muscle.

The present study investigated whether maximal in vitro Na-K-ATPase activity in human skeletal muscle is changed with exercise and whether it was altered by acute hypoxia. Needle biopsies from 14 subjects were obtained from vastus lateralis before and after 4 min of intense muscle activity. In addition, six subjects exercised also in hypoxia (12.5% oxygen). The Na-K-ATPase assay revealed a 19% increase (P < 0.05) in maximal velocity (Vmax) for Na⁺-dependent Na-K-ATPase activity after exercise and a tendency (P < 0.1) toward a decrease in Km for Na⁺ (increased Na⁺ affinity) in both normoxia and hypoxia. In contrast, the in vitro Na-K-ATPase activity determined with the 3-O-MFPase technique was 11-32% lower after exercise in normoxia (P < 0.05) and hypoxia (P < 0.1). Based on the different results obtained with the Na-K-ATPase assay and the 3-O-MFPase technique, it was suggested that the 3-O-MFPase method is insensitive to changes in Na-K-ATPase activity. To test this possibility, changes in Na-K-ATPase activity was induced by protein kinase C activation. The changes quantified with the Na-K-ATPase assay could not be detected with the 3-O-MFPase method. In addition, purines stimulated Na-K-ATPase activity in rat muscle membranes; these changes could not be detected with the 3-O-MFPase method. Therefore, the 3-O-MFPase technique is not sensitive to changes in Na⁺ sensitivity, and the method is not suited to detecting changes in Na-K-ATPase activity with exercise. In conclusion, muscle activity in humans induces an increased in vitro Na⁺-dependent Na-K-ATPase activity, which contributes to the upregulation of the Na-K-ATPase in association with exercise both in normoxia and hypoxia.