Physical training increases skeletal muscle Na+,K+-ATPase (NKA) and improves exercise performance, but the effects of inactivity per se on NKA in human muscle are unknown. We investigated the effects of 23-days unilateral lower limb suspension (ULLS) and subsequent 4-weeks resistance training (RT) on muscle function and NKA in six healthy adults, measuring quadriceps muscle peak torque; fatigue and venous [K+] during intense one-legged cycling exercise; and skeletal muscle NKA content ([3H]ouabain binding) and NKA isoform abundances (immunoblotting) in muscle homogenates (α1-3, β1-2) and in single fibers (α1-3, β1). In the unloaded leg after ULLS, quadriceps peak torque and cycling time to fatigue declined by 22% and 23%, respectively, which were restored with RT. Whole muscle NKA content and homogenate NKA α1-3 and β1-2 isoform abundances were unchanged with ULLS or RT. However, in single muscle fibers, NKA α3 in Type I (-66%, p=0.006) and β1 in Type II fibers (-40%, p=0.016) decreased after ULLS, with other NKA isoforms unchanged. After RT, NKA α1 (79%, p=0.004) and β1 (35%, p=0.01) increased in Type II fibers, while α2 (76%, p=0.028) and α3 (142%, p=0.004) increased in Type I fibers compared to post-ULLS. Despite considerably impaired muscle function and earlier fatigue onset, muscle NKA content, and homogenate α1 and α2 abundances were unchanged, thus being resilient to inactivity induced by ULLS. Nonetheless, fiber-type specific downregulation with inactivity and upregulation with RT of several NKA isoforms indicates complex regulation of muscle NKA expression in humans.
- strength training
- Copyright © 2016, Journal of Applied Physiology