Tissue-specific Role of the Na,K-ATPase α2 Isozyme in Skeletal Muscle

The Na,K-ATPase alpha 2 isozyme is the major Na, K-ATPase of mammalian skeletal muscle. This distribution is unique compared with most other cells, which express mainly the Na, K-ATPase alpha 1 isoform, but its functional significance is not known. We developed a gene-targeted mouse (sk alpha 2(-/-)) in which the alpha 2 gene (Atp1a2) is knocked out in the skeletal muscles, and examined the consequences for exercise performance, membrane potentials, contractility, and muscle fatigue. Targeted knockout was confirmed by genotyping, Western blot, and immunohistochemistry. Skeletal muscle cells of sk alpha 2(-/-) mice completely lack alpha 2 protein and have no alpha 2 in the transverse tubules, where its expression is normally enhanced. The alpha 1 isoform, which is normally enhanced on the outer sarcolemma, is up-regulated 2.5-fold without change in subcellular targeting. sk alpha 2(-/-) mice are apparently normal under basal conditions but show significantly reduced exercise capacity when challenged to run. Their skeletal muscles produce less force, are unable to increase force to match demand, and show significantly increased susceptibility to fatigue. The impairments affect both fast and slow muscle types. The subcellular targeting of alpha 2 to the transverse tubules is important for this role. Increasing Na, K-ATPase alpha 1 content cannot fully compensate for the loss of alpha 2. The increased fatigability of sk alpha 2(-/-) muscles is reproduced in control extensor digitorum longus muscles by selectively inhibiting alpha 2 enzyme activity with ouabain. These results demonstrate that the Na, K-ATPase alpha 2 isoform performs an acute, isoform-specific role in skeletal muscle. Its activity is regulated by muscle use and enables working muscles to maintain contraction and resist fatigue.