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Expression of Na,K‐ATPase α1 and α2 isoforms in the skeletal muscle tissues

The alpha catalytic isoforms of the Na,K‐ATPase are distributed in a highly cell‐ and tissue‐specific manner. Skeletal muscles express the α1 and α2 Na,K‐ATPase isoforms, with α2 being the major isoform of adult muscle based on measurements of alpha subunit protein and mRNA. The muscle organ contains a complex mix of fiber types as well as vascular, neuronal and other tissues. However, the distribution of alpha isoforms within the muscle unit is not known. In this study, we examine their distribution by immuno cytochemistry on fresh frozen skeletal muscle sections, using specific monoclonal and polyclonal antibodies. We show that α1 and α2 expression is most abundant in fibers having a fatigue resistant oxidative‐glycolytic phenotype and MHC IIA expression; and, is less abundant in glycolytic fibers expressing MHC IIB. Both α1 and α2 isoforms are expressed in nerve fibers, and the α2 isoform is preferentially localized in a sheath surrounding the nerve bundle. Within muscle spindles, both isoforms are expressed in the plasma membrane of intrafusal fibers with obvious fiber type specificity, and α1 is preferentially expressed in the outer capsule. In the muscle vasculature, α1 is more abundant than α2. This information will direct hypotheses regarding the physiological roles of the different alpha Na,K‐ATPase isoforms in skeletal muscle function. (Supported by the Physiology Research Fund)