Cellular localisation of NaK-ATPase isoforms in human skeletal muscle is muscle fibre-type specific

C.H. Steward and M.J. McKenna, Institute of Sport Exercise and Active Living (ISEAL), College of Sport and Exercise Science, Victoria University, Melbourne, VIC 8001, Australia.

The six isoforms of Na⁺,K⁺-ATPase (NKA) expressed in skeletal muscle have been detected in both the sarcolemmal and t-tubular systems of muscle cells using various techniques, however this has mostly been investigated in animal muscle. The aim of this study was to determine localisation patterns of NKA isoforms (α₁-₃ and β₁-₃) in human skeletal muscle, as well as examining for fibre-type specific distribution.

A vastus lateralis muscle biopsy was taken from seven healthy, young adults. Isoform localisation in the plasma membrane (PM) and intracellular (IC) regions, defined as all areas/structures within the plasma membrane envelope including the t-tubules, was investigated via immunofluorescence microscopy.

The fibre-type distribution was approximately 55% type I and 45% type II fibres for each NKA isoform analysed. The density of NKA α₁ was 24% higher in type II compared to type I fibres (p<0.01). The NKA α₂ density was over 2-fold greater in PM compared to IC (p<0.05), with no difference between fibre types. NKA α₃ was 63% greater in type I than type II fibres (p<0.05). The density of NKA β₁ was 58% greater in the PM (p<0.05), and NKA β₂ was 25% greater in the PM compared to IC (p<0.05). Co-localisation analysis of NKA β₂ with nuclei found a strong correlation (0.69), and showed 65% of the NKA β₂ co-localized with nuclei. The density of NKA β₃ was 21% greater in the PM compared to IC (p<0.01). These results demonstrate a higher density of NKA α_2, β₁ and β₃ in the PM compared to the IC region, and a higher density of NKA α₁ in type II fibres and α₃ in type I fibres.