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		· Introduction · The Na+,K+ pump in skeletal muscle · Questions · Analysis of the concentration of Na+,K+ pumps in skeletal muscle · Thyroid hormones · Food restriction · Training and immobilisation · Perspectives for future research · Concluding remarks · References
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The Na⁺,K⁺ pump in skeletal muscle
To maintain the high concentration of K⁺ and the low concentration of Na⁺ that exists in animal cells, relative to the external environment, a specific transport system is required. Skou's discovery of an enzyme that hydrolyses ATP only when Na⁺ and K⁺ are present (in addition to Mg²⁺ required by all ATP enzymes), was the beginning of our understanding of the Na⁺K⁺ pump [37]. The enzyme, known as Na⁺,K⁺-ATPase, forms an integral part of the Na⁺,K⁺ pump and the splitting of ATP provides the energy required to drive the active transport of the cations.

Fig. 1 A - D Schematic representation of the component parts of a skeletal (striated) muscle. (A) whole muscle. (B) a small part of the muscle magnified to show the individual muscle cells or fibres. (C) a part of the muscle fibre magnified. (D) an individual myofibril. A group of myofibrils together forms a muscle fibre or cell. The Na+,K+ pump is located within the plasma membrane of these muscle cells. E - Diagrammatic representation of the subunit structure and orientation of the Na+,K+ pump in the plasma membrane.

The pump is located within the sarcolemma of skeletal muscle cells (Figure 1, A-D) and is a so-called ab heterodimer, consising of two a and two b subunit proteins that have a fixed orientation within the lipid bilayer of the cell membrane (Figure 1; Part E). It is the larger a subunit that has the three receptor sites for binding sodium ions on the portion of the protein protruding to the interior of the cell, the two potassium ion receptor sites found on the exterior and, adjacent or near to the sodium ion binding sites, it has the ATPase activity, hence the terms 'pump' and 'enzyme' are often used interchangeably. Like many enzymes, Na⁺,K⁺-ATPase exists in various isoforms, giving rise to tissue-specific expression and differential regulation of the molecule. Skeletal muscle expresses a1, a2, b1 and b2 isoforms and thus has the possibility of four ab combinations [41]. The role of the Na⁺,K⁺ pump in the restoration of membrane potential after exitation [4,5,6], and its confirmational changes during a transport cycle, in which sodium ions are transported out and potassium ions in to the cell, are illustrated in Figure 2.

Fig. 2 (Flash plug-in required)
Animation showing a transport cycle of the Na+,K+ pump. During the cycle, the Na+,K+ pump is first phosphorylated and subsequently de-phosphorylated. Binding of intracellular Na+ ions to the pump initiate phosphorylation of the enzyme, while the binding of extracellular K+ ions trigger the dephosphorylation reaction. After a complete cycle, three Na+ ions have been extruded from and two K+ ions have entered into the cell. (Adapted from an illustration by Kjell Lundin, featured in a special publication to mark the award of the 1997 Nobel Prize in Chemistry to Professor Jens Christian Skou).

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