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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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Thus, as a result of repetitive action potentials, exercise induces a loss of K⁺ from the muscle cells into the extracellular space, giving rise to an increase in plasma K⁺ [4,5,27]. In man, hyperkalemia occurs during both dynamic and static exercise and is believed to play a role in the development of muscular fatigue [27,36]. While the long-term control of plasma K⁺ concentrations depends ultimately on kidney function, achieved by increasing the concentration of Na⁺,K⁺ pumps in the cell membrane (for example, by thyroid hormones or training), it is skeletal muscle that plays the dominant role in its acute adjustment, by increasing the activity of the Na⁺,K⁺ pump (for example, by adrenaline; Figure 3), or by increasing the concentration of Na⁺,K⁺ pumps in the cell membrane (for example, by thyroid hormones or training). These muscles represent the body's largest pool of K⁺ and Na⁺,K⁺ pumps, and therefore provide an enormous capacity for rapid Na⁺,K⁺ exchange [4,5,6].

Fig. 3 Diagram of regulatory factors controlling the activity and the concentration of Na+,K+ pumps in a skeletal muscle cell. Left: factors eliciting acute stimulation of the Na+,K+ pump (not discussed in text). Right: factors known to influence the concentration of Na+,K+ pumps by stimulation or inhibition of their synthesis and insertion into the plasma membrane. The first two factors (thyroid hormones and training) are discussed in this review. The lower two muscle cells indicate that K+ leaves the cell during excitation through ATP or Ca2+-dependent K+ channels.
Abbreviations: IGF-1 - insulin-like growth factor I; CGRP - calcitonin generelated peptide. (Modified from Reference 4).

An increase in the capacity for active Na⁺,K⁺ transport in skeletal muscle should, however, lead to a 'blunted', or 'dampened', rise in plasma K⁺ during exercise (Figure 4), and hence to an improvement in muscle endurance. Indeed, this does occur in man after sprint training [17,29]. Furthermore, a correlation exists between maximum O₂ uptake, running distance and Na⁺,K⁺ pump concentration in skeletal muscle [14]. It has also been reported that a large increase in the capacity for active Na⁺,K⁺ transport occurs in the skeletal muscle of patients suffering from hyperthyroidism [4,7,22], despite the condition being associated normally with increased fatigability and reduced endurance [13,19].

Fig. 4 Changes in plasma K+ concentrations in 4-year old horses (n=3) during an exercise test performed before and after a 10-day training period. The rise in plasma K+ during exercise is significantly "blunted" after training.
* P<0.05, after training compared with before training.

Questions
In 1997, the author began investigating the regulation of the Na⁺,K⁺ pump and K⁺ homeostasis during exercise in cats, dogs, horses and cattle. Three questions were addressed:

Is thyroid hormone a major determinant in the concentration of Na⁺,K⁺ pumps in skeletal muscle of domestic animal species, as it is in man?
Does a reduction in Na⁺,K⁺ pump concentration lead to hyperkalemia during exercise? And,
Does training lead to an upregulation in the Na⁺,K⁺ pump concentration in skeletal muscle?