Food restriction
Apart from thyroid disorders, thyroid hormone levels may change dramatically as a result of other diseases or food restriction [10] and may lead to a change in Na⁺,K⁺-ATPase concentration in skeletal muscle. For example, rats receiving one third to half their normal food supply, for 3 consecutive weeks, revealed a 50% reduction in total plasma triiodothyronine (T₃) in association with a 25% reduction in Na⁺,K⁺-ATPase concentration. This effect proved to be reversible; after just one week of being fed normal (full) rations, the rats' plasma T₃ and Na⁺,K⁺-ATPase concentrations had returned to normal [4,5]. Similar observations could not be reported in a group of Shetland ponies subjected to severe, long-term (2.5 years) food restriction; they showed a reduction in total and free T₃, (30 and 50%, respectively), a proportional loss of body weight, but only a modest (14%) decrease in Na⁺,K⁺-ATPase concentration in skeletal muscle [38]. This raised the questions whether skeletal muscle Na⁺,K⁺-ATPase isoforms are identical between species and to what extent thyroid hormone regulates specific isoforms [46].

Training and immobilisation
Depending on its intensity, exercise is accompanied by a rise in plasma K⁺ concentrations [27,36], most probably originating from the working muscles. It is believed that inadequate sarcolemmal Na⁺,K⁺ -ATPase activity and a failure to restore Na⁺,K⁺ gradients across the sarcolemma during excitation are responsible [4,5,6,27]. Exercise-induced hyperkalemia is reduced by training in man [17,29], dogs [25], cattle [16] and horses [28] and is most likely due to an increase in skeletal muscle Na⁺,K⁺ pump concentration; an observation made in many species including rats [23], guinea pigs [26], man [14,17,29], horses [28,40] and cattle [44]. Alternatively, the early release of K⁺ from cells may occur in association with H⁺ exchange [45]; in other words, training induces a reduction in the K⁺/H⁺ exchange, if the blunted rise in plasma K⁺ witnessed during exercise is to be explained (Figure 4).

Training and immobilisation in rodents
Studies in which the concentration of Na⁺,K⁺-ATPase was measured in the skeletal muscles of different species (rats, guinea pigs, horses, cattle and man) before and after training, showed a relative effect of between 15 and 50%. It remains to be seen, however, whether this difference is related to the muscle type, to the relative size of the animals concerned, or to the duration and type of training.

One study looked at the combined effect of immobility and training on Na⁺K⁺-ATPase concentration in the fast, gastrocnemius, and slow, soleus, muscles of guinea pigs [26]. Within one to three weeks, the gastrocnemius muscle Na⁺,K⁺-ATPase concentration had decreased to a maximum of 25% its original value. However, during a fourth week of immobilisation these levels returned spontaneously to their normal value. After three weeks of daily running exercise on a treadmill, the Na⁺,K⁺-ATPase concentration increased by 50% in fast muscle but only by 15% in slow muscle. In rats, six weeks of swimming was found to induce a comparable (40%) increase in [³H]ouabain binding site concentration in slow (soleus) and fast (extensor digitorum longus) muscles [23].

Training studies in man
Studies in man investigating the effects of training on Na⁺,K⁺-ATPase concentration in skeletal muscle, often involve the collection of biopsies from the easily accessible vastus lateralis muscle, which consists of mixed types of fibre. Invariably, these studies use bicycle training as the preferred form of exercise. It not only works the relevant muscle group sufficiently, but it is also easily standardised in a laboratory setting. Two simultaneous studies showed an increase of 14% [17] and 16% [29] in the concentration of [³H]ouabain binding sites in the vastus lateralis muscle of male subjects, aged 18 to 20 years. The first of these studies demonstrated this effect after only six, two-hour daily training sessions [17]. In the second study, in which subjects performed short bouts of sprint work three times a week, biopsies were not taken until seven weeks after the start of training [29]. Thus, although the rise in Na⁺,K⁺-ATPase concentration was similar after endurance and sprint training, a longer period of sprint training was required to attain this effect. Due to the characteristic mixed fibre type of the vastus lateralis muscle, the increase in Na⁺,K⁺-ATPase concentration cannot be ascribed to one type of muscle fibre and, because biopsies were taken only at the end of the seven-week sprint training period, neither can it be established at what time point changes in Na⁺,K⁺-ATPase concentration occurred first.

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