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].

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].

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:

1. 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?

2. Does a reduction in Na⁺,K⁺ pump concentration lead to hyperkalemia during exercise? And,

3. Does training lead to an upregulation in the Na⁺,K⁺ pump concentration in skeletal muscle?

Analysis of the concentration of Na⁺, K⁺ pumps in skeletal muscle
Quantitative analysis of membrane bound enzymes, such as Na⁺,K⁺-ATPase, is often performed on plasma membrane fractions that have been isolated from cell homogenates using differential centrifugation. However, this procedure requires large amounts of tissue, making it impractical for both medical and veterinary clinical studies, it may result in a loss of > 95% of Na⁺,K⁺ pumps and recovery rates between preparations vary enormously [18]. Thus, the development of techniques to measure the concentration of Na⁺,K⁺ pumps in small samples of intact skeletal muscle has proved invaluable to physiological studies [30].

Cardiac glycosides, such as digoxin and ouabain, bind specifically to the outer surface of the Na⁺,K⁺ pump, a stoichiometric process: one molecule of cardiac glycoside binds to one Na⁺,K⁺ pump molecule. The concentration of Na⁺,K⁺ pumps is measured using radioactively-labelled [³H] ouabain, provided the isozyme of the Na⁺,K⁺ pump in a specific tissue has a high affinity for the molecule. Analysis of mRNA coding for the Na⁺,K⁺ pump in skeletal muscle has revealed that most of it does, indeed, code for an isozyme with a high affinity for ouabain [41].

Binding of [³H]ouabain to the Na⁺,K⁺ pump is facilitated by the presence of the phosphate analogue vanadate (Figure 5). Using this anion, a simple and rapid assay has been developed for the measurement of Na⁺,K⁺ pump concentration in muscle samples weighing as little as 5 mg [30]. The recovery of such small samples has the considerable advantage of enabling multiple biopsies to be taken from a tissue and therefore duplicate, triplicate or quadruplicate measurements may be made.

Thyroid hormones
For about 30 years, it has been known that Na⁺,K⁺-ATPase activity in skeletal muscle and other tissues increases as a function of thyroid status; hyperthyroidism gives rise to an increase in pump activity, while hypothyroidism results in its decreased activity [20]. The increase in Na⁺,K⁺ transport associated with hyperthyroidism was once thought to account for the calorigenic action of the thyroid hormones, however, only 5-10% of the total heat produced in skeletal muscle of eu-, hypo- and hyperthyroid animals can be attributed to active Na⁺,K⁺ transport [7].

Na⁺,K⁺-ATPase in rat muscle
Thyroid hormones largely determine the concentration of Na⁺,K⁺ pumps in skeletal muscle through a general endocrine effect [5,7], which is in stark contrast to observations made during training (see later). In rat skeletal muscle, the Na⁺,K⁺-ATPase concentration is approximately the same whether it consists predominantly of slow (eg. soleus) or fast (eg. gastrocnemius) fibres [21]. However, gastrocnemius muscle recovered from hyperthyroid rats contained five times the concentration of Na⁺,K⁺ pumps compared to equivalent samples recovered from hypothyroid animals. This difference rose to as much as ten times when soleus muscle samples were compared. These findings suggest that muscles show a greater response to an alteration in thyroid status when they consist predominantly of slow fibres.

Fatigability and Na⁺,K⁺ pump capacity
Contrary to expectations, the soleus muscle of hyperthyroid rats shows a greater susceptibility to fatigue and less endurance [13,19] than its increased capacity for active Na⁺,K⁺-transport suggests [12,19,21]. However, when this phenomenon is considered in relation to the increased influx of Na⁺ through specialised channels, it is likely that muscle endurance is determined by the leak-to-pump ratio of Na⁺, not by the Na⁺,K⁺ pump concentration alone. Furthermore, studies in which the time course of the effects of thyroid hormone on Na⁺ influx and K⁺ efflux was compared with that on Na⁺,K⁺-ATPase activity in skeletal muscle, have shown that the rise in the unidirectional flux of cations preceded the rise in Na⁺,K⁺ pump concentration [12,19]. Thus, increased permeability of the sarcolemma to cations after thyroid hormone treatment may be the driving force for the synthesis of Na⁺,K⁺ pumps.

Na⁺,K⁺-ATPase in cats
The concentration of Na⁺,K⁺ pumps in the skeletal muscles of hypo- and hyperthyroid dogs and cats has also been determined. Hypothyroidism is the most frequent thyroid disorder encountered in dogs, while hyperthyroidism is observed more often in cats [33]. Studies in both these species retrieved samples from the sternothyroid muscle due to its easy accessibility during surgical thyroidectomy

Total thyroxine (T₄) concentrations were approximately 400% higher, and Na⁺,K⁺ pump concentrations around 75% higher, in hyperthyroid compared to euthyroid cats (Schaafsma et al, unpublished data). In both groups of cats, the apparent dissociation constant for ouabain was of the same order of magnitude as that measured in rats with comparable thyroid status [21]. An intriguing observation made recently on a cat that was treated for 10 days with the anti-thyroid drug Strumazol (company, town and country), showed a high concentration of [³H]ouabain binding sites had been maintained while the total plasma T₄ returned to normal (Schaafsma et al, unpublished data).

Na⁺,K⁺-ATPase and K⁺ homeostasis in dogs
Recently, the concentration of Na⁺,K⁺ pumps was measured in the sternothyroid muscle of Beagle dogs, before and after thyroidectomy [34]. In euthyroid Beagles the Na⁺,K⁺ pump concentration was almost twice that recorded in euthyroid cats, but fell by 40% after thyroidectomy. The decrease in [³H]ouabain binding capacity was not due to the Na⁺,K⁺ pump's reduced affinity for ouabain.

Total plasma T₄ concentrations were about 20 nmol/l in euthyroid and <2 nmol/l in hypothyroid dogs. The resting plasma K⁺ concentration was significantly higher in hypothyroid compared to euthyroid dogs and remained higher throughout the experiment, including the work and recovery phases of the exercise test (Figure 6). In addition, hypothyroid dogs showed a significant exercise-induced hyperkalemia. The most likely explanation for this was a decrease in the muscle's capacity to pump K⁺ back into the tissue, since neither muscle damage nor kidney failure was apparent [34].

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.

Training studies in young and adult horses
Young horses, sprint-trained from birth until five months of age [42], showed an increase in [3H]ouabain binding capacity in gluteus medius and semitendinosus muscles of 30% and 20%, respectively [40]. Adult horses also revealed a 36% rise in Na⁺,K⁺-ATPase concentration in the gluteus medius muscle [20]. In adult horses, this rise was associated with a significant reduction in the plasma K⁺ concentration during an exercise test [28].

Measurements of Na⁺,K⁺-ATPase concentrations in the gluteus medius muscle of young and adult horses affected by periodic hyperkalemic paralysis have been compared with those of age-matched control horses [31]. It was concluded that the cell membrane events underlying the periodic episodes of paralysis in hyperkalemic horses could not be attributed to changes in the Na⁺,K⁺ pump in either the Na⁺,K⁺ number or affinity. In addition, the decrease in Na⁺,K⁺-ATPase concentration measured in skeletal muscle showed an age-dependent decrease. This is true also for rat muscle, in which the concentration of Na⁺,K⁺ pumps rises five-fold from birth to four weeks of age and then falls due to an increase in the diameter of mature muscle cells [4,5]. Finally, when Na⁺,K⁺ pump concentrations were compared in gluteus muscle samples taken from horses of similar age but of different breeds, including the Quarter horse, Thoroughbred and Dutch warmblood, they were found to be similar [28,31,40].

Cattle
When endurance-trained Hereford calves were exercised at a maximum sustainable rate, they showed a rise in peak arterial plasma K⁺ concentrations due to an increased maximum work capacity [16]. However, when they were exercised at a similar work load before and after physical conditioning, the rise was significantly reduced [16]. Young male and female Mozambican Angoni cattle, subjected to two hours of draught work every day for two weeks, showed increases in the concentration of Na⁺,K⁺-ATPase in semitendinosus muscle of 16 and 30%, respectively. When plasma K⁺ concentrations were measured regularly during the daily two-hour training periods, the rise in concentration was lower at the end of the two weeks than it was after only eight days of training. This difference was not significant however [44].

Persistence of the training effect
How long does the training-induced rise in Na⁺,K⁺-ATPase concentration persist when intensive training is discontinued? Rats, subjected to six weeks of swim training, revealed a large rise in [³H]ouabain binding site concentration, in both soleus (slow) and extensor digitorum longus (fast) muscles [23], which was almost completely reversed within three weeks of training being stopped. However, when a five month training period for young horses was followed by a six month period of rest, the concentration of [³H]ouabain binding sites in semitendinosus muscle remained the same and in gluteus medius muscle was reduced by 10% [39]. Whether this discrepancy is due to species differences or the type of exercise performed is difficult to conclude, but the topic warrants further studies.

Is the training effect due to a general or a specific effect?
In trained rats, swimming induced up-regulation of the Na⁺,K⁺-ATPase in all hind limb and spinal muscles, but not in the diaphragm [23]. This result provides evidence against the existence of a non-specific endocrine factor, such as thyroid hormone, resposible for eliciting the training effect on the concentration of Na⁺,K⁺-ATPase [5]. A recent study in young foals has confirmed this idea by demonstrating that the training-induced rise in Na⁺,K⁺-ATPase was apparent in the gluteus and semitendinosus muscles of the hind limb, but not in the masseter muscle of the jaw [39]. Considered together, these observations suggest that the factor eliciting an up-regulation in the Na⁺,K⁺ pump numbers during training is located in the muscle itself.

Perspectives for future research
In addition to being essential for locomotion, skeletal muscle from some animals is consumed as meat by man. A muscle's movement and meat quality are determined by the growth and composition of its composite fibres, as well as by the maintenance of ion gradients. The physiological and morphological properties of adult skeletal muscle are the combined result of genetic predisposition, diet, hormonal influences and the workload that the muscle has been exposed to.

During development skeletal muscles not only hypertrophy but also adapt to their required mechanical functions, such as rapid short-lasting movements (fast muscles) or prolonged actions (slow muscles). With respect to meat quality, slow muscles have better water holding capacity but lower colour stability than fast muscles [24].

A muscle's functional adaptation during development is evident through changes in the cation transport activity [8] and in the myosin heavy chain isoform expression during the postnatal period [9]. Both parameters are strongly affected by thyroid hormones and by exercise [5,32,35]. These effects are not easy to investigate independently since standardising exercise regimes is difficult and maintaining animals with relatively long growth periods is costly.

Muscle cell culture
The use of tissue culture techniques to study the adaptive behaviour and growth of muscle cells has obvious advantages [1]. Foetal myoblasts and adult muscle satellite cells are readily isolated and grown in vitro (Figure 7). After an initial phase of proliferation they fuse to form myotubes and then differentiate to become spontaneously contracting myofibres. During further growth, the satellite cells divide and their nuclei are added to the fibres, mimicking the processes of muscle growth and regeneration after injury.

A better understanding of the involvement of satellite cells in postnatal myogenesis and in muscle hypertrophy will be essential to improve the efficiency of muscle growth in meat producing animals [3]. Muscle cells from pigs [11] and cattle [3] have been cultured successfully and directed towards differentiation or proliferation by growth factors [15] and hormones [3]. Electrical and mechanical stimulation have also been applied to the cells in vitro, as a means of mimicking the application of a 'workload' [43]. Cultured human skeletal muscle cells have also been analysed for their degree of maturity by measuring Na⁺,K⁺-ATPase activity [2].

Future Research
We are currently developing an in vitro model to test the hypothesis that exercise, hormones and growth factors together determine the variations found in growth and fibre composition of skeletal muscle. Two fundamental questions are under consideration. First, do slow and fast muscle fibres respond differently to physical and hormonal stimulation? And second, when during embryonic and post-natal development are the growth and composition of skeletal muscle fibres most affected by these stimuli?

Concluding remarks
The concentration of Na⁺,K⁺ pumps in the skeletal muscle of cats, dogs, horses and cattle is regulated by mechanisms similar to those described in rodents and man. We already know that hyperthyroidism and physical training increase the number of Na⁺,K⁺ pumps in skeletal muscle and that hypothyroidism and immobility reduce their number. We know, too, that the rise in Na⁺,K⁺ pump concentration after training is associated with a blunted rise in plasma K⁺ during exercise. However, the question remains whether the mechanism responsible for the up-regulation of the Na⁺,K⁺ pump during hyperthyroidism is the same as that during training.

References
1. Allen, R.A. and Rankin, L.A. (1990) Regulation of satellite cells during skeletal muscle growth and development. Proc. Soc. exp. Biol. Med. 194, 81-86.

2. Benders, A.A.G.M., van Kuppevelt, T.H.M.S.M., Oosterhof, A., Wevers, R.A., Veerkamp, J.H. (1992) Adenosine triphosphatase during maturation of cultured human skeletal muscle cells and in adult human muscle. Biochim. Biophys. Acta 1112, 89-98.

3. Cassar-Malek, I., Langlois, N., Picard, B. and Geay, Y. (1999) Regulation of bovine satellite cell proliferation and differentiation by insulin and triiodothyronine. Domest. Anim. Endocrinol. 17, 373-388.

4. Clausen, T. (1996) Long and short-term regulation of the Na⁺,K⁺ pump in skeletal muscle. News Physiol. Sci. 11, 24-30.

5. Clausen, T. and Everts, M.E. (1989) Regulation of the Na⁺,K⁺ pump in skeletal muscle. Kidney Int. 35, 1-13.

6. Clausen, T. and Nielsen, O.B. (1999) The Na⁺,K⁺-pump in muscle and CNS - physiology and pathophysiology. NS 2, 4-8.

7. Clausen, T., van Hardeveld, C. and Everts, M.E. (1991) Significance of cation transport in control of energy metabolism and thermogenesis. Physiol. Rev. 71, 733-774.

8. Dauncey, M.J. and Harrison, A.P. (1996) Developmental regulation of cation pumps in skeletal and cardiac muscle. Acta Physiol. Scand. 156, 313-323.

9. Dingboom, E.G., Dijkstra, G., Enzerink, E., van Oudheusden, H.C. and Weijs, W.A. (1999) Postnatal muscle fibre composition of the gluteus medius muscle of Dutch warmblood foals; maturation and the influence of exercise. Equine vet. J. Supplement 31, 95-100.

10. Docter, R., Krenning, E.P. de Jong, M. and Hennemann, G. (1993) The sick euthyroid syndrome: Changes in thyroid hormone serum parameters and hormone metabolism. Clin. Endocrinol. 39, 499-518.

11. Doumit,M.E. and Merkel, R.A. (1992) Conditions for isolation and culture of porcine myogenic satellite cells. Tissue and Cell 24, 253-262.

12. Everts, M.E. and Clausen, T. (1988) Effects of thyroid hormone on Na⁺,K⁺-transport in resting and stimulated rat skeletal muscle. Am. J. Physiol. 255, E604-E612.

13. Everts, M.E., Simonides, W.S., Leijendekker, W.J. and van Hardeveld, C. (1987) Fatigability and recovery of rat soleus muscle in hyperthyroidism. Metabolism 36, 444-450.

14. Evertsen, F., Medbø, J.I., Jebens, E. and Nicolaysen, K. (1997) Hard training for 5 months increases Na⁺,K⁺ pump concentration in skeletal muscle of cross-country skiers. Am. J. Physiol. 272, R1417-R1424.

15. Fligger, J.M., Malven, P.V., Doumit, M.E., Merkel, R.A. and Grant, A.L. (1998) Increases in insulin-like growth factor binding protein-2 accompany decreases in proliferation and differentiation when porcine muscle satellite cells undergo multiple passages. J. Anim. Sci. 76, 2086-2093.

16. Fosha-Dolezal, S.R. and Fedde, M.R. (1988) Serum potassium during exercise in Hereford calves: influence of physical conditioning. J. appl. Physiol. 65, 1360-1366.

17. Green, H.J., Chin, E.R., Ball-Burnett, M. and Ranney, D. (1993) Increases in human skeletal muscle Na⁺,K⁺-ATPase concentration with short-term training. Am. J. Physiol. 264, C1538-C1541.

18. Hansen, O. and Clausen, T. (1988) Quantitative determination of the Na⁺,K⁺-ATPase and other sarcolemmal components in muscle cells. Am. J. Physiol. 254, C1-C7.

19. Harrison, A.P. and Clausen, T. (1998) Thyroid hormone-induced upregulation of Na⁺ channels and Na⁺,K⁺ pumps: implications for contractility. Am. J. Physiol. 274, R864-R867.

20. Ismail-Beigi, F. and Edelman, I.S. (1970) Mechanism of thyroid calorigenesis: Role of active sodium transport. Proc. Natl. Acad. Sci. USA 67, 1071-1078.

21. Kjeldsen, K., Everts, M.E. and Clausen, T. (1986a) The effects of thyroid hormones on ³H-ouabain binding site concentration, Na⁺,K⁺ contents and ⁸⁶Rb-efflux in rat skeletal muscle. Pflugers Arch. 406, 529-535.

22. Kjeldsen, K., Norgaard, A., Gotzsche, C.O., Thomassen, A., Clausen, T. (1984) Effect of thyroid function on number of Na⁺-K⁺ pumps in human skeletal muscle. Lancet II, 8-10.

23. Kjeldsen, K., Richter, E.A., Galbo, H., Lortie, G. and Clausen, T. (1986b) Training increases the concentration of 3H-ouabain binding sites in rat skeletal muscle. Biochim. Biophys. Acta 860, 708-712.

24. Klont, R.E., Brocks, L. and Eikelenboom, G. (1998) Muscle fibre type and meat quality. Meat Science 49 Supplement 1, S219-S229.

25. Knochel, J.P., Blachley, J.D., Johnson, J.H. and Carter, N.W. (1985) Muscle cell electrical hyperpolarization and reduced exercise hyperkalemia in physically conditioned dogs. J. clin. Invest. 75, 740-745.

26. Leivseth, G., Clausen, T., Everts, M.E. and Bjorda, E. (1992) Effects of reduced joint mobility and training on Na⁺,K⁺-ATPase and Ca2⁺-ATPase in skeletal muscle. Muscle and Nerve 15, 843-849.

27. Lindinger, M.I. and Sjøgaard, G. (1991) Potassium regulation during exercise and recovery. Sports Med. 11, 382-401.

28. McCutcheon, L.J., Geor, R.J. and Shen, H. (1999) Skeletal muscle Na⁺,K⁺-ATPase and K⁺ homeostasis during exercise: effects of short-term training. Equine vet. J. Supplement 30, 303-310.

29. McKenna, M.J., Schmidt, T.A., Hargreaves, M., Cameron, L., Skinner, S.L. and Kjeldsen, K. (1993) Sprint training increases human skeletal muscle Na⁺,K⁺-ATPase concentration and improves K⁺ regulation. J. appl. Physiol. 75, 173-180.

30. Nørgaard, A., Kjeldsen, K. and Clausen, T. A. (1984) Method for the determination of the total number of 3H-ouabain binding sites in biopsies of human skeletal muscle. Scand. J. clin. lab. Invest. 44, 509-518.

31. Pickar, J.G., Spier, S.J. Harrold, D. and Carlsen, R.C. (1993) [³H]Ouabain binding in skeletal muscle from horses with hyperkalemic periodic paralysis. Am. J. vet. Res. 54, 783-787.

32. Putman, C.T., Dusterhoft, D. and Pette, D. (1998) Changes in satellite cell content and myosin isoforms in low-frequency-stimulated fast muscle of hypothyroid rats. J. appl. Physiol. 86, 40-51.

33. Rijnberk, A. (1996) The thyroids. In: Clinical endocrinology of dogs and cats: an illustrated text. Ed. A. Rijnberk. Kluwer Academic Publishers, Amsterdam, pp. 35-59.

34. Schaafsma, I.A., van Emst, M.G., Kooistra, H.S., Verkleij, C.B., Peeters, M.E., Rijnberk, A. and Everts, M.E. (2000) Exercise induced hyperkalemia in hypothyroid dogs. Abstracts of the World Congress of WSAVA/FECAVA, Amsterdam, 2000.

35. Simonides, W.S. and van Hardeveld, C. (1989) The postnatal development of sarcoplasmic reticulum Ca2⁺-transport activity in skeletal muscle of the rat is critically dependent on thyroid hormone. Endocrinology 124, 1145-1153.

36. Sjøgaard, G. (1990) Exercise-induced muscle fatigue: the significance of potassium. Acta Physiol. Scand. 140, Suppl. 593, 1-63.

37. Skou, J.C. (1957) The influence of some cations on an adenosine triphosphatase from peripheral nerves. Biochim. Biophys. Acta 23, 394-401.

38. Suwannachot, P., Verkleij, C.B., Kocsis, S., Enzerink, E. and Everts, M.E. (2000) Prolonged food restriction and mild exercise in Shetland ponies: effects on weight gain, thyroid hormone concentrations, and muscle Na⁺,K⁺-ATPase. J. Endocrinol. In press.

39. Suwannachot, P., Verkleij, C.B., Kocsis, S., van Weeren, P.R. and Everts, M.E. (2000) Specificity and reversibility of the training effects on the concentration of Na⁺,K⁺-ATPase in foal muscle. Equine vet. J. In press.

40. Suwannachot, P., Verkleij, C.B., Weijs, W.A., van Weeren, P.R. and Everts, M.E. (1999) Effects of training on the concentration of Na⁺,K⁺-ATPase in foal muscle. Equine vet. J. Suppl 31, 101-105.

41. Sweadner, K.J. (1989) Izozymes of the Na⁺,K⁺-ATPase. Biochim. Biophys. Acta 988, 185-220.

42. van Weeren, P.R. and Barneveld, A. (1999) The influence of exercise during the first months of life on the development of the equine musculoskeletal system with special attention to osteochondrosis: Experimental set-up and scope of the study. Equine vet. J. Suppl 31, 4-8.

43. Vandenburgh, H.H. (1992) Mechanical forces and their second messengers in stimulating cell growth in vitro. Am. J. Physiol. 262, R350-R355.

44. Veeneklaas, R.J., Verkleij, C.B., van Schie, B., Harun, M.A.S. and Everts, M.E. (2000) Na⁺,K⁺-ATPase in skeletal muscle of draught animals in Mozambique: effect of race, gender, age and training. Submitted for publication.

45. Wasserman, K., Stringer, W.W. Casaburi, R. and Zhang, Y-Y. (1997) Mechanism of the exercise hyperkalemia: an alternate hypothesis. J. appl. Physiol. 83, 631-643.

46. Yalcin, Y., Carman, D., Shao, Y., Ismail-Beigi, F., Klein, I. and Ojamaa, K. (1999) Regulation of Na⁺/K⁺-ATPase gene expression by thyroid hormone and hyperkalemia in the heart. Thyroid 9, 53-59.


© 2000 Veterinary Sciences Tomorrow