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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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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higher, and Na⁺,K⁺ pump concentrations around 75% higher, in hyperthyroid compared to euthyroid cats (Schaafsma et al, unpublished data).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of [³H]ouabain binding sites had been maintained while the total plasma T₄ returned to normal (Schaafsma et al, unpublished data).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.

Fig.6 Plasma K+ levels in age-matched euthyroid and hypothyroid Beagle dogs, before and after exercise. Exercise consisted of a single 5 min run at a speed of 4.5 km/h. Each point on the graph represents the mean of 6, or in some cases 7, observations. The bars denote the standard error (SE) for each mean value;
* P<0.05 or better within groups; † P<0.05 or better between groups.

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].

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