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.

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