These results indicate that both exercise modalities have an effect on a hindlimb fast-twitch muscle in mice, with the greatest impact seen with forced treadmill running.

In horses, show jumping training at an early age resulted in increased Na(+),K(+)-ATPase content of the deep portions of the gluteus medius muscle. The lack of training effects on the other muscle characteristics can partly be explained by the fact that an appropriate (aerobic) fiber type composition was already established at training commencement. These data also suggested that the developmental changes in equine muscle represent sufficient adaptation to meet the demands of this specific training.

Uptake of tri-iodothyronine (T 3 ) was compared with that of thyroxine (T 4 ) in the embryonic heart cell line H9c2 (2-1). These cells propagate as myoblasts and form differentiated myotubes upon reduction of the serum concentration, as indicated by a 31-fold increase in creatine kinase activity. Protein and DNA content per well were around 2-fold higher in myotubes than in myoblasts. When expressed per well, T 3 and T 4 uptake were, compared with myoblasts, 1·9-to 2-fold and 3·1-to 4-fold higher in myotubes respectively. On the other hand, the characteristics of T 3 and T 4 uptake were similar in myoblasts and myotubes. At any time-point, T 4 uptake was 2-fold higher than that of T 3 , and both uptakes were energy but not Na + dependent. T 3 and T 4 uptake exhibited mutual inhibition in myoblasts and myotubes: 10 µM unlabeled T 3 reduced T 4 uptake by 51-60% (P<0·001), while 10 µM T 4 inhibited T 3 uptake by 48-51% (P<0·001). Furthermore, T 3 and T 4 uptake in myoblasts was dose-dependently inhibited by tryptophan (maximum inhibition around 70%; P<0·001). Exposure of the cells to T 3 or T 4 during differentiation significantly increased the fusion index (35 and 40%; P<0·01). Finally, both myoblasts and myotubes showed a small deiodinase type I activity, while deiodinase type II activity was undetectable. In conclusion, T 3 and T 4 share a common energy-dependent transport system in H9c2(2-1) cells, that may be important for the availability of thyroid hormone during differentiation.

In horses, the activation of skeletal muscle Na+,K(+)-ATPase during exercise is likely to decrease with age. Training appears to result in an increase in Na+,K(+)-ATPase activity in skeletal muscle with subsequent upregulation of Na+,K(+)-ATPase concentration if the existing Na+,K(+)-ATPase capacity cannot meet requirements.

Conjugation reactions are important pathways in the peripheral metabolism of thyroid hormones. Rat cardiac fibroblasts produce and secrete glucuronidated thyroxine (T4G) and 3,3 ,5-triiodothyronine (T3G). We here show that, compared to fibroblasts from other anatomical locations, the capacity of cardiofibroblasts to secrete T4G and T3G is highest. H9c2(2-1) myotubes, a model system for cardiomyocytes, take up T4G and T3G at a rate that is 10-15 times higher than that for the unconjugated thyroid hormones. T3 and T4, and their glucuronides, stimulate H9c2(2-1) myoblast-to-myotube differentiation. A substantial ␤-glucuronidase activity was measured in H9c2(2-1) myotubes, and this confers a deconjugating capacity to these cells, via which native thyroid hormones can be regenerated from glucuronidated precursors. This indicates that the stimulatory effects on myoblast differentiation are exerted by the native hormones. We suggest that glucuronidation represents a mechanism to uncouple local thyroid hormone action in the heart from that in other peripheral tissues and in the systemic circulation. This could represent a mechanism for the local fine-tuning of cardiac thyroid hormone action.