Different time course of changes in sarcoplasmic reticulum and myosin isoforms in rat soleus muscle at early stage of hyperthyroidism

Aims and methods: We studied changes in the expression of myosin and sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) and in Ca2+ uptake and release rates by the SR in rat slow-twitch soleus muscles treated with thyroid hormone (3,5,3'-triiodo-L-thyronine, T-3) for 1, 3, 7 and 21 days. Results: The content of fast SR Ca2+-ATPase isoform SERCA1a protein significantly increased as early as 3 days of T-3 treatment, while a rise in fast myosin heavy chain isoforms was found only 21 days after the onset of T-3 administration. T-3-induced slow-to-fast transitions were much less pronounced in myosin light chain than in myosin heavy chain. Moreover, in vitro measurements of SR Ca2+-handling capacity with indo-1 indicated that increases in uptake rate preceded those of release rate (7 vs. 21 days). Conclusion: These data suggest that changes in the myosin and SERCA expressions seem not to occur in a coordinated manner at the early stage of hyperthyroidism and that intervention with T-3 may temporarily produce slow-twitch type I fibres which contain, in addition to slow SERCA2a isoform, the appreciable amounts of fast SERCA1a isoform. Moreover, it is speculated that some fibres in the hyperthyroid soleus muscles may temporarily acquire unique contractile properties distinct from normal fast and slow fibres.