Thyroid hormone effects on contractility and myosin composition of soleus muscle and single fibres from young and old rats.

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Abstract
1. Young (3‐6 months) and old (20‐24 months) male Wistar rat soleus muscles were examined for myosin isoform composition, fibre type, contractility and sarcoplasmic reticulum (SR) Ca2+ release properties either in control rats or in rats treated with thyroid hormone (3,5,3'‐triiodothyronine, T3) for 4 weeks. 2. T3 treatment had a strong impact on myosin heavy chain (MyHC) and light chain (MyLC) isoform composition in both young and old rats. That is, all single fibres co‐expressed type I and IIA (type I/IIA fibres) or type I, IIA and IIX MyHCs (type I/IIAX fibres) after treatment. Slow and fast MyLC isoforms, i.e. MyLC1s, MyLC1f, MyLC2s, MyLC2f and MyLC3, co‐existed in each of the type I/IIA and I/IIAX fibres in variable proportions. 3. In old rats the maximum velocity of unloaded shortening (V0) was related to MyHC isoform composition: V0 for type I fibres was less than that for type I/IIA fibres which was less than that for type I/IIAX fibres. In young rats, on the other hand, V0 did not differ between pure type I fibres from controls and those co‐expressing type I and type II MyHC isoforms from T3‐treated rats. 4. Contraction and half‐relaxation times of the isometric twitch were significantly longer in old than in young controls. This was paralleled by an age‐related decrease in the caffeine threshold of the SR. Four weeks of T3 treatment eliminated the age‐related differences in both speed of twitch contraction and caffeine thresholds. V0, on the other hand, was slower in old than in young animals, both control and T3‐treated, when cells with a similar MyHC composition were compared. 5. In conclusion, thyroid hormone can substantially reverse at least some of the changes that occur in ageing muscle. Further, the age‐related decline in V0 in soleus fibres from control and hyperthyroid rats suggests that: (1) the identification of beta/slow myosin isoforms is incomplete; or (2) the molecular characteristics of MyHC differ between young and old age; or (3) MyHC is not the only determinant of V0.

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