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) Ca2P 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 (AMyLC) isoform 3. In old rats the maximum velocity of unloaded shortening (VO) 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, VO 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. VOX 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 f/slow myosin isoforms is incomplete; or (2) the molecular characteristics of MyHC differ between young and old age; or (3) M1yHC is not the only determinant of VO.It is well known that thyroid hormone (3,5,3'-triiodo-total amount of SR, in the Ca2P transport activity of SR, thyronine, T3) is very important in normal development of and in the percentage of fibres expressing the fast-type SR vertebrate skeletal muscle, and an intact thyroid gland is Ca2+-ATPase in rat skeletal muscle (Fitts, Winder, Brooke, required both for the dev7elopment of muscle mass and for the Kaiser & Holloszy, 1980;Muller, van (Izumo, Nadal-Ginard & 1iahdavi, 1986 (Fig. IB). In all conditions there was a strong correlation between the fibre types identified by enzyme-histochemical staining and the MyHC-reactive antibodies with which they stained.In the present study, using immunocvtochemical staining, we did not find any difference between the slow M-vHC (Fig. 1A and B). Correlations of the sort shown in Fig. 1 (Staron & Pette, 1986). These data suggest that a continuum of fibre types exists in which loss of A4.951+ slow MyHC is paralleled by an increase in A4.74+ type IIA MyHC. Only in those fibres that make a more marked shift towards faster contractile characteristics does the remaining A4.840+ slow AMvHC isoform decline. This is the reverse of the normal developmental progression where A4.951+ slow MIyHC succeeds A4.840+A4.951-slow 1\iyHC in slow fibres around the third postnatal week (Hughes et al. 1993).
Electrophoretic s...
