Effects of ageing on contractility and myosin composition of rat and man skeletal muscles and muscle cells : with special reference to muscle adaptability

Sammanfattning: The aims of this thesis were to characterize the effects of ageing on the contractile and biochemical properties of fast- and slow-twitch skeletal muscles at the whole muscle and single fibre level in an attempt to improve our understanding of the mechanisms underlying the age-related impairments of muscle fibres. Specific interest was in the adaptabiliy of skeletal muscle and the regulation of speed of shonening at single-cell level in the rat and man. At the whole-muscle level in the rat, an age-related increase in contraction and half-relaxation times of the isometric twitch was observed in both fast- and slow-twitch muscles, as well as an age-related increase in the proportion of type I, and decrease in type IIA, fibres in the slow-twitch soleus. Four weeks of T3 treatment eliminated the age-related differences in isometric twitch propenies in both soleus amd EDL muscles. T3 induced a dramatic decrease in the proportion of type I fibres from young and old soleus, with a concomitant increase in the relative number of type IC and IIC fibres. But the fibre type composition of EDL was not affected either by ageing or by T3 treatment. In the rat soleus, the type I MyHC content was higher and type IIA content lower in old rats than in the young. Type IIXMyHC was observed in some young control soleus muscles, but not in old ones. After four weeks of T3 treatment, the content of type I MyHC decreased and type IIA and IIX increased in both young and old animals, resulting in an elimination of the age-related difference in the contents of type I and 11 MyHCs. In the EDL, the content of type IIX MyHC was significantly higher in the old animals, at the expense of a lower content of type IIB MyHC. MyHC composition was not affected significantly by T, treatment in EDL, irrespective of age. In single skinned fibre preparations, V0 was significantly lower in soleus fibres from old control rats than from young ones, despite the fact that all fibres expressed the type I MyHC and slow MyLC isoforms. In the EDL, V0 values in single fibres did not differ between young and old animals. Further, the co-ordinated expression of MyLC and MyHC isoforms in type IIXB fibres, as well as the correlation between V0 and the compositions of MyLC and MyHC, that was observed in young rats was lost in old animals. We speculate that there could be more than one beta/slow MyHC isoform and an age-related transition within these isoforms. Thyroid hormone induced a dramatic up-regulation of type IIA or/and IIX MyHCs as well as the fast isoforms of MyLCs insoleus muscle fibres, but variable amounts of type I MyHC were still observed in almost all single soleus cells in both young and old animals. T3 had no detectable effect on V0 in young soleus fibres, but significantly increased VO in old soleus fibres. However, the age-related decline in V0 still persisted in the soleus fibres. Myosin composition and VO of EDL fibres were notaffected by T3 treatment, either in young or old rats. There could be slow MyHC diversity not detected by our methods or theMyHC ATPase rate may be regulated by mechanisms other than isoform transitions as fibres become older, and this effect ispanially reversed by T3. In man, muscle biopsies were obhined from young, sedentary old and well trained old male volunteers. Type I MyHC fibres were more frequent in the old subjects than in the young and well trained old ones. Myofibrillar ATPase (mATPase) activities were lower in all fibre types in the old as compared to the young subjects, while mATPase in fibres from well-trained old men were intermediate between those of young and old conuols. MyLC3 was observed in many type I MyHC fibres, irrespectiveof age. The type I fibres expressing both slow and fast isoforms of MyLC2 were more frequent in the young subjects than in the old. In parallel with the age-related decrease in mATPase activiy, age-related declines in V0 and specific tension were observed in type I and IIA MyHC fibres according to 6% SDS-PAGE. Physical activiy in the old men counteracted the age-related atrophy of fast muscle cells, but V0 as well as specific tension was still lower in the physically active old subjects tbanin young controls. In conclusion, the altered contractile properties in type I and IIA MyHC fibres are in pan related to changes in enzymatic properties of myosin and in the composition of MyLC isoforms. However, other etiological factors also have tobe considered such as an incomplete characterization of MyHCs or the fact that other thick and thin filament proteins influencing contractility are altered in old age.