Effects of immobilization on the rat soleus muscle in relation to age.

A hind limb of young adult, adult and old male Wistar rats (4-5, 6-7 and 20-21 months, respectively) was immobilized for 4 weeks by a plaster cast with the knee and ankle joints in a resting position. Enzyme-histochemical, morphometrical and contractile characteristics of the soleus muscle were compared with those in age-matched controls. A pronounced decrease in muscle mass and cross-sectional muscle fibre area was found at all ages. The degree of atrophy after immobilization did not differ between different fibre types in each age group, but the decrease in fibre area was less pronounced in old animals (i.e. the fibre area was decreased by 49-64, 53-66 and 27-38% in young adult, adult and old animals, respectively). The maximum tetanus force was decreased in all age groups (by 73, 78 and 69% in young adult, adult and old rats, respectively) as was the tetanus tension (i.e. tetanus force divided by muscle fibre cross-sectional area). The contraction time of the isometric twitch was significantly altered, i.e. decreased, only in the youngest age group, although it also tended to decrease in old age. A significant increase in the number and proportion of fibre types intermediate to types I and IIA, was found in the immobilized muscle of 4-5- and 6-7-month-old animals, but not in that of old ones (i.e. the proportion of intermediate fibres increased by 14, 13 and 2% in young adult, adult and old animals, respectively). Thus, in contrast to the atrophic changes, the contractile alterations after immobilization were not markedly different between young and old age. It is further concluded that the age-related fast-to-slow muscle fibre transition that occurs in normal soleus during maturation and growth can be partly reversed by restrictions of the normal muscle activity and that the ability of the soleus to modulate its fibre-type composition in response to a change in activity may be diminished in old age.