DIFFERENCES IN ULTRASTRUCTURAL AND METABOLIC PROFILES WITHIN THE SAME TYPE OF FIBERS IN VARIOUS MUSCLES OF YOUNG AND ADULT-RATS

Single fibres from tibialis anterior, extension digitorum longus, gastrocnemius and soleus muscles in young (4-week-old) and adult (35-week-old) Wistar male rats were classified into three types on the basis of their enzyme-histochemical features; slow-twitch oxidative (SO), fast-twitch oxidative and glycolytic (FOG) and fast-twitch glycolytic (FG) fibres. Ultrastructural (volume density of mitochondria: V-mt and Z line width) and metabolic (phosphofructokinase: PFK and succinate dehydrogenase: SDH activities) profiles were measured. PFK activity in all types of fibres was higher in adult rats, and the difference between the two age-groups (adult/young) was largest between FG, FOG and SO fibres respectively. SDH activity and V-mt were lower in adult rats in a similar way in all fibres. A significant positive correlation was observed between the V-mt and SDH activity in both age-groups. This positive correlation was very specific in fast-twitch and slow-twitch fibres. Changes in the V-mt did not relate directly to the changes in fibre cross-sectional area. The overall pattern indicates that glycolytic capacity of fast-twitch fibres in flexor muscles (TA and EDL) is higher than in extensor muscles (GC and SOL), and that oxidative capacity of all types of fibre in extensor muscles is higher than in flexor muscles. These profiles were changed by growth, and may be related to the specific differences in pattern of activity of each skeletal muscle, and may reflect differences in the recruitment of different muscles.