Effects of ageing and chronic dietary restriction on the morphology of fast and slow muscles of the rat.

The soleus and extensor digitorum longus muscles of the hindlimb and the flexor digitorum profundus muscle of the forelimb were studied in ad libitum-fed control and age-matched diet-restricted male rats at various ages from weaning to senescence. Growth of individual muscles was accomplished by fibre hypertrophy and not hyperplasia. Between weaning and one year, fibre numbers remained constant in the soleus but fell by 50% in the extensor digitorum longus. Both muscles displayed increasingly oxidative fibre type profiles with advancing age, irrespective of dietary status. This was particularly noticeable in the soleus, which transformed its fibre population from one containing 35% fast fibres at weaning to one with no fast fibres at 91 weeks. In senility, however, the fibre type population again displayed 25% fast fibres. The capillary: fibre ratio and the capillary density were correlated with muscle fibre size in both hindlimb muscles. Although capillarity increased with age, expected differences between fast and slow muscles were probably minimised by the high proportion of FOG fibres in the extensor digitorum longus. Both hindlimb muscles displayed significant increases in the ratio of connective: muscle tissue with increasing age. The soleus invariably contained more connective tissue than the extensor digitorum longus. Dietary restriction reduced the rate of increase, so that the connective tissue content was approximately one half that found in control muscles at one year. Various pathological features associated with old age were delayed considerably in the muscles of the diet-restricted rats. It is concluded that chronic dietary restriction imposed directly after weaning has a dramatic effect on the normal growth and ageing of skeletal muscle.