EFFECTS OF AGE ON SARCOPLASMIC-RETICULUM PROPERTIES AND HISTOCHEMICALCOMPOSITION OF FAST-AND SLOW-TWITCH RAT MUSCLES

Calcium release activity of sarcoplasmic reticulum and enzyme-histoche mical properties were investigated in extensor digitorum longus (e.d.l .) and soleus muscles in young (4 months) and old (24 months) male rat s. With age, the caffeine threshold concentration for calcium release from the sarcoplasmic reticulum of soleus skinned muscle fibres showed only minor modifications. On the other hand, in e.d.l. skinned muscle fibres, the caffeine threshold concentration decreased significantly (P < 0.05). The histochemical fibre type composition changed with age both in soleus and in e.d.l. muscles, showing a common transformation toward a more oxidative histochemical profile. In fact, in aged soleus , a significant (P < 0.05) increase was observed of type 1 fibres to r epresent almost the totality of the muscle fibres (more than 98%), whi le types 2C and 2A were reduced in proportion. In aged e.d.l. the perc entage of type 1 (P < 0.05), 2A and 2X (a recently identified fourth c omponent of the fast-twitch muscle types) fibres increased, with a red uction of type 2B (P < 0.01) fibres. The present results suggest that the changes in contractile properties of aged muscles may be related t o the changes not only in fibre composition but also in the mechanism of calcium release from sarcoplasmic reticulum.