Effects of age on sarcoplasmic reticulum properties and histochemical composition of fast- and slow-twitch XXXX muscles.

@entity35 @entity5834 @entity460
@entity460 release activity of sarcoplasmic reticulum and enzyme-histochemical properties were investigated in extensor digitorum longus (e.d.l.) and soleus muscles in young (4 months and old (24 months) male @entity35 . With age, the @entity5834 threshold concentration for @entity460 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 @entity5834 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 represent almost the totality of the muscle fibres (more than 98%), while types 2C and 2A were reduced in proportion. In aged e.d.l. the percentage of type 1 (P < 0.05), 2A and 2X (a recently identified fourth component of the fast-twitch muscle types) fibres increased, with a reduction of type 2B (P < 0.01) fibres. The present results suggest that the changes in contractile properties of aged muscles may be related to the changes not only in fibre composition but also in the mechanism of @entity460 release from sarcoplasmic reticulum.
