Developmental changes of glycogen enzymes in fast and slow muscles of the rat.

Glycogen synthase (GS), Branching enzyme (BE), Glycogen Phosphorylase (GPho) and the Debranching enzyme (DE) activities were studied in the white extensor digitorum longus (ELD) and in the red soleus (S) muscles of rats during the developmental period from birth to the young adulthood. During the first days of postnatal life all enzyme activities similarly increase in both ELD and in S. BE and DE seem to rise faster and reach the adult levels earlier than GS and GPho. Significant differences can be seen between ELD and S by age 14--21 days: glycogenolytic enzymes rise higher in the ELD, GS activity is similar in both muscles whereas BE activity is higher in S. All these develop changes are completed in 25 days. Thereafter, only GPho activity in the ELD continues to slowly increase.