Protein synthesis in fast and slow muscles of developing cockerels loaded with 2G for 3 weeks

In our previous study, the increment of RNA and protein content ratios against body weight in the whole skeletal muscle were determined in young developing cockerels under 2G hypergravity for 3 weeks. In the present study, protein synthesis rate, RNA activity and protein content were investigated in different types of muscles [slow muscle (soleus) and fast muscle (extensor digitorum longus; EDL)] in similar experimental conditions.

It is believed that slow and fast muscle will show different responses to hypergravity.

Young cockerels were raised standing vertically in a centrifuge and lived in a hypergravity (2G) environment for 3 weeks. At the end of the 3 weeks, the soleus and EDL muscles in their legs were immediately removed and the protein and RNA extracted. The fractional rate of protein synthesis in the muscles were determined by incorporation of L-[4-3H]-phenylalanine.

The feed intake of centrifuged cockerels was reduced on the first day but recovered on the second day. Thus, the growth curve was parallel although 2G-loaded cockerels were smaller in size. In a slow muscle (soleus) the protein synthesis rate, protein content and muscle mass were significantly increased after 2G loading. While fast muscle (EDL) protein synthesis rate (Ks) and RNA activity decreased, muscle mass and protein content had no significant change in the 2G loading group.

Soleus, which supports the body against gravity, apparently responded to hypergravity, while EDL may be repressed in protein turnover under the same condition. These findings imply that slow and fast muscles show different responses to hypergravity according to their function.