The influence of suspension hypokinesia on contractile properties of slow and fast twitch muscles of young growing and adult rats.

The influence of the lack of the gravity force on contractile properties of the slow-twitch soleus (SOL) and the fast-twitch extensor digitorum longus (EDL) muscles was investigated in 3 week or 6-month-old rats suspended by their tail. In both groups suspension hypokinesia (SH) lasting 3 weeks or longer leads to a marked decrease of the mass and of the mean cross-sectional area of the SOL. The SOL of suspended animals shows decreased contraction and half-relaxation times and a lower tension of the single twitch, a decreased maximum tetanic force output, an increased fusion frequency and a decreased tetanus-half-rise time. The force developed per unit cross-sectional area and the twitch-tetanus ratio are unchanged. The atrophy and the changes in contractile properties are more pronounced in young developing rats than in adult ones. The SOL of suspended young rats does not develop into a slow-twitch muscle and shows a posttetanic potentiation and a cooling potentiation of the single twitch, properties observed in fast-twitch muscles only. In comparison to controls SH has no influence on the wet weight, mean cross-sectional area, and contractile properties of the EDL of young or adult rats. The data presented here and supported by histochemical and biochemical results demonstrates the importance of the gravity force for slow-twitch muscles especially during development.