Arrest of developmental conversion of type II to type I fibres after suspension hypokinesia

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Abstract
The effects of hypokinesia and of the lack of gravity on muscle fibres, fibre type composition and myosin light chain pattern, as well as on muscle mechanoreceptors were investigated in the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles of young growing and adult rats after suspension hypokinesia (SH) of their hind limbs. The animals were suspended by their tail so that their hind limbs were relieved of their normal weight-bearing function for 3–6 weeks. In normal 3-to 4-week-old rats the SOL contained about 50% type I fibres and their percentage increased up to about 80% until the 10th week, with simultaneous reduction of type IIA fibres. After 3 to 6 weeks of suspension treatment maintained from 3-to, 4-week-old rats up to 6 to 10 weeks of age, the SOL still only contained about 50% of type I fibres. The content of fast LC1 and LC2 in the SOL of 6-week-old rats after 3 weeks of suspension was higher than that of control litter-mates reflecting the higher occurrence of IIA fibres in the suspended solei. No changes in fibre type composition were observed after SH performed in adult rats. SH thus leads, in young animals, to the arrest of conversion of type IIA to type I fibres resulting in the persistence of the fibre type composition and of the myosin light chain pattern corresponding to those present in the SOL at the time of the onset of suspension. In both young and adult rats, SH markedly decreased the mass and the mean cross-sectional area of the SOL, mainly due to the severe atrophy of type I fibres. We observed no signs indicating conversion of type I back to type IIA muscle fibres due to the SH either in young or adult animals. In contrast to profound changes in the SOL, no significant differences were found in the EDL in any of the parameters studied. No changes in the investigated parameters of muscle spindles and tendon organs were observed after SH, performed either in young or in adult rats. We thus conclude that SH leads to muscle atrophy and that it influences mainly or exclusively type I fibres in muscles with a postural function such as the SOL. It is suggested that in young rats SH arrests changes in the SOL motoneurones, which remain unable to ensure the normal developmental transformation of type IIA into type I fibres, thus preventing conversion of the SOL into a typical slow-twitch muscle.