Evolution of contractile and elastic properties of rat soleus muscle fibres under unloading conditions.

Rats were submitted to 14 days of hindlimb suspension in order to examine the contractile and elastic properties of the soleus muscles under disuse conditions. The calcium/strontium activation properties, the maximal shortening velocity (V0), as well as the time behaviour of force transients following quick releases and the T1 curves characterizing the active part of the series elastic elements, were determined on single chemically skinned fibres. After the functional measurements, the fibres were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis in order to analyse both the myosin heavy (MHC) and light (MLC) chain isoforms. According to the MHC and MLC composition, two groups of fibres were defined after hindlimb suspension: a group of slow fibres expressing the slow set of both MHC and MLC isoforms, and a group of fast fibres co-expressing the slow and fast MHC and MLC isoforms with a predominant expression of the fast ones. For the first group, the contractile as well as the elastic properties were found to be close to those of control slow soleus fibres. For the second group, both contractile and elastic properties were modified insofar as they became close to those found in a fast muscle such as the extensor digitorum longus. We suggested that between the two populations found in the soleus muscle after hindlimb suspension the modifications in the contractile properties, as well as the alterations in the elastic characteristics, were concomitant to the changes in both MHC and MLC compositions.