Recovery of the soleus muscle after short- and long-term disuse induced by hindlimb unloading: effects on the electrical properties and myosin heavy chain profile.

The hindlimb unloading (HU) rat is a model of muscle disuse characterized by atrophy and slow-to-fast phenotype transition of the postural muscles, such as the soleus. We previously found that the resting sarcolemmal chloride conductance (gCl) that is typically lower in slow-twitch myofibers than in fast ones increased in soleus fibers following 1 to 3 weeks of HU in accord with the slow-to-fast transition of myosin heavy chain (MHC) isoforms. Nevertheless, the gCl already raised after a 3-day HU, whereas no change in MHC expression was detected. The present work evaluates the ability of soleus muscle to recover on return to normal load after a short (3 days) or long (2 weeks) disuse period. The changes observed after a 2-week HU were slowly reversible, since 3-4 weeks of reloading were needed to completely recover gCl, fiber diameter, MHC expression pattern, as well as the mechanical threshold Rheobase, an index of calcium homeostasis. After 3-day HU, the gCl increased homogeneously in most of the soleus muscle fibers and gCl recovery was rapidly completed after 4-day reloading. These results suggest different induction mechanisms for gCl augmentation after the short and long HU periods, as well as a possible role for gCl in the slow muscle adaptation to disuse.

Change of chloride ion channel conductance is an early event of slow-to-fast fibre type transition during unloading-induced muscle disuse.

Disuse of postural slow-twitch muscles, as it occurs in hypogravity, induces a slow-to-fast myofibre type transition. Nothing is known about the effects of weightlessness on the resting membrane chloride conductance (gCl), which controls sarcolemma excitability and influences fibre type transition during development and adult life. Using the current-clamp method, we observed that rat hindlimb unloading (HU) for 1-3 weeks increased gCl in fibres of the slow-twitch soleus (Sol) muscle toward values found in fast muscle. Northern blot analysis suggested that this effect resulted from an increased ClC-1 chloride channel mRNA level. In the meantime, a 4-fold increase in fibres expressing fast isoforms of the myosin heavy chain (MHC) was observed by immunostaining of muscle sections. Also, Sol muscle function evolved toward a fast phenotype during HU, as demonstrated by the positive shift of the threshold potential for contraction. After 3-days HU, Sol muscle immunostaining and RT-PCR experiments revealed no change in MHC protein and mRNA expression, whereas the gCl was already maximally increased, due to a pharmacologically probed, increased activity of ClC-1 channels. Thus the increase in gCl is an early event in Sol muscle experiencing unloading, suggesting that gCl may play a role in muscle adaptation to modified use. Pharmacological modulation of ClC-1 channels may help to prevent disuse-induced muscle impairment.