Cellular adaptations in soleus muscle during recovery after hindlimb unloading Y. Oishi, 1 T. Ogata, 2 K.-i. Yamamoto, 1 M. Terada, 3 T. Ohira, 3 Y. Ohira, 3,4 K. Taniguchi 1 and R. R. Roy 5 1 Laboratory of Muscle Physiology, Faculty of Education, Kumamoto University, Kumamoto, Japan 2 Faculty of Sport Sciences, Waseda University, Tokorozawa, Japan 3 Graduate School of Frontier Bioscience, Osaka University, Osaka, Japan 4 Graduate School of Medicine, Osaka University, Osaka, Japan 5 Brain Research Institute, University of California, Los Angeles, CA, USA Received 23 April 2007, revision requested 19 June 2007, revision received 19 July 2007, accepted 23 July 2007 Correspondence: Y. Oishi, Laboratory of Muscle Physiology, Faculty of Education, Kumamoto University, Kumamoto 860-8555, Japan. E-mail: oishi@gpo.kumamoto-u.ac.jp Abstract Aim: We used a model of chronic unloading followed by reloading to examine the apoptotic responses associated with soleus muscle atrophy and subsequent recovery. Methods: Male Wistar rats were subjected to hindlimb unloading (HU) for 2 weeks and subsequent reloading for 0, 3, 7 and 14 days. One-half of the HU-reloaded rats were administered cyclosporine A (CsA), a calcineurin (CaN) inhibitor. Results: There was fibre atrophy (73%) and a decrease in slow type I fibre/ myosin heavy chain (MyHC) composition in the soleus muscle after 2 weeks of HU. Fibre size and type I MyHC composition recovered to near the age-matched control levels by recovery day 14 in non-treated, but not in CsA-treated, rats. Myonuclear number was lower and the number of apop- totic nuclei higher in 2-week HU than control rats. These values returned to control levels after 7 and 14 days of recovery, respectively, in both HU-recovery groups. After 2 weeks of HU, the levels of heat shock proteins (Hsp) 60 and 72, mitochondrial cytochrome c oxidase subunit IV (Cox IV), and peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1) proteins were lower than control. The levels of all of these proteins gradually increased to or above the control levels during cage recovery in both groups. Conclusion: Our results indicate that apoptotic mechanisms are involved in the modulation of myonuclear number during chronic unloading and sub- sequent reloading. Furthermore, it appears that CaN is related to fibre size and phenotype adaptations, but not to apoptotic responses. Keywords apoptosis, calcineurin, fibre atrophy, heat shock proteins, myosin heavy chain, PGC-1. Skeletal muscle atrophy associated with periods of decreased neuromuscular activity (loading and/or acti- vation), such as that occurring with ageing (Machida & Narusawa 2006, Marzetti & Leeuwenburgh 2006), hindlimb unloading (HU) (Edgerton & Roy 1996), spinal cord injury (Roy et al. 1991) and spaceflight (Edgerton & Roy 1996) is well documented. Recent studies indicate that apoptosis of myonuclei plays a role in this atrophic response (Allen et al. 1996, 1997, Lawler et al. 2003, Jackman & Kandarian 2004, Leeuwenburgh et al. 2005, Powers et al. 2005). For example, HU results in a decrease in the number of myonuclei and an increase in apoptotic myonuclei in the predominantly slow rat soleus (Allen et al. 1996, 1997, Acta Physiol 2008, 192, 381–395 Ó 2007 The Authors Journal compilation Ó 2007 Scandinavian Physiological Society, doi: 10.1111/j.1748-1716.2007.01747.x 381