Effects of heat stress on muscle mass and the expression levels of heat shock proteins and lysosomal cathepsin L in soleus muscle of young and aged mice Yoshitaka Ohno • Sumio Yamada • Ayumi Goto • Akihiro Ikuta • Takao Sugiura • Yoshinobu Ohira • Toshitada Yoshioka • Katsumasa Goto Received: 13 February 2012 / Accepted: 12 June 2012 / Published online: 24 June 2012 Ó Springer Science+Business Media, LLC. 2012 Abstract Effects of heat stress on skeletal muscle mass in young and aged mice were investigated. Young (7-week) and aged (106-week) male C57BL/6J mice were randomly assigned to control and heat-stressed groups in each age. Mice in heat-stressed group were exposed to heat stress (41 °C for 60 min) in an incubator without anesthesia. Seven days after the exposure, soleus muscles were dis- sected from both hindlimbs. Protein content and the rela- tive composition of Type II fibers in aged soleus were lower than those in young muscle. In aged soleus, higher baseline expression levels of HSP25, HSP72, and cathepsin L were observed compared with those in young muscle (p \ 0.05). However, there were no significant differences in the expression levels of phosphorylated p70 S6 kinase (p-p70S6K), calpain 1, and calpain 2 of soleus between two age groups. A significant increase in muscle mass of both age groups was induced by heat stress (p \ 0.05). Heat stress also upregulated the expressions of HSP25, HSP72, and p-p70S6K in both ages (p \ 0.05). On the other hand, a significant decrease in cathepsin L expression by heating was observed in aged soleus, but not in young (p \ 0.05). Both the percentage of Type I fibers and the expression of calpains in both age groups were unchanged following heat stress. Heat stress-associated downregulation of cathepsin L may be attributed to the upregulation of HSP72, which stabilizes lysosomal membranes (p \ 0.05). Upregulations of HSP25, HSP72, and p-p70S6K and/or the downregula- tion of cathepsin L may play a role in heat stress-associated muscle hypertrophy in aged soleus muscle. Keywords Skeletal muscle Á Heat stress Á Aging Á Heat shock proteins Á Lysosomal enzyme Introduction Skeletal muscle has a large plasticity to environmental stimuli. It is well known that mechanical loading, such as stretch as well as anti-gravitational activity, is one of the anabolic stimuli which stimulate muscle protein synthesis, resulting in muscle hypertrophy [1]. On the other hand, inactivity as well as unloading causes skeletal muscle atrophy, especially anti-gravitational soleus muscles [2–4]. Aging is also one of atrophic factors for skeletal muscles. Aging-associated muscle atrophy, the so-called sarcopenia, exhibits the reduction of muscle strength, the impairment of physical function, resulting in a significant reduction in the quality of life [5–7]. Skeletal muscle mass depends on the net protein balance, which is attributed to a dynamic Y. Ohno Á K. Goto Laboratory of Physiology, School of Health Sciences, Toyohashi SOZO University, Toyohashi 440-8511, Japan S. Yamada School of Health Sciences, Nagoya University, Nagoya 461-8673, Japan A. Goto Á A. Ikuta Á K. Goto (&) Department of Physiology, Graduate School of Health Sciences, Toyohashi SOZO University, Toyohashi, 20-1 Matsushita, Ushikawa, Toyohashi, Aichi 440-8511, Japan e-mail: gotok@sepia.ocn.ne.jp T. Sugiura Faculty of Education, Yamaguchi University, Yamaguchi 753-8513, Japan Y. Ohira Graduate School of Medicine, Osaka University, Toyonaka 560-0043, Japan T. Yoshioka Hirosaki Gakuin University, Hirosaki 036-8577, Japan 123 Mol Cell Biochem (2012) 369:45–53 DOI 10.1007/s11010-012-1367-y