Abstract
We investigated cytosolic calcium (Ca2+) and sarcoplasmic reticulum Ca2+ regulation in skeletal muscle fibers of hibernating Daurian ground squirrels (Spermophilus dauricus), non-hibernating hindlimb-unloaded (HLU) squirrels, and HLU rats to clarify the molecular mechanisms involved in preventing muscle atrophy in hibernators. The Na+, K+-ATPase and Ca2+-ATPase activities in the soleus muscle (SOL) of squirrels were maintained in hibernation, decreased during interbout arousal (IB-A), and increased to autumn/pre-hibernation (AUT/Pre-H) levels in torpor after interbout arousal (Post-IBA), whereas activities in the extensor digitorum longus muscle (EDL) were stable during hibernation, but increased during post-hibernation (Post-H). Activities increased in the SOL of HLU rats, but were stable in HLU squirrels. Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) activity in the SOL decreased in IB-A squirrels, but returned to AUT/Pre-H levels in the Post-IBA group; no significant changes were found in the EDL. SERCA activity increased in the EDL of HLU squirrels and SOL of HLU rats. Compared with AUT/Pre-H, SERCA type 2 protein expression increased in the SOL and EDL of IB-A and Post-IBA squirrels, but increased in the SOL only in HLU animals. We also describe the protein kinase A changes in this paper. Thus, hibernating ground squirrels displayed remarkable Na+, K+-ATPase, Ca2+-ATPase, and SERCA plasticity.
Highlights
Following sustained unloading or disuse, mammalian skeletal muscles can show considerable loss or deficiency in contractile proteins
Post-H groups increased by 43.93% (P < 0.05) and 27.78% (P < 0.05), respectively; compared with the AUT/ Pre-H and H group, Na+, K+-ATPase activity in the interbout arousal (IB-A) group decreased by 30.30% (P < 0.05) and 29.46% (P < 0.05), respectively; compared with the IB-A group, Na+, K+-ATPase activity in the Post-IBA and Post-H groups increased by 60.34% (P < 0.05) and 41.79% (P < 0.01), respectively (Fig. 1a)
Ca2+-ATPase activity in the IB-A group was lowest, with significant differences found in the other groups (P < 0.05); Ca2+-ATPase activity in the Post-IBA group was highest, and compared with the IB-A group, activity in the summer group (SUM), AUT/Pre-H, H, Post-IBA, and Post-H groups increased by 70.02% (P < 0.001), 73.25% (P < 0.001), 64.20% (P < 0.001), 141.26% (P < 0.001), and 64.18% (P < 0.001), respectively
Summary
Following sustained unloading or disuse, mammalian skeletal muscles can show considerable loss or deficiency in contractile proteins. Skeletal muscle disuse induces cytosolic Ca2+ overload, which can activate ubiquitin proteasome by activating calpains, thereby accelerating skeletal muscle protein degradation and muscle atrophy[9, 10]. The expression of MuRF1, which is stable in the SOL muscle of Daurian ground squirrels during hibernation, might help prevent atrophy by regulating the ubiquitination of muscle proteins[13]. Daurian ground squirrels did not demonstrate significant cytosolic Ca2+ overload induced by disuse during hibernation inactivity, as has been observed in non-hibernating animals, and cytosolic Ca2+ homeostasis was maintained in their slow-twitch SOL and fast-twitch EDL muscles[19]. The remarkable preservation of Ca2+ homeostasis in skeletal muscles of hibernators could prevent the activation of calpains, which accelerate protein degradation and skeletal muscle atrophy[9, 10]. The dynamic release of Ca2+ from these organelles is mediated by the RYR
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