Predominant synthesis of giant myofibrillar proteins in striated muscles of the long-tailed ground squirrel Urocitellus undulatus during interbout arousal

Abstract

Molecular mechanisms underlying muscle-mass retention during hibernation have been extensively discussed in recent years. This work tested the assumption that protein synthesis hyperactivation during interbout arousal of the long-tailed ground squirrel Urocitellus undulatus should be accompanied by increased calpain-1 activity in striated muscles. Calpain-1 is known to be autolysed and activated in parallel. Western blotting detected increased amounts of autolysed calpain-1 fragments in the heart (1.54-fold, p < 0.05) and m. longissimus dorsi (1.8-fold, p < 0.01) of ground squirrels during interbout arousal. The total protein synthesis rate determined by SUnSET declined 3.67-fold in the heart (p < 0.01) and 2.96-fold in m. longissimus dorsi (p < 0.01) during interbout arousal. The synthesis rates of calpain-1 substrates nebulin and titin in muscles did not differ during interbout arousal from those in active summer animals. A recovery of the volume of m. longissimus dorsi muscle fibres, a trend towards a heart-muscle mass increase and a restoration of the normal titin content (reduced in the muscles during hibernation) were observed. The results indicate that hyperactivation of calpain-1 in striated muscles of long-tailed ground squirrels during interbout arousal is accompanied by predominant synthesis of giant sarcomeric cytoskeleton proteins. These changes may contribute to muscle mass retention during hibernation.

Torpor versus interbout arousal: what is more important for protein metabolism and regeneration?

“…In discussing these new and interesting results, it can be assumed, however, that a significant contribution to the formation of hypertrophic or atrophic muscle phenotype could have been made by changes in anabolic and catabolic processes occurring in interbout arousals, as indicated by the results of recent studies (Chang et al., 2020; Popova et al., 2020 ). Further research is needed to show how important interbout arousals are in the regulation of protein turnover and regeneration in different striated muscles in hibernating animals.…”