Priority Strategy of Intracellular Ca(2+) Homeostasis in Skeletal Muscle Fibers during the Multiple Stresses of Hibernation

Intracellular calcium (Ca(2+)) homeostasis plays a vital role in the preservation of skeletal muscle. In view of the well-maintained skeletal muscle found in Daurian ground squirrels (Spermophilus dauricus) during hibernation, we hypothesized that hibernators possess unique strategies of intracellular Ca(2+) homeostasis. Here, cytoplasmic, sarcoplasmic reticulum (SR), and mitochondrial Ca(2+) levels, as well as the potential Ca(2+) regulatory mechanisms, were investigated in skeletal muscle fibers of Daurian ground squirrels at different stages of hibernation. The results showed that cytoplasmic Ca(2+) levels increased in the skeletal muscle fibers during late torpor (LT) and inter-bout arousal (IBA), and partially recovered when the animals re-entered torpor (early torpor, ET). Furthermore, compared with levels in the summer active or pre-hibernation state, the activity and protein expression levels of six major Ca(2+) channels/proteins were up-regulated during hibernation, including the store-operated Ca(2+) entry (SOCE), ryanodine receptor 1 (RyR1), leucine zipper-EF-hand containing transmembrane protein 1 (LETM1), SR Ca(2+) ATPase 1 (SERCA1), mitochondrial calcium uniporter complex (MCU complex), and calmodulin (CALM). Among these, the increased extracellular Ca(2+) influx mediated by SOCE, SR Ca(2+) release mediated by RyR1, and mitochondrial Ca(2+) extrusion mediated by LETM1 may be triggers for the periodic elevation in cytoplasmic Ca(2+) levels observed during hibernation. Furthermore, the increased SR Ca(2+) uptake through SERCA1, mitochondrial Ca(2+) uptake induced by MCU, and elevated free Ca(2+) binding capacity mediated by CALM may be vital strategies in hibernating ground squirrels to attenuate cytoplasmic Ca(2+) levels and restore Ca(2+) homeostasis during hibernation. Compared with that in LT or IBA, the decreased extracellular Ca(2+) influx mediated by SOCE and elevated mitochondrial Ca(2+) uptake induced by MCU may be important mechanisms for the partial cytoplasmic Ca(2+) recovery in ET. Overall, under extreme conditions, hibernating ground squirrels still possess the ability to maintain intracellular Ca(2+) homeostasis.