*3.3. SR Ca2*<sup>+</sup> *Level in Single Skeletal Muscle Fibers during Di*ff*erent Hibernation Periods*

In contrast to the results produced for cytoplasmic Ca2<sup>+</sup>, the SR Ca2<sup>+</sup> level in single PL and AM muscle fibers decreased significantly during hibernation and almost recovered to SA levels post-hibernation (Figure 4). During the torpor-arousal cycle, compared with that in the LT and IBA groups, the SR Ca2<sup>+</sup> level in the PL muscle was significantly elevated by 27–34% when animals re-entered the torpor state, indicating that the SR Ca2<sup>+</sup> level partially recovered in the ET group.

**Figure 4.** Changes in the SR Ca2<sup>+</sup> concentration in PL and AM muscles during different periods. (**A**) Representative fluorescence images of single PL and AM muscle fibers. Scale bar = 100 μm. (**B**) Histogram depicting the cytoplasmic Ca2<sup>+</sup> fluorescence intensity in PL and AM muscles during different periods. SR, sarcoplasmic reticulum; PL, plantaris; AM, adductor magnus. SA, summer active group; PRE, pre-hibernation group; LT, late torpor group; IBA, inter-bout arousal group; ET, early torpor group; POST, post-hibernation group. Values are means ± SEM, n = 6–8. \* *p* < 0.05, \*\* *p* < 0.01, and \*\*\* *p* < 0.001 compared with SA; # *p* < 0.05, ## *p* < 0.01, and ### *p* < 0.001 compared with PRE; &&& *p* < 0.001 compared with LT; \$ *p* < 0.05, \$\$ *p* < 0.01, and \$\$\$ *p* < 0.001 compared with IBA; +++ *p* < 0.001 compared with ET.
