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

In comparison with that in the SA and PRE groups, the cytoplasmic Ca2<sup>+</sup> level in the single PL and AM muscle fibers increased significantly during hibernation, and almost recovered to SA levels post-hibernation. During the torpor-arousal cycle, the cytoplasmic Ca2<sup>+</sup> level partially recovered when animals re-entered the torpor state. Compared with the results in the LT group, significant decreases in cytoplasmic Ca2<sup>+</sup> levels were observed in the PL (24%) and AM muscles (32%) of the ET group (Figure 3).

**Figure 3.** Changes in cytoplasmic 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. 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* < 0.05, && *p* < 0.01, and &&& *p* < 0.001, compared with LT; \$\$ *p* < 0.01 and \$\$\$ *p* < 0.001 compared with IBA; +++ *p* < 0.001 compared with ET.
