In Arctic ground squirrels, diet and torpor alter skeletal muscle relaxation kinetics but not force development

• Published in: The FASEB Journal
• Main Authors: Krishnan, Jishnu K., Rice, Sarah, Mikes, Monica, Sugiura, M. Hoshi, Drew, Kelly, Barati, Zeinab, Oliver, S. Ryan
• Other Authors: National Institutes of Health
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2022
• Subjects: Arctic
• Online Access: http://dx.doi.org/10.1096/fasebj.2022.36.s1.r5525

During the hibernation season, Arctic ground squirrels (AGS) experience extreme temperature fluctuations (body temperature, Tb, as low as ‐3°C), during which they are mostly physically inactive. Once Tb reaches ~15°C during interbout arousals, hibernators recruit skeletal muscle (SkM) for shivering thermogenesis to reach Tb of ~35°C. Polyunsaturated fatty acids (PUFA) in the diet are known to influence SkM function and metabolism. Recent studies in the cardiac muscle of hibernators have revealed that increased levels of ω‐6 and the ω‐6:ω‐3 PUFA ratio correlates with sarco/endoplasmic reticulum calcium ATPase (SERCA) activity and hibernation status. Muscular activity is characterized by contraction and relaxation mediated by calcium ions cycling released from SR stores. Ryanodine receptors (RyR1) on SR membranes release calcium ions into the cytosol, facilitating muscle contraction via actin‐myosin cross‐bridging. Whereas SERCA catalyzes Ca 2+ reuptake back into the SR, releasing Ca 2+ from actin/myosin filaments, completing the calcium cycle, and causing muscle relaxation. We hypothesized that diet (increased ω‐6:ω‐3 PUFA ratio) and torpor status are important in the regulation of the SERCA pump and that this may improve SkM performance during hibernation. Ex‐vivo functional assays were used to characterize performance changes in SkM (diaphragm) from AGS fed the following diets. 1) Standard rodent chow with an ω‐6:ω‐3 ratio of 5:1, and 2) a balanced diet with an ω‐6:ω‐3 ratio of 1:1 that roughly mimics wild diet. We collected diaphragms at three different stages of hibernation (early torpor, late torpor, and arousal) and evaluated muscle function under hypothermic temperature stress at 4°C, 15°C, 25°C, and 37°C to determine functional resilience. Our data show that torpid animals fed standard rodent chow appear to have an increase in SERCA activity, resulting in a significant increase in calcium mobilization from the sarcoplasm to the sarcoplasmic reticulum by the SERCA pump, allowing faster SkM relaxation when ...