Data from: Managing anabolic steroids in pre-hibernating Arctic ground squirrels: obtaining their benefits and avoiding their costs

Androgens have benefits, such as promoting muscle growth, but also significant costs, including suppression of immune function. In many species, these trade-offs in androgen action are reflected in regulated androgen production, which is typically highest only in reproductive males. However, all non-reproductive Arctic ground squirrels, irrespective of age and sex, have high levels of androgens prior to hibernating at sub-zero temperatures. Androgens appear to be required to make muscle in summer, which, together with lipid, is then catabolized during overwinter. By contrast, most hibernating mammals catabolize only lipid. We tested the hypothesis that androgen action is selectively enhanced in Arctic ground squirrel muscle because of an upregulation of androgen receptors (ARs). Using Western blot analysis, we found that Arctic ground squirrels have AR in skeletal muscle more than four times that of Columbian ground squirrels, a related southern species that overwinters at approximately 0°C and has low pre-hibernation androgen levels. By contrast, AR in lymph nodes was equivalent in both species. Brain AR was also modestly but significantly increased in Arctic ground squirrel relative to Columbian ground squirrel. These results are consistent with the hypothesis that tissue-specific AR regulation prior to hibernation provides a mechanism whereby Arctic ground squirrels obtain the life-history benefits and mitigate the costs associated with high androgen production.

雄激素（androgens）兼具促进肌肉生长等诸多益处，但同时也会带来显著的代价，例如免疫功能抑制。许多物种中，雄激素作用的这种权衡效应会体现在受调控的雄激素合成过程中——这类合成过程通常仅在繁殖期雄性个体中达到峰值。然而，所有非繁殖期的北极地松鼠（Arctic ground squirrels），无论年龄与性别，在进入亚零温度环境进行冬眠前，体内雄激素水平均处于较高水平。研究表明，夏季合成肌肉需要雄激素，而这些肌肉与脂质（lipid）会一同在越冬过程中被分解代谢；与之形成对比的是，大多数冬眠哺乳动物仅会分解代谢脂质。本研究针对下述假说展开验证：由于雄激素受体（androgen receptors，AR）的上调，北极地松鼠的肌肉组织中雄激素作用会被选择性增强。通过蛋白质免疫印迹（Western blot）分析，本研究发现北极地松鼠骨骼肌中的AR含量是哥伦比亚地松鼠（Columbian ground squirrels）的四倍以上；后者是栖息于南部的近缘物种，越冬环境温度约为0℃，且冬眠前雄激素水平较低。与之相反，两种松鼠的淋巴结中AR含量并无显著差异。相较于哥伦比亚地松鼠，北极地松鼠脑组织中的AR水平虽增幅较小，但仍存在显著提升。上述结果与该假说相符：冬眠前的组织特异性AR调控，为北极地松鼠获取生命史层面的益处、抵消高雄激素水平带来的相关代价提供了一种潜在机制。