Department of Evolutionary and Physiological Ecology, Faculty of Biology, University of Białystok, Białystok, Poland.
Department of Biological Sciences, College of Arts and Sciences, University of Alaska Anchorage, Anchorage, USA.
J Comp Physiol B. 2024 Dec;194(6):909-924. doi: 10.1007/s00360-024-01579-9. Epub 2024 Sep 5.
Energy conservation associated with hibernation is maximized at the intersection of low body temperature (T), long torpor bouts, and few interbout arousals. In the arctic ground squirrel (Urocitellus parryii), energy conservation during hibernation is best achieved at ambient temperatures (T) around 0 °C; however, they spend the majority of hibernation at considerably lower T. Because arctic ground squirrels switch to mixed fuel metabolism, including protein catabolism, at extreme low T of hibernation, we sought to investigate how microbial urea-nitrogen recycling is used under different thermal conditions. Injecting squirrels with isotopically labeled urea (C/N) during hibernation at T's of - 16 °C and 2 °C and while active and euthermic allowed us to assess the ureolytic activity of gut microbes and the amount of liberated nitrogen incorporated into tissues. We found greater incorporation of microbially-liberated nitrogen into tissues of hibernating squirrels. Although ureolytic activity appears higher in euthermic squirrels, liberated nitrogen likely makes up a smaller percentage of the available nitrogen pool in active, fed animals. Because non-lipid fuel is a limiting factor for torpor at lower T in this species, we hypothesized there would be greater incorporation of liberated nitrogen in animals hibernating at - 16 °C. However, we found higher microbial-ureolytic activity and incorporation of microbially-liberated nitrogen, particularly in the liver, in squirrels hibernating at 2 °C. Likely this is because squirrels hibernating at 2 °C had higher T and longer interbout arousals, a combination of factors creating more favorable conditions for gut microbes to thrive and maintain greater activity while giving the host more time to absorb microbial metabolites.
与冬眠相关的能量节约在体温（T）低、长时间蛰伏和较少苏醒之间的交点达到最大化。在北极地松鼠（Urocitellus parryii）中，冬眠期间的能量节约在环境温度（T）约为 0°C 时最佳；然而，它们在冬眠期间的大部分时间里体温都要低得多。由于北极地松鼠在冬眠的极低 T 下切换到混合燃料代谢，包括蛋白质分解代谢，我们试图研究在不同热条件下微生物尿素氮循环是如何被利用的。在 T 为-16°C 和 2°C 的冬眠期间以及在活跃和恒温时向松鼠注射同位素标记的尿素（C/N），使我们能够评估肠道微生物的脲酶活性以及释放到组织中的游离氮量。我们发现冬眠的松鼠组织中微生物释放的氮的掺入量更大。尽管在恒温的松鼠中脲酶活性似乎更高，但在活跃、进食的动物中，释放的氮可能只占可用氮库的一小部分。由于在这种物种中，非脂类燃料是在较低 T 下进入蛰伏的限制因素，我们假设在-16°C 下冬眠的动物中会有更多的释放氮被掺入。然而，我们发现冬眠的松鼠在 2°C 时具有更高的微生物脲酶活性和微生物释放氮的掺入量，特别是在肝脏中。这可能是因为在 2°C 下冬眠的松鼠体温较高，苏醒时间较长，这些因素的组合为肠道微生物创造了更有利的条件，使其能够更活跃地生长并保持更长的时间，同时为宿主提供更多的时间来吸收微生物代谢物。
