Platelet Dynamics during Natural and Pharmacologically Induced Torpor and Forced Hypothermia

Hibernation is an energy-conserving behavior in winter characterized by two phases: torpor and arousal. During torpor, markedly reduced metabolic activity results in inactivity and decreased body temperature. Arousal periods intersperse the torpor bouts and feature increased metabolism and euthermic body temperature. Alterations in physiological parameters, such as suppression of hemostasis, are thought to allow hibernators to survive periods of torpor and arousal without organ injury. While the state of torpor is potentially procoagulant, due to low blood flow, increased viscosity, immobility, hypoxia, and low body temperature, organ injury due to thromboembolism is absent. To investigate platelet dynamics during hibernation, we measured platelet count and function during and after natural torpor, pharmacologically induced torpor and forced hypothermia. Splenectomies were performed to unravel potential storage sites of platelets during torpor. Here we show that decreasing body temperature drives thrombocytopenia during torpor in hamster with maintained functionality of circulating platelets. Interestingly, hamster platelets during torpor do not express P-selectin, but expression is induced by treatment with ADP. Platelet count rapidly restores during arousal and rewarming. Platelet dynamics in hibernation are not affected by splenectomy before or during torpor. Reversible thrombocytopenia was also induced by forced hypothermia in both hibernating (hamster) and non-hibernating (rat and mouse) species without changing platelet function. Pharmacological torpor induced by injection of 5′-AMP in mice did not induce thrombocytopenia, possibly because 5′-AMP inhibits platelet function. The rapidness of changes in the numbers of circulating platelets, as well as marginal changes in immature platelet fractions upon arousal, strongly suggest that storage-and-release underlies the reversible thrombocytopenia during natural torpor. Possibly, margination of platelets, dependent on intrinsic platelet functionality, governs clearance of circulating platelets during torpor.冬眠是冬季的节能行为，可分为蛰伏（torpor）与觉醒（arousal）两个阶段。蛰伏阶段机体代谢活动显著降低，表现为活动停滞与体温下降；觉醒间期穿插于各蛰伏周期之间，此时代谢水平回升，体温恢复至正常恒温状态。现有研究认为，止血功能（hemostasis）受抑等生理参数改变，可使冬眠动物在蛰伏与觉醒周期中免受器官损伤。尽管蛰伏状态因血流缓慢、血液黏度升高、机体不动、缺氧及低体温等因素存在潜在促凝风险，但血栓栓塞（thromboembolism）所致的器官损伤并未出现。为探究冬眠过程中的血小板动态变化，我们检测了自然蛰伏、药物诱导蛰伏以及强制性低体温状态下及恢复期的血小板计数与功能。我们通过脾切除术（splenectomy）探究蛰伏期血小板的潜在储存位点。本研究证实，仓鼠蛰伏期间体温下降可引发血小板减少症（thrombocytopenia），但循环血小板的功能得以维持。有趣的是，蛰伏期仓鼠血小板不表达P选择素（P-selectin），但经二磷酸腺苷（ADP）处理后可诱导其表达。觉醒及复温过程中，血小板计数可快速恢复至正常水平。蛰伏前后或蛰伏期间进行的脾切除术，并未对冬眠仓鼠的血小板动态变化产生影响。强制性低体温还可在冬眠物种（仓鼠）与非冬眠物种（大鼠、小鼠）中诱发可逆性血小板减少症，且不会改变血小板功能。向小鼠注射5'-腺苷一磷酸（5'-AMP）诱导的药物性蛰伏，并未引发血小板减少症，这可能是因为5'-AMP可抑制血小板功能。循环血小板数量变化的快速性，以及觉醒期未成熟血小板分数（immature platelet fractions）的微小变化，强烈提示储存-释放机制是自然蛰伏期间可逆性血小板减少症的成因。或许依赖于血小板固有功能的血小板边集现象（margination），调控了蛰伏期循环血小板的清除过程。