Wiebler James M, Kumar Manisha, Muir Timothy J
Biology Department, Augustana College, Rock Island, IL, 61201, USA.
Department of Biology, Miami University, Oxford, OH, 45056, USA.
J Comp Physiol B. 2017 Dec;187(8):1163-1172. doi: 10.1007/s00360-017-1099-0. Epub 2017 Apr 13.
Although seasonal increases in cold hardiness are well documented for temperate and polar ectotherms, relatively little is known about supplemental increases in cold hardiness during winter. Because many animals are exposed to considerable thermal variation in winter, they may benefit from a quick enhancement of cold tolerance prior to extreme low temperature. Hatchling painted turtles (Chrysemys picta) overwintering in their natal nests experience substantial thermal variation in winter, and recently, it was found that brief subzero chilling of winter-acclimated hatchlings decreases subsequent chilling-induced mortality, increases blood concentrations of glucose and lactate, and protects the brain from cryoinjury. Here, we further characterize that phenomenon, termed 'cold conditioning', by exposing winter-acclimated hatchling turtles to -3.5, -7.0, or -10.5 °C gradually or repeatedly via daily thermal fluctuations over the course of 5 days and assessing their survival of a subsequent cold shock to a discriminating temperature of -12.7 °C. To better understand the physiological response to cold conditioning, we measured changes in glucose and lactate concentrations in the liver, blood, and brain. Cold conditioning significantly increased cold-shock survival, from 9% in reference turtles up to 74% in cold-conditioned turtles, and ecologically relevant daily thermal fluctuations were at least as effective at conferring cryoprotection as was gradual cold conditioning. Cold conditioning increased glucose concentrations, up to 25 μmol g, and lactate concentrations, up to 30 μmol g, in the liver, blood, and brain. Turtles that were cold conditioned with daily thermal fluctuations accumulated more glucose in the liver, blood, and brain, and had lower brain lactate, than those gradually cold conditioned. Given the thermal variation to which hatchling painted turtles are exposed in winter, we suggest that the supplemental protection conferred by cold conditioning, especially that induced by daily thermal fluctuations, may be important for their overwinter survival. Investigation into the duration of the cold-conditioning induced protection and its occurrence in natural field conditions is needed to better understand its ecological significance. We suggest that future work exploring the underlying mechanisms of cold conditioning should focus on non-colligative effects of glucose, expression of small Hsps, changes in membrane structure, and ion homeostasis.
尽管温带和极地变温动物耐寒性的季节性增强已有充分记录，但对于冬季耐寒性的额外增强却知之甚少。由于许多动物在冬季会经历相当大的温度变化，它们可能会在极端低温来临之前迅速提高耐寒能力而受益。在出生巢穴中越冬的彩龟（Chrysemys picta）幼龟在冬季会经历显著的温度变化，最近发现，对冬季适应的幼龟进行短暂的零下低温处理可降低随后低温诱导的死亡率，提高血液中葡萄糖和乳酸的浓度，并保护大脑免受冷冻损伤。在此，我们通过在5天的过程中，通过每日温度波动将冬季适应的幼龟逐渐或反复暴露于-3.5、-7.0或-10.5°C，并评估它们在随后-12.7°C的鉴别温度下冷休克的存活率，进一步描述了这种称为“冷驯化”的现象。为了更好地理解对冷驯化的生理反应，我们测量了肝脏、血液和大脑中葡萄糖和乳酸浓度的变化。冷驯化显著提高了冷休克存活率，从对照龟的9%提高到冷驯化龟的74%，与逐渐冷驯化相比，与生态相关的每日温度波动在提供冷冻保护方面至少同样有效。冷驯化使肝脏、血液和大脑中的葡萄糖浓度提高到25 μmol g，乳酸浓度提高到30 μmol g。与逐渐冷驯化的龟相比，通过每日温度波动进行冷驯化的龟在肝脏、血液和大脑中积累了更多的葡萄糖，且大脑中的乳酸含量更低。鉴于彩龟幼龟在冬季所经历的温度变化，我们认为冷驯化提供的额外保护，尤其是由每日温度波动诱导的保护，可能对它们的越冬存活很重要。需要研究冷驯化诱导保护的持续时间及其在自然野外条件下的发生情况，以更好地理解其生态意义。我们建议，未来探索冷驯化潜在机制的工作应集中在葡萄糖的非依数性效应、小分子热休克蛋白的表达、膜结构的变化和离子稳态上。
