Effects of acute warming on cardiovascular performance of Antarctic fishes

Abstract: We tested the hypothesis that blackfin icefish (Chaenocephalus aceratus), one of the six species in the family Channichthyidae (the icefishes) that do not express haemoglobin and myoglobin, lack regulatory cardiovascular flexibility during acute warming and activity. The experimental protocols were designed to optimize the surgical protocol and minimize stress. First, minimally invasive heart rate (fH) measurements were made during a thermal ramp until cardiac failure in C. aceratus and compared with those from the closely related red-blooded black rockcod (Notothenia coriiceps). Then, integrative cardiovascular adjustments were more extensively studied using flow probes and intravascular catheters in C. aceratus during acute warming (from 0 to 8°C) at rest and after imposed activity. Chaenocephalus aceratus had a lower routine fH than N. coriiceps (9 beats min-1 versus 14 beats min-1) and a lower peak fH during acute warming (38 beats min-1 versus 55 beats min-1) with a similar cardiac breakpoint temperature (13 and 14°C, respectively). Routine cardiac output (Q̇) for C. aceratus at ∼0°C was much lower (26.6 ml min-1 kg-1) than previously reported, probably because fish in the present study had a low fH (12 beats min-1) indicative of a high routine vagal tone and low stress. Chaenocephalus aceratus increased oxygen consumption during acute warming and with activity. Correspondingly, Q̇ increased considerably (maximally 86.3 ml min-1 kg-1), as did vascular conductance (5-fold). Thus, unlike earlier suggestions, these data provide convincing evidence that icefish can mount a well-developed cardiovascular regulation of heart rate, cardiac output and vascular conductance, and this regulatory capacity provides flexibility during acute warming.摘要：我们验证了如下假说：黑鳍冰鱼（*Chaenocephalus aceratus*，鳄冰鱼科（Channichthyidae）6种不表达血红蛋白（haemoglobin）与肌红蛋白（myoglobin）的冰鱼之一）在急性升温与运动状态下缺乏心血管调节灵活性。本实验方案旨在优化手术流程并最大程度降低应激反应。首先，我们对黑鳍冰鱼开展温度梯度升温下的微创心率（heart rate, fH）监测，直至其出现心脏衰竭，并将监测结果与亲缘关系较近的红血黑岩鳕（*Notothenia coriiceps*）的对应数据进行对比。随后，我们通过血流探头（flow probes）与血管内导管（intravascular catheters），对静息状态下及强制运动后的黑鳍冰鱼在0℃至8℃的急性升温过程中的整合性心血管调节功能进行了更深入的研究。黑鳍冰鱼的常规心率（9次·min⁻¹ 对比 14次·min⁻¹）与急性升温时的最大心率（38次·min⁻¹ 对比 55次·min⁻¹）均低于红血黑岩鳕，但二者的心脏临界温度相近（分别为13℃与14℃）。在约0℃的环境中，黑鳍冰鱼的常规心输出量（Q̇）为26.6 ml·min⁻¹·kg⁻¹，远低于此前的报道值，这可能与本研究受试鱼的心率较低（12次·min⁻¹）有关——该心率指标提示其迷走紧张程度较高且应激水平较低。黑鳍冰鱼在急性升温及运动过程中耗氧量均有所升高。与之对应，其心输出量显著提升（最大值达86.3 ml·min⁻¹·kg⁻¹），血管传导率（vascular conductance）也提升了5倍。因此，与此前的学术观点不同，本研究数据提供了确凿证据，证明冰鱼可对心率、心输出量与血管传导率实现完善的心血管调节，且该调节能力可使其在急性升温过程中获得适应性灵活性。