Acute exposure to hypoxic conditions is a frequent natural event during the development of bird eggs. However, little is known about the effect of such exposure on the ability of young embryos in which cardiovascular regulation is not yet developed to maintain a normal heart rate (HR). To address this question, we studied the effect of 10–20 min of exposure to moderate or severe acute hypoxia (10% or 5% O 2 , respectively) on the HR of day 4 (D4) chicken embryos. In ovo, video recording of the beating embryo heart inside the egg revealed that severe, but not moderate, hypoxia resulted in significant HR changes. The HR response to severe hypoxia consisted of two phases: the first phase, consisting of an initial decrease in HR, was followed by a phase of partial HR recovery. Upon the restoration of normoxia, after an overshoot period of a few minutes, the HR completely recovered to its basal level. In vitro (isolated heart preparation), the first phase of the HR response to severe hypoxia was strengthened (nearly complete heart silencing) compared to that in ovo, and the HR recovery phase was greatly attenuated. Furthermore, neither an overshoot period nor complete HR recovery after hypoxia was observed. Thus, the D4 chicken embryo heart can partially maintain its rhythm during hypoxia in ovo, but not in vitro. Some factors from the egg, such as catecholamines, are likely to be critical for avian embryo responding to hypoxic condition and survival.

中文翻译：

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鸡胚胎在孵化第4天缺氧时可以保持心率

急性暴露于缺氧条件是鸟蛋发育过程中常见的自然事件。然而，关于这种暴露对尚未形成心血管调节以维持正常心率 (HR) 的年轻胚胎能力的影响知之甚少。为了解决这个问题，我们研究了暴露于中度或重度急性缺氧（分别为 10% 或 5% O 2 ）10-20 分钟对第 4 天（D4）鸡胚胎 HR 的影响。在卵内，卵子内跳动的胚胎心脏的视频记录显示，严重但不中等的缺氧会导致心率发生显着变化。HR 对严重缺氧的反应由两个阶段组成：第一阶段包括 HR 的初始下降，然后是部分 HR 恢复阶段。恢复常氧后，经过几分钟的超调期后，HR 完全恢复到其基础水平。在体外（离体心脏准备），与卵内相比，HR 对严重缺氧反应的第一阶段得到加强（几乎完全的心脏沉默），并且 HR 恢复阶段大大减弱。此外，在缺氧后既没有观察到超调期，也没有观察到完全的 HR 恢复。因此，D4 鸡胚心脏在卵内缺氧期间可以部分保持其节律，但在体外则不行。来自鸡蛋的一些因素，例如儿茶酚胺，可能对鸟类胚胎对缺氧条件和存活的反应至关重要。与卵内相比，HR 对严重缺氧反应的第一阶段得到加强（几乎完全的心脏沉默），并且 HR 恢复阶段大大减弱。此外，在缺氧后既没有观察到超调期，也没有观察到完全的 HR 恢复。因此，D4 鸡胚心脏在卵内缺氧期间可以部分保持其节律，但在体外则不行。来自鸡蛋的一些因素，例如儿茶酚胺，可能对禽类胚胎对缺氧条件和存活的反应至关重要。与卵内相比，HR 对严重缺氧反应的第一阶段得到加强（几乎完全的心脏沉默），并且 HR 恢复阶段大大减弱。此外，在缺氧后既没有观察到超调期，也没有观察到完全的 HR 恢复。因此，D4 鸡胚心脏在卵内缺氧期间可以部分保持其节律，但在体外则不行。来自鸡蛋的一些因素，例如儿茶酚胺，可能对鸟类胚胎对缺氧条件和存活的反应至关重要。