Winmill Rachel E, Chen Anna K, Hedrick Michael S
Department of Biological Sciences, California State University, Hayward, Hayward, CA 94542, USA.
J Exp Biol. 2005 Jan;208(Pt 2):213-22. doi: 10.1242/jeb.01399.
The aim of this study was to examine the effects of cellular hypoxia, and the contribution of anaerobic metabolism, on respiratory activity in bullfrogs at different stages of development. Respiratory-related neural activity was recorded from cranial nerve rootlets in isolated brainstem preparations from pre-metamorphic (Taylor-Kollros (T-K) stages VIII-XVI) and postmetamorphic tadpoles (T-K stages XXIV-XXV) and adults. Changes in fictive gill/lung activity in brainstems from pre-metamorphic tadpoles and lung activity in postmetamorphic tadpoles and adults were examined during superfusion with control (98% O(2)/2% CO(2)) or hypoxic (98% N(2)/2% CO(2)) artificial cerebrospinal fluid (aCSF). Iodoacetate (IAA; 100 micromol l(-1)) was used in conjunction with hypoxic aCSF to inhibit glycolysis. Gill burst frequency in pre-metamorphic brainstems did not change over a 3 h exposure to hypoxia and fictive lung burst frequency slowed significantly, but only after 3 h hypoxia. Blockade of glycolysis with IAA during hypoxia significantly reduced the time respiratory activity could be maintained in pre-metamorphic, but not in adult, brainstems. In brainstems from post-metamorphic tadpoles and adults, lung burst frequency became significantly more episodic within 5-15 min hypoxic exposure, but respiratory neural activity was subsequently abolished in every preparation. The cessation of fictive breathing was restored to control levels upon reoxygenation. Neither tadpole nor adult brainstems exhibited changes in neural bursts resembling 'gasping' that is observed in mammalian brainstems exposed to severe hypoxia. There was also a significant increase in the frequency of 'non-respiratory' bursts in hypoxic postmetamorphic and adult brainstems, but not in pre-metamorphic brainstems. These results indicate that pre-metamorphic tadpoles are capable of maintaining respiratory activity for 3 h or more during severe hypoxia and rely to a great extent upon anaerobic metabolism to maintain respiratory motor output. Upon metamorphosis, however, hypoxia results in significant changes in respiratory frequency and pattern, including increased lung burst episodes, non-ventilatory bursts and a reversible cessation of respiratory activity. Adults have little or no ability to maintain respiratory activity through glycolysis but, instead, stop respiratory activity until oxygen is available. This 'switch' in the respiratory response to hypoxia coincides morphologically with the loss of gills and obligate air-breathing in the postmetamorphic frog. We hypothesize that the cessation of respiratory activity in post-metamorphic tadpoles and adults is an adaptive, energy-saving response to low oxygen.
本研究的目的是考察细胞缺氧的影响以及无氧代谢对处于不同发育阶段牛蛙呼吸活动的作用。在来自变态前（泰勒 - 科尔罗斯（T - K）八至十六期）、变态后蝌蚪（T - K二十四至二十五期）及成体的离体脑干标本中，从颅神经根记录与呼吸相关的神经活动。在用对照（98% O₂/2% CO₂）或低氧（98% N₂/2% CO₂）人工脑脊液（aCSF）灌流期间，检测变态前蝌蚪脑干中假想鳃/肺活动的变化以及变态后蝌蚪和成体中肺活动的变化。碘乙酸（IAA；100 μmol l⁻¹）与低氧aCSF联合使用以抑制糖酵解。在暴露于低氧3小时的过程中，变态前脑干中的鳃爆发频率没有变化，假想肺爆发频率显著减慢，但仅在低氧3小时后出现。在低氧期间用IAA阻断糖酵解显著缩短了变态前脑干而非成体脑干中呼吸活动能够维持的时间。在变态后蝌蚪和成体的脑干中，在低氧暴露5 - 15分钟内肺爆发频率变得明显更呈阵发性，但随后每个标本中的呼吸神经活动都被消除。复氧后，假想呼吸的停止恢复到对照水平。蝌蚪和成体脑干均未表现出在暴露于严重低氧的哺乳动物脑干中观察到的类似“喘气”的神经爆发变化。在低氧的变态后和成体脑干中，“非呼吸”爆发的频率也显著增加，但在变态前脑干中未增加。这些结果表明，变态前蝌蚪在严重低氧期间能够维持呼吸活动3小时或更长时间，并且在很大程度上依赖无氧代谢来维持呼吸运动输出。然而，在变态后，低氧会导致呼吸频率和模式发生显著变化，包括肺爆发次数增加、非通气性爆发以及呼吸活动的可逆性停止。成体通过糖酵解维持呼吸活动的能力很小或没有，而是在有氧气可用之前停止呼吸活动。这种对低氧的呼吸反应“转变 ”在形态学上与变态后青蛙鳃的丧失和专性空气呼吸相吻合。我们推测，变态后蝌蚪和成体中呼吸活动的停止是对低氧的一种适应性节能反应。
