Cellular CO2 Transduction In Avian Intrapulmonary Chemoreceptors

二氧化碳是细胞代谢的主要废物，也是所有呼吸空气的脊椎动物体液pH值的重要决定因素。 身体CO2水平由CO2敏感神经元（呼吸化学感受器）检测，这些神经元提供对肺部CO2消除的关键反馈控制。 CO2调节失败会导致代谢障碍或死亡，不幸的是，这是严重心肺疾病或神经损伤的常见并发症。 由于CO2传感和调节的问题是活跃的呼吸空气的脊椎动物的基础，动物已经进化出各种CO2传感神经元（呼吸化学感受器）在不同的位置，包括脑干，全身动脉系统，和肺气道。 我们正在研究禽肺中的CO2敏感性肺内化学感受器（IPC），以了解细胞CO2转导的基本步骤（即当CO2水平变化时改变IPC动作电位放电速率的机制，从而编码有关肺CO2的神经信息）。鸟类IPC是呼吸化学感受的强大细胞模型，因为它们对CO2非常敏感，它们的轴突易于测量动作电位，它们感觉末梢周围的CO2可以在实验室中精确控制，并且它们对CO2具有独特的逆反应（即低PCO 2兴奋IPC，高PCO 2抑制）。 在这里，我们测试了以下关于IPC CO2转导的新假设：IPC通过细胞内pH的变化感知CO2;并且在给定的CO2水平下，IPC的细胞内pH唯一地由碳酸酐酶催化的细胞内CO2水合速率，细胞内H+缓冲速率，以及通过泵送机制和反向端口的跨膜H+和HCO 3-挤出速率。 我们还假设IPC膜兴奋性和动作电位的产生与细胞内pH值的变化有关。 这项研究的具体目标包括：（1）开发CO2化学转导的数学模型以测试动力学转导假说，（2）肺中IPC感觉末梢和结状神经节中细胞体的组织化学鉴定，（3）使用靶向特定离子通道、反向转运体和神经递质的分子拮抗剂、激动剂和神经毒素以确定它们在CO2转导中的作用，以及（4）改变细胞内pH缓冲以测试其在CO2转导中的作用。 北方亚利桑那大学强调以实验室为基础的本科生物学教育。 因此，这个项目是向学生介绍神经生物学研究的有效工具（到目前为止，已有10名学生参与其中，其中一半是少数民族学生，包括两名纳瓦霍印第安人）。 这仍然是继续赠款周期的一个优先事项。

Carbon dioxide is a major waste product of cellular metabolism and an important determinant of body fluid pH in all air-breathing vertebrates. Body CO2 levels are detected by CO2 sensitive neurons (respiratory chemoreceptors) that provide critical feedback control of CO2 elimination by the lungs. Failure of CO2 regulation causes metabolic impairment or death, and is, unfortunately, a common complication of severe cardiopulmonary disease or neurological injury. Because the problem of CO2 sensing and regulation is fundamental for active air-breathing vertebrates, animals have evolved a variety of CO2-sensing neurons (respiratory chemoreceptors) in diverse locations including the brainstem, systemic arterial system, and lung airways. We are studying CO2-sensitive intrapulmonary chemoreceptors (IPC) in avian lungs to understand the fundamental steps of cellular CO2 transduction (i.e. mechanisms that alter IPC action potential discharge rate when CO2 levels change, thereby encoding neural information about lung CO2). Avian IPC are a powerful cellular model of respiratory chemoreception because they are extremely CO2 sensitive, their axons are easily accessible for measuring action potentials, the CO2 around their sensory endings can be precisely controlled in the laboratory, and they have a unique inverse response to CO2 (i.e. low PCO2 excites IPC, high PCO2 inhibits). Here we test the following novel hypotheses about IPC CO2 transduction: that IPC sense CO2 through changes in intracellular pH; and that at a given CO2 level the intracellular pH of IPC is uniquely determined by a dynamic kinetic balance between the rate of intracellular CO2 hydration catalyzed by carbonic anhydrase, the rate of intracellular H+ buffering, and the rates of transmembrane H+ and HCO3- extrusion by pumping mechanisms and antiports. We also hypothesize that IPC membrane excitability and action potential generation are coupled to changes in intracellular pH. Specific aims for this study include: (1) development of a mathematical model of CO2 chemotransduction to test the kinetic transduction hypothesis, (2) histochemical identification of IPC sensory endings in the lung and cell bodies in the nodose ganglia, (3) use of molecular antagonists, agonists and neurotoxins targeting specific ion channels, antiports and neurotransmitters to determine their role in CO2 transduction, and (4) altering intracellular pH buffering to test its role in CO2 transduction. Northern Arizona University emphasizes laboratory-based undergraduate biology education. Accordingly, this project is an efficient vehicle for introducing students to neurobiological research (so far ten students have been significantly involved, half of whom have been minority students--including two Navajo Native Americans). This remains a priority for the continuing grant cycle.