Cellular CO2 Transduction In Avian Intrapulmonary Chemoreceptors

Northern Arizona University, Flagstaff AZ

Investigators

Abstract

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.