Mechanisms and consequences of carbon dioxide sensing in fish

Fish possess chemoreceptors able to sense increasing levels of ambient CO(2) and initiate various cardiorespiratory reflexes including hyperventilation and bradycardia. These chemoreceptors are localized predominantly to the gills, are oriented to sense the external environment and typically are stimulated by changes in environmental molecular CO(2) rather than H(+) (although increasing H(+) may be the proximate intracellular stimulus). In zebrafish, a subset of branchial neuroepithelial cells (NECs) act as bimodal sensors of CO(2) and O(2), similar to the Type I (glomus) cells of the mammalian carotid body. Like O(2) sensing, the mechanisms underlying CO(2) detection involve the inhibition of a background K(+) current leading to membrane depolarization and subsequent elevation of intracellular Ca(2+) levels. Carbonic anhydrase, by catalysing the hydration of CO(2) to H(+) and HCO(3)(-), appears to play a critical role in reducing NEC response times and increasing the magnitude of membrane depolarization accompanying hypercapnia. In larval zebrafish, CA activity is essential for the rapid development of hypercapnic bradycardia.