AMP-activated Protein Kinase Mediates Carotid Body Excitation by Hypoxia^*

Abstract

Early detection of an O₂ deficit in the bloodstream is essential to initiate corrective changes in the breathing pattern of mammals. Carotid bodies serve an essential role in this respect; their type I cells depolarize when O₂ levels fall, causing voltage-gated Ca²⁺ entry. Subsequent neurosecretion elicits increased afferent chemosensory fiber discharge to induce appropriate changes in respiratory function (1). Although depolarization of type I cells by hypoxia is known to arise from K⁺ channel inhibition, the identity of the signaling pathway has been contested, and the coupling mechanism is unknown (2). We tested the hypothesis that AMP-activated protein kinase (AMPK) is the effector of hypoxic chemotransduction. AMPK is co-localized at the plasma membrane of type I cells with O₂-sensitive K⁺ channels. In isolated type I cells, activation of AMPK using 5-aminoimidazole-4-carboxamide riboside (AICAR) inhibited O₂-sensitive K⁺ currents (carried by large conductance Ca²⁺-activated (BK_Ca) channels and TASK (tandem pore, acid-sensing potassium channel)-like channels, leading to plasma membrane depolarization, Ca²⁺ influx, and increased chemosensory fiber discharge. Conversely, the AMPK antagonist compound C reversed the effects of hypoxia and AICAR on type I cell and carotid body activation. These results suggest that AMPK activation is both sufficient and necessary for the effects of hypoxia. Furthermore, AMPK activation inhibited currents carried by recombinant BK_Ca channels, whereas purified AMPK phosphorylated theα subunit of the channel in immunoprecipitates, an effect that was stimulated by AMP and inhibited by compound C. Our findings demonstrate a central role for AMPK in stimulus-response coupling by hypoxia and identify for the first time a link between metabolic stress and ion channel regulation in an O₂-sensing system.