Gaseous messengers in oxygen sensing

The carotid body is a sensory organ that detects acute changes in arterial blood oxygen (O-2) levels and reflexly mediates systemic cardiac, vascular, and respiratory responses to hypoxia. This article provides a brief update of the roles of gas messengers as well as redox homeostasis by hypoxia-inducible factors (HIFs) in hypoxic sensing by the carotid body. Carbon monoxide (CO) and nitric oxide (NO), generated by heme oxygenase-2 (HO-2) and neuronal nitric oxide synthase (nNOS), respectively, inhibit carotid body activity. Molecular O-2 is a required substrate for the enzymatic activities of HO-2 and nNOS. Stimulation of carotid body activity by hypoxia may reflect reduced formation of CO and NO. Glomus cells, the site of O-2 sensing in the carotid body, express cystathionine gamma-lyase (CSE), an H2S generating enzyme. Cth(-/-) mice, which lack CSE, exhibit severely impaired hypoxia-induced H2S generation, sensory excitation, and stimulation of breathing in response to low O-2. Hypoxia-evoked H2S generation in the carotid body requires the interaction of CSE with HO-2, which generates CO. Carotid bodies from Hif1a(+/-) mice with partial HIF-1 alpha deficiency do not respond to hypoxia, whereas carotid bodies from mice with partial HIF-2 alpha deficiency are hyper-responsive to hypoxia. The opposing roles of HIF-1 alpha and HIF-2 alpha in the carotid body have provided novel insight into molecular mechanisms of redox homeostasis and its role in hypoxia sensing. Heightened carotid body activity has been implicated in the pathogenesis of autonomic morbidities associated with sleep-disordered breathing, congestive heart failure, and essential hypertension. The enzymes that generate gas messengers and redox regulation by HIFs represent potential therapeutic targets for normalizing carotid body function and downstream autonomic output in these disease states.