Potential interaction of heme oxygenase-2- (HO-2-) generated CO with cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS) in glomus cells of the carotid body. (a) Under normoxia, HO-2-generated CO is relatively high, resulting in inhibition of both CSE and CBS activity. H2S production is relatively low, contributing to low sensory activity in glomus cells. (b) Under hypoxia, CO generation from HO-2 is reduced, resulting in removal of CO inhibition on CSE and CBS activity. H2S levels are increased due to enhanced CSE and CBS activities. Increased H2S may activate Ca2+ channels due to membrane depolarization via the inhibition of K+ channels, which in turn excites sensory responses in glomus cells.