Autocrine and paracrine actions of ATP in rat carotid body

Carotid bodies are peripheral chemoreceptors that detect lowering of arterial blood O-2 level. The carotid body comprises clusters of glomus (type I) cells surrounded by glial-like sustentacular (type II) cells. Hypoxia triggers depolarization and cytosolic [Ca2+] ([Ca2+](i)) elevation in glomus cells, resulting in the release of multiple transmitters, including ATP. While ATP has been shown to be an important excitatory transmitter in the stimulation of carotid sinus nerve, there is considerable evidence that ATP exerts autocrine and paracrine actions in carotid body. ATP acting via P2Y(1) receptors, causes hyperpolarization in glomus cells and inhibits the hypoxia-mediated [Ca2+](i) rise. In contrast, adenosine (an ATP metabolite) triggers depolarization and [Ca2+](i) rise in glomus cells via A(2A) receptors. We suggest that during prolonged hypoxia, the negative and positive feedback actions of ATP and adenosine may result in an oscillatory Ca2+ signal in glomus cells. Such mechanisms may allow cyclic release of transmitters from glomus cells during prolonged hypoxia without causing cellular damage from a persistent [Ca2+](i) rise. ATP also stimulates intracellular Ca2+ release in sustentacular cells via P2Y(2) receptors. The autocine and paracrine actions of ATP suggest that ATP has important roles in coordinating chemosensory transmission in the carotid body.