Pivotal role of nucleotide P2X2 receptor subunit of the ATP-gated ion channel mediating ventilatory responses to hypoxia

In mammals, the ventilatory response to decreased oxygen tension in the arterial blood is initiated by excitation of specialized O-2-sensitive chemoreceptor cells in the carotid body that release neurotransmitters to activate endings of the sinus nerve afferent fibers. We investigated the role of ATP acting via ionotropic P2X receptors in the carotid body function and ventilatory response to hypoxia in mice. Mice deficient in P2X(2) receptor subunit showed a markedly attenuated ventilatory response to hypoxia, whereas the response to hypoxia in P2X(3)-deficient mice was comparable with that seen in wild-type controls. P2X(2) and P2X(3) receptor subunit deficiency did not affect the ventilatory responses to hypercapnia. P2X(2) subunit deficiency resulted in a dramatic reduction in the responses of the carotid sinus nerve to hypoxia in the in vitro carotid body - sinus nerve preparation. ATP and its stable analog alpha,beta-methylene ATP both evoked rapid excitation of sinus nerve afferents, and the P2 receptor antagonist PPADS (pyridoxal-5'-phosphate-6-azophenyl-2', 4'-disulphonic acid) (100 muM) blocked hypoxia-induced increase in sinus nerve discharge. Immunoreactivities for P2X(2) and P2X(3) subunits were both detected on afferent terminals surrounding clusters of glomus cells in the wild-type animals but were absent in mice deficient in P2X(2) and P2X(3) receptor subunits. These observations provide the first definitive evidence that, in the carotid body, ATP is a key transmitter released by chemoreceptor cells to activate endings of the sinus nerve afferent fibers. We conclude that P2X receptors containing the P2X(2) subunit play a pivotal role in carotid body function and in mediating ventilatory responses to hypoxia.