Pivotal Role of Nucleotide P2X₂Receptor Subunit of the ATP-Gated Ion Channel Mediating Ventilatory Responses to Hypoxia

Abstract

In mammals, the ventilatory response to decreased oxygen tension in the arterial blood is initiated by excitation of specialized O2-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 P2X2 receptor subunit showed a markedly attenuated ventilatory response to hypoxia, whereas the response to hypoxia in P2X3-deficient mice was comparable with that seen in wild-type controls. P2X2 and P2X3 receptor subunit deficiency did not affect the ventilatory responses to hypercapnia. P2X2 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-methyleneATP both evoked rapid excitation of sinus nerve afferents, and the P2 receptor antagonist PPADS (pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulphonic acid) (100 microm) blocked hypoxia-induced increase in sinus nerve discharge. Immunoreactivities for P2X2 and P2X3 subunits were both detected on afferent terminals surrounding clusters of glomus cells in the wild-type animals but were absent in mice deficient in P2X2 and P2X3 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 P2X2 subunit play a pivotal role in carotid body function and in mediating ventilatory responses to hypoxia.

Mecanismos purinérgicos no bulbo ventrolateral rostral modulam respostas cardiovasculares e respiratórias promovidas pela ativação dos quimiorreceptores centrais e periféricos.

“…In vitro, loose-patch recordings from RTN neurons show that P2 receptor blockers decreased responsiveness to both 10% and 15% CO 2 also by 30%. In the slice, the contribution of purinergic signalling to RTN chemoreception did not increase with temperature (22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) • C) and was retained in low extracellular Ca 2+ medium. Conversely, the gap junction blockers carbenoxolone and cobalt decreased neuronal CO 2 /H + sensitivity by an amount similar to P2 receptor antagonists.…”

“…For example, as part of the first step in peripheral chemotrans duction glomus cells-the chemosensory unit of the carotid bodies-release ATP to activate P2X2 and P2X3 receptors on sensory nerve endings, which then relays this excitatory drive to neurons in the cNTS. 22 In the cNTS, application of ATP in awake rats mimicked cardiorespiratory responses (ie, bradycardia, hypertension, and tachypnea) to peripheral che moreceptor activation.…”

“…The transmitter basis for this drive is thought to depend largely on glutamate because NTS terminals in this region are immu noreactive for vesicular glutamate transporter 2 (VGLUT2, a marker of glutamatergic cells) 16,17 , and bilateral injections of kynurenic acid (glutamate receptor blocker) blunted peripheral chemoreceptor-mediated activation of breathing and blood pressure. 9,18 However, purinergic signaling has also been impli cated in the peripheral chemoreflex, [19][20][21][22] including at the level of the RVLM. 23 For example, chemosensitive RTN neurons 24,25 and presympathetic RVLM neurons 26 are activated by puriner gic agonists, application of purinergic agonists into the RVLM increased breathing 27,28 and blood pressure 29,30 in anesthetized rats, and inhibition of P2 receptors in the nearby Bötzinger and Abstract-Catecholaminergic C1 cells of the rostral ventrolateral medulla (RVLM) are key determinants of the sympathoexcitatory response to peripheral chemoreceptor activation.…”

“…For example, as part of the first step in peripheral chemotrans duction glomus cells-the chemosensory unit of the carotid bodies-release ATP to activate P2X2 and P2X3 receptors on sensory nerve endings, which then relays this excitatory drive to neurons in the cNTS. 22 In the cNTS, application of ATP in awake rats mimicked cardiorespiratory responses (ie, bradycardia, hypertension, and tachypnea) to peripheral che moreceptor activation.49,50 Also consistent with a role of puri nergic signaling in peripheral chemoreflex, cNTS injections of glutamate receptor antagonists did not block sympathetic or bradycardic components of the chemoreceptor reflex in awake rats 51 or in the working heartbrain stem prepara tion, 52 whereas simultaneous antagonism of glutamate and P2 receptors in this region reduced pressor and sympathetic responses to chemoreflex activation. 19 In addition, presym pathetic neurons in the RVLM are activated by exogenous application of ATP analogs, 26,29,30 and application of puri nergic agonists into this region increased sympathetic tone 29 and blood pressure 29,30 in anesthetized rats.…”

“…9,18 However, purinergic signaling has also been impli cated in the peripheral chemoreflex, [19][20][21][22] including at the level of the RVLM. 23 For example, chemosensitive RTN neurons 24,25 and presympathetic RVLM neurons 26 are activated by puriner gic agonists, application of purinergic agonists into the RVLM increased breathing 27,28 and blood pressure 29,30 in anesthetized rats, and inhibition of P2 receptors in the nearby Bötzinger and Abstract-Catecholaminergic C1 cells of the rostral ventrolateral medulla (RVLM) are key determinants of the sympathoexcitatory response to peripheral chemoreceptor activation.…”