ATP evoked responses from the carotid body in Wistar and spontaneously hypertensive rats

Aims: ATP acting on P2X3 receptors expressed in chemoreceptive afferent neurones causes chemoreflex hyperexcitability in the spontaneously hypertensive rat (SHR). However, whether this reflects differences in the amount of ATP released and/or its break down remains elusive. We tested the hypothesis that more ATP is released in SHR carotid body (CB), and this is associated with a down regulation of ectonucleotide degrading enzymes.

Methods and Results: First, we quantified the amount of ATP released from CBs of Wistar and SHRs in vitro; second, we quantified the gene expression of six enzymes, which are all involved with the extracellular metabolism of ATP, comprising: Ectonucleotide pyrophosphatase/phosphodiesterase, Enpp1-3; Ectonucleoside triphosphate diphosphohydrolase, Entpd2-3; ecto-5′-nucleotidase, Nt5e. Third, to assess hyperplasia, we quantified the gene expression for tyrosine hydroxylase (Th) and Pannexin 1 channel (Panx 1). Fourth, using live cell confocal imaging of ATP vesicular release, we quantified the amount of ATP released from dissociated glomus cells. Lastly, we used the working heart-brainstem preparation for functional analysis of ATP-evoked chemoreflex responses in both strains.

CBs of SHR released more ATP than normotensive rats (P<0.05). This result was associated with a paradoxical upregulation of enzyme transcripts Enpp2-3 and Nt5e (P<0.05); Th was upregulated as expected due to CB hyperplasia (p<0.05). There was no difference in the evoked ATP released from dissociated glomus cells in vitro between rat strains. Focal topical application of ATP or α, β-methylene ATP to the CB produced tachypnoea and sympathetic activation; the latter was greater in SHR relative to Wistars rats (P<0.05). P2X3 receptor antagonism produced a greater reduction in CB sensory firing evoked by cyanide in SHR than in Wistar rats (P<0.05).

Conclusions: The increased ATP release from the CB of SHR may reflect its larger size. ATP acting on P2X3 receptors drives CB-mediated sympathetic overactivity in SHR and remains an attractive target to treat cardiovascular disease.