Molecular Characterization of Equilibrative Nucleoside Transporters in the Rat Carotid Body and Their Regulation by Chronic Hypoxia

The mammalian carotid body (CB) is the main peripheral arterial chemoreceptor organ that is excited by decreases in blood PO2 (hypoxia) and increases in blood PCO2/H+. An increase in CB afferent carotid sinus nerve (CSN) discharge results in respiratory and cardiovascular reflex responses that help maintain homeostasis. The CB consists mainly of innervated clusters of the chemoreceptive type I (glomus) cells that are associated with the processes of glial-like type II cells. Extracellular ATP and adenosine (ADO) levels increase in response to acute hypoxia and there is evidence that during chronic sustained hypoxia ADO elevation plays a major role in regulating CB chemosensitivity and CSN discharge. We recently characterized the molecular identities of ectonucleotidase enzymes involved in regulating extracellular ATP hydrolysis to produce ADO in the rat CB. In the present study, we focus on a molecular characterization of the equilibrative nucleoside transporter (ENT) system that is known to regulate extracellular ADO concentrations in the rat CB based on pharmacological studies. Examination of ENT expression using quantitative PCR (qPCR) analysis revealed the expression of both ENT1 and ENT2 mRNAs in whole CB extracts from ~2-week-old juvenile rats. In dissociated rat CB cultures, both ENT1 and ENT2 immunoreactivity was localized to type I cell clusters. Furthermore, we show that ENT1 and ENT2 mRNA expression is downregulated in CBs isolated from rat pups exposed to chronic hypobaric hypoxia (~1 week). These findings reveal the molecular identities of the ENT system expressed in the rat CB and are consistent with the proposed shift to ADO signaling during chronic hypoxia.