Adenosine stimulates depolarization and rise in cytoplasmic [Ca²⁺] in type I cells of rat carotid bodies

Abstract

During hypoxia, the level of adenosine in the carotid bodies increases as a result of ATP catabolism and adenosine efflux via adenosine transporters. Using Ca2+ imaging, we found that adenosine, acting via A2A receptors, triggered a rise in cytoplasmic [Ca2+] ([Ca2+]i) in type I (glomus) cells of rat carotid bodies. The adenosine response could be mimicked by forskolin (but not its inactive analog), and could be abolished by the PKA inhibitor H89. Simultaneous measurements of membrane potential (perforated patch recording) and [Ca2+]i showed that the adenosine-mediated [Ca2+]i rise was accompanied by depolarization. Ni2+, a voltage-gated Ca2+ channel (VGCC) blocker, abolished the adenosine-mediated [Ca2+]i rise. Although adenosine was reported to inhibit a 4-aminopyridine (4-AP)-sensitive K+ current, 4-AP failed to trigger any [Ca2+]i rise, or to attenuate the adenosine response. In contrast, anandamide, an inhibitor of the TWIK-related acid-sensitive K+-1 (TASK-1) channels, triggered depolarization and [Ca2+]i rise. The adenosine response was attenuated by anandamide but not by tetraethylammonium. Our results suggest that adenosine, acting via the adenylate cyclase and PKA pathways, inhibits the TASK-1 K+ channels. This leads to depolarization and activation of Ca2+ entry via VGCC. This excitatory action of adenosine on type I cells may contribute to the chemosensitivity of the carotid body during hypoxia.

Hypoxia. 4. Hypoxia and ion channel function

“…Later reports demonstrated that other K ϩ channel subtypes, including Ca 2ϩ -dependent K ϩ (K Ca ), human ether-a-go-gorelated gene (hERG), a subtype of inwardly rectifying K ϩ channels, and twin pore acid-stimulated K ϩ (TASK) channellike background currents can be modulated by O 2 (28, 162,169,170,252), and since application of inhibitors of TASK-1 (257) and hERG (162) channels caused significant depolarization of the resting E m and elevation of [Ca 2ϩ ] i , these channels appeared to be excellent candidates for mediating hypoxiainduced depolarization. Consistent with this possibility, inhibitors of hERG and TASK-1 mimic the effects of hypoxia on glomus cells from rabbit (162) and rat (257), respectively, and genetic deletion of TASK-1, but not TASK-3, markedly atten- uates the ventilatory response to hypoxia in mice (222). One explanation tying these various reports together is that initiation of depolarization by hypoxia occurs due to inhibition of K ϩ channels open at the resting E m (i.e., hERG or TASK channels), and that the inhibitory effects of hypoxia on K V and K Ca channels, which normally open with a shift to more positive E m and/or elevations in [Ca 2ϩ ] i , serves to limit their activity and maintain depolarization.…”

Signal processing at mammalian carotid body chemoreceptors

“…Adenosine, acting via A2A receptors, produces a PKA-dependent inhibition of TASK-like (background) K + channels in the rat carotid body. This depolarizes type I cells [60]. Blocking A2A receptors with nanomolar concentrations of the selective antagonist SCH58261 reduces hypoxia-evoked receptor potentials in rat type I carotid body cells in some cases (e.g.…”

Enhanced adenosine A2breceptor signaling facilitates stimulus-induced catecholamine secretion in chronically hypoxic carotid body type I cells

“…For instance, Ado was shown to inhibit TASK-1 K ϩ channels, leading to membrane depolarization, and voltage-gated Ca 2ϩ entry in rat type I cells (59). On the other hand, Ado has also been shown to inhibit voltage-dependent Ca 2ϩ currents and partially inhibit the hypoxia-evoked increase in [Ca 2ϩ ] i (26).…”

“…Also, acute hypoxia leads to Ado accumulation (7), and exogenous Ado increases cAMP content (41) and stimulates catecholamine secretion (6) from whole CBs. In studies on isolated CB type I cells, Ado was found to inhibit voltage-dependent K ϩ channels (55), as well as background K ϩ channels, which led to membrane depolarization and a rise in intracellular Ca 2ϩ ([Ca 2ϩ ] i ) (59). However, whether these presynaptic actions of Ado occur via the low-affinity A 2b receptors or the high-affinity A 2a receptors remains controversial.…”