Acute hypoxia depolarizes carotid body chemoreceptor (glomus) cells and elevates intracellular Ca2+ concentration ([Ca2+]i). Recent studies suggest that hydrogen sulfide (H2S) may serve as an oxygen sensor/signal in the carotid body during acute hypoxia. To further test such a role for H2S, we studied the effects of H2S on the activity of TASK channel and [Ca2+]i, which are considered important for mediating the glomus cell response to hypoxia. Like hypoxia, NaHS (a H2S donor) inhibited TASK activity and elevated [Ca2+]i. To inhibit the production of H2S, glomus cells were incubated (3 hr) with inhibitors of cystathionine-β-synthase and cystathionine-γ-lyase (DL-propargylglycine, aminooxyacetic acid, β-cyano-L-alanine; 0.3 mM). SF7 fluorescence was used to assess the level of H2S production. The inhibitors blocked L-cysteine- and hypoxia-induced elevation of SF7 fluorescence intensity. In cells treated with the inhibitors, hypoxia produced an inhibition of TASK activity and a rise in [Ca2+]i, similar in magnitude to those observed in control cells. L-cysteine produced no effect on TASK activity or [Ca2+]i and did not affect hypoxia-induced inhibition of TASK and elevation of [Ca2+]i. These findings suggest that under normal conditions, H2S is not a major signal in hypoxia-induced modulation of TASK channels and [Ca2+]i in isolated glomus cells.

Key pointsr Hypoxia is thought to depolarize glomus cells by inhibiting the outward K + current, which sets in motion a cascade of ionic events that lead to transmitter secretion, increased afferent carotid sinus nerve activity and increased ventilation.r Our study of Na + -permeable channels in glomus cells has revealed that hypoxia not only inhibits TASK background K + channels but also indirectly activates a non-selective cation channel with a single channel conductance of 20 pS. Under physiological conditions, the reversal potential of the cation channel is ß -28 mV, indicating that Na + influx is also involved in hypoxia-induced excitation of glomus cells.

Large-conductance K (BK) and other voltage-dependent K channels (Kv) are highly expressed in carotid body (CB) glomus cells, but their role in hypoxia-induced excitation is still not well defined and remains controversial. We addressed this issue by studying the effects of inhibitors of BK (IBTX) and BK/Kv (TEA/4-AP) on [Ca ] responses to a wide range of hypoxia at different levels of resting cell membrane potential (E ). IBTX and TEA/4-AP did not affect the basal [Ca ] in isolated glomus cells bathed in 5 mm KCl , but elicited transient increases in [Ca ] in cells that were moderately depolarized (11-20 mV) by elevation of [KCl] (12-20 mm). Thus, BK and Kv were mostly closed at rest and activated by depolarization. Four different levels of hypoxia (mild, moderate, severe, anoxia) were used to produce a wide range of [Ca ] elevation (0-700 nm). IBTX did not affect the rise in [Ca ] , but TEA/4-AP strongly (∼3-fold) enhanced [Ca ] rise by moderate and severe levels of hypoxia. Guangxitoxin, a Kv2 blocker, inhibited the whole-cell current by ∼50%, and enhanced 2-fold the [Ca ] rise elicited by moderate and severe levels of hypoxia. Anoxia did not directly affect BK, but activated BK via depolarization. Our findings do not support the view that hypoxia inhibits BK/Kv to initiate or maintain the hypoxic response. Rather, our results show that BK/Kv are activated as glomus cells depolarize in response to hypoxia, which then limits the rise in [Ca ] . Inhibition of Kv may provide a mechanism to enhance the chemosensory activity of the CB and ventilation.

No abstract

Recent study showed that hypoxia activates a Ca2+-sensitive, Na+-permeable non-selective cation channel (NSC) in carotid body glomus cells. We studied the effects of mitochondrial inhibitors that increase Ca2+ influx via Ca2+ channel (Cav), and receptor agonists that release Ca2+ from endoplasmic reticulum (ER) on NSC. Mitochondrial inhibitors (NaCN, FCCP, H2S, NO) elevated [Ca2+]i and activated NSC. Angiotensin II and acetylcholine that elevate [Ca2+]i via the Gq-IP3 pathway activated NSC. However, endothelin-1 (Gq) and 5-HT (Gq) showed little or no effect on [Ca2+]i and did not activate NSC. Adenosine (Gs) caused a weak rise in [Ca2+]i but did not activate NSC. Dopamine (Gs) and γ-aminobytyric acid (Gi) were ineffective in raising [Ca2+]i and failed to activate NSC. Store-operated Ca2+ entry (SOCE) produced by depletion of Ca2+ stores with cyclopiazonic acid activated NSC. Our results show that Ca2+ entry via Cav, ER Ca2+ release and SOCE can activate NSC. Thus, NSC contributes to both voltage- and receptor-mediated excitation of glomus cells.