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Severe levels of acidosis (pH < 6.8) have been shown to cause a sustained rise in cytosolic Ca 2+ concentration in carotid body Type 1 (glomus) cells. To understand how physiologically relevant levels of acidosis regulate Ca 2+ signaling in glomus cells, we studied the effects of small changes in extracellular pH (pH o ) on the kinetics of Ca 2+ oscillations. A decrease in pH o from 7.4 to 7.3 (designated mild) and 7.2 (designated moderate) acidosis produced significant increases in the frequency and amplitude of Ca 2+ oscillations. These effects of acidosis on Ca 2+ oscillations were not blocked by NS383 and amiloride [acid-sensing ion channel (ASIC) inhibitors]. Mild and moderate levels of acidosis, however, caused a small but significant inhibition of two-pore domain acid-sensing K + channels (TASK) (TASK-1- and TASK-3-like channels) and depolarized the cell by 6-13 mV. Acidosis-induced increase in Ca 2+ oscillations was inhibited by nifedipine (1 µM; L-type Ca v inhibitor) and by TTA-P2 (20 µM; T-type Ca v inhibitor). Mild inhibition of TASK activity by N -[(2,4-difluorophenyl)methyl]-2'-[[[2-(4methoxyphenyl)acetyl]amino]methyl][1,1'-biphenyl]-2-carboxamide (A1899) (0.3 µM) and 1-[1-[6-[[1,1'-biphenyl]-4-ylcarbonyl)-5,6,7,8-tetrahydropyrido[4,3- d ]pyrimidine-4-yl]-4-piperidinyl]-1-butanon (PK-THPP) (0.1 µM) increased Ca 2+ oscillation frequency to levels similar to those observed with mild-moderate acidosis. Mild acidosis (pH o 7.3) and mild hypoxia (∼5%O 2 ) produced similar levels of changes in the kinetics of Ca 2+ oscillations. Block of tetraethylammonium (TEA)-sensitive K v channels did not affect acid-induced increase in Ca 2+ oscillations. Our study shows that mild and moderate levels of acidosis increase the frequency and amplitude of Ca 2+ oscillations primarily by inhibition of TASK without involving ASICs, and suggests a major role of TASK for signal transduction in response to a physiological change in pH o .

We studied the mechanisms by which carotid body glomus (type 1) cells produce spontaneous Ca 2+ oscillations in normoxia and hypoxia. In cells perfused with normoxic solution at 37°C, we observed relatively uniform, low-frequency Ca 2+ oscillations in >60% of cells, with each cell showing its own intrinsic frequency and amplitude. The mean frequency and amplitude of Ca 2+ oscillations were 0.6 ± 0.1 Hz and 180 ± 42 nM, respectively. The duration of each Ca 2+ oscillation ranged from 14 to 26 s (mean of ∼20 s). Inhibition of inositol (1,4,5)-trisphosphate receptor and store-operated Ca 2+ entry (SOCE) using 2-APB abolished Ca 2+ oscillations. Inhibition of endoplasmic reticulum Ca 2+ -ATPase (SERCA) using thapsigargin abolished Ca 2+ oscillations. ML-9, an inhibitor of STIM1 translocation, also strongly reduced Ca 2+ oscillations. Inhibitors of L- and T-type Ca 2+ channels (Ca v ; verapamil>nifedipine>TTA-P2) markedly reduced the frequency of Ca 2+ oscillations. Thus, Ca 2+ oscillations observed in normoxia were caused by cyclical Ca 2+ fluxes at the ER, which was supported by Ca 2+ influx via Ca 2+ channels. Hypoxia (2-5% O 2 ) increased the frequency and amplitude of Ca 2+ oscillations, and Ca v inhibitors (verapamil>nifedipine>>TTA-P2) reduced these effects of hypoxia. Our study shows that Ca 2+ oscillations represent the basic Ca 2+ signaling mechanism in normoxia and hypoxia in CB glomus cells.