Ryanodine receptor-mediated [Ca]i release in glomus cells is independent of natural stimuli and does not participate in the chemosensory responses of the rat carotid body

The hypothesis that intracellular calcium (Ca²⁺]_i) release in glomus cells via ryanodine receptor (RyR) activation by caffeine may be independent of natural stimuli and chemosensory discharge was tested in the rat carotid body (CB). CB type I cells were isolated, plated and preloaded with calcium-sensitive fluorescent probe, Indo-1AM. With the increase of caffeine dose (0–50 mM) cytosolic calcium (Ca²⁺]_c) increased from 85±15 nM to 1933±190 nM (n=6) at normoxia (P ₂=125–130 Torr, P ₂=25–30 Torr, pH 7.30–7.35). Hypoxia (P ₂=10–15 Torr) increased and hypocapnia (P ₂=7–9 Torr) decreased the cytoplasmic calcium Ca²⁺]_c levels, independent of caffeine. Caffeine-related Ca²⁺]_c increase was the same in the presence and the absence of extracellular calcium (Ca²⁺]_o), indicating the source of Ca²⁺ ions is the cellular store. Permeabilization of the cell membrane with saponin (25 μg/ml) retained the caffeine response. Additional treatment of the cells with 50 μM ryanodine (an inhibitor of the caffeine-activated RyR site) abolished caffeine-stimulated response. In vitro CB chemosensory (carotid sinus nerve, CSN) responses to hypoxia (P ₂=35–40 Torr) were not altered by caffeine. These results suggest that Ca²⁺]_i stores in CB cells, mobilized by RyR activation, do not participate in the CSN responses to natural stimuli.