Coexistence of functional IP(3) and ryanodine receptors in vagal sensory neurons and their activation by ATP.

Intracellular photorelease of caged D-myo-inositol 1,4,5-trisphosphate (IP(3)), caffeine application, and immunofluorescence confocal microscopy were used to determine that D-myo-inositol 1,4,5-trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) coexist in rabbit vagal sensory nodose ganglion neurons (NGNs). ATP, an extracellular physiological signaling molecule, consistently evoked robust transient increases in cytosolic free Ca(2+) concentration (Ca(2+) transients). ATP applied in Ca(2+)-free physiological saline elicited Ca(2+) transients that averaged approximately 70% of the amplitude of transients evoked in the presence of extracellular Ca(2+). The component of the ATP-evoked Ca(2+) transient that was independent of extracellular Ca(2+) corresponds to Ca(2+) release from intracellular stores. This release component was sensitive to the pharmacological antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), U73122, neomycin, and heparin (13.5-15 kD), indicating that P2 purinoreceptors (P2Y) and the IP(3) signaling pathway are required for ATP-evoked Ca(2+) release. Additionally, a portion of ATP-evoked Ca(2+) release was inhibited by ryanodine, a selective blocker of RyRs. The ryanodine-insensitive component (approximately 70%) of ATP-evoked Ca(2+) release corresponds to IP(3)-induced Ca(2+) release via IP(3)Rs, while the ryanodine-sensitive component (approximately 30%) corresponds to consequent Ca(2+)-induced Ca(2+) release (CICR) via RyRs. These results indicate that functional IP(3)Rs and RyRs coexist in nodose neurons and that both IP(3)-induced Ca(2+) release and CICR can be activated by ATP.