Abstract: | The biological characteristics of the globular substance, aprecursor of otoconia, are unclear. In the present study, the ATP-induced internal free Ca2+ concentration(Ca2+]i) changes of the globular substanceand the ATP distribution in the vestibular organ were investigatedusing a Ca2+ indicator, fluo 3, and an adeninenucleotide-specific fluorochrome, quinacrine, by means of confocallaser scanning microscopy. Ca2+]i showed arapid and dose-dependent increase in response to ATP with a 50%effective concentration (EC50) of 16.7 µM. Thisreaction was independent of external Ca2+, indicating thepresence of an internal Ca2+ reservoir. Neither adenosine,,-methylene-ATP, 3'-O-(4-benzoylbenzoyl)-ATP, ADP, norUTP evoked this reaction, whereas 2-methylthio-ATP induced an increaseof Ca2+]i with an EC50 of 14.4 µM. Moreover, P2 antagonists, reactive blue 2 and suramin, and aphospholipase C inhibitor, U-73122, inhibited the ATP-inducedCa2+]i increase. These findings indicate thepresence of a P2Y purinoceptor on the globular substance. In addition,granular fluorescence was observed in the quinacrine-stained macularsensory epithelium, indicating the presence of ATP-containing granulesin this tissue. These results suggest that a paracrine mechanisminvolving ATP may exist in the macula and that this mechanism regulatesthe biological behavior of the globular substance. |