Calcium mobilization and entry induced by extracellular ATP in the non-sensory epithelial cell of the cochlear lateral wall.

Effects of external ATP application on the intracellular Ca2+ concentration ([Ca2+]i) of the epithelial lining cells of the cochlear lateral wall, the stria vascularis (SV), spiral prominence (SP), and external sulcus (ES) cells, were examined by the fluorescent Ca2+ indicator, Fura-2. ATP induced an increase in [Ca2+]i of these epithelial cells loaded with Fura-2 in a dose-dependent manner (1-100 microM). The strongest response was observed in SP and ES cells, whereas SV cells showed a weak response. The increase in [Ca2+]i was a biphasic response consisting of a rapid transient peak followed by a sustained phase. Removal of the external Ca2+ caused a slight transient increase in [Ca2+]i without a subsequent sustained phase. The Mn2(+)-quenching method revealed the Ca2+ entry across the plasma membrane immediately after the ATP application. The initial peak results from both the release of Ca2+ from intracellular stores and the Ca2+ influx from the extracellular space. The sustained phase is totally derived from the external Ca2+. The effective order of purinergic agonists was 2-methylthio ATP > or = ATP > 3'-O-(4-benzoyl)benzoyl ATP > alpha, beta-methylene ATP > or = ADP, but adenosine or UTP showed no response. The ATP-induced [Ca2+]i response was inhibited by reactive blue 2. The [Ca2+]i was partially dependent on the concentration of the fully ionized form, ATP-4. These findings indicate the presence of both P2y- and P2z-purinergic receptors in the non-sensory epithelial cells of the lateral wall.