Hearing and its protection is regulated by ATP-evoked Ca2+ signaling in the supporting
cells of the organ of Corti, however, the unique anatomy of the cochlea hampers observing
these mechanisms. For the first time, we have performed functional ratiometric Ca2+
imaging (fura-2) in three different supporting cell types in the hemicochlea preparation
of hearing mice to measure purinergic receptor-mediated Ca2+ signaling in pillar,
Deiters' and Hensen's cells. Their resting [Ca2+]i was determined and compared in
the same type of preparation. ATP evoked reversible, repeatable and dose-dependent
Ca2+ transients in all three cell types, showing desensitization. Inhibiting the Ca2+
signaling of the ionotropic P2X (omission of extracellular Ca2+) and metabotropic
P2Y purinergic receptors (depletion of intracellular Ca2+ stores) revealed the involvement
of both receptor types. Detection of P2X2,3,4,6,7 and P2Y1,2,6,12,14 receptor mRNAs
by RT-PCR supported this finding and antagonism by PPADS suggested different functional
purinergic receptor population in pillar versus Deiters' and Hensen's cells. The sum
of the extra- and intracellular Ca2+-dependent components of the response was about
equal with the control ATP response (linear additivity) in pillar cells, and showed
supralinearity in Deiters' and Hensen's cells. Calcium-induced calcium release might
explain this synergistic interaction. The more pronounced Ca2+ leak from the endoplasmic
reticulum in Deiters' and Hensen's cells, unmasked by cyclopiazonic acid, may also
suggests the higher activity of the internal stores in Ca2+ signaling in these cells.
Differences in Ca2+ homeostasis and ATP-induced Ca2+ signaling might reflect the distinct
roles these cells play in cochlear function and pathophysiology.
