ATP-Induced Ca(2+) release in cochlear outer hair cells: localization of an inositol triphosphate-gated Ca(2+) store to the base of the sensory hair bundle.
We used a high-performance fluorescence imaging system to visualize rapid changes in intracellular free Ca(2+) concentration ([Ca(2+)](i)) evoked by focal applications of extracellular ATP to the hair bundle of outer hair cells (OHCs): the sensory-motor receptors of the cochlea. Simultaneous recordings of the whole-cell current and Calcium Green-1 fluorescence showed a two-component increase in [Ca(2+)](i). After an initial entry of Ca(2+) through the apical membrane, a second and larger, inositol triphosphate (InsP(3))-gated, [Ca(2+)](i) surge occurred at the base of the hair bundle. Electron microscopy of this intracellular Ca(2+) release site showed that it coincides with the localization of a unique system of endoplasmic reticulum (ER) membranes and mitochondria known as Hensen's body. Using confocal immunofluorescence microscopy, we showed that InsP(3) receptors share this location. Consistent with a Ca(2+)-mobilizing second messenger system linked to ATP-P2 receptors, we also determined that an isoform of G-proteins is present in the stereocilia. Voltage-driven cell shape changes and nonlinear capacitance were monitored before and after ATP application, showing that the ATP-evoked [Ca(2+)](i) rise did not interfere with the OHC electromotility mechanism. This second messenger signaling mechanism bypasses the Ca(2+)-clearance power of the stereocilia and transiently elevates [Ca(2+)](i) at the base of the hair bundle, where it can potentially modulate the action of unconventional myosin isozymes involved in maintaining the hair bundle integrity and potentially influence mechanotransduction.
Mammano
F
F
Biophysics Sector and Istituto Nazionale di Fisica della Materia Unit, International School for Advanced Studies, 34014 Trieste, Italy.
Frolenkov
G I
GI
Lagostena
L
L
Belyantseva
I A
IA
Kurc
M
M
Dodane
V
V
Colavita
A
A
Kachar
B
B
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D006168:Guinea Pigs; D018072:Hair Cells, Auditory, Outer / Q000187:drug effects* / Q000378:metabolism; D015544:Inositol 1,4,5-Trisphosphate / Q000502:physiology*; D015640:Ion Channel Gating; D008465:Mechanoreceptors / Q000502:physiology*; D008854:Microscopy, Electron; D018408:Patch-Clamp Techniques; D015398:Signal Transduction / Q000187:drug effects; D014741:Video Recording
D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D002118:Calcium