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Journal of Neuroscience, Vol 10, 1388-1397, Copyright © 1990 by Society for Neuroscience
Increasing intracellular free calcium induces circumferential contractions in isolated cochlear outer hair cells
D Dulon, G Zajic and J Schacht
Kresge Hearing Research Institute, University of Michigan, Ann Arbor 48109.
The relationship between intracellular free calcium and the motile
responses of outer hair cells isolated from the guinea pig cochlea was
examined. Calcium levels were modulated by the addition of the calcium
ionophores ionomycin or A23187 to the incubation medium and monitored with
the fluorescent calcium indicator fluo-3. In the presence of 1.25 mM
external calcium, the application of either ionophore (10 microM) led to an
increase in intracellular free calcium from 157 +/- 76 nM to 1200 +/- 500
nM within 30-60 sec. Concurrently, cells elongated by 1-2 microns, cell
diameter decreased, and cell volume shrank by 269 +/- 220 microns 3 (5.0
+/- 4.1%). The reduction in diameter was most pronounced in the middle
portion of the cell (4.4% +/- 4.2%), also evident in the apical region
(3.1% +/- 4.8%) but not significant in the basal region near the nucleus.
This response was observed in outer hair cells from basal and apical turns
of the cochlea and was reversed when the cells were rinsed with
calcium-free medium supplemented with 2 mM EGTA. Optical imaging of the
cell membrane with the potentiometric dye 1-(3- sulfonatopropyl)-4-[beta]
[2-(di-n-butylaminol)-6-naphthyl vinyl] pyridinium betaine during the
elevation of intracellular calcium demonstrated features of contractility
at the lateral cell membrane. A rise in intracellular calcium as well as
the motile response was still observed after a 5-min exposure of the cells
to a calcium-free solution (supplemented with 2 mM EGTA), indicating that
the ionophore was also able to liberate calcium from intracellular sites.
However, depletion of calcium stores through prolonged incubation of the
cells in calcium- free medium (30-60 min) suppressed both the calcium
signal and the cell response. The calmodulin inhibitors trifluoperazine and
pimozide (30 microM) blocked the cell motility induced by ionomycin while
they left the increase of intracellular calcium unaffected. These
observations suggest that calcium-dependent circumferential contractions in
outer hair cells are mediated by calmodulin. The application to the
extracellular medium of putative neurotransmitters of the cochlear efferent
system such as acetylcholine and GABA led to neither an increase in
intracellular calcium nor a modification of cell shape. Therefore, these
neurotransmitters may not be directly involved in calcium-induced
contractions in outer hair cells. The circumferential contractions altered
the stiffness of the plasma membrane and the turgor of the cell. Under
normal conditions, changes in cell volume were inversely proportional to
the osmotic pressure of the extracellular medium following van't Hoff's
law.(ABSTRACT TRUNCATED AT 400 WORDS)
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