Regulation of outer hair cell cytoskeletal stiffness by intracellular Ca2+: underlying mechanism and implications for cochlear mechanics.
Two Ca(2+)-dependent mechanisms have been proposed to regulate the mechanical properties of outer hair cells (OHCs), the sensory-motor receptors of the mammalian cochlea. One involves the efferent neurotransmitter, acetylcholine, decreasing OHC axial stiffness. The other depends on elevation of intracellular free Ca(2+) concentration ([Ca(2+)](i)) resulting in OHC elongation, a process known as Ca(2+)-dependent slow motility. Here we provide evidence that both these phenomena share a common mechanism. In whole-cell patch-clamp conditions, a fast increase of [Ca(2+)](i) by UV-photolysis of caged Ca(2+) or by extracellular application of Ca(2+)-ionophore, ionomycin, produced relatively slow (time constant approximately 20s) cell elongation. When OHCs were partially collapsed by applying minimal negative pressure through the patch pipette, elevation of the [Ca(2+)](i) up to millimole levels (estimated by Fura-2) was unable to restore the cylindrical shape of the OHC. Stiffness measurements with vibrating elastic probes showed that the increase of [Ca(2+)](i) causes a decrease of OHC axial stiffness, with time course similar to that of the Ca(2+)-dependent elongation, without developing any measurable force. We concluded that, contrary to a previous proposal, Ca(2+)-induced OHC elongation is unlikely to be driven by circumferential contraction of the lateral wall, but is more likely a passive mechanical reaction of the turgid OHC to Ca(2+)-induced decrease of axial stiffness. This may be the key phenomenon for controlling gain and operating point of the cochlear amplifier.
Frolenkov
Gregory I
GI
Section on Structural Cell Biology, National Institute on Deafness and Other Communication Disorders (NIDCD), National Institutes of Health (NIH), Bldg. 50, Room 4346, Bethesda, MD 20892-8027, USA. gregory.frolenkov@nih.gov
Mammano
Fabio
F
Kachar
Bechara
B
D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002465:Cell Movement / Q000187:drug effects / Q000502:physiology; D048429:Cell Size / Q000187:drug effects / Q000502:physiology; D003599:Cytoskeleton / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D004548:Elasticity / Q000187:drug effects; D016257:Fura-2; D006168:Guinea Pigs; D018072:Hair Cells, Auditory, Outer / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D006309:Hearing / Q000502:physiology*; D007424:Intracellular Fluid / Q000187:drug effects / Q000378:metabolism*; D007476:Ionophores / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology; D013314:Stress, Mechanical; D009435:Synaptic Transmission / Q000502:physiology; D015854:Up-Regulation / Q000187:drug effects / Q000502:physiology
D007476:Ionophores; D000109:Acetylcholine; D002118:Calcium; D016257:Fura-2