Regulation of outer hair cell cytoskeletal stiffness by intracellular Ca²⁺: Underlying mechanism and implications for cochlear mechanics

Two Ca²⁺-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 Ca²⁺ concentration ([Ca²⁺]_i) resulting in OHC elongation, a process known as Ca²⁺-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 [Ca²⁺]_i by UV-photolysis of caged Ca²⁺ or by extracellular application of Ca²⁺ -ionophore, ionomycin, produced relatively slow (time constant ∼ 20 s) cell elongation. When OHCs were partially collapsed by applying minimal negative pressure through the patch pipette, elevation of the [Ca²⁺]_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 [Ca²⁺]_i causes a decrease of OHC axial stiffness, with time course similar to that of the Ca²⁺-dependent elongation, without developing any measurable force. We concluded that, contrary to a previous proposal, Ca²⁺-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 Ca²⁺-induced decrease of axial stiffness. This may be the key phenomenon for controlling gain and operating point of the cochlear amplifier.