Characterization of cochlear outer hair cell turgor

Asymmetric vibrations in the organ of Corti by outer hair cells measured from excised gerbil cochlea

Pending questions regarding cochlear amplification and tuning are hinged upon the organ of Corti (OoC) active mechanics: how outer hair cells modulate OoC vibrations. Our knowledge regarding OoC mechanics has advanced over the past decade thanks to the application of tomographic vibrometry. However, recent data from live cochlea experiments often led to diverging interpretations due to complicated interaction between passive and active responses, lack of image resolution in vibrometry, and ambiguous measurement angles. We present motion measurements and analyses of the OoC sub-components at the close-to-true cross-section, measured from acutely excised gerbil cochleae. Specifically, we focused on the vibrating patterns of the reticular lamina, the outer pillar cell, and the basilar membrane because they form a structural frame encasing active outer hair cells. For passive transmission, the OoC frame serves as a rigid truss. In contrast, motile outer hair cells exploit their frame structures to deflect the upper compartment of the OoC while minimally disturbing its bottom side (basilar membrane). Such asymmetric OoC vibrations due to outer hair cell motility explain how recent observations deviate from the classical cochlear amplification theory.

3D Ultrastructure of the Cochlear Outer Hair Cell Lateral Wall Revealed By Electron Tomography

Outer Hair Cells (OHCs) in the mammalian cochlea display a unique type of voltage-induced mechanical movement termed electromotility, which amplifies auditory signals and contributes to the sensitivity and frequency selectivity of mammalian hearing. Electromotility occurs in the OHC lateral wall, but it is not fully understood how the supramolecular architecture of the lateral wall enables this unique form of cellular motility. Employing electron tomography of high-pressure frozen and freeze-substituted OHCs, we visualized the 3D structure and organization of the membrane and cytoskeletal components of the OHC lateral wall. The subsurface cisterna (SSC) is a highly prominent feature, and we report that the SSC membranes and lumen possess hexagonally ordered arrays of particles. We also find the SSC is tightly connected to adjacent actin filaments by short filamentous protein connections. Pillar proteins that join the plasma membrane to the cytoskeleton appear as variable structures considerably thinner than actin filaments and significantly more flexible than actin-SSC links. The structurally rich organization and rigidity of the SSC coupled with apparently weaker mechanical connections between the plasma membrane (PM) and cytoskeleton reveal that the membrane-cytoskeletal architecture of the OHC lateral wall is more complex than previously appreciated. These observations are important for our understanding of OHC mechanics and need to be considered in computational models of OHC electromotility that incorporate subcellular features.

Influence of acute hyperglycemia on otoacoustic emissions and the medial olivocochlear reflex

Stimulus-frequency (SF) otoacoustic emission (OAE) amplitude and the amplitude of medial olivocochlear (MOC) inhibition of SF OAEs for ipsilateral, contralateral and bilateral MOC reflex elicitors were recorded in six subjects with type 2 diabetes during a glucose tolerance test (GTT). Five of the six subjects were tested twice for a total of 11 trials and three subjects were tested in a control experiment. During the GTT experiment, the subjects' blood glucose was elevated from a euglycemic level below 150 mg/dL to a hyperglycemic level above 160 mg/dL following the consumption of a bolus of 80 g of sugar. A subset of three subjects were tested in a control experiment during which SF OAE and MOC reflex measurements were made while blood sugar levels remained constant within the euglycemic region. Mean SF OAE amplitudes were elevated following glucose consumption. A statistically significant increase in MOC inhibition amplitude was observed during elevated sugar levels for the 11 GTT trials. Maximum inhibition occurred about an hour after glucose consumption when blood glucose levels peaked. Results indicate that acute hyperglycemia influences efferent control of the cochlea in people with type 2 diabetes.

Biophysical mechanisms underlying outer hair cell loss associated with a shortened tectorial membrane

The tectorial membrane (TM) connects to the stereociliary bundles of outer hair cells (OHCs). Humans with an autosomal dominant C1509G mutation in alpha-tectorin, a protein constituent of the TM, are born with a partial hearing loss that worsens over time. The Tecta(C1509/+) transgenic mouse with the same point mutation has partial hearing loss secondary to a shortened TM that only contacts the first row of OHCs. As well, Tecta(C1509G/+) mice have increased expression of the OHC electromotility protein, prestin. We sought to determine whether these changes impact OHC survival. Distortion product otoacoustic emission thresholds in a quiet environment did not change to 6 months of age. However, noise exposure produced acute threshold shifts that fully recovered in Tecta (+/+) mice but only partially recovered in Tecta(C1509G/+) mice. While Tecta(+/+) mice lost OHCs primarily at the base and within all three rows, Tecta(C1509G/+) mice lost most of their OHCs in a more apical region of the cochlea and nearly completely within the first row. In order to estimate the impact of a shorter TM on the forces faced by the stereocilia within the first OHC row, both the wild type and the heterozygous conditions were simulated in a computational model. These analyses predicted that the shear force on the stereocilia is ~50% higher in the heterozygous condition. We then measured electrically induced movements of the reticular lamina in situ and found that while they decreased to the noise floor in prestin null mice, they were increased by 4.58 dB in Tecta(C1509G/+) mice compared to Tecta(+/+) mice. The increased movements were associated with a fourfold increase in OHC death as measured by vital dye staining. Together, these findings indicate that uncoupling the TM from some OHCs leads to partial hearing loss and places the remaining coupled OHCs at higher risk. Both the mechanics of the malformed TM and the increased prestin-related movements of the organ of Corti contribute to this higher risk profile.

Insulin-induced hypoglycemia increases hepatic sensitivity to glucagon in dogs

In individuals with type 1 diabetes, hypoglycemia is a common consequence of overinsulinization. Under conditions of insulin-induced hypoglycemia, glucagon is the most important stimulus for hepatic glucose production. In contrast, during euglycemia, insulin potently inhibits glucagon's effect on the liver. The first aim of the present study was to determine whether low blood sugar augments glucagon's ability to increase glucose production. Using a conscious catheterized dog model, we found that hypoglycemia increased glucagon's ability to overcome the inhibitory effect of insulin on hepatic glucose production by almost 3-fold, an effect exclusively attributable to marked enhancement of the effect of glucagon on net glycogen breakdown. To investigate the molecular mechanism by which this effect comes about, we analyzed hepatic biopsies from the same animals, and found that hypoglycemia resulted in a decrease in insulin signaling. Furthermore, hypoglycemia and glucagon had an additive effect on the activation of AMPK, which was associated with altered activity of the enzymes of glycogen metabolism.

Osmotic properties of auditory hair cells in the leopard frog: evidence for water-permeable channels

When amphibian papillar hair cells (APHCs) of the leopard frog, Rana pipiens pipiens, are osmotically challenged, they exhibit a characteristically asymmetric (rectifying) response: small decreases (5%, or less) in the extracellular solution's osmolarity do not significantly affect the cells' volume; larger decreases produce a relatively slow volume increase in APHCs, while exposure to a hyperosmotic medium leads to rapid shrinking of these cells. Furthermore, the rate of volume change appears to be a function of the rate of extracellular osmotic change. These characteristics make the application of methods devised for the estimation of the osmotic permeability coefficient (P(f)) for semipermeable membranes - i.e., those with significant permeability only to water - to APHC membrane rather futile. We have, therefore, devised a method that takes both the permeability to solutes as well as the kinetics of the osmolarity change into consideration, in order to obtain estimates of P(f) that are to a large degree independent of these factors. We have compared the new and earlier methods. Using the new method, we have estimated the P(f) of APHCs' plasma membrane to be in the 10(-2)-cm/s range, and thus significantly larger than those reported for lipid bilayers. APHC's membrane P(f) appears to be cell-size independent and insensitive to extracellular mercury. These results suggest that APHCs express water-permeable channels in their plasma membrane. Furthermore, we suggest that asymmetric and rate dependent shape changes produced by osmolarity changes in APHCs imply the presence of significant permeability to solutes. The significance of transmembrane solute transport and water channel expression in amphibian auditory hair cells is discussed.

Cell membrane tethers generate mechanical force in response to electrical stimulation

Living cells maintain a huge transmembrane electric field across their membranes. This electric field exerts a force on the membrane because the membrane surfaces are highly charged. We have measured electromechanical force generation by cell membranes using optically trapped beads to detach the plasma membrane from the cytoskeleton and form long thin cylinders (tethers). Hyperpolarizing potentials increased and depolarizing potentials decreased the force required to pull a tether. The membrane tether force in response to sinusoidal voltage signals was a function of holding potential, tether diameter, and tether length. Membrane electromechanical force production can occur at speeds exceeding those of ATP-based protein motors. By harnessing the energy in the transmembrane electric field, cell membranes may contribute to processes as diverse as outer hair cell electromotility, ion channel gating, and transport.

Compartmentalization of the outer hair cell demonstrated by slow diffusion in the extracisternal space

In the outer hair cell (OHC), the extracisternal space (ECiS) is a conduit and reservoir of the molecular and ionic substrates of the lateral wall, including those necessary for electromotility. To determine the mechanisms through which molecules are transported in the ECiS of the OHC, we selectively imaged the time-dependent spatial distribution of fluorescent molecules in a <100 nm layer near the cell/glass interface of the recording chamber after their photolytic activation in a diffraction-limited volume. The effective diffusion coefficient was calculated using the analytical solution of the diffusion equation. It was found that diffusion in the ECiS is isotropic and not affected by depolarizing the OHC. Compared with free solution, the diffusion of 10 kDa dextran was slowed down in both the ECiS and the axial core by a factor of 4.6 and 1.6, respectively.

Perilymph osmolality modulates cochlear function

OBJECTIVES/HYPOTHESIS

The cochlear amplifier is required for the exquisite sensitivity of mammalian hearing. Outer hair cells underlie the cochlear amplifier and they are unique in that they maintain an intracellular turgor pressure. Changing the turgor pressure of an isolated outer hair cells through osmotic challenge modulates its ability to produce electromotile force. We sought to determine the effect of osmotic challenge on cochlear function.

STUDY DESIGN

In vivo animal study.

METHODS

Hypotonic and hypertonic artificial perilymph was perfused through the scala tympani of anesthetized guinea pigs. Cochlear function was assessed by measuring the compound action potential, distortion product otoacoustic emissions, the cochlear microphonic, and the endocochlear potential.

RESULTS

Hypotonic perilymph decreased and hypertonic perilymph increased compound action potential and distortion product otoacoustic emission thresholds in a dose-dependent and reversible manner. The cochlear microphonic quadratic distortion product magnitude increased after hypotonic perfusion and decreased with hypertonic perfusion. There were no changes in the stimulus intensity growth curve of the low-frequency cochlear microphonic. The endocochlear potential was not affected by perilymph osmolality.

CONCLUSIONS

These data demonstrate that perilymph osmolality can modulate cochlear function and are consistent with what would be expected if outer hair cells turgor pressure changes the gain of the cochlear amplifier in vivo.

Slow motility in hair cells of the frog amphibian papilla: myosin light chain-mediated shape change

Using video, fluorescence and confocal microscopy, quantitative analysis and modeling, we investigated intracellular processes mediating the calcium/calmodulin (Ca(2+)/CaM)-dependent slow motility in hair cells dissociated from the rostral region of amphibian papilla, one of the two auditory organs in frogs. The time course of shape changes in these hair cells during the period of pretreatment with several specific inhibitors, as well as their response to the calcium ionophore, ionomycin, were recorded and compared. These cells respond to ionomycin with a tri-phasic shape change: an initial phase of iso-volumetric length decrease; a period of concurrent shortening and swelling; and the final phase of increase in both length and volume. We found that both the myosin light chain kinase inhibitor, ML-7, and antagonists of the multifunctional Ca(2+)/CaM-dependent kinases, KN-62 and KN-93, inhibit the iso-volumetric shortening phase of the response to ionomycin. The type 1 protein phosphatase inhibitors, calyculin A and okadaic acid induce minor shortening on their own, but do not significantly alter phase 1 response. However, they appear to counter effects of the inhibitors of Ca(2+)/CaM-dependent kinases. We hypothesize that an active actomyosin-based process mediates the iso-volumetric shortening in the frog rostral amphibian papillar hair cells.

Functional prestin transduction of immature outer hair cells from normal and prestin-null mice

Prestin is a membrane protein in the outer hair cell (OHC) that has been shown to be essential for electromotility. OHCs from prestin-null mice do not express prestin, do not have a nonlinear capacitance (the electrical signature of electromotility), and are smaller in size than wild-type OHCs. We sought to determine whether prestin-null OHCs can be transduced to incorporate functional prestin protein in a normal fashion. A recombinant helper-dependent adenovirus expressing prestin and green fluorescent protein (HDAd-prestin-GFP) was created and tested in human embryonic kidney cells (HEK cells). Transduced HEK cells demonstrated membrane expression of prestin and nonlinear capacitance. HDAd-prestin-GFP was then applied to cochlear sensory epithelium explants harvested from wild-type and prestin-null mice at postnatal days 2-3, the age at which native prestin is just beginning to become functional in wild-type mice. At postnatal days 4-5, we investigated transduced OHCs for (1) their prestin expression pattern as revealed by immunofluorescence; (2) their cell surface area as measured by linear capacitance; and (3) their prestin function as indicated by nonlinear capacitance. HDAd-prestin-GFP efficiently transduced OHCs of both genotypes and prestin protein localized to the plasma membrane. Whole-cell voltage clamp studies revealed a nonlinear capacitance in transduced wild-type and prestin-null OHCs, but not in non-transduced cells of either genotype. Prestin transduction did not increase the linear capacitance (cell surface area) for either genotype. In peak nonlinear capacitance, voltage at peak nonlinear capacitance, charge density of the nonlinear capacitance, and shape of the voltage-capacitance curves, the transduced cells of the two genotypes resembled each other and previously reported data from adult wild-type mouse OHCs. Thus, prestin introduced into prestin-deficient OHCs segregates normally to the cell membrane and generates a normal nonlinear capacitance, indicative of normal prestin function.

Controlled microaspiration for high-pressure freezing: a new method for ultrastructural preservation of fragile and sparse tissues for TEM and electron tomography

High-pressure freezing is the preferred method to prepare thick biological specimens for ultrastructural studies. However, the advantages obtained by this method often prove unattainable for samples that are difficult to handle during the freezing and substitution protocols. Delicate and sparse samples are difficult to manipulate and maintain intact throughout the sequence of freezing, infiltration, embedding and final orientation for sectioning and subsequent transmission electron microscopy. An established approach to surmount these difficulties is the use of cellulose microdialysis tubing to transport the sample. With an inner diameter of 200 microm, the tubing protects small and fragile samples within the thickness constraints of high-pressure freezing, and the tube ends can be sealed to avoid loss of sample. Importantly, the transparency of the tubing allows optical study of the specimen at different steps in the process. Here, we describe the use of a micromanipulator and microinjection apparatus to handle and position delicate specimens within the tubing. We report two biologically significant examples that benefit from this approach, 3D cultures of mammary epithelial cells and cochlear outer hair cells. We illustrate the potential for correlative light and electron microscopy as well as electron tomography.

Cochlear outer hair cell motility

Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential. The forces depend on the presence of a motor protein in the lateral membrane of the cells. This protein, known as prestin, is a member of a transporter superfamily SLC26. The functional and structural properties of prestin are described in this review. Whether outer hair cell motility might account for sound amplification at all frequencies is also a critical question and is reviewed here.

Hypotonic swelling of salicylate-treated cochlear outer hair cells

The outer hair cell (OHC) is a hydrostat with a low hydraulic conductivity of Pf=3x10(-4) cm/s across the plasma membrane (PM) and subsurface cisterna that make up the OHC's lateral wall. The SSC is structurally and functionally a transport barrier in normal cells that is known to be disrupted by salicylate. The effect of sodium salicylate on Pf is determined from osmotic experiments in which isolated, control and salicylate-treated OHCs were exposed to hypotonic solutions in a constant flow chamber. The value of Pf=3.5+/-0.5x10(-4) cm/s (mean+/-s.e.m., n=34) for salicylate-treated OHCs was not significantly different from Pf=2.4+/-0.3x10(-4) cm/s (mean+/-s.e.m., n=31) for untreated OHCs (p=.3302). Thus Pf is determined by the PM and is unaffected by salicylate treatment. The ratio of longitudinal strain to radial strain epsilonz/epsilonc=-0.76 for salicylate-treated OHCs was significantly smaller (p=.0143) from -0.72 for untreated OHCs, and is also independent of the magnitude of the applied osmotic challenge. Salicylate-treated OHCs took longer to attain a steady-state volume which is larger than that for untreated OHCs and increased in volume by 8-15% prior to hypotonic perfusion unlike sodium alpha-ketoglutarate-treated OHCs. It is suggested that depolymerization of cytoskeletal proteins and/or glycogen may be responsible for the large volume increase in salicylate-treated OHCs as well as the different responses to different modes of application of the hypotonic solution.

A Novel Glucagon Receptor Antagonist, NNC 25-0926, Blunts Hepatic Glucose Production in the Conscious Dog

Elevated glucagon is associated with fasting hyperglycemia in type 2 diabetes. We assessed the effects of the glucagon receptor antagonist (2R)-N-[4-({4-(1-cyclohexen-1-yl)[(3,5-dichloroanilino)carbonyl]anilino}methyl)benzoyl]-2-hydroxy-b-alanine (NNC 25-0926) on hepatic glucose production (HPG) in vivo, using arteriovenous difference and tracer techniques in conscious dogs. The experiments consisted of equilibration (-140 to -40 min), control (40-0 min), and experimental [0-180 min, divided into P1 (0-60 min) and P2 (60-180 min)] periods. In P1, NNC 25-0926 was given intragastrically at 0 (veh), 10, 20, 40, or 100 mg/kg, and euglycemia was maintained. In P2, somatostatin, basal intraportal insulin, and 5-fold basal intraportal glucagon (2.5 ng/kg/min) were infused. Arterial plasma insulin levels remained basal throughout the study in all groups. Arterial plasma glucagon levels remained basal during the control period and P1 and then increased to approximately 70 pg/ml in P2 in all groups. Arterial plasma glucose levels were basal in the control period and P1 in all groups. In P2, the arterial glucose level increased to 245+/-22 and 172+/-15 mg/dl in the veh and 10 mg/kg groups, respectively, whereas in the 20, 40, and 100 mg/kg groups, there was no rise in glucose. Net hepatic glucose output was approximately 2 mg/kg/min in all groups during the control period. In P2, it increased by 9.4+/-2 mg/kg/min in the veh group. In the 10, 20, 40, and 100 mg/kg groups, the rise was only 4.1+/-0.9, 1.6+/-0.6, 2.4+/-0.7, and 1.5+/-0.3 mg/kg/min, respectively, due to inhibition of glycogenolysis. In conclusion, NNC 25-0926 effectively blocked the ability of glucagon to increase HGP in the dog.

Electromechanical models of the outer hair cell composite membrane

The outer hair cell (OHC) is an extremely specialized cell and its proper functioning is essential for normal mammalian hearing. This article reviews recent developments in theoretical modeling that have increased our knowledge of the operation of this fascinating cell. The earliest models aimed at capturing experimental observations on voltage-induced cellular length changes and capacitance were based on isotropic elasticity and a two-state Boltzmann function. Recent advances in modeling based on the thermodynamics of orthotropic electroelastic materials better capture the cell's voltage-dependent stiffness, capacitance, interaction with its environment and ability to generate force at high frequencies. While complete models are crucial, simpler continuum models can be derived that retain fidelity over small changes in transmembrane voltage and strains occurring in vivo. By its function in the cochlea, the OHC behaves like a piezoelectric-like actuator, and the main cellular features can be described by piezoelectric models. However, a finer characterization of the cell's composite wall requires understanding the local mechanical and electrical fields. One of the key questions is the relative contribution of the in-plane and bending modes of electromechanical strains and forces (moments). The latter mode is associated with the flexoelectric effect in curved membranes. New data, including a novel experiment with tethers pulled from the cell membrane, can help in estimating the role of different modes of electromechanical coupling. Despite considerable progress, many problems still confound modelers. Thus, this article will conclude with a discussion of unanswered questions and highlight directions for future research.

Slow motility in hair cells of the frog amphibian papilla: Ca2+-dependent shape changes

We investigated the process of slow motility in non-mammalian auditory hair cells by recording the time course of shape change in hair cells of the frog amphibian papilla. The tall hair cells in the rostral segment of this organ, reported to be the sole recipients of efferent innervation, were found to shorten in response to an increase in the concentration of the intracellular free calcium. These shortenings are composed of two partially-overlapping phases: an initial rapid iso-volumetric contraction, followed by a slower length decrease accompanied with swelling. It is possible to unmask the iso-volumetric contraction by delaying the cell swelling with the help of K+ or Cl- channel inhibitors, quinine or furosemide. Furthermore, it appears that the longitudinal contraction in these cells is Ca2+-calmodulin-dependent: in the presence of W-7, a calmodulin inhibitor, only a slow, swelling phase could be observed. These findings suggest that amphibian rostral AP hair cells resemble their mammalian counterparts in expressing both a Ca2+-calmodulin-dependent contractile structure and an "osmotic" mechanism capable of mediating length change in response to extracellular stimuli. Such a mechanism might be utilized by the efferent neurotransmitters for adaptive modulation of mechano-electrical transduction, sensitivity enhancement, frequency selectivity, and protection against over-stimulation.

Extraction of prestin-dependent and prestin-independent components from complex motile responses in guinea pig outer hair cells

Electromotility of cochlear outer hair cells (OHC) is associated with conformational changes in the integral membrane protein prestin. We have recently reported that electrical stimulation evokes significant prestin-dependent changes in the length, width, and area of the longitudinal section of OHCs, but not in their volume. In contrast, prestin-independent responses elicited at constant membrane potential are associated with changes in cell length, width, and volume without significant changes in their longitudinal section area. In this report we describe a novel analytical technique, based on a simple theoretical model and continuous measurement of changes in cell length and longitudinal section area, to evaluate the contribution of each one of these mechanisms to the motile response of OHCs. We demonstrate that if the relative change in OHC length (L) during the motile response is expressed as L = A2 x V(-1) (with A and V being the relative changes in longitudinal section area and volume, respectively), A2 will describe the contribution of the prestin-dependent mechanism whereas V(-1) will describe the contribution of the prestin-independent mechanism. Thus, relative changes in any two of these cellular morphological parameters (L, A, or V) would be necessary and sufficient for characterizing any OHC motile response. This simple approach provides access to information previously unavailable, and may become a novel and important tool for increasing our understanding of the cellular and molecular mechanisms of OHC motility.

Prestin-dependent and prestin-independent motility of guinea pig outer hair cells

The motile response of isolated guinea pig outer hair cells (OHCs) was investigated using a combination of whole-cell patch clamp recording and continuous video image analysis. OHC's length, width, and area were measured from video images and the cell volume estimated from these values. Morphological data was then correlated with electrophysiological recordings of whole-cell current, membrane potential and voltage-dependent non-linear capacitance. Electromotility was evoked either by manipulating the membrane potential under voltage-clamp conditions or by exposing OHCs to high K+ solutions. Other motile responses were investigated in voltage-clamp experiments at constant holding potential, or exposing OHCs to solutions that did not affect the membrane potential. We found that electrical stimulation evoked voltage-dependent changes in OHC's length, width and area but not in cell volume regardless of the time course of stimulation. Moreover, changes in cell area were always associated with both voltage-dependent motility and non-linear capacitance, suggesting prestin dependency. In contrast, voltage-independent motile responses at constant membrane potential, which are presumed to be prestin-independent, were associated with changes in cell length, width and volume without significant changes in area. Area measurements, then, become a tool to investigate the simultaneous occurrence of both prestin-dependent and prestin-independent OHC motilities, and for evaluating the individual contribution of each mechanism to the total cell movement.

Evidence for a highly elastic shell-core organization of cochlear outer hair cells by local membrane indentation

Cochlear outer hair cells (OHCs) are thought to play an essential role in the high sensitivity and sharp frequency selectivity of the hearing organ by generating forces that amplify the vibrations of this organ at frequencies up to several tens of kHz. This tuning process depends on the mechanical properties of the cochlear partition, which OHC activity has been proposed to modulate on a cycle-by-cycle basis. OHCs have a specialized shell-core ultrastructure believed to be important for the mechanics of these cells and for their unique electromotility properties. Here we use atomic force microscopy to investigate the mechanical properties of isolated living OHCs and to show that indentation mechanics of their membrane is consistent with a shell-core organization. Indentations of OHCs are also found to be highly nonhysteretic at deformation rates of more than 40 microm/s, which suggests the OHC lateral wall is a highly elastic structure, with little viscous dissipation, as would appear to be required in view of the very rapid changes in shape and mechanics OHCs are believed to undergo in vivo.

Immunohistochemical localization of urea transporters A and B in the rat cochlea

Urea is present in the inner ear, and when it is administered, induces rapid changes in the volume and osmolality of the inner ear fluid. However, the regulating mechanisms are unknown. Two groups of urea transporters (UTs), the renal urea transporter (UT-A) and the erythrocyte urea transporter (UT-B) have been cloned recently. The aims of the current study were to investigate the cellular localization of UTs in the cochlea of male Sprague-Dawley rats by immunohistochemistry. Both UT-A1 and UT-B were expressed in the inner and outer pillar cells, inner and outer hair cells, Boettcher's cells, and Deiters' cells in the organ of Corti. Immunoreactivity for UT-A3 was localized only in the mesothelial cells underlying the basilar membrane. In the stria vascularis, UT-A1 was expressed only in the marginal cells, whereas UT-B was expressed only in the basal cells. In the spiral ganglion, most cells had strong UT-A1 immunoreactivity whereas UT-B was not expressed. In the spiral limbus, UT-B was expressed in the interdental cells whereas UT-A was not expressed. In the crista ampullaris, UT-A1 was expressed in the dark cells, and UT-B expressed in the apical membrane of supporting cells in the neuroepithelium. The distribution of UT-A and UT-B in the inner ear suggests that the cells that surround the inner ear fluids may be involved in urea transport and thus play an important role in fluid homeostasis in the inner ear. The expression of UT-A and UT-B in the hair cells raises the possibility that UTs may be involved in volume regulation in these cells and mediate hair cell turgor.

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.

Excess plasma membrane and effects of ionic amphipaths on mechanics of outer hair cell lateral wall

The interaction between the outer hair cell (OHC) lateral wall plasma membrane and the underlying cortical lattice was examined by a morphometric analysis of cell images during cell deformation. Vesiculation of the plasma membrane was produced by micropipette aspiration in control cells and cells exposed to ionic amphipaths that alter membrane mechanics. An increase of total cell and vesicle surface area suggests that the plasma membrane possesses a membrane reservoir. Chlorpromazine (CPZ) decreased the pressure required for vesiculation, whereas salicylate (Sal) had no effect. The time required for vesiculation was decreased by CPZ, indicating that CPZ decreases the energy barrier required for vesiculation. An increase in total volume is observed during micropipette aspiration. A deformation-induced increase in hydraulic conductivity is also seen in response to micropipette-applied fluid jet deformation of the lateral wall. Application of CPZ and/or Sal decreased this strain-induced hydraulic conductivity. The impact of ionic amphipaths on OHC plasma membrane and lateral wall mechanics may contribute to their effects on OHC electromotility and hearing.

Micro- and nanomechanics of the cochlear outer hair cell

Outer hair cell electromotility is crucial for the amplification, sharp frequency selectivity, and nonlinearities of the mammalian cochlea. Current modeling efforts based on morphological, physiological, and biophysical observations reveal transmembrane potential gradients and membrane tension as key independent variables controlling the passive and active mechanics of the cell. The cell's mechanics has been modeled on the microscale using a continuum approach formulated in terms of effective (cellular level) mechanical and electric properties. Another modeling approach is nanostructural and is based on the molecular organization of the cell's membranes and cytoskeleton. It considers interactions between the components of the composite cell wall and the molecular elements within each of its components. The methods and techniques utilized to increase our understanding of the central role outer hair cell mechanics plays in hearing are also relevant to broader research questions in cell mechanics, cell motility, and cell transduction.

Action of 2,3-butanedione monoxime on capacitance and electromotility of guinea-pig cochlear outer hair cells

1. Whole-cell patch-clamp recordings were obtained from isolated cochlear outer hair cells (OHCs) while applying 2,3-butanedione monoxime (BDM) by pressure. BDM (5 mM) shifted the range of voltage sensitivity of membrane capacitance and cell length in the hyperpolarised direction by -49.6 +/- 4.0 mV (n = 12; mean +/- S.E.M.), without appreciable effects on membrane conductance. The shift was completely reversible and dose dependent, with a Hill coefficient of 1.8 /- 0.4 and a half-maximal dose of 3.0 +/- 0.8 mM (values +/- S.D). 2. The shift of the capacitance curve was also reproducible in cells whose natural turgor had been removed. BDM had no detectable effect on the capacitance of Deiters' cells, a non-sensory cell type of the organ of Corti. 3. The effect of BDM on membrane capacitance was faster than that of salicylate. At similar saturating concentrations (20 mM), the time constant of the capacitance changes was 1.8 +/- 0.3 s (n = 3) for salicylate and 0.75 +/- 0.06 s (n = 3) for BDM. The recovery periods were 13 +/- 1 s and 1.7 +/- 0.4 s, respectively (means +/- S.E.M.). 4. The effect of BDM, a known inorganic phosphatase, was compared to the effects of okadaic acid, trifluoperazine and W-7, which are commonly used in studies of protein phosphorylation. Incubation of OHCs with okadaic acid (1 microM, 30-60 min) shifted the voltage sensitivity of the membrane capacitance in the hyperpolarised direction. Incubation with trifluoperazine (30 microM) and W-7 (150 microM) shifted it in the opposite, depolarised direction. BDM induced hyperpolarising shifts even in the presence of W-7. 5. Simultaneous measurement of membrane capacitance and intracellular free Ca2+ concentration ([Ca2+]i) showed that BDM action on OHC voltage-dependent capacitance and electromotility is not mediated by changes of [Ca2+]i. 6. Our results suggest that: (a) the effects of BDM are unrelated to its inorganic phosphatase properties, cell turgor conditions or Ca2+ release from intracellular stores; and (b) BDM may target directly the voltage sensor of the OHC membrane motor protein.

Water permeability of cochlear outer hair cells: characterization and relationship to electromotility

The distinguishing feature of the mammalian outer hair cells (OHCs) is to elongate and shorten at acoustic frequencies, when their intracellular potential is changed. This "electromotility" or "electromechanics" depends critically on positive intracellular pressure (turgor), maintained by the inflow of water through yet uncharacterized water pathways. We measured the water volume flow, J(v), across the plasma membrane of isolated guinea pig and rat OHCs after osmotic challenges and estimated the osmotic water permeability coefficient, P(f), to be approximately 10(-2) cm/sec. This value is within the range reported for osmotic flow mediated by the water channel proteins, aquaporins. J(v) was inhibited by HgCl(2), which is known to block aquaporin-mediated water transport. P(f) values that were estimated for OHCs from neonatal rats were of the order of approximately 2 x 10(-3) cm/sec, equivalent to that of membranes lacking water channel proteins. In an immunofluorescence assay we showed that an anti-peptide antibody specific for aquaporins labels the lateral plasma membrane of the OHC in the region in which electromotility is generated. Using patch-clamp recording, we found that water influx into the OHC is regulated by intracellular voltage. We also found that the most pronounced increases of the electromotility-associated charge movement and of the expression of OHC water channels occur between postnatal days 8 and 12, preceding the onset of hearing function in the rat. Our data indicate that electromotility and water transport in OHCs may influence each other structurally and functionally.

Distinguishing cochlear pathophysiology in 4-aminopyridine and furosemide treated ears using a nonlinear systems identification technique

To test the adequacy of physiologic indices derived from a third-order polynomial model quantifying cochlear mechano-electric transduction (MET), 24 Mongolian gerbils were exposed to either 250-mM glucose (control), 150-mM 4-aminopyridine (4-AP), or 30-mM furosemide solutions applied to the round window (RW) membrane. The cochlear microphonic (CM) was recorded from the RW in response to 68- and 88-dB SPL Gaussian noise. A nonlinear systems identification technique (NLID) provided the frequency-domain parameters and physiologic indices of the polynomial model of MET. The control group showed no change in both compound action potential (CAP) thresholds and CM. Exposure to 4-AP and furosemide resulted in a similar elevation in CAP thresholds and a reduction in CM. However, the polynomial model of MET showed different changes. The operating point, slope, and symmetry of the MET function, the polynomial model parameters, and related nonlinear coherences differed between the experimental groups. It is concluded that the NLID technique is sensitive and specific to alterations in the cochlear physiology.

Two distinct Ca(2+)-dependent signaling pathways regulate the motor output of cochlear outer hair cells

The outer hair cells (OHCs) of the cochlea have an electromotility mechanism, based on conformational changes of voltage-sensitive "motor" proteins in the lateral plasma membrane. The translocation of electrical charges across the membrane that accompanies electromotility imparts a voltage dependency to the membrane capacitance. We used capacitance measurements to investigate whether electromotility may be influenced by different manipulations known to affect intracellular Ca(2+) or Ca(2+)-dependent protein phosphorylation. Application of acetylcholine (ACh) to the synaptic pole of isolated OHCs evoked a Ca(2+)-activated apamin-sensitive outward K(+) current. It also enhanced electromotility, probably because of a phosphorylation-dependent decrease of the cell's axial stiffness. However, ACh did not change the voltage-dependent capacitance either in conventional whole-cell experiments or under perforated-patch conditions. The effects produced by the Ca(2+) ionophore ionomycin mimicked those produced by ACh. Hyperpolarizing shifts of the voltage dependence of capacitance and electromotility were induced by okadaic acid, a promoter of protein phosphorylation, whereas trifluoperazine and W-7, antagonists of calmodulin, caused opposite depolarizing shifts. Components of the protein phosphorylation cascade-IP(3) receptors and calmodulin-dependent protein kinase type IV-were immunolocalized to the lateral wall of the OHC. Our results suggest that two different Ca(2+)-dependent pathways may control the OHC motor output. The first pathway modulates cytoskeletal stiffness and can be activated by ACh. The second pathway shifts the voltage sensitivity of the OHC electromotile mechanism and may be activated by the release of Ca(2+) from intracellular stores located in the proximity of the lateral plasma membrane.

On the mechanoelectrical coupling in the cochlear outer hair cell

Outer hair cell electromotility, a manifestation of the interconnection between the mechanical and electrical processes occurring in outer hair cells, is believed to be an important contribution to the active cochlea. Two modes of mechanoelectrical coupling in the outer hair cell wall are studied: the potential shift caused by mechanical loading under the wall charge preservation conditions and the current (transferred charge) caused by mechanical loading under the voltage-clamp conditions. By using the previously reported elastic moduli of the wall and components of the active force, the potential shift under the charge preservation conditions is derived. This shift is expressed in terms of the wall strains and the active force derivatives with respect to the wall potential. The magnitudes of the potential shift corresponding to the conditions of cell inflation, axial stretch (compression), and the micropipet aspiration are estimated. In the last case, the distribution of the potential shift along the cell wall is also demonstrated. The potential shift can reach -20(-)-40 mV under the conditions of the micropipet aspiration or cell inflation. Such shift is much smaller under the condition of cell stretch (compression). The current and the charge transfer caused by the cell stretch under the voltage-clamp conditions is analyzed, and shows good agreement of predictions with experimental data.

Salicylate induced changes in outer hair cell lateral wall stiffness

Micropipette aspiration was used to study the lateral wall stiffness of isolated guinea pig outer hair cells (OHCs) perfused with a sodium salicylate solution. Salicylate treatment significantly decreased lateral wall stiffness as measured by a stiffness parameter (S) compared to cells perfused with a standard bathing solution (S = 0.68 +/- 0.26 vs. S= 1.09 +/- 0.25, P < 0.05). The effect was reversible cells treated with salicylate and then with bathing solution exhibited a lateral wall stiffness similar to control cells (S = 1.10 +/- 0.40. P=0.94). Salicylate perfusion diminishes electromotile responses in isolated OHCs and physiologic doses of salicylate produce hearing loss and tinnitus in human subjects. The OHC lateral wall, the locus of electromotility, consists of three concentric layers: (1) an outermost plasma membrane, (2) a cytoskeletal network of actin and spectrin called the cortical lattice and (3) an innermost collection of flattened membranes called the subsurface cisternae (SSC). Ultrastructural studies have shown that salicylate treatment dilates and vesiculates the lateral wall subsurface cisternae (SSC) in guinea pig OHCs. In addition, salicylate causes an outward curvature of plasma membranes in human erythrocytes. The reversible, salicylate induced increase in lateral wall compliance may result from a direct action on the SSC and/or the plasma membrane.

Mechanically evoked shortening of outer hair cells isolated from the guinea pig organ of Corti

Outer hair cells isolated from the mammalian hearing organ have been shown to respond to mechanical stimuli at acoustic frequencies by expressing a change in cell length (e.g. Canlon et al., 1988). The acoustically evoked response is characterised by both a tonic length change following the envelope of the stimulus, and a frequency-dependent phasic component. We show here that mechanical stimulation at much lower frequencies directed at the cell body also elicits length changes of the outer hair cells. When the apical pole of isolated outer hair cells was compressed with a quartz fibre, a shortening or contraction at the basal pole was observed. Transverse indentation at the lateral membrane elicited shortenings at both ends of the cells. The sensitivity to the mechanical manipulation was changed by an altered tonicity of the external solution, or exposure to salicylate. As the response occurs at very low stimulus frequencies, it may account for the mechanism by which the hearing organ responds to the low frequency modulation component in complex signals like speech.

Effect of temperature on lateral wall mechanics of the guinea pig outer hair cell

The outer hair cell is thought to enhance the sensitivity of mammalian hearing. Its lateral wall consists of 3 concentric layers: an outermost plasma membrane, a cortical lattice, and an innermost collection of flattened membranes called the subsurface cisternae. The cytoplasm requires positive pressure for full expression of the outer hair cell's electromotility. Using micropipette aspiration, we investigated the mechanics of the guinea pig's outer hair cell lateral wall at room temperature (22 degrees C) and at the guinea pig's body temperature (39 degrees C). Although there was a 10% decrease in stiffness parameter with an increase from room to body temperature, the difference was not statistically significant; values ranged from 0.45 to 0.65 dyne/cm. With sufficient negative pressure, the cytoplasmic membrane is separated from the rest of the outer hair cell's lateral wall, a process that leads to vesiculation of the plasma membrane. Vesiculation occurs at a lower pressure than at body temperature. Our results demonstrate that the stiffness parameter of the outer hair cell lateral wall at body temperature is similar to that at room temperature. However, the plasma membrane's attachment to the cortical lattice is greatly altered by temperature. The decrease in strength of membrane attachment at body temperature may result from a change in membrane fluidity, making it more easy for membrane attachment sites to break free and permit vesiculation. Whether the tethering of the plasma membrane to the cortical lattice is lost under clinically pathologic conditions deserves future study.

Differentiation of cochlear pathophysiology in ears damaged by salicylate or a pure tone using a nonlinear systems identification technique

Mongolian gerbils were exposed to either alpha-ketoglutarate, salicylate, or an 8-kHz pure tone. Cochlear microphonic (CM) was recorded from the round window in response to 68 and 88 dB SPL Gaussian noise. A nonlinear systems identification technique provided the frequency-domain parameters of a third-order polynomial model characterizing cochlear mechano-electric transduction (MET). A series of physiologic indices were derived from further exploration of the model. Exposure to the 8-kHz pure tone and round window application of salicylate resulted in different changes in the polynomial parameters and physiologic indices even though the threshold shifts were similar. A general reduction of CM magnitude was found after the tone exposure, and an increase at low-mid frequencies was demonstrated in the salicylate group especially at the lower signal level. The slope of the MET curve was reduced by the acoustic overstimulation. The root or the operating point of the MET was shifted in opposite directions after the two treatments. Sound-pressure levels that saturate MET expanded in the tone exposure group and narrowed in the salicylate group. The signal level also had effects on these indices.

Analysis of the micropipet experiment with the anisotropic outer hair cell wall

The in-plane and bending stiffnesses of the outer hair cell wall are characteristics crucial to the understanding of force and energy transmission between cochlear elements. A mathematical interpretation is given of the micropipet experiment directed at determining the outer hair cell wall stiffness. On the basis of the microstructural observation of the wall showing that it comprises protein networks with different elastic properties, the model of an orthotropic cylindrical shell is used. The boundary-value problem is analyzed corresponding to the stress-strain of the wall (shell) caused by the action of the micropipet. The solution is given in terms of Fourier series with respect to the circumferential coordinate. An asymptotic analysis of the solution is developed and an approximate formula for the length of the tongue aspirated within the pipet is derived. This leads to an analytical expression for the stiffness parameter measured in the micropipet experiment in terms of Young's moduli and Poisson's ratios of the wall. This expression is an important part of the estimation of the elastic constants of the wall.

An ancient conserved gene expressed in the human inner ear: identification, expression analysis, and chromosomal mapping of human and mouse antiquitin (ATQ1)

We constructed and screened a human fetal cochlear cDNA library to identify genes involved in hearing and deafness. From this library we isolated a cDNA corresponding to the highly conserved ancient gene antiquitin (ATQ1). The plant homolog of ATQ1 is thought to be involved in regulating turgor pressure, a function that also would be essential for cells of the mammalian cochlea. Northern blots of 13 human fetal tissues show antiquitin to be highly expressed in cochlea, ovary, eye, heart, and kidney. Using RT-PCR of rat cochlear hair cell-specific cDNA libraries, we detect antiquitin expression in outer hair cells, but not in inner or vestibular type 1 hair cells, suggesting that antiquitin is not expressed ubiquitously in the cochlea. Human ATQ1 was mapped to human chromosome region 5q31 using fluorescence in situ hybridization, and mouse ATQ1 was mapped to mouse chromosome 18 by single-strand conformation polymorphism mapping of interspecific backcross progeny DNAs. Four human antiquitin-like sequences, possibly pseudogenes, were also identified and mapped.

Correspondence between middle frequency auditory loss in vivo and outer hair cell shortening in vitro

The aromatic hydrocarbon, toluene, has been reported to disrupt auditory system function both in occupational epidemiological and in laboratory animal investigations. This agent, along with several other organic solvents, impairs hearing preferentially at middle frequencies - a finding that distinguishes these agents from the traditional high frequency impairment observed with ototoxic drugs such as aminoglycoside antibiotics and cisplatin. Prior investigations performed in vivo have identified the outer hair cell as a probable target for toluene exposure. The purpose of this investigation was to determine directly whether outer hair cells isolated from the guinea pig cochlea show morphological alterations consistent with the toxic response seen in physiological studies with toluene exposure. The effect of toluene superfusion on outer hair cell shortening was assessed for cells harvested from different locations within the cochlea. Control studies included assessment of cell shortening among outer hair cells exposed to trimethyltin and cells exposed to benzene. Trimethyltin disrupts high frequency hearing preferentially and benzene does not produce hearing loss in vivo. Toluene at a concentration of 100 microM produced a marked shortening of outer hair cells although the effect was significantly greater among cells isolated from the apical half of the cochlea than from the basal half of the cochlea. By contrast, trimethyltin at the same concentration produced a preferential shortening among outer hair cells from the base of the cochlea. Benzene (100 microM) did not disrupt outer hair cell length of cells harvested from the apex. The results indicate that intrinsic features of outer hair cells contribute significantly to the site of ototoxic impairment observed in vivo for toluene.

Acetylcholine, outer hair cell electromotility, and the cochlear amplifier

The dominant efferent innervation of the cochlea terminates on outer hair cells (OHCs), with acetylcholine (ACh) being its principal neurotransmitter. OHCs respond with a somatic shape change to alterations in their membrane potential, and this electromotile response is believed to provide mechanical feedback to the basilar membrane. We examine the effects of ACh on electromotile responses in isolated OHCs and attempt to deduce the mechanism of ACh action. Axial electromotile amplitude and cell compliance increase in the presence of the ligand. This response occurs with a significantly greater latency than membrane current and potential changes attributable to ACh and is contemporaneous with Ca2+ release from intracellular stores. It is likely that increased axial compliance largely accounts for the increase in motility. The mechanical responses are probably related to a recently demonstrated slow efferent effect. The implications of the present findings related to commonly assumed efferent behavior in vivo are considered.

Hyposmotic swelling induces magnitude and gain change in the electromotile performance of isolated outer hair cells

OHCs were inserted into a partitioning microchamber. Reversible swelling was induced by hyposmotic incubation medium (270 mos.kg H2O-1). The transfer curve of electromotility was measured by stimulating the OHCs with a series of brief (2.5 ms) square-pulses with both polarities and graded magnitudes. Response magnitude and gain of electromotility were increased in swollen OHCs. On the basis of their electromotile performance-characteristics, swelling appeared to depolarize the OHCs. Transition from contraction dominated asymmetry of motility was observed towards a symmetric response. Depolarization, by itself, cannot explain the gain and magnitude increase. Acetylcholine (ACh) responsiveness of swollen OHCs preserves the pre-challenged pattern: ACh further increases gain and magnitude.

Measurements and a model of the outer hair cell hydraulic conductivity

The hydraulic conductivity of the cochlear outer hair cell (OHC) is central to the maintenance of the positive intracellular pressure necessary for its function as the cochlear amplifier. A mathematical model of osmotic water transport across the OHC membrane is formulated. The model relates the OHC hydraulic conductivity, Lp, to the rate of volume change in response to osmotic stimuli. Lp is evaluated from osmotic experiments in which isolated OHCs are exposed to an hypotonic solution. The rate of volume increase in response to the hypotonic challenge was determined by a morphometric analysis of video images of cells. Lp was found to be about 10(-14) m s-1 Pa-1 or equivalently, Pf approximately 10(-4) cm s-1. This is on the low side of values reported for different lipid bilayers and is 2 orders of magnitude lower than the hydraulic conductivity of red blood cells. The relation of the low OHC hydraulic conductivity to the composition and morphology of its membranes is discussed.

Membrane tension directly shifts voltage dependence of outer hair cell motility and associated gating charge

The unique electromotility of the outer hair cell (OHC) is believed to promote sharpening of the passive mechanical vibration of the mammalian basilar membrane. The cell also presents a voltage-dependent capacitance, or equivalently, a nonlinear gating current, which correlates well with its mechanical activity, suggesting that membrane-bound voltage sensor-motor elements control OHC length. We report that the voltage dependence of the gating charge and motility are directly related to membrane stress induced by intracellular pressure. A tracking procedure was devised to continuously monitor the voltage at peak capacitance (VpkCm) after obtaining whole cell voltage clamp configuration. In addition, nonlinear capacitance was more fully evaluated with a stair step voltage protocol. Upon whole cell configuration, VpkCm was typically near -20 mV. Negative patch pipette pressure caused a negative shift in VpkCm, which obtained a limiting value near the normal resting potential of the OHC (approximately -70 mV) at the point of cell collapse. Positive pressure in the pipette caused a positive shift that could reach values greater than 0 mV. Measures of the mechanical activity of the OHC mirrored those of charge movement. Similar membrane-tension dependent peak shifts were observed after the cortical cytoskeletal network was disrupted by intracellular dialysis of trypsin from the patch pipette. We conclude that unlike stretch receptors, which may sense tension through elastic cytoskeletal elements, the OHC motor senses tension directly. Furthermore, since the voltage dependence of the OHC nonlinear capacitance and motility is directly regulated by intracellular turgor pressure, we speculate that modification of intracellular pressure in vivo provides a mechanism for controlling the gain of the mammalian "cochlear amplifier".

Direct effects of reactive oxygen species on cochlear outer hair cell shape in vitro

Reactive oxygen species (ROS) have been implicated in the ototoxicity of various agents. This study examines the effects of superoxide anion (O2), hydroxyl radical (OH.) and hydrogen peroxide (H2O2), on isolated cochlear outer hair cell (OHC) morphology. OHCs were superfused with artificial perilymph (AP) or AP containing a specific ROS scavenger, and then with AP, ROS system or scavenger plus ROS system for 90 min. The generation of ROS as well as the scavenging properties of other agents were confirmed by specific biochemical assays. Control cells decreased 4.8% in mean length, and showed no obvious membrane damage. Generation of O2. or OH. resulted in high rates (85.7 and 42.9%, respectively) of bleb formation at the synaptic pole, and decreased (O2., 15.2%; OH., 17.3%) mean cell length. Length change and bleb formation rate were H2O2 concentration-dependent. 20 mM H2O2 led to 33.3% decreased mean cell length, and only 20% bleb formation; 0.1 mM H2O2 led to 83.3% bleb formation, with no length decrease. Superoxide dismutase, deferoxamine and catalase protected against O2., OH. and H2O2 effects, respectively. Bleb formation and diminished cell length likely represent differential lipid peroxidative outcomes at supra- and infranuclear membranes, and are consistent with effects of certain ototoxicants.