Reduction of the Endocochlear Potential by the New “Loop” Diuretic, Bumetanide

Identifying targets to prevent aminoglycoside ototoxicity

Aminoglycosides are potent antibiotics that are commonly prescribed worldwide. Their use carries significant risks of ototoxicity by directly causing inner ear hair cell degeneration. Despite their ototoxic side effects, there are currently no approved antidotes. Here we review recent advances in our understanding of aminoglycoside ototoxicity, mechanisms of drug transport, and promising sites for intervention to prevent ototoxicity.

Slow oscillatory changes of DPOAE magnitude and phase after exposure to intense low-frequency sounds

Sensitive sound detection within the mammalian cochlea is performed by hair cells surrounded by cochlear fluids. Maintenance of cochlear fluid homeostasis and tight regulation of intracellular conditions in hair cells are crucial for the auditory transduction process but can be impaired by intense sound stimulation. After a short, intense low-frequency sound, the cochlea shows the previously described "bounce phenomenon," which manifests itself as slow oscillatory changes of hearing thresholds and otoacoustic emissions. In this study, distortion product otoacoustic emissions (DPOAEs) were recorded after Mongolian gerbils were exposed to intense low-frequency sounds (200 Hz, 100 dB SPL) with different exposure times up to 1 h. After all sound exposure durations, a certain percentage of recordings (up to 80% after 1.5-min-long exposure) showed oscillatory DPOAE changes, similar to the bounce phenomenon in humans. Changes were quite uniform with respect to size and time course, and they were independent from sound exposure duration. Changes showed states of hypo- and hyperactivity with either state preceding the other. The direction of changes was suggested to depend on the static position of the cochlear operating point. As assessed with DPOAEs, no indication for a permanent damage after several or long exposure times was detected. We propose that sensitivity changes occur due to alterations of the mechanoelectrical transduction process of outer hair cells. Those alterations could be induced by different challenged homeostatic processes with slow electromotility of outer hair cells being the most plausible source of the bounce phenomenon. NEW & NOTEWORTHY Low-frequency, high-intensity sound can cause slowly cycling activity changes in the mammalian cochlea. We examined the effect of low-frequency sound duration on the degree of these alterations. We found that cochlear changes showed a stereotypical biphasic pattern independent of sound exposure duration, but the probability that significant changes occurred decreased with increasing sound duration. Despite exposure durations of up to 1 h, no permanent or transient impairments of the cochlea were detected.

Age-related changes in Na, K-ATPase expression, subunit isoform selection and assembly in the stria vascularis lateral wall of mouse cochlea

Due to the critical role of cochlear ion channels for hearing, the focus of the present study was to examine age-related changes of Na, K-ATPase (NKA) subunits in the lateral wall of mouse cochlea. We combined qRT-PCR, western blot and immunocytochemistry methodologies in order to determine gene and protein expression levels in the lateral wall of young and aged CBA/CaJ mice. Of the seven NKA subunits, only the mRNA expressions of α1, β1 and β2 subunit isoforms were detected in the lateral wall of CBA/CaJ mice. Aging was accompanied by dys-regulation of gene and protein expression of all three subunits detected. Hematoxylin and eosin (H&E) staining revealed atrophy of the cochlear stria vascularis (SV). The SV atrophy rate (20%) was much less than the ∼80% decline in expression of all three NKA isoforms, indicating lateral wall atrophy and NKA dys-regulation are independent factors and that there is a combination of changes involving the morphology of SV and NKA expression in the aging cochlea which may concomitantly affect cochlear function. Immunoprecipitation assays showed that the α1-β1 heterodimer is the selective preferential heterodimer over the α1-β2 heterodimer in cochlea lateral wall. Interestingly, in vitro pathway experiments utilizing cultured mouse cochlear marginal cells from the SV (SV-K1 cells) indicated that decreased mRNA and protein expressions of α1, β1 and β2 subunit isoforms are not associated with reduction of NKA activity following in vitro application of ouabain, but ouabain did disrupt the α1-β1 heterodimer interaction. Lastly, the association between the α1 and β1 subunit isoforms was present in the cochlear lateral wall of young adult mice, but this interaction could not be detected in old mice. Taken together, these data suggest that in the young adult mouse there is a specific, functional selection and assembly of NKA subunit isoforms in the SV lateral wall, which is disrupted and dys-regulated with age. Interventions for this age-linked ion channel disruption may have the potential to help diagnose, prevent, or treat age-related hearing loss.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

Acute blockade of inner ear marginal and dark cell K+ secretion: Effects on gravity receptor function

Specific pharmacological blockade of KCNQ (Kv7) channels with XE991 rapidly (within 20 min) and profoundly alters inner ear gravity receptor responses to head motion (Lee et al., 2017). We hypothesized that these effects were attributable to the suppression of K⁺ secretion following blockade of KCNQ1-KCNE1 channels in vestibular dark cells and marginal cells. To test this hypothesis, K⁺ secretion was independently inhibited by blocking the Na⁺-K⁺-2Cl^- cotransporter (NKCC1, Slc12a2) rather than KCNQ1-KCNE1 channels. Acute blockade of NKCC1 with ethacrynic acid (40 mg/kg) eliminated auditory responses (ABRs) within approximately 70 min of injection, but had no effect on vestibular gravity receptor function (VsEPs) over a period of 2 h in the same animals. These findings show that, vestibular gravity receptors are highly resistant to acute disruption of endolymph secretion unlike the auditory system. Based on this we argue that acute suppression of K⁺ secretion alone does not likely account for the rapid profound effects of XE991 on gravity receptors. Instead the effects of XE991 likely require additional action at KCNQ channels located within the sensory epithelium itself.

A microsensing system for the in vivo real-time detection of local drug kinetics

Real-time recording of the kinetics of systemically administered drugs in in vivo microenvironments may accelerate the development of effective medical therapies. However, conventional methods require considerable analyte quantities, have low sampling rates and do not address how drug kinetics correlate with target function over time. Here, we describe the development and application of a drug-sensing system consisting of a glass microelectrode and a microsensor composed of boron-doped diamond with a tip of around 40 μm in diameter. We show that, in the guinea pig cochlea, the system can measure-simultaneously and in real time-changes in the concentration of bumetanide (a diuretic that is ototoxic but applicable to epilepsy treatment) and the endocochlear potential underlying hearing. In the rat brain, we tracked the kinetics of the drug and the local field potentials representing neuronal activity. We also show that the actions of the antiepileptic drug lamotrigine and the anticancer reagent doxorubicin can be monitored in vivo. Our microsensing system offers the potential to detect pharmacological and physiological responses that might otherwise remain undetected.

NKCCs in the fibrocytes of the spiral ligament are silent on the unidirectional K⁺ transport that controls the electrochemical properties in the mammalian cochlea

Unidirectional K(+) transport across the lateral cochlear wall contributes to the endocochlear potential (EP) of +80 mV in the endolymph, a property essential for hearing. The wall comprises two epithelial layers, the syncytium and the marginal cells. The basolateral surface of the former and the apical membranes of the latter face the perilymph and the endolymph, respectively. Intrastrial space (IS), an extracellular compartment between the two layers, exhibits low [K(+)] and a potential similar to the EP. This IS potential (ISP) dominates the EP and represents a K(+) diffusion potential elicited by a large K(+) gradient across the syncytial apical surface. The K(+) gradient depends on the unidirectional K(+) transport driven by Na(+),K(+)-ATPases on the basolateral surface of each layer and the concomitant Na(+),K(+),2Cl(-)-cotransporters (NKCCs) in the marginal cell layer. The NKCCs coexpressed with the Na(+),K(+)-ATPases in the syncytial layer also seem to participate in the K(+) transport. To test this hypothesis, we examined the electrochemical properties of the lateral wall with electrodes measuring [K(+)] and potential. Blocking NKCCs by perilymphatic perfusion of bumetanide suppressed the ISP. Unexpectedly and unlike the inhibition of the syncytial Na(+),K(+)-ATPases, the perfusion barely altered the electrochemical properties of the syncytium but markedly augmented [K(+)] of the IS. Consequently, the K(+) gradient decreased and the ISP declined. These observations resembled those when the marginal cells' Na(+),K(+)-ATPases or NKCCs were blocked with vascularly applied inhibitors. It is plausible that NKCCs in the marginal cells are affected by the perilymphatically perfused bumetanide, and these transporters, but not those in the syncytium, mediate the unidirectional K(+) transport.

Temporal expression pattern of Fkbp8 in rodent cochlea

BACKGROUND

FKBP8 is a multifunctional protein involved in many distinct processes like formation of central nervous system, viral RNA replication and inhibition of apoptosis. Fkbp8 expression was reported in different tissues, various cell lines and malignancies, in the latter displaying changes during carcinogenesis. Loss of Fkbp8 leads to substantial neurodegenerations during regular mouse development, thus hearing onset in mice could also potentially depend on Fkbp8 expression. Since Fkbp8 is crucial for patterning of neuronal function, we studied its expression during maturation of the rodent auditory function.

METHODS

Fkbp8 gene expression in rodent cochlear samples was studied by RT-PCR, qPCR, and western blot. Localization of Fkbp8 transcripts and protein was analyzed by in-situ hybridization and immunohistochemistry.

RESULTS

Studies of auditory organ demonstrate that Fkbp8 gene activity is increasing just before hearing onset and gradually decreasing after onset of hearing. Western blot analysis suggests substantial levels of Fkbp8 protein before hearing onset, and slow degradation after onset of hearing. The Fkbp8 mRNA is localized in spiral ganglion of cochlea but its distribution changes over time to the stria vascularis, a finding supported by immunohistochemistry staining. Additionally, in pre-hearing time Fkbp8-specific signal was also observed in the tectorial membrane, whose α- and β-Tectorin components show similar time-dependent expression of mRNA as Fkbp8.

CONCLUSION

These results indicate a temporal shift in expression of Fkbp8 which correlates with cochlear maturation, strongly suggesting a contribution of Fkbp8 to the onset of the rodent hearing processes.

Sox2 and JAGGED1 expression in normal and drug-damaged adult mouse inner ear

Inner ear hair cells detect environmental signals associated with hearing, balance, and body orientation. In humans and other mammals, significant hair cell loss leads to irreversible hearing and balance deficits, whereas hair cell loss in nonmammalian vertebrates is repaired by the spontaneous generation of replacement hair cells. Research in mammalian hair cell regeneration is hampered by the lack of in vivo damage models for the adult mouse inner ear and the paucity of cell-type-specific markers for non-sensory cells within the sensory receptor epithelia. The present study delineates a protocol to drug damage the adult mouse auditory epithelium (organ of Corti) in situ and uses this protocol to investigate Sox2 and Jagged1 expression in damaged inner ear sensory epithelia. In other tissues, the transcription factor Sox2 and a ligand member of the Notch signaling pathway, Jagged1, are involved in regenerative processes. Both are involved in early inner ear development and are expressed in developing support cells, but little is known about their expressions in the adult. We describe a nonsurgical technique for inducing hair cell damage in adult mouse organ of Corti by a single high-dose injection of the aminoglycoside kanamycin followed by a single injection of the loop diuretic furosemide. This drug combination causes the rapid death of outer hair cells throughout the cochlea. Using immunocytochemical techniques, Sox2 is shown to be expressed specifically in support cells in normal adult mouse inner ear and is not affected by drug damage. Sox2 is absent from auditory hair cells, but is expressed in a subset of vestibular hair cells. Double-labeling experiments with Sox2 and calbindin suggest Sox2-positive hair cells are Type II. Jagged1 is also expressed in support cells in the adult ear and is not affected by drug damage. Sox2 and Jagged1 may be involved in the maintenance of support cells in adult mouse inner ear.

Functional significance of channels and transporters expressed in the inner ear and kidney

A number of ion channels and transporters are expressed in both the inner ear and kidney. In the inner ear, K+cycling and endolymphatic K+, Na+, Ca2+, and pH homeostasis are critical for normal organ function. Ion channels and transporters involved in K+cycling include K+channels, Na+-2Cl−-K+cotransporter, Na+/K+-ATPase, Cl−channels, connexins, and K+/Cl−cotransporters. Furthermore, endolymphatic Na+and Ca2+homeostasis depends on Ca2+-ATPase, Ca2+channels, Na+channels, and a purinergic receptor channel. Endolymphatic pH homeostasis involves H+-ATPase and Cl−/HCO3−exchangers including pendrin. Defective connexins (GJB2 and GJB6), pendrin (SLC26A4), K+channels (KCNJ10, KCNQ1, KCNE1, and KCNMA1), Na+-2Cl−-K+cotransporter (SLC12A2), K+/Cl−cotransporters (KCC3 and KCC4), Cl−channels (BSND and CLCNKA + CLCNKB), and H+-ATPase (ATP6V1B1 and ATPV0A4) cause hearing loss. All these channels and transporters are also expressed in the kidney and support renal tubular transport or signaling. The hearing loss may thus be paralleled by various renal phenotypes including a subtle decrease of proximal Na+-coupled transport (KCNE1/KCNQ1), impaired K+secretion (KCNMA1), limited HCO3−elimination (SLC26A4), NaCl wasting (BSND and CLCNKB), renal tubular acidosis (ATP6V1B1, ATPV0A4, and KCC4), or impaired urinary concentration (CLCNKA). Thus, defects of channels and transporters expressed in the kidney and inner ear result in simultaneous dysfunctions of these seemingly unrelated organs.

Computational model of vectorial potassium transport by cochlear marginal cells and vestibular dark cells

Cochlear marginal cells and vestibular dark cells transport potassium into the inner ear endolymph, a potassium-rich fluid, the homeostasis of which is essential for hearing and balance. We have formulated an integrated mathematical model of ion transport across these epithelia that incorporates the biophysical properties of the major ion transporters and channels located in the apical and basolateral membranes of the constituent cells. The model is constructed for both open- and short-circuit situations to test the extremes of functional capacity of the epithelium and predicts the steady-state voltages, ion concentrations, and transepithelial currents as a function of various transporter and channel densities. We validate the model by establishing that the cells are capable of vectorial ion transport consistent with several experimental measurements. The model indicates that cochlear marginal cells do not make a significant direct contribution to the endocochlear potential and illustrates how changes to the activity of specific transport proteins lead to reduced K(+) flux across the marginal and dark cell layers. In particular, we investigate the mechanisms of loop diuretic ototoxicity and diseases with hearing loss in which K(+) and Cl(-) transport are compromised, such as Jervell and Lange-Nielsen syndrome and Bartter syndrome, type IV, respectively. Such simulations demonstrate the utility of compartmental modeling in investigating the role of ion homeostasis in inner ear physiology and pathology.

Gastric type H+,K+-ATPase in the cochlear lateral wall is critically involved in formation of the endocochlear potential

Cochlear endolymph has a highly positive potential of approximately +80 mV known as the endocochlear potential (EP). The EP is essential for hearing and is maintained by K(+) circulation from perilymph to endolymph through the cochlear lateral wall. Various K(+) transport apparatuses such as the Na(+),K(+)-ATPase, the Na(+)-K(+)-2Cl(-) cotransporter, and the K(+) channels Kir4.1 and KCNQ1/KCNE1 are expressed in the lateral wall and are known to play indispensable roles in cochlear K(+) circulation. The gastric type of the H(+),K(+)-ATPase was also shown to be expressed in the cochlear lateral wall (Lecain E, Robert JC, Thomas A, and Tran Ba Huy P. Hear Res 149: 147-154, 2000), but its functional role has not been well studied. In this study we examined the precise localization of H(+),K(+)-ATPase in the cochlea and its involvement in formation of EP. RT-PCR analysis showed that the cochlea expressed mRNAs of gastric alpha(1)-, but not colonic alpha(2)-, and beta-subunits of H(+),K(+)-ATPase. Immunolabeling of an antibody specific to the alpha(1) subunit was detected in type II, IV, and V fibrocytes distributed in the spiral ligament of the lateral wall and in the spiral limbus. Strong immunoreactivity was also found in the stria vascularis. Immunoelectron microscopic examination exhibited that the H(+),K(+)-ATPase was localized exclusively at the basolateral site of strial marginal cells. Application of Sch-28080, a specific inhibitor of gastric H(+),K(+)-ATPase, to the spiral ligament as well as to the stria vascularis caused prominent reduction of EP. These results may imply that the H(+),K(+)-ATPase in the cochlear lateral wall is crucial for K(+) circulation and thus plays a critical role in generation of EP.

Bumetanide-induced enlargement of the intercellular space in the stria vascularis critically depends on Na+ transport

The intercellular space in the stria vascularis (intrastrial space) is a closed space and isolated from both the endolymph and the perilymph in normal tissue. Loop diuretics such as bumetanide and furosemide cause an acute enlargement of the intrastrial space in association with a decline in the endocochlear potential. It is known that bumetanide inhibits the Na+-K+-2Cl- cotransporter, which is expressed abundantly in the basolateral membrane of marginal cells. We studied ionic mechanisms underlying the bumetanide-induced enlargement of the intrastrial space using perilymphatic perfusion in guinea pigs. Perilymphatic perfusion with artificial perilymph containing 100 microM bumetanide caused marked enlargement of the intrastrial space, as reported previously. Removal of K+ from the perilymph did not affect the bumetanide-induced enlargement, whereas removal of Na+ from the perilymph inhibited it almost completely. Perilymph containing 1 mM amiloride also inhibited the enlargement of the intrastrial space almost completely. These results indicate that perilymphatic Na+, but not K+, and amiloride-sensitive pathways are essential to the bumetanide-induced enlargement of the intrastrial space. Two possible pathways could yield these results. Na+ in the perilymph could enter the endolymph via Reissner's membrane or the basilar membrane; Na+ in the endolymph would then be taken up by marginal cells via the apical membrane and secreted into the intrastrial space by Na+-K+-ATPase in the basolateral membrane of them. Another, less likely possibility is that Na+ in the perilymph is transported into basal cells or fibrocytes in the spiral ligament, then into intermediate cells via gap junctions, and finally secreted into the intrastrial space via Na+-K+-ATPase of intermediate cells.

Ultrastructure of the inner ear of NKCC1-deficient mice

The transduction of the auditory signal is dependent on the flow of ions within the inner ear. We have generated mice deficient in NKCC1, an ion cotransporter that is thought to be involved in the secretion of K+ by the strial marginal cells. Inner ear histology revealed partial to almost total absence of the scala media and collapse of Reissner's membrane. Ultrastructural analysis showed that Reissner's membrane consists of 3-4 cell layers instead of the usual two, and a substance of unknown composition is present between Reissner's membrane and underlying structures. Within the tunnel of Corti, hair cells and supporting cells were difficult to identify. The location of the tectorial membrane was altered, and a precipitate was observed surrounding it. Severe structural defects were noted in the interdental cells and Boettcher cells, and mild defects were observed in the stria vascularis and in type II and type IV fibrocytes. The finding that major defects occur predominantly in cells that are not known to express NKCC1 suggests that loss of NKCC1 results in functional defects in cells expressing NKCC1 and a morphological effect on cell populations downstream in the proposed K+ recycling pathway.

Sodium, potassium, chloride and calcium concentrations measured in pigeon perilymph and endolymph

According to Davis' (1965) model of the inner ear, a potential difference between the endocochlear potential and the hair cell resting potential drives the transduction current across the apical hair cell membrane. It is assumed that the endocochlear potential (EP) consists of two components. The first is a diffusion potential, which depends on the ionic composition of endolymph and perilymph and on the permeability of the perilymph-endolymph barrier. The second is an electrogenic component which is determined by active ion transport across the perilymph-endolymph barrier. In birds, the EP is between +8 and +20 mV. Little is known about the underlying mechanisms responsible for the measured EP in birds. The present paper studies whether ionic compositions of endo- and perilymph might explain the EP in birds. Concentrations of Na+, K+, Ca2+ and Cl- in pigeon scala vestibuli, scala tympani and scala media were determined with ion-selective microelectrodes. Na+, K+, Ca2+ and Cl- were 150.0, 4.2, 1.4 and 117.0 mM in perilymph (scala tympani and scala vestibuli). In scala media, the concentrations of K+, Ca2+ and Cl- were 140.6, 0.23 and 142.1 mM.

Endocochlear potential in focal lesions of the guinea pig cochlea

To estimate the correlation between the cochlear lateral wall and endocochlear potential (EP), the EP was measured at different time intervals up to 3 weeks at cochlear focal lesions made in the guinea pig. Lesions were produced by a photochemical reaction between systemically administered Rose Bengal and green light illumination in the second cochlear turn. Focal strial lesions (mean diameter 975 microm) became apparent under a scanning electron microscope 60 min after illumination, and degeneration of the organ of Corti was recognized at 3 days. The EP was measured continuously for 1 h following onset of the reaction in eight ears. In the other ears, EP measurements were done after various intervals of time up to 3 weeks. The EP showed two declines from pre-illumination level. The first started soon after illumination, reached a minimum value at 25 min, and then slightly recovered by 1 h. The second decline appeared at 3 days after illumination. In contrast to evident focal morphological degeneration 2 weeks post illumination, the EP values had recovered to almost normal. We suggest that the sequential changes in the EP were produced depending on the degeneration and then repairing processes of the endolymphatic boundary tissues.

Na-K-Cl cotransporter expression in the developing and senescent gerbil cochlea

Changes in the cellular expression pattern of the Na-K-Cl cotransporter (NKCC) were investigated during postnatal development and with advancing age in the gerbil cochlea. At birth, faint immunostaining for NKCC was discernable in the developing stria vascularis (StV), Reissner's membrane, interdental cells and some relatively undifferentiated cells lining the cochlear partition. Between 2 and 4 days after birth (DAB) immunostaining persisted and increased in the future interdental, inner and outer sulcus and claudius cells but then disappeared from these sites by 8 DAB. In contrast, NKCC immunoreactivity in the StV increased progressively during development and approached adult levels by 12 DAB. Immunostaining for NKCC in subpopulations of fibrocytes in the inferior portion of the spiral ligament, the suprastrial region and the spiral limbus was first detectable between 10 and 12 DAB and staining intensity reached adult levels around 16 DAB. Changes in NKCC expression with advancing age generally mimicked those previously observed for Na,K-ATPase in focal regions of atrophic lateral wall. Diminished immunostaining was first seen in the StV, presumably associated with the involution of the marginal cell's basolateral processes. Further atrophy culminated in complete loss of immunostaining in the StV and an associated down-regulation of NKCC expression in spiral ligament transport fibrocytes. The marked similarities in the developmental and age-related expression patterns of NKCC and Na,K-ATPase point to a high level of functional cooperativity between these two ion transport mediators, which together provide an efficient mechanism for generating and maintaining high K+ levels in endolymph and the endocochlear potential.

Changes in the volume of marginal cells induced by isotonic 'Cl- depletion/restoration': involvement of the Cl- channel and Na+-K+-Cl- cotransporter

Marginal cells constitute the endolymph-facing epithelium responsible for the secretion of endolymph by the stria vascularis in the inner ear. We have studied the possible involvement of Cl- conductance and Na+-K+-Cl- cotransport in the mechanism of changes in cell volume upon isotonic Cl- depletion/restoration. Changes in cell volume were estimated from video-microscopic images with the aid of an image processor. Marginal cells shrank to approximately 80% of their original volume in 30 s and to 65-70% in 90 s upon total replacement of [Cl]o (approximately 150 mM) by gluconate-, and the original volume of the shrunken cells was restored within 2 min after restoration of Cl-. The order of potency of anions to induce isotonic shrinkage was gluconate > I- > F- > Br-. The cell shrinkage caused by Cl- depletion was partially inhibited by 5-Nitro-2-(3-phenyl-propylamino)-benzoic acid (NPPB, 0.2 mM), but not by either 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid (SITS, 0.5 mM), bumetanide (10 microM) or ouabain (1 mM). The cell shrinkage caused by a reduction of [Cl]o from approximately 150 mM to 7.5 mM was not affected by [K]o in the range of 3.6 mM to 72 mM. These results suggest that the main efflux pathway(s) responsible for the 'Cl removal'-induced shrinkage depends on volume-correlated Cl- conductance (Takeuchi and Irimajiri, J. Membrane Biol. 150, 47-62, 1996) and that this pathway(s) is essentially independent of the Na+-K+-Cl- cotransporter, the Na+,K+-ATPase, and the K+-Cl- cotransporter. With regard to volume recovery after isotonic shrinkage, its critical dependence on the simultaneous presence of Na+, K+ and Cl- in the bath and its substantial inhibition by bumetanide (10 microM) both indicate a major role for Na+-K+-Cl- cotransport. The strong influence on cell volume of solute fluxes working through the Cl- channel and the Na+-K+-Cl- cotransporter implies an essential role for these pathways in the ion transport mechanism(s) of the marginal cell.

Decline in the endocochlear potential corresponds to decreased Na,K-ATPase activity in the lateral wall of quiet-aged gerbils

The ion transport-mediating enzyme, Na,K-ATPase, is abundantly present in the cochlear lateral wall. This enzyme is essential for the generation and maintenance of the endocochlear potential. Diminished enzyme activity has been observed previously in the lateral wall of quiet-aged gerbils. The present study was designed to investigate the impact of the age-related decline in Na,K-ATPase specific activity upon auditory function. Measures of the resting endocochlear potential value and the level of Na,K-ATPase specific activity were made in cochleae obtained from gerbils aged in quiet conditions. Analysis revealed a high degree of correspondence between the level of lateral wall Na,K-ATPase specific activity and the value of the endocochlear potential measured in the round window/turn 1 region of the cochlea. Nonlinear regression models showed a strong relationship between the age-related reductions in enzyme activity and the magnitude of the endocochlear potential. The data suggest that during metabolic presbyacusis a decrease in Na,K-ATPase specific activity can explain most, but not all, of the decline in the endocochlear potential.

Molecular and clinical implications of loop diuretic ototoxicity

Recent advances in molecular biology have been applied to inner ear research. Loop diuretic ototoxicity has been suggested, but not proven, to share a common mechanism with diuretic effects on renal tubules. The discovery of the molecular nature of the Na-K-2Cl cotransporter in the cochlea provided a better understanding of loop diuretic ototoxicity. In this review, we describe clinical reports of loop diuretic ototoxicity and other information obtained by physiological, biochemical and morphological investigations related to the mechanism sensitive to loop diuretics. Based on recent evidence for the molecular nature of the Na-K-2Cl cotransporter expressed in the mammalian cochlea, the underlying mechanisms of ototoxicity induced by loop diuretics are described.

Ultrastructural localization of the Na-K-Cl cotransporter in the lateral wall of the rabbit cochlear duct

Localization of the immunoreactivity in the lateral wall of the rabbit cochlear duct was examined using a post-embedding immunogold method with a polyclonal antiserum raised against the rabbit parotid Na-K-Cl cotransporter. In the stria vascularis, the labeling was significant on the basolateral membrane infolding of marginal cells, whereas no labeling was seen on the luminal membrane of these cells. Immunoreactivity was also detected on the cell membranes of various other cells. These include fibrocytes of the spiral ligament and the spiral prominence, and vascular endothelial cells in the stria vascularis and the spiral ligament. In contrast, virtually no gold particles were seen on the membrane of intermediate cells, basal cells of the stria vascularis, the epithelial cells of the spiral prominence, or Reissner's membrane. Our result on the localization of the Na-K-Cl cotransporter in marginal cells is consistent with electrophysiological studies (Wangemann et al. (1995) Hear. Res. 84, 19-29). Our result on fibrocytes is discussed in relation to K+ circulation into endolymph from perilymph (Schulte and Steel (1994) Hear. Res. 78, 65-76).

Comparison of ion transport mechanisms between vestibular dark cells and strial marginal cells

Morphologic similarities between strial marginal cells and vestibular dark cells have long been recognized and it has long been accepted that both of these cell types are involved in the secretion of K+ into endolymph. Functional similarities of these two epithelia however, were considered unlikely as long as strial marginal cells were assumed to generate the endocochlear potential which has no equivalent in the vestibular labyrinth. The recently introduced concept that strial marginal cells transport K+ but that the mechanism for the generation of the endocochlear potential is located in another cell type provided the basis to hypothesize that ion transport mechanisms and their regulation are similar in vestibular dark and strial marginal cells. The present review compiles evidence in support of this hypothesis.

Ion transport mechanisms responsible for K+ secretion and the transepithelial voltage across marginal cells of stria vascularis in vitro

It has long been accepted that marginal cells of stria vascularis are involved in the generation of the endocochlear potential and the secretion of K+. The present study was designed to provide evidence for this hypothesis and for a cell model proposed to explain K+ secretion and the generation of the endocochlear potential. Stria vascularis from the cochlea of the gerbil was isolated and mounted into a micro-Ussing chamber such that the apical and basolateral membrane of marginal cells could be perfused independently. In this preparation, the transepithelial voltage (Vt) and resistance (Rt) were measured across marginal cells and the resulting equivalent short circuit current (Isc) was calculated (Isc = Vt/Rt). Further, K+ secretion (JK+,probe) was measured with a K(+)-selective vibrating probe in the vicinity of the apical membrane. In the absence of extrinsic chemical driving forces, when both sides of the marginal cell epithelium were bathed with a perilymph-like solution, Vt was 8 mV (apical side positive), Rt was 10 ohm-cm2 and Isc was 850 microA/cm2 (N = 27). JK+,probe was outwardly directed from the apical membrane and reversibly inhibited by basolateral bumetanide, a blocker of the Na+/Cl-/K+ cotransporter. On the basolateral but not apical side, oubain and bumetanide each caused a decline of Vt and an increase of Rt suggesting the presence of the Na,K-ATPase and the Na+/Cl-/K+ cotransporter in the basolateral membrane. The responses to [Cl-] steps demonstrated a significant Cl- conductance in the basolateral membrane and a small Cl- conductance in the paracellular pathway or the apical membrane. The responses to [Na+] steps demonstrated no significant Na+ conductance in the basolateral membrane and a small Na+ or nonselective cation conductance in the apical membrane or paracellular pathway. The responses to [K+] steps demonstrated a large K+ conductance in the apical membrane. Apical application of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and basolateral elevation of K+ caused an increase in Vt and a decrease in Rt consistent with stimulation of the apical K+ conductance. Similar observations have been made in vestibular dark cells, which suggest that strial marginal cells and vestibular dark cells are homologous and transport ions by the same pathways. Taken together, these observations are incompatible with a model for the generation of the endocochlear potential which ascribes the entire potential to the strial marginal cells [Offner et al. (1987) Hear. Res. 29, 117-124].(ABSTRACT TRUNCATED AT 400 WORDS)

Role of endolymphatic anion transport in forskolin-induced Cl- activity increase of scala media

To determine the role of anion transport in the forskolin-induced Cl- increase of scala media (SM), effects of forskolin on the EP (endocochlear potential) and Cl- activity (ACl) in SM were examined with double-barrelled Cl(-)-selective microelectrodes. The experiments were carried out on guinea pig cochleae, using a few anion transport inhibitors: IAA-94 for a Cl- channel blocker, bumetanide (BU) for an Na+/K+/2Cl- cotransport blocker, and SITS and DIDS for Cl-/HCO3- exchange blockers. The application of forskolin (200 microM) into scala vestibuli (SV) caused a 20 mEq increase of endolymphatic ACl and a 15 mV elevation of EP, and IAA-94 with forskolin completely abolished these responses. Although each application of BU, SITS or DIDS did not completely suppress EP elevation, the concurrent application of these inhibitors completely suppressed EP with endolymphatic ACl increase. The results indicate the involvement of Cl- channels, Na+/K+/2Cl- cotransport and Cl-/HCO3- exchange in forskolin-induced increase of ACl and EP. The role of adenylate cyclase activation and Cl- transport in endolymph homeostasis was discussed.

The effect of furosemide on the endocochlear potential in ears with experimentally induced endolymphatic hydrops

The endocochlear potential (EP) was measured in 38 guinea pigs with experimentally induced endolymphatic hydrops at the 3rd, 6th, 12th and 24th postoperative weeks, and the effects of furosemide (FUR, 50 or 80 mg/kg) on the EP were examined. A time-related reduction of the EP from the normal value and increased susceptibility to FUR were disclosed in the hydropic animals. Furthermore, 24-week animals given 80 mg/kg FUR showed a significantly slower recovery rate of the EP than the other groups, indicating impairment of the strial function progressive with post-operative time. The negative component of the EP was considered to be unimpaired until at least 12 weeks after the surgery.

Ototoxic effect of potassium canrenoate on the guinea pig cochlea

Potassium canrenoate (PC) is a diuretic with antialdosterone action, reducing the reabsorption of sodium in the efferent renal tubules. This drug has been reported to damage only the marginal cells of the stria vascularis in the cochlea. The perilymphatic space of the guinea pig cochlea was perfused with an artificial perilymph containing 5 x 10(-3) M potassium canrenoate. Following the onset of perfusion, the endocochlear dc potential (EP) gradually declined to around 10 mV but did not become negative even when the perfusion was continued. A similar decrease in EP was observed in kanamycin-deafened guinea pigs. When the respirator was turned off and perfusion discontinued, a large negative EP appeared during 5 min of anoxia in normal guinea pigs but not in the kanamycin-deafened guinea pigs. The decreased EP recovered to preanoxic level after resumption of ventilation. The cochlear microphonics (CM) also gradually declined in parallel with the EP. The summating potential (SP) showed only a minor change. The potassium ion activity in the endolymph decreased slightly but the sodium ion activity remained unchanged during perfusion. These findings suggest that the main target of the PC is the cells of the stria vascularis.

Ototoxicity of indacrinone is stereospecific

Indacrinone (MK-196) is a loop diuretic which consists of a racemic mixture. The purpose of this study was to evaluate the individual enantiomers in the chinchilla model to determine whether these compounds affect auditory function and whether a difference in ototoxic potency exists. Very little change of endocochlear potential (EP) or compound action potential (CAP) was noted in animals receiving the (+)-enantiomer. On the other hand, chinchillas injected with the (-)-enantiomer were found to have a dose related reduction in both CAP and EP. These findings suggest the possibility that the diuretic receptor in the kidney and the receptor mediating ototoxicity in the cochlea, may have similar steric requirements for interacting with loop diuretics.

N1 latency prolongation in the guinea pig cochlea treated with nitrogen mustard-N-oxide studied by narrow band analysis

The effect of nitrogen mustard-N-oxide (NMO) upon the click and tone burst-evoked N1 latency was examined in 14 albino guinea pigs. In all animals except one, the pseudothresholds of action potentials were elevated, especially in the high tone area. In addition to the amplitude reduction, the N1 latency was prolonged in 12 animals. The narrow band analysis of N1 revealed that the latency was equally prolonged in all frequency areas, although the amount of the amplitude reduction was much larger in the high frequency area. It was concluded that the prolongation of the N1 latency in NMO-treated animals was due to dysfunction of outer hair cells along the entire cochlear partition.

Sidedness of action of loop diuretics and ouabain on nonsensory cells of utricle: a micro-Ussing chamber for inner ear tissues

It is known that nonsensory tissues of the utricle produce a lumen-positive transepithelial electrical potential difference (VT). This potential has been shown previously to be inhibited by ouabain and bumetanide applied to the bathing medium in vitro. In order to more fully characterize the origin of this potential we mounted the utricle as a flat sheet in a newly designed Ussing chamber and measured the VT and transepithelial resistance (RT) while perfusing the endolymphatic and perilymphatic surfaces independently with identical solutions. The aperture of the chamber was 1.5 X 10(-4) cm2. VT averaged 5.6 +/- 0.46 mV and RT was 24.0 +/- 2.43 omega X cm2 (n = 45). Ouabain and loop diuretics of the furosemide family were found to inhibit the VT only from the serosal side. The KI for ouabain was 7.63 X 10(-5) M. The loop diuretics tested inhibited the VT in the same order as in other tissues known to contain a Na/2 Cl/K cotransporter (KI: 2-benzylamino-4-cyclohexylsulfonyl-5-sulfamoylbenzolsulfonate++ + (BCSB), 1.72 X 10(-7) M; bumetanide, 1.10 X 10(-6) M; piretanide, 5.67 X 10(-6) M; furosemide, 4.14 X 10(-5) M). It is concluded that this tissue produces a lumen-positive VT (i) in the absence of a transepithelial chemical gradient; the generation of which is dependent upon the activity of (ii) Na,K-ATPase and (iii) a Na/2 Cl/K cotransporter; (iv) in the basolateral membranes of the nonsensory cells; (v) which is not depressed by luminal application of inhibitors of these transporters.

Transepithelial electrical potential of nonsensory region of gerbil utricle in vitro

Transepithelial electrical potential difference (VT) was measured across the vestibular labyrinth of the inner ear in vitro by puncturing the epithelial wall of the utricle with a glass microelectrode. A region of nonsensory cells of the utricle was isolated from the sensory regions by introducing columns of liquid Sylgard 184. Under control conditions, the VT of this region was +7.5 +/- 0.3 mV (means +/- SE), lumen positive. This potential difference was rapidly reduced by either 1 mM ouabain, 10-100 microM bumetanide, 0.5-5.0 mM Ba (in the bathing solution), or cooling, but not by the disulfonic stilbene, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. Changes in VT due to reductions of Cl or Na or to increases of K in the bathing solution in exchange for presumably impermeant ions were observed in this region and were compared with those in a preparation in which the insulating seals were absent. The K-induced voltage change was significantly higher in the unblocked preparation, a finding consistent with a high K permeability of the sensory cells. The voltage change due to reduction of Cl was not inhibited by Cl channel blockers (9-anthracenecarboxylate and diphenylamine-2-carboxylate) in the bathing solution. These results represent the first direct demonstration that the nonsensory cells of the utricle produce a lumen-positive active-transport potential and characterize some of the properties of the cell membranes in terms of their pharmacological sensitivities and net voltage responses to changes in the bathing medium ions Na, K, and Cl.

The effect of transient anoxia upon the cochlear potentials

To contribute to the analysis of the electrocochleographical findings in humans, the reversibility of the cochlear potentials (the AP, CM, and SP) after transient anoxia was examined using 24 albino guinea pigs. The durations of anoxia ranged from 5 to 120 min, and the AP, CM and SP were examined one hour after restoration of the blood supply. The mildest form of cochlear damage after transient anoxia was the disappearance of the L-part of the AP and the severest was the complete abolition of these potentials. The severity of the damage closely correlated with the duration of anoxia. The AP was poorer in reversibility than the CM and the SP. These results were analysed and discussed in the light of literature on clinical and basic studies.