Evidence for a medial K+ recycling pathway from inner hair cells

K+ effluxed from outer hair cells and their nerves is thought to flow laterally to strial marginal cells for recycling into scala media. Observations reported here provide evidence that K+ effluxed from inner hair cells and inner radial nerves travels medially through border cells, inner sulcus cells (ISCs), limbal fibrocytes and interdental cells (IDCs) for return to endolymph. Morphologic features of ISCs in the medial route resembled those of Hensen and Claudius cells in the lateral indicating an ion transport role for ISCs like that of Hensen and Claudius cells. Na,K-ATPase in plasmalemma of IDCs testified to their capacity to resorb and transport K+ through their known gap junctions. IDCs were differentiated into three subgroups. The most lateral IDCs formed short and long columns. Long columns contacted the medialmost ISC inferiorly and the undersurface of the tectorial membrane superiorly providing thereby a potential transcellular route for K+ transit from ISCs to endolymph. Short columns faced inner sulcus below and tectorial membrane above and accordingly possessed cells with opposite polarity at the bottom and top of the column. Short columns thus appeared situated to resorb electrolytes from limbal stroma for release into inner sulcus and beneath tectorial membrane at opposite ends of the column. The central IDCs were positioned for resorbing and transporting K+ effluxing from the Na,K-ATPase-rich stellate fibrocytes which spread toward the IDCs from near the inner sulcus. The most medial IDCs lined cuplike invaginations near the attachment of Reissner's membrane and lay apposed to light fibrocytes located between supralimbal fibrocytes and the medial IDCs. Content of Na,K-ATPase and position in the K+ transport route likened the limbal stellate fibrocytes to the spiral ligament type II fibrocytes and supralimbal fibrocytes to suprastrial fibrocytes in the lateral wall. From content of creatine kinase and position in the transport path, limbal light fibrocytes appeared analogous to spiral ligament type I fibrocytes. The additional finding that limbal fibrocytes showed unchanged or upregulated Na,K-ATPase immunoreactivity in aged gerbils with strial atrophy provided further evidence for an independent medial transport route and for the survival of inner hair cells in presbyacusis.

Effect of locally applied drugs on the endolymphatic sac potential

In Ménière's disease, an inner ear disorder related to an endolymphatic hydrops, an alteration of the functioning of the endolymphatic sac has been proposed. The endolymphatic sac is assumed to be involved in the secretion/resorption of endolymph. The epithelial transport systems have been indirectly studied by the recording of the endolymphatic sac transepithelial potential (ESP) in control conditions and after the local injection of drugs such as diuretics that have been proposed in the treatment of Ménière's disease. The ESP was recorded, in vivo, in guinea pigs up to 150 minutes after the perisaccular injection of 5 microL of a 150 mmol/L (mM) NaCl solution containing various drugs known to inhibit ionic transport systems. The initial ESP was +8.4+/-0.3 mV (mean +/- SEM, n = 78). The basolateral injection of 5 microL of 150 mM NaCl induced an ESP decrease of 64%+/-6.0% (n = 12), 5 minutes after the end of the injection. Then ESP increased, returning to its initial value at 60 minutes and surpassing it at 120 minutes. Diuretics such as acetazolamide (10[-3] mol/L [M]), an inhibitor of carbonic anhydrase, and amiloride (10[-4] M), an inhibitor of Na channel or Na/H exchanger, decreased the ESP recovery. At variance, bumetanide (10[-6] M, 10[-4] M), the Na-K-Cl cotransport inhibitor, and chlorothiazide (10[-4] M), a Na-Cl cotransporter inhibitor, failed to alter the ESP as compared with the control group. Ouabain (10[-3] M), the Na+,K+-adenosine triphosphatase (ATPase) inhibitor, prevented the ESP recovery otherwise observed 60 minutes after the NaCl injection. Bafilomycin A1, the inhibitor of the vacuolar-type H+-ATPase, prevented the recovery of the ESP with a log-dose/effect (10[-5] M, 10[-6] M, 10[-8] M). Disulfonic acid stilbene (DIDS) (10[-4] M), an inhibitor of transporters involving HCO3-, also prevented the ESP recovery. These results suggest that the genesis of the ESP was highly dependent on acid-base transport systems including carbonic anhydrase, a vacuolar-type H+-ATPase, and an anionic transport system blocked by DIDS. Further studies are needed to confirm the alteration of the acid-base balance in this epithelium and its possible involvement in the pathogenesis of Ménière's disease.

Mechanisms of endolymph secretion

In the inner ear, the membranous labyrinth, a tight heterogeneous sensory epithelium, separates two compartments that are filled with fluids of completely different composition. The lumen of the membranous labyrinth is filled with endolymph, a K-rich, positively polarized fluid, whereas the surrounding spaces are filled with perilymph, with a composition similar to an usual extracellular fluid. The inner ear fluids play a major role in the cochlear and vestibular physiology by the transmission of the mechanical stimulus to the hair cells, on the one hand, and by the transduction of this signal to a nerve potential, on the other hand. Numerous studies have been performed in order to know the chemical and physical characteristics of the inner ear fluids. A high, positive transepithelial potential has been evidenced in the cochlea together with a high K concentration and a low Na concentration. During the last years, the composition of the inner ear fluids, the origins of endolymph and perilymph, and the cellular mechanisms involved in the secretion of these fluids have been a great part elucidated. The present paper is a review of the contribution of the lab to the understanding of the physiology of the inner ear.

Interaction of Methylprednisolone and Transient Asphyxia on the Inner Ear of the Adrenalectomized Rat

Methylprednisolone has been shown clinically to have beneficial effects on certain types of hearing loss. In the current study, compound action potential (CAP) thresholds, endocochlear potentials (EPs), and potassium concentration (CK +) values in the endolymph were determined under conditions of transient asphyxia (45 seconds) and methylprednisolone treatment (24 hours) in bilateral adrenalectomized rats. Treatment with methylprednisolone significantly reduced the effect of transient asphyxia on CAP thresholds as compared with nontreated animals. Methylprednisolone did not alter the dramatic short-term reduction in the EPs produced by anoxia. Potassium concentrations in treated adrenalectomized rats were significantly lower before transient asphyxia than in nontreated adrenalectomized rats. In the nontreated rats, transient asphyxia induced a reduction in CK + levels that was not seen in the methylprednisolone-treated animals. The data support the clinical application of methylprednisolone for certain forms of hearing loss and for potassium imbalance in the endolymph.

Calcium permeation of the turtle hair cell mechanotransducer channel and its relation to the composition of endolymph

1. Recordings of mechanoelectrical transducer currents were combined with calcium imaging of hair bundles in turtle auditory hair cells located near the high-frequency end of the cochlea. The external face of the hair bundles was perfused with a range of Ca2+ concentrations to study the quantitative relationship between Ca2+ influx and transducer adaptation. 2. With Na+ as the monovalent ion, the peak amplitude of the transducer current decreased monotonically as the external [Ca2+] was raised from 25 microns to 20 mm. When Na+ was replaced with the impermeant Tris the transducer current increased with external [Ca2+]. These results indicate that Ca2+ can both permeate and block the transducer channels. The Ca2+ concentration for half-block of the monovalent current was 1 mm. 3. To quantify the Ca2+ influx, the fraction of transducer current carried by Ca2+ was measured using the change in bundle fluorescence in cells loaded with 1 mm Calcium Green-1. The fluorescence change was calibrated by substituting an impermeable monovalent ion to render Ca2+ the sole charge carrier. 4. In the presence of Na+, the fractional Ca2+ current was approximately 10% in 50 microns Ca2+, a concentration similar to that in endolymph, which bathes the hair bundles in vivo. The amount of Ca2+ entering was dependent on the identity of the monovalent ion, and was larger with K+, suggesting that the transducer channel is a multi-ion pore. 5. Over a range of ionic conditions, the rate of transducer adaptation was proportional to Ca2+ influx indicating that adaptation is driven by a rise in intracellular [Ca2+]. 6. Shifts in the current-displacement function along the displacement axis in different external Ca2+ concentrations were predictable from variation in the resting Ca2+ influx. We suggest that changes in the resting open probability of the transducer channels adjust the entry of Ca2+ to keep its concentration constant at an internal site. 7. The results demonstrate that endolymph containing high K+, 50 microns Ca2+ and low Mg2+ concentrations, maximizes the transducer current while still allowing sufficient Ca2+ entry to drive adaptation. The hair cell mechanotransducer channel, in its permeation and block by Ca2+, shows behaviour similar to the voltage-gated Ca2+ channel and the cyclic nucleotide-gated channel.

Localization of pH regulating proteins H+ATPase and Cl-/HCO3- exchanger in the guinea pig inner ear

Mechanisms that regulate endolymphatic pH are unknown. It has long been recognized that, because of the large positive endolymphatic potential in the cochlea, a passive movement of protons would be directed out of endolymph leading to endolymphatic alkalization. However, endolymphatic pH is close to that of blood, suggesting that H+ is being secreted into endolymph. Since the kidney and the inner ear are both actively engaged in fluid and electrolyte regulation, we attempted to determine whether proteins responsible for acid secretion in the kidney also exist in the guinea pig inner ear. To that end, a monoclonal antibody against a 31 kDa subunit of a vacuolar vH+ATPase and a polyclonal, affinity purified antibody against the AE2 Cl-/HCO3- exchanger (which can also recognize AE1 under some conditions) were used. In the cochlea, the strongest immunoreactivity for the vH+ATPase was found in apical plasma membranes and apical cytoplasm of strial marginal cells. These cells were negative for the Cl-/HCO3- exchanger. Certain cells of the inner ear demonstrated both apical staining for vH+ATPase and basolateral staining for the Cl-/HCO3- exchanger; these included interdental cells and epithelial cells of the endolymphatic sac. Cochlear cell types with diffuse cytoplasmic staining for vH+ATPase and a basolaterally localized Cl-/HCO3- exchanger included inner hair cells, root cells and a subset of supporting cells in the organ of Corti. Hair cells of the utricle, saccule and cristae ampullaris also expressed both vH+ATPase and the Cl-/HCO3- exchanger, but immunostaining for the vH+ATPase was less intense and less polarized than in the cochlea. These immunocytochemical results support a role for the vH+ATPase and Cl-/HCO3- exchanger in the regulation of endolymphatic pH and suggest that certain cells (including strial marginal cells and epithelial cells of the endolymphatic sac) may be specialized for this regulation.

Effects of high intensity impulse noise on ionic concentrations in cochlear endolymph of the guinea pig

OBJECTIVE

To investigate the effect of acoustic overstimulation on concentrations of cations in cochlear endolymph and analyze the relationship between the ionic changes in endolymph and the hearing loss.

METHODS

The endocochlear potentials (EP), K+, Na+ and Ca2+ concentration in cochlear endolymph were examined in vivo for normal and 167 +/- 2 dB SPL impulse noise exposed groups of guinea pig bored on time course by means of double-barreled ion-selective microelectrodes. Brain stem auditory evoked potential (BAEP) was used to evaluate the auditory function. Ca(2+)-ATPase activity was demonstrated cytochemically in the lateral cochlear wall as induced by Ando et al with slight modification.

RESULTS

The K+ and Ca2+ concentration exhibited significant changes in 8-hour groups (P < 0.05 for K+ and P < 0.01 for Ca2+). Then the K+ concentration was eventually resumed to the initial levels in accordance with the EP recovery in 7 days, while during the same period Ca2+ concentration was always significantly higher than that in control group (P < 0.01). The BAEP threshold shifts were correlated well with changes in ionic concentrations, especially Ca2+ (P < 0.001), in the endolymph induced by acoustic trauma. Although the normal positive EP was observed 7 days after noise exposure, the function of the vascular stria was not completely restored as revealed by the fact that the Ca(2+)-ATPase was diffused to the apical membrane surface.

CONCLUSIONS

Endolymph compartment intrinsic mechanism for maintaining ionic composition is seriously deteriorated after high impulse noise stimulation. The changes of the unique environment of endolymph may play an important role in the mechanism of sensorineural hearing loss induced by acoustic trauma.

Quantitative differences in endolymphatic calcium and endocochlear potential between pigmented and albino guinea pigs

A number of previous studies have suggested that melanin may play a role in Ca2+ homeostasis of endolymph. In the present study, endolymph Ca2+ levels and endocochlear potential (EP) were measured in all four cochlear turns of pigmented or albino guinea pigs. Auditory sensitivity was also evaluated using cochlear action potential (AP) thresholds. In pigmented animals we found that endolymph Ca2+ tended to increase from base to apex of the cochlea, while EP systematically decreased towards the apex. In contrast, no significant Ca2+ gradient was found in albinos and the EP decline was far less. As a result, the apical turn of albino animals had significantly lower Ca2+ and significantly higher EP than in pigmented animals. AP thresholds pooled across all test frequencies were significantly lower in albino animals although no differences at individual frequencies reached significance. Even after correction for EP differences, the endolymph Ca2+ levels in albino animals were significantly lower than in pigmented ones. These results confirm that there are significant physiologic differences between pigmented and albino animals, which are a likely consequence of the absence of melanin in the albino cochlea. They are consistent with the involvement of melanin in the active transport of Ca2+ into endolymph.

Breakdown of Stria Vascularis Blood-Labyrinth Barrier in C3H/lpr Autoimmune Disease Mice

Sensorineural hearing loss related to autoimmune disease is a well-recognized condition, although the exact pathophysiologic mechanisms remain unclear. One current theory postulates immune complex-induced interference with blood-labyrinth barrier integrity in the stria vascularis. The C3H/ lpr autoimmune mouse was chosen to study the permeability of capillaries in the stria vascularis because this mouse model has demonstrated abnormalities of the stria vascularis and shifts in the auditory brain stem response threshold during active disease. C3H/ lpr mice with active disease were compared with younger mice without disease, as well as age-matched C3H/HeJ control mice. The mice were injected with the tracer ferritin and examined by transmission electron microscopy to evaluate the integrity of the capillary tight junctions in the stria vascularis. Four of five mice with active disease were noted to have extensive leakage of ferritin into the perivascular tissues. Neither the young, disease-free autoimmune mice nor the nonautoimmune control mice demonstrated vessel leakage. Thickening of the basement membrane was also noted in the diseased animals. The results imply that active disease leads to a breakdown in the blood-endolymph barrier, which could underlie the hearing loss accompanying autoimmune and other immune diseases.

Two types of K+ currents in marginal cells cultured from rat stria vascularis

Membrane currents in marginal cells cultured from rat stria vascularis were studied using the whole-cell patch-clamp technique. Two types of voltage-dependent whole-cell currents were observed in the voltage range from -150 mV to +50 mV: an outwardly rectifying current and an inwardly rectifying current. The outwardly rectifying current, which was activated by depolarizing pulses more positive than -30 mV, was sensitive to TEA (20 mM) and relatively not to Ba2+ (0.5 mM). Tail current analysis revealed that the outward currents were primarily K+-selective. The conductance of the current was half-maximal at 0.5 mV and a substantial portion of current was not inactivated by the depolarizing prepulses from -30 mV to +20 mV. The inwardly rectifying current with rapid exponential activation was observed with hyperpolarizing voltage pulses. The zero-current potential of this current was dependent on extracellular K+ concentration. In contrast to the outwardly rectifying current, this current was blocked by extracellular application of Ba2+, not by TEA. The conductance of this current increased with the increase in external K+ concentration. Our data suggest that marginal cells cultured from rat stria vascularis express at least two types of voltage-dependent K+ currents which may serve as K+ secretory pathways into endolymph.

[Effect of furosemide on the ampullar endolymphatic potential and endolymphatic Ca2+ activity]

The ampullar endolymphatic potential (AEP) and the endolymphatic Ca2+ activity were measured with double-barred Ca2+ selective microelectrodes in the experimental hydrops of guinea pigs. They showed 3.16 +/- 0.42 mV (n = 9) and (4.10 +/- 1.22) x 10(-4) M, respectively. The AEP decreased significantly at about 6 min after intravenous administration of 10 mg/kg furosemide. Its minimal value was 3.04 +/- 0.53 mV at about 40 min. But there was no significant difference between that and the value before injection. The Ca2+ activity increased significantly and reached (7.10 +/- 1.32) x 10(-4) M at about 60 min. The changes indicate that Ca2+ may play an important role in the mechanism of the furosemide test.

Paracellular transport properties of inner ear barriers do not account for cisplatin toxicity in the rat

The interindividual variability for the ototoxic effect of the antineoplastic drug cisplatin has still to be explained. To examine if the variability can be related to differences in drug kinetics, the effect of cisplatin on the paracellular transport properties of the inner ear barriers was studied in vivo in cisplatin treated Long-Evans rats. The concentration of [3H]mannitol was followed in plasma, scala vestibuli perilymph, and endolymph after an intravenous infusion of the tracer. Cisplatin had no effect on paracellular transport of the inner ear barriers 3 days after administration of 8 mg/kg cisplatin. However, an interindividual variability for the transport of [3H]mannitol across the blood-perilymph barrier was evident, indicating a variability for the passive transport of solutes to the inner ear.

Stretch-activated nonselective cation, Cl- and K+ channels in apical membrane of epithelial cells of Reissner's membrane

Ion channels on the apical membrane of epithelial cells (the surface facing the endolymph) of acutely isolated Reissner's membrane from guinea-pig cochlea were investigated by using patch-clamp technique in cell-attached and inside-out configurations. Three types of ion channel were identified: namely, a stretch-activated nonselective cation, a chloride and a potassium channel. When the pipette was filled with high-K+ endolymph-like solution, the most significant channel activity was nonselective cation channels (85/110, 77% patches). The current versus voltage relationship was linear with a unitary conductance of 22.1 +/- 0.4 pS and reversal potential (Vr) of 2.3 +/- 0.8 mV (n = 18). The channel exhibited a lower conductance (14.0 +/- 0.6 pS, n = 8) to Ca2+. The open probability was low (NPo approximately 0.1) in cell-attached configuration under +60 mV pipette potential and increased when the membrane was stretched with negative pressure. The channel was blocked by 10 microM extracellular Gd3+. The two other types of channels were a small voltage-sensitive Cl- channel (6.0 +/- 0.3 pS; 91/99, 92% patches) and a K+ channel (approximately 30 pS; 29/191, 15% patches). These channels might play roles in the regulation of cell volume, in balancing the hydrostatic pressure across Reissner's membrane and in maintaining the electrochemical composition of endolymph.

The effects of calcium buffering and cyclic AMP on mechano-electrical transduction in turtle auditory hair cells

1. The effects of intracellular Ca2+ buffering on hair cell mechanotransduction were studied in an intact cochlear epithelium where the endolymphatic and perilymphatic surfaces could be separately perfused with different Ca2+ solutions. 2. The speed and extent of transducer adaptation increased as the concentration in the patch electrode of the Ca2+ buffer BAPTA was lowered. In 0.1 mM BAPTA or less, the transducer adapted almost completely, with a mean time constant of 0.8 ms. 3. For a fixed internal BAPTA concentration, the transducer conductance varied with hair cell location, increasing towards the high-frequency end of the cochlea, and the time constant of adaptation decreased proportionally. At a given cochlear location, hair cells with larger transducer conductances displayed faster adaptation. We suggest that transducer adaptation accounts for a variable high-pass filter observed in the acoustic tuning curve. 4. The effects of perfusion of 50 microM Ca2+ endolymph depended on the BAPTA concentration of the electrode: with 3 mM BAPTA, adaptation was abolished, but in most recordings with 0.01 or 0.1 mM BAPTA, rapid adaptation was retained. The current-displacement curve was also shifted less the lower the intracellular BAPTA concentration. Cells in the high-frequency half of the papilla retained adaptation at a higher BAPTA concentration. 5. Treatment with the cAMP agonist, 8-bromo-cAMP, or with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, caused a rightward shift in the current-displacement curve which was independent of the internal BAPTA concentration. 6. We conclude that the free Ca2+ and cyclic nucleotide concentrations of the hair bundle modulate the position of the activation curve of the transducer. The factors which may be important for the correct functioning of adaptation in vivo are discussed.

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).

Changes in cochlear function after double-membrane rupture in the guinea pig

We measured the transiently evoked otoacoustic emissions (TEOAEs), compound action potentials (CAPs) and cochlear microphonics (CMs) in guinea pigs after rupture of the round window membrane alone (n = 5) or of the round window membrane with localized cochlear damage (n = 10). The localized cochlear damage entailed rupture of Reissner's membrane with damage to the stria vascularis. We determined the time course of changes in the total echo power (TEP) in TEOAEs and the minimal detectable levels of CAPs and CMs. The endocochlear potential (EP) was measured in the cochlea with localized damage. There were no changes in TEOAEs, CAPs or CMs in the guinea pigs subjected to round window membrane rupture alone, but the minimal detectable levels of CAPs and CMs were increased in all the guinea pigs in which TEOAEs were absent after rupture of the round window membrane with localized cochlear damage. Our results suggest that double-membrane rupture (rupture of the round window membrane with localized cochlear damage) produces acute sensorineural hearing loss. The hearing loss appeared to be related to damage to the cochlea, which may be induced by influx of potassium-rich endolymph into the perilymph, and by morphological damage to the scala media.

Anionic sites of charge barrier in the guinea pig crista ampullaris

In present studies we obtained anionic sites in the epithelial and capillary basement membranes in the dark cell area of the crista ampullaris in the guinea pig. The immersion method with cationic tracer polyethyleneimine (PEI) was applied. Electronmicroscopically, the arrangement of PEI particles was observed as two strata along the basement membrane. The number of particles could be counted and compared in each portion. The control test with protamine sulfate showed that the number of PEI particles decreased in both the epithelium and capillaries of the dark cell area. In the experiment using furosemide, the stria vascularis and the dark cells had changed, with pathological findings of interstitial edema and PEI particles reduced in number. It is suggested that the PEI particles reflect different conditions of charge in the basement membrane, which influences the production or absorption of the inner ear fluid.

Vibrating probes: new technology for investigation of endolymph homeostasis

It is well known that the function of the cochlear and vestibular labyrinth depends on the high concentration of potassium (K+) in the luminal fluid, endolymph. Homeostasis of endolymphatic ion composition has been attributed to the stria vascularis and vestibular dark cells but with little prior experimental basis. The extremely small domain of each epithelial cell type bounding the endolymphatic space has precluded study of ion fluxes from these cells. The voltage-sensitive and (+)-selective vibrating probes were adapted recently for the demonstration of electrogenic K+ secretion and its regulation by stria vascularis and vestibular dark cell epithelium. The isolated stria vascularis and vestibular dark cell epithelium are known to produce a transepithelial current directed toward the endolymphatic side and this current has been shown to be sensitive to bumetanide, an inhibitor of the Na(+)-Cl(-)-K+ cotransporter. The vibrating probes were used to demonstrate that this current is carried by K+ and that the K+ flux is also sensitive to bumetanide. Several other agents and maneuvers which alter the transepithelial current (e.g. apical DIDS and basolateral hypotonic challenge) were found to produce similar changes in the K+ flux. The technique holds the promise of discovery of the contribution to the homeostasis of endolymph of other cell types in the inner ear.

Effects of aging on potassium homeostasis and the endocochlear potential in the gerbil cochlea

Previous work has shown that the endocochlear potential (EP) decreases with age in the gerbil. Concomitant with the EP decrease is an age-related loss of activity of Na,K-ATPase in the lateral wall and stria vascularis. We hypothesized that the EP decrease is associated with a similar decrease in the endolymphatic potassium concentration [Ke+]. This hypothesis was tested using double-barrelled, K(+)-selective electrodes introduced into scala media through the round window in young and quiet-aged gerbils. Results show that the means (+/- S.D.) of the [Ke+] in young and aged gerbils were not significantly different (178.2 +/- 14.2 mM and 171.2 +/- 34.4 mM, respectively), although the intersubject variability was much greater in the aged animals than in the young. These values of [Ke+] are slightly higher than those found for other mammals and may reflect the higher plasma osmolarity found in the gerbil. The concentration of perilymphatic potassium [Kp+] in scala tympani at the round window was also similar for the young and aged groups (3.57 +/- 1.17 mM and 4.18 +/- 2.03 mM, respectively). On the other hand, mean EP values in the young and aged gerbils were 92.0 +/- 5.7 mV and 64.8 +/- 15.8 mV, respectively and were statistically different (P < 0.001). Overall, EP and [Ke+] showed little correlation (R2 = 0.23), except that when [Ke+] fell below 150 mM, the EP was always less than 60 mV. An analysis of the chemical potential for Ke+ with respect to Kp+ shows that it was similar for young and aged gerbils (overall mean of 103.1 +/- 13.7 mV) and remained constant with respect to the EP, in spite of an overall electrochemical potential of Ke+ that varied from 120 to 210 mV. Thus, the system maintains Ke+ homeostasis at the expense of the EP, even when the EP is on the verge of collapse.

Influence of chronic kanamycin administration on basement membrane anionic sites in the labyrinth

We studied the effect of chronic treatment with kanamycin on the basement membrane (BM) anionic sites in the cochlea and endolymphatic sac using polyethyleneimine (PEI) as a cationic tracer. Albino guinea pigs weighing 250-300 g received kanamycin (400 mg/kg/day, i.m.) for 10 or 17 consecutive days. The number of BM anionic sites as derived from the PEI area was not affected in Reissner's membrane, spiral prominence, basilar membrane or endolymphatic sac, whereas it was significantly decreased in the stria vascularis and spiral limbus, being more marked in the guinea pigs treated for 17 days than in those treated for 10 days. The number of BM anionic sites in these regions did not recover until 6 weeks after kanamycin treatment. These findings suggest that chronically administered kanamycin may selectively and progressively affect the BM anionic sites in the stria vascularis and spiral limbus, resulting in disruption of a barrier function in the cochlea, and that severely impaired BM anionic sites in the cochlea may not recover.

Morphological and functional characteristics of the different cell types in the stria vascularis: a comparison between cells obtained from fresh tissue preparations and cells cultured in vitro

Endolymph is the only extracellular fluid in the body which is characterized by an intracellular-like ion composition. The molecular mechanisms responsible for the production of endolymph and the regulation of endolymph composition are still unknown to a large extent, although the stria vascularis (SV) is believed to play an important role for these functions. A basic requirement for investigating the function of different cell types in the SV is the establishment of a method, which increases the accessibility of the tissue with maintained cell viability and function. In this study, fresh tissue preparations and cultured cells from SV, harvested from pigmented guinea pigs, were established. Marginal cells, intermediate/melanocyte-like cells and fibroblasts could be discerned in the cell cultures with bright-field microscopy, transmission and scanning electron microscopy as well as immunohistochemistry, using polyclonal antibodies against cytokeratin and vimentin. In order to study functional characteristics of the fresh tissue preparations and the cell cultures, changes in the cytoplasmic free Ca2+ concentration were determined with the fura-2 method. The cultured cells, of different types in the SV, are a suitable model for future studies of the molecular mechanisms behind the production of endolymph and the regulation of endolymph composition.

Combined effects of acute lead acetate exposure and tone exposure of the guinea pig cochlea

Lead acetate exposure to humans can induce various disorders of the cranial nerves. Although vertigo and sensorineural deafness have been reported in lead workers, the dose effects of lead acetate on the cochlea and eighth cranial nerve are not well documented. We investigated the effects of lead acetate on the male albino Hartley guinea pig cochlea by measuring cochlear microphonics (CM), whole nerve action potential (AP), endocochlear potential (EP) and K+ ion concentration of the endolymph. Animals were given lead acetate by intraperitoneal injection as 20 mg/week for 4 consecutive weeks. A total dose < 80 mg did not induce electrophysiological changes in the cochlea. However, the AP output voltage (N1) decreased if the 80 mg lead acetate treatment was followed by an 80 dB tone exposure at 6 kHz during 24 h. A change was observed in CM and EP but not K+ ion concentration in the scala media.

K(+)-induced stimulation of K+ secretion involves activation of the IsK channel in vestibular dark cells

Vestibular dark cells in the inner ear secrete K+ from perilymph containing 4 mM K+ to endolymph containing 145 mM K+. Sensory transduction causes K+ to flow from endolymph to perilymph, thus threatening the homeostasis of the perilymphatic K+ concentration which is crucial for maintaining sensory transduction since the basolateral membranes of the sensory cells and adjacent neuronal elements need to be protected from K(+)-induced depolarization. The present study addresses the questions (1) whether increases in the perilymphatic K+ concentration by as little as 1 mM are sufficient to stimulate KCl uptake across the basolateral membrane of vestibular dark cells, (2) whether K(+)-induced stimulation of KCl uptake causes stimulation of the IsK channel in the apical membrane, and (3) whether the rate of transepithelial K+ secretion depends on the perilymphatic (basolateral) K+ concentration when the apical side of the epithelium is bathed with a solution containing 145 mM K+, as in vivo. Uptake of KCl was monitored by measuring cell height as an indicator for cell volume. The current (IIsK), conductance (gIsK) and inactivation time constant (tau IsK) of the IsK channel as well as the apparent reversal potential of the apical membrane (Vr) were obtained with the cell-attached macro-patch technique. Vr was corrected for the membrane voltage previously measured with microelectrodes. The rate of transepithelial K+ secretion JK was obtained as equivalent short circuit current from measurements of the transepithelial voltage (Vt) and resistance (Rt) measured in the micro-Ussing chamber. Cell height of vestibular dark cells was 7.2 microns (average). Elevations of the extracellular K+ concentration from 3.5 to 4.5 mM caused cell swelling with an initial rate of cell height change of 11 nm/s. With 3.6 mM K+ in the pipette IIsK was outwardly directed and elevation of the extracellular K+ concentration from 3.6 to 25 mM caused an increase of IIsK from 12 to 65 pA, gIsK from 152 to 950 pS and tau IsK from 278 to 583 ms as well as a hyperpolarization of Vr from -50 to -60 mV. With 150 mM K+ in the pipette IIsK was inwardly directed and the elevation of the extracellular K+ concentration caused an increase of IIsK from -1 to -143 pA, gIsK from 141 to 1833 pS and tau IsK from 248 to 729 ms. Vr remained within +/- 10 mV from zero. JK was 4.8 nmol x cm-2 x s-1 when the both the apical side and the basolateral side of the epithelium were perfused with a solution containing 3.5 mM K+. Elevation of the basolateral K+ concentration by 1 mM caused JK to increase by 1.1 nmol x cm-2 x s-1 or 23%. When the basolateral side of the epithelium was perfused with a solution containing 3.5 mM K+ and the apical side with a solution containing 145 mM K+, as in vivo, JK was 0.8 nmol x cm-2 x s-1 and elevation of the basolateral K+ concentration by 1 mM caused JK to increase by 0.8 nmol x cm-2 x s-1 or 100%. These data suggest that physiologically relevant increases in the perilymphatic K+ concentration increase JK by increasing KCl uptake across the basolateral membrane and activation of K+ release via the IsK channel in the apical membrane. Thus, the data demonstrate that vestibular dark cells adjust the rate of K+ secretion into endolymph according to the perilymphatic K+ concentration.

Electrochemical potentials and potassium concentration profiles recorded from perilymph, endolymph and associated inner ear tissues in adrenalectomized rats

This study evaluated the electrochemical potentials and potassium concentration (Ck+) profiles in the perilymph, endolymph, marginal cells, and spiral ligament of adrenalectomized rats in which endogenous corticosteroids had been removed. Electrochemical potentials recorded at the four cochlear sites were not affected by adrenalectomy (ADX). Ck+ was greater in the endolymph of the ADX animals as compared to control animals. Additionally, there was an increase of Ck+ in the marginal cells, perilymph, and spiral ligament tissues of the ADX animals as compared to control animals, although the observed increases were not statistically significant. In a previous study (Ma et al., 1995a), it was found that potassium levels in the blood plasma of ADX animals were higher than those identified in normal rats; thus, ADX may have a systemic effect on Ck+ that is detectable in both tissues and fluids within the cochlea. Even though Ck+ was elevated within the cochlea in the ADX model, the functional response of the inner ear, as assessed electrophysiologically, was not altered.