Recent and Planned Presentations by our group

Last updated November 25, 1996

Recent

Evidence for two components of endolymph volume regulation involving local and flow-based mechanisms
A.N. Salt and J.E. DeMott
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1996.

Perilymph potassium increases during acute endolymph volume enlargement.
J.E. DeMott and A.N. Salt
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1996.

Endocochlear potential and cochlear fluid pH changes induced by respiratory CO2 manipulations
M. Suzuki and A. N. Salt
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1996.

Endolymph calcium homeostasis in pigmented and albino guinea pigs
S.S.Gill and A. N. Salt
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1996.

Endolymph volume regulation mechanisms revealed by microinjections into scala media..
Alec N. Salt and John E. DeMott
130th meeting of the Acoustical Society, St. Louis, November 1995.

Measurement of longitudinal fluid movements in the cochlea under normal and abnormal conditions.
Alec N. Salt
Symposium on "Intracranial and intralabyrinthine fluids - Basic aspects and clinical applications.", Seeheim, Germany, September 1995

Small longitudinal movements of endolymph are induced by volume disturbances.
Alec N. Salt and John E. DeMott
16th Danavox symposium on Meniere's disease, Kolding, Denmark, September 1995

Longitudinal movments and area changes of endolymph produced by osmotic dehydration.
J.E. DeMott and A.N. Salt
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1995.

Quantification of endolymphatic hydrops in the cochlea by magnetic resonance microscopy
Salt, A.N., Henson, M.H. Gewalt, S.L, Keating, A. W., DeMott, J.E. and Henson, O.W.
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1995.

Endolymph calcium differences between pigmented and albino guinea pigs.
S.S. Gill and A.N. Salt
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1995.

Endolymph Composition Changes in the Dominant Spotting (Wv) Mouse.
D. M. Miller and A.N. Salt
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1995.

Planned

Undetectable pressure increase during induction of acute endolymphatic hydrops by microinjection.
J.E. DeMott and A.N. Salt
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1997.

Fixation-induced shrinkage of Reissner's membrane and its potential influence on the assessment of endolymphatic volume
A.S. Brunschwig, A.N. Salt, J.E. DeMott, M.M. Henson and S.L. Gewalt
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1997.

Dependence of endolymph pH on perilymph bicarbonate concentration
Y.L. Ma, A.N. Salt and M. Suzuki
Association for Research in Otolaryngology, St. Petersberg Beach, Florida, February 1997.

Undetectable pressure increase during induction of acute endolymphatic hydrops by microinjection.


J.E. DeMott and A.N. Salt
Department of Otolaryngology, Washington University School of Medicine,
St. Louis, MO, 63110, USA.

Prior studies by have shown that injection of artificial endolymph into the cochlea at rates less than 60 nl/min for periods up to 15 min produced acute endolymphatic hydrops and /or basally-directed flow of endolymph (Salt & DeMott ARO 19: #543, 1996). In the present study, microinjections into the endolymphatic or perilymphatic spaces were performed while hydrostatic pressure was monitored simultaneously in both compartments using micropressure instrumentation. Time averaging was performed to reduce background pressure fluctuations due to respiration. During injections, virtually identical pressure changes were recorded in both the endolymphatic and perilymphatic compartments. Digital subtraction of the traces demonstrated no detectable change in pressure of endolymph with respect to perilymph during injection into endolymph. It was estimated that less than 0.1 mm Hg pressure increase of endolymph with respect to perilymph occurred while acute hydrops was produced. These results demonstrate that the terms endolymphatic hydrops and endolymphatic hypertension are not synonymous. In the normal animal, the boundary membranes of the endolymphatic system appear to be highly compliant so that pressure differences across the membranes are extremely low. These results are in agreement with those of Takeuchi et al. (Ann. Otol. Rhinol. Laryngol. 100, 244, 1991) in which larger volumes of artificial endolymph were injected more rapidly than in the present study. The predominantly basally-directed endolymph flow observed during endolymphatic injections in our prior study indicates that a structure outside the cochlea, probably the saccule or endolymphatic sac, is even more compliant than the structures bounding scala media in the cochlea.

Study supported by NIH/NIDCD DC01368

Fixation-induced shrinkage of Reissner's membrane and its potential influence on the assessment of endolymphatic volume


A.S. Brunschwig, A.N. Salt, J.E. DeMott, M.M. Henson and S.L. Gewalt
Department of Otolaryngology, Washington University School of Medicine,
St. Louis, MO, 63110, USA.

The quantification of endolymph volume by histological techniques or by magnetic resonance (MR) microscopy could be influenced significantly by shrinkage of Reissner's membrane during fixation. Since Reissner's membrane is anchored to bone at its medial and lateral sides, shrinkage of a distended membrane would tend to pull it towards a straight position. The goal of this study was to determine the amount of shrinkage of Reissner's membrane induced by various fixation protocols and to establish a protocol suitable for MR microscopy which minimizes soft tissue shrinkage. Fresh guinea pig cochleae were dissected in an artificial perilymph, and fragments of Reissner's membrane were isolated. Each individual specimen was observed in a petri dish using an inverted microscope and its size documented. A selected fixative was then infused while the specimen was observed and recorded on video tape. Size changes of the specimen were quantified, usually over a 20 minute period. Heidenhain-Susa, a fixative widely used for histological study of cochleae, caused substantial shrinkage of Reissner's membrane during the first few minutes. The mean decrease in specimen length was 15.1 % ▒ 1.1% (SE; n=8). It can be shown that the distension of Reissner's membrane in mild to moderate hydrops would be nullified by a 15% shrinkage of the tissue. Other fixation protocols produced far less shrinkage. The protocol showing least shrinkage was 3.1% glutaraldehyde in Hanks' buffered salt solution, which produced a mean length decrease of 0.3% ▒ 1.5% (SE; n=4).

For MR microscopy, mercuric chloride (HgCl2), a component of Heidenhain-Susa, greatly enhances the contrast of Reissner's membrane against the background of the cochlear fluid spaces, which appear white in the MR image. We have found that Reissner's membrane looks black in specimens fixed with HgCl2-containing solutions, whereas it appears white in formalin-fixed specimens. However, the inclusion of HgCl2 in fixation media results in greater shrinkage of Reissner's membrane, when used as either a primary or secondary fixative. Protocols for MR microscopy are presently being optimized to maximize contrast while minimizing Reissner's membrane shrinkage.

In conclusion, the degree of Reissner's membrane shrinkage shown here to be caused by some fixatives could result in systematic errors in the quantification of endolymph volume.

This study was supported by NIH grants numbered DC01368 (AS), DC00114 (MH) and 1P41RR05959 (SG)

Dependence of endolymph pH on perilymph bicarbonate concentration


Y.L. Ma, A.N. Salt and M. Suzuki
Department of Otolaryngology, Washington University School of Medicine,
St. Louis, MO, 63110, USA.

In previous studies, we found endolymph pH to be highly sensitive to respiratory CO2 status. Hypercapnea decreased endolymph pH while hypocapnea elevated pH. When the perilymphatic scalae of the cochlea are perfused with CO2-equilibrated artificial perilymph, we consistently observe an elevation in endolymph pH. We have found that one factor which contributes significantly to perfusion-induced endolymph pH changes is the concentration of bicarbonate in the perfusion medium. Since bicarbonate provides the dominant pH buffering in the perfusate, solutions of different bicarbonate content, equilibrated to physiological CO2 levels (approximately 5 %), will vary in pH, with pH increasing as bicarbonate increases. When the cochlea was perfused with CO2-equilibrated high-bicarbonate medium (50 mM HCO3), which has a higher pH than normal (pH 7.65), endolymph pH was decreased by an average of 0.05 (n=4). Conversely, perfusion with a nominally HCO3-free solution, caused endolymph pH to rise by an average of 0.11 (n=6). Both high- and low-bicarbonate manipulations generally reduced the endocochlear potential and elevated cochlear action potential thresholds. The observation that the change in endolymph pH is opposite in direction to the change in perilymph pH suggests that mechanisms of bicarbonate or pH homeostasis are involved. Far smaller endolymph pH changes were measured when comparable perilymph pH changes were induced by perfusing HCO3-free, HEPES-buffered solutions of varying pH. This supports the view that the observed endolymph pH changes with HCO3-modified media may be related to perilymph bicarbonate homeostasis. One explanation of the observations is that when perilymph bicarbonate is high, the secretion of acid to neutralize the excess perilymph bicarbonate, would release additional CO2, which, by rapidly equilibrating across membranous boundaries, would produce an acidification of endolymph. Conversely, under low-HCO3 conditions, the generation of bicarbonate from the reaction of CO2 and water, catalyzed by carbonic anhydrase, would reduce CO2 levels, resulting in an alkalinization of endolymph. These studies demonstrate that in the cochlea there are strong interrelationships between CO2, bicarbonate and fluid pH, in which the homeostasis of bicarbonate plays a highly prominent role.

This study was supported by NIH/NIDCD, grant number DC01368

Measurement of longitudinal fluid movements in the cochlea under normal and abnormal conditions.


Alec N. Salt
Department of Otolaryngology, Washington University School of Medicine,
St. Louis, MO, 63110, USA.

Cochlear fluid movements can be accurately quantified with the use of chemical flow markers. In our technique, we injected small amounts of ionic markers (hexafluoro- arsenate or tetramethylammonium) into one of the cochlear fluids of pigmented guinea pigs. Movements of the marker were monitored by one or more ion selective micro- electrodes, which were sensitive enough to detect micromolar, non-toxic, marker concentrations. Results were interpreted using mathematical models that combined the effects of marker movements by diffusion, by longitudinal flow and by clearance from the compartment. In endolymph of the normal cochlea, longitudinal flow rates were extremely small, with a mean rate that was basally-directed at 4 Ám/min. This rate was not significantly different from zero. However, when endolymph volume was disturbed, by osmotic dehydration or by direct volume injection, longitudinal volume flows were induced which contributed to the homeostatic process. Osmotic dehydration of cochlear endolymph produced apically-directed flow at a rate that averaged 43 Ám/min. Direct micro-injections into cochlear endolymph at rates from 20-60 nl/min produced basally- directed endolymph flow in the basal turn. A somewhat similar situation exists for cochlear perilymph. When marker injection and recording electrodes were meticulously sealed into the cochlea, longitudinal movements of perilymph were extremely low, averaging only 1.6 nl/min in an apical direction along the basal turn of scala tympani. In contrast, when the otic capsule was perforated, releasing cochlear fluid pressure, the rate of longitudinal flow averaged 1.0 Ál/min, which was over 600 times the normal rate. This high rate was due to cerebrospinal fluid entering the cochlea through the cochlear aqueduct, which rapidly displaced perilymph. We can conclude that in the normal state, longitudinal fluid movements make virtually no contribution to the homeostasis of the major ions in cochlear endolymph and perilymph. However, longitudinal flows may be induced in some situations, the effects of which may far exceed the normal, local homeostatic processes. Since the chemical compositions of inner ear fluids are known to vary in different regions, it is likely that chemical disturbances of cochlear tissues will occur under these conditions. (Work supported by NIH grant DC01368).
key words : cochlea, endolymph, perilymph, endolymphatic hydrops, fistula
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Small longitudinal movements of endolymph are induced by volume disturbances

Alec N. Salt and John E. DeMott, Department of Otolaryngology, Washington University School of Medicine, 517 South Euclid Avenue, St. Louis, MO, 63110, USA.

We have previously shown that in the normal cochlea, longitudinal endolymph movements are extremely small and make a negligible contribution to the homeostasis of endolymph. In the present study, longitudinal endolymph movements were measured in guinea pigs during experimentally-induced disturbances of endolymph volume. In separate studies, volume was either decreased by perilymphatic perfusion of hypertonic medium, or was increased by direct injection of an artificial endolymph into the second turn. Induced changes of endolymph flow and area were measured in the basal turn using an ionic marker technique. The method involved the placement of three electrodes into endolymph of the basal turn. One electrode contained a volume marker ion (tetramethylammonium or hexafluoroarsenate), from which a trace amount was continually injected into endolymph by iontophoresis. Two ion-selective electrodes, sensitive to the marker ion, were inserted into endolymph, one apical and one basal to the injection site. The measured time courses of volume marker concentration during experimental manipulations were interpreted with the aid of a computer model, which simulated ionic movements in the endolymph. Our results show that when endolymph volume is disturbed, longitudinal movements of endolymph are induced which contribute to the measured ionic changes. Osmotic dehydration results in an apically-directed endolymph movement, while injection into the second turn produced basally-directed endolymph movement. The movements appear to be mechanically-induced, resulting from distension of the boundary structures of endolymph. These findings help reconcile previous apparent contradictions in the literature and support the concept that some aspect of endolymph volume regulation occurs outside the cochlea, in structures such as the endolymphatic sac.
(Work supported by NIH grant DC01368)

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Endolymph volume regulation mechanisms revealed by microinjections into scala media.

Alec N. Salt (Dept. of Otolaryngology, Washington University School of Medicine, 517 South Euclid Avenue, St. Louis, MO, 63110) and John E. DeMott (Dept. of Otolaryngology, Washington University School of Medicine)

Longitudinal movements of endolymph were quantified in vivo in the basal turn of the guinea pig cochlea during injections of artificial endolymph into the second turn at rates up to 70 nl/min. Flow measurements utilized the marker ion tetramethylammonium (TMA), which was iontophoretically injected into the basal turn. TMA dispersion was measured by two ion- selective microelectrodes, one placed 0.5 mm apical, and the other 0.5 mm basal to the TMA injection site. Longitudinal endolymph movements were calculated from the recorded TMA time courses . Prior to injections, endolymph flow rates were extremely low. Volume injection into the second turn induced basally-directed flows in the basal turn. The relationship between the measured rate and the injection rate was nonlinear, in which low injection rates produced proportionately less flow than higher rates. These data show that for injections at low rates, volume disturbances have primarily local effects, presumably generating local distension or being compensated by local homeostatic mechanisms. At higher rates, basally-directed endolymph flow becomes increasingly significant. These findings suggest that two, independent processes may be involved in the regulation of endolymph volume. [Work supported by NIH grant DC01368]

Evidence for two components of endolymph volume regulation involving local and flow-based mechanisms.

Experimentally-induced longitudinal movements and area changes of endolymph were quantified using an ionic marker technique. The method used the ionic marker tetramethylammonium (TMA), which was iontophoresed into endolymph and monitored by two ion-selective electrodes, one placed basally and the other placed apically to the injection site at distances of approximately 0.5 mm. Area and flow changes induced in the basal turn were quantified during microinjections of artificial endolymph into the second turn at rates from 15 to 60 nl/min. The recorded TMA concentration time courses were interpreted in terms of endolymph area and flow changes with the aid of a mathematical model of tracer dispersal. Prior to injections, endolymph flow rates were extremely low. During volume injections into the second turn, an increasing rate of basally-directed flow was observed, the time course of which varied with the injection rate. Toward the end of the injection period the flow rate correlated well with the injection rate. These results demonstrate that in the case of excess endolymph volume, longitudinal flow out of the cochlea provides one mechanism contributing to homeostasis. This supports the concept that structures outside the cochlea, such as the endolymphatic sac, play a role in the correction of endolymph volume disturbances. However, the total volume passing the basal turn was systematically lower than the volume injected. The difference can be accounted for by local distension of scala media in the cochlea or by local ion regulation mechanisms, acting to remove solutes, and presumably water, from endolymph. In experiments which used low rates of volume injection, increases of endolymph cross-sectional area in the basal turn could be demonstrated. However, for higher injection rates the longitudinal flow rates in the basal turn became so high that area could not be reliably quantified. Nevertheless, it is likely that local ion transport processes also contribute to the correction of volume disturbance, especially for the lower injection rates used in this study.
Work supported by NIH grant DC01368.

Perilymph potassium increases during acute endolymph volume enlargement.

An acute increase of endolymph volume was produced in anesthetized guinea pigs by microinjection of an artificial endolymph into scala media. Injections were performed in the second turn at rates of 15 to 60 nl/min for periods up to 15 min. During injections, the endocochlear potential (EP) increased by up to 14 mV and action potential thresholds were temporarily elevated, with greatest elevations for low frequency stimuli.
Simultaneous with these changes, the potassium concentration in scala vestibuli perilymph (KSV) showed a systematic increase by 0.5 to 2.0 mM. The magnitude of the KSV increase did not correlate well with the injected volume but did correlate with the magnitude of EP increase (r2=0.723). It is likely that the KSV increase represents the response of local ion transport processes to the induced endolymph volume increase. A mechanism which permits K+ and corresponding anions to be released from endolymph would contribute to the return of endolymph towards normal volume. This result therefore provides support for the concept that local homeostatic mechanisms contribute to the recovery of normal endolymph volume after disturbance. Acute endolymphatic injections may provide a valuable tool for the study of endolymph volume regulation. The present study demonstrates that substantial differences exist between animals made hydropic by ablation of the endolymphatic sac and those made hydropic by acute endolymphatic injection. In the chronic model of hydrops, EP is reduced by 8-10 mV within 4 days of sac ablation (Salt & DeMott Hearing Research 74:165, 1994) while the present study showed EP to increase with acute volume injection.
This study supported by NIH grant DC01368

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Endocochlear potential and cochlear fluid pH changes induced by respiratory CO2 manipulations

Endocochlear potential (EP), endolymph pH (pHe) and scala vestibuli perilymph pH (pHv) were monitored in the second turn of the guinea pig cochlea during manipulation of the inspired CO2 level. End-tidal CO2 was monitored as an index of the induced vascular change. End-tidal CO2 was normally set to 5%. Increases and decreases in vascular CO2 were induced by varying the CO2 content of inspired air while the animal was slightly hyperventilated. The end-tidal CO2 could be decreased to a reading of 2%, or increased to a reading of 15% without major changes in vascular O2. Increasing the end-tidal CO2 generated an EP increase and reduction of end-tidal CO2 decreased the EP. An EP increase resulting from increased respiratory CO2 was previously reported by Prazma et al. (Ann Otol 88, 222,1979). Simultaneous with the EP changes, changes of pHe and pHv were also recorded, in which the pH of both fluids decreased with end-tidal CO2 elevation and increased as end-tidal CO2 was reduced. The measured pH changes were less than those calculated assuming constant fluid bicarbonate concentration. Similar fluid pH changes were monitored when respiratory manipulations were performed during perilymphatic perfusion at 5 Ál/min with bicarbonate-buffered artificial perilymph. This demonstrated that CO2 levels in cochlear fluids were dominated by the CO2 level set by the vascular system. These results show that EP and cochlear fluids pH are extremely sensitive to the respiratory status of the animal. In order to maintain the cochlea in a stable state it is necessary to maintain vascular CO2 at a constant level.
Work supported by NIH grant DC01368.

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Endolymph calcium homeostasis in pigmented and albino guinea pigs

It was previously reported that systematic differences exist between the endocochlear potential (EP) and endolymph calcium concentration (Cae) in the apical turns of pigmented and albino guinea pig cochleae (Gill and Salt, ARO #560, 1995). In the pigmented animal, EP declines from base to apex and Cae is higher at the apex than at the base. In albino animals, the EP declines far less towards the apex and Cae remains uniform with distance. As a result, EP is higher and Cae is lower in the apical turn of albino animals compared to pigmented animals. In the present study we have further investigated the mechanisms of endolymph Cae homeostasis by iontophoretically injecting Ca into endolymph of the fourth turn of pigmented and albino animals. In each case, the time course of Cae concentration change was recorded with a Ca-selective electrode placed in endolymph close to the injection site. From the measured concentration time courses the rate of Ca clearance from scala media was determined. In both pigmented and albino animals, Ca was found to be cleared rapidly from endolymph. The mean clearance half-times were 7.15 mins and 7.00 mins for pigmented and albino groups respectively. The difference between these values was not statistically significant. The Cae differences between pigmented and albino animals are therefore not accounted for by differences in clearance rates. Rather, it is more likely that they arise from different characteristics of the processes responsible for Ca transport into endolymph. If melanin plays a role in this transport, then its absence in the albino cochlea may impair the process by which Ca is transported into endolymph.
This study supported by NIH training grant DC00022 and by the Deafness Research Foundation.

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Longitudinal movments and area changes of endolymph produced by osmotic dehydration

Previous studies have shown that in the normal cochlea the rate of longitudinal endolymph flow is extremely low. In the present study, movements of endolymph associated with endolymph volume disturbances were investigated. Changes of endolymph volume were induced in the guinea pig by perfusion of the perilymphatic space for 20 min with hypertonic medium (up to 400 mOsm, by addition of sucrose). Endolymph cross-sectional area and longitudinal movements were quantified simultaneously by a technique which used two ion-selective electrodes to monitor concentration of a volume marker at each side of an injection site. The injected markers were tetramethylammonium (TMA+) or arsenic hexafluoride (AsF) which were iontophoresed into the second turn. By measuring the tracer time course from the two electrodes two components could be identified; area changes which generated similar changes at the two recording sites, and longitudinal movements, which produced opposite changes at the two sites. From these measurements we found that during dehydration the area of scala media decreased by approximately 20%. During this area reduction there was an apically-directed movement of endolymph by a distance of approximately 0.8 mm. This small movement is of sufficient magnitude to contribute to the increase in concentration of electrolytes in endolymph during dehydration. When perilymph osmolarity was returned to normal, endolymph area increased, but in most cases did not return to the pre-treatment value. This was accompanied by a movement of endolymph towards the basal turn of larger magnitude than that during dehydration. Such longitudinal movements, though small, represent a significant contribution to changes in endolymph composition during dehydration and recovery and contribute to endolymph volume regulation.
This work supported by NIH grant DC01368.

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Quantification of endolymphatic hydrops in the cochlea by magnetic resonance microscopy

Magnetic resonance (MR) microscopy was used to image the fluid spaces of the guinea pig cochlea. Tissue specimens were fixed by whole head perfusion with Heidenhain-Susa, followed by immersion in buffered formaldehyde. Intact, isolated cochleas were imaged as serial slices with a voxel resolution of 25Ám3. At this resolution, Reissner's membrane was clearly delineated throughout the length of the cochlea. Custom software was developed to generate three dimensional reconstructions of the endolymphatic and perilymphatic scalae. This enabled the area of each scala to be quantified throughout the length of the cochlea. Scala areas were quantified for normal cochleas, and cochleas in which endolymphatic hydrops has been induced by surgical obstruction of the vestibular aqueduct 8 weeks previously. In normal cochleas, the areas of all scalae, including scala media, decreased from base to apex. In hydropic cochleas, the area of scala media varied considerably with location and large changes in the degree of distension of Reissner's membrane could occur over short distances. MR microscopy and subsequent computer analysis of the isotropic data appears to be an excellent tool for quantifying the areas of cochlear fluid compartments.
This work was supported by NIH grants DC01368 (Alec Salt) and DC00114 and 1P411RR05959 (Miriam Henson).

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Endolymph calcium differences between pigmented and albino guinea pigs.

Melanocytes are considered to be necessary for the development of the stria vascularis and production of the endocochlear potential (EP). However, the precise role of the melanin polymer, which increases from base to apex within the stria vascularis, remains unclear. It has been shown that endolymphatic Ca++ concentration increases and EP decreases along the length of the cochlea in pigmented animals. Recent studies have found that albino animals with amelanotic stria have relatively constant EP in all four turns of the cochlea when compared to their pigmented counterparts (Conlee & Bennett, Hear. Res. 65:141, 1993). Since it has been suggested that melanin pigmentation may be involved in endolymphatic Ca++ homeostasis, the present study was designed to determine whether endolymph Ca++ composition differs between the pigmented and albino inner ear. Endolymph Ca++ and EP were measured in all four cochlear turns of 10 pigmented and 11 albino guinea pigs using double barreled Ca++-selective microelectrodes. Both pigmented and albino groups had similar EP values at the basal turn (mean values 85.5 and 86.8 mV respectively). The EP was significantly lower for the pigmented animals at the apical turn (mean 62.5) when compared to the albino group (mean 73.6), p=0.0079. Basal endolymphatic Ca++ concentrations were also similar for pigmented and albino animals (mean 23.3 and 21.9 ÁM respectively). Ca++ increased nearly two-fold for the pigmented group along the length of the cochlea (apex mean 39.1 ÁM) but showed a far smaller increase for the albino group (apex mean 24.9 ÁM). The third and fourth turn Ca++ concentrations were significantly different between the two groups with p-values of 0.033 and 0.014 respectively. The marked increase in endolymphatic Ca++ concentration from base to apex for the pigmented group parallels the strial melanin gradation along the length of the cochlea and is not present to the same degree in albino animals.
This work supported by NIH Training Grant DC00022 and the Deafness Research Foundation.

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Endolymph Composition Changes in the Dominant Spotting (Wv) Mouse

It has been previously reported that mice homozygous for the Wv (dominant spotting) mutation show impaired auditory function and lack the normal positive endocochlear potential (Steel, et al. Hear. Res. 27:11-26, 1987). The present study investigated the chemical composition of endolymph in the Wv mutant bred in C57BL and CBA/Ca backgrounds. Endolymph K+, Ca++ and endocochlear potential (EP) were measured in the basal turn using double- barreled ion-selective microelectrodes. In addition, auditory sensitivity was assessed using compound action potential thresholds and the cochleas were fixed for histological examination. The median value of each measure in the two groups for the in a CBA/Ca background were:

		 EP	 K+	 Ca++	16kHz threshold 
		(mV)	(mM)	(ÁM)	   (dB SPL)

+/+ (Control)	101.6	168.5	20.3		39.9
Wv/Wv (Mutant)	0.3	144.9	984.0		81.4
It is notable that while EP, Ca++ and auditory sensitivity were abnormal in the mutant group, endolymph K+ levels remained high. This demonstrates that the endolymph-perilymph barrier must remain intact and that endolymph K+ homeostasis must be near-normal in the Wv mutant. However, the functional loss did not correlate well with either EP or Ca++ levels, suggesting that the deficit is of complex origins. In some animals, the effect of anoxia on the EP was evaluated. Mutant animals which showed near-zero EP demonstrated a negative EP of comparable magnitude to that seen in controls. Histologically, we confirmed that in mutants, the presence of melanocytes in stria vascularis correlated with the presence of EP and vice versa. In animals in which melanocytes were absent, other histological changes, including hair cells loss, were observed. The findings of this study add support to the concept that endolymph K+ homeostasis and EP generation are accomplished by separate processes and possibly different cells within stria vascularis.

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