Endolymphatic hydrops and therapeutic effects are visualized in 'atypical' Meniere's disease

A 53-year-old male with fluctuating low frequency sensorineural hearing loss and tinnitus, but without vertigo, was evaluated by MRI obtained by intratympanic injection of a gadolinium-based contrast agent (GBCA) before and after the administration of isosorbide. The endolymphatic hydrops was semi-quantitatively evaluated by a 3.0-T MR scanner. For quantification, the affected side/contralateral side ratios were calculated. A gadodiamide (a kind of GBCA)-enhanced space surrounding the endolymph in the affected side with a 0.50 ratio (which may have represented endolymphatic hydrops) improved after isosorbide therapy to a 0.98 ratio. Thus, endolymphatic hydrops was demonstrated in a patient with 'atypical' Meniere's disease (MD), suggesting that at least some atypical MD may share similar etiology with, and therefore be a continuum of, MD. Also, therapeutic effects could be visualized by using MRI. Therefore, MRI-based diagnosis of MD-related disease will be a powerful tool not only because of its precision but also its usefulness for therapeutic evaluation.

Ca(2+) regulation of endocochlear potential in marginal cells

We examined the effect of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) in marginal cells on the asphyxia- or furosemide-induced decrease in the endocochlear potential (EP) by perfusing the endolymph with or without a Ca(2+) chelator or inhibitors of Ca(2+)-permeable channels or Ca(2+)-pump during transient asphyxia or intravenous administration of furosemide. We obtained the following results. (1) Endolymphatic administration of SKF96365 (an inhibitor of TRPC and L-type Ca(2+) channels) or EGTA-acetoxymethyl ester (EGTA-AM) significantly inhibited both the transient asphyxia-induced decrease in EP (TAID) and the furosemide-induced decrease in EP (FUID). (2) Endolymphatic perfusion with nifedipine significantly inhibited the TAID but not the FUID. (3) The recovery from the FUID was significantly suppressed by perfusing the endolymph with EGTA-AM, nifedipine, or SKF96365. (4) Endolymphatic administration of thapsigargin inhibited both the FUID and TAID. (5) The recovery rate from the FUID was much slower than that from the TAID, indicating that furosemide may inhibit the Ca(2+)-pump. (6) A strong reaction in immunohistochemical staining for TRPC channels was observed in the luminal and basolateral membranes of marginal cells. (7) A positive staining reaction for the gamma subunit of epithelial Na(+) channels was observed in the luminal and basolateral membranes of marginal cells. (8) Positive EP was diminished toward 0 mV by the endolymphatic perfusion with 10 muM amiloride or 10 muM phenamil. Taken together, these findings suggest that [Ca(2+)](c) regulated by endoplasmic Ca(2+)-pump and Ca(2+)-permeable channels in marginal cells may regulate the positive EP, which is partly produced by the diffusion potential of Na(+) across the basolateral membrane in marginal cells.

Simulation of cupulolithiasis and canalolithiasis by an animal model

The physical mechanisms responsible for cupulolithiasis and canalolithiasis have been investigated by two groups of experiments in isolated posterior semicircular canal (SCC) of frog (Rana esculenta L.). First, clouds of 10-30 isolated otoconia were let to fall (snowfall of otoconia) either through the ampulla onto the cupula, or inside the long arm of the canal, opposite to the cupula. Second, microspheres ranging 30 to 350 microm in diameter were gently moved to and fro inside the long arm of the canal by a micromanipulator. The effects were evaluated by recording the firing rate (Nfr) of the ampullary nerve. Snowfall of otoconia produced detectable changes of Nfr only when otoconia got in contact with the cupula, but not when falling through the endolymph. Movement of the microspheres in the canal long arm induced Nfr changes only if the microsphere diameter exceeded about 50 microm. Although the exact microsphere size needed for receptor stimulation may depend on the experimental conditions, these results strongly suggest that debris moving inside a SCC (canalolithiasis) can produce transcupular pressures able to stimulate ampullar receptors only if they have suitable size, whereas isolated otoconia cannot, except when lying on the cupula (cupulolithiasis).

Does BAPTA leave outer hair cell transduction channels closed?

The calcium chelator BAPTA was iontophoresed into the scala media of the second turn of the guinea pig cochlea. This produced a reduction in low frequency cochlear microphonic (CM) measured in scala media and an elevation of the cochlear action potential (CAP) threshold that lasted for the duration of the experiment. Using two pipettes, one filled with KCl and the other KCl and BAPTA (50, 20 and 5 mM) it was possible to observe the effect of passing current through one electrode while measuring the endolymphatic potential (EP) with the other. The results demonstrated that current passed via the BAPTA pipette caused a sustained increase in EP of 8.2, 12.9 and 7.8 mV in the three animals used. This increase coincided with the decrease in low frequency CM that indicated a causal connection between the two. In a second series of experiments, pipettes with larger tips were inserted into scala media in the first cochlear turn and BAPTA was allowed to diffuse from the pipette. The results confirmed the relationship between EP increase and the fall of scala media CM. One interpretation of these results is that lowering the Ca2+ concentration of endolymph with BAPTA inhibits mechano-electrical transduction in outer hair cells (OHCs) and leaves the hair cell transduction channels in a closed state, thus increasing the resistance across OHCs and increasing the EP. These findings are consistent with a model of hair cell transduction in which tension on stereo cilia opens the transduction channels.

Effects of CO2/HCO3− in Perilymph on the Endocochlear Potential in Guinea Pigs

The effect of CO(2)/HCO(3)(-) on the endocochlear potential (EP) was examined by using both ion-selective and conventional microelectrodes and the endolymphatic or perilymphatic perfusion technique. The main findings were as follows: (i) A decrease in the EP from approximately +75 to approximately +35 mV was produced by perilymphatic perfusion with CO(2)/HCO(3)(-)-free solution, which decrease was accompanied by an increase in the endolymphatic pH (DeltapH(e), approximately 0.4). (ii) Perilymphatic perfusion with a solution containing 20 mM NH(4)Cl produced a decrease in the EP (DeltaEP, approximately 20 mV) with an increase in the pH(e) (DeltapH(e), approximately 0.2), whereas switching the perfusion solution from the NH(4)Cl solution to a 5% CO(2)/25 mM HCO(3)(-) solution produced a gradual increase in the EP to the control level with the concomitant recovery of the pH(e). (iii) The perfusion with a solution of high or low HCO(3)(-) with a constant CO(2) level within 10 min produced no significant changes in the EP. (iv) Perfusion of the perilymph with 10 microg/ml nifedipine suppressed the transient asphyxia-induced decrease in EP slightly, but not significantly. (v) By contrast, the administration of 1 microg/ml nifedipine via the endolymph inhibited significantly the reduction in the EP induced by transient asphyxia or perilymphatic perfusion with CO(2)/HCO(3)(-)-free or 20 mM NH(4)Cl solution. These findings suggest that the effect of CO(2) removal from perilymphatic perfusion solution on the EP may be mediated by an increase in cytosolic Ca(2+) concentration induced by an elevation of cytosolic pH in endolymphatic surface cells.

Effects of lithium on endolymph homeostasis and experimentally induced endolymphatic hydrops

There is evidence to suggest that water homeostasis in the inner ear is regulated via the vasopressin (VP)-aquaporin 2 (AQP2) system in the same fashion as in the kidney. The VP-AQP2 system in the kidney is well known to be inhibited by lithium, resulting in polyuria due to a decrease in reabsorption of water in the collecting duct of the kidney. Therefore, lithium is also likely to inhibit the VP-AQP2 system in the inner ear, and consequently exert some influence on inner ear fluid homeostasis. In this study, we investigated the effects of lithium on AQP2 expression in the rat inner ear, and on the cochlear fluid volume in hydropic ears of guinea pigs. A quantitative PCR study revealed that lithium reduced AQP2 mRNA expression in the cochlea and endolymphatic sac. Lithium application also decreased the immunoreactivity of AQP2 in the cochlea and endolymphatic sac. In a morphological study, lithium intake significantly reduced endolymphatic hydrops dose-dependently. These results indicate that lithium acts on the VP-AQP2 system in the inner ear, consequently producing a dehydratic effect on the endolymphatic compartment.

[Effect of adriamycin on expression of mdr1 gene in guinea pig cochlea lateral wall]

OBJECTIVE

To study whether ADM affect the expression of mdr1 gene in guinea pig cochlea lateral wall.

METHOD

Sixty white guinea pigs with red eyes were divided into three groups: control group, NS group and ADM group. Guinea pigs were administered an intravenous injection of 30 mg/kg ADM. Four hoars later,ten animals were then immediately sacrificed, and the bony labyrinths were removed. The remaining ten animals were sacrificed after 24 h. The effect of ADM, an ototoxicity drug, on expression of mdrl gene in guinea pigs cochlea lateral walls were observed with RT-PCR. Furthermore, the ADM concentration in endolymph were measured by high performance liquid chromatography.

RESULT

For ADM group, the density ratio of mdr1 to beta-action of the 24 h was significantly greater compared to that of the 4 h or controls; In contrast, the ADM concentration of 4 h was significantly higher than that of the 24 h.

CONCLUSION

mdr1 gene in normal guinea pig cochlea lateral wall is a stress gene. It can be induced by ADM; it may be not only related to reduce the accumulation of ototoxicity drug in endolymph, but also associated with the protective function of inner ear.

Ca2+ current-driven nonlinear amplification by the mammalian cochlea in vitro

An active process in the inner ear expends energy to enhance the sensitivity and frequency selectivity of hearing. Two mechanisms have been proposed to underlie this process in the mammalian cochlea: receptor potential-based electromotility and Ca(2+)-driven active hair-bundle motility. To link the phenomenology of the cochlear amplifier with these cellular mechanisms, we developed an in vitro cochlear preparation from Meriones unguiculatus that affords optical access to the sensory epithelium while mimicking its in vivo environment. Acoustic and electrical stimulation elicited microphonic potentials and electrically evoked hair-bundle movement, demonstrating intact forward and reverse mechanotransduction. The mechanical responses of hair bundles from inner hair cells revealed a characteristic resonance and a compressive nonlinearity diagnostic of the active process. Blocking transduction with amiloride abolished nonlinear amplification, whereas eliminating all but the Ca(2+) component of the transduction current did not. These results suggest that the Ca(2+) current drives the cochlear active process, and they support the hypothesis that active hair-bundle motility underlies cochlear amplification.

Endolymphatic Perfusion with EGTA-Acetoxymethyl Ester Inhibits Asphyxia- and Furosemide-Induced Decrease in Endocochlear Potential in Guinea Pigs

We examined the effect of the Ca(2+) concentration in the endolymph ([Ca](e)) or in the endolymphatic surface cells ([Ca](i)) on the endocochlear potential (EP) by using an endolymphatic or perilymphatic perfusion technique, respectively. (i) A large increase in [Ca](e) up to approximately 10(-3) M with a fall in the EP was induced by transient asphyxia ( approximately 2 min) or by the intravenous administration of furosemide (60 mg/kg), and a significant correlation was obtained between the EP and p[Ca](e) (= -log [Ca](e), r = 0.998). (ii) Perfusion of the endolymph with 10 mM EGTA for 5 min neither produced any significant change in the EP nor altered the asphyxia-induced change in EP (DeltaEP(asp)), suggesting that neither [Ca](e) nor the Ca(2+) concentration gradient across the stria vascularis contributed directly to the generation of the EP in the condition of low [Ca](e). In contrast, endolymphatic perfusion with high Ca(2+) (more than 10 mM) produced a decrease in EP and a significant correlation was obtained between the EP and the Ca(2+) concentration of perfusion solution (r = 0.982), suggesting that Ca(2+) permeability may exist across the stria vascularis. (iii) The administration of a Ca(2+) chelator, EGTA-acetoxymethyl ester (AM, 0.3 mM), to the endolymph, which produced a gradual increase in EP, suppressed significantly, by 60-80%, DeltaEP(asp) or furosemide-induced changes in EP. In contrast, perilymphatic administration of 0.5 mM EGTA-AM caused no significant suppression of the DeltaEP(asp). These findings suggest that [Ca](i) plays an important role in generating/maintaining a large positive EP.

BAPTA Induces Frequency Shifts in vivo of Spontaneous Otoacoustic Emissions of the Bobtail Lizard

Spontaneous otoacoustic emissions (SOAEs) are indicators of active processes in the inner ear and are found in all classes of land vertebrates. In the Australian bobtail lizard, earlier work showed that otoacoustic emissions are generated by an active motility process in the hair-cell bundle. This is likely to be driven by calcium-sensitive mechanisms implicated in other non-mammalian hair cell systems. If so, it should be fundamentally influenced by the extracellular calcium concentration. In in vitro studies, the rate of force generation in hair cell stereovilli is linked to the extracellular calcium concentration. In such preparations, low-calcium solutions, buffered by the calcium chelator BAPTA, were reported to change the frequency of hair cell bundle oscillations. In the present study, BAPTA was iontophoresed into the endolymph of the bobtail skink in vivo, and SOAEs were monitored. Application of BAPTA resulted in a prolonged downward shift in the frequency of individual SOAE spectral peaks. Recovery took more than 1 h, consistent with a slow clearance of BAPTA from endolymph. SOAE peak amplitudes were most often enhanced, suggesting there was no functional disruption of tip links. The direction and degree of frequency shifts were consistent with in vitro and in vivo data showing the effects of changing calcium concentrations in the endolymph directly.

Time Course of Dehydrating Effects of Isosorbide on Experimentally Induced Endolymphatic Hydrops in Guinea Pigs

Osmotic diuretics are therapeutic agents used to reduce endolymphatic hydrops. However, glycerol-induced change in endolymph volume is followed by a rebound phenomenon. In this study, we investigated the rebound phenomenon occurring with isosorbide, an osmotic diuretic used as a therapeutic agent for Ménière's disease in Japan. Forty guinea pigs underwent surgical obliteration of the endolymphatic sac. Thirty received isosorbide orally 1 month after surgery. These animals were sacrificed 3, 6, or 12 h after isosorbide intake. The remaining 10 animals served as controls. Quantitative assessment of changes in the endolymphatic space was performed light-microscopically. Isosorbide reduced cochlear endolymph volume, with a peak reduction 6 h after intake. Thereafter, no prominent rebound phenomenon was noted. Clinically, since isosorbide is orally administered every 8 h, rebound phenomenon need not be considered in the treatment with isosorbide.

Endolymphatic sodium homeostasis by Reissner's membrane

Cochlear sensory transduction depends on active extrusion of sodium ion (Na(+)) from the luminal fluid, endolymph. Reissner's membrane epithelium forms much of the barrier between cochlear endolymph and perilymph and we hypothesized that Reissner's membrane might be responsible for this function. We found that Reissner's membrane isolated from gerbil produced a short circuit current (I(sc)) directed into the apical side, consistent with cation absorption and/or anion secretion. I(sc) was inhibited by amiloride analogs in the potency sequence benzamil>amiloride>>ethylisopropylamiloride, consistent with Na(+) absorption through an epithelial sodium channel in the apical cell membrane. I(sc) was also inhibited by an inhibitor of Na(+),K(+)-ATPase, ouabain, and by the K(+) channel blockers Ba(2+), 4-aminopyridine and quinine but not tetraethylammonium nor glibenclamide, consistent with the presence of a voltage-activated K(+) channel. Bumetanide, an inhibitor of the Na(+),2Cl(-),K(+)-cotransporter, had no effect on I(sc). Contrary to previous hypotheses, no evidence was found for electrogenic secretion of Cl(-) under control of cAMP since neither forskolin nor genistein affected I(sc) when Na(+) absorption was blocked. These results provide the first direct evidence that Reissner's membrane contributes to normal cochlear function by absorption of Na(+) from endolymph.

Asphyxia and diuretic-induced changes in the Ca2+ concentration of endolymph

Using Ca2+ -selective microelectrodes based on the neutral carrier, ETH-1001 with polyvinyl chloride (PVC), we have measured changes in the free Ca2+ concentration of guinea pig cochlear endolymph ([Ca](e)) after transient asphyxia or intravenous administration of diuretics. Under the control conditions, the endocochlear potential (EP) was +80 mV, and the [Ca](e) was in the range 1.4 x 10(-7)-2.4 x 10(-6) M (n = 16). Transient asphyxia (1-1.5 min) produced an increase in the [Ca](e) with a fall in the EP, whereas the cessation of the asphyxia led to a quick recovery of both [Ca](e) and EP to their control levels. Intravenous administration of furosemide (60 mg/kg) or bumetanide (30 mg/kg) also caused an increase in the [Ca](e) with a fall in the EP, followed by a gradual recovery of both [Ca](e) and EP. From these results, we obtained a significant correlation between EP and p[Ca](e) (= -log[Ca](e)), and conclude that (1) the [Ca](e) is extremely low, around 10(-6) M or less, under normal conditions and (2) the [Ca](e) is directly correlated with EP under physiological conditions.

Otolith growth in trout Oncorhynchus mykiss: supply of Ca2+ and Sr2+ to the saccular endolymph

Kinetic and pharmacological characteristics of Ca2+ fluxes across the saccular epithelium of trout were studied using a perfused isolated inner ear. 45Ca2+ influx from the Ringer solution to the endolymph was 3-4 nmoles h-1μl-1 endolymph, which corresponds to a global turnover rate of the endolymph calcium of 200 %h-1. Ca2+ entry into the proximal endolymph was faster than into the distal fluid. Net Ca2+ movement across the saccular epithelium depended on the direction and intensity of the chemical gradient of calcium between the Ringer solution and the endolymph. Increasing the calcium concentration in the Ringer solution up to 4.4 mmol l-1 provoked an accumulation of Ca2+ in both proximal and distal endolymphs, and equilibrium was reached about 30 min after the beginning of perfusion. Perfusion with calcium-free Ringer partially emptied the proximal compartment of calcium, whereas the calcium levels in the distal endolymph did not vary during 70 min of perfusion. Verapamil (10-5 mol l-1) and cyanide (CN, 10-3 mol l-1) did not modify the accumulation of Ca2+ within the endolymph in the presence of a favourable calcium chemical gradient. Furthermore the relationship between Ca2+ net fluxes and the chemical calcium gradient across the saccular epithelium was linear, indicating a passive diffusional mechanism via a paracellular pathway. Similar relationships were found for Sr2+ fluxes across the saccular epithelium in the presence of positive chemical gradients (1, 2 and 4 mmol l-1 Sr2+). In vivo experiments in which trout were intraperitoneously injected with CaCl2 solution confirmed the tight relationship between the calcium levels in plasma and endolymph (both proximal and distal). Sampling proximal and distal endolymphs in trout and turbot saccules revealed a decreasing proximo—distal calcium gradient in endolymph of both fish species. The present results strongly suggest that the endolymph is supplied with Ca2+ and Sr2+via a paracellular pathway located in the proximal area of the saccular epithelium.

Expression of p-glycoprotein is associated with that of multidrug resistance protein 1 (MRP1) in the vestibular labyrinth and endolymphatic sac of the guinea pig

Expression of p-glycoprotein (p-gp) and multidrug resistance protein 1 (MRP1) was detected in the vestibular labyrinth and endolymphatic sac (ES) of the guinea pig by immunohistochemical staining using anti-p-gp monoclonal antibody (mAb) C219 and anti-MRP mAb MRPr1. P-gp was detected in capillary endothelial cells of the crista ampullaris, utricle, saccule and ES. MRP1 was detected in the epithelial lining of the crista ampullaris, utricle, saccule, and epithelial cells of the ES. Since p-gp and MRP1 act as extrusion pumps, they may coordinate with each other in vestibular organs and ES and play an important role in the blood-labyrinth barrier.

Objective method for differentiating between drug-induced vestibulotoxicity and cochleotoxicity

HYPOTHESIS

An objective direct method is proposed to differentiate between drug-induced functional vestibulotoxicity and cochleotoxicity.

BACKGROUND

Many substances are ototoxic. Although there are objective methods to directly evaluate functional cochlear toxicity (auditory nerve brainstem responses [ABR]), it is more difficult to assess direct functional ototoxicity to the various vestibular end organs.

METHODS

Short-latency vestibular evoked potentials (VsEP) from different vestibular end organs and ABR, were used to assess functional impairment of the vestibular and cochlear end organs caused by daily injections of the aminoglycoside amikacin (known to be preferentially cochleotoxic) in guinea pigs.

RESULTS

There was no significant change in the various VsEPs. whereas ABR thresholds were elevated, confirming the selective functional cochleotoxicity previously reported, as evaluated by other (mainly nondirect) methods.

CONCLUSION

This study demonstrates the feasibility in general of using short-latency evoked potentials to evaluate functional cochleotoxicity and vestibulotoxicity of ototoxic drugs and to differentiate between them.

Effects of enzyme and anion transport inhibitors on in vitro incorporation of inorganic carbon and calcium into endolymph and otoliths in salmon Oncorhynchus masou

The transepithelial transport of inorganic carbon to endolymph and its subsequent deposition on otoliths were pharmacologically examined by incubating the sacculus containing an otolith with NaH(14)CO(3). Calcium incorporation was also studied. Carbon incorporation into endolymph and otoliths was saturated with increased concentrations of bicarbonate ions in the incubation medium and was followed by the Michaelis-Menten equation with a K(m) of 26.3 mM and 0.4 mM, respectively. Carbon incorporation decreased with an increase in chloride concentrations in the medium. Calcium incorporation was not affected by chloride and bicarbonate ions up to 10 mM. Higher concentrations of bicarbonate ions reduced calcium incorporation into both fractions. Carbon incorporation into endolymph and otoliths was inhibited by acetazolamide, disulfonate stilbenes (DIDS and SITS), thiocyanate, and ouabain. Calcium incorporation was not affected by these inhibitors. Amiloride inhibited carbon incorporation into otoliths alone. These results suggest that HCO(3)(-)-ATPase and Cl(-)/HCO(3)(-)-exchangers are involved in the transepithelial transport of bicarbonate ions to the endolymph. Carbonic anhydrase was also suggested to play a role in carbonate production for otolith calcification.

[Endolymphatic analgesia after the surgery for an acute mechanical ileus]

It was established in experiment that, while endolymphatic infusion of analgin with droperidol, the maximal analgin concentration in central lymph, blood and cerebrospinal fluid holds out much longer than while intramuscular injection. In 39 patients, operated on for an acute mechanical ileus, the endolymphatic analgin and droperidol infusion have permitted to achieve a stable anesthesia.

Effect of acetazolamide on cation concentration in the endolymph of the endolymphatic sac

Acetazolamide (ACTZ), a carbonic anhydrase inhibitor, has been reported to decrease the endolymphatic sac (ES) DC potential (ESP) in the guinea pig. To assess the involvement of cation transport in the ESP change by ACTZ we examined the effect of ACTZ upon the K+ and Na+ activities of the ES endolymph in the guinea pig using ion-sensitive microelectrode. ACTZ (10 mg/kg), a dose that produces the ESP maximum reduction, produced a significant increase in Na+ activity of the ES endolymph with no change in K+ activity. The results suggest that Na+ transport may be directly or indirectly involved in ESP reduction by ACTZ, and that a Na(+)-H+ exchanger may be involved in Na+ influx pathway from endolymph to the ES epithelial cells.

Cytochemical and patch-clamp studies of calcium influx through voltage-dependent Ca2+ channels in vestibular supporting cells of guinea pigs

To clarify whether or not vestibular supporting cells have voltage-dependent Ca2+ channels, cytochemical and patch-clamp studies were performed using cells isolated from the ampullae of the semicircular canal of the guinea pig. Image analysis used fura-2 as a Ca(2+)-sensitive fluorescence dye and showed that the intracellular Ca2+ concentration ([Ca2+]i) increased with bath application of high (150 mM)K+, but was unaffected by 80 mM K+. The increase in [Ca2+]i induced by high K+ was completely blocked by 1 microM nifedipine as an L-type Ca2+ channel antagonist. In the patch-clamp whole-cell recording of the isolated supporting cells, the voltage-dependent inward current was induced by a depolarizing pulse lasting 2 s in a high (50 mM) Ca2+ and tetraethylammonium-containing external solution replaced by choline chloride and a Cs(+)-containing internal solution. The inward current was obtained when the membrane was depolarized to -50 mV and maximum current was observed at -10 to +10 mV. This inward current was completely blocked by 1 microM nifedipine. These findings strongly suggest that voltage-dependent Ca2+ channels exist in the vestibular supporting cells and regulate Ca2+ concentration in the vestibular endolymph.

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.

Induction of endolymphatic hydrops in the guinea pig by perisaccular deposition of sepharose beads carrying and not carrying immune complexes

We tried to induce endolymphatic hydrops in guinea pig cochleas by unilateral, perisaccular deposition of sepharose beads carrying immune complexes. Controls consisted of the deposition of sepharose beads without immune complexes and the contralateral, untreated ear. The effects of the treatment were studied by light microscopy and electrophysiological recordings of the gross cochlear potentials 1, 2, and 6 weeks after treatment. Each condition included six animals. Analysis of variance of the morphometric data concerning the ears treated with deposition of the beads showed a statistically significant difference (P = 0.04) between the degree of hydrops found for the beads with immune complexes and for those without. The difference between the treated ears and the contralateral untreated ears was significant (P = 0.01) for the beads with immune complexes and not significant (P = 0.8) for those without immune complexes while there was no significant effect of post-treatment time interval. Analysis of variance of the electrophysiological data, collected in response to tone bursts at the apex of the cochlea, showed no significant differences between the results for the beads with and without immune complexes. Therefore these results were pooled. One week after treatment the pooled results for the compound action potential showed a small decrease in amplitude, just significant at 2 kHz, but not at 4 and 8 kHz. This decrease disappeared completely after 6 weeks. The pooled results for the negative summating potential (SP) showed a significant increase in magnitude at all frequencies decreasing with post-treatment interval. The cochlear microphonics did not demonstrate any change in amplitude after treatment. The results indicate that deposition of sepharose beads with immune complexes induces endolymphatic hydrops. Also, deposition of the sepharose beads itself may have induced hydrops together with enhancement of the SP. SP enhancement may be related to the development of endolymphatic hydrops rather than to the presence of hydrops as such. This may be based on pressure build-up while hydrops develops.

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.

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.

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.

Effect of glycerol on electrochemical composition of endolymph and perilymph in the rat

Glycerol (2 g/kg body weight), or 0.15 M NaCl for control animals, was administered to rats by i.v. injection. The dose was chosen in order to obtain an osmolarity increase in plasma of about 15 mosm/l 1 h after the glycerol administration, an increase which is similar to that observed in the human glycerol dehydration test. Endolymph and perilymph were sampled from the basal turn of the cochlea; cerebrospinal fluid (CSF) was sampled from cisterna magna. Plasma osmolarity, endocochlear potential, Na and K concentrations in endolymph, perilymph and CSF were determined 1 and 2 h after the glycerol injection. Compared with control animals, glycerol induced an increase in Na and K concentration in perilymph and endolymph, respectively, 1 and 2 h after the glycerol injection. No modification of the endocochlear potential was observed. These results are compatible with an increase in inner ear fluids osmolarity induced by glycerol.

Dexamethasone perfusion of the labyrinth plus intravenous dexamethasone for Ménière's disease

Recent clinical and laboratory evidence indicates that Meniere's disease is an immune-mediated disease. Dexamethasone perfusion of the inner ear through the round window plus intravenous dexamethasone often will stop the dizzy spells, reduce the fullness and low-frequency tinnitus, and sometimes improve the hearing in patients with Meniere's disease. The dexamethasone must act mostly on the endolymphatic sac and, to a lesser extent, on the stria vascularis and spiral ligament, the known targets of immune response in the inner ear, to reduce the endolymphatic hydrops and restore the fluid dynamics of the endolymph. Despite the good results with streptomycin perfusion, the number of patients with further hearing loss is large, so dexamethasone perfusion with intravenous dexamethasone should be tried first. The initial response to dexamethasone perfusion plus intravenous dexamethasone has been very good, with very little risk of further hearing loss, and it holds great promise for the future.

Extracellular adenosine 5'-triphosphate (ATP) in the endolymphatic compartment influences cochlear function

There is strong evidence for the presence of P2 purinoceptors on cochlear tissues, but the role of extracellular ATP in cochlear function is still unclear. Our previous studies have determined the presence of ATP in the cochlear fluids and indicated that the purinoceptors are substantially localized to the tissues lining the endolymphatic compartment. This implies that extracellular ATP may have an humoral role confined to the endolymphatic space. In order to study the influence of extracellular ATP in the endolymphatic space, a series of studies were undertaken in which ATP (10 microM to 10 mM) in artificial endolymph (EL) (test solution: 2-12.5 nl) was injected into the scala media and the effect on the cochlear microphonic (CM) and endocochlear potential (EP) evaluated. A double-barrelled pipette, with one barrel containing the test solution and the other artificial EL (control solution) was inserted into scala media of the third turn of the guinea-pig cochlea. A known volume (2-12.5 nl) of test or control solution was then pressure-injected into the space. ATP had a significant dose-dependent suppressive effect on both EP and CM with a threshold of approximately 2 x 10(-14) mol; the response was readily reversible, also in a dose-dependent fashion. Artificial EL of the same volume had no effect on EP and CM. The ATP effect on EP was blocked by the P2 purinoceptor antagonists suramin and reactive blue 2 (RB2). Neither adenosine (2 x 10(-13) to 2 x 10(-11) mol) nor suramin or RB2 on their own had any effect on EP and CM. This study provides the first evidence for an effect of extracellular ATP in the endolymphatic compartment on cochlear function which is mediated via P2 purinoceptors. This provides supporting evidence for an humoral role for extracellular ATP in the modulation of cochlear function.

Role of ATP-sensitive K+ channels in anoxia-sensitive negative potential of endolymph

To investigate the possible role of the ATP-sensitive K+ channels (KATP channels) in the generation of the anoxia-sensitive negative potential (ASNP), the effects of the treatment with glybenclamide and diazoxide on the endocochlear potential (EP) and K+ activity (AK) in the scala media were examined with double-barrelled K(+)-selective microelectrodes. The experiments were carried out in guinea pig cochleae, using glybenclamide as a KATP channel blocker and diazoxide as a KATP channel opener. Perilymphatic perfusion of glybenclamide decreased the amplitude of the ASNP and shortened its duration, whereas perfusion of diazoxide increased ASNP amplitude and prolonged its duration. Glybenclamide enhanced the decrease of endolymphatic AK by anoxia, whereas diazoxide suppressed this AK decrease. The results suggest that KATP channels may be involved in the generation of the ASNP.

Luminal non-selective cation and outwardly rectifying chloride channels in cultured strial marginal cells from gerbil

Ionic channels located on the luminal side of strial marginal cells (MCs) of gerbil in culture were investigated using the patch-clamp technique. Two types of channels were identified. The most frequently recorded single-channel activity corresponded to a non-selective cation (NSC) channel with a conductance of 23.7 +/- 0.2 pS (n = 18) in symmetrical NaCl conditions. The channel was activated by internal Ca2+ and inhibited by internal adenine nucleotides and flufenamic acid. Spontaneous activity of NSC channels was found in 16% of the cell-attached patches and with a very high density (9 +/- 2 levels/patch, n = 28) in 100% of the excised patches. An outwardly rectifying chloride (ORC) channel was also identified in 14% of the patches but only after excision. The channel exhibited at 0 mV a unit conductance of 26.8 +/- 1.3 pS (n = 8) and a strong outward rectification in symmetrical NaCl conditions, and the open probability increased with depolarization. The luminal NSC channel and the ORC channel evidenced in this study might participate in the production of endolymph. Although extrapolation of the presents results to the in vivo situation should be made with caution, this study suggests that culture of strial MCs may be a suitable model for investigation of endolymph physiology.

Effect of intravenously applied canrenoate on ampullar endolymphatic potential

The effect of canrenoate, an aldosterone antagonist, on the ampullar endolymphatic potential (AEP) was examined to assess a possible role for aldosterone in the ampulla. Intravenous administration of canrenoate increased the AEP amplitude in a dose-dependent manner, with a significant reduction of the AEP negative component induced by anoxia. Pretreatment with aldosterone attenuated the AEP change produced by canrenoate. The results suggest that aldosterone may be involved in the modulation of ampullar function.

Effect of cisplatin administration on the electrochemical composition of endolymph in the rat cochlea

The effect of cisplatin on the electrochemical composition of the cochlear endolymph was studied in Long-Evans rats three days after a single intraperitoneal injection (8 mg/kg b.w.). A dose 2/3 of LD50 induced a decrease of the endolymphatic concentration of potassium whereas the endocochlear potential was unaffected. The discrepancy between these two findings indicated that cisplatin did not alter the mechanisms involved in the genesis of the endocochlear potential but modified the passive K transport into endolymph.

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.

Mineralocorticoid type I receptor in the rat cochlea: mRNA identification by polymerase chain reaction (PCR) and in situ hybridization

Expression of mineralocorticoid type I receptor (MR) gene in the rat cochlea was determined using molecular biological techniques. We synthesized complementary DNA (cDNA) from rat cochlear total RNA and then amplified MR cDNA fragments by polymerase chain reaction (PCR). The amplified cDNA fragments were subcloned into an expression vector and the nucleotide sequence was analyzed to confirm the expression of mRNA encoding MR in the cochlea. We then synthesized digoxigenin-labeled riboprobes with this cloned DNA template and examined the localization of MR mRNA in the cochlea by in situ hybridization. The amino acid sequence of MR cDNA expressed in the cochlea was identical to that of the MR first cloned in the rat hippocampus. In situ hybridization showed the expression of MR mRNA in marginal cells of the stria vascularis, suggesting that aldosterone may regulate microhomeostasis of the endolymph, presumably by modulating Na, K-ATPase activity. Intense MR signal was also identified in spiral ganglion cells, the function of which remains to be determined.

The effect of nimodipine on cochlear potentials and Na+/K(+)-ATPase activity in normal and hydropic cochleas of the albino guinea pig

In experimental endolymphatic hydrops (EEH) a decrease in the endocochlear potential (EP) has been reported and is thought to be due to decreased activity of the enzyme Na+/K(+)-ATPase in the stria vascularis. By stimulating Na+/K(+)-ATPase, the EP, and thereby cochlear function as a whole, might be restored. On the other hand, stimulation of stria vascularis Na+/K(+)-ATPase might result in excessive production of endolymph and thus produce or augment hydrops. In this study we have investigated the effect of intraperitoneally applied nimodipine on cochlear potentials and on Na+/K(+)-ATPase activity in the stria vascularis, both in normal cochleas (control) and in cochleas with EEH. Nimodipine is an L-type Ca(2+)-channel blocking agent with Na+/K(+)-ATPase stimulating properties at concentrations as low as 1.5 nM. The compound action potential (CAP), evoked by 2,4 and 8 kHz tone bursts was found to be depressed in the EEH ears with and without nimodipine treatment, and in the nimodipine treated control ears. Statistical analysis (ANOVA) showed that the effects of EEH and nimodipine on the CAP were additive. The negative summating potential (SP), measured extracochlearly at the apex, in response to 4 and 8 kHz tone bursts was significantly enhanced in the EEH ears. Nimodipine treatment did not affect the SP, neither in the control, nor in the EEH ears. Cytochemically, Na+/K(+)-ATPase activity appeared to be decreased in the oedematous stria vascularis of hydropic cochleas. No effect of nimodipine on Na+/K(+)-ATPase activity could be established ultracytochemically, neither in the controls nor in the EEH ears. In the lower turns of some of the nimodipine treated control cochleas a mild hydrops was seen during light-microscopic evaluation. Although it was not possible to prove a stimulatory effect of nimodipine on the enzyme Na+/K(+)-ATPase cytochemically, the finding of mild endolymphatic hydrops in nimodipine treated control ears suggests (a history of) increased endolymph production. This hydrops might be responsible for the depression of the CAP in the nimodipine treated ears.

Pharmacokinetics of furosemide in endolymph

To investigate the pharmacokinetics of organic anions in the endolymph of the guinea pig, 100 mg/kg furosemide, an organic anion, was intravenously given to measure the concentration in the cochlear endolymph by high-performance liquid chromatography with fluorescence detection. In the endolymph, the concentration of the furosemide increased slowly for 1 hr to 1.6 micrograms/ml and gradually declined thereafter. Pretreatment with 200 mg/kg probenecid, an anion transport inhibitor, had no effect on the furosemide elimination in the endolymph except on the concentration at 2 hr. This was contrary to the drastic change observed in the perilymph of the scala tympani by the same pretreatment. Analogous to the effect in the endolymph, probenecid showed no change in the concentration of the serum, while a pronounced gradient of furosemide concentration existed between them. The present results suggest that the furosemide passively transfers from blood to the endolymph at a relatively low penetrability.

Furosemide VOR test for the detection of endolymphatic hydrops

In order to detect endolymphatic hydrops, we applied three neuro-otologic examination methods, including the furosemide VOR test, the glycerol dehydration test, and electrocochleography in 61 patients with unilateral Menière's disease, 21 patients with delayed endolymphatic hydrops, and 10 patients with syphilitic labyrinthitis. The positive rate of the furosemide VOR test in patients with endolymphatic hydrops was more than 50%. Inconsistencies were observed between the results of the furosemide VOR test, the glycerol test and/or electrocochleography. Therefore, combined test battery of these tests is useful clinically for detecting endolymphatic hydrops in the vestibular and cochlear systems.

Changes in the electrochemical composition of cochlear fluids after intracisternal application of doxorubicin in the rat

Since doxorubicin (Adriamycin) is known to bind on membranous negative surface charges, its effect on the electrochemical composition of the cochlear fluids was studied in rats. Doxorubicin was infused into the cerebrospinal fluid via the lateral cerebral ventricle. The endocochlear resting potential was recorded, and endolymph and perilymph of the scala vestibuli were collected from the basal cochlear turn before and 1, 2, and 4 h after the drug application. Na+, K+, and Cl- concentrations and osmolality of the endolymph and perilymph were measured in 1 nl aliquots. In perilymph, Cl- concentration increased 4 h after doxorubicin treatment to reach a concentration 8 mM higher than recorded initially. In endolymph, the endocochlear potential decreased by 5 mV/h while its K+ concentration and osmolality increased by about 3 mM/h and 4 mosmol/kg H2O per hour, respectively. These results suggest that the negative surface charges demonstrated on Reissner's membrane may play a role in the homeostasis of endolymph.

Cochlear blood flow and microvascular resistance changes in response to hypertonic glycerol, urea, and mannitol infusions

The effect of hyperosmotic agents on cochlear blood flow (CBF) was tested in normal guinea pigs and in guinea pigs having prior unilateral operations to ablate the endolymphatic duct. Laser-Doppler-measured CBF was normalized to remove apparent changes related directly to systemic blood pressure. Hyperosmotic fluids were given via venous infusion: glycerol (20% and 40% solutions), urea (10%, 30%, and 40% solutions), and mannitol (40% solution). All agents were dissolved in 0.9% saline and the mixtures were given at a rate of 0.3 to 0.6 mL/min for 5 minutes. Control infusions were of 0.9% saline and isotonic dextran 70 (Pharmacia). All hyperosmotic infusions resulted in similar increases in normalized cochlear blood flow (nCBF) that extended to a maximum of 300% of the baseline value in a dose-dependent way during the infusion time period. Within approximately 30 minutes following infusions, nCBF had returned to baseline levels. Saline infusion alone had little effect on nCBF, but isotonic dextran 70 gave a sustained increase to 122% of the baseline levels. There was no difference between the responses of nCBF in hydropic and normal cochleas for either control or hyperosmotic solutions. Measurements of systemic hematocrit at time intervals during and following the infusions showed that transient reductions of up to approximately 8% (for the maximum osmotic challenge) occurred during the infusion. It is concluded that the hyperosmotic treatments tested here are equally effective for short-term enhancements of nCBF in both normal and hydropic cochleas. The basis of the flow increase is partially rheologic and partially due to a local vasodilation.

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.

Effect of glycerol on pressure difference between perilymph and endolymph

The pressure difference between the perilymph and the endolymph following administration of 50% glycerol (12 ml/kg) was studied in guinea pigs. The perilymphatic and endolymphatic pressures were measured simultaneously with two sets of a servo-nulling system. Glycerol was administered for about one minute via a gastrocatheter. Both the perilymphatic and endolymphatic pressures began to decrease about 5 min after the administration of glycerol, and thereafter the decrease continued for about one hour with no significant difference between the two pressures. We concluded from the results that the glycerol-induced pressure difference between the perilymph and the endolymph, if present, is only a very small one, although a collapse of the scala media after glycerol intake was reported in guinea pigs and chinchillas.

[Dehydration therapy in low tone hearing loss. An alternative to rheologic therapy?]

Patients with acute low-tone sensorineural hearing loss were treated by rheology or dehydration after recording of the summation and compound action potentials by electrocochleography. Patients treated in the acute phase, as well as patients with chronic disease (that is the interval between onset of hearing loss and therapy was more than 1 week) clearly responded better to dehydration than to conventional rheological treatment. This was especially striking in those patients showing an enhanced SP/AP ratio indicating endolymphatic hydrops.

[Effect of potassium canrenoate on the EP and Na+, K+ activities in endolymph]

The perilymphatic space was perfused with artificial perilymph containing 5 x 10(-3)M potassium canrenoate. The EP, K+ and Na+ activities in the scala media were measured with double barrelled K+ or Na+ selective microelectrode. Following the onset of the perfusion, the EP gradually declined and was stable after about 20 minutes. K+ activity also declined gradually but Na+ activity was unchanged. When the EP became stable, the artificial ventilation was stopped. The EP dropped to a large negative potential and K+ activity decreased gradually, but Na+ activity increased by degrees. The same results were observed in the untreated animals when the ventilation was stopped. There are no pathological changes both in the TEM view of the stria vascularis and in the SEM view of the hair cell. These results suggest that this drug may affect K+ conductance of the stria vascularis specifically.

The preparation of acetic acid for use in otic drops and its effect on endocochlear potential and pH in inner ear fluid

The ototoxicity of an otic drop preparation containing 2% acetic acid and 3% propylene glycol (VoSol, Denver Chemical Co., Humacao, PR) was investigated according to measurements of endocochlear potential (EP) and inner ear fluid pH. The application of this preparation to the round window membrane for 30 minutes caused a depression in EP from 80.5 +/- 2.5 mV (mean +/- SD; n = 6) to 11.7 +/- 7.7 mV, and lowered inner ear fluid pH from 7.55 +/- 0.09 to 5.06 +/- 0.19 (n = 6) in perilymph and from 7.52 +/- 0.07 to 5.88 +/- 0.63 (n = 6) in endolymph. Two percent acetic acid produced similar changes after 30 minutes: EP was reduced from 83.0 +/- 2.2 mV to 34.0 +/- 2.9 mV and endolymphatic pH from 7.49 +/- 0.04 to 6.83 +/- 0.21 (n = 4). However, the application of artificial perilymph of pH 4 titrated with HCl induced no significant changes in either EP or endolymphatic pH. We suggest that the mechanisms of ototoxicity in the otic drop preparation are Na+ and K+-ATPase inhibition, and that such inhibition is due to the intracellular acidification of strial cells resulting from the penetration of acetic acid across the cell membrane, and to the direct and synergistic actions of propylene glycol.

Turnover of sulphated macromolecules in the murine endolymphatic sac after long-term kanamycin treatment

Radioactively labeled sulphur was injected into 12 mice of the NMRI strain 20 days after pretreatment with kanamycin. The animals were decapitated after intervals ranging from 2 minutes to 24 hours after injection. After a routine autoradiographic procedure, darkly stained silver grains were detected in the endolymphatic sac (ES) and its surroundings. One hour after injection, the grains were found in the surrounding blood vessels. Eleven hours later, maximal uptake was seen in the epithelial cells of the ES in the kanamycin-treated animals. Twenty-four hours after injection, a faint S35 uptake in the ES could still be detected. An increase in the ES activity, indicated by an increased number of free-floating cells and the secretion of a sulphur-containing intraluminal substance, did not occur. The spreading pattern of sulphur, as shown in this investigation, does not support the theory that waste products from the inner ear, transported to the ES by the longitudinal flow, produces an increased activity in the sac after long pretreatment with kanamycin. In 12 control animals, no difference in the distribution of labeling was observed, but there was an additional uptake in the free intraluminal cells which was not seen in the kanamycin group.

Sodium transfer from endolymph through a luminal amiloride-sensitive channel

An in vitro preparation of frog semicircular canal was devised to study the mechanisms of Na transport across the labyrinthine epithelium. When the lumen of the semicircular canal was filled with perilymph-like solution, the structure was able to secrete K into and to absorb Na from the lumen and to generate a lumen-positive transepithelial potential. When the lumen of the semicircular canal was filled with endolymph-like solution, the electrochemical composition of the luminal fluid was partly maintained up to 2 h. In this last experimental condition net and unidirectional fluxes were calculated in absence or presence of transport inhibitors, separately for the ampulla and for the nonampullar part of the canal. Amiloride (10(-5) M) but not dimethyl amiloride (10(-5) M) inhibited 60% of the unidirectional Na efflux out of endolymph; this Na efflux decrease resulted in an increase of the inward net Na flux. The net Na flux was also increased after abluminal application of ouabain (10(-3) M), furosemide (10(-4) M), and bumetanide (10(-6) M). This study validates this isolated preparation as a suitable tool for the study of endolymph secretion, confirms that the secretion of endolymph is achieved in the ampulla, and provides evidence for an apical amiloride-sensitive Na channel through which Na is transferred out of endolymph along an electrochemical gradient provided by the activity of the abluminal Na+-K+-ATPase.

Lack of effect of carbonic anhydrase inhibition on direct measurements of endolymph bicarbonate

In the past, sodium, potassium, and chloride have been measured in endolymph directly, but bicarbonate has been measured only indirectly. We sampled endolymph directly while monitoring endocochlear potentials in normal and methazolamide-treated guinea pigs. Bicarbonate was determined in samples by use of a method that depends on reduction of NADH to NAD linked to malate formation from oxaloacetate. In 11 normal animals, the bicarbonate in endolymph was 20.2 mM +/- 4,4 mM (mean +/- standard deviation); in six of these, plasma bicarbonate was 23.1 mM +/- 3.5 mM. Nine animals treated with methazolamide (carbonic anhydrase inhibitor) had an endolymph bicarbonate of 19.5 mM +/- 3.9 mM; plasma bicarbonate in five of these was 25 mM +/- 3.2 mM. Carbonic anhydrase inhibition did not significantly affect endolymphatic bicarbonate levels.