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.

Differential effect of the loop diuretic furosemide on short latency auditory and vestibular-evoked potentials

OBJECTIVE

This study aimed to investigate the differential effect of the loop diuretic furosemide on the auditory and vestibular (otolith) end organs in the same animals simultaneously.

DESIGN AND METHODS

Auditory nerve-brain stem-evoked responses (ABR-generated in the cochlea) and short latency vestibular-evoked responses to linear acceleration impulses (L-VsEP-generated in the otolith organs) were recorded from albino Sabra rats both before and at minute intervals after intravenous injections of the loop diuretic furosemide. In some animals, an equal volume of saline was injected to control for the effect of the injection itself. In most animals, more than one injection of saline or furosemide was possible (furosemide, N = 17 injections in 10 rats; saline, N = 18 injections in 9 rats). Peak-to-peak amplitude and peak latency changes in the first wave in each recording (representing end-organ activity) as a function of postinjection time were compared between the two evoked potentials using analysis of variance and repeated t-tests.

RESULTS

Saline injections caused only minor changes in the amplitude of the ABR and the L-VsEP. After administration of furosemide, the amplitude of the L-VsEP hardly changed. However, there was a noticeable decrease in the amplitude of the ABR.

CONCLUSIONS

Although furosemide has a major depressant effect on cochlear function, vestibular end-organ activity is hardly altered.

pH regulation of the globular substance in the otoconial membrane of the guinea-pig inner ear

Physiological and pharmacological characteristics of the globular substance, a precursor of otoconia, are not well understood. In the present study, we investigated the variations and regulation of internal pH (pHi) of the globular substance of the guinea-pig inner ear. The otoconial membrane was dissected out from the utricular macula and loaded with the pH-sensitive fluorophore, carboxyseminaphthorhodafluor-1. Dynamic changes of fluorescence were directly observed under a confocal laser scanning microscope, and pHi was calculated from dual emission ratio. In the NaCl standard solution buffered with 5 mM HEPES/Tris at pH 7.4, the pHi of the globular substance varied from 6.26 to 8.55 with an average of 7.21 (n = 270). Exposure to 25 mM NH+4 induced a rapid increase of the pHi followed by a slow relaxation. The, wash-out of NH+4 caused a prompt and pronounced acidification followed by a gradual pHi recovery to the initial level. This gradual pHi recovery was significantly inhibited by the absence of external Na+, indicating the presence of an external Na(+)-dependent H+ extrusion mechanism. This pHi recovery was also inhibited by 1 mM amiloride and 10 microM 3-amino-N-(aminoiminomethyl)-6-[ethyl(2-propyl)amino]pyrazine-2-ca rboxamide. These results suggest the presence of an Na(+)-H+ exchanger in the globular substance of the guinea pig. However, HCO3(-)-transporting mechanisms were not determined. The working hypothesis for the otoconial formation is discussed.

Why do benign paroxysmal positional vertigo episodes recover spontaneously?

It is well known that most episodes of benign paroxysmal positional vertigo (BPPV), even in untreated, recover spontaneously in 2 to 6 weeks. In the present study, we put forward the hypothesis that this is mainly due to the fact that endolymph, owing to its low calcium content (20 microM) is able to dissolve otoconia. To support this, the fate of frog saccular otoconia immersed in normal endolymph (Ca2+ content 20 microM) and in Ca2+-rich endolymphatic fluids (up to 500 microM) was studied by observing the crystals at regular intervals for 3 weeks. The results demonstrated that normal endolymph can dissolve otoconia very rapidly (in about 20 hours). When the endolymphatic Ca2+ content was increased (50 to 200 microM) otoconia dissolution time was slowed down (about 100 to 130 hours, respectively) and completely stopped when the endolymphatic Ca2+ content was of 500 microM. The present results therefore suggest that the major process involved in the spontaneous recovery of BPPV episodes is the capability of the endolymph to dissolve dislodged otoconia.

P2 purinoceptor of the globular substance in the otoconial membrane of the guinea pig inner ear

The biological characteristics of the globular substance, a precursor of otoconia, are unclear. In the present study, the ATP-induced internal free Ca2+ concentration ([Ca2+]i) changes of the globular substance and the ATP distribution in the vestibular organ were investigated using a Ca2+ indicator, fluo 3, and an adenine nucleotide-specific fluorochrome, quinacrine, by means of confocal laser scanning microscopy. [Ca2+]i showed a rapid and dose-dependent increase in response to ATP with a 50% effective concentration (EC50) of 16.7 microM. This reaction was independent of external Ca2+, indicating the presence of an internal Ca2+ reservoir. Neither adenosine, alpha, beta-methylene-ATP, 3'-O-(4-benzoylbenzoyl)-ATP, ADP, nor UTP evoked this reaction, whereas 2-methylthio-ATP induced an increase of [Ca2+]i with an EC50 of 14.4 microM. Moreover, P2 antagonists, reactive blue 2 and suramin, and a phospholipase C inhibitor, U-73122, inhibited the ATP-induced [Ca2+]i increase. These findings indicate the presence of a P2Y purinoceptor on the globular substance. In addition, granular fluorescence was observed in the quinacrine-stained macular sensory epithelium, indicating the presence of ATP-containing granules in this tissue. These results suggest that a paracrine mechanism involving ATP may exist in the macula and that this mechanism regulates the biological behavior of the globular substance.

Effect of streptomycin on the otoconial layer of the guinea pig

The effects of streptomycin on the otoconial layer of the adult guinea pig were investigated using scanning electron microscopy. Administration of streptomycin induced a reduction of otoconia with formation of giant otoconia. These phenomena reached their maximum 4 weeks after the cessation of streptomycin intoxication in the utricle and between 2 and 6 weeks after in the saccule. Otoconia began to recover in the utricle 4 weeks and in the saccule 6 weeks after the cessation of streptomycin intoxication with an increasing number of small otoconia. Eight or 10 weeks after the cessation of streptomycin intoxication, the otoconial layer has completely recovered in both the utricle and saccule.

[Formation and calcium incorporation of giant otoconia of the guinea pig after streptomycin intoxication]

The mechanisms for the formation and fate of giant otoconia following streptomycin (SM) intoxication were investigated in adult pigmented guinea pigs by scanning electron microscopy. Calcium turnover into otoconia has also been studied by using tetracycline as a tracer. The administration of SM induced the reduction of otoconia with the formation of giant otoconia. The giant otoconia had a multifaceted morphology in their early developmental period. This type of otoconia showed entire fluorescence indicating existence of calcium uptake. They then grew up to the transitional type and finally to the cylindrical type. The giant otoconia were thought to be formed mainly by dissolution of normal otoconia due to the loss of environmental calcium followed by recrystallization as giant crystals. The transitional type of giant otoconia showed less calcium ion uptake and the removal of calcium from the giant otoconia caused their quick disappearance. These phenomena might be closely related to the otoconial dynamics which may regulate calcium ion homeostasis of endolymph.

Formation and fate of giant otoconia of the guinea pig following streptomycin intoxication

Formation and fate of abnormal (giant) otoconia of the guinea pig following streptomycin intoxication were investigated using scanning electron microscopy. The giant otoconia formed as multifaceted morphology in their early developmental period. They grew up the the transitional type and finally to the cylindrical type. It has been suggested that the giant otoconia found following streptomycin intoxication may be formed mainly by dissolution of normal otoconia due to the loss of environmental calcium, followed by recrystallization as giant crystals. These phenomena seemed to be closely related to the otoconial dynamics which may regulate calcium ion homeostasis of the endolymph.

In vitro calcium ion turnover in otoconia of the guinea pig

In vitro calcium ion turnover into otoconia of adult guinea pigs was investigated by the use of tetracycline. The incubation in tetracycline in vitro clearly demonstrated the deposition of tetracycline (fluorescence) on the outer surface of otoconia. Levels of Ca2+ uptake in utricular otoconia were higher than those in saccular otoconia. In vitro uptake and/or exchange of Ca2+ was greater in fluid with a high K+/Na+ ratio than in fluid in which the ratio was low. It has been thus indicated that a high K+/Na+ ratio in normal endolymph should be more suitable for Ca2+ uptake and/or exchange to maintain the ionic environment in the inner ear. Sodium dodecyl sulfate treatment of otoconia increased the Ca2+ uptake. The ability of the otoconia to take up Ca2+ appears to decline with age. It has been indicated that the organic matrix of otoconia may be one of the inhibitory factors for Ca2+ uptake in old animals.

Incorporation of tetracycline into otoconia of the guinea-pig following streptomycin intoxication

The effects of streptomycin on the calcium ion turnover into otoconia of adult guinea-pigs investigated by the use of tetracycline. The oral administration of tetracycline induced the deposition of tetracycline (fluorescence) on the outer surface of otoconia indicating the existence of dynamic exchange and/or uptake of calcium ions in the otoconia. The significant finding is that streptomycin specifically interfered with calcium uptake into the otoconia which indicated that the decrease in calcium uptake caused by streptomycin may be closely related to the loss of otoconia as well as to a decrease in the calcium contents of otoconia. The decrease in calcium incorporation into otoconia caused by streptomycin was recovered within 6 weeks after the last injection of streptomycin. The number of otoconia with fluorescence in the central portion as well as their outer surface was increased. It is therefore suggested that the recovery of calcium uptake as well as new otoconial regeneration may play an important role for the recovery from loss of otoconia.

Carbohydrate distribution in the living utricular macula of the guinea pig detected by lectins

Carbohydrate distribution in the fresh utricular macula of the guinea pig was analysed using lectins such as Concanavalin A (ConA), Dolichos biflorus agglutinin (DBA), peanut agglutinin (PNA), soybean agglutinin (SBA), Ulex europeus agglutinin (UEA-1) and wheat germ agglutinin (WGA) by means of confocal laser scanning microscopy. The ciliary bundle was strongly reactive to ConA, PNA, SBA and WGA but not to DBA and UEA-I, showing that the ciliary bundle has abundant D-galactose (GaI), N-acetyl-D-glucosamine (GlcNAc), D-mannose (Man) and sialic acid(s) (Sia) but not detectable amounts of L-fucose (Fuc) and terminal N-acetyl-D-galactosamine (GalNAc). Similar patterns of lectin bindings with moderate-to-weak intensities were observed on the non-cilial apical surface, on the surface of the otoconia and in the gelatinous layer of the otoconial membrane. On the contrary, the globular substance, a precursor of the otoconia, was scarcely reactive to any lectin examined, implying that it lacks glycoconjugates on its surface. Previous histochemical studies reported that the otoconial membrane possesses a much higher affinity for lectins that does the sensory epithelium (including the cilia) in the vestibular organ. This discrepancy suggests that factors in the preparation process may affect the otoconial membrane or the surface coat of the cilia to change their lectin affinity. Meanwhile, sialidase treatment augmented the affinity of the ciliary bundle for DBA and PNA, indicating that sialylated GalNAc and Gal are present on the vestibular ciliary bundle.

Electron probe microanalysis of gentamicin-induced changes on ionic composition of the vestibular gelatinous membrane

Gentamicin-induced changes in ionic composition in the otolithic membrane of adult OF1 mice were evaluated in the gelatinous layers of the saccule and utricle by quantitative electron probe X-ray microanalysis. The otolithic membranes were plunge-frozen and freeze-dried to prevent the redistribution of elements. Quantitative analysis was carried out with an energy dispersive detector using the peak-to-background (P/B) ratio method and different salts dissolved in dextran as standards to calibrate the P/B ratio against the concentration of the elements P, S and K in the microprobe. Gentamicin selectively decreased the concentrations of P (P < 0.001) and S (P < 0.01) in the gelatinous membrane of the saccule, and had no effect in the utricle. The concentration of K also increased in the utricular gelatinous membrane (P < 0.05). The mechanism of ototoxicity in the gelatinous membrane is unknown, but the ability of aminoglycosides to block calcium channels may induce disturbances in the ionic equilibrium of the endolymphatic fluid, and thus affect the biochemical composition of the gelatinous membrane. This technique can be useful to evaluate the distribution of ions in the process of drug-induced ototoxicity.

Gentamicin ototoxicity in otoconia: quantitative electron probe X-ray microanalysis

Chronic gentamicin ototoxicity was evaluated in the otolithic membrane of adult OF1 mice at the otoconial layer of the saccule and utricle by quantitative electron probe X-ray microanalysis of Ca and K. The otolithic membranes were plunge-frozen and freeze-dried. The analysis was carried out with an energy dispersive detector using the peak-to-back-ground ratio method and different inorganic salts of Ca and K as standards to calibrate the microprobe. Ca and K in the otoconia are related via a linear function in both the saccule and the utricle. This association is not maintained after exposure to gentamicin, which suggests that this aminoglycoside antibiotic interferes with the Ca-K equilibrium in the otoconia. A dose of 200 mg/kg gentamicin twice a day for 5 days did not affect Ca in the mineral phase of the otoconia, but did increase K in both saccular (p < 0.05) and utricular (p < 0.01) otoconia. These increases in K may reflect a modification in the composition of the endolymph, resulting from cellular damage at the plasma membrane.

Effects of carbonic anhydrase inhibitor on the otolithic organs of developing chick embryos

Carbonic anhydrase appears to be involved in the process of otoconial formation. The purpose of this investigation was to observe the morphologic change in the surface structure of the otolithic organ in developing chick embryos after injection of the carbonic anhydrase inhibitor, acetazolamide. Acetazolamide (1.5, 3, or 6 mg/0.06 mL/egg) was injected into the yolk sac of the embryo of the fifth day of incubation. Embryo specimens were collected on the 11th, 13th, and 18th days of incubation. The chicks were killed on the third day posthatching, and the surfaces of the otolithic organs were observed under a scanning electron microscope. A marked disturbance in otoconial formation was noted in both utricle and saccule, marked by a decrease or absence of otoconia. A widely exposed meshwork structure of otolithic membrane was observed, with sensory cilia penetrating the meshwork small holes in many instances. There were also several otoconial abnormalities, such as the appearance of only a single giant otoconium, or from several to dozens of giant otoconia, and rough, spongy-surfaced global substances entirely covering the maculae. Clearly, carbonic anhydrase inhibitor (acetazolamide) injected into the yolk sac of developing chick embryos alters and inhibits normal otoconial morphogenesis.

Bilateral vestibular paresis: diagnosis and treatment

Patients with bilateral peripheral vestibular dysfunction present a substantial diagnostic and therapeutic challenge to the otologist. Caloric irrigations that suggest bilateral weakness may mislead the clinician when considerable peripheral function remains. Rotary chair testing at several frequencies is helpful in confirming the degree of residual function and may identify patients who have a better prognosis in rehabilitation. Forty-nine patients with bilateral vestibular paresis were evaluated. The diagnosis was based on caloric responses of less than 5 degrees/sec in both ears and reduced gain on rotary chair trials. Oscillopsia occurred in the minority of patients (35%) and was rare unless severe paresis was present. Episodic vertigo occurred in 43%. All patients had abnormal posturography results, with vestibular dysfunction and severe dysfunction patterns predominating. The treatment outcomes of 22 patients who underwent vestibular rehabilitation are discussed. Prevention of bilateral vestibular paresis remains a high priority.

Effects of dinocap on otolith development: evaluation of mouse and hamster fetuses at term

The morphology of otoliths in CD-1 mouse and Syrian hamster fetuses exposed to the fungicide dinocap were evaluated at the end of gestation. Pregnant mice were dosed by gavage with 0, 10, 15, 30, or 60 mg/kg/day dinocap in corn oil on days 7-16 of gestation. Pregnant hamsters were dosed by the same route with 0, 50, 100, or 200 mg/kg/day on days 7-14 of gestation. At the end of gestation (day 18 in mice, day 15 in hamsters) dams were killed and all fetuses were removed and fixed overnight in 70% ethanol. Fetal heads were then removed, left in 70% ethanol for at least 3 days, and then dehydrated in a graded ethanol series and cleared with methyl salicylate. Otoliths were examined by darkfield microscopy, and each otolith was scored for morphological completeness on a scale of 0 to 3. Otolith development was complete by day 18 of gestation in control mouse fetuses. Otolith development was complete in many, but not all, of the hamster fetuses by day 15 of gestation. In the mouse, dinocap exposure inhibited fetal otolith formation in a dose-related manner, with a significant effect on total otolith score occurring at 10 mg/kg/day and above. Dinocap affected otolith formation in the hamster only at 100 mg/kg/day (200 mg/kg/day was embryolethal), concomitant with severe maternotoxicity and fetotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Preliminary findings of a reduction of otoconia in the inner ear of adult rats prenatally exposed to phenytoin

Pregnant Sprague-Dawley CD rats were administered phenytoin by gavage on days 7-18 of gestation in doses of 0 or 200 mg/kg. Following completion of a series of behavioral tests (20), progeny (18 months of age) were examined for otoconia in the vestibular labyrinth of the inner ear. None of the controls (N = 22) had reduced otoconia, while 23.3% of phenytoin-treated offspring (N = 43) had reductions. None of the controls, but 44.2% of phenytoin-treated offspring exhibited abnormal circling behavior during systematic examinations conducted in dry and swimming environments. Of the phenytoin-treated offspring exhibiting circling, 21.0% had reduced otoconia in either the utricle or saccule of one ear, while 25.0% of phenytoin-treated offspring not circling exhibited similar reductions. Conversely, 79% of phenytoin-treated offspring exhibiting circling did not exhibit any otoconial reductions. Thus, otoconial reduction cannot account for the majority of the cases of circling. The 21% vs. 25% otoconial reduction difference was not significant, however, when ratings of the magnitude of reduction were analyzed, circling offspring had significantly lower scores in their utricles than those not circling. More specifically, otoconial reduction in the right utricle and circling behavior were significantly related, although the number of concordant cases was small. Otoconial ratings did not differ for saccules. No differences in regional brain weights were found at the time of otoconial examination (560 days). The evidence provide preliminary support for the idea that prenatal exposure to phenytoin induces a reduction in otoconial crystals of the vestibular labyrinth in some of the exposed offspring, but it cannot account for most of the behavioral effects that have been observed in these offspring.

Prenatal dinocap exposure alters swimming behavior in mice due to complete otolith agenesis in the inner ear

Exposure to the fungicide dinocap during gestation produces behavioral abnormalities in the house mouse that are not apparent at birth but become obvious at weaning. Pregnant mice (CD-1) were exposed on Days 7 to 16 of gestation to dinocap at 0, 6, 12, or 25 mg/kg/day and the postnatal behavioral development of the offspring was assessed. Torticollis (head-tilting) appears in the treated offspring at 3 weeks of age (4.4% at 12 and 25.3% at 25 mg/kg/day) and, during a test of swimming ability, many of the mice (6.8% at 12 and 47.2% at 25 mg/kg/day) sink below the surface or are unstable and swim on their side in the water. These behavioral abnormalities are the result of agenesis of the otoliths in the inner ears. These were the only developmental defects noted in the 12 mg/kg/day dosage group. In this group 4.4% of the mice displayed torticollis, 9.2% did not swim normally, 19% were missing one or more whole otoliths (7.7% were missing all four otoliths), and partial agenesis of the crystalline material was seen in an additional 11.6% of the mice. The frequency of behavioral and inner ear defects increased in the higher dosage group, but the order of sensitivity of the effects did not change.

Scanning electron microscopic studies of the membranous labyrinth after transtympanic infusion of local anesthetics (lidocain)

Scanning electron microscopic observation of the transtympanic infusion of 10% xylocain revealed twisting and disorders of outer hairs especially from the 2nd and 3rd rows of the basal to the 2nd turn of the organ of Corti, and adhesion and fusion of the sensory hairs of the crista ampullaris and otolith organ 1 and 4 days after injection. Four percent xylocain however demonstrated very minor alteration of the sensory hairs and no missing sensory hairs were detected from any cases. The authors hypothesized from electron microscopic observation that adhesion and fusion of vestibular sensory hairs or twisting and disorders of the sensory hairs of the organ of Corti might act to intercept harmful circulation of exclusion of abnormal excitement of hair cells and the innovation of electrical discharge of sensory hairs due not only to the pharmacological effects of xylocain but also to osmotic pressure.

Otoconia and neural junctions of type I hair cells in amikacin-treated guinea pigs presenting saccular acoustic responses

Using the experimental guinea pig model, in which total and selective destruction of the cochlear receptor revealed saccular acoustic responses, ultrastructural observations were made on otolithic organs using electron microscopy. Scanning electron microscope studies of otoconia from the saccule and utricle of experimental animals did not show any differences with control animals. Quantitative and qualitative observations of sensory neural junctions of Type I cells also did not show any abnormality. These results confirm the extremely selective toxicity of amikacin for the cochlea, and thus strongly suggest that an acoustic reception by the saccule is a normal feature of the guinea pig inner ear.

Normal and altered otoliths of guinea pigs. Scanning electron microscopy observations

Normal morphology and structural alterations of the otoliths were observed in 20 young albino guinea pigs. Ten animals had been exposed to long-term treatment with an aminoglycoside antibiotic and ten others, which had not been treated, have been analyzed as a control group. The observations were carried out using scanning electron microscopy. All the animals survived the period of stabling and treatment. They were sacrificed and the temporal bones were dissected and fixed with 2% osmium tetroxide buffered at pH 7.4. The inner ear was subsequently dehydrated, microdissected, dried with CO2, and coated with gold. Different kinds of deterioration were observed and valued according to the appearance of the otoliths, which looked as if they had been melted, with many pores and wide and prominent cavitations probably due to chemical etching.