Endocochlear potential and scala media oxygen tension during partial anoxia

The endocochlear potential and the scala media oxygen tension were measured from the basal coil of the guinea pig cochlea during levels of partial respiratory anoxia. This was accomplished by simultaneously positioning two electrodes (for endocochlear potential and PO2) into the endolymph by a transbasilar membrane approach, in order to minimize blood capillary damage. The intracochlear pO2 was stable until the respiratory oxygen concentration was reduced to 16 per cent on the average. Thereafter, a further reduction of oxygen in the respiratory mix reduced both the endolymph pO2 and the endocochlear potential. The coupling between the endocochlear potential and endolymph pO2 is proportional over a 70 to 80 per cent reduction in endolymph pO2, and at this point the endocochlear potential is reduced by 50 per cent. For partial levels of respiratory oxygen that are not too low (e.g., > 12 per cent oxygen) it is possible to maintain partial levels of intracochlear pO2 for at least six minutes. During this time period the endocochlear potential is also held at a stable but reduced level.

Mechanism of the Ménière attack

A survey of experimental evidence showing the influence of an increase in potassium concentration in the perilymph on cochlear and vestibular nerve branches is discussed. Informations have indicated that a concentration lower than 20--30 mM produced an increase in action potential frequency in nerve preparations and that a low potassium concentration in animal experiments produced an ipsilateral nystagmus. Increasing the concentration of potassium in the endolymph to values not over the normal endolymph concentration, blocked the conduction of action potentials in the vestibular nerve branches resulting in a reversible contralateral nystagmus. From these results it may be concluded that all objective symptoms of a Ménière attack can be experimentally reproduced by one single factor: the potassium concentration in the perilymph. The known ruptures in the distended walls in a hydrops, with a diffusion of endolymph potassium into the perilymph surrounding the nerve branches to the vestibular and cochlear areas have consequently been assumed to produce the basis for Ménière attacks.

Effects of exposure to noise on ion movement in guinea pig cochlea

Healthy guinea pigs were exposed to broad band noise at levels between 95 and 115 dBA for 7 days. A significant decrease of the sound-induced cochlear responses, together with a substantial increase of the endocochlear potential, was observed in guinea pigs exposed to noise at 105 or 115 dBA. Microsamples of the endolymph obtained from these guinea pigs showed a significant increase of K+ and Cl- concentrations and a decrease of Na+ concentration, when compared with those from control animals. The K+, Na+ and Cl- concentrations in the perilymph were not markedly affected by noise exposure. When the perilymphatic space was perfused with artificial perilymph containing 43K, 22Na or 36 Cl, the uptake of radiotracers into the endolymph showed a single exponential function of the perfusion time. When compared with rate constants in normal animals, the value of rate constant for K+ was significantly decreased in animals exposed to noise. These results indicate that ionic permeability changes of the endolymph-perilymph barrier are a significant factor in the physiological mechanisms underlying noise-induced hearing loss.

The nature of the negative endocochlear potentials produced by anoxia and ethacrynic acid in the rat and guinea-pig

1. The alterations in the Na+ and K+ concentrations of the cochlear endolymph and in the endocochlear potential were followed simultaneously by means of ion-sensitive and conventional micro-electrodes during simple anoxia, during anoxia after i.v. ethacrynic acid and after i.v. ethacrynic acid alone. The endolymphatic pH changes were measured separately and the effect of perilymphatic ethacrynic acid upon the endocochlear potential was investigated. 2. The over-all Na+:K+ permeability ratio for the endolymph system was determined in individual animals for the first time using an indirect method. The normal mean values of 0.27 (rat) and 0.38 (guinea-pig) were increased after ethacrynic acid. Permeability changes occurred during anoxia but were delayed in onset. 3. The negative endocochlear potentials in each situation behaved quantitatively like modified K+ diffusion potentials largely dependent upon the K+ and Na+ gradients between endolymph and perilymph.

Variation of endocochlear PO2 and cochlear potentials by breathing carbon dioxide

The effect of carbon dioxide on oxygen dioxide tension in the endolymph was determined by the micropolarographic technique. Different concentrations (5% and 10% CO2) and different exposure times (3, 5, and 20 minutes) were investigated. The highest levels of PO2 in the endolymph (101.7, 93.9 and 69.5 mm Hg) were accomplished by respiration of 10% CO2, 90% O2, for 20, 5 and 3 minutes consecutively. The lowest PO2 increase, 50.7 mm Hg was observed after breathing 5% CO2, 90% O2 for 20 minutes. Extreme hypercapnia caused an increase of endocochlear potentials (EP) in all groups. In the second group EP increased from +79.3 to +84.9 and in all groups they had returned to the pretreatment level after CO2 discontinuation. These results support the theory that carbonic anhydrase participates in the generation of EP. At the same time that EP increased, cochlear microphonics declined and opposite after the breathing mixture was discontinued. The results permit the conclusion that high levels of PO2 in endolymph is achievable even with short periods of respiration with high CO2 mixture, and suggest the role of carbonic anhydrase during EP generation.

The uptake of methyl mercury in guinea pig cochlea in relation to its ototoxic effect

Guinea pigs were treated for 7 days by daily subcutaneous injection of methyl mercury chloride labeled with 14C, the total dose of which was 17 mg Hg/kg. In these animals the cochlear microphonics and whole-nerve action potentials were suppressed in the basal turn but there was no marked losses in the third turn of the cochlea. The endocochlear potential was not decreased in magnitude. At the end of the treatment there was no accumulation of mercury in the perilymph, endolymph and cerebrospinal fluid. Uptake and elimination of mercury in the cochlear fluids were studied in guinea pigs which were treated by a single intravenous injection of 203Hg-labeled methyl mercury, the dose of which ranged from 0.2 to 17 mg Hg/kg. The results indicated that mercury concentration ratio of the blood relative to cochlear fluids was comparable with the blood to plasma ratio reported previously. In contrast to lack of accumulation in the extra cellular environment, it is likely that tissues of the sensory end organs in the cochlea accumulated methyl mercury.

The effects of ethacrynic acid upon the potassium concentration in guinea pig cochlear fluids

After i.v. injection of 50 mg/kg ethacrynic acid (EA), potassium concentration in the endolymph (Ke+) measured with K+-specific microelectrodes decreases by 10 mM at the most and endocochlear potential falls to negative values. Potassium concentration in the perilymph (Kp+) generally does not change, but sometimes a transient decrease in Kp+ level of about 0.5 mM was observed, presumably due to the electrogenic effect of the time-related decrease of the endocochlear potential. When anoxia is induced approximately 120 min after EA administration Ke+ slowly decreases. The decrease in Ke+ 50 min after the arrest of ventilation is smaller when compared with the Ke+ anoxic decrease without preceding EA administration. The endocochlear potential, which falls to negative values during anoxia after EA administration, does not return to the zero level as in the case when only anoxia is applied. Similarly, during anoxia, which follows EA administration, the perilymphatic Ke+ concentration increases more slowly than in the case when only anoxia is introduced. It is assumed from the results that EA abolishes activity of the positive electrogenic K+ pump and reduces the passive permeability of the walls of the cochlear duct to the potassium ions.

A correlation of the effects of normoxia, hyperoxia and anoxia on PO2 of endolymph and cochlear potentials

Change in PO2 in endolymph, endocochlear potentials and cochlear microphonics have been tested in normoxia, hyperoxia and anoxia on 24 guinea pigs. The polarographic method and construction of oxygen-sensitive microelectrodes is described in detail. The normal level of PO2 in endolymph vaires between 20 and 30 mm Hg. One-minute anoxia induced by breathing 100% N2 caused a decline in PO2, EP and CM, but during recovery only PO2 returned with over-correction to the preexposure level. Hyperoxia evoked by breathing 100% oxygen failed to increase the cochlear potentials and the PO2 in the endolymph. This suggests that vasoconstriction most likely occurs proximal to the capillaries bed of the stria vascularis.

Current status of surgical decompression and drainage procedures upon the endolymphatic system

At the present time, neither the etiologic factors nor the pathophysiologic mechanisms that lead to the development of endolymphatic hydrops or Ménière's disease are known; however, the anatomic-pathologic alterations in the labyrinth that are produced by the disease are well-documented. Undoubtedly, what is not known is responsible for the controversial status and questionable benefit that surgical decompression and drainage procedures have upon the endolymphatic system.

Pathology of Ménière's disease as it relates to the sac and tack procedures

The rationale for endolymphatic sac and tack operations is to prevent the further accumulation of endolymph, the former by draining endolymph into extralabyrinthine tissues and the latter by shunting endolymph into the perilymphatic space. While the basic concepts are reasonable enough, the probability of achieving these objectives seems remote. There appears to be adequate clinical evidence, however, to show that these procedures are of therapeutic value in selected cases. It is tempting to speculate that these surgical insults to the labyrinth, with the associated inflammatory and biochemical changes, alter the function of cells which control fluid physiology and thus improve the symptoms of Ménière's disease.

Physiology and chemistry of cerebrospinal fluid, aqueous humor and endolymph in Squalus acanthias

By means of the appropriate isotopes injected into the spiny dogfish, Squalus acanthias, the transfer of all major ions into cerebrospinal fluid (CSF), aqueous humor (A) and endolymph (E) was studied. In addition, the effect of raising pCO2 in sea-water upon HCO3- concentration of these fluids was measured. In the several types of experiments, acetazolamide or methazolamide was used to inhibit completely carbonic anhydrase. The rates of fluid formation and ion transfer in CSF and A were fairly close, but those for E were far slower. The general pattern of ion transport in the three fluids were the same, Na+ (or Na+ + K+ in E) entry greater than Cl - entry, and the difference was HCO3-. The greater rate constants for HCO3-, increase in its entry rate by elevation of pCO2, and inhibition of its appearance by the sulfonamides, show that this is a special case of transport; the ion is formed in secretory cells from gaseous CO2 + OH-. Secretory cells at sites of formation of all the fluids contain both carbonic anhydrase and Na+-K+-ATP-ase, which subserve HCO3- formation and Na+ (or K+) transport. Comparison of these results with studies in mammals show that the vertebrate pattern for secretion of these three fluids is well established in the elasmobranch.

Time course of anoxia-induced K+ concentration changes in the cochlea measured with K+ specific microelectrodes

The endocochlear potential (EP), potassium concentration in the endolymph (Ke+) and in the perilymph (Kp+) were measured in guinea-pigs during anoxia of different duration. Specific K+ double-barrel microelectrodes with liquid ion exchanger were used. The resting K+ concentration in the endolymph was 146.8 +/- 9.2 mM and in the perilymph 3.2 +/- 0.5 mM. The following time course of events was observed in the cochlea during anoxia: 40-50 s after the arrest of ventilation the K+ concentration decreased by 0.1-0.2 mM in the scala vestibuli, which was time related to a rapid fall of EP to negative values. Perilymphatic K+ started to increase in both scalae with a latency of 2-2.5 min, reaching a concentration of about 14 mM 60 min after the arrest of ventilation. The endolymphatic K+ began to decrease after a latency of 2.5-3 min, and 60 min after the arrest of ventilation an 80% concentration (average 112 mM K+) was reached as compared to the initial value. From the comparison of K+ concentration changes with the experimental values of the negative EP, it may be assumed that the negative EP is mainly generated by the K+ gradient between the perilymph and endolymph.

An experimental model for the study of endolymphatic hydrops in sharks: implications for the clinician

In recent years, attention has focused on the role of the endolymphatic sac (ELS) and the endolymphatic duct (ELD) in the pathogenesis of endolymphatic hydrops (ELH). Changes have been noted surgically and radiographically by others in the ELS and ELD in patients with ELH. This report summarizes the development of a shark model with which to study the pathophysiology of ELH. The background material and overall results of anatomic, histopathologic, and ultrastructural studies using the model are presented. Possible implications for the clinical handling of ELH as a result of this work is emphasized.

Pharmacokinetics and ototoxicity of gentamicin, tobramycin, and amikacin

The pharmacokinetics of gentamicin, tobramycin, and amikacin in inner ear fluids, serum, cerebrospinal fluid, and the compartments of the eye were studied and compared in guinea pigs. The concentrations of antibiotic were determined by microbiologic methods and were confirmed by the use of 14C-labeled gentamicin. Retention was clearly demonstrated in perilymph, in which the half-lives of gentamicin, tobramycin, and amikacin were 12, 11, and 10 hr, respectively. The concentrations of drug in perilymph were symmetrical and were many times higher than the concentrations of antibiotic in the brain. There was no difference between the concentration of drug in endolymph and that in perilymph. A linear relation between concentrations in the perilymph and the dosage of gentamicin was ascertained. Long-term treatment did not influence the pharmacokinetics of the three antibiotics in the inner ear. However, increased levels of drug in the inner ear in animals with uremia and in some animals with otitis media explained the increased ototoxicity that occurs in treatment of these two conditions. Suboccipital puncture and diuresis did not change the concentrations of aminoglycoside antibiotics in the inner ear. Antibiotics applied locally in the middle ear had high degrees of ototoxicity.

Observations on the stria vascularis of the guinea pig cochlea and the changes resulting from the administration of the diuretic furosemide

An electron microscope examination of the stria vascularis of guinea pigs and the structural changes occurring after administration of furosemide has been made. The use of ruthenium-red, which stains cell coats, has shown that the atria is impermeable to the passive diffusion of material from both the endolymph and the spiral ligament. The first changes after administration of furosemide are observed 4 hours after injection; small spaces develop between the cells. After 5--6 hours the spaces have enlarged but the tight junctions between marginal cells remain intact. The intermediate cells show signs of atrophy. In severely affected animals the intermediate cells have so shrunk that the spaces are very large and the marginal cells clearly resolved. These latter cells show no structural damage. In this severely affected tissue it is seen that the blood vessels are entirely surrounded by marginal cell extensions. It is suggested that the intermediate cells may have a regulatory function. The damage appears to be recoverable, since 6 hours after injection the spaces have decreased in size although some signs of intermediate cell atrophy are still apparent.

Effects of cholera toxin on cochlear endolymph production: model for endolymphatic hydrops

To evaluate a possible role for adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] and adenosine 3':5'-cyclic monophosphate in the secretion of endolymph, we studied the effect of an intra-scala media injection of purified cholera toxin (an adenylate cyclase stimulant) on cochlear endolymph volume, endolymphatic potential, and endolymphatic Na and K concentrations.

Effect of phlorizin on glucose transport in the inner ear

14C-glucose was used as a tracer to study the effect of phlorizin on glucose transport into perilymph and endolymph. The transport was found to be lowered in phlorizin-injected animals compared with that in normal controls. This inhibitory effect corresponding to a phlorizin-sensitive part of glucose transport was greater in the perilymphatic system than in the endolymphatic system. Pathways which are not affected by phlorizin, are considered to exist in both compartments of perilymph and endolymph.