Actions of "loop" diuretics and mercurials upon the cochlea

Effects of Furosemide and Ethacrynic Acid on the Endocochlear Direct Current Potential in Normal and Kanamycin Sulfate-Treated Guinea Pigs

The effects of loop diuretics on the endocochlear direct current (DC) potential and the effective electrical resistance of the cochlear partition were studied. The effective electrical resistance was increased, and the endocohlear DC potential was decreased. The decrease in the endocohlear DC potential must not be caused by breaks in the electrical insulation of the cochlear partition. With loop diuretics the endocochlear DC potential decreased less in guinea pigs treated with kanamycin sulfate than it did in the control group. Two interpretations of these phenomena are presented.

Effects of Furosemide and Ethacrynic Acid on The Endocochlear Direct Current Potential in Normal and Kanamycin Sulfate-Treated Guinea Pigs

The effects of loop diuretics on the endocochlear direct current (DC) potential and the effective electrical resistance of the cochlear partition were studied. The effective electrical resistance was increased, and the endocohlear DC potential was decreased. The decrease in the endocohlear DC potential must not be caused by breaks in the electrical insulation of the cochlear partition. With loop diuretics the endocochlear DC potential decreased less in guinea pigs treated with kanamycin sulfate than it did in the control group. Two interpretations of these phenomena are presented.

Cochlear outer hair cell electromotility can provide force for both low and high intensity distortion product otoacoustic emissions

It is generally believed that the force for the otoacoustic emission (OAE) generation is provided by a mechanism of electromotility, observed in isolated cochlear outer hair cells (OHCs). OHC electromotility is resistant to several ototoxic reagents, it does not depend on ATP hydrolysis, but it can be blocked by specific sulfhydryl reagents: p-chloromercuriphenylsulfonic acid (pCMPS) and p-hydroxymercuriphenylsulfonic acid (pHMPS). We have used these reagents to test whether they also affect OAE. Application of pCMPS and pHMPS on the round window membrane of anesthetized guinea pigs produced a dose-dependent inhibition of the cubic (2F1-F2) distortion product OAE (DPOAE). The inhibition developed progressively from high to low frequencies, reflecting the diffusion of the drugs through the cochlear compartment. The effect of pCMPS and pHMPS was different from the effects of furosemide and lethal anoxia, which impair cochlear function but do not block OHC electromotility. pHMPS suppressed DPOAE completely at all sound intensities tested (45-80 dB SPL), whereas furosemide or lethal anoxia caused DPOAE to disappear at low-level stimulation (45-60 dB SPL) only. Our results suggest that the OHC electromotility might provide the force for DPOAE generation not only at low, but also at high stimulus intensities.

Molecular and clinical implications of loop diuretic ototoxicity

Recent advances in molecular biology have been applied to inner ear research. Loop diuretic ototoxicity has been suggested, but not proven, to share a common mechanism with diuretic effects on renal tubules. The discovery of the molecular nature of the Na-K-2Cl cotransporter in the cochlea provided a better understanding of loop diuretic ototoxicity. In this review, we describe clinical reports of loop diuretic ototoxicity and other information obtained by physiological, biochemical and morphological investigations related to the mechanism sensitive to loop diuretics. Based on recent evidence for the molecular nature of the Na-K-2Cl cotransporter expressed in the mammalian cochlea, the underlying mechanisms of ototoxicity induced by loop diuretics are described.

Inhibition of the carbachol-evoked synthesis of inositol phosphates by ototoxic drugs in the rat cochlea

The ability of amikacin, neomycin, ethacrynate, mercuric chloride and cisplatin to alter the inositol phosphate (IP) signalling pathway was assessed in the 12-day-old rat cochlea, where the turnover of IPs is coupled to muscarinic receptors. This study was motivated by: (1) the demonstration of neomycin binding to phosphatidylinositol 4,5-biphosphate, the precursor of IPs, and (2) the fact that ototoxic drugs induce some common symptoms in outer hair cells. At concentrations below 1 mM, none of the compounds changed the control 3H-IP formation. Mercuric chloride, cisplatin and ethacrynate inhibited the carbachol-induced formation of IPs in a dose-dependent manner with IC50 values of 74,340 and 430 microM, respectively. The aminoglycosides were less efficient in reducing the carbachol-stimulated accumulation of IPs, since neither amikacin nor neomycin, both at 1 mM, had any significant effect. However, neomycin applied at 15 and 30 microM induced 29% and 43% of inhibition of the stimulated IP response. Finally, additive effects are obtained between some of the toxic drugs. The results suggest that a block of the IP transduction system, associated with the cholinergic efferent innervation of the organ of Corti, is a feature that may be involved in some types of ototoxicity. The inefficiency of aminoglycosides and the putative targets of the ototoxic agents are discussed.

Inhibition of adenylate-cyclase-coupled G protein complex by ototoxic diuretics and cis-platinum in the inner ear of the guinea pig

The enzyme adenylate cyclase produces the second messenger cAMP and is located in the mammalian inner ear, predominantly in the stria vascularis and to a lesser extent in the organ of Corti. It is coupled to hormone receptors and regulating G proteins in the outer cell membrane. By means of immunofluorescence in cryostat sections of the guinea pig cochlea, we could demonstrate the G proteins Gs and Gi, which belong to the adenylate cyclase complex. These proteins had their highest density in the stria vascularis. In membrane preparations of this tissue, the adenylate cyclase complex was inhibited by ototoxic drugs (furosemide, ethacrynic acid and cis-platinum). Stimulation at different sites of the enzyme system showed that the target of these drugs was probably the regulating G protein complex and not the enzyme molecule itself. Inhibition depended on the concentration of the drug and the incubation time.

Ototoxic and nephrotoxic drugs inhibit agonist-induced inositol phosphate formation in rat brain synaptoneurosomes

Neomycin (an aminoglycoside antibiotic), ethacrynate (a loop diuretic), cisplatin (an anticancer drug) and mercuric chloride are chemically unrelated drugs which present similar ototoxic and nephrotoxic properties. We have found that all these molecules inhibit inositol phosphate turnover induced by carbachol or glutamate in rat brain synaptoneurosomes. Since this second messenger system appears to be a key mechanism for cell functioning and even survival, our observations raise the possibility that the expression of the specific toxicity of these compounds may result from excessive inhibition of the phosphoinositide cascade.

The effect of furosemide on glucose oxidation of the cochlea and other tissues

We tested the hypothesis that furosemide interferes with energy generation in the cochlea, and determined its effect on CO2 formation from glucose and glyceroaldehyde-3-phosphate dehydrogenase (GAPDH) activity by examining biochemical and histochemical changes in the cochlea, the kidney, and the liver. We found that furosemide suppressed CO2 formation in vitro at relatively low concentrations in all tissues examined. GAPDH was inhibited as soon as 2 min after its administration (80 mg/kg, i.v.). Recovery of this enzyme activity was most rapid in the liver. We concluded that furosemide does interfere with energy generation in the cochlea, kidney, and liver as a result of its inhibition of GAPDH.

The effect of bumetanide on the stria vascularis: a stereological analysis of cell volume density

The purpose of this research was to determine the effect of bumetanide on the volume density (Vv) of the cells, capillaries and intercellular spaces of the stria vascularis (SV). 29 chinchillas were divided into seven groups. There were 3 experimental groups, three control groups and one normal, untreated, group of animals. After either a 20 mg/kg intravenous injection of bumetanide or an injection of a control solution, the animals were killed at 10 min, 1 h and 24 h. One complete radial section of the SV was analyzed in each animal. This section was located at 70% of the length of the basilar membrane as measured from the cochlear apex. Marginal cell volume decreased by 24% and 15% at 10 min and 1 h, respectively, after bumetanide administration. Intermediate cell volume increased by 31% and 27% at 10 min and 1 h, respectively, after bumetanide administration. Intercellular space volume increased by 14% and 21% at 10 min and 1 h, respectively, after bumetanide administration. No significant alteration in the Vv was observed in the strial capillaries or basal cells. A hypothetical model of the ion transporting properties of the SV is presented.

Structure side-effect sorting of drugs. VI. Ototoxicities

From a literature survey, over 130 (about 7.8%) drugs and chemicals have been associated with ototoxicities. The major classes are basic aminoglycoside and other antibiotics, anti-inflammatory drugs, antimalarials, beta-blockers, antineoplastic agents, heavy metals, diuretics, some topical agents and various miscellaneous drugs. Possible mechanisms of action are presented and discussed. These include inhibition of protein synthesis, the glycolytic cycle, the TCA cycle, energy utilization, energy generation and the respiratory system within the mitochondria membrane of the hair cell, and also alteration of the permeability of the endolymphatic membrane or alteration of the excretion system for the basic aminoglycosides in the lateral wall of the membranous cochlea. The relative rank order of ototoxicity and reactivity toward mucopolysaccharides of five aminoglycosides is found to be related to the number of basic groups in each molecule.

[Morphologic studies of the toxicity of ethacrynic acid in the spiral prominence]

Ultrastructural changes in the guinea pig spiral prominence were studied at various times after a single intravenous injection of ethacrynic acid (40 mg/kg body-weight). Initial swelling of endolymph-facing epithelial cells was followed by dilatation of the intercellular spaces and marked shrinkage of the stroma cells surrounding the spiral prominence vessel. While the changes in the stria vascularis and the spiral prominence progressed at about the same pace, the regression to normal ultrastructure set in earlier in the spiral prominence.

Application of loop diuretics for treatment of sensorineural hearing impairment. Experimental and clinical study

In a series of animal experiments on guinea pigs and rats, conducted in an attempt to find a better solution for the treatment of sensorineural hearing impairment and tinnitus-known as diseases almost impossible to heal-it was revealed electrophysiologically, biochemically and electronmicroscopically that loop diuretics, referred to as ototoxic drugs, cause an increased amount of the therapeutic preparation to transfer to the inner ear fluid. It is concluded, on the basis of the encouraging results of these experiments, that combined medication with metabolic activator, steroid hormone and loop diuretics is significantly more effective in improving the symptoms of patients with inner ear deafness and tinnitus.

The pharmacology of ototoxic drugs

This article briefly reviews the nature of the toxic effects of drugs on the inner ear and the incidence of ototoxic side effects in man. There follows a more detailed discussion of the most important groups of ototoxic drugs which are identified as the aminoglycoside antibiotics, the "loop" diuretics, quinine and chloroquine, the salicylates and some antitumour drugs. Attention is drawn to the synergistic interaction between aminoglycoside antibiotics and "loop" diuretics and the predisposition to ototoxicity if the drugs are given to subjects with renal impairment. The comparative ototoxicological potential of individual aminoglycosides is discussed and their toxic effects on the kidney and the neuromuscular junction summarized. The importance of an understanding of the pharmacokinetics of aminoglycosides both in relation to toxicity and the rational control of therapy is emphasized.

Specificity of action of vanadate to the organ of corti

Although vanadate strongly inhibits Na/K-ATPase activity of the stria vascularis in vitro, it initially causes no depression of the ouabain-sensitive endocochlear potential (EP) when perfused perilymphatically or via the vasculature. However, when the perilymph of scala tympani is replaced with artificial media containing 0.1 to 1 mM vanadate, there is a large (about 17 mV) increase in the EP of the second cochlear turn. Further experiments showed that the cochlear microphonics declined during the time in which the EP increased, and that the response of these two potentials to vanadate is greater in the second turn than in the first. Injection of 50 n1 of 1 mM vanadate (in artificial endolymph) into the endolymphatic space of the second turn caused no increase in the EP. These results support the notion that the early effects of vanadate are on the contra-luminal membranes of cells of the organ of Corti rather than on the stria vascularis. By superimposing anoxia or furosemide (i.v.) upon vanadate intoxication, we determined that the initial increase of the compound EP due to vanadate alone was due to a reduction in magnitude of the negative component of the EP. It is argued that of the three prevalent theories concerning the generation of the negative EP, the data tend to support the hypothesis that the intracellular potential of the hair cells gives rise to the negative EP.

Brain cortex gangliosides and (Na+, K+)ATPase system of the stria vascularis in guinea pig

This paper presents the results of an investigation into the possibility that gangliosides (brain cortex glycosphingolipids) are capable of being functionally incorporated into cell membranes and of interfering with enzymatic activity of the ATPase system on the stria vascularis and spiral ligament in the guinea pig. Labelled gangliosides (3H-GM1) were incorporated into the cell membranes of the tissues under examination. (Na+, K+)ATPase activity increased l0 minutes after intravenous injection of gangliosides and prevented the decrease in (Na+, K+)ATPase system produced by ethacrynic acid. The action of gangliosides on the (Na+, K+)ATPase system is discussed.

Ultrastructural histopathology in a case of human ototoxicity due to loop diuretics

The temporal bones of a patient who suffered sudden deafness and ataxia after administration of both furosemide and ethacrynic acid, were prepared for light and electron microscopy. There was no loss of hair or supporting cells. However, some hair cells, in both the vestibular neuroepithelium and the organ of Corti, particularly in the basal turn, were more densely staining and more granular than normal. Membrane whorls also were common within mitochondria of such cells. The endoplasmic reticulum of some spiral ganglion cells was dilated. The major cytologic changes were found in the stria vascularis of the cochlea and dark cell areas of the vestibular system. There was marked dilatation of the intercellular fluid spaces, consistent with the biochemical observation that loop diuretics interfere first with enzyme systems responsible for fluid transport within the inner ear.

Adenylate cyclase activity in the fetal and the early postnatal inner ear of the mouse

The adenylate cyclase activity was analyzed in fetal, early postnatal and adult inner ears of the CBA/CBA mouse and also in approximately one month old inner ears from Shaker -1 and Shaker -2 mice. A comparison was made with the maturation of potassium levels in endolymph as investigated with the X-ray energy dispersive technique. Adenylate cyclase activity in the developing normal inner ear shows two significant periods of increases: from the 16th to the 19th gestational day in both the cochlear and vestibular parts of the labyrinth, and from birth to day 6 after birth in the lateral wall tissues of the scala media. During the first period the anatomical boundaries of the secretory epithelia are developing. The postnatal rise in adenylate cyclase activity correlates with the morphological maturation of stria vascularis at the cellular and subcellular levels and the rise in potassium content of endolymph. The rise of enzyme activity in the cochlear during the maturation of endolymph supports a link between adenylate cyclase and the control of inner ear fluids. Adenylate cyclase activity in stria vascularis/spiral ligament of Shaker -1 and Shaker -2 mice were at normal levels and correlated better with the rather normal morphology of the tissues than the abnormal composition of endolymph in these mutants.

Characteristics and drug responses of cochlear and vestibular adenylate cyclase

Adenylate cyclase activity of dissected cochlear and vestibular structures was assayed with the ATP-analog adenylyl imidodiphosphate as substrate. High activities (per mg protein) were found in stria vascularis and in vestibular preparations, lower activities in spiral ligament, VIIIth nerve, and organ of Corti. The enzyme from all structures was stimulated by fluoride, guanylyl imidodiphasphate or manganese(II) ions, and strongly inhibited by ethacrynate and lead ion. The anti-cancer drug cis-diammine-dichloro platinum significantly inhibited adenylate cyclase from stria vascularis.

The Davis theory: a review, and implications of recent electrophysiological evidence

The Davis theory of mechano-electrical transduction asserts that the endocochlear potential and the hair cell resting potential summate to provide a driving force for current flow through the hair cell. However, while a variety of agents which depress the endocochlear potential simultaneously depress auditory nerve sensitivity and reduce the cochlear microphonic, recent reports suggest that the hair cells may be depolarised without such effects ensuing. The relevant literature is reviewed.

Maintenance of cochlear function with artificial oxygen carriers

By means of vascular perfusion via the anterior inferior cerebellar artery with a blood substitute containing the perfluorochemical FC 47 as oxygen carrier, it is possible to maintain normal or near normal levels of the cochlear microphonics and the endolymphatic potential of the guinea pig for perios of 90 min, or longer. Following 60 min. of perfusion with artificial blood, the levels of ATP and 5' AMP in the stria vascularis and the organ of Corti are comparable to those of nonperfused control animals maintained at optimal metabolic conditions. Following the same period of perfusion, the appearance of the organ of Corti is normal, but small vacuoles, presumably deposits of FC 47, are visible in the marginal cells of the stria vascularis. Preliminary experiments concerning the survival time and the revival time of the cochlear potentials, as well as the response to furosemide, ouabain, and mersalyl are presented to illustrate the value of this method in elucidating various biochemical and pharmacological problems of the cochlea.

The relationship between the cytotoxicity of kanamycin and ethacrynic acid for mammalian cells in vitro and their ototoxicity in vivo

Dose-effect curves for inhibition of growth of P388/P mouse lymphoma cells by ethacrynic acid and kanamycin used alone and in combination were determined in vitro. Ethacrynic acid was 600 times more potent than kanamycin and combinations of the drugs resulted in overall additive effects. These results were compared with known dose-effect data on the ototoxicity of these drugs in vivo. Kanamycin was highly selective in its toxicity for cochlear hair cells compared to cultured cells. The dose-effect data for ethacrynic acid was coincident with that reported for functional and biochemical effects on the cochlea following perilymphatic perfusion with the drug. The potentiation observed following the ototoxic interaction the two drugs in vivo was not observed following combinations of the drugs in vitro.

Reduction of the endocochlear potential by the new "loop" diuretic, bumetanide

The effect of bumetanide upon the endocochlear potential (EP) was examined in 46 guinea pigs. The EP was reduced with dosages of 5 mg/kg or more. The maximum depression of the EP (reduction to -30 to -40 mV) was obtained at a dosage of 30 mg/kg. The recovery of the potential was incomplete at any dosage within three hours and the response pattern of the EP to bumetanide was similar to that to ethacrynic acid. The present experiments revealed that bumetanide, by weight, has a stronger ototoxic potency than the other "loop" diuretics--furosemide and ethacrynic acid. However, the diuretic effect of 1 mg bumetanide is equivalent to 40 to 60 mg furosemide or ethacrynic acid. Therefore, the relative ototoxic potency of bumetanide is much smaller suggesting that from a clinical standpoint bumetanide is much safer than the other "loop" diuretics.

Noxious effects upon cochlear metabolism

The influence of various toxic substances and of drugs with ototoxic side effects upon energy generation, energy utilization, and membrane processes of the cochlea were studied. None of the drugs tested interfered with energy generation to as great an extent as did anoxia or cyanide and 2,4-dinitrophenol. Ouabain produced a pronounced interference with energy utilization of the stria vascularis. The "loop" diuretics ethacrynic acid and furosemide produced a reduction of energy utilization of a lesser degree than did ouabain. The "loop" diuretics do not seem to exert their toxic action upon strial Na+K+-ATPase, but may act by interfering with strial adenylate cyclase. Aminoglycoside antibiotics and diuretic and nondiuretic mercurials seem to exert their primary noxious action upon cochlear function by interfering with membrane processes of the structures bounding the cochlear duct.