Morphological alteration of the stria vascularis after administration of the diuretic bumetanide

Ototoxic effects and mechanisms of loop diuretics

Over the past two decades considerable progress has been made in understanding the ototoxic effects and mechanisms underlying loop diuretics. As typical representative of loop diuretics ethacrynic acid or furosemide only induces temporary hearing loss, but rarely permanent deafness unless applied in severe acute or chronic renal failure or with other ototoxic drugs. Loop diuretic induce unique pathological changes in the cochlea such as formation of edematous spaces in the epithelium of the stria vascularis, which leads to rapid decrease of the endolymphatic potential and eventual loss of the cochlear microphonic potential, summating potential, and compound action potential. Loop diuretics interfere with strial adenylate cyclase and Na+/K+-ATPase and inhibit the Na-K-2Cl cotransporter in the stria vascularis, however recent reports indicate that one of the earliest effects in vivo is to abolish blood flow in the vessels supplying the lateral wall. Since ethacrynic acid does not damage the stria vascularis in vitro, the changes in Na+/K+-ATPase and Na-K-2Cl seen in vivo may be secondary effects results from strial ischemia and anoxia. Recent observations showing that renin is present in pericytes surrounding stria arterioles suggest that diuretics may induce local vasoconstriction by renin secretion and angiotensin formation. The tight junctions in the blood-cochlea barrier prevent toxic molecules and pathogens from entering cochlea, but when diuretics induce a transient ischemia, the barrier is temporarily disrupted allowing the entry of toxic chemicals or pathogens.

NKCCs in the fibrocytes of the spiral ligament are silent on the unidirectional K⁺ transport that controls the electrochemical properties in the mammalian cochlea

Unidirectional K(+) transport across the lateral cochlear wall contributes to the endocochlear potential (EP) of +80 mV in the endolymph, a property essential for hearing. The wall comprises two epithelial layers, the syncytium and the marginal cells. The basolateral surface of the former and the apical membranes of the latter face the perilymph and the endolymph, respectively. Intrastrial space (IS), an extracellular compartment between the two layers, exhibits low [K(+)] and a potential similar to the EP. This IS potential (ISP) dominates the EP and represents a K(+) diffusion potential elicited by a large K(+) gradient across the syncytial apical surface. The K(+) gradient depends on the unidirectional K(+) transport driven by Na(+),K(+)-ATPases on the basolateral surface of each layer and the concomitant Na(+),K(+),2Cl(-)-cotransporters (NKCCs) in the marginal cell layer. The NKCCs coexpressed with the Na(+),K(+)-ATPases in the syncytial layer also seem to participate in the K(+) transport. To test this hypothesis, we examined the electrochemical properties of the lateral wall with electrodes measuring [K(+)] and potential. Blocking NKCCs by perilymphatic perfusion of bumetanide suppressed the ISP. Unexpectedly and unlike the inhibition of the syncytial Na(+),K(+)-ATPases, the perfusion barely altered the electrochemical properties of the syncytium but markedly augmented [K(+)] of the IS. Consequently, the K(+) gradient decreased and the ISP declined. These observations resembled those when the marginal cells' Na(+),K(+)-ATPases or NKCCs were blocked with vascularly applied inhibitors. It is plausible that NKCCs in the marginal cells are affected by the perilymphatically perfused bumetanide, and these transporters, but not those in the syncytium, mediate the unidirectional K(+) transport.

Supporting cell characteristics in long-deafened aged mouse ears

Significant sensory hair cell loss leads to irreversible hearing and balance deficits in humans and other mammals. Future therapeutic strategies to repair damaged mammalian auditory epithelium may involve inserting stem cells into the damaged epithelium, inducing non-sensory cells remaining in the epithelium to transdifferentiate into replacement hair cells via gene therapy, or applying growth factors. Little is currently known regarding the status and characteristics of the non-sensory cells that remain in the deafened auditory epithelium, yet this information is integral to the development of therapeutic treatments. A single high-dose injection of the aminoglycoside kanamycin coupled with a single injection of the loop diuretic furosemide was used to kill hair cells in adult mice, and the mice were examined 1 year after the drug insult. Outer hair cells are lost throughout the entire length of the cochlea and less than a third of the inner hair cells remain in the apical turn. Over 20% and 55% of apical organ of Corti support cells and spiral ganglion cells are lost, respectively. We examined the expression of several known support cell markers to investigate for possible support cell dedifferentiation in the damaged ears. The support cell markers investigated included the microtubule protein acetylated tubulin, the transcription factor Sox2, and the Notch signaling ligand Jagged1. Non-sensory epithelial cells remaining in the organ of Corti retain acetylated tubulin, Sox2 and Jagged1 expression, even when the epithelium has a monolayer-like appearance. These results suggest a lack of marked SC dedifferentiation in these aged and badly damaged ears.

Potentiation of Chemical Ototoxicity by Noise

High-intensity and/or prolonged exposure to noise causes temporary or permanent threshold shifts in auditory perception. Occupational exposure to solvents or administration of clinically important drugs, such as aminoglycoside antibiotics and cisplatin, also can induce permanent hearing loss. The mechanisms by which these ototoxic insults cause auditory dysfunction are still being unraveled, yet they share common sequelae, particularly generation of reactive oxygen species, that ultimately lead to hearing loss and deafness. Individuals are frequently exposed to ototoxic chemical contaminants (e.g., fuel) and noise simultaneously in a variety of work and recreational environments. Does simultaneous exposure to chemical ototoxins and noise potentiate auditory dysfunction? Exposure to solvent vapor in noisy environments potentiates the permanent threshold shifts induced by noise alone. Moderate noise levels potentiate both aminoglycoside- and cisplatin-induced ototoxicity in both rate of onset and in severity of auditory dysfunction. Thus, simultaneous exposure to chemical ototoxins and moderate levels of noise can potentiate auditory dysfunction. Preventing the ototoxic synergy of noise and chemical ototoxins requires removing exposure to ototoxins and/or attenuating noise exposure levels when chemical ototoxins are present.

Sox2 and JAGGED1 expression in normal and drug-damaged adult mouse inner ear

Inner ear hair cells detect environmental signals associated with hearing, balance, and body orientation. In humans and other mammals, significant hair cell loss leads to irreversible hearing and balance deficits, whereas hair cell loss in nonmammalian vertebrates is repaired by the spontaneous generation of replacement hair cells. Research in mammalian hair cell regeneration is hampered by the lack of in vivo damage models for the adult mouse inner ear and the paucity of cell-type-specific markers for non-sensory cells within the sensory receptor epithelia. The present study delineates a protocol to drug damage the adult mouse auditory epithelium (organ of Corti) in situ and uses this protocol to investigate Sox2 and Jagged1 expression in damaged inner ear sensory epithelia. In other tissues, the transcription factor Sox2 and a ligand member of the Notch signaling pathway, Jagged1, are involved in regenerative processes. Both are involved in early inner ear development and are expressed in developing support cells, but little is known about their expressions in the adult. We describe a nonsurgical technique for inducing hair cell damage in adult mouse organ of Corti by a single high-dose injection of the aminoglycoside kanamycin followed by a single injection of the loop diuretic furosemide. This drug combination causes the rapid death of outer hair cells throughout the cochlea. Using immunocytochemical techniques, Sox2 is shown to be expressed specifically in support cells in normal adult mouse inner ear and is not affected by drug damage. Sox2 is absent from auditory hair cells, but is expressed in a subset of vestibular hair cells. Double-labeling experiments with Sox2 and calbindin suggest Sox2-positive hair cells are Type II. Jagged1 is also expressed in support cells in the adult ear and is not affected by drug damage. Sox2 and Jagged1 may be involved in the maintenance of support cells in adult mouse inner ear.

Rapid hair cell loss: a mouse model for cochlear lesions

In comparison to other mammals, mice have proved extremely resistant to aminoglycoside-induced hair cell ablation in vivo. In this paper we examine the pattern and extent of cochlear lesions rapidly induced with a combination of a single dose of aminoglycoside (kanamycin) followed by a loop diuretic (bumetanide). With this protocol, the vestibular system was unaffected, but in the cochlea, there was extensive loss of outer hair cells (OHC) that commenced in the basal coil and progressed apically so that, by 48 h, OHC loss was almost complete. TUNEL-positive nuclei and activated caspase-3 labeling demonstrated that most OHC died via a classical apoptotic pathway. However, scattered debris within the OHC region suggested that many apoptotic cells ruptured prior to completion of apoptosis. Following lesion repair, supporting cells retained characteristics of differentiated cells but positional shift occurred. In comparison to OHC loss, inner hair cell (IHC) death was delayed and only observed in 50% of all cochleae examined even after extensive reorganization of the tissue. The coadmininstration of diuretic with FM1-43, used as a tracer for aminoglycoside uptake, indicated entry into IHC as readily as OHC, suggesting that the differential response to aminoglycoside was not due to differential uptake. Where IHC death was ongoing, there were indications of different modes of cell death: cells with morphological features of autophagy, necrosis, and apoptosis were apparent. In addition to damage to the organ of Corti, there was a significant and progressive decrease in strial thickness beginning as early as 7 days posttreatment. This was due predominantly to degeneration of marginal cells. The strial pathology resembled that reported after noise damage and with aging. This in vivo protocol provides a robust model in which to obtain extensive OHC loss in the mature cochleae of mice and is a means with which to examine different aspects of cochlear pathology in transgenic or mutant strains.

Bumetanide-induced enlargement of the intercellular space in the stria vascularis critically depends on Na+ transport

The intercellular space in the stria vascularis (intrastrial space) is a closed space and isolated from both the endolymph and the perilymph in normal tissue. Loop diuretics such as bumetanide and furosemide cause an acute enlargement of the intrastrial space in association with a decline in the endocochlear potential. It is known that bumetanide inhibits the Na+-K+-2Cl- cotransporter, which is expressed abundantly in the basolateral membrane of marginal cells. We studied ionic mechanisms underlying the bumetanide-induced enlargement of the intrastrial space using perilymphatic perfusion in guinea pigs. Perilymphatic perfusion with artificial perilymph containing 100 microM bumetanide caused marked enlargement of the intrastrial space, as reported previously. Removal of K+ from the perilymph did not affect the bumetanide-induced enlargement, whereas removal of Na+ from the perilymph inhibited it almost completely. Perilymph containing 1 mM amiloride also inhibited the enlargement of the intrastrial space almost completely. These results indicate that perilymphatic Na+, but not K+, and amiloride-sensitive pathways are essential to the bumetanide-induced enlargement of the intrastrial space. Two possible pathways could yield these results. Na+ in the perilymph could enter the endolymph via Reissner's membrane or the basilar membrane; Na+ in the endolymph would then be taken up by marginal cells via the apical membrane and secreted into the intrastrial space by Na+-K+-ATPase in the basolateral membrane of them. Another, less likely possibility is that Na+ in the perilymph is transported into basal cells or fibrocytes in the spiral ligament, then into intermediate cells via gap junctions, and finally secreted into the intrastrial space via Na+-K+-ATPase of intermediate cells.

Ultrastructure of the inner ear of NKCC1-deficient mice

The transduction of the auditory signal is dependent on the flow of ions within the inner ear. We have generated mice deficient in NKCC1, an ion cotransporter that is thought to be involved in the secretion of K+ by the strial marginal cells. Inner ear histology revealed partial to almost total absence of the scala media and collapse of Reissner's membrane. Ultrastructural analysis showed that Reissner's membrane consists of 3-4 cell layers instead of the usual two, and a substance of unknown composition is present between Reissner's membrane and underlying structures. Within the tunnel of Corti, hair cells and supporting cells were difficult to identify. The location of the tectorial membrane was altered, and a precipitate was observed surrounding it. Severe structural defects were noted in the interdental cells and Boettcher cells, and mild defects were observed in the stria vascularis and in type II and type IV fibrocytes. The finding that major defects occur predominantly in cells that are not known to express NKCC1 suggests that loss of NKCC1 results in functional defects in cells expressing NKCC1 and a morphological effect on cell populations downstream in the proposed K+ recycling pathway.

D-Methionine protects against cisplatin damage to the stria vascularis

D-Methionine (D-met) protects against cisplatin (CDDP)-induced hearing loss and outer hair cell loss (Campbell et al., 1996). However, D-met's protective effects on the stria vascularis has not been previously investigated. The purpose of this study was to examine, using semi-quantitative analysis, whether D-met also protects the stria vascularis. We removed a basal turn section of the stria vascularis from five groups of five male Wistar rats each: (1) a CDDP-treated control group receiving a 30 min i.p. infusion of 16 mg/kg CDDP, (2) a saline-injected control group receiving an equivalent volume of saline, and (3) three groups injected with either 75, 150, or 300 mg/kg D-methionine (D-met) i.p. 30 min prior to receiving the 16 mg/kg CDDP dosing. Using transmission electron microscopy and light microscopy, we analyzed strial volume (i.e. edema), marginal cell damage classification (bulging and/or compression), and relative optical density (ROD) ratios (i.e. depletion of marginal cell cytoplasmic organelles). All three levels of D-met provided complete protection against marginal cell bulging and/or compression but only partial protection against strial edema. At 300 mg/kg, D-met significantly reduced ROD ratio degradation in the spiral prominence and middle stria vascularis regions. In Reissner's membrane region, values from the D-met pretreated group were not significantly different from either the treated or untreated control groups suggesting only partial protection for that area. Protection of marginal cell cytoplasmic organelles was also noted. In summary, D-met partially or fully protects the stria vascularis from several types of CDDP-induced damage.

Distribution of canalicular reticulum in Deiters cells and pillar cells of gerbil cochlea

Postfixation with an osmium tetroxide-potassium ferrocyanide solution revealed in supporting cells in the organ of Corti a network of canaliculi termed canalicular reticulum (CR). In Deiters cells (DCs), the CR filled cytosol at the base of the phalanx and under plasmalemma apposed to either the outer hair cells' (HCs) basal surface or nerve terminals. From these locations the CR, accompanied by dense fibrillar substance, descended along microtubule bundles and terminated by surrounding the rosette complex in the apical cytosol. Canalicular profiles protruding from the reticulum penetrated the loose meshwork comprising the periphery of the rosette complex to contact at intervals branches of the dense trabeculae that make up the core of the complex. This arrangement disclosed a structural and presumably functional relationship between outer HCs and the CR and rosette complex. Inner pillar cells (PCs) exhibited moderately abundant to sparse profiles of CR interspersed between microtubule bundles of the microtubule stalk that connected head and foot regions. More elaborate CR extended as a network upward from the top of the microtubule stalk part was into the head body and downward into a conical expansion of the stalk at the base of the cell. Cytosol on the medial side of the basal microtubule expansion contained abundant CR which in conjunction with CR between basal microtubule bundles lay situated for possible uptake of ions or neurotransmitter released from numerous adjoining nerves. CR in outer PCs resembled that in inner PCs but appeared less prevalent in the head and foot regions and did not occur in cytosol beside the basal microtubule stalk. Characteristically small Golgi complexes accompanied the reticulum in DCs and were prevalent in the upper regions but absent in the mid and lower part of inner PCs. Short cisternae in the Golgi stacks associated with CR contrasted with the lengthier cisternae in the complexes infrequently observed in cytosol outside the microtubule stalk of inner PCs.

A semiquantitative analysis of the effects of cisplatin on the rat stria vascularis

Cisplatin (CDDP) is a very effective chemotherapeutic agent but is highly ototoxic. Most studies have focused on the effects of CDDP on the outer hair cells. The purpose of this study was to examine changes in the stria vascularis in cisplatin treated male Wistar rats and to provide semiquantitative analysis of the results. We removed a section of the stria vascularis from the basal turn of five control and five CDDP (16 mg/kg) treated rats. Using transmission electron microscopy (TEM) we analyzed: (1) changes to the strial tissue as a whole; and (2) intracellular changes in the marginal cells. We also subjected the samples to semiquantitative analysis using the MCID, focusing on three aspects of strial profile abnormalities; the number of abnormal marginal cells in CDDP treated tissue, intracellular strial edema and densitometry. Controls appeared normal, but many pathologic changes were apparent in the experimental group. Results from the semiquantitative analysis indicate cisplatin has a deleterious effect on the stria vascularis including strial edema; bulging, rupture and/or compression of the marginal cells and depletion of the cytoplasmic organelles.

Quantification of the stria vascularis and strial capillary areas in quiet-reared young and aged gerbils

The area of the stria vascularis (StV) and of StV capillaries was measured in radial sections from regions corresponding to 0.5, 2, 4, 10, 20 and 40 kHz. In young gerbils, StV area ranged from 3700 to 8500 microm2 and that of individual StV capillaries from 70 to 110 microm2. The maximal StV area as well as the largest number of capillaries occurred at the 20 kHz region. In quiet-aged gerbils, the StV area also varied with frequency and was 28-67% smaller than corresponding measures in young gerbils. The decrease in StV area was statistically significant at all but the 2 and 4 kHz regions. The area of individual StV capillaries declined also (8-29%) with age even when the StV area remained near normal. Reductions in capillary area were statistically significant at the 2, 20 and 40 kHz regions. The large variance in StV radial area among aged gerbils reflects the patchy nature of strial degeneration previously observed in this species. The data agree with those of our previous studies and indicate alterations in StV capillaries are a primary cause of presbyacusis in the gerbil.

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.

Effects of organic acids on the edema of the stria vascularis induced by furosemide

Furosemide is a loop diuretic which is ototoxic. Investigations have shown the stria vascularis to be the target tissue of this ototoxic drug. The purpose of the present study was to investigate the effects of furosemide on the stria vascularis in chinchillas, in controls and in animals pretreated with the above organic acids. Control animals were injected with 0.5 ml alkalinized saline followed by furosemide IV 30 min later. Experimental animals received probenecid, penicillin or sodium salicylate IV. Thirty minutes later, furosemide was injected in the same dose as in the controls. The basal turn of the stria vascularis was rapidly removed at various times from 10 to 30 min after furosemide administration and processed for transmission electron microscopy. Control animals were found to have reversible edema of the stria vascularis. Experimental animals had variable findings. Those animals pretreated with penicillin had virtually no edema of the stria vascularis at any time. Salicylate and probenecid pretreated animals had significantly less edema from one to 10 min after furosemide injection, but more edema than controls at later times. These findings suggest a discrepancy between ultrastructural pathology and functional status of the cochlea in experimental animals pretreated with probenecid or sodium salicylate followed by furosemide. On the other hand, good structure function correlations were seen in controls and in experimental animals pretreated with penicillin.

The ototoxic mechanism of cisplatin

The ototoxic mechanism of cisplatin was investigated. Potentiation of cisplatin ototoxicity by furosemide and amino-oxyacetic acid (AOAA) was observed. Substantial hearing loss in cisplatin-deafened animals was accompanied by normal values of the endocochlear potential and a reduction in the sensitivity of the 2f1-f2 distortion products. The loss in dB of the sensitivity of the distortion products correlated extremely well with the loss of the neural sensitivity in dB. There was also a relationship between the fractional reduction of the low frequency (1000 Hz) microphonic potential and hearing loss in dB. Iontophoresis of cisplatin into scala media resulting in the immediate loss of neural thresholds at the site of iontophoresis. It is concluded that cisplatin caused the hearing loss by blocking OHC transduction channels.

Ototoxicity of indacrinone is stereospecific

Indacrinone (MK-196) is a loop diuretic which consists of a racemic mixture. The purpose of this study was to evaluate the individual enantiomers in the chinchilla model to determine whether these compounds affect auditory function and whether a difference in ototoxic potency exists. Very little change of endocochlear potential (EP) or compound action potential (CAP) was noted in animals receiving the (+)-enantiomer. On the other hand, chinchillas injected with the (-)-enantiomer were found to have a dose related reduction in both CAP and EP. These findings suggest the possibility that the diuretic receptor in the kidney and the receptor mediating ototoxicity in the cochlea, may have similar steric requirements for interacting with loop diuretics.

Comparative ototoxicity of furosemide and piretanide

The purpose of these studies was to compare the ototoxicity of piretanide to that of furosemide in the chinchilla. Chinchillas weighing 400 to 700 g were anesthetized with ketamine plus pentobarbital. Endocochlear potential (EP) was measured continuously by the round window approach using glass microelectrodes. Piretanide or furosemide injected through a jugular vein catheter in adults range from 10-100 mg/kg. Little or no change in EP was noted with doses below 15 mg/kg of either diuretic. The overall pattern of decline of EP was rather similar in piretanide-treated and furosemide-injected animals, and it was interesting to find that the dose-ototoxicity response curve for piretanide was similar to that for furosemide. The diuretic effect of equal ototoxic doses of either diuretic was equivalent. Both diuretics cause a decrease of endocochlear potential when applied locally to the round window membrane of the chinchilla as well as by systemic administration. These findings suggest that piretanide and furosemide have approximately the same propensity for ototoxicity.

Effects of noise and quinine on the vessels of the stria vascularis: an image analysis study

Surface preparations of the stria vascularis from guinea pigs exposed to wide-band noise or intoxicated with quinine monohydrochloride dihydrate were studied by light microscopy and computerized image analysis in order to evaluate quantitatively the effects of these agents on two characteristics of the strial vasculature: vascular density and erythrocyte distribution. An image analyzer was used to measure the area of strial vessel lumen and erythrocyte distribution as a fraction of the total area of strial tissue under observation. The results demonstrate that changes in the strial vessels do occur in guinea pigs exposed to noise or given large doses of quinine. Localized vessel narrowings caused by swollen endothelial cells and possibly by contraction of pericytes were found in both experimental groups, but there was no apparent tonotopical relationship between these effects and the reduction in cochlear potentials. A significant reduction in the number of erythrocytes was found in all turns of the cochlea in both experimental groups. Although a significant difference in vascular density was found among turns of the cochlea in both experimental and control animals, there was no widespread change in vascular density as a result of either noise exposure or quinine treatment.

Cochlear Vessel Permeability to Horseradish Peroxidase in the Normal and Acoustically Traumatized Chinchilla: A Reevaluation

A method involving incubation of intact cochleae using the small protein, horseradish peroxidase, has revealed that a barrier exists between the blood and the stria vascularis. This barrier is more tenuous than the barrier that exists across other cochlear vessels. The stria — strial vessel barrier can be altered physiologically by acoustic trauma or artifactually by dissection.

Effect of high intensity impulse noise on the vascular system of the chinchilla cochlea

Changes in the vasculature of the chinchilla cochlea were quantitatively assessed 45 days after impulse noise exposure of either 155- or 160-dB peak sound pressure level. The condition of the vessels in the lateral wall and spiral lamina of each cochlea was evaluated in terms of 20 morphological parameters. Multivariate statistical analysis identified the parameters that were significantly affected by the noise exposure. Evaluation of these significant parameters indicated a net reduction in blood flow to the cochlea. All of the noise-exposed cochleas showed vascular changes when compared to controls, and the magnitude of these changes was related to the amount of hair cell loss. The vascular changes are presumed to be permanent.

A morphometric technique for analysis of cochlear vessels

A computer based system was developed for morphometric analysis of the cochlear lateral wall vasculature. Variables measured were vascular density, RBC density, vessel width, aggregation density, lumen compression count, WBC count and pigment density. Vessels were divided into three systems based on the lateral wall structures that they supplied. The results indicated that these three vessel systems were morphologically distinct in terms of their vascular density, RBC density, and vessel width. How these differences relate to functional needs of the lateral wall is discussed.

Bumetanide. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic use

Bumetanide is a potent 'loop' diuretic for the treatment of oedema associated with congestive heart failure, hepatic and renal diseases, acute pulmonary congestion and premenstrual syndrome and in forced diuresis during and after surgery. Bumetanide may be given orally, intravenously or intramuscularly and produces a rapid and marked diuresis, and increased urinary excretion of sodium, chloride and other electrolytes (within 30 minutes) which persists for 3 to 6 hours. Its principal site of action is on the ascending limb of the loop of Henle, with a secondary action on the proximal tubule. Pharmacologically, bumetanide is about 40-fold more potent than frusemide (furosemide), with the exception of its effects on urinary potassium excretion where its potency is lower. Studies in patients with oedema due to congestive heart failure, pulmonary oedema or hepatic disease show that oral or intravenous bumetanide 0.5 to 2 mg/day produces results comparable to those with frusemide 20 to 80 mg/day. In acute pulmonary oedema, intravenous bumetanide produces a very rapid diuresis. Higher doses of bumetanide may be required (up to 15 mg/day) in patients with chronic renal failure or nephrotic syndrome. In these patients muscle cramps are not uncommon with bumetanide, but glomerular filtration rates are unaffected. In most studies, diuretic effects were accompanied by decreased bodyweight, abdominal girth and improvements in a variety of haemodynamic parameters. Comparison of bumetanide with frusemide at a dose ratio of 1 : 40 reveals no significant differences in clinical response with the exception of renal disease, where patients with oedema appear to respond better to bumetanide. Combination with thiazide diuretics enhances the clinical response to bumetanide. Potassium supplements and spironolactone may be beneficial additions to bumetanide where patients at risk of hypokalaemia can be identified. Clinically important side effects are infrequent, with audiological impairment occurring to a lesser extent than with frusemide. Bumetanide thus offers an important alternative to frusemide when a 'loop' diuretic is indicated.

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.

The volume density of cells and capillaries of the normal stria vascularis

The purpose of this study was to provide morphometric (i.e. quantitative anatomical) data on the normal chinchilla stria vascularis. Five normal chinchillas were used in the present investigation and four regions of the cochlea were examined in each animal. The width, radial area and number of marginal cells across the stria's width increased from the cochlear apex toward the base. The increase in strial width and area appeared to be due to hyperplasia of the marginal cells. The mean total endolymphatic surface area of the stria vascularis was estimated to be 7.4 mm2 (S.E. = 1.23). The mean total volume of the stria vascularis was estimated to be 0.15 microliter (S.E. = 0.01). In addition, using a stereological method we found that the volume density of the cells and capillaries of the stria vascularis was constant along the length of the scala media. The mean (+/- S.E.) volume density of the stria cells and capillaries was estimated to be: marginal cells = 0.528 (0.013), intermediate cells = 0.212 (0.026), basal cells = 0.163 (0.009) and capillaries = 0.097 (0.009).

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.

A survey of the effects of loop diuretics on the zonulae occludentes of the perilymph-endolymph barrier by freeze fracture

The acute (30 min) effects of the loop diuretics piretanide, ethacrynic acid, bumetanide, furosemide, and azosemide, and the chronic (8 days) effects of furosemide and bumetanide on the zonulae occludentes (tight junctions) of the perilymph-endolymph barrier in the stria vascularis and in Reissner's membrane of the basal cochlear turns were studied by freeze-fracture. Quantitative analysis of their effects indicated that the structural integrity of the barrier was modified by either as increase or a decrease in the number, depth, and density of the strands of the tight junctions of the strial marginal cells. Only azosemide appeared to modify the tight junctions between the epithelial cells of Reissner's membrane, but it had little effect on the strial junctions. The tight junctions between the basal cells of the stria appeared to be the least affected by the various loop diuretics, although piretanide appeared to increase randomly the intercellular spaces lying between the strands.

Ultrastructural and electrophysiological studies of acute ototoxic effects of furosemide

Previous studies of the effects of ethacrynic acid on the inner ear following intraperitoneal injection of the diuretic have shown a progression of reversible changes occurring in the stria vascularis. The time course of these changes approximately parallels alterations in endolymphatic potential (EP). In this report, some preliminary findings concerning the effects of furosemide after intraperitoneal injection of 80 mg/kg are described. EP declined over a longer time course than that recorded with intravenous injection. Cochlear microphonic (CM) and compound action potential (CAP) also declined but to differing degrees. In the stria vascularis a progression of changes was apparent. In general, the changes were similar to those observed following ethacrynic acid intoxication and affected marginal cells, intermediate cells and strial capillaries. The upper basal turn of the cochlea was affected first and the damage spread apically. In the organ of Corti, stereocilia on the outermost row of outer hair cells were disorganized. This was apparent in approximately the same region as initial strial effects and was only observed when strial derangement was quite marked.

Ultrastructure in the stria vascularis of the guinea pig following intraperitoneal injection of ethacrynic acid

Following intraperitoneal injection of ethacrynic acid, progressive, mainly reversible changes occurred in the stria vascularis, affecting all three cell types and the capillary basal laminae. Both marginal and intermediate cells showed abnormalities early, at a time when EP was just beginning to decline. Progressive changes in marginal cells culminated in apical bulging followed by recession of the swelling and stretching of the cells concomitant with gross interstitial oedema. Marginal cell mitochondria showed damage and the transcellular tubule system was dilated. Intermediate cells also showed a progression of changes, culminating in a marked, but reversible, shrinkage. The time of appearance of severe strial derangement correlated with the time of maximal depression of EP. The ability of the stria to regain rapidly an almost normal morphology appeared to be due partly to the distribution and orientation of microtubules in marginal and intermediate cells preventing major disruption of stria vascularis architecture.

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.

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.