Effects of Ca2+ antagonists and aminoglycoside antibiotics on Ca2+ current in isolated outer hair cells of guinea pig cochlea

Stretch induced endothelin-1 secretion by adult rat astrocytes involves calcium influx via stretch-activated ion channels (SACs)

The expression of endothelins (ETs) and ET-receptors is often upregulated in brain pathology. ET-1, a potent vasoconstrictor, also inhibits the expression of astrocyte glutamate transporters and is mitogenic for astrocytes, glioma cells, neurons, and brain capillary endothelia. We have previously shown that mechanical stress stimulates ET-1 production by adult rat astrocytes. We now show in adult astrocytes that ET-1 production is driven by calcium influx through stretch-activated ion channels (SACs) and the ET-1 production correlates with cell proliferation. Mechanical stimulation using biaxial stretch (<20%) of a rubber substrate increased ET-1 secretion, and 4 μM GsMTx-4 (a specific inhibitor of SACs) inhibited secretion by 30%. GsMTx-4 did not alter basal ET-1 levels in the absence of stretch. Decreasing the calcium influx by lowering extracellular calcium also inhibited stretch-induced ET-1 secretion without effecting ET-1 secretion in unstretched controls. Furthermore, inhibiting SACs with the less specific inhibitor streptomycin also inhibited stretch-induced ET-1 secretion. The data can be explained with a simple model in which ET-1 secretion depends on an internal Ca(2+) threshold. This coupling of mechanical stress to the astrocyte endothelin system through SACs has treatment implications, since all pathology deforms the surrounding parenchyma.

Effects of electrical stimulation of the inferior colliculus on 2f1-f2 distortion product otoacoustic emissions in anesthetized guinea pigs

The effects of electrical stimulation of the inferior colliculus (IC) on the activation of olivocochlear nerve fibers were investigated in guinea pigs in which the 2f1-f2 distortion product otoacoustic emissions (DPOAE) were recorded. Animals were anesthetized with ketamine (33 mg/kg) and xylazine (6.6 mg/kg). Bipolar electrical stimulation of the IC by a train of pulses with currents less than the threshold for evoking muscle twitches resulted in a small depression of the DPOAE amplitude by 0.1-2 dB. The maximal effect was observed when the stimulating electrodes were located in the rostro-medial or ventral parts of the IC. The suppression of electrically evoked DPOAE was similar to the DPOAE suppression produced by acoustical stimulation of the contralateral ear by a broad-band noise. Suppression of DPOAE amplitude in response to both acoustical and electrical stimulation was abolished 1-2 h after a single intramuscular injection of gentamicin (210-250 mg/kg). The results indicate that electrical stimulation of the IC can activate the efferent system and produce DPOAE changes by similar mechanisms as does acoustical stimulation of the contralateral ear.

Voltage-dependent channels in dissociated outer hair cells of the guinea pig

Voltage-dependent channels in outer hair cells (OHCs) dissociated from the guinea pig cochlea were investigated by the use of a whole-cell patch-clamp technique. Two types of K+ current were recorded from OHCs. One was a slowly inactivating K+ current that was activated at a potential more positive than -30 mV. Another is a K+ current that was already activated at resting membrane potential. After suppressing both K+ currents, depolarizing voltage steps elicited a slowly inactivating inward current that was dependent on external Ca2+ and was indicative of an L-type Ca2+ channel in OHCs. Aminoglycoside antibiotics known to be ototoxic selectively inhibited the Ca2+ current.

A new aspect of aminoglycoside ototoxicity: impairment of cochlear dopamine release

Aminoglycoside ototoxicity is a well-documented process via several pathophysiological pathways. The protective role of cochlear dopamine, released from the lateral olivocochlear efferents, was implicated previously in case of ischemia or acoustic trauma, as it postsynaptically inhibits the effect of excessively released glutamate from the hair cells. In our in vitro superfusion experiments we showed that neomycin dose- dependently inhibits the dopamine release from isolated guinea pig cochlea, while gentamicin and kanamycin was ineffective on it. After chronic application of neomycin the dopamine outflow did not change significantly, suggesting an adaptive process. In our experiments we have found a possibly new action site of one of the aminoglycoside antibiotics, neomycin.

Effects of gentamicin and pH on [Ca2+]i in apical and basal outer hair cells from guinea pigs

Aminoglycosides are widely used antibiotics and frequently produce acute ototoxicity. In this study we attempted to comparatively investigate the effects of gentamicin on Ca2+ influx of apical and basal outer hair cells (OHCs) isolated from guinea-pig cochlea. Since the solution of gentamicin sulfate salt is acidic (pH 3.1-3.3), we also explored the effect of external acidification on Ca2+ influx. By means of fura-2 microspectrofluorimetry, we measured the intracellular calcium concentration ([Ca2+]i) of OHCs bathed in Hanks' balanced salt solution (pH 7.40) during either a resting state or high K+-induced depolarization. Our results show that at the resting state, the baseline [Ca2+]i in apical OHCs (94+/-2.0 nM) was slightly lower than that in basal OHCs (101.1+/-2.4 nM). By contrast, the increase in [Ca2+]i evoked by high K+ depolarization in apical OHCs was about two-fold greater than that in basal OHCs. Nifedipine (30 microM) abolished the increased [Ca2+]i in both types of OHCs, suggesting that Ca2+ influx was mainly through L-type Ca2+ channels of OHCs. While gentamicin and extracellular acidification (pH 7.14) can separately attenuate this increase in [Ca2+]i in both types of OHCs, their suppressive effects are additive in basal OHCs, but not in apical OHCs. The implications of these findings are that: (1) apical and basal OHCs behave differently in response to depolarization-increased [Ca2+]i, and (2) basal OHCs are more vulnerable to the impairment of Ca2+ entry during depolarization by a combination of gentamicin and extracellular acidification, which is correlated with the clinical observation that ototoxicity of aminoglycosides at the basal coil of OHCs is more severe than that at the apical coils. Moreover, the possibility that extracellular acidification may enhance the acute ototoxic effects of aminoglycosides should be considered especially in topical applications.

Block of the alpha9 nicotinic receptor by ototoxic aminoglycosides

In the present study, we report that the alpha9 nicotinic acetylcholine receptor (nAChR) expressed in Xenopus laevis oocytes is reversibly blocked by aminoglycoside antibiotics. The aminoglycosides tested blocked the alpha9 nAChR in a concentration-dependent manner with the following rank order of potency: neomycin>gentamicin>streptomycin>amikacin>kanamycin. The antagonistic effect of gentamicin was not overcome by increasing the concentration of acetylcholine (ACh), indicative of a non-competitive type of block. Blockage of ACh-evoked currents by gentamicin was found to be voltage-dependent, being more potent at hyperpolarized than at depolarized holding potentials. Furthermore, gentamicin blockage was dependent upon the extracellular Ca(2+) concentration, shown by the fact that increments in extracellular Ca(2+) significantly reduced the potency of this aminoglycoside to block the alpha9 nAChR. Possible mechanisms of blockage by the aminoglycosides are discussed. The present results suggest that the initial reversible actions of aminoglycosides at the organ of Corti, such as the elimination of the olivocochlear efferent function, are due in part to the interaction with the native alpha9-containing cholinergic receptor of the outer hair cells.

Gentamicin blocks ACh-evoked K+ current in guinea-pig outer hair cells by impairing Ca2+ entry at the cholinergic receptor

Aminoglycoside antibiotics such as gentamicin are known to block the medial olivocochlear efferent system. In order to determine whether this inhibition takes place at the postsynaptic cholinergic receptors in outer hair cells (OHCs), we studied the effects of these polycationic molecules on cholinergic currents evoked in isolated guinea-pig OHCs. The cholinergic response of OHCs involves nicotinic-like receptors (nAChRs) permeable to Ca2+ ions that activate nearby Ca2+-sensitive K+ channels (KCa(ACh) channels). The extracellular application of gentamicin and neomycin reversibly blocked ACh-evoked K+ current (IK(ACh)) with IC50 values of 5.5 and 3.2 microM, respectively. The results showed that the blocking mechanism of IK(ACh) was due to inhibition of Ca2+ influx via nAChRs. Our study also provides interesting insights into the functional coupling between nAChRs and KCa(ACh) channels in OHCs. By directly recording the cation current flowing through nAChRs (In(ACh)) using an intracellular solution containing 10 mM BAPTA, we measured an EC50 near 110 microM for ACh-evoked In(ACh). This EC50 for ACh is one order of magnitude higher than that measured indirectly on IK(ACh). This reveals a rather low affinity of ACh for its receptor but a very efficient coupling between nAChRs and KCa(ACh) channels. We also show that a high external Ca2+ concentration reverts the gentamicin inhibition of IK(ACh) and that gentamicin directly alters the cation current flowing through the nAChRs of OHCs. We propose that gentamicin acts as a non-competitive cholinergic blocker by displacing Ca2+ from specific binding sites at the nAChRs. This block of the nAChRs at the level of the postsynaptic membrane in OHCs could explain the inhibitory effect of gentamicin reported on the crossed medial olivocochlear efferent system in vivo.

Gentamicin blocks both fast and slow effects of olivocochlear activation in anesthetized guinea pigs

The medial olivocochlear (MOC) efferent system, which innervates cochlear outer hair cells, suppresses cochlear responses. MOC-mediated suppression includes both slow and fast components, with time courses differing by three orders of magnitude. Pharmacological studies in anesthetized guinea pigs suggest that both slow and fast effects on cochlear responses require an initial acetylcholine activation of alpha-9 nicotinic receptors on outer hair cells and that slow effects require additional intracellular events downstream from those mediating fast effects. Gentamicin, an aminoglycoside antibiotic, has been reported to block fast effects of sound-evoked OC activation following intramuscular injection in unanesthetized guinea pigs, without changing slow effects. In the present study, we show that electrically evoked fast and slow effects in the anesthetized guinea pig are both blocked by either intramuscular or intracochlear gentamicin, with similar time courses and/or dose-response curves. We suggest that sound-evoked slow effects in unanesthetized animals are fundamentally different from electrically evoked slow effects in anesthetized animals, and that the former may arise from effects of the lateral OC system.

Characterization of voltage dependent calcium channels on the lateral olivocochlear efferent fibers of the guinea pig

Using in vitro superfusion technique the release of [3H]-dopamine from the lateral olivocochlear efferent fibers of the cochlea was investigated. Our previous study gave the first neurochemical evidence for the transmitter role of dopamine and proved its neuronal origin. Using specific antagonists now we characterized the voltage-dependent calcium channels (VDCCs) involved in the release of dopamine evoked by electrical stimulation of the cochlear tissue. Verapamil or nifedipine, and Ni2+ failed to affect the release, indicating that neither L-, nor T-type VDCCs are essential for the release process. The fact that omega-conotoxin inhibited the release of dopamine from lateral olivocochlear efferent fibers suggests, that N-type VDCCs are required for the calcium influx during electrical stimulation. These VDCCs could be presynaptic targets of modulation of the dopamine release under pathological conditions or in therapy.

Stretch-activated cation channels of leech neurons: characterization and role in neurite outgrowth

The goal of this study was to characterize the stretch-activated ion channels (SACs) of adult identified neurons of the leech Hirudo medicinalis and to test the role of SACs in neurite outgrowth of isolated cells. Using cell-attached patch recording, we established that SACs are densely distributed in the growth cone membrane of cultured neurons. In excised patches, we found that these channels are permeable to Ca2+, as well as to monovalent cations. The channels are blocked by the extracellular application of gadolinium (Gd3+), amiloride and gentamicin. Amiloride and gentamicin, respectively, induce a partial and complete voltage-dependent block. Time-lapse video recordings of neurite outgrowth from single cultured neurons were used to study the effects of blocking SACs with gentamicin. Within 20 h of plating in the presence of the aminoglycoside, the total length of neuronal arborization was significantly greater than that measured in its absence. The amount of assembled axon per unitary surface area remained constant over 40 h and did not differ significantly with or without gentamicin. Our findings show that SACs of leech neurons admit Ca2+, are densely distributed in the growth cone membrane and exhibit typical pharmacological features of mechanotransducer ion channels. In addition, our data suggest that these cation channels participate in the early interaction between growing neurites and culture substrate.

Once daily aminoglycoside therapy. Is it less toxic than multiple daily doses and how should it be monitored?

After 50 years of clinical experience with the aminoglycoside agents, there is continuing debate over the most appropriate administration regimen for these drugs. In recent years, once daily administration has been used increasingly, in the hope of both improving efficacy and reducing toxicity. At least 30 controlled clinical trials have compared once versus conventional multiple daily administration. Efficacy was assessed in some, but not all, studies using clinical and/or bacteriological cure. Toxicity was generally determined using rather nonsensitive end-points such as measurement of serum creatinine for nephrotoxicity and clinically detectable hearing loss for ototoxicity. The results of individual clinical trials and subsequent meta-analyses have been variable. However, 5 of 9 meta-analyses found clinical efficacy to be significantly better with once daily administration, and in 3 of the 9 there were significantly less nephrotoxicity with once daily administration. The results were not significant for ototoxicity in any of the meta-analyses. There is debate about how therapeutic drug monitoring should be performed, and whether it is still required with once daily administration. Previous experience with the aminoglycosides, especially in patients with impaired drug clearance caused by renal impairment, suggests that monitoring is still prudent. Results from the once daily administration trials appear to support this. Various methods of monitoring and dose adjustment have been proposed. The most common is to measure a 24-hour trough concentration and to adjust the dose to maintain the trough concentration below a value of 2, 1 or 0.5 mg/L. However, this method allows for greater total aminoglycoside exposure than has been permitted with conventional dosages, increasing the likelihood of toxicity in patients with impaired aminoglycoside clearance. Other methods measure drug concentrations at a time-point or points within the dose interval (when the concentration is still measurable), and adjust the dose according to concentration-time curve nomograms or to a target area under the concentration-time curve. This allows the use of higher doses in those with high drug clearance. Furthermore, in patients with impaired clearance, drug exposure is limited to the same extent as, or less than, that with conventional multiple daily administration. To date no controlled trials have compared methods of dose-individualisation. In summary, in addition to a slight overall improvement in efficacy, once daily administration has resulted in a small reduction in nephrotoxicity. In the studies using more sensitive measures of toxicity, the differences in toxicity were greater, strengthening the case for once daily administration. Therapeutic drug monitoring is probably required with once daily administration. Methods which use mid-dosage interval concentrations to gauge drug exposure would seem to be preferable over trough concentration measurement.

Experimental model of cisplatin ototoxicity in chinchillas

Cisplatin is an important antineoplastic agent. Its ototoxicity has been well defined, both in human and animal studies. However, animal models of systemic cisplatin administration have been complicated by multiple toxic effects. We studied cisplatin ototoxicity in an animal model involving topical application of cisplatin to the round-window membrane. Adult chinchillas were anesthetized with ketamine and pentobarbital, and auditory function was tested with the use of auditory brain-stem responses to various stimuli (clicks and 8-and 16-kHz tone bursts). Each animal was used as its own control. The middle-ear cavity was exposed through the bulla. In the experimental ear, a 25-microl solution of 0.25 mg cisplatin/1.0 ml normal saline solution was applied to the round-window membrane. In the control ear, 25 microl normal saline solution was applied to the round-window membrane. Follow-up auditory brain-stem response testing was conducted 7 days after treatment. A significant increase in threshold in the experimental ears was seen on comparison with the control ears. This finding suggests that application of cisplatin to the round-window membrane is a useful animal model in which to study cisplatin ototoxicity.

Mapping the distribution of outer hair cell voltage-dependent conductances by electrical amputation

The mammalian outer hair cell (OHC) functions not only as sensory receptor, but also as mechanical effector; this unique union is believed to enhance our ability to discriminate among acoustic frequencies, especially in the kilohertz range. An electrical technique designed to isolate restricted portions of the plasma membrane was used to map the distribution of voltage-dependent conductances along the cylindrical extent of the cell. We show that three voltage-dependent currents, outward K, I(K,n), and I(Ca) are localized to the basal, synaptic pole of the OHC. Previously we showed that the lateral membrane of the OHC harbors a dense population of voltage sensor-motor elements responsible for OHC motility. This segregation of membrane molecules may have important implications for auditory function. The distribution of OHC conductances will influence the cable properties of the cell, thereby potentially controlling the voltage magnitudes experienced by the motility voltage sensors in the lateral membrane, and thus the output of the "cochlear amplifier."

Cisplatin metabolites and their toxicity on isolated cochlear outer hair cells in vitro

The exact mechanism of ototoxicity by cisplatin remains obscure. In the present study, the toxicity of cisplatin metabolites was evaluated using isolated outer hair cells of guinea pig cochlea. Cisplatin metabolites were produced by reaction of cisplatin and S9 fraction from guinea pig liver after administration with phenobarbital. The viability of the outer hair cells did not decrease after the incubation in non-metabolized cisplatin solutions or S9 fraction only, demonstrating the absence of toxic effect from non-metabolized cisplatin and the absence of intrinsic toxic substances in S9 fraction. However, metabolized cisplatin reacted with S9 fraction at 37 degrees C significantly decreased the viability. Cisplatin reacted with heat-treated S9 fraction did not produce a cytotoxic effect, suggesting an enzymatic basis for the formation of metabolites. When NADP was omitted, outer hair cells remained viable, suggesting the significant role of oxygenases in the metabolism of cisplatin. The existence of a toxic metabolites could account for the poor correlation between drug peak level and pathologic damage to the inner ear, and a delay in the onset of ototoxicity could represent the time necessary for induction of the metabolizing enzymes or for reaching a critical point in toxicity by metabolites.

Suppression of a nonselective cation conductance by substance P in cochlear outer hair cells

Substance P (SP) hyperpolarizes outer hair cells (OHCs) of guinea pig cochlea. The cellular mechanisms of the SP response were investigated with the whole cell patch-clamp technique. SP induced outward currents in a dose-dependent manner at a holding potential of -60 mV in a concentration range between 3 x 10(-6) and 10(-4) M. SP decreased slope conductance between -60 and +20 mV. Ion substitutions in the external medium revealed that SP suppresses nonselective cation conductance with high permeability for Ca2+. The relative ion permeability of the channel modulated by SP was as follows: Ca2+ > Li+ approximately Cs+ approximately Na+ > Tris+. The potency of the agonist action was as follows: SP >> neurokinin A > neurokinin B. Peptide antagonists induced currents similar to those of SP. CP-96345, a selective nonpeptide antagonist for the neurokinin type 1 receptor, did not inhibit the SP-induced current. Intracellular dialysis of guanosine 5'-O-(2-thiodiphosphate) and pertussis toxin (PTX) suggests that a PTX-insensitive G protein is involved in the SP response. Neither the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid nor staurosporine (10(-6) M) affects the SP response. Local application of SP by a puffer pipette indicates that the SP receptors are distributed along the side of the OHC. These results suggest the possibility that the action of SP on the OHCs may not be mediated by the tachykinin receptors but rather by a tachykinin receptor-independent pathway. It is proposed that SP suppresses the nonselective cation conductance in the lateral wall of OHCs via a PTX-insensitive G protein.

Transplatin blocks voltage-dependent calcium current in isolated cochlear outer hair cells but is not ototoxic in vivo

The effects of cisplatin (cis-DDP) and transplatin (trans-DDP) on the voltage-dependent calcium current (ICa) of outer hair cells isolated from the guinea pig cochlea were compared using the whole-cell patch-clamp technique. Both cis-DDP and trans-DDP blocked ICa in a concentration-dependent manner. Trans-DDP did not show ototoxicity or nephrotoxicity in vivo. From these results, it was suggested that the effect of cis-DDP on ICa does not directly lead to cell death.

ATP modulation of L-type calcium channel currents in guinea pig outer hair cells

Ca2+ channel currents and their modulation by adenosine 5'-triphosphate (ATP) in acutely isolated guinea pig outer hair cells (OHCs) were investigated using the whole-cell patch-clamp technique. The current-voltage (I-V) relation of OHCs indicated that the Ca2+ channel opened near -30 mV, and the current reached a maximum at +10 and 0 mV in 20 mM Ca2+ and Ba2+ external solutions, respectively. BayK 8644 (BayK, 2 microM) caused a 3.5-fold increase in peak Ca2+ currents and shifted the I-V curves toward more negative potentials. These results suggest that the majority of Ca2+ channels in OHCs have L-type characteristics. The effects of ATP on Ca2+ channels of OHCs were heterogenous. ATP (100 microM) decreased Ca2+ channel currents by 31.7 +/- 5.6% at 0 mV and shifted Ca2+ tail activation curves toward more depolarized potentials in some cells (N = 6). By contrast, in others, ATP enhanced the currents by 43.5 +/- 12.5% at +10 mV (N = 6). In the presence of BayK, however, ATP-induced inhibition or enhancement of Ca2+ channel currents was attenuated. In addition, 100 microM ATP produced little effect on Ca2+ channel currents in another subpopulation of cells (N = 12). This heterogenous neuromodulation of Ca2+ channel currents by ATP may reflect a functional diversity among OHCs.

Caffeine rapidly decreases potassium conductance of dissociated outer hair cells of guinea-pig cochlea

The effects of caffeine on the outer hair cells (OHCs) freshly dissociated from guinea-pig cochlea were investigated with the whole-cell patch-clamp technique, in both the conventional and the nystatin perforated patch-clamp configurations under voltage-clamp condition. Application of caffeine (> 1 mM for 10-30 s) induced an inward current (Icaffeine) with decrease of conductance in a dose-dependent manner at a holding potential (VH) of -60 mV. The reversal potential of Icaffeine (Ecaffeine) was close to the K+ equilibrium potential. The Icaffeine was not affected by Ca(2+)-free external solution. The internal perfusion of the Ca2+ chelator BAPTA had no effect on Icaffeine. The Icaffeine was not modulated by the external application of H-8 or staurosporine and by the internal perfusion of GDP-beta S. The amplitude of Icaffeine was the largest at the basal region of OHCs when caffeine was locally applied by the 'puffer' method. These results suggest that caffeine induces a decrease in membrane potassium conductance of the OHCs mainly at the basal region without mediating the intracellular signaling pathway.

Ionic properties of IK,n in outer hair cells of guinea pig cochlea

The ionic properties of voltage-dependent K+ current activated at the resting membrane potential (IK,n) of outer hair cells (OHC) isolated from the guinea pig cochlea were studied using a patch-clamp technique in a whole-cell recording mode. The reversal potential of IK,n indicated a high selectivity for K+, and the relative permeability ratios for various monovalent cations were K+:Rb+:NH4+ = 1:1.21:0.13. Decrease in extracellular Cl- inhibited the IK,n. IK,n was blocked by Cs+ and Ba2+, although the inhibitory manner of Cs+ and Ba2+ were voltage-dependent and voltage-independent, respectively. By the use of puff-application method, the local application of Ba2+ to basolateral surface of OHC shifted the holding current level in an inward direction, whereas the application to apex and hair showed little change. Indicating that the IK,n channels preferentially locate at the basolateral region of cell membrane.

Cisplatin blocks voltage-dependent calcium current in dissociated outer hair cells of guinea-pig cochlea

The effect of the ototoxic molecule cisplatin (cis-DDP) on the voltage-dependent Ca2+ channel in dissociated outer hair cells (OHCs) of guinea-pig cochlea was investigated using a whole-cell patch-clamp technique. Cis-DDP had antagonistic effect on the Ca2+ channel and reversibly suppressed the Ca2+ current in a concentration-dependent manner. These results suggested that one of the ototoxic mechanisms of cis-DDP is involved in the inhibition of the Ca2+ channel in OHCs.

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.