S. Komune

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

The effects of various Ca2+ antagonists and aminoglycoside antibiotics on the Ca2+ channel in isolated outer hair cells of the guinea pig were investigated using a whole-cell patch-clamp technique. The inhibitory action was in the order of La3+ much greater than Cd2+ much greater than Ni2+ greater than Co2+ for inorganic Ca2+ antagonists, and flunarizine = nicardipine greater than omega-conotoxin greater than methoxyverapamil = diltiazem much greater than amiloride for organic ones. Aminoglycoside antibiotics also had antagonistic effects on the Ca2+ channel.

The mechanism underlying maintenance of the endocochlear potential by the K+ transport system in fibrocytes of the inner ear

•  The endocochlear potential (EP) of +80 mV in cochlear endolymph is essential for audition and controlled by K+ transport across the lateral cochlear wall composed of two epithelial barrier layers, the syncytium containing the fibrocytes and the marginal cells. •  The EP depends upon the diffusion potential elicited by a large K+ gradient across the apical surface of the syncytium. •  We examined by electrophysiological approaches an involvement of Na+,K+‐ATPase, which occurs at the syncytiums basolateral surface comprising the fibrocytes’ membranes and would mediate K+ transport across the lateral wall, in maintenance of the EP. •  We show that the Na+,K+‐ATPase sustains the syncytiums high [K+] that is crucial for the K+ gradient across the apical surface of the syncytium. •  The results help us better understand the mechanism underlying the establishment of the EP as well as the pathophysiological process for deafness induced by dysfunction of the ion transport apparatus.

Disturbance of Regulation of Sodium by CIS-Diamminedichloroplatinum in Perilymph of the Guinea Pig Cochlea

We studied the acute effects of cis-diamminedichloroplatinum (CDDP) on the cochlear partition and inner ear fluid in the guinea pig. At 48 hours after the administration of a single intramuscular injection of CDDP, 12.5 mg/kg of body weight, the endocochlear resting potential (EP) was significantly decreased to 32.1 ± 1.8 mV in the treated animals, versus 80.6 ±.1.0 mV in the control animals. There was a significant rise in potassium (K+), sodium (Na+), and chlorine (Cl−) in the endolymph of the animals treated with CDDP as compared with the control animals. Only Na+ was found to increase significantly in the perilymph, reaching more than twice the level of the control animals; both K+ and Cl− remained within the normal range. Serum electrolytes also remained within the normal range. Evaluation of modified ionic permeabilities across the endolymph-perilymph barrier showed an apparent increase in Na+ permeability and a normal range of K+ and Cl− permeabilities. Histopathologic examination of the cochlea showed a moderate collapse of the endolymphatic space, with atrophy of the stria vascularis and destruction of the outer hair cells. The findings suggest that the acute changes produced in the cochlea by administration of CDDP were attributable to a breakdown in the regulation of Na+ metabolism in the perilymph.

Single Channel Recordings of Calcium Currents in Chick Cochlear Hair Cells

Single Ca2+ channel currents were recorded from chick cochlear hair cells by using cell-attached patches. The elementary current was about -2 pA at 70 mV positive to the resting potential (R.P.+ 70 mV); the slope conductance over a 40 mV voltage range was 24 pS. The open time histogram was reasonably well expressed by a single exponential function, while the closed time histogram was expressed by a sum of two exponential functions. The Ca2+ channel was activated around R.P.+ 60 mV and the average ensemble current did not decay during 130 ms depolarization, suggesting that the Ca2+ channel in chick cochlear hair cells is the L-type. The Ca2+ channel has three modes of gating behaviour, expressed as current records with brief openings (mode I), no openings (mode 0), or long-lasting openings and very brief closings (mode 2).

Congenital malformation of the inner ear associated with recurrent meningitis

Abstract Congenital deformities of the labyrinth of the inner ear can be associated with meningitis and varying degrees of hearing loss or deafness. A recurrence of meningitis is due to the development of a fistulous communication between the subarachnoid space and the middle ear cavity, and can prove lethal. An illustrative case of a 4-year-old Japanese girl with bilateral severe hearing loss, recurrend meningitis and malformations of the inner ear and stapes footplate is presented. Removal of the stapes during tympanotomy provoked a gush of cerebrospinal fluid. The defect was repaired successfully, and there has been no further episodes of meningitis to date.

A 16-year survey of changes in bacterial isolates associated with chronic suppurative otitis media

Bacteria were isolated and cultured from the ears of outpatients with chronic suppurative otitis media (OMC) who attended the otologic clinics of Kyushu University Hospital from 1976 to 1991. Bacterial isolates were analyzed retrospectively to determine their incidences and antibiotic sensitivities.Staphylococcus aureus andPseudomonas aeruginosa were the most commonly isolated organisms. The incidence ofS. aureus infections gradually rose over 16 years, while the incidence ofProteus infections gradually declined. The frequency of isolation of glucose non-fermenting gram-negative rod (NF-GNR) species, fungi and anaerobic bacteria gradually increased. The species of bacteria isolated from OMC cases with acute and chronic infections, cholesteatoma and postoperative infections were compared. Over the 16 years, the isolation ofS. aureus increased in frequency. The differences in each type of OMC ceased to exist.S. aureus were quite sensitive to most of the antibiotics tested, whereasPs. aeruginosa and NF-GNR showed poor sensitivity.

Regulation of inner ear fluid in the guinea pig cochlea after the application of saturated NaCl solution to the round window membrane

Abstract The regulation of K+ and Na+ in the inner ear fluid of the guinea pig was studied after the application of saturated NaCl solution to the round window membrane. K+ and Na+ activities in the scala tympani increased rapidly and then decreased. K+ activity in the scala media increased immediately, but Na+ activity continued to increase during the period of observation. K+ activity in the scala vestibuli continued to increase in the observation period. Na+ activity in the scala vestibuli increased and then decreased. The endocochlear potential decreased immediately to approximately 20% of its initial level. Total activities of K+ and Na+ increased immediately and then decreased in both the scala tympani and scala media. The total activity of K+ and Na+ increased slowly and showed no regulatory decrease in the scala vestibuli. Thus, changing patterns in the total activity of K+ and Na+ were similar for the scala tympani and scala media, but not for the scala media and the scala vestibuli. Different patterns of K+ and Na+ activities among the three scalae indicate that their mechanisms for regulating inner ear fluid differ.

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.

The effects of ototoxic drugs on mechano-electric transduction channels in chick cochlear hair cells.

The effects of ototoxic drugs on mechano-electrical transduction (MET) currents were investigated in dissociated cochlear hair cells of the chick, using a wholecell patch-electrode voltage-clamp technique. Dihydrostreptomycin (DHSM) and cisplatin (cis-dichlorodiammine platinum II, CDDP) blocked the MET channel in a dose- and voltage-dependent manner. In contrast, acetyl salicylate did not suppress the MET current. At −50 mV, DHSM and CDDP blocked the MET channel with a Hill coefficient of 0.93 and 2.1, respectively. These findings suggest that a single DHSM molecule or more than one CDDP molecule binds to a single MET channel.

Susceptibility of the endocochlear potential to pH and osmolarity changes in the perilymph of the cochlea in the guinea pig

SummaryThe effects of artificial perilymph at various pH levels and osmolarities on the endocochlear potential (EP) were investigated in a guinea pig model. In 47 ears, the mean EP was 74.5 ± 0.5 mV in the second turn of the cochlea before perilymphatic perfusion. The artificial perilymph with a pH ranging from 5 to 9 and tonicity less than 1.25 (osmolarity, 345 mosmol/1) did not alter the EP when testings were recorded for at least 1 h. However, after the start of a perfusion with a pH less than 4, the EP declined gradually but recovered to the pre-perfusion level rapidly when the perfusion was stopped. The artificial perilymph with a pH over 10 decreased the EP considerably but showed no recovery, even after cessation of the perfusion. Tonicity more than 1.35 (osmolarity, 372.6 mosmol/1) also decreased the EP, with this decrease paralleling further increases in the tonicity used. From these data, we have concluded that a perilymph perfusate with a pH from 5 to 9 and an osmolarity less than 350 mosmol/I can be used in animal testing without any noxious effects on the EP.