The gastric H,K-ATPase in stria vascularis contributes to pH regulation of cochlear endolymph but not to K secretion

BACKGROUND

Disturbance of acid-base balance in the inner ear is known to be associated with hearing loss in a number of conditions including genetic mutations and pharmacologic interventions. Several previous physiologic and immunohistochemical observations lead to proposals of the involvement of acid-base transporters in stria vascularis.

RESULTS

We directly measured acid flux in vitro from the apical side of isolated stria vascularis from adult C57Bl/6 mice with a novel constant-perfusion pH-selective self-referencing probe. Acid efflux that depended on metabolism and ion transport was observed from the apical side of stria vascularis. The acid flux was decreased to about 40 % of control by removal of the metabolic substrate (glucose-free) and by inhibition of the sodium pump (ouabain). The flux was also decreased a) by inhibition of Na,H-exchangers by amiloride, dimethylamiloride (DMA), S3226 and Hoe694, b) by inhibition of Na,2Cl,K-cotransporter (NKCC1) by bumetanide, and c) by the likely inhibition of HCO3/anion exchange by DIDS. By contrast, the acid flux was increased by inhibition of gastric H,K-ATPase (SCH28080) but was not affected by an inhibitor of vH-ATPase (bafilomycin).  K flux from stria vascularis was reduced less than 5 % by SCH28080.

CONCLUSIONS

These observations suggest that stria vascularis may be an important site of control of cochlear acid-base balance and demonstrate a functional role of several acid-base transporters in stria vascularis, including basolateral H,K-ATPase and apical Na,H-exchange. Previous suggestions that H secretion is mediated by an apical vH-ATPase and that basolateral H,K-ATPase contributes importantly to K secretion in stria vascularis are not supported. These results advance our understanding of inner ear acid-base balance and provide a stronger basis to interpret the etiology of genetic and pharmacologic cochlear dysfunctions that are influenced by endolymphatic pH.

Epithelial cell stretching and luminal acidification lead to a retarded development of stria vascularis and deafness in mice lacking pendrin

Loss-of-function mutations of SLC26A4/pendrin are among the most prevalent causes of deafness. Deafness and vestibular dysfunction in the corresponding mouse model, Slc26a4^−/−, are associated with an enlargement and acidification of the membranous labyrinth. Here we relate the onset of expression of the HCO₃⁻ transporter pendrin to the luminal pH and to enlargement-associated epithelial cell stretching. We determined expression with immunocytochemistry, cell stretching by digital morphometry and pH with double-barreled ion-selective electrodes. Pendrin was first expressed in the endolymphatic sac at embryonic day (E) 11.5, in the cochlear hook-region at E13.5, in the utricle and saccule at E14.5, in ampullae at E16.5, and in the upper turn of the cochlea at E17.5. Epithelial cell stretching in Slc26a4^−/− mice began at E14.5. pH changes occurred first in the cochlea at E15.5 and in the endolymphatic sac at E17.5. At postnatal day 2, stria vascularis, outer sulcus and Reissner's membrane epithelial cells, and utricular and saccular transitional cells were stretched, whereas sensory cells in the cochlea, utricle and saccule did not differ between Slc26a4^+/− and Slc26a4^−/− mice. Structural development of stria vascularis, including vascularization, was retarded in Slc26a4^−/− mice. In conclusion, the data demonstrate that the enlargement and stretching of non-sensory epithelial cells precedes luminal acidification in the cochlea and the endolymphatic sac. Stretching and luminal acidification may alter cell-to-cell communication and lead to the observed retarded development of stria vascularis, which may be an important step on the path to deafness in Slc26a4^−/− mice, and possibly in humans, lacking functional pendrin expression.

Endocytosis of microperoxidase in the marginal cells of stria vascularis

OBJECTIVE

Endocytosis has been thought to control entry into the cell and play a crucial role in the development, immune response, neurotransmission, intercellular communication, signal transduction, and cellular and organismal homeostasis. We investigated the basic properties of endocytosis in the marginal cells of stria vascularis (SV) to discuss whether marginal cells have a potential to maintain the endolymph homeostasis.

METHODS

We perfused microperoxidase (MPO), an endocytosis tracer, into the cochlear duct. After 5-60 min of endolymphatic perfusion, the tissues were fixed and the distribution of MPO within the marginal cell was observed by transmission electron microscopy.

RESULTS

Endocytosis started already at 5 min after MPO perfusion. Small MPO-loaded endosomes were observed up to 30 min after MPO perfusion. The small tubulovesicular endosomes and the plasma membrane invagination were not decorated by an electron dense bristle structure. After endocytosis, MPO labeled preendosomes were quickly transported to the large vacuolar endosomes that connected with tubular endosomes. At 60 min after MPO perfusion, MPO-loaded large vesicles that have small vesicles in the lumen were observed.

CONCLUSION

The time-course of MPO-loaded endosomes was similar to that of CF-loaded endosomes in the marginal cells of SV. The strial marginal cells have vigorous endocytotic activity both in clathrin-independent and clathrin-dependent endocytosis. This high activity of endocytosis in SV seems to be needed to maintain the homeostasis of endolymph via membranous channels and/or receptors regulations.

Gastric type H+,K+-ATPase in the cochlear lateral wall is critically involved in formation of the endocochlear potential

Cochlear endolymph has a highly positive potential of approximately +80 mV known as the endocochlear potential (EP). The EP is essential for hearing and is maintained by K(+) circulation from perilymph to endolymph through the cochlear lateral wall. Various K(+) transport apparatuses such as the Na(+),K(+)-ATPase, the Na(+)-K(+)-2Cl(-) cotransporter, and the K(+) channels Kir4.1 and KCNQ1/KCNE1 are expressed in the lateral wall and are known to play indispensable roles in cochlear K(+) circulation. The gastric type of the H(+),K(+)-ATPase was also shown to be expressed in the cochlear lateral wall (Lecain E, Robert JC, Thomas A, and Tran Ba Huy P. Hear Res 149: 147-154, 2000), but its functional role has not been well studied. In this study we examined the precise localization of H(+),K(+)-ATPase in the cochlea and its involvement in formation of EP. RT-PCR analysis showed that the cochlea expressed mRNAs of gastric alpha(1)-, but not colonic alpha(2)-, and beta-subunits of H(+),K(+)-ATPase. Immunolabeling of an antibody specific to the alpha(1) subunit was detected in type II, IV, and V fibrocytes distributed in the spiral ligament of the lateral wall and in the spiral limbus. Strong immunoreactivity was also found in the stria vascularis. Immunoelectron microscopic examination exhibited that the H(+),K(+)-ATPase was localized exclusively at the basolateral site of strial marginal cells. Application of Sch-28080, a specific inhibitor of gastric H(+),K(+)-ATPase, to the spiral ligament as well as to the stria vascularis caused prominent reduction of EP. These results may imply that the H(+),K(+)-ATPase in the cochlear lateral wall is crucial for K(+) circulation and thus plays a critical role in generation of EP.

Cu/Zn superoxide dismutase and age-related hearing loss

Mice, in which the genetics can be manipulated and the life span is relatively short, enable evaluation of the effects of specific gene expression on cochlear degeneration over time. Antioxidant enzymes such as Cu/Zn superoxide dismutase (SOD1) protect cells from toxic, reactive oxygen species and may be involved in age-related degeneration. The effects of SOD1 deletion and over-expression on the cochlea were examined in Sod1-null mice, Sod1 transgenic mice and in age- and genetics-matched controls. Auditory brainstem responses (ABR) were measured and cochleae were histologically examined. The absence of SOD1 resulted in hearing loss at an earlier age than in wildtype or heterozygous mice. The cochleae of the null mice had severe spiral ganglion cell degeneration at 7-9 months of age. The stria vascularis in the aged, null mice was thinner than in the heterozygous or wildtype mice. Over-expression of SOD1 did not protect against hearing loss except at 24 months of age. In conclusion, SOD1 seems important for survival of cochlear neurons and the stria vascularis, however even half the amount is sufficient and an over abundance does not provide much protection from age-related hearing loss.

Matrix metalloproteinase dysregulation in the stria vascularis of mice with Alport syndrome: implications for capillary basement membrane pathology

Alport syndrome results from mutations in genes encoding collagen alpha3(IV), alpha4(IV), or alpha5(IV) and is characterized by progressive glomerular disease associated with a high-frequency sensorineural hearing loss. Earlier studies of a gene knockout mouse model for Alport syndrome noted thickening of strial capillary basement membranes in the cochlea, suggesting that the stria vascularis is the primary site of cochlear pathogenesis. Here we combine a novel cochlear microdissection technique with molecular analyses to illustrate significant quantitative alterations in strial expression of mRNAs encoding matrix metalloproteinases-2, -9, -12, and -14. Gelatin zymography of extracts from the stria vascularis confirmed these findings. Treatment of Alport mice with a small molecule inhibitor of these matrix metalloproteinases exacerbated strial capillary basement membrane thickening, demonstrating that alterations in basement membrane metabolism result in matrix accumulation in the strial capillary basement membranes. This is the first demonstration of true quantitative analysis of specific mRNAs for matrix metalloproteinases in a cochlear microcompartment. Further, these data suggest that the altered basement membrane composition in Alport stria influences the expression of genes involved in basement membrane metabolism.

Probable function of Boettcher cells based on results of morphological study: localization of nitric oxide synthase

Boettcher cells lie on the basilar membrane beneath Claudius cells. The cells are considered supporting cells for the organ of Corti, and present only in the lower turn of the cochlea, which responds to high-frequency sound. Boettcher cells interdigitate with each other, and project microvilli into the intercellular space. Their structural specialization suggests that Boettcher cells may play a significant role in the function of the cochlea. Nitric oxide synthase (NOS) has previously been detected in substructures of the cochlea. In the cochlea, it is believed that nitric oxide plays an important role in neurotransmission, blood flow regulation, and induction of cytotoxicity under pathological conditions. Findings concerning detection of NOS on Boettcher cells are rare. We demonstrated here the localization of NOS on Boettcher cells of the rat by immunohistochemistry using polyclonal antibody to NOS. On observation with the light microscope using DAB staining, positive immunostaining to NOS was observed in Boettcher cells. In immunoelectron micrographs, NOS was detected abundantly in the cytoplasm of the interdigitations. This suggests that the interdigitations may play significant roles by using NOS. It follows from this that the nitric oxide (NO) on Boettcher cells may influences neighboring Boettcher cells. The ultrastructure of Boettcher cells suggests that they may be active cells, which perform both secretory and absorptive functions.

Upregulated iNOS and oxidative damage to the cochlear stria vascularis due to noise stress

Our previous work has revealed increased nitric oxide (NO) production in the cochlear perilymph following noise stress. However, it is not clear if the increase of NO is related to iNOS and whether NO-related oxidative stress can cause vascular tissue damage. In this study, iNOS immunoreactivity, NO production, and reactive oxygen species (ROS) in the lateral wall were examined in normal mice and compared with similar animals exposed to 120 dBA broadband noise, 3 h/day, for 2 consecutive days. In the normal animals, iNOS expression was not observed in the vascular endothelium of the stria vascularis and only weak iNOS immunoactivity was detected in the marginal cells. However, expression of iNOS in the wall of the blood vessels of stria vascularis and marginal cells was observed after loud sound stress (LSS). Relatively low levels of NO production and low ROS activity were detected in the stria vascularis in the unstimulated condition. In contrast, NO production was increased and ROS activity was elevated in the stria vascularis after LSS. These changes were attenuated by the iNOS inhibitor, GW 274150. To explore whether noise induces apoptotic processes in the stria vascularis, we examined morphological changes in endothelial- and marginal-cells. In vitro, annexin-V phosphatidylserine (PS) (to label and detect early evidence of apoptosis) was combined with propidium iodide (PI) (to probe plasma membrane integrity). PI alone was used in fixed tissues to detect later stage apoptotic cells by morphology of the nuclei. Following LSS, PS was expressed on cell surfaces of endothelial cells of blood vessels and marginal cells of the stria vascularis. Later stage apoptosis, characterized by irregular nuclei and condensation of nuclei, was also observed in these cells. The data indicate that increased iNOS expression and production of both NO and ROS following noise stress may lead to marginal cell pathology, and the dysfunction of cochlear microcirculation by inducing blood vessel wall damage.

Nuclear-factor kappa B (NF-kappa B)-inducible nitric oxide synthase (iNOS/NOS II) pathway damages the stria vascularis in cisplatin-treated mice

BACKGROUND

Cisplatin is reported to damage the stria vascularis of the cochlea. Free radicals, especially large amounts of nitric oxide catalyzed by inducible nitric oxide synthase, are considered to have an important role in this toxicity. The induction of inducible nitric oxide synthase is regulated by nuclear-factor kappa B (NF-kappa B). We examined the damage of the stria vascularis by immunohistochemical techniques.

MATERIALS AND METHODS

Cisplatin (15 mg/kg b.w.) was injected intraperitoneally into the mice. Three days after the injection, the cochleas were immunohistochemically-stained using specific antibodies for nuclear-factor kappa B (NF-kappa B), inducible nitric oxide synthase (iNOS) or single-stranded DNA.

RESULTS

NF-kappa B was expressed in the cisplatin-treated cochlea, especially in the stria vascularis and the spiral ligament. iNOS was also expressed in the stria vascularis and the spiral ligament. Fragments of DNA were observed only in the stria vascularis.

CONCLUSION

The large amounts of NO catalyzed by iNOS led to inner ear dysfunction. Our results indicate that apoptosis is triggered by iNOS and that it mediates the ototoxicity induced by cisplatin.

Localization of nitric oxide synthase isoforms in the human cochlea

The location of nitric oxide (NO) in the structures of the cochlea is a topical issue. Nitric oxide synthase (NOS) has been detected previously in mammalian cochleae, but information on its presence in the human cochlea is still sparse. The location of NOS isoforms I, II and III in substructures of the human cochlea was studied by immunohistochemistry (fluorescein isothiocyanate technique) using monoclonal antibodies to NOS I, II and III. NOS I was the predominant isoform and staining could be observed in cells of the spiral ganglion (SG), in nerve fibres and in the outer hair cells (OHC). Furthermore, the supporting cells of the organ of Corti and the stria vascularis showed a fluorescent reaction to NOS I. Staining for NOS III was less intense and was located in the OHC, supporting cells and SG cells, while the stria vascularis remained unstained. By contrast, NOS II showed weak staining in a few neuron fibres only. The results imply that NO in the human cochlea could act as a neurotransmitter/neuromodulator at the level of neural cells and may be involved in the physiology of the supporting cells and stria vascularis. Moreover, because NO is both a mediator of excitotoxicity and a non-specifically toxic radical, it may also play a role in neurotoxicity of the human cochlea.

Occurrence of NaK-ATPase isoforms during rat inner ear development and functional implications

This study examined the presence of NaK-ATPase isoforms in the developing inner ear of the rat and studied the importance of functional subunit combinations in endolymph homeostasis. The findings were: (a) the combination alpha 1 beta 1 is found in epithelial, mesenchymal, and neural inner ear cells with an early starting expression 14 days postconception (dpc) in some endolymphatic sac cells; (b) from 1 day after birth (dab) expression of alpha 1 beta 2 is observed in marginal cells, vestibular dark cells, and certain vestibular nonsensory cells; (c) a transient expression of alpha 2 beta 1 is found in suprastrial fibrocytes and spiral ligament fibrocytes type II between 10 and 15 dab; (d) starting at 16 dpc the combination alpha 3 beta 1 is uniquely expressed in inner ear neural cells (as in other neural tissues). In conclusion, during development a switch from alpha 2 beta 1 towards alpha 1 beta 1 is observed in suprastrial fibrocytes and in spiral ligament fibrocytes type II. Thus, according to the biochemical characteristics of these combinations, a switch towards a NaK-ATPase with higher capacity takes place. In addition, prominent expression of the alpha 1 beta 2 combination in predominantly K+ ion transporting marginal and dark cells is in accordance with the characteristic of this combination and thus with the presumed function of these cells as important K+ suppliers for the endolymph. We believe this combination in certain vestibular nonsensory cells to be involved in K+ sensing. Early expression of the alpha 1 beta 1 combination in the endolymphatic sac, prior to that in the other parts of the inner ear, suggests that this structure may be involved to some extent in the development of the vestibulum and cochlea.

[Effect of injectio Salvia Miltiorrhiza on gentamicin ototoxicity-induced activity of nitric oxide synthase in cochlear stria vascularis of guinea pig]

AIM

to investigate the change of nitric oxide synthase (NOS) activity in cochlear stria vascularis (SV) of guinea pig after gentamicin (GM) and Salvia Miltiorrhiza (SM) injection, and to explore the protective role of injectio SM on GM ototoxicity.

METHODS

NADPH-diaphorase histochemistry staining and image quantitative analysis technique, combined with auditory brainstem response (ABR) measurement.

RESULTS

SM + GM significantly reduced NOS activity in cochlear SV and ABR threshold as compared with CM along (P < 0.01); and ABR threshold shift was in high correlation with NOS activity (rControl = -0.9464; rGM = -0.9117; rSM + GM = -0.8958; P < 0.01).

CONCLUSION

SM can reduce NOS activity in cochlear SV so as to alleviate GM ototoxicity, thus ameliorate hearing function.

Effect of a beta-stimulant on the inner ear stria vascularis

In vivo and in vitro experiments were conducted to investigate the effect of alpha-isoproterenol on the inner ear stria vascularis with intracellular cytochrome oxidase activity used as an index. Intraperitoneal injection of alpha-isoproterenol (5 mg/kg) was performed in 10 rats, and that of physiological saline in 4 rats, for 21 consecutive days. After the 3-week treatment, bilateral cochleas were excised for frozen sections and stained for cytochrome oxidase. The staining density of the stria vascularis for the enzyme was analyzed with a computer. Electron microscopic observation was also performed for some specimens. As for the in vitro experiments, bilateral cochleas from 6 normal rats were excised for cell culture. Cochlear cells from the right ear were cultured with medium containing alpha-isoproterenol (10-micromol/L concentration), and those from the left ear with medium alone. After 3-day culture, the enzyme activity of cytochrome oxidase in the stria vascularis was quantified by the same method used for the in vivo experiments. Cytochrome oxidase activity was markedly elevated in the alpha-isoproterenol group. The activity tended to be higher in the lower turns of the cochlea. Electron microscopy revealed that numerous mitochondria were present in marginal cells that protruded into the endolymphatic space. The enzyme activity was also elevated in the stria vascularis from cochlear specimens in the alpha-isoproterenol group of the in vitro experiment. The above results suggest that alpha-isoproterenol accelerated the metabolic activity of the cells that constitute the stria vascularis. The increase in activity was probably attributable to direct pharmaceutical effects of the beta-stimulant, rather than an increase in blood flow. It is possible that the cells that constitute the stria vascularis may have beta-receptors.

Lipopolysaccharide-induced expression of inducible nitric oxide synthase in the guinea pig organ of Corti

The purpose of the investigation was to ascertain whether inoculation of bacterial lipopolysaccharide (LPS) into the cochlea of the guinea pig could elicit formation of inducible nitric oxide synthase (iNOS). Immunohistochemical study revealed that immunoreactivity to iNOS was seen below outer hair cells representing nerve fibers and synaptic nerve endings. iNOS-staining could also be observed in phalangeal dendrites of Deiter's cells pointing to the cuticular membrane, Hensen's cells and on stria vascularis 48 h after inoculation with LPS. Immunohistochemical investigation with a specific anti-nitrotyrosine antibody also revealed intense immunoreactivity identical to that of iNOS, suggesting formation of peroxynitrite in the organ of Corti by the reaction of NO with O(2)(-). On the basis of these findings, it can be concluded that NO together with O(2)(-), which form the more reactive peroxynitrite, are the most important pathogenic agents in LPS-induced damage of cochlea in the guinea pig.

Effects of epinephrine on ouabain-sensitive, K(+) -dependent P-nitrophenylphosphatase activity in strial marginal cells of guinea pigs

In strial marginal cells, Na+/K(+)-ATPase activity is abundant, and contributes to maintain the characteristic electrolyte composition of the cochlear endolymph. In the present study, to clarify the relationship between epinephrine and strial Na+/K(+)-ATPase activity, the ouabain-sensitive, K+-dependent p-nitrophenylphosphatase (K(+)-NPPase) activity of strial marginal cells was investigated with a cerium-based method in normal guinea pigs and guinea pigs treated with reserpine, epinephrine, and reserpine plus epinephrine. In our previous study, K(+)-NPPase activity had almost completely decreased 3 to 20 days after reserpine administration. In the present study, at 10 days after reserpinization and following repeated epinephrine treatment, enzyme activity was detectable. These results suggest that exogenous epinephrine was able to restore strial K(+)-NPPase activity in the reserpine-treated animals, and that epinephrine might increase strial Na+/K(+)-ATPase activity.

Effect of norepinephrine on ouabain-sensitive, K+-dependent p-nitrophenylphosphatase activity in strial marginal cells of the cochlea in normal and reserpinized guinea pigs

On the basolateral infoldings of the strial marginal cells in the cochlea, Na K ATPase activity is abundant. To clarify the humoral control by norepinephrine, K-NPPase activity of strial marginal cells in the cochlea was investigated in normal, reserpine, norepinephrine (NE), reserpine plus NE-treated guinea pigs using a cerium-based method. K-NPPase activity was almost completely decreased 3-20 days after reserpine administration. At 10 days after reserpinization and following NE repeated treatment, enzyme activity was detectable. These results suggested that norepinephrine might restore and regulate strial K-NPPase activity.

Effect of acoustic trauma on cytochrome oxidase activity in stria vascularis

The present study was undertaken to determine the role of metabolic disturbance in noise-induced hearing loss by histochemical studies of cytochrome oxidase activity. Adult normal albino guinea pigs were used. The experimental animals were exposed to broad-band noise at 105 dB SPL for 24 h. The control animals were not exposed to the noise. The thresholds of the auditory brainstem response (ABR) of all guinea pigs were measured 3 times: before noise exposure, 1 day and 1 month later. The difference between the ABR thresholds before and after noise exposure was statistically significant. Vibratome sections of decalcified cochleae of the noise-exposed (n = 8) and control groups (n = 4) were incubated with Spector's medium and embedded with Epon. Thin sections (2 microm) and ultrathin sections (100 nm) were cut to observe cytochrome oxidase activity in the stria vascularis under light and electron microscopes, respectively. A decreased activity of cytochrome oxidase was consistently shown in the normal-appearing stria vascularis of most noise-exposed ears. Acoustic trauma has an adverse effect on cytochrome oxidase activity in the stria vascularis as well as on hearing. A decrease in the activity of cytochrome oxidase implicates that metabolic damage may play a role in noise-induced hearing loss.

[The experimental research of inner ear metabolism and electrical physiology of autoimmune sensorineural hearing loss]

Inquire into the mechanism of inner ear pathological physiology in autoimmune sensorineural hearing loss (ASHL). With the auditory electric-physiological techniques and enzyme-histochemical method, the change of inner ear hearing function and enzyme activity were observed. These animals, which threshold of auditory nerve compound active potential (CAP) and cochlear microphonic potential(CM) heightening evidently, showed that the amplitude of endolymphatic potential(EP) (include-EP) bring down in various degrees, which was related to the change of the active of Na(+)-K(+)-ATPase and SDH in vascularis stria and endolymphatic sac. The abnormality of enzymes metabolism in inner ear tissues, which following autoimmune inflammation damage, is the pathological foundation of hearing dysfunction.

Intercellular junctional maturation in the stria vascularis: possible association with onset and rise of endocochlear potential

The postnatal maturation of intercellular junctions of marginal and basal cells of the stria vascularis was examined in the gerbil using thin sections and freeze fracture techniques. Immunohistochemical methods were used to determine the presence of Na,K-ATPase postnatally. The onset and growth of endocochlear potential (EP) was also measured. In marginal cells, the apical surface and junctional region around the apical pole of the cell was found to have adult-like characteristics by the time of onset of EP, whilst the increase in staining for Na.K-ATPase temporally coincided with an increasing density of intra-membrane protein particles on the infoldings of marginal cell lateral membranes. Maturation of the junctional specialisations of the basal cells was found to correspond temporally with the period of onset and rise of EP. Tight junctions between basal cells first appeared as small, broken strands composed of widely spaced particles at 6 days after birth (DAB). These junctional strands increased in number and in particle density until adult-like at 16 DAB when they covered large areas of the basal cell lateral membrane. Gap junctions on the apical membrane of basal cells first appeared as small patches of loosely packed junctional elements at 6 DAB. Between 8 and 16 DAB the area of membrane occupied by the gap junctions increased, reaching a mature conformation by 18 DAB. The results suggest that EP maturation is dependent upon the development of sealing between the basal cells by tight junctions and also the establishment and development of gap junctions in the apical plasma membrane of basal cells, associated with intermediate cells.

Effect of serotonin on ouabain-sensitive, K+-dependent, p-nitrophenylphosphatase activity in strial marginal cells of normal and reserpinized guinea pigs

Na+,K+-ATPase activity is abundant on the basolateral infoldings of the strial marginal cells and contributes to the maintenance of the characteristic electrolyte composition of the endolymph. However, the stria vascularis of the cochlea is known not to be innervated. In order to clarify its humoral regulation by serotonin, the K+-p-nitrophenylphosphatase activity of strial marginal cells was investigated with a cerium-based method in normal guinea pigs and in guinea pigs treated with reserpine, 5-hydroxytryptamine or reserpine plus 5-hydroxytryptamine. K+-p-nitrophenylphosphatase activity was almost completely depressed 3-20 days after reserpine administration. Ten days after reserpinization, followed by repeated 5-hydroxytryptamine treatment, the enzyme activity was detectable. These results suggest that 5-hydroxytryptamine increases the phosphatase activity. Thus, the function of the stria vascularis in producing cochlear endolymph may be regulated by 5-hydroxytryptamine.

Changes in cochlear antioxidant enzyme activity after sound conditioning and noise exposure in the chinchilla

Exposure to low level noise prior to a high level exposure reduces noise-induced hearing loss in mammals. This phenomenon is known as sound conditioning or 'toughening'. Reactive oxygen intermediates have been implicated in noise-induced cochlear damage. To evaluate if in situ antioxidant processes may play a role in the toughening phenomenon initiated by low level noise exposure we analyzed glutathione reductase, gamma-glutamyl cysteine synthetase, and catalase in stria vascularis and organ of Corti fractions from cochleae of chinchillas exposed to a sound conditioning paradigm. Chinchillas were either (A) kept in quiet cages (control), (B) exposed to conditioning noise of a 0.5 kHz octave band (90 dB for 6 h/day for 10 days), (C) exposed to high level noise (105 dB for 4 h) or (D) exposed to conditioning noise (B) followed by exposure to the higher level noise (C). Each of the noise exposure conditions (B, C, D) induced changes in the levels of these three antioxidant enzymes. The enzyme-specific activity data for the four subject groups support the following two hypotheses. (1) Changes in glutathione reductase, gamma-glutamyl cysteine synthetase, and catalase play a role in attenuating hearing loss associated with sound conditioning followed by high level noise. (2) Hair cells in the organ of Corti are protected from noise-induced damage by increasing stria vascularis levels of catalase, a hydrogen peroxide scavenging enzyme, and of enzymes involved in maintaining glutathione in the reduced state. The model formulated by these hypotheses suggests that agents that protect or augment the glutathione system in the cochlea may be protective against noise-induced hearing loss.

Expression patterns of ion transport enzymes in spiral ligament fibrocytes change in relation to strial atrophy in the aged gerbil cochlea

Fibrocytes of the lateral wall function in conjunction with the stria vascularis (StV) to mediate cochlear ion homeostasis. Age-related changes in the expression patterns of ion transport enzymes in spiral ligament fibrocytes were investigated to ascertain their relation to metabolic presbyacusis in the gerbil. Immunoreactivity of fibrocytes for Na,K-ATPase (Na,K), carbonic anhydrase isozyme II (CA) and creatine kinase isozyme BB (CK) varied among and within cochleas from aged but not from young gerbils. The variable immunostaining was related to the extent and location of StV atrophy. Age-dependent degeneration and loss of Na,K in the StV occurred predominantly in the apex and lower base and hook of the cochlea, largely sparing more central regions. Immunostaining intensity for Na,K, CK, and CA in fibrocytes changed in relation to declines in strial marginal cell Na,K initially showing upregulation followed by downregulation. Spiral ligament fibrocytes in cochleas with more than two remaining normal turns often disclosed overexpression of CK in regions of strial atrophy. Conversely, CA in such cochleas was often increased in regions of normal StV adjacent to foci of atrophic StV. Senescent cochleas with two or fewer functional turns generally contained fibrocytes with diminished CK or CA immunoreactivity in regions of atrophic StV but in isolated instances exhibited fibrocytes with enhanced staining. Heightened staining for CK in type Ia fibrocytes underlying regions of complete or partial strial atrophy indicated an increased metabolic demand in fibrocytes in response to strial insufficiency. The findings provide further support for the role of spiral ligament fibrocytes in cochlear fluid and ion homeostasis.

Alteration in capillary permeability of horseradish peroxidase in the stria vascularis and movement of leaked horseradish peroxidase after administration of furosemide

The permeability of horseradish peroxidase (HRP) from the capillaries of the stria vascularis and the movement of leaked HRP in this site were investigated over time after the administration of furosemide, a loop diuretic, for the purpose of clarifying the function of the stria vascularis. Guinea pigs were used as experimental animals. The stria vascularis became markedly edematous at 10 min after the administration of furosemide, while the vascular permeability of HRP was decreased. This edema was thought to result not from the high permeation of strial capillaries, but from the blockage of water transport into the ductus cochlearis by inhibition of the Na+/2Cl-/K+ cotransport system on the cell membrane of the marginal cells and the exudation from the intermediate cells due to change of cell membrane permeability. A large amount of the leaked HRP from the strial capillaries was taken into vacuoles of marginal cells 1 h after furosemide administration. In the present study, no leakage of HRP was observed in the ductus cochlearis. Destroyed or degenerated cells were observed in vacuoles of marginal cells 2 and 3 h after furosemide administration, suggesting that marginal cells have a phagocytic function. Two hours after furosemide administration, the vascular permeability of HRP and the function of intermediate cells was still at a decreased level, although the stria vascularis was almost restored to normal morphology. Three hours after furosemide treatment, the vascular permeability of HRP and the intake of HRP into intermediate cell vesicles were generally normalized, suggesting near restoration of stria vascularis function. In the non-furosemide-treated control group, no vacuolation was observed in marginal cells, nor was HRP intake observed in these cells.

Ca(2+)-ATPases in the cochlear duct

Differing levels of the Ca(2+)-ATPase enzymes that reside on the plasma membrane (PM) and on the endoplasmic reticulum (ER) were identified in individual rat cochlear tissues by the use of a semi-quantitative enzyme-linked immunosorbent assay (ELISA). Unlike other studies, a specific antibody to PM Ca(2+)-ATPase was used to detect significantly greater levels (about 2x) of PM Ca(2+)-ATPase in the stria vascularis (SV) than that in the spiral ligament (SL) and organ of Corti (OC) tissues. Similarly, levels of ER Ca(2+)-ATPase were also significantly higher in the SV than in the SL and OC tissues. The presence of ER Ca(2+)-ATPase in the tissues of the SV has not been demonstrated previously. Given the importance of Ca2+ homeostasis in the inner ear, the statistically significantly higher densities of both PM and ER Ca(2+)-ATPase measured in the SV relative to the SL and OC regions would indicate tissue-specific responses to fluctuations in systemic and local Ca2+ concentrations.

Protein kinase and protein phosphatase presence in the stria vascularis

Na+, K+-ATPase contributes to the high potassium concentration in the endolymph and the resulting endocochlear potential, which are both essential for the function of the sensory part of the inner ear. Na+, K+-ATPase is present in the stria vascularis and it has lately been suggested that its activity is hormonally regulated. The intracellular signalling system for hormonal short-term regulation of Na+, K+-ATPase activity by phosphorylation in renal tubular cells has been well described. In this study, the presence of the intracellular components of this phosphorylation system in the stria vascularis from guinea-pig has been investigated with immunoblotting. The concentrations found were related to those in renal medullary tissue or the corpus striatum. Protein kinase C was present with isoforms alpha, delta and zeta in the stria vascularis. Calcium- and calmodulin-dependent protein kinase II and protein phosphatase-1 isoforms alpha and gamma were found in the stria vascularis. Protein phosphatase-2B, on the other hand, could not be detected. I-1, an inhibitor of protein phosphatase activity, was present, whereas the phosphatase inhibitor dopamine- and cAMP-regulated phosphoprotein (DARPP-32), was not present in the stria vascularis. These results demonstrate that several intracellular components of the phosphorylation/dephosphorylation system are present in the stria vascularis, and suggest that hormonal short-term regulation of Na+, K+-ATPase activity is also possible in the stria vascularis.

Effect of gentamycin on the melanosomes in the stria vascularis of the pigmented guinea pig: an ultrastructural study

Melanosomes of the stria vascularis in gentamycin (GM)-intoxicated guinea pigs were examined ultrastructurally. Experimental animals were given GM sulfate intramuscularly in a daily adjusted dose of 100 mg/kg for 15 consecutive days (group A), and 150 mg/kg for 5 consecutive days (group B). Melanosomes of the intermediate cells in group B significantly increased in number in comparison with those in group A and the control group. Melanosomes in the apical turn outnumbered those in the basal turn in all three groups. Cytochemically, Na+, K(+)-ATPase activity was also demonstrated in the marginal cells. There was little difference in the degree of enzyme activity between groups A, B and the control group. The role and significance of melanosomes of the stria vascularis are discussed.

Effects of aging on potassium homeostasis and the endocochlear potential in the gerbil cochlea

Previous work has shown that the endocochlear potential (EP) decreases with age in the gerbil. Concomitant with the EP decrease is an age-related loss of activity of Na,K-ATPase in the lateral wall and stria vascularis. We hypothesized that the EP decrease is associated with a similar decrease in the endolymphatic potassium concentration [Ke+]. This hypothesis was tested using double-barrelled, K(+)-selective electrodes introduced into scala media through the round window in young and quiet-aged gerbils. Results show that the means (+/- S.D.) of the [Ke+] in young and aged gerbils were not significantly different (178.2 +/- 14.2 mM and 171.2 +/- 34.4 mM, respectively), although the intersubject variability was much greater in the aged animals than in the young. These values of [Ke+] are slightly higher than those found for other mammals and may reflect the higher plasma osmolarity found in the gerbil. The concentration of perilymphatic potassium [Kp+] in scala tympani at the round window was also similar for the young and aged groups (3.57 +/- 1.17 mM and 4.18 +/- 2.03 mM, respectively). On the other hand, mean EP values in the young and aged gerbils were 92.0 +/- 5.7 mV and 64.8 +/- 15.8 mV, respectively and were statistically different (P < 0.001). Overall, EP and [Ke+] showed little correlation (R2 = 0.23), except that when [Ke+] fell below 150 mM, the EP was always less than 60 mV. An analysis of the chemical potential for Ke+ with respect to Kp+ shows that it was similar for young and aged gerbils (overall mean of 103.1 +/- 13.7 mV) and remained constant with respect to the EP, in spite of an overall electrochemical potential of Ke+ that varied from 120 to 210 mV. Thus, the system maintains Ke+ homeostasis at the expense of the EP, even when the EP is on the verge of collapse.

Cytochemical effects of in vitro dopamine treatment on the Na-KATPase activity in strial marginal cells

Dopamine is often used clinically for the treatment of patients with shock. In a previous study, the Na-KATPase (K-NPPase) activity of strial marginal cells was inhibited after the repeated in vivo administration of dopamine hydrochloride. In the present study, the K-NPPase activity of strial marginal cells was determined using a cerium-based cytochemistry after an in vitro incubation with dopamine. The enzyme reaction product was found in untreated normal strial marginal cells, but it was almost completely undetectable after an in vitro treatment with 10 mM dopamine. This finding suggested that dopamine directly inhibited the Na-KATPase activity of strial marginal cells, and that these cells might have a dopamine receptor.

The fine structure of spiral ligament cells relates to ion return to the stria and varies with place-frequency

Ultrastructural analysis of cells in the cochlea's lateral wall was undertaken to investigate morophologic features relevant to the route of K+ cycling from organ of Corti (OC) to stria vascularis (StV) and to the question of a transcellular versus an extracellular path. The fine structure of outer sulcus cells (OSCs) evidenced their capacity for uptake of K+ from Claudius cells and from perilymph in inferior spiral ligament. Plasmalemmal amplification and mitochondrial density together with known content of Na,K-ATPase testified to activity of type II, IV and V fibrocytes in resorbing K+. Location and fine structure afforded a basis for distinguishing subtypes among the type I, II and IV cells. The type II, IV and V fibrocytes can be viewed as drawing K+ from surrounding perilymph and from OSCs and generating an intracellular downhill diffusion gradient for K+ flow through gap junctions to subtype Ib and Ia fibrocytes and strial basal cells. Pumping action enabled by extreme structural specialization of type II fibrocytes is considered to mediate K+ translocation across the interruption between the gap junction connected epithelial and gap junction connected fibrocyte systems and to explain ion flow directed toward StV through OSCs and fibrocytes despite their lack of polarity. The OSC bodies shrank, their root bundles expanded and the gap junction contact between OSCs and Claudius cells increased toward the base of the cochlea. Expanding root bundles and type I and IIb fibrocyte populations contrasted with shrinking OHCs and Deiters and tectal cells from the apex to the base of the cochlea. These differences indicated an increased magnitude and alternate route of K+ transport toward the StV in high as compared to low-frequency regions. The augmented K+ transport through spiral ligament in basal cochlea correlates with and provides a possible basis for the larger endocochlear potential in the base. The findings appear consistent with current flow extracellularly through scalae tympani and vestibuli and transcellularly through OC, OSCs and class I, II, IV and V fibrocytes.

Na,K-ATPase alpha- and beta-isoforms in the developing cochlea of the mouse

Immunohistochemistry was used to investigate the presence of Na,K-ATPase alpha- and beta-subunits isoforms (alpha 1, alpha 2, alpha 3, beta 1 and beta 2) in the cochlea of the mouse at different ages between embryological day (E) 19 and postnatal day (P) + 30. alpha 1 was mainly found in the stria vascularis and in the spiral ligament; it increased steadily from p+4. These data correlates well with the morphological and electrophysiological maturation of the cochlea. alpha 3 predominated in the spiral ganglia and the cochlear nerve. This finding is well in accordance with reports that alpha 3 seems to be associated with the nervous system. The beta-subunit was found mainly in those tissues where staining of the alpha-subunit also was seen. Both subunits were localized in tissue regions where fluid regulation is expected to play an important role. For some isoforms, the expression pattern of Na,K-ATPase during development in the mouse is different from that in the rat. The expression of Na,K-ATPase and that of glucocorticoid receptors during development in the inner ear of the mouse show a similar pattern, which may indicate that glucocorticoid receptors could be involved in regulating the expression of Na,K-ATPase.

Detection and regulation of Cu/Zn-SOD and Mn-SOD in rat cochlear tissues

Relative levels of copper/zinc-superoxide dismutase (Cu/Zn-SOD) and manganese-superoxide dismutase (Mn-SOD) in individual cochlear tissues were detected by the use of an enzyme-linked immunosorbent assay (ELISA). A heterogeneous distribution of Cu/Zn-SOD was observed in the individual tissues of control animals: high levels were measured in the stria vascularis (SV), intermediate levels of enzyme were measured in the spiral ligament (SL), and low levels were measured in the organ of Corti region (OC); collectively, these levels were not statistically significant (P = 0.0645). Levels of Mn-SOD in individual tissues of the control group were statistically significant (P < 0.05): high levels were measured in the SV, medium levels were detected in the SL, and low levels were identified in the OC. Following the administration of methylprednisolone (MP), a significant reduction of Cu/Zn-SOD in the SV (P < 0.05) and a non-significant, but noticeable, increase (> 30%) of Mn-SOD in the OC were observed. These results indicate that levels of SOD are tissue specific and that SOD is subject to glucorticoid regulation.

Ultrastructural localization of G-protein GS in the lateral wall of the guinea pig cochlear duct

Immunocytochemical localization of a GTP-binding protein, Gs, in the various cells of the lateral wall of guinea pig cochlear duct was investigated using a post-embedding immunogold method with antibody raised against a synthetic decapeptide (RMHLRQYELL) encoding the C-terminus of the alpha-subunit of Gs. In the stria vascularis, labeling was observed on the basolateral membrane infoldings of marginal cells, on the juxtaposed membrane of intermediate cells, and on the cell membrane of basal cell. In contrast, no significant labeling was observed on the luminal membrane of marginal cells. Immunoreactivity also was detected on the cell membranes of various other cells. These include spiral prominence epithelial cells, fibrocytes of spiral ligament, external sulcus cells, and epithelial and mesothelial cells of Reissner's membrane. Adenylylcyclase has been functionally implicated in some of the cell types with membranes labeled in this study. The significance of these findings is briefly discussed.

Na,K-ATPase in the cochlear lateral wall of human temporal bones with endolymphatic hydrops

Meniere's disease has traditionally been thought to arise from a disruption in longitudinal endolymphatic flow. This view has been brought into question by recent experimental studies that have focused attention on derangements of cochlear fluid and electrolyte homeostatic mechanisms in Meniere's disease, including abnormalities in Na,K-ATPase enzymes found in the cochlear lateral wall. The current study examined the immunohistochemical labeling pattern of the major ion-transporting enzyme of the stria vascularis, Na,K-ATPase, in archival sections of hydropic and nonhydropic human temporal bones for increased density of label that could indicate overproduction of fluid. The results showed good labeling of the stria vascularis in the celloidin sections. The hydropic ears tended to have darker label, but the difference was not statistically significant. The findings are consistent with normal functioning of the stria vascularis in cases of Meniere's disease.

Localization of ornithine decarboxylase (ODC) in the cochlea of the immature rat

Ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine synthesis, is important in cochlear development. Whereas tissue specific differences in cochlear ODC activity have been demonstrated, cellular localization of ODC protein in the inner ear of the immature rat has not. ODC was localized in inner ear structures using an ODC polyclonal antibody and the effects of cycloheximide on ODC immunoreactivity and enzymatic activity were determined. Tissues demonstrating elevated enzymatic activity contained cells with the strong immunoreactivity. ODC activity was highest in the organ of Corti and lateral wall followed by the cochlear nerve. Immunoreactivity was demonstrated throughout the cochlea with intense staining of the hair cells, pillar cells, Deiter's cells, inner sulcus cells, basilar membrane, stria vascularis, spiral ganglion cell bodies and cochlear nerve fibers. Cycloheximide rapidly diminished cochlear ODC activity and expression of ODC protein. The half-life of cochlear ODC was 30 min. Localization of cellular sites of ODC is important in understanding the role of the ODC-polyamine pathway in cochlear development and will be a valuable marker for tissue damage from ototoxic agents.

Na,K-ATPase activity decreases in the cochlear lateral wall of quiet-aged gerbils

Alterations in the distribution and activity of Na,K-ATPase have been implicated in declining cell function with age. However, the location, size and anatomical complexity of the cochlea have limited study of this essential enzyme. Here we describe a micro-colorimetric assay which measures Na,K-ATPase activity in subregions of individual cochleae. Na,K-ATPase activity was determined in lateral wall and organ of Corti tissues by measuring liberation of inorganic phosphate (Pi) from ATP against a standard phosphate curve. Na,K-ATPase specific activity, expressed as mu mol Pi liberated/mg protein/h, was calculated as the difference between total Pi liberated versus Pi liberated in the presence of ouabain. Na,K-ATPase specific activity and total protein content in the lateral wall significantly exceeded those of the organ of Corti. Although lateral wall protein content remained constant with age, Na,K-ATPase specific activity declined in some older gerbils, suggesting a basis for age-related reductions in magnitude of the endocochlear potential and confirming previous histochemical results. This microassay offers a sensitive, reliable means to assay enzyme activity in subregions or single turns of the cochlea that unlike other methods does not rely on use of radioisotopes, enzymatic cycling or sample pooling.

Expression of Na+,K(+)-ATPase alpha 1 subunit mRNA in the developing rat cochlea

The distribution of Na+,K(+)-ATPase alpha 1 subunit mRNA was identified using in situ hybridization in the developing rat cochlea. The expression of alpha 1 subunit mRNA in stria vascularis (SV) was observed in all time points studied, 1 to 30 postnatal day (pnd) rats. The adult expression level was attained between 11 to 14 pnd. Surprisingly, alpha 1 subunit mRNA in spiral ligament (SL) and spiral limbus (SLi) was expressed in a more distinct time-dependent manner. At 7 pnd, the alpha 1 subunit mRNA expression was observed initially in the tissues of the SL. At 11 pnd, alpha 1 subunit expression appeared in SLi. Between 11 and 14 pnd, an adult-like pattern of Na+,K(+)-ATPase alpha 1 subunit mRNA expression was attained in the SL and SLi. These data suggest that the expression of Na+,K(+)-ATPase alpha 1 subunit mRNA in these areas are closely related to the development of the rat EP, as its expression in the stria vascularis.

Development of melanosomes and cytochemical observation of tyrosinase activity in the inner ear

The ultrastructure and tyrosinase activity of melanocytes in the inner ear of pigmented guinea pigs were observed. Melanocytes/melanocyte-like cells were seen in the stria vascularis, the vestibular dark cell area and the endolymphatic sac. In the stria vascularis, melanosomes in several stages of maturation were seen in the cytoplasm of the intermediate cells and melanin-laden endosomes existed in the basal cells. Only the intermediate cells contained tyrosinase-positive cytoorganelles; Golgi sacs and neighboring small vesicles. Numerous melanocytes containing many melanosomes were observed under the epithelium of the vestibular dark cell area, and they showed tyrosinase activity. Melanocytes were seen in the endolymphatic sac, and they also showed tyrosinase activity. However, the epithelial cells of the endolymphatic sac, which had vacuoles containing melanin, did not show tyrosinase activity. Based on these findings, it can be said that (1) most of the intermediate cells of the stria vascularis must be melanocytes, (2) melanogenesis is vigorous in melanocytes of the inner ear, and (3) melanin in the epithelial cells of the endolymphatic sac is transferred in from melanocytes and is never synthesized in the epithelial cells.

Na,K-ATPase subunit isoform expression in the guinea pig endolymphatic sac

Na,K-ATPase subunit isoform expression was studied by immunocytochemistry in the guinea pig endolymphatic sac, using subunit isoform-specific polyclonal antibodies. Epithelial cells of the guinea pig endolymphatic sac were observed to contain the alpha 1- and beta 2-subunit isoforms, and to a lesser extent the beta 1-subunit isoform, of Na,K-ATPase. The alpha 1- and beta 2-subunit isoforms of Na,K-ATPase have been observed previously in other ion and fluid transporting regions of the membranous labyrinth, e.g., stria vascularis and vestibular dark cells. Combined data indicate that the alpha 1-, beta 2-form of Na,K-ATPase plays a role in the microhomeostasis of endolymph. The alpha 1 beta 2 Na,K-ATPase subunit isoform combination is different from typical ion and fluid transporting tissues, e.g., kidney and colon, and may reflect distinctive characteristics of inner ear Na,K-ATPase.

Effects of low-sodium, high-potassium dietary intake on cochlear lateral wall Na+,K(+)-ATPase

The effect of a low Na+, high K+ diet on Na+,K(+)-ATPase levels in cochlear lateral wall tissues was investigated in laboratory rats by using an enzyme-linked immunosorbent assay. The low Na+, high K+ diet induced high aldosterone plasma levels in the animals as well as changes in plasma cation levels. Animals that received a low Na+, high K+ diet demonstrated a statistically significant (97%) increase in Na+,K(+)-ATPase levels in the stria vascularis when compared to animals that received a control diet. This increase in strial Na+,K(+)-ATPase levels was blocked only 70% by administration of the aldosterone antagonist, spironolactone. Findings therefore indicate that strial Na+,K(+)-ATPase may be modulated by both aldosterone and Na+,K+ plasma levels. Na+,K(+)-ATPase levels in the spiral ligament were not affected by the experimental treatment. These findings suggest that spiral ligament Na+,K(+)-ATPase levels may be regulated by factors other than aldosterone and Na+,K+ plasma levels. This study provides further insight into the mechanisms of the beneficial effects of salt restriction and potassium loading in patients with Méniére's disease.

Expression of alpha and beta subunit isoforms of Na,K-ATPase in the mouse inner ear and changes with mutations at the Wv or Sld loci

Mice homozygous for mutations at the viable dominant spotting (Wv) and Steel-dickie (Sld) loci exhibit a similar phenotype which includes deafness. The auditory dysfunction derives from failure of the stria vascularis to develop normally and to generate a high positive endocochlear potential (EP). Because strial function is driven by Na,K-ATPase its expression was investigated in inner ears of Wv/Wv and Sld/Sld mice and their wild-type littermates by immunostaining with antisera against four of the enzyme's subunit isoforms. Wild-type mice from two different genetic backgrounds showed an identical distribution of subunit isoforms among inner ear transport cells. Several epithelial cell types coexpressed the alpha 1 and beta 1 subunits. Vestibular dark cells showed no reactivity for beta 1 but expressed abundant beta 2, whereas, strial marginal cells stained strongly for both beta isoforms. The only qualitative difference between mutant and wild-type mice was the absence of beta 1 subunit in marginal cells of the mutant's stria. However, it is unlikely that this difference accounts for failure of mutants to generate a high EP because the beta 1 subunit is not present in the stria vascularis of either rats or gerbils with normal EP values. Strong immunostaining for Na,K-ATPase in lateral wall fibrocytes of normal mice along with diminished immunoreactivity in the mutants supports the concept that these strategically located transport fibrocytes actively resorb K+ leaked across Reissner's membrane into scala vestibuli or effluxed from hair cells and nerves into scala tympani. It is further speculated that the resorbed K+ normally is siphoned down its concentration gradient into the intrastrial space through gap junctions between fibrocytes and strial basal and intermediate cells where it is recycled back to endolymph via marginal cells. Thus, failure of mutants to generate a positive EP could be explained by the absence of intermediate cells which may form the final link in the conduit for moving K+ from perilymph to the intrastrial compartment.

Na,K-ATPase alpha and beta subunit isoform distribution in the rat cochlear and vestibular tissues

The distribution of five Na,K-ATPase subunit isoforms (alpha 1, alpha 2, alpha 3, beta 1 and beta 2) in rat cochlear and vestibular tissues was determined by immunocytochemical techniques using subunit isoform specific polyclonal antibodies. The expression of Na,K-ATPase alpha and beta subunit isoforms varied among different cell regions of the inner ear. The alpha 1 subunit isoform was more extensively distributed in all inner ear tissues than the alpha 2 or alpha 3 subunit isoforms. The beta 1 subunit isoform was distributed primarily in spiral ligament and inner hair cells of the cochlea, and in crista ampullaris and macula of the saccule. The beta 2 subunit isoform was most abundant in the stria vascularis, dark cells of the ampullae and utricle. The alpha 1 beta 1 subunit combination of Na,K-ATPase was most commonly found in the spiral ligament, while the alpha 1 beta 2 combination was most abundant in the stria vascularis. Similarly, alpha 1 beta 2 was confined more to the dark cells of the ampullae and utricle. The alpha 3 beta 1 subunit combination of Na,K-ATPase was identified in the inner hair cells of the cochlea and the sensory regions of the vestibular end organs. These observations may reflect functional diversity of Na,K-ATPase in the individual inner ear regions and may provide insight into the differences between fluid and ion transport in the inner ear and that of other transporting tissues. Overall, the distribution pattern further indicates that the different isoform combinations have specific roles.

Na-K-ATPase activity in the guinea pig stria vascularis in experimentally-induced endolymphatic hydrops

The effect of endolymphatic hydrops on the Na-K-ATPase activity in the guinea pig stria vascularis was electron microscopically and enzyme cytochemically investigated one year after experimental induction. The morphological observations revealed intercellular dropsy in the basal infoldings of the marginal cells, and shrinkage and disappearance of intermediate cells. Moreover, shrinkage of the marginal cells, especially of the basal infoldings, was occasionally observed. In spite of these morphological alterations, the Na-K-ATPase activity was still detected on the plasma membrane of the basal infoldings of most marginal cells. No remarkable differences were found among the cochlear turns of the specimens examined. However, no reaction product was detected on the basolateral plasma membrane of severely degenerated marginal cells. The present results indicate that the Na-K-ATPase of the plasma membrane of the basal infoldings of the marginal cells plays an important role in the maintenance of the unique ion concentration of the endolymph even in the endolymphatic hydropic condition, and that the Na-K-ATPase activity is attenuated in severely atrophic cells.

Reserpine inhibits the NaK ATPase activity of the stria vascularis in the cochlea

The function of the stria vascularis in the cochlea is believed to be the production of endolymph. The mechanisms that maintain or control the function of the stria vascularis, however, remain unclear. In the present study, the effects of one-shot reserpine administration on the NaK ATPase activity of the stria vascularis in guinea pigs were investigated. NaK ATPase activity was shown to be completely inhibited 3 to 20 days after reserpinization, and was detectable again 60 days after reserpinization. As reserpine is an adrenergic neuron blocker, these results suggest that catecholamines may play an essential role in the maintenance or control of NaK ATPase activity, and that the stria vascularis may be one of the target organs of catecholamines.

Dynamics of Na,K-ATPase sites in lateral cochlear wall tissues of the rat

The objective of this study was to determine whether varying levels of either the synthetic glucocorticoid, dexamethasone, or the mineralocorticoid, aldosterone, modulate the quantity of Na,K-ATPase sites in stria vascularis and spiral ligament tissues. Surgically adrenalectomized male rats were administered different dosages of dexamethasone or aldosterone for 7 days and subsequently were sacrificed. Na,K-ATPase alpha-subunits of stria vascularis and spiral ligament homogenates were determined quantitatively by enzyme-linked immunosorbent assay, using a monoclonal antibody shown to react with lateral wall Na,K-ATPase alpha-subunit. Elevated serum levels of dexamethasone were correlated with significantly increased Na,K-ATPase alpha-subunit levels in both the stria vascularis and spiral ligament (P < 0.05). Elevated serum levels of aldosterone were correlated with increased Na,K-ATPase alpha-subunits in the stria vascularis, but not in the spiral ligament (P < 0.1). These data further indicate a positive correlation between increased serum levels of adrenal steroids and induction of Na,K-ATPase synthesis by such steroids, particularly by dexamethasone.

Morphological aspects of transport of potassium ion in the marginal cell

Structure and function of the marginal cell in the stria vascularis were studied by freeze-fracture, cytochemistry and immunohistochemistry with special regard to the ion transport of potassium. Freeze-fracture showed that marginal cells were connected by tight junctions beneath the scala media K(+)-NPPase cytochemistry showed that Na+, K(+)-ATPase was abundant on the basolateral infoldings of the marginal cell. Immunohistochemistry of a rat Isk protein, which has a property of a potassium channel, revealed that the rat Isk protein was localized at the endolymphatic surface of the marginal cell. These findings supported the 'one-pump' theory (Offner et al. Hear Res 1987; 29: 117-24).

Expression of carbonic anhydrase isoenzyme-like immunoreactivity in the limbus spiralis of the human fetal cochlea

The distribution of carbonic anhydrase isoenzymes (CA I, II, III, V) was analyzed the 14-, 15- and 16-week-old human fetal cochleae using immunohistochemical methods. The interdental cells in the limbus spiralis were strongly labelled with antibodies against CA I and III and the spiral ligament was also positive for CA I and III. The stria vascularis and organ of Corti were however negative for CA I, II, III and V. These results suggest that the interdental cells in the human fetus may be related to fluid and ion transport of endolymph especially in the early stages of development.

Development of endocochlear potential and compound action potential in the rat

The present study was designed to investigate the developmental changes of the endocochlear potential and compound action potential simultaneously from rat pups of various ages. Animals were anesthetized with ketamine/xylazine, and the endocochlear potential was measured with a glass microelectrode. At the same time, a wire electrode was placed on the round window to record the click-evoked compound action potential. The endocochlear potential was found to be very low during the first few days of postnatal life. A rapid increase in the value of the endocochlear potential was noted between eleven and thirteen days of age, and adult-like values were recorded by seventeen days of age. Compound action potential responses were recorded at thirteen days of age to high intensity clicks, followed by a progressive improvement of thresholds and reduction of latencies. The development of the endocochlear potential and compound action potential was found to be reciprocally related - as the magnitude of the endocochlear potential increased, the compound action potential threshold declined with increasing age. The development of the endocochlear potential was found to closely approximate the development of enzymatic activity of sodium, potassium-ATPase in the stria vascularis reported by Kuijpers (1974).

Prolonged antinociceptive activity of pseudodipeptide analogues of Lys-Trp(Nps) and Trp(Nps)-Lys

Peptide bond substitution in the molecules of Lys-Trp(Nps) (LTN) and Trp(Nps)-Lys (TNL) by an aminomethylene and ketomethylene bond, respectively, afforded pseudodipeptides with analgesic activity. The new compounds Lys psi(CH2NH)-Trp(Nps)-OMe (LTNAM) and Trp(Nps)psi(COCH2)(R,S)-Lys (TNLKM) induced a dose-dependent and naloxone-reversible analgesia following intracerebroventricular (ICV) administration to mice. The antinociceptive effects were longer lasting compared to those induced by the parent compounds. The pseudodipeptides protected Met-enkephalin degradation by rat striatal slices and, combined with an ineffective dose of the opioid peptide, induced analgesia. LTNAM and TNLKM were as potent as LTN to inhibit brain aminopeptidase in vitro and ex vivo. An increased resistance to proteolysis of the pseudodipeptides may explain their prolonged analgesic activity.

Modulation of the rat stria vascularis in the absence of circulating adrenocorticosteroids

Structural changes in the cellular morphology of the rat stria vascularis from a standardized region of the basal region and from a standardized region in the apical region of the rat cochlear duct were measured using stereological methods after removal of endogenous levels of adrenal steroids by bilateral adrenalectomy. Although there were some inconsistent and insignificant alterations in the volume density of intermediate and basal cells, a decreased volume density of marginal cells in both the basal region and in the apical region in adrenalectomized (ADX) animals as compared to sham animals was consistent with a concomitant significant increased (p less than or equal to 0.05) volume density of intercellular space as observed in both the basal and apical regions of the stria vascularis of ADX animals. Findings of this study indicate that the strial cells of the stria vascularis react differently and independently in response to the removal of adrenal steroids, and such strial responses occur uniform in both the base and apex.

Quantitative immunocytochemical localization of Na+,K+-ATPase alpha-subunit in the lateral wall of rat cochlear duct

Ultrastructural localization of the alpha-subunit of Na+,K+-ATPase on the lateral wall of rat cochlear duct was investigated quantitatively by the protein A-gold method, using affinity-purified antibody against the alpha-subunit of rat kidney Na+,K+-ATPase. In the stria vascularis, gold particles were sparse over the endolymphatic luminal surface of the marginal cells but were numerous over the basolateral membrane. The labeling density of the basolateral membrane was almost equal to that of the same domain of the distal tubule cells of kidney. The intermediate cells were studded with a large number of gold particles on the plasma membrane domain facing the basolateral domain of the marginal cells. On the luminal surfaces of the other epithelial cells, including those of Reissner's membrane, no significant amount of gold particles was found. Many gold particles were localized on all the plasma membranes of the spiral prominence stromal cells and on the intracellular membrane domain of the external sulcus cells.