Cell volume control in vestibular dark cells during and after a hyposmotic challenge

Stria vascularis and vestibular dark cells: characterisation of main structures responsible for inner-ear homeostasis, and their pathophysiological relations

The regulation of inner-ear fluid homeostasis, with its parameters volume, concentration, osmolarity and pressure, is the basis for adequate response to stimulation. Many structures are involved in the complex process of inner-ear homeostasis. The stria vascularis and vestibular dark cells are the two main structures responsible for endolymph secretion, and possess many similarities. The characteristics of these structures are the basis for regulation of inner-ear homeostasis, while impaired function is related to various diseases. Their distinct morphology and function are described, and related to current knowledge of associated inner-ear diseases. Further research on the distinct function and regulation of these structures is necessary in order to develop future clinical interventions.

Functional IsK/KvLQT1 potassium channel in a new corticosteroid-sensitive cell line derived from the inner ear

Endolymph, a high K(+)/low Na(+) fluid, participates in mechanoelectrical transduction in inner ear. Molecular mechanisms controlling endolymph ion homeostasis remain elusive, hampered by the lack of appropriate cellular models. We established an inner ear cell line by targeted oncogenesis. The expression of SV40 T antigen was driven by the proximal promoter of the human mineralocorticoid receptor (MR) gene, a receptor expressed in the inner ear. The EC5v cell line, microdissected from the semicircular canal, grew as a monolayer of immortalized epithelial cells forming domes. EC5v cells exhibited on filters of high transepithelial resistance and promoted K(+) secretion and Na(+) absorption. Functional MR and the 11beta-hydroxysteroid dehydrogenase type 2, a key enzyme responsible for MR selectivity were identified. Expression of the epithelial sodium channel and serum glucocorticoid-regulated kinase 1 was shown to be up-regulated by aldosterone, indicating that EC5v represents a novel corticosteroid-sensitive cell line. Ionic measurements and (86)Rb transport assays revealed an apical secretion of K(+) at least in part through the I(sK)/KvLQT1 potassium channel under standard culture conditions. However, when cells were exposed to high apically K(+)/low Na(+) fluid, mimicking endolymph exposure, I(sK)/KvLQT1 actually functioned as a strict apical to basolateral K(+) channel inhibited by clofilium. Quantitative reverse transcriptase-PCR further demonstrated that expression of KvLQT1 but not of I(sK) was down-regulated by high K(+) concentration. This first vestibular cellular model thus constitutes a valuable system to further investigate the molecular mechanisms controlling ionic transports in the inner ear and the pathophysiological consequences of their dysfunctions in vertigo and hearing loss.

Sodium- and potassium-activated ATPase in the mammalian vestibular system

Autoradiographic and cytochemical procedures were employed to determine the cellular distribution of the Na,K-ATPase enzyme in the mammalian vestibular system. A light-microscope survey of vestibular tissues incubated with [(3)H]ouabain shows high densities of ouabain binding sites within the dark cell epithelium (DC) of the ampullae of the semi-circular canals, and to a lesser extent, the DC of the utricular macula. A moderate number of binding sites was found in nerve fibers penetrating the connective tissue beneath the sensory epithelium (SE) of the ampullae and the maculae. A small number of binding sites is distributed in the deep portion of the SE, both in the ampullae and in the maculae. These latter binding sites seem to be associated with nerve terminals and receptor cells. At the ultrastructural level, the vestibular dark cells exhibit extensive basolateral membrane infolding, a morphological hallmark of cells engaged in trans-epithelial ion transport. The cytochemical reaction product is K(+)-dependent, ouabain inhibitable, and is restricted to the basolateral membrane extensions, with little or no product on the luminal membrane. The extent of membrane infolding in dark cells of the utricle is less pronounced than that of the ampullar dark cells and the intensity of the cytochemical reaction appears to correlate with the extent of membrane infolding. The results support the widely held hypothesis that the vestibular dark cells play a role in endolymph production. They also suggest that the vestibular sensory epithelia may be a site of ion exchange.

Noninvasive measurement of hydrogen and potassium ion flux from single cells and epithelial structures

This review introduces new developments in a technique for measuring the movement of ions across the plasma membrane. With the use of a self-referencing ion-selective (Seris) probe, transport mechanisms can be studied on a variety of preparations ranging from tissues to single cells. In this paper we illustrate this versatility with examples from the vas deferens and inner ear epithelium to large and small single cells represented by mouse single-cell embryos and rat microglia. Potassium and hydrogen ion fluxes are studied and pharmacological manipulation of the signals are reported. The strengths of the self-referencing technique are reviewed with regard to biological applications, and the expansion of self-referencing probes to include electrochemical and enzyme-based sensors is discussed.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Changes in the volume of marginal cells induced by isotonic 'Cl- depletion/restoration': involvement of the Cl- channel and Na+-K+-Cl- cotransporter

Marginal cells constitute the endolymph-facing epithelium responsible for the secretion of endolymph by the stria vascularis in the inner ear. We have studied the possible involvement of Cl- conductance and Na+-K+-Cl- cotransport in the mechanism of changes in cell volume upon isotonic Cl- depletion/restoration. Changes in cell volume were estimated from video-microscopic images with the aid of an image processor. Marginal cells shrank to approximately 80% of their original volume in 30 s and to 65-70% in 90 s upon total replacement of [Cl]o (approximately 150 mM) by gluconate-, and the original volume of the shrunken cells was restored within 2 min after restoration of Cl-. The order of potency of anions to induce isotonic shrinkage was gluconate > I- > F- > Br-. The cell shrinkage caused by Cl- depletion was partially inhibited by 5-Nitro-2-(3-phenyl-propylamino)-benzoic acid (NPPB, 0.2 mM), but not by either 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid (SITS, 0.5 mM), bumetanide (10 microM) or ouabain (1 mM). The cell shrinkage caused by a reduction of [Cl]o from approximately 150 mM to 7.5 mM was not affected by [K]o in the range of 3.6 mM to 72 mM. These results suggest that the main efflux pathway(s) responsible for the 'Cl removal'-induced shrinkage depends on volume-correlated Cl- conductance (Takeuchi and Irimajiri, J. Membrane Biol. 150, 47-62, 1996) and that this pathway(s) is essentially independent of the Na+-K+-Cl- cotransporter, the Na+,K+-ATPase, and the K+-Cl- cotransporter. With regard to volume recovery after isotonic shrinkage, its critical dependence on the simultaneous presence of Na+, K+ and Cl- in the bath and its substantial inhibition by bumetanide (10 microM) both indicate a major role for Na+-K+-Cl- cotransport. The strong influence on cell volume of solute fluxes working through the Cl- channel and the Na+-K+-Cl- cotransporter implies an essential role for these pathways in the ion transport mechanism(s) of the marginal cell.

Endothelin-A receptors mediate vasoconstriction of capillaries in the spiral ligament

The purpose of this study was to determine whether endothelin-1 (ET-1), endothelin-2 (ET-2) or endothelin-3 (ET-3) alter the vascular diameter of capillaries in the spiral ligament. Changes in vascular tone were measured in capillaries from the isolated spiral ligament in vitro. Capillaries were occluded on one end and opened on the other end. Red blood cells trapped in the capillaries served as markers for a luminal volume defined by the red cell itself, the capillary wall and the occluder. Movement of the red cell toward the open end was taken as evidence for vasoconstriction and movement of the red cell toward the occluder was taken as evidence for vasodilation. The inner diameter of the capillaries was 7.0 microm and decreased maximally by a factor of 0.8 in response to ET-1 and ET-2 (both 10(-8) M). Vasoconstriction induced by ET-1 and ET-2 was concentration-dependent in the range between 10(-12) and 10(-8) M whereas ET-3 (10(-8) M) had no effect. The EC50s for ET-1 and ET-2 were 1.2 x 10(-10) M and 1.4 x 10(-9) M, respectively. Thus, the potency order was ET-1 > ET-2 >> ET-3. Vasoconstriction induced by ET-1 and ET-2 was completely inhibited by the competitive antagonist 10(-6) M BQ-123 (cyclic D-Asp-L-Pro-D-Val-L-Leu-D-Trp). Vasoconstriction induced by ET-1 or ET-2 continued for more than 1 min after removal of agonist from the perfusate. Rapid vasodilation of capillaries preconstricted by ET-1 was observed in response to 10(-3) M sodium nitroprusside. Sodium nitroprusside, however, had no significant effect on the vascular diameter of resting capillaries. These results demonstrate that capillaries in the spiral ligament can constrict and the endothelin-mediated vasoconstriction occurs via ET(A) receptors.

Expression of voltage-dependent chloride channels in the rat cochlea

Voltage-dependent chloride (ClC) channels have not yet been identified in the cochlea. In this study, an approach utilizing the reverse transcription-polymerase chain reaction (RT-PCR) was devised to clone the cDNA of ClC channels. PCR was performed using degenerate primers corresponding to two highly conserved regions of the ClC channels. By Southern hybridization and sequencing studies, the sequences corresponding to ClC-2 and ClC-3 were found in the cochlear lateral wall, while ClC-1 was not detected. These results suggest that ClC-2 and ClC-3 might be involved in Cl- transport in the cochlear lateral wall.

Inner ear defects induced by null mutation of the isk gene

The isk gene is expressed in many tissues. Pharmacological evidence from the inner ear suggests that isk mediates potassium secretion into the endolymph. To examine the consequences of IsK null mutation on inner ear function, and to produce a system useful for examining the role(s) IsK plays elsewhere, we have produced a mouse strain that carries a disrupted isk locus. Knockout mice exhibit classic shaker/waltzer behavior. Hair cells degenerate, but those of different inner ear organs degenerate at different times. Functionally, we show that in mice lacking isk, the strial marginal cells and the vestibular dark cells of the inner ear are unable to generate an equivalent short circuit current in vitro, indicating a lack of transepithelial potassium secretion.

K(+)-induced stimulation of K+ secretion involves activation of the IsK channel in vestibular dark cells

Vestibular dark cells in the inner ear secrete K+ from perilymph containing 4 mM K+ to endolymph containing 145 mM K+. Sensory transduction causes K+ to flow from endolymph to perilymph, thus threatening the homeostasis of the perilymphatic K+ concentration which is crucial for maintaining sensory transduction since the basolateral membranes of the sensory cells and adjacent neuronal elements need to be protected from K(+)-induced depolarization. The present study addresses the questions (1) whether increases in the perilymphatic K+ concentration by as little as 1 mM are sufficient to stimulate KCl uptake across the basolateral membrane of vestibular dark cells, (2) whether K(+)-induced stimulation of KCl uptake causes stimulation of the IsK channel in the apical membrane, and (3) whether the rate of transepithelial K+ secretion depends on the perilymphatic (basolateral) K+ concentration when the apical side of the epithelium is bathed with a solution containing 145 mM K+, as in vivo. Uptake of KCl was monitored by measuring cell height as an indicator for cell volume. The current (IIsK), conductance (gIsK) and inactivation time constant (tau IsK) of the IsK channel as well as the apparent reversal potential of the apical membrane (Vr) were obtained with the cell-attached macro-patch technique. Vr was corrected for the membrane voltage previously measured with microelectrodes. The rate of transepithelial K+ secretion JK was obtained as equivalent short circuit current from measurements of the transepithelial voltage (Vt) and resistance (Rt) measured in the micro-Ussing chamber. Cell height of vestibular dark cells was 7.2 microns (average). Elevations of the extracellular K+ concentration from 3.5 to 4.5 mM caused cell swelling with an initial rate of cell height change of 11 nm/s. With 3.6 mM K+ in the pipette IIsK was outwardly directed and elevation of the extracellular K+ concentration from 3.6 to 25 mM caused an increase of IIsK from 12 to 65 pA, gIsK from 152 to 950 pS and tau IsK from 278 to 583 ms as well as a hyperpolarization of Vr from -50 to -60 mV. With 150 mM K+ in the pipette IIsK was inwardly directed and the elevation of the extracellular K+ concentration caused an increase of IIsK from -1 to -143 pA, gIsK from 141 to 1833 pS and tau IsK from 248 to 729 ms. Vr remained within +/- 10 mV from zero. JK was 4.8 nmol x cm-2 x s-1 when the both the apical side and the basolateral side of the epithelium were perfused with a solution containing 3.5 mM K+. Elevation of the basolateral K+ concentration by 1 mM caused JK to increase by 1.1 nmol x cm-2 x s-1 or 23%. When the basolateral side of the epithelium was perfused with a solution containing 3.5 mM K+ and the apical side with a solution containing 145 mM K+, as in vivo, JK was 0.8 nmol x cm-2 x s-1 and elevation of the basolateral K+ concentration by 1 mM caused JK to increase by 0.8 nmol x cm-2 x s-1 or 100%. These data suggest that physiologically relevant increases in the perilymphatic K+ concentration increase JK by increasing KCl uptake across the basolateral membrane and activation of K+ release via the IsK channel in the apical membrane. Thus, the data demonstrate that vestibular dark cells adjust the rate of K+ secretion into endolymph according to the perilymphatic K+ concentration.

Vestibular dark cells contain the Na+/H+ exchanger NHE-1 in the basolateral membrane

Vestibular dark cells and strial marginal cells transport K+ by similar mechanisms. We have shown that K+ transport in vestibular dark cells is sensitive to the cytosolic pH (pHi) (Wangemann et al. (1995a): J. Membrane Biol. 147: 255-262). The present study addresses pharmacologically the questions whether vestibular dark cells and strial marginal cells from the gerbil contain a Na+/H+ exchanger (NHE) and in which membrane, apical or basolateral, NHE is located. Further, the study addresses the question which NHE subtype is present in vestibular dark cells. pHi was measured micro-fluorometrically with the pH-sensitive dye 2',7'-bicarboxyethyl-5(6)-carboxyfluorescein (BCECF), cell volume which is a measure of the net balance between ion influx and efflux was monitored as cell height (CH) and the equivalent short circuit current (Isc) which is a measure of transepithelial K+ secretion was calculated from measurements of the transepithelial voltage (Vt) and the transepithelial resistance (Rt). Changes in pHi were induced by 20 or 40 mM propionate. In the presence of propionate a transient acidification of pHi was observed in vestibular dark cells as well as a subsequent alkalinization to pHi values exceeding those under control conditions. The alkalinization of pHi in the presence of propionate was inhibited by the NHE blockers amiloride and EIPA. Propionate-induced swelling of vestibular dark cells was inhibited by amiloride. The NHE blocker amiloride caused in vestibular dark cells an acidification of pHi and a decrease in CH. Amiloride caused in both vestibular dark cells and strial marginal cells a transient stimulation of Isc when added to the basolateral side but not added to the apical side. Similar effects and pHi were observed in vestibular dark cells with the amiloride analog ethyl-isopropyl-amiloride (EIPA) and similar effects on Isc were observed with EIPA and the NHE blocker HOE694 when applied to the basolateral side of vestibular dark cell epithelium. The IC50 for these basolateral effects of EIPA, HOE694 and amiloride on Isc in vestibular dark cells were 2 x 10(-7) M, 8 x 10(-7) M and 4 x 10(-5) M. These observation suggest that vestibular dark cells and strial marginal cells contain NHE in their basolateral membrane, that K+ transport in strial marginal cells in pHi sensitive similar to K+ transport in vestibular dark cells and that NHE in vestibular dark cells consists of the subtype NHE-1.

Osmotic water permeability of capillaries from the isolated spiral ligament: new in-vitro techniques for the study of vascular permeability and diameter

Perilymph is separated from blood by a barrier called the blood-labyrinth or blood-perilymph barrier in analogy to the blood-brain or blood-cerebrospinal fluid barrier. These barriers consist mainly of vascular endothelial cells. To characterize the blood-labyrinth barrier we developed in vitro techniques for the quantitative determination of the osmotic water permeability and for the determination of changes in the diameter of isolated inner ear capillaries. Both techniques rely on measurement of the velocity of marker red cells trapped in the lumen of capillaries. The velocity of marker red cells is a measure for the capillary permeability when a water flux across the capillary wall is induced by an osmotic gradient or a measure for a change in the capillary diameter. With these techniques the osmotic water permeability coefficient (Pf) and the pH sensitivity of isolated capillaries from the spiral ligament of the inner ear was determined. Pf at 23 degrees C was (1.49 +/- 0.17) 10(-3) cm/s at pH 7.4 and (1.61 +/- 0.23) 10(-3) cm/s at pH 6.8 (n = 12: mean +/- SEM: n = number of tissues). Pf at 37 degrees C was (2.26 +/- 0.23) 10(-3) cm/s at pH 7.4 and (2.35 +/- 0.17) 10(-3) cm/s at pH 6.8 (n = 13). No change in capillary diameter was observed when the pH of the interstitial fluid was lowered from pH 7.4 to 6.8. These data demonstrate that Pf and the capillary diameter of spiral ligament capillaries are pH independent and suggest that water crosses the blood-labyrinth barrier via an aqueous pathway. Further, these data suggest that the relatively low Pf is another characteristic shared by the blood-labyrinth and the blood-brain barrier.

Ion selectivity of volume regulatory mechanisms present during a hypoosmotic challenge in vestibular dark cells

Volume regulation during a hypoosmotic challenge (RVD) in vestibular dark cells from the gerbilline inner ear has previously been shown to depend on the presence of cytosolic K+ and Cl-, suggesting that it involves KCl efflux. The aim of the present study was to characterize hypoosmotically-induced KCl transport under conditions where a hypoosmotic challenge causes KCl influx via the pathways normally used for efflux. Net osmolyte movements were monitored as relative changes in cell volume measured as epithelial cell height (CH). A hypoosmotic challenge (298 to 154 mosM) in the presence of 3.6 or 25 mM K+ and loop-diuretics (piretanide or bumetanide) caused an increase in CH by about a factor of 1.2 presumably due to the net effect of primary swelling defined as osmotic dilution of the cytosol and RVD involving KCl efflux. A hypoosmotic challenge in the presence of 79 mM K+ and loop-diuretics, however, caused CH to increase by a factor of over 2.4. Presumably, this large increase in CH was due to the sum of primary and secondary swelling. Secondary swelling depended on the presence of extracellular K+ and Cl- suggesting that it involved KCl influx followed by water. The ion selectivity of secondary swelling was K+ = Rb+ > Cs+ >> Na+ = NMDG+ and Cl- = NO3- = SCN- >> gluconate-. Secondary swelling was not inhibited by Ba2+, tetraethylammonium, quinidine, lidocaine, amiloride, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, 4-acetamido-4'-diisothiocyanatostilbene-2,2'-disulfonic acid, 4,4'-dinitrostilbene-2,2'-disulfonic acid, 5-nitro-2(3-phenylpropylamino)benzoic acid, acetazolamide, or ethoxyzolamide. These data define a profile of the hypoosmotically-induced KCl transport pathways. The ion selectivity and the blocker insensitivity are consistent with the involvement of the apical slowly activating K+ channel (IsK or minK channel) and the basolateral 360 pS Cl- channel. The involvement of these channels, however, remains to be demonstrated.

Hypo-osmotic challenge stimulates transepithelial K+ secretion and activates apical IsK channel in vestibular dark cells

Volume regulation of vestibular dark cells from the gerbilline inner ear in response to a hypo-osmotic challenge depends on the presence of cytosolic K+ and Cl-. The present study addresses the questions: (i) whether and by what mechanism K+ is released during volume regulation, (ii) whether the osmolarity of the basolateral medium has an effect on the steady-state rate of transepithelial K+ transport and (iii) whether there is cross-talk between the basolateral membrane responsible for K+ uptake and the apical membrane responsible for K+ release. K+ secretion (JK+,probe) and current density (Isc,probe) were measured with vibrating probes in the vicinity of the apical membrane and the transepithelial potential (Vt) and resistance (Rt) were measured in a micro-Ussing chamber. The equivalent short-circuit current (Isc) was calculated. The current (IIsK), conductance (gIsK) and inactivation time constant (tau IsK) of the IsK channel and the apparent reversal potential of the apical membrane (Vr) were obtained with the cell-attached macropatch technique. Vr was corrected (Vrc) for the membrane voltage (Vm) measured separately with microelectrodes. A hypo-osmotic challenge (294 to 154 mosM by removal of 150 mM mannitol) on the basolateral side of the epithelium increased JK+,probe and Isc,probe by a factor of 2.7 and 1.6. When this hypo-osmotic challenge was applied to both sides of the epithelium Vt and Isc increased from 5 to 14 mV and from 189 to 824 microA/cm2 whereas Rt decreased from 27 to 19 omega-cm2. With 3.6 mM K+ in the pipette IIsK was outwardly directed, tau IsK was 267 msec and the hypo-osmotic challenge caused IIsK and gIsK to increase from 14 to 37 pA and from 292 to 732 pS. Vrc hyperpolarized from -44 to -76 mV. With 150 mM K+ in the pipette IIsK was inwardly directed, tau IsK was 208 msec and the hypo-osmotic challenge caused IIsK and gIsK to increase in magnitude from 0 to -21 pA and from 107 to 1101 pS. Vrc remained unchanged (-2 vs. 1 mV). These data demonstrate that a hypo-osmotic challenge stimulates transepithelial K+ secretion and activates the apical IsK channel. The hypo-osmotically-induced increase in K+ secretion exceeded the estimated amount of K+ release necessary for the maintenance of constant cell volume, suggesting that the rate of basolateral K+ uptake was upregulated in the presence of the hypo-osmotic challenge and that cross-talk exists between the apical membrane and the basolateral membrane.

Molecular mechanisms for passive and active transport of water

Water crosses cell membranes by passive transport and by secondary active cotransport along with ions. While the first concept is well established, the second is new. The two modes of transport allow cellular H2O homeostasis to be viewed as a balance between H2O leaks and H2O pumps. Consequently, cells can be hyperosmolar relative to their surroundings during steady states. Under physiological conditions, cells from leaky epithelia may be hyperosmolar by roughly 5 mosm liter-1, under dilute conditions, hyperosmolarities up to 40 mosm liter-1 have been recorded. Most intracellular H2O is free to serve as solvent for small inorganic ions. The mechanism of transport across the membrane depends on how H2O interacts with the proteinaceous or lipoid pathways. Osmotic transport of H2O through specific H2O channels such as CHIP 28 is hydraulic if the pore is impermeable to the solute and diffusive if the pore is permeable. Cotransport of ions and H2O can be a result of conformational changes in proteins, which in addition to ion transport also translocate H2O bound to or occlude in the protein. A cellular model of a leaky epithelium based on H2O leaks and H2O pumps quantitatively predicts a number of so-far unexplained observations of H2O transport.