Voltage-dependent outward K(+) current in intermediate cell of stria vascularis of gerbil cochlea

Role of bile acids and their receptors in gastrointestinal and hepatic pathophysiology

Bile acids (BAs) can regulate their own metabolism and transport as well as other key aspects of metabolic homeostasis via dedicated (nuclear and G protein-coupled) receptors. Disrupted BA transport and homeostasis results in the development of cholestatic disorders and contributes to a wide range of liver diseases, including nonalcoholic fatty liver disease and hepatocellular and cholangiocellular carcinoma. Furthermore, impaired BA homeostasis can also affect the intestine, contributing to the pathogenesis of irritable bowel syndrome, inflammatory bowel disease, and colorectal and oesophageal cancer. Here, we provide a summary of the role of BAs and their disrupted homeostasis in the development of gastrointestinal and hepatic disorders and present novel insights on how targeting BA pathways might contribute to novel treatment strategies for these disorders.

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

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

Mice deficient in H+-ATPase a4 subunit have severe hearing impairment associated with enlarged endolymphatic compartments within the inner ear

Mutations in the ATP6V0A4 gene lead to autosomal recessive distal renal tubular acidosis in patients, who often show sensorineural hearing impairment. A first Atp6v0a4 knockout mouse model that recapitulates the loss of H⁺-ATPase function seen in humans has been generated and recently reported (Norgett et al., 2012). Here, we present the first detailed analysis of the structure and function of the auditory system in Atp6v0a4^−/− knockout mice. Measurements of the auditory brainstem response (ABR) showed significantly elevated thresholds in homozygous mutant mice, which indicate severe hearing impairment. Heterozygote thresholds were normal. Analysis of paint-filled inner ears and sections from E16.5 embryos revealed a marked expansion of cochlear and endolymphatic ducts in Atp6v0a4^−/− mice. A regulatory link between Atp6v0a4, Foxi1 and Pds has been reported and we found that the endolymphatic sac of Atp6v0a4^−/− mice expresses both Foxi1 and Pds, which suggests a downstream position of Atp6v0a4. These mutants also showed a lack of endocochlear potential, suggesting a functional defect of the stria vascularis on the lateral wall of the cochlear duct. However, the main K⁺ channels involved in the generation of endocochlear potential, Kcnj10 and Kcnq1, are strongly expressed in Atp6v0a4^−/− mice. Our results lead to a better understanding of the role of this proton pump in hearing function.

Exposure to low-dose barium by drinking water causes hearing loss in mice

PURPOSE

We continuously ingest barium as a general element by drinking water and foods in our daily life. Exposure to high-dose barium (>100mg/kg/day) has been shown to cause physiological impairments. Direct administration of barium to inner ears by vascular perfusion has been shown to cause physiological impairments in inner ears. However, the toxic influence of oral exposure to low-dose barium on hearing levels has not been clarified in vivo. We analyzed the toxic influence of oral exposure to low-dose barium on hearing levels and inner ears in mice.

EXPERIMENTAL DESIGN

We orally administered barium at low doses of 0.14 and 1.4 mg/kg/day to wild-type ICR mice by drinking water. The doses are equivalent to and 10-fold higher than the limit level (0.7 mg/l) of WHO health-based guidelines for drinking water, respectively. After 2-week exposure, hearing levels were measured by auditory brain stem responses and inner ears were morphologically analyzed. After 2-month exposure, tissue distribution of barium was measured by inductively coupled plasma mass spectrometry.

RESULTS

Low-dose barium in drinking water caused severe hearing loss in mice. Inner ears including inner and outer hair cells, stria vascularis and spiral ganglion neurons showed severe degeneration. The Barium-administered group showed significantly higher levels of barium in inner ears than those in the control group, while barium levels in bone did not show a significant difference between the two groups. Barium levels in other tissues including the cerebrum, cerebellum, heart, liver and kidney were undetectably low in both groups.

CONCLUSIONS

Our results demonstrate for the first time that low-dose barium administered by drinking water specifically distributes to inner ears resulting in severe ototoxicity with degeneration of inner ears in mice.

In vivo imaging of hepatic excretory function in the rat by fluorescence microscopy

Applying intravital fluorescence microscopy, we assessed sinusoidal delivery and biliary clearance of two different polymethine dyes. DY635, a benzopyrylium-based hemocyanine dye with shorter excitation wavelength than indocyanine green (ICG), was validated for assessment of hepatic excretory function. Decrease of DY635 and ICG reflecting transcellular transport was 83 ± 4% (DY635) and 14 ± 2% (ICG; p < 0.05) over 35 minutes, respectively. In cholestasis, hepatobiliary excretion of DY635 was markedly impaired (control 3176 ± 148 pmol vs. cholestatic 1929 ± 179 pmol; p < 0.05). DY635 even enabled an analysis at high resolution suggesting 1.) hepatocyte uncoupling and 2.) failure of primarily the canalicular pole, allowing in vivo insights into molecular mechanisms of this critical facet of hepatobiliary function.

Computational model of a circulation current that controls electrochemical properties in the mammalian cochlea

Sound-evoked mechanical stimuli permit endolymphatic K(+) to enter sensory hair cells. This transduction is sensitized by an endocochlear potential (EP) of +80 mV in endolymph. After depolarizing the cells, K(+) leaves hair cells in perilymph, and it is then circulated back to endolymph across the lateral cochlear wall. In theory, this process entails a continuous and unidirectional current carried by apical K(+) channels and basolateral K(+) uptake transporters in both the marginal cell and syncytial layers of the lateral wall. The transporters regulate intracellular and extracellular [K(+)], allowing the channels to form K(+) diffusion potentials across each of the two layers. These diffusion potentials govern the EP. What remains uncertain is whether these transport mechanisms accumulating across diverse cell layers make up a continuous circulation current in the lateral wall and how this current might affect the characteristics of the endolymph. To address this question, we developed an electrophysiological model that incorporates channels and transporters of the lateral wall and channels of hair cells that derive a circulation current. The simulation replicated normal experimental EP values and reproduced experimentally measured changes in the EP and intra- and extracellular [K(+)] in the lateral wall when different transporters and channels were blocked. The model predicts that, under these different conditions, the circulation current's contribution to the EP arises from different sources. Finally, our model also accurately simulated EP loss in a mouse model of a chloride channelopathy associated with deafness.

How is the highly positive endocochlear potential formed? The specific architecture of the stria vascularis and the roles of the ion-transport apparatus

Cochlear endolymph, an extracellular solution containing 150 mM K(+), exhibits a positive potential of +80 mV. This is called the endocochlear potential (EP) and is essential for audition. The mechanism responsible for formation of the EP has been an enigma for the half century since its first measurement. A key element is the stria vascularis, which displays a characteristic tissue structure and expresses multiple ion-transport apparatus. The stria comprises two epithelial layers: a layer of marginal cells and one composed of intermediate and basal cells. Between the two layers lies an extracellular space termed the intrastrial space (IS), which is thus surrounded by the apical membranes of intermediate cells and the basolateral membranes of marginal cells. The fluid in the IS exhibits a low concentration of K(+) and a positive potential similar to the EP. We have demonstrated that the IS is electrically isolated from the neighboring extracellular fluids, perilymph, and endolymph, which allows the IS to sustain its positive potential. This IS potential is generated by K(+) diffusion across the apical membranes of intermediate cells, where inwardly rectifying Kir4.1 channels are localized. The low K(+) concentration in the IS, which is mandatory for the large K(+)-diffusion potential, is maintained by Na(+),K(+)-ATPases and Na(+),K(+),2Cl(-)-cotransporters expressed at the basolateral membranes of marginal cells. An additional K(+)-diffusion potential formed by KCNQ1/KCNE1-K(+) channels at the apical membranes of marginal cells also contributes to the EP. Therefore, the EP depends on an electrically isolated space and two K(+)-diffusion potentials in the stria vascularis.

Inflammation mediated down-regulation of hepatobiliary transporters contributes to intrahepatic cholestasis and liver damage in murine biliary atresia

To investigate the hypothesis that during the development of biliary atresia, early changes in hepatobiliary transport are mainly related to the inflammatory process and lead to intrahepatic cholestasis and subsequent liver injury, livers from mice with rhesus rotavirus-induced biliary atresia were analyzed for mRNA expression of hepatobiliary transporters, nuclear receptors, and inflammatory cytokines. Seven days after inoculation, despite high bile acid concentrations in the liver, gene expression of canalicular and basolateral hepatobiliary transporters and their regulatory nuclear receptors was down-regulated with concomitant increase in gene expression of inflammatory cytokines and rise in serum unconjugated bilirubin. At 14 d, hepatobiliary transporters and nuclear receptors remained down-regulated although the inflammatory response subsided. The percentage of conjugated bilirubin started to increase as extrahepatic biliary obstruction occurred. At 18 d, expression of hepatobiliary transporters remained low, expression of nuclear receptors returned to normal, while expression of inflammatory cytokines decreased further. Moreover, histology demonstrated progressive inflammation, bile duct damage, ductular proliferation, and hepatocyte necrosis. In conclusion, intrahepatic cholestasis due to inflammation-related down-regulation of basolateral and canalicular hepatobiliary transporters is an early event in the development of biliary atresia. Intrahepatic cholestasis contributes to the development of jaundice and liver injury.

The endocochlear potential depends on two K+ diffusion potentials and an electrical barrier in the stria vascularis of the inner ear

An endocochlear potential (EP) of +80 mV is essential for audition. Although the regulation of K(+) concentration ([K(+)]) in various compartments of the cochlear stria vascularis seems crucial for the formation of the EP, the mechanism remains uncertain. We have used multibarreled electrodes to measure the potential, [K(+)], and input resistance in each compartment of the stria vascularis. The stria faces two fluids, perilymph and endolymph, and contains an extracelluar compartment, the intrastrial space (IS), surrounded by two epithelial layers, the marginal cell (MC) layer and that composed of intermediate and basal cells. Fluid in the IS exhibits a low [K(+)] and a positive potential, called the intrastrial potential (ISP). We found that the input resistance of the IS was high, indicating this space is electrically isolated from the neighboring extracellular fluids. This arrangement is indispensable for maintaining positive ISP. Inhibiting the K(+) transporters of the stria by anoxia, ouabain, or bumetanide caused the [K(+)] of the IS to increase and the intracellular [K(+)] of MCs to decrease, reducing both the ISP and the EP. Calculations indicate that the ISP represents the K(+) diffusion potential across the apical membranes of intermediate cells through Ba(2+)-sensitive K(+) channels. The K(+) diffusion potential across the apical membranes of MCs also contributes to the EP. Because the EP depends on two K(+) diffusion potentials and an electrical barrier in the stria vascularis, interference with any of these elements can interrupt hearing.

Generation of the endocochlear potential: a biophysical model

The generation and maintenance of the endocochlear potential (EP) by the stria vascularis is essential for proper function of the cochlea. We present a mathematical model that captures the critical biophysical interactions between the distinct cellular layers that generate the EP. By describing the relationship between the K(+) concentration in the intrastrial space and the intermediate cell transmembrane potential, we rationalize the presence of a large intermediate cell K(+) conductance and predict that the intrastrial [K(+)] is approximately 4 mM at steady state. The model also predicts that the stria vascularis is capable of buffering the EP against external perturbations in a manner modulated by changes in intrastrial [K(+)], thus facilitating hearing sensitivity across the broad dynamic range of the auditory system.

Intraabdominal bacterial infections significantly alter regeneration and function of the liver in a rat model of major hepatectomy

BACKGROUND

No systematic investigations of interactions of postoperative infections and liver regeneration after resection are available.

MATERIALS AND METHODS

Male Sprague-Dawley rats underwent sham operation, 70% partial hepatectomy (PH), cecal ligation and puncture (CLP), or synchronous PH + CLP and were killed at regular intervals. Liver regeneration and function were measured by the mitotic index, Bromo-deoxy-uridine labeling, and Ki-67 as well as bilirubin, albumin, and indocyanine green plasma disappearance rate. The inflammatory response was evaluated by determination of IL-1beta and myeloperoxidase (MPO) activity. Bacterial concentrations in different organs were quantified.

RESULTS

Simultaneous CLP + PH resulted in a significantly delayed regeneration kinetic, which was most pronounced at 24 h. This was preceded by hyperinflammation with increased liberation of pro-inflammatory cytokines in the PH + CLP group at 6 h. After 48 h, the pro-inflammatory response declined, and regeneration proceeded also in the PH + CLP group. Liver function was found impaired in both groups; however, it was significantly worse in the PH + CLP group. Especially after 48 h, when regeneration peaked in this group, liver function significantly declined. At 96 h, only minor differences were seen, but the persistently elevated proliferative activity indicated the delay of regeneration after PH + CLP.

CONCLUSION

The present analysis shows that infectious conditions delay liver regeneration. Our data suggest a cross-linkage of both conditions via the functional liver capacity. A direct role of microorganisms seems unlikely; however, the inhibitory effect of the pro-inflammatory cytokines may be involved.

Plasma disappearance of indocyanine green: a marker for excretory liver function?

OBJECTIVE

To investigate whether the plasma disappearance rate of indocyanine green (ICG) assessed using a commercially available bedside monitor provides an accurate estimation of cumulative biliary ICG excretion in a clinically relevant model of long-term, hyperdynamic porcine endotoxemia.

DESIGN AND SETTING

Prospective experimental study in the animal laboratory in a university hospital.

SUBJECTS

Fifteen domestic pigs.

INTERVENTIONS

Pigs were anesthetized, mechanically ventilated, and instrumented. Intravenous endotoxin was continuously infused over 12 h concomitant with fluid resuscitation. Measurements were performed before and 12 h after the start of endotoxin infusion.

MEASUREMENTS AND RESULTS

All animals developed hyperdynamic circulation characterized by a sustained increase in cardiac output. Despite well maintained portal venous and consequently total liver blood flow endotoxemia decreased hepatic lactate uptake, which was accompanied by a significant fall in portal and hepatic venous pH. Both the cumulative bile flow and biliary ICG and bicarbonate excretion measured during 1 h after intravenous bolus of 25 mg ICG fell significantly. By contrast, neither the plasma disappearance rate of ICG nor the rate corrected for liver blood flow exhibited any changes over time.

CONCLUSIONS

In hyperdynamic porcine endotoxemia the plasma disappearance rate of ICG failed to accurately substitute for direct short-term measures of biliary ICG excretion. Hence normal values of plasma disappearance rate of ICG should be interpreted with caution in early, acute inflammatory conditions.

Novel structures in marginal and intermediate cells presumably relate to functions of apical versus basal strial strata

Prior ultrastructural studies showed that K+ supplied to the stria vascularis came from recycling ions from the organ of Corti or perilymph to strial basal cells. A newly distinguished basal subtype of intermediate cell (BIC) completely covered the basal cells with a leaf-like horizontal process and appeared situated to absorb from them all of the recycled K+. The basal region of marginal cells (MCs) projected foot-like and enlarged processes to border BICs opposite an unique ca. 150 angstroms space. These basal MC processes appeared positioned to resorb part of the K+ recycled to BICs. A second, upper subtype of IC (UIC), occupying middle to upper strial strata, contacted BIC's extensively. UICs were thus located to resorb from BICs the portion of the recycled K+ not forwarded to basal MC processes. The apical segment of MCs projected mitochondria-filled primary processes and numerous associated secondary processes. The Na,K-ATPase-rich secondary processes populated mid to upper stria where they could siphon K+ from UICs and resorb and secrete the ions thus generating the 150 mM [KCl] of endolymph. The morphologic relationship of basal marginal cell processes to BICs differed so strikingly from the relation of upper MC processes to UICs as to suggest a different function for basal stria, one possibly concerned with generating the endocochlear potential.

Lipopolysaccharide-mediated regulation of hepatic transporter mRNA levels in rats

The function of hepatic transporters is to move organic substances across sinusoidal and canalicular membranes. During extrahepatic cholestasis, transporters involved in the movement of substances from blood to bile, such as sodium/taurocholate-cotransporting polypeptide (Ntcp) and multidrug resistance protein 2 (Mrp2), are down-regulated, whereas others that transport chemicals from liver to blood, such as Mrp3, are up-regulated. Unlike extrahepatic cholestasis, where transporter expression responds to the stress of accumulating bile constituents, lipopolysaccharide (LPS)-induced intrahepatic cholestasis may be directly caused by alterations in transporter expression. The aim of this study was to quantitatively determine the effect of LPS on transporter expression and study the mechanism(s) by which LPS alters mRNA levels of major hepatic transporters in Sprague-Dawley rats. Hepatic mRNA levels of Mrp2, Mrp6, multiple drug resistance protein 1a (Mdr1a), organic anion-transporting polypeptide 1 (Oatp1), Oatp2, Oatp4, Ntcp, bile salt export pump, organic cation transporter 1 (Oct1), and organic anion transporter 3 (Oat3) were dramatically decreased, beginning approximately 6 h after LPS administration, whereas Mrp5 and Oat2 levels were unchanged. In contrast, LPS increased mRNA levels of Mrp1, Mrp3, and Mdr1b concurrently with the down-regulated transporters. Pretreatment with dexamethasone, which decreases the release of cytokines, reversed the reduction of Mdr1a, Oatp1, Oatp2, Oct1, and Ntcp mRNA following LPS administration. Furthermore, dexamethasone pretreatment also prevented the LPS-mediated increase in Mrp1, Mrp3, and Mdr1b, whereas pretreatment with aminoguanidine or gadolinium chloride, an inhibitor of inducible nitric oxide synthetase and a Kupffer cell toxicant, respectively, had no effect on the LPS-induced changes. The concurrent repression and induction of various transporters, as well as dexamethasone abatement of both LPS-mediated repression and induction, indicates that these responses may be mediated through similar pathways.

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

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

Loss of KCNJ10 protein expression abolishes endocochlear potential and causes deafness in Pendred syndrome mouse model

BACKGROUND

Pendred syndrome, a common autosomal-recessive disorder characterized by congenital deafness and goiter, is caused by mutations of SLC26A4, which codes for pendrin. We investigated the relationship between pendrin and deafness using mice that have (Slc26a4+/+) or lack a complete Slc26a4 gene (Slc26a4-/-).

METHODS

Expression of pendrin and other proteins was determined by confocal immunocytochemistry. Expression of mRNA was determined by quantitative RT-PCR. The endocochlear potential and the endolymphatic K+ concentration were measured with double-barreled microelectrodes. Currents generated by the stria marginal cells were recorded with a vibrating probe. Tissue masses were evaluated by morphometric distance measurements and pigmentation was quantified by densitometry.

RESULTS

Pendrin was found in the cochlea in apical membranes of spiral prominence cells and spindle-shaped cells of stria vascularis, in outer sulcus and root cells. Endolymph volume in Slc26a4-/- mice was increased and tissue masses in areas normally occupied by type I and II fibrocytes were reduced. Slc26a4-/- mice lacked the endocochlear potential, which is generated across the basal cell barrier by the K+ channel KCNJ10 localized in intermediate cells. Stria vascularis was hyperpigmented, suggesting unalleviated free radical damage. The basal cell barrier appeared intact; intermediate cells and KCNJ10 mRNA were present but KCNJ10 protein was absent. Endolymphatic K+ concentrations were normal and membrane proteins necessary for K+ secretion were present, including the K+ channel KCNQ1 and KCNE1, Na+/2Cl-/K+ cotransporter SLC12A2 and the gap junction GJB2.

CONCLUSIONS

These observations demonstrate that pendrin dysfunction leads to a loss of KCNJ10 protein expression and a loss of the endocochlear potential, which may be the direct cause of deafness in Pendred syndrome.

BK channels mediate the voltage-dependent outward current in type I spiral ligament fibrocytes

Recent experimental and clinical studies have provided considerable evidence to support the phenomenon of K(+) recycling in the mammalian cochlea. However, the precise cellular and molecular mechanisms underlying and regulating this process remain only partially understood. Here, we report that cultured type I spiral ligament fibrocytes (SLFs), a major component of the K(+) recycling pathway, have a dominant K(+) membrane conductance that is mediated by BK channels. The averaged half-maximal voltage-dependent membrane potential for the whole-cell currents was 70+/-1.2 mV at 1 nM intracellular free Ca(2+) and shifted to 38+/-0.2 mV at 20 microM intracellular free Ca(2+) (n=4-6). The reversal potential of whole-cell tail currents against different bath K(+) concentrations was 52 mV per decade (n=3-6). The sequence of relative ion permeability of the whole-cell conductance was K(+)>Rb(+)z.Gt;Cs(+)>Na(+) (n=5-17). The whole-cell currents were inhibited by extracellular tetraethylammonium and iberiotoxin (IbTx) with IC(50) values of 0.07 mM and 0.013 microM, respectively (n=3-7). The membrane potentials of type I SLFs measured with conventional zero-current whole-cell configuration were highly K(+)-selective and sensitive to IbTx (n=4-9). In addition, the BK channels in these cells exhibited voltage-dependent and incomplete inactivation properties and the recovery time was estimated to be approximately 6 s with repetitive voltage pulses from -70 to 80 mV (n=3). These data suggest that BK channels in type I SLFs play a major role in regulating the intracellular electrochemical gradient in the lateral wall syncytium responsible for facilitating the K(+) movement from perilymph to the stria vascularis.

Effects of proinflammatory cytokines on rat organic anion transporters during toxic liver injury and cholestasis

Hepatobiliary transporters are down-regulated in toxic and cholestatic liver injury. Cytokines such as tumor necrosis factor alpha (TNF-alpha) and interleukin 1 beta (IL-1 beta) are attributed to mediate this regulation, but their particular contribution in vivo is still unknown. Thus, we studied the molecular mechanisms by which Ntcp, Oatp1, Oatp2, and Mrp2 are regulated by proinflammatory cytokines during liver injury. Rats were injected intraperitoneally with either carbon tetrachloride or endotoxin. Inactivation of TNF-alpha and IL-1 beta was achieved by repetitive intraperitoneal injection of etanercept and anakinra, respectively. Messenger RNA (mRNA) levels of transporters and binding activities as well as nuclear protein levels of Ntcp, Oatp2, and Mrp2 transactivators were determined 20 to 24 hours later. In contrast to IL-1 beta, TNF-alpha inactivation alone fully prevented down-regulation of Ntcp, Oatp1, and Oatp2 mRNA as well as reduced binding activity of hepatocyte nuclear factor 1 (HNF-1) in CCl(4)-induced toxic injury. In endotoxemia, down-regulation of Mrp2, and partially in case of Ntcp, could be prevented by IL-1 beta but not TNF-alpha blockade. However, inactivation of either cytokine led to preservation of HNF1 and partially of retinoid X receptor/retinoic acid receptor (RXR/RAR) binding activity. No effect of anticytokines was seen on pregnane X receptor (PXR) and constitutive androstane receptor (CAR) binding activity as well as nuclear protein mass. In conclusion, TNF-alpha represents the master cytokine responsible for HNF1-dependent down-regulation of Ntcp, Oatp1, and Oatp2 in CCl(4)-induced toxic liver injury. IL-1 beta predominates in a complex signaling network of Ntcp and Mrp2 regulation in cholestatic liver injury. In contrast to in vitro studies, HNF1 and RXR/RAR-independent mechanisms appear to be more important in regulation of Mrp2 and Ntcp gene expression in endotoxemia.

Aquaporin-1 (AQP1) is expressed in the stria vascularis of rat cochlea

Cochlea endolymph, produced by the stria vascularis, is essential for normal inner ear function. Abnormal endolymphatic volumes correlate closely with pathological conditions such as Ménière's disease. The critical roles played by aquaporins, which facilitate osmotic movement of water molecules, are known in a variety of tissues. We investigated the expression of aquaporin-1 (AQP1) in the rat inner ear using reverse transcription polymerase chain reaction and immunohistochemical methods. We obtained novel data showing that not just AQP1 mRNA but also AQP1 protein is expressed in the stria vascularis, in addition to other data confirming previous reports. AQP1 immunoreactivity localized to the intermediate cells in the stria vascularis. The above finding suggests that AQP1 may play a role in the water distribution associated with vigorous ion transport in the stria vascularis since the intermediate part of the stria vascularis contains both intermediate cells and the basolateral parts of marginal cells, both of which express ion transporters abundantly.

Chronic Staphylococcal enterotoxin B and lipopolysaccharide induce a bimodal pattern of hepatic dysfunction and injury

OBJECTIVE

To determine the effect of chronic exposure to endotoxin (lipopolysaccharide) and Staphylococcal enterotoxin B on hepatic injury and function.

DESIGN

Prospective, controlled trial.

SETTING

Research laboratory in a university hospital.

SUBJECTS

Male Sprague-Dawley rats weighing 325-350 g with chronic vascular and bile catheters.

INTERVENTIONS

Chronically catheterized rats were treated daily with saline, 50 microg/kg Staphylococcal enterotoxin B alone, 1000 microg/kg lipopolysaccharide alone, 1000 microg/kg lipopolysaccharide with 50 microg/kg Staphylococcal enterotoxin B, or 100 microg/kg lipopolysaccharide with 50 microg/kg Staphylococcal enterotoxin B for 10 days. Serum and biliary measures of hepatic injury and dysfunction were measured before and then 6 hrs and 1, 2, 3, 7, and 10 days after the start of treatment. The animals were killed at 10 days and the livers examined histologically.

MEASUREMENTS AND MAIN RESULTS

Mean rates of bile flow, biliary indocyanine green excretion, and bile acid flux were significantly decreased immediately after treatment (6 hr, 1 and 2 days) and then at 10 days. Increases in biliary and serum gamma-glutamyltransferase and serum bile acids also occurred in a similar bimodal pattern. Animals treated with lipopolysaccharide or Staphylococcal enterotoxin B alone became tolerant and did not develop the bimodal pattern of hepatic dysfunction. Histologic examination of the liver at 10 days revealed periportal inflammation and fibrosis.

CONCLUSIONS

The combination of lipopolysaccharide and Staphylococcal enterotoxin B leads to late liver injury, whereas either toxin alone does not. These data may explain the frequent development of liver dysfunction in patients exposed to multiple bacterial toxins such as in sepsis, multiple-system organ failure, and other diseases with altered intestinal permeability.

Reduction in the endocochlear potential caused by Cs(+) in the perilymph can be explained by the five-compartment model of the stria vascularis

In an earlier publication (Takeuchi et al., Biophys. J. 79 (2000) 2572-2582), we proposed that K(+) channels in intermediate cells within the stria vascularis may play an essential role in the generation of the endocochlear potential (EP), and we presented an extended version of the five-compartment model of the stria vascularis. In search of further evidence supporting the five-compartment model, we studied the effects of Cs(+) added to the perilymph on guinea pig EP. Cs(+) is known as a competitive K(+) channel blocker. Both the scala tympani and the scala vestibuli of four cochlear turns were perfused at a flow rate of 10 microl/min, and the EP was recorded from the second cochlear turn. Cs(+) at 30 mM caused a biphasic change in the EP; the EP increased transiently from a control level of 89.6 mV to 94.8 mV within 10 min, and then decreased to a steady level of 24.5 mV within the next 40 min. We propose that the initial transient increase in the EP results from Cs(+)-mediated blockade of K(+) conductance in the basolateral membrane of hair cells, and that the subsequent EP decrease is due to effects of Cs(+) on the stria vascularis. We believe that Cs(+) in the perilymph is able to access the stria vascularis by being taken up by fibrocytes in the spiral ligament and then being transported to intermediate cells because it is known that Cs(+) is taken up via Na(+),K(+)-ATPase and that gap junctions connect fibrocytes in the spiral ligament to basal cells and basal cells to intermediate cells. To clarify the effect of intracellular Cs(+) on the electrophysiological properties of intermediate cells, these cells were dissociated from guinea pigs and studied by the whole-cell patch-clamp method. Intracellular Cs(+) depolarized intermediate cells in a dose-dependent manner. In addition, efflux of Cs(+) from the intermediate cell was much less than the efflux of K(+). Thus, Cs(+) may accumulate in the intermediate cell, which depolarizes the cell, which in turn decreases the EP. We conclude that the five-compartment model of the stria vascularis can explain the EP decrease caused by Cs(+) in the perilymph.

KCNJ10 (Kir4.1) potassium channel knockout abolishes endocochlear potential

Stria vascularis of the cochlea generates the endocochlear potential and secretes K(+). K(+) is the main charge carrier and the endocochlear potential the main driving force for the sensory transduction that leads to hearing. Stria vascularis consists of two barriers, marginal cells that secrete potassium and basal cells that are coupled via gap junctions to intermediate cells. Mice lacking the KCNJ10 (Kir4.1) K(+) channel in strial intermediate cells did not generate an endocochlear potential. Endolymph volume and K(+) concentration ([K(+)]) were reduced. These studies establish that the KCNJ10 K(+) channel provides the molecular mechanism for generation of the endocochlear potential in concert with other transport pathways that establish the [K(+)] difference across the channel. KCNJ10 is also a limiting pathway for K(+) secretion.

Three-dimensional and ultrastructural relationships between intermediate cells and capillaries in the gerbil stria vascularis

Structural relationships between intermediate cells and capillaries in the stria vascularis of gerbils were examined by confocal laser microscopy and electron microscopy. Immunostaining for an inward rectifier K(+) channel (Kir4.1), which was localized to intermediate cells, was used to determine the three-dimensional distribution of intermediate cells. These cells constituted a honeycomb-like network, and their dendritic processes surrounded not only capillaries but also the basolateral surface of epithelial marginal cells. On the basis of the above finding and the large K(+) conductance in intermediate cells, we propose that the network composed of intermediate cells has a spatial K(+) buffering function. Transmission electron microscopy revealed the absence of the basal lamina in some regions and the presence of a gap junction-like membrane association between intermediate cells and pericytes and/or endothelial cells. This result supported our previous finding that intermediate cells were dye-coupled with pericytes and endothelial cells. The presence of gap junctions between intermediate cells and pericytes and/or endothelial cells suggests that endothelial cells and pericytes may play roles other than forming a structural route for blood circulation.

Location and function of the epithelial Na channel in the cochlea

In the cochlea, endolymph is a K-rich and Na-poor fluid. The purpose of the present study was to check the presence and to assess the role of epithelial Na channel (ENaC) in this organ. alpha-, beta-, and gamma-ENaC subunit mRNA, and proteins were detected in rat cochlea by RT-PCR and Western blot. alpha-ENaC subunit mRNA was localized by in situ hybridization in both epithelial (stria vascularis, spiral prominence, spiral limbus) and nonepithelial structures (spiral ligament, spiral ganglion). The alpha-ENaC-positive tissues were also positive for beta-subunit mRNA (except spiral ganglion) or for gamma-subunit mRNA (spiral limbus, spiral ligament, and spiral ganglion), but the signals of beta- and gamma-subunits were weaker than those observed for alpha-subunit. In vivo, the endocochlear potential was recorded in guinea pigs under normoxic and hypoxic conditions after endolymphatic perfusion of ENaC inhibitors (amiloride, benzamil) dissolved either in K-rich or Na-rich solutions. ENaC inhibitors altered the endocochlear potential when Na-rich but not when K-rich solutions were perfused. In conclusion, ENaC subunits are expressed in epithelial and nonepithelial cochlear structures. One of its functions is probably to maintain the low concentration of Na in endolymph.

Distinguishing cochlear pathophysiology in 4-aminopyridine and furosemide treated ears using a nonlinear systems identification technique

To test the adequacy of physiologic indices derived from a third-order polynomial model quantifying cochlear mechano-electric transduction (MET), 24 Mongolian gerbils were exposed to either 250-mM glucose (control), 150-mM 4-aminopyridine (4-AP), or 30-mM furosemide solutions applied to the round window (RW) membrane. The cochlear microphonic (CM) was recorded from the RW in response to 68- and 88-dB SPL Gaussian noise. A nonlinear systems identification technique (NLID) provided the frequency-domain parameters and physiologic indices of the polynomial model of MET. The control group showed no change in both compound action potential (CAP) thresholds and CM. Exposure to 4-AP and furosemide resulted in a similar elevation in CAP thresholds and a reduction in CM. However, the polynomial model of MET showed different changes. The operating point, slope, and symmetry of the MET function, the polynomial model parameters, and related nonlinear coherences differed between the experimental groups. It is concluded that the NLID technique is sensitive and specific to alterations in the cochlear physiology.

Mechanism generating endocochlear potential: role played by intermediate cells in stria vascularis

The endocochlear DC potential (EP) is generated by the stria vascularis, and essential for the normal function of hair cells. Intermediate cells are melanocytes in the stria vascularis. To examine the contribution of the membrane potential of intermediate cells (E(m)) to the EP, a comparison was made between the effects of K(+) channel blockers on the E(m) and those on the EP. The E(m) of dissociated guinea pig intermediate cells was measured in the zero-current clamp mode of the whole-cell patch clamp configuration. The E(m) changed by 55.1 mV per 10-fold changes in extracellular K(+) concentration. Ba(2+), Cs(+), and quinine depressed the E(m) in a dose-dependent manner, whereas tetraethylammonium at 30 mM and 4-aminopyridine at 10 mM had no effect. The reduction of the E(m) by Ba(2+) and Cs(+) was enhanced by lowering the extracellular K(+) concentration from 3.6 mM to 1.2 mM. To examine the effect of the K(+) channel blockers on the EP, the EP of guinea pigs was maintained by vascular perfusion, and K(+) channel blockers were administered to the artificial blood. Ba(2+), Cs(+) and quinine depressed the EP in a dose-dependent manner, whereas tetraethylammonium at 30 mM and 4-aminopyridine at 10 mM did not change the EP. A 10-fold increase in the K(+) concentration in the artificial blood caused a minor decrease in the EP of only 10.6 mV. The changes in the EP were similar to those seen in the E(m) obtained at the lower extracellular K(+) concentration of 1.2 mM. On the basis of these results, we propose that the EP is critically dependent on the voltage jump across the plasma membrane of intermediate cells, and that K(+) concentration in the intercellular space in the stria vascularis may be actively controlled at a concentration lower than the plasma level.

Endotoxin-stimulated macrophages decrease bile acid uptake in WIF-B cells, a rat hepatoma hybrid cell line

Endotoxemia leads to cytokine-mediated alterations of the hepatocellular sodium-taurocholate-cotransporting polypeptide (ntcp). We hypothesized that stimulated macrophages are essential transducers for down-regulating hepatocellular bile salt uptake in response to endotoxin (lipopolysaccharide [LPS]) exposure. Using an in vitro model, we exposed mouse macrophages (IC-21 cell line) to LPS for 24 hours. Concentrations of cytokines tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, and IL-6 increased 10.6-fold, 12.5-fold, and 444-fold, respectively, in LPS-conditioned IC-21 medium (CM) versus unconditioned IC-21 medium (UM). WIF-B rat hepatoma hybrid cells were incubated with either CM or UM or treated directly with medium containing recombinant TNF-alpha, IL-1beta, and IL-6. [(3)H]Taurocholate ([(3)H]TC) uptake decreased in WIF-B cells exposed to either TNF-alpha (54% of control), IL-1beta (78%), IL-6 (55%) as single additives, or in triple combination (TCC) (43%). A virtually identical decrease was observed after exposing WIF-B cells to CM (52%, P <.001). LPS had no direct effect on [(3)H]TC uptake. CM treatment did not decrease L-alanine transport in WIF-B cells. Blocking antibodies against TNF-alpha, IL-1beta, and IL-6 restored the diminished [(3)H]TC uptake in cells exposed to TCC and CM to 87% and 107% of controls, respectively. Northern blotting revealed that ntcp messenger RNA (mRNA) expression was significantly reduced in WIF-B cells after exposure to CM, and in primary rat hepatocytes exposed to CM or TNF-alpha (68%, 14%, and 29% of control, respectively). We conclude that macrophages and their ability to secrete the cytokines TNF-alpha, IL-1beta, and IL-6 may be essential in mediating the endotoxin-induced cholestatic effect of decreased hepatocellular bile salt uptake.

Sound needs sound melanocytes to be heard

Intermediate cells in the stria vascularis of the mammalian cochlea are melanocytes, which contain melanin pigments and are capable of synthesizing melanin. These melanocytes are required for normal development of the cochlea, as evidenced by studies of mutant mice with congenital melanocyte anomalies. Melanocytes are also needed for developed cochleae to function normally, as evidenced by studies of mutant mice with late-onset melanocyte anomaly and humans with acquired melanocyte anomaly. Melanin, per se, does not seem to be essential for normal hearing function, but it may protect against traumata to the cochlea, e.g., noise and ototoxic aminoglycosides. Recent electrophysiological studies have revealed that strial intermediate cells are provided with specific ionic channels, such as inwardly rectifying K+ channels (Kir4.1) and voltage-dependent outwardly rectifying K+ channels. These channels may play central roles in strial function and thus in normal hearing.