DIDS increases K+ secretion through an IsK channel in apical membrane of vestibular dark cell epithelium of gerbil

4,4'-Diisothiocyanatostilbene-2,2'-disulfonate modulates voltage-gated K+ current and influences cell cycle arrest in androgen sensitive and insensitive human prostate cancer cell lines

The stilbene derivative, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), an anion channel blocker is used in the present study to evaluate its modulatory effect on voltage-gated K⁺ current (I_K) in human prostate cancer cell lines (LNCaP and PC-3). Voltage-gated K⁺ (K_V) channels in the plasma membrane are critically involved in the proliferation of tumor cells. Therefore, K_V channels are considered as a novel potential target for cancer treatment. The results of the present study show that the external perfusion of DIDS activates I_K in a concentration-dependent manner, although the known K⁺ channel blocker TEA failed to block the DIDS activated I_K in PC-3 cells. Whereas, in LNCaP cells, the higher concentration of DIDS blocked I_K, though this effect was not completely recovered after washout. The difference in function of DIDS might be due to the expression of different Kv channel isoforms in LNCaP and PC-3 cells. Further, the anticancer studies show that treatment of DIDS significantly induced G2/M phase cell cycle arrest and induced moderate and low level of cell death in LNCaP and PC-3 cells respectively. This finding reveals that DIDS modulates I_K and exerts cell cycle arrest and cell death in LNCaP and PC-3 cells.

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.

The KCNE Tango - How KCNE1 Interacts with Kv7.1

The classical tango is a dance characterized by a 2/4 or 4/4 rhythm in which the partners dance in a coordinated way, allowing dynamic contact. There is a surprising similarity between the tango and how KCNE β-subunits "dance" to the fast rhythm of the cell with their partners from the Kv channel family. The five KCNE β-subunits interact with several members of the Kv channels, thereby modifying channel gating via the interaction of their single transmembrane-spanning segment, the extracellular amino terminus, and/or the intracellular carboxy terminus with the Kv α-subunit. Best studied is the molecular basis of interactions between KCNE1 and Kv7.1, which, together, supposedly form the native cardiac I(Ks) channel. Here we review the current knowledge about functional and molecular interactions of KCNE1 with Kv7.1 and try to summarize and interpret the tango of the KCNEs.

Reactive blue 2, an antagonist of rat P2Y4, increases K+ secretion in rat cochlea strial marginal cells

Extracellular ATP decreases K+ secretion in strial marginal cells via apical P2Y4 receptors. We investigated the effect of reactive blue 2 (RB-2), an antagonist of rat P2Y4, on rat strial marginal cells using a voltage-sensitive vibrating probe. The application of RB-2 increased K+ secretion in a dose-dependent manner, and this increase was characterized as a peak followed by a partial relaxation to a steady-state. Moreover, this response was similar to that caused by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). Suramin had no similar effect, except at high concentration. Thus, we tested the effects of these chemicals on P2Y4 receptors in strial marginal cells. Both RB-2 and DIDS had antagonistic activities at P2Y4, and the antagonist potency at P2Y4 paralleled the potency of K+ secretion. Interestingly, 2'- and 3'-O-(4-benzoyl-benzoyl)adenosine 5'-triphosphate (BzATP) exhibited an agonistic effect at P2Y4 receptor, which was blocked by RB-2, but not by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). Based on these results, we speculate that direct and/or indirect inhibitory mechanisms between P2Y4 and KENQ1/KCNE1 K+ channels exist in strial marginal cell.

Chloride secretion by semicircular canal duct epithelium is stimulated via beta 2-adrenergic receptors

The ductal epithelium of the semicircular canal forms much of the boundary between the K+-rich luminal fluid and the Na+-rich abluminal fluid. We sought to determine whether the net ion flux producing the apical-to-basal short-circuit current (I(sc)) in primary cultures was due to anion secretion and/or cation absorption and under control of receptor agonists. Net fluxes of 22Na, 86Rb, and 36Cl demonstrated a basal-to-apical Cl- secretion that was stimulated by isoproterenol. Isoproterenol and norepinephrine increased I(sc) with an EC50 of 3 and 15 nM, respectively, and isoproterenol increased tissue cAMP of native canals with an EC50 of 5 nM. Agonists for adenosine, histamine, and vasopressin receptors had no effect on I(sc). Isoproterenol stimulation of I(sc) and cAMP was inhibited by ICI-118551 (IC50 = 6 microM for I(sc)) but not by CGP-20712A (1 microM) in primary cultures, and similar results were found in native epithelium. I(sc) was partially inhibited by basolateral Ba2+ (IC50 = 0.27 mM) and ouabain, whereas responses to genistein, glibenclamide, and DIDS did not fully fit the profile for CFTR. Our findings show that the canal epithelium contributes to endolymph homeostasis by secretion of Cl- under beta 2 adrenergic control with cAMP as second messenger, a process that parallels the adrenergic control of K+ secretion by vestibular dark cells. The current work points to one possible etiology of endolymphatic hydrops in Meniere's disease and may provide a basis for intervention.

Ototoxic side-effects of the I(Ks)-channel blocker HMR1556

The inhibitor of I(Ks)-channels, HMR1556, a potentially antiarrhythmic drug, might possess ototoxic side-effects. The I(Ks)-channels are not only expressed in the heart but also in the stria vascularis of the inner ear and in the dark cells of the vestibular organ. Therefore possible effects of HMR1556 on hearing were studied in cats. Thresholds and intensity function of the cochlear action potential (CAP) were used as criteria. In addition to effects of the drug on heart rate and ECG, a substantial elevation of hearing thresholds and a shift in CAP intensity functions were observed. There was a clear dose-effect relationship. The hearing impairment observed showed a tendency for recovery. It is concluded that inhibitors of I(Ks)-channels may generally exert ototoxic effects provided they can reach the cochlear spaces.

Application of physiological genomics to the study of hearing disorders

Although the biophysical principles of how the ear operates are reasonably well understood, little is known about the specific genes that confer normal function to the inner ear. Nevertheless, the recent implementation of genomic tools has led to extraordinary progress in the identification of mutated genes that cause non-syndromic and syndromic forms of deafness. Part of this success is directly related to the sequencing of the human and mouse genomes and improved gene annotation methods. This review discusses how physiological genomic tools, such as genomic databases, expressed sequence tag databases and DNA arrays have been applied to find candidate genes for important molecular processes in the inner ear. It also illustrates, using the discovery of genes encoding essential components of cochlear K+ homeostasis as an example, how the combination of physiological genomic tools with physiological and morphological information has led to an in-depth understanding of cochlear ion homeostasis. Finally, it discusses how the use of applied genomic tools, such as gene arrays, will further advance our knowledge of how the inner ear works, develops, ages and regenerates.

Neurotransmitter-stimulated ion transport by cultured porcine vas deferens epithelium

A collagenase-based dissociation technique has been developed to routinely establish monolayer cultures of freshly isolated porcine vas deferens epithelium. Cells isolated from each tissue are transferred to 25-cm(2) tissue culture flasks and grown in a standard cell culture medium. Flasks reach confluency in 3-4 days, and cells are subsequently seeded onto permeable supports. Cultured cells display a monolayer cobblestone appearance and are immunoreactive to anti-ZO-1 and anti-cytokeratin antibodies. Electron microscopy is employed to demonstrate the presence of junctional complexes and microvilli. When evaluated in modified Ussing chambers, cultured monolayers exhibit a basal lumen negative potential difference, high electrical resistance (>1,000 Omega. cm(2)), and respond to norepinephrine, vasopressin, ATP, adenosine, and histamine, with changes in short-circuit current indicative of anion secretion. Responses are significantly attenuated in Cl(-)- and/or HCO-free solutions. Attempts to further optimize culture conditions have shown that chronic exposure to insulin increases proliferation rates. Thus the culture method described will reliably produce viable neurotransmitter-responsive cell monolayers that will allow for the characterization of vas deferens epithelial function and associated control mechanisms.

Estrogen acutely inhibits ion transport by isolated stria vascularis

We investigated the nongenomic effects of female sex steroid hormones on the short circuit current (I(sc,probe)) across gerbil stria vascularis using the voltage-sensitive vibrating probe. The strial marginal cell epithelial layer produces I(sc,probe) by secreting K+ via I(Ks) channels in the apical membrane. Application of 17beta-estradiol (E2) caused a decrease of I(sc,probe) in a dose-dependent manner (10 nM-10 microM) within seconds. Tamoxifen, a competitive inhibitor of the intracellular estrogen receptor, did not change the inhibitory effect of E2. Activation of I(Ks) channels by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid in the presence and absence of E2 was used to test the mechanism of action. The results were consistent with a direct inhibitory effect of E2 on the I(Ks) channels. By contrast, progesterone caused a transient increase of I(sc,probe). These results suggest that E2 decreases secretion of K+ by inhibition of I(Ks) channels via a nongenomic mechanism at concentrations near those occurring under some physiologic conditions while progesterone caused only transient effects on I(sc,probe).

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.

Pharmacology of CFTR chloride channel activity

Pharmacology of CFTR Chloride Channel Activity. Physiol. Rev. 79, Suppl.: S109-S144, 1999. - The pharmacology of cystic fibrosis transmembrane conductance regulator (CFTR) is at an early stage of development. Here we attempt to review the status of those compounds that modulate the Cl- channel activity of CFTR. Three classes of compounds, the sulfonylureas, the disulfonic stilbenes, and the arylaminobenzoates, have been shown to directly interact with CFTR to cause channel blockade. Kinetic analysis has revealed the sulfonylureas and arylaminobenzoates interact with the open state of CFTR to cause blockade. Suggestive evidence indicates the disulfonic stilbenes act by a similar mechanism but only from the intracellular side of CFTR. Site-directed mutagenesis studies indicate the involvement of specific amino acid residues in the proposed transmembrane segment 6 for disulfonic stilbene blockade and segments 6 and 12 for arylaminobenzoate blockade. Unfortunately, these compounds (sulfonylureas, disulfonic stilbenes, arylaminobenzoate) also act at a number of other cellular sites that can indirectly alter the activity of CFTR or the transepithelial secretion of Cl-. The nonspecificity of these compounds has complicated the interpretation of results from cellular-based experiments. Compounds that increase the activity of CFTR include the alkylxanthines, phosphodiesterase inhibitors, phosphatase inhibitors, isoflavones and flavones, benzimidazolones, and psoralens. Channel activation can arise from the stimulation of the cAMP signal transduction cascade, the inhibition of inactivating enzymes (phosphodiesterases, phosphatases), as well as the direct binding to CFTR. However, in contrast to the compounds that block CFTR, a detailed understanding of how the above compounds increase the activity of CFTR has not yet emerged.

Protein kinase C mediates P2U purinergic receptor inhibition of K+ channel in apical membrane of strial marginal cells

Strial marginal cells (SMC) electrogenically secrete K+ via slowly activating K+ (I[sK]) channels, consisting of I(sK) regulatory and KvLQT1 channel subunits, and the associated short circuit current (I[sc]) is inhibited by agonists of the apical P2U receptor [Liu et al., Audit. Neurosci. 2 (1995) 331-340]. Measurements of relative K+ flux (JK) with a self-referencing K+-selective probe demonstrated a decrease in JK after apical perfusion of 100 microM ATP. On-cell macro patch recordings from the apical membrane of gerbil SMC showed a decrease of the I(sK) channel current (I[IsK]) by 88 +/- 8% during pipette perfusion of 100 microM ATP. The magnitude of the decrease of L(sc) by ATP was diminished in the presence of inhibitors of phospholipase C (PLC) and protein kinase C (PKC), U-73122 and GF109203X. Activation of PKC by phorbol 12-myristate 13-acetate (20 nM) decreased I(IsK) (gerbil: by 62 +/- 10%; rat: by 72 +/- 6%) in perforated-patch whole-cell recordings while the inactive analog, 4alphaPMA, had no effect. By contrast, elevation of cytosolic [Ca2+] by A23187 increased the whole-cell I(IsK). The expression of the isk gene transcript was confirmed and the serine responsible for the species-specific response to PKC was found to be present in the gerbil I(sK) sequence. These data provide evidence consistent with a direct effect of the PKC branch of the PLC pathway on the I(sK) channel of SMC in response to activation of the apical P2U receptor and predict that the secretion of endolymph in the human cochlea may be controlled by PKC in the same way as in our animal model.

Functional evidence for a monocarboxylate transporter (MCT) in strial marginal cells and molecular evidence for MCT1 and MCT2 in stria vascularis

The transport of lactate, pyruvate and other monocarboxylates across plasma membranes of metabolically active cells such as strial marginal cells (SMC) may be important under aerobic conditions as well as under ischemic and hypoxic conditions. This study addresses the question whether SMC from the gerbil contain a membrane transport mechanism for monocarboxylates. The type of transporter was identified in functional studies by monitoring uptake of monocarboxylates into SMC through measurement of the cytosolic pH (pHi) with the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Further, subtypes of the functionally identified transporter which are present in stria vascularis were identified as transcripts by cloning and sequencing the reverse-transcription polymerase chain reaction (RT-PCR) products. All functional experiments were performed under nominally HCO3--free conditions. The monocarboxylates acetate and pyruvate (each 20 mM) induced an acidification of pHi. In contrast, the dicarboxylate malonate (20 mM) had no significant effect on pHi. Alpha-cyano-4-hydroxycinnamate (CHC; 5 mM), a blocker of H+/monocarboxylate cotransporter (MCT), reduced reversibly the acidification induced by 5 mM pyruvate. In contrast, 1 microM DIDS, a blocker of band-3 protein, had no significant effect on the acidification induced by 20 mM acetate. The presence of the transcripts for each of the MCT subtypes, MCT1 and MCT2, was determined by RT-PCR of stria vascularis from gerbil. RT-PCR performed with primers for the MCT1 and MCT2 subtypes on total RNA from stria vascularis revealed PCR products of the predicted sizes. Sequence analysis confirmed that amplified MCT1 and MCT2 cDNA fragments encoded a nucleotide sequence of MCT1 and MCT2, respectively. These observations suggest that SMC contain a MCT and that stria vascularis contains RNA for the subtypes MCT1 and MCT2 subtypes.

P2U purinergic receptor inhibits apical IsK/KvLQT1 channel via protein kinase C in vestibular dark cells

Vestibular dark cells (VDC) are known to electrogenically secrete K+ via slowly activating K+ (IsK) channels, consisting of IsK regulatory and KvLQT1 channel subunits, and the associated short-circuit current (Isc) is inhibited by agonists of the apical P2U (P2Y2) receptor (J. Liu, K. Kozakura, and D. C. Marcus. Audit. Neurosci. 2: 331-340, 1995). Measurements of relative K+ flux (JK) with a self-referencing K(+)-selective probe demonstrated a decrease in JK after apical perfusion of 100 microM ATP. On-cell macropatch recordings from gerbil VDC showed a decrease of the IsK channel current (IIsK) by 83 +/- 7% during pipette perfusion of 10 microM ATP. The magnitude of the decrease of Isc by ATP was diminished in the presence of inhibitors of phospholipase C (PLC) and protein kinase C (PKC), U-73122 and GF109203X. Activation of PKC by phorbol 12-myristate 13-acetate (PMA, 20 nM) decreased IIsK by 79 +/- 3% in perforated-patch whole cell recordings, whereas the inactive analog, 4 alpha-PMA, had no effect. In contrast, elevation of cytosolic Ca2+ concentration by A-23187 increased the whole cell IIsK. The expression of the isk gene transcript was confirmed, and the serine responsible for the species-specific response to PKC was found to be present in the gerbil IsK sequence. These data provide evidence consistent with a direct effect of the PKC branch of the PLC pathway on the IsK channel of VDC in response to activation of the apical P2U receptor and predict that the secretion of endolymph in the human vestibular system may be controlled by PKC in the same way as in our animal model.

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.

Inhibitory effect of erythromycin on ion transport by stria vascularis and vestibular dark cells

A previous study showed that systemic administration of erythromycin caused a reversible decline in the endocochlear and cochlear microphonic potentials. Those data were thought to suggest that erythromycin caused hearing loss by interference with ion transport processes in the stria vascularis. The present study was undertaken to test this hypothesis by measuring the effects of erythromycin perfused on either the apical or basolateral side on the transepithelial short circuit current (Isc), a measure of the K+ secretion rate. Isc was measured from preparations of strial marginal cells and the homologous vestibular dark cells in vitro with a micro-Ussing chamber. Erythromycin was found to have no effect when perfused on the apical side but to cause a reversible decrease in Isc when perfused on the basolateral side for both epithelia. These data are consistent with the notion that at least one ototoxic effect of erythromycin is the inhibition of K+ secretion in the inner ear.