Amiloride directly inhibits the Na,K-ATPase activity of rabbit kidney proximal tubules

4-(Azolyl)-Benzamidines as a Novel Chemotype for ASIC1a Inhibitors

Acid-sensing ion channels (ASICs) play a key role in the perception and response to extracellular acidification changes. These proton-gated cation channels are critical for neuronal functions, like learning and memory, fear, mechanosensation and internal adjustments like synaptic plasticity. Moreover, they play a key role in neuronal degeneration, ischemic neuronal injury, seizure termination, pain-sensing, etc. Functional ASICs are homo or heterotrimers formed with (ASIC1-ASIC3) homologous subunits. ASIC1a, a major ASIC isoform in the central nervous system (CNS), possesses an acidic pocket in the extracellular region, which is a key regulator of channel gating. Growing data suggest that ASIC1a channels are a potential therapeutic target for treating a variety of neurological disorders, including stroke, epilepsy and pain. Many studies were aimed at identifying allosteric modulators of ASIC channels. However, the regulation of ASICs remains poorly understood. Using all available crystal structures, which correspond to different functional states of ASIC1, and a molecular dynamics simulation (MD) protocol, we analyzed the process of channel inactivation. Then we applied a molecular docking procedure to predict the protein conformation suitable for the amiloride binding. To confirm the effect of its sole active blocker against the ASIC1 state transition route we studied the complex with another MD simulation run. Further experiments evaluated various compounds in the Enamine library that emerge with a detectable ASIC inhibitory activity. We performed a detailed analysis of the structural basis of ASIC1a inhibition by amiloride, using a combination of in silico approaches to visualize its interaction with the ion pore in the open state. An artificial activation (otherwise, expansion of the central pore) causes a complex modification of the channel structure, namely its transmembrane domain. The output protein conformations were used as a set of docking models, suitable for a high-throughput virtual screening of the Enamine chemical library. The outcome of the virtual screening was confirmed by electrophysiological assays with the best results shown for three hit compounds.

The Medical Management of Cerebral Edema: Past, Present, and Future Therapies

Cerebral edema is commonly associated with cerebral pathology, and the clinical manifestation is largely related to the underlying lesioned tissue. Brain edema usually amplifies the dysfunction of the lesioned tissue and the burden of cerebral edema correlates with increased morbidity and mortality across diseases. Our modern-day approach to the medical management of cerebral edema has largely revolved around, an increasingly artificial distinction between cytotoxic and vasogenic cerebral edema. These nontargeted interventions such as hyperosmolar agents and sedation have been the mainstay in clinical practice and offer noneloquent solutions to a dire problem. Our current understanding of the underlying molecular mechanisms driving cerebral edema is becoming much more advanced, with differences being identified across diseases and populations. As our understanding of the underlying molecular mechanisms in neuronal injury continues to expand, so too is the list of targeted therapies in the pipeline. Here we present a brief review of the molecular mechanisms driving cerebral edema and a current overview of our understanding of the molecular targets being investigated.

Protein Carbonylation of an Amino Acid Residue of the Na/K-ATPase α1 Subunit Determines Na/K-ATPase Signaling and Sodium Transport in Renal Proximal Tubular Cells

BACKGROUND

We have demonstrated that cardiotonic steroids, such as ouabain, signaling through the Na/K-ATPase, regulate sodium reabsorption in the renal proximal tubule. By direct carbonylation modification of the Pro222 residue in the actuator (A) domain of pig Na/K-ATPase α1 subunit, reactive oxygen species are required for ouabain-stimulated Na/K-ATPase/c-Src signaling and subsequent regulation of active transepithelial (22)Na(+) transport. In the present study we sought to determine the functional role of Pro222 carbonylation in Na/K-ATPase signaling and sodium handling.

METHODS AND RESULTS

Stable pig α1 knockdown LLC-PK1-originated PY-17 cells were rescued by expressing wild-type rat α1 and rat α1 with a single mutation of Pro224 (corresponding to pig Pro222) to Ala. This mutation does not affect ouabain-induced inhibition of Na/K-ATPase activity, but abolishes the effects of ouabain on Na/K-ATPase/c-Src signaling, protein carbonylation, Na/K-ATPase endocytosis, and active transepithelial (22)Na(+) transport.

CONCLUSIONS

Direct carbonylation modification of Pro224 in the rat α1 subunit determines ouabain-mediated Na/K-ATPase signal transduction and subsequent regulation of renal proximal tubule sodium transport.

Characterization of Rat Meibomian Gland Ion and Fluid Transport

Purpose

We establish novel primary rat meibomian gland (MG) cell culture systems and explore the ion transport activities of the rat MG.

Methods

Freshly excised rat MG tissues were characterized as follows: (1) mRNA expression of selected epithelial ion channels/transporters were measured by RT-PCR, (2) localization of epithelial sodium channel (ENaC) mRNAs was performed by in situ hybridization, and (3) protein expression and localization of βENaC, the Na⁺/K⁺/Cl⁻ cotransporter (NKCC), and the Na⁺/K⁺ ATPase were evaluated by immunofluorescence. Primary isolated rat MG cells were cocultured with 3T3 feeder cells and a Rho-associated kinase (ROCK) inhibitor (Y-27632) for expansion. Passaged rat MG cells were cultured as planar sheets under air-liquid interface (ALI) conditions for gene expression and electrophysiologic studies. Passaged rat MG cells also were cultured in matrigel matrices to form spheroids, which were examined ultrastructurally by transmission electron microscopy (TEM) and functionally using swelling assays.

Results

Expression of multiple ion channel/transporter genes was detected in rat MG tissues. β-ENaC mRNA and protein were localized more to MG peripheral acinar cells than central acinar cells or ductular epithelial cells. Electrophysiologic studies of rat MG cell planar cultures demonstrated functional sodium, chloride, and potassium channels, and cotransporters activities. Transmission electron microscopic analyses of rat MG spheroids revealed highly differentiated MG cells with abundant lysosomal lamellar bodies. Rat MG spheroids culture-based measurements demonstrated active volume regulation by ion channels.

Conclusions

This study demonstrates the presence and function of ion channels and volume transport by rat MG. Two novel primary MG cell culture models that may be useful for MG research were established.

Hexamethylene amiloride engages a novel reactive oxygen species- and lysosome-dependent programmed necrotic mechanism to selectively target breast cancer cells

Anticancer chemotherapeutics often rely on induction of apoptosis in rapidly dividing cells. While these treatment strategies are generally effective in debulking the primary tumor, post-therapeutic recurrence and metastasis are pervasive concerns with potentially devastating consequences. We demonstrate that the amiloride derivative 5-(N,N-hexamethylene) amiloride (HMA) harbors cytotoxic properties particularly attractive for a novel class of therapeutic agent. HMA is potently and specifically cytotoxic toward breast cancer cells, with remarkable selectivity for transformed cells relative to non-transformed or primary cells. Nonetheless, HMA is similarly cytotoxic to breast cancer cells irrespective of their molecular profile, proliferative status, or species of origin, suggesting that it engages a cell death mechanism common to all breast tumor subtypes. We observed that HMA induces a novel form of caspase- and autophagy-independent programmed necrosis relying on the orchestration of mitochondrial and lysosomal pro-death mechanisms, where its cytotoxicity was attenuated with ROS-scavengers or lysosomal cathepsin inhibition. Overall, our findings suggest HMA may efficiently target the heterogeneous populations of cancer cells known to reside within a single breast tumor by induction of a ROS- and lysosome-mediated form of programmed necrosis.

Fluid Shear Stress Effects on Endothelial Cell Cytosolic pH

Fluid flow can modulate endothelial cell intracellular pH (pH(i)). Venous and arterial shear stresses of 1.4 and 14 dyn/cm2, respectively, induced intracellular acidification. The kinetics of the process and magnitude of acidification were dependent on the level of shear stress. Endothelial cells exposed to a venous shear stress were able to recover from the acidification, whereas cells exposed to an arterial shear stress remained acidic. Addition of SITS (1 mM), a HCO(3) (-)/CI(-) exchange inhibitor, greatly reduced the shear stress induced acidification, suggesting that the HCO(3) (-)/C1(-) exchanger is activated by shear stress. Shear stress may activate the exchanger by lowering the [HCO(3) (-)] at the cell surface via convective mass transfer. Altering the HCO(3) (-) gradient across the cell membrane activates the exchanger and, as a consequence, results in intracellular acidification. Perfusion with media containing ATP (10 microM) altered the kinetics of flow-induced acidification observed at both shear stress levels. ATP modulation of pH(i) may be coupled to the rise in [Ca(2+)](j) known to occur with ATP stimulation. To summarize, media perfusion induces intracellular acidification in endothelial cells, and there is evidence to suggest that pH(i) may serve as a second messenger to modulate flow associated changes in endothelial cell metabolism.

A unique modulator of endoplasmic reticulum stress-signalling pathways: the novel pharmacological properties of amiloride in glial cells

BACKGROUND AND PURPOSE

Stress on the endoplasmic reticulum (ER) can trigger rescuer responses such as the unfolded protein response (UPR). However, pharmacological modulators of these ER-regulated stress responses are not well understood. In the present study, we found that amiloride, a potassium-sparing diuretic, has unique properties relating to such stress.

EXPERIMENTAL APPROACH

We treated mouse primary cultured glial cells with amiloride, in the absence and presence of the ER stress-inducing reagents tunicamycin (Tm) or dithiothreitol, and measured UPR and ER stress-induced cell death. IRE1alpha phosphorylation, eIF2alpha phosphorylation, X-box binding protein 1 (XBP1) splicing, glucose regulated protein 78 (GRP78) and CCAAT/enhancer-binding protein homologous protein (CHOP) expression by reverse transcription-polymerase chain reaction and Western blotting were used to assess UPR and lactate dehydrogenase activity was determined to measure ER stress-induced cell death.

KEY RESULTS

Amiloride completely inhibited ER stress-induced activation of IRE1alpha, an ER-localized stress sensor protein, splicing of XBP1, and subsequent expression of GRP78 at the mRNA and protein levels. ER stress induces the phosphorylation of eIF2alpha, leading to the expression of CHOP or an attenuation of translation in cells. Surprisingly, treatment with amiloride alone markedly promoted the phosphorylation but actually inhibited ER stress-induced CHOP expression. Finally, we found that amiloride (200 microM) synergistically enhanced ER stress-induced cell death, which was mediated through caspases. On the other hand, a low dose of amiloride (20 microM) significantly prevented Tm-induced cell death.

CONCLUSIONS AND IMPLICATIONS

These results suggest that amiloride can modulate UPR. They also suggest amiloride to be an important pharmacological agent and provide basic information for understanding and preventing ER stress-related diseases.

Effect of epithelial sodium channel blockade on the myogenic response of rat juxtamedullary afferent arterioles

The mechanotransduction mechanism underlying the myogenic response is poorly understood, but evidence implicates participation of epithelial sodium channel (ENaC)-like proteins. Therefore, the role of ENaC on the afferent arteriolar myogenic response was investigated in vitro using the blood-perfused juxtamedullary nephron technique. Papillectomy was used to isolate myogenic influences by eliminating tubuloglomerular feedback signals. Autoregulatory responses were assessed by manipulating perfusion pressure in 30-mm Hg steps. Under control conditions, arteriolar diameter increased by 15% from 13.0+/-1.3 to 14.7+/-1.2 microm (P<0.05) after reducing perfusion pressure from 100 to 70 mm Hg. Diameter decreased to 11.3+/-1.1 and 10.6+/-1.0 microm after increasing pressure to 130 and 160 mm Hg (88+/-1 and 81+/-2% of control diameter, P<0.05), respectively. Pressure-mediated autoregulatory responses were significantly inhibited by superfusion of 10 micromol/L amiloride (102+/-2, 97+/-4, and 94+/-3% of control diameter), or 10 micromol/L benzamil (106+/-5, 100+/-3, and 103+/-3% of control diameter), and when perfusing with blood containing 5 micromol/L amiloride (106+/-2, 97+/-4, and 97+/-4% of control diameter). Vasoconstrictor responses to 55 mmol/L KCl were preserved as diameters decreased by 67+/-4, 55+/-8, and 60+/-4% in afferent arterioles superfused with amiloride or benzamil, and perfused with amiloride, respectively. These responses were similar to responses obtained from control afferent arterioles (64+/-6%, P>0.05). Immunofluorescence revealed expression of the alpha, beta, and gamma subunits of ENaC in freshly isolated preglomerular microvascular smooth muscle cells. These results demonstrate that selective ENaC inhibitors attenuate afferent arteriolar myogenic responses and suggest that ENaC may function as mechanosensitive ion channels initiating pressure-dependent myogenic responses in rat juxtamedullary afferent arterioles.

Stimulation of renal sulfate secretion by metabolic acidosis requires Na+/H+exchange induction and carbonic anhydrase

The acute effect of metabolic acidosis on SO42−secretion by the marine teleost renal proximal tubule was examined. Metabolic acidosis was mimicked in primary cultures of winter flounder renal proximal tubule epithelium (fPTCs) mounted in Ussing chambers by reducing interstitial pH to 7.1 (normally 7.7). fPTCs with metabolic acidosis secreted SO42−at a net rate that was 40% higher than in paired isohydric controls (pH 7.7 on interstitium). The stimulation was completely blocked by the carbonic anhydrase inhibitor methazolamide (100 μM). Although Na+/H+exchange (NHE) isoforms 1, 2, and 3 were identified in fPTCs by immunoblotting, administering EIPA (20 μM) to the interstitial and luminal bath solutions had no effect on net SO42−secretion by fPTCs with a normal interstitial pH of 7.7. However, EIPA (20 μM) blocked most of the stimulation caused by acidosis when applied to the lumen but not interstitium, demonstrating that induction of brush-border NHE activity is important. In the intact flounder, serum pH dropped 0.4 pH units (pH 7.7 to 7.3, at 2–3 h) when environmental pH was lowered from 7.8 to ∼4.3. Whereas serum [SO42−] was not altered by acidosis, renal tubular SO42−secretion rate was elevated 200%. Thus metabolic acidosis strongly stimulates renal sulfate excretion most likely by a direct effect on active renal proximal tubule SO42−secretion. This stimulation appears to be dependent on inducible brush-border NHE activity.

Amiloride augments TRAIL-induced apoptotic death by inhibiting phosphorylation of kinases and phosphatases associated with the P13K-Akt pathway

We have previously shown that low extracellular pH (pHe) promotes cell killing by the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). In this study, we examined whether amiloride, an inhibitor of the Na(+)/H(+) antiporter capable of lowering the intracellular pH (pHi), can potentiate TRAIL-induced apoptotic death. Human prostate adenocarcinoma DU-145 cells were treated with various concentrations of TRAIL (10-200 ng/ml) and/or amiloride (0.1-1 mM) for 4 h. Amiloride, which caused little or no cytotoxicity by itself, enhanced TRAIL-induced apoptosis. The TRAIL-mediated activation of caspase, and PARP (poly (ADP-ribose) polymerase) cleavage were both promoted by amiloride. Western blot analysis showed that combined treatment with TRAIL and amiloride did not change the levels of TRAIL receptors (death receptor (DR)4, DR5, and DcR2 (decoy recepter 2) or antiapoptotic proteins (FLICE-inhibitory protein (FLIP), inhibitor of apoptosis (IAP), and Bcl-2). However, unlike pHe, amiloride promoted the dephosphorylation of Akt. Interestingly, amiloride also induced the dephosphorylation of P13K (phosphatidylinositol 3-kinase) and PDK-1 (phosphoinositide-dependent kinase-1) kinases along with PTEN (phosphatase and tensin homolog deleted on chromosome 10) and PP1alpha phosphatases. In vitro kinase assays revealed that amiloride inhibited phosphorylation of kinases and phosphatases by competing with ATP. Taken together, the present studies suggest that amiloride enhances TRAIL-induced cytotoxicity by inhibiting phosphorylation of the PI3K-Akt pathway-associated kinases and phosphatases.

Role of HER-2/neu signaling in sensitivity to tumor necrosis factor-related apoptosis-inducing ligand: Enhancement of TRAIL-mediated apoptosis by amiloride

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been shown to induce apoptosis in numerous transformed cell lines but not in most normal cells. Although this selectivity offers a potential therapeutic application in cancer, not all cancers are sensitive to TRAIL-mediated apoptosis. In this study, we observed that amiloride, a current clinically used diuretic drug, which had little or no cytotoxicity, sensitized TRAIL-resistant human prostate adenocarcinoma LNCaP and human ovarian adenocarcinoma SK-OV-3 cells. The TRAIL-mediated activation of caspase, and PARP cleavage, were promoted in the presence of amiloride. Western blot analysis showed that combined treatment with TRAIL and amiloride did not change the levels of TRAIL receptors (DR4, DR5, and DcR2) and anti-apoptotic proteins (FLIP, IAP, and Bcl-2). However, amiloride dephosphorylated HER-2/neu tyrosine kinase as well as Akt, an anti-apoptotic protein. Interestingly, amiloride also dephosphorylated PI3K and PDK-1 kinases along with PP1alpha phosphatase. In vitro kinase assay revealed that amiloride inhibited phosphorylation of kinase as well as phosphatase by competing with ATP. Taken together, the present studies suggest that amiloride enhances TRAIL-induced cytotoxicity by inhibiting phosphorylation of the HER-2/neu-PI3K-Akt pathway-associated kinases and phosphatase.

CO2 transduction in avian intrapulmonary chemoreceptors is critically dependent on transmembrane Na+/H+ exchange

Avian intrapulmonary chemoreceptors (IPC) are vagal respiratory afferents that are inhibited by high lung Pco(2) and excited by low lung Pco(2). Previous work suggests that increased CO(2) inhibits IPC by acidifying intracellular pH (pH(i)) and that pH(i) is determined by a kinetic balance between the rate of intracellular carbonic anhydrase-catalyzed CO(2) hydration/dehydration and transmembrane extrusion of acids and/or bases by various exchangers. Here, the role of amiloride-sensitive Na(+)/H(+) exchange (NHE) in the IPC CO(2) response was tested by recording single-unit action potentials from IPC in anesthetized ducks, Anas platyrhynchos. For each of the IPC tested, blockade of the NHE using dimethyl amiloride (DMA) elicited a marked (>50%) dose-dependent decrease in mean IPC discharge (P < 0.05), suggesting that NHE is important for pH(i) regulation and CO(2) transduction in IPC. In addition, activation of the NHE using 12-O-tetradecanoylphorbol 13-acetate stimulated six of the seven IPC tested, although the overall effect was not statistically significantly (P = 0.07). Taken together, these findings suggest that CO(2) transduction in IPC is dependent on transmembrane NHE although it is likely to be much slower than carbonic anhydrase-catalyzed hydration-dehydration of CO(2).

Catecholamines increase lung edema clearance in rats with increased left atrial pressure

During hydrostatic pulmonary edema, active Na(+) transport and alveolar fluid reabsorption are decreased. Dopamine (DA) and isoproterenol (ISO) have been shown to increase active Na(+) transport in rat lungs by upregulating Na(+)-K(+)-ATPase in the alveolar epithelium. We studied the effects of DA and ISO in isolated rat lungs with increased left atrial pressure (Pla = 15 cmH(2)O) compared with control rats with normal Pla (Pla = 0). Alveolar fluid reabsorption decreased from control value of 0.51 +/- 0.02 to 0.27 +/- 0.02 ml/h when Pla was increased to 15 cmH(2)O (P < 0.001). DA and ISO increased the alveolar fluid reabsorption back to control levels. Treatment with the D(1) antagonist SCH-23390 inhibited the stimulatory effects of DA (0.30 +/- 0.02 ml/h), whereas fenoldopam, a specific D(1)-receptor agonist, increased alveolar fluid reabsorption in rats exposed to Pla of 15 cmH(2)O (0.47 +/- 0.04 ml/h). Propranolol, a beta-adrenergic-receptor antagonist, blocked the stimulatory effects of ISO; however, it did not affect alveolar fluid reabsorption in control or DA-treated rats. Amiloride (a Na(+) channel blocker) and ouabain (a Na(+)-K(+)-ATPase inhibitor), either alone or together, inhibited the stimulatory effects of DA. Colchicine, which disrupts the cellular microtubular transport of ion-transporting proteins to the plasma membrane, inhibited the stimulatory effects of DA, whereas the isomer beta-lumicolchicine did not block the stimulatory effects of DA. These data suggest that DA and ISO increase alveolar fluid reabsorption in a model of increased Pla by regulating active Na(+) transport in rat alveolar epithelium. The effects of DA and ISO are mediated by the activation of dopaminergic D(1) receptors and the beta-adrenergic receptors, respectively.

Effects of glucagon on in vitro liquid production by lungs from fetal guinea pigs

BACKGROUND

Lung liquid reabsorption in newborns with respiratory distress syndrome can be deficient. Respiratory distress syndrome is often seen in infants of diabetic mothers, in whom the neonatal surge of glucagon is suppressed.

AIM

To investigate the possible effects of glucagon on lung liquid reabsorption.

METHODS

Lungs from near term fetal guinea pigs (62 (2) days gestation; term = 67 days) were supported in vitro for three hours; lung liquid production and reabsorption were monitored by a dye dilution method.

RESULTS

Untreated control preparations produced fluid at 1.75 (0.33) ml/h per kg body weight, and did not change significantly in three hours; those immersed in 10(-12) M glucagon during the middle hour showed no significant change, but those given higher concentrations all showed significant reductions in fluid production or even reabsorption (65.6 (10.3)% fall at 10(-11) M, 70.0 (6.3)% fall at 10(-10) M, and 90.6 (11.1)% fall at 10(-9) M; based on 54 preparations). At 10(-9) M glucagon, 12 out of 30 preparations reabsorbed fluid. The linear log dose-response curve (r(2) = 0.94) gave a theoretical threshold at 4 x 10(-15) M glucagon. Responses appeared to involve the amiloride sensitive Na(+) based reabsorptive system: responses to 10(-9) M glucagon appeared to be reduced by 10(-6) M amiloride, and were abolished by 10(-5) M amiloride (based on 72 preparations).

CONCLUSIONS

The results suggest that the surge of glucagon at birth may help to drain the lungs of fluid. As glucagon liberates cAMP, which also stimulates surfactant, glucagon is worth consideration for possible use in neonatal respiratory distress.

Effect of amiloride on ischaemia and reperfusion injury in isolated, perfused rat hearts

The effect of amiloride, a potent inhibitor of Na+/H+ exchange, on ischaemic reperfused rat hearts was studied in order to investigate whether Na+/H+ exchange or Na+/Ca2+ exchange is involved in ischaemia-reperfusion injury, When hearts were pre-ischaemically loaded with 100 microM amiloride, recovery of left ventricular developed pressure was significantly better than in control hearts, whereas recovery of heart rate at 30-min reperfusion was unaffected. Amiloride pretreatment also decreased creatine phosphokinase activity in the coronary effluent and completely abolished occurrence of ventricular arrhythmias during reperfusion. It also inhibited intracellular Na+ accumulation early in reperfusion (within 5 min), whereas in the late stage (from 5 to 30 min), Ca2+ overload was inhibited. The findings suggest that Na+/H+ exchange participates mainly in the early stage of reperfusion injury and the Na+/Ca2+ exchange system, secondary to Na+/H+ exchange, in the late stage. The reduction in post-ischaemic cardiac dysfunction induced by amiloride pretreatment may be attributable to inhibition of the resultant Ca2+ accumulation during reperfusion.

Modulation of lung liquid clearance by isoproterenol in rat lungs

beta-Adrenergic agonists have been reported to increase lung liquid clearance by stimulating active Na+ transport across the alveolar epithelium. We studied mechanisms by which beta-adrenergic isoproterenol (Iso) increases lung liquid clearance in isolated perfused fluid-filled rat lungs. Iso perfused through the pulmonary circulation at concentrations of 10(-4) to 10(-8) M increased lung liquid clearance compared with that of control lungs (P < 0.01). The increase in lung liquid clearance was inhibited by the beta-antagonist propranolol (10(-5) M), the Na(+)-channel blocker amiloride (10(-4) M), and the antagonist of Na-K-ATPase, ouabain (5 x 10(-4) M). Colchicine, which inhibits cell microtubular transport of ion-transporting proteins to the plasma membrane, blocked the stimulatory effects of Iso on active Na+ transport, whereas the isomer lumicolchicine, which does not affect cell microtubular transport, did not inhibit Na+ transport. In parallel with these changes, the Na-K-ATPase alpha 1-subunit protein abundance and activity increased in alveolar type II cells stimulated by 10(-6) M Iso. Colchicine blocked the stimulatory effect of Iso and the recruitment of Na-K-ATPase alpha 1-protein to the basolateral membrane of alveolar type II cells. Accordingly, Iso increased active Na+ transport and lung liquid clearance by stimulation of beta-adrenergic receptors and probably by upregulation of apical Na+ channels and basolateral Na-K-ATPase mechanisms. Recruitment from intracellular pools and microtubular transport of Na+ pumps to the plasma membrane participate in beta-adrenergic stimulation of lung liquid clearance in rat lungs.

Potassium transport in opossum kidney cells: effects of Na-selective and K-selective ionizable cryptands, and of valinomycin, FCCP and nystatin

The effects of two ionizable cryptands, the Na-selective (221)C10 and the K-selective (222)C10, and of valinomycin, FCCP and nystatin on K+ fluxes in opossum kidney (OK) cells have been quantified. The Na,K-ATPase (ouabain-sensitive 86Rb influx) was stimulated by nystatin (> or = 20%), and inhibited by the other ionophores (50-80%), by barium (K-channel blocker) (61%) and by amiloride (Na entry blocker) (34%). The Vmax of the Na,K-ATPase phosphatase activity was unmodified by the ionophores, indicating the absence of direct interaction with the enzyme. The ATPi content was unmodified by the inhibitors and nystatin, but was lowered by (221)C10 (47%), (222)C10 (75%), valinomycin (72%) and FCCP (88%). Amiloride was found to partially remove the inhibition caused by (222)C10 (51%) and valinomycin (49%). Rb efflux was stimulated by nystatin (32%), unmodified by valinomycin, and was inhibited by (221)C10 (19%), (222)C10 (19%) and FCCP (10%). Barium (39%) and amiloride (32%) inhibited this efflux and, in their presence, the nystatin effect persisted, whereas that of the other ionophores vanished. At pH 6.4, the Rb efflux decreased by 14% of its value at pH 7.4, with no additional inhibition by cryptands. Cryptands are shown to inhibit the pH-sensitive K+-conductance, probably by inducing a K+-H+ exchange at the plasma membrane, and by uncoupling oxidative phosphorylation by inducing the entry of K+ and H+ (and possibly Ca2+) ions into the mitochondria.

Dopamine stimulates sodium transport and liquid clearance in rat lung epithelium

Pulmonary edema clearance is driven primarily by active sodium transport out of the alveoli, mediated predominantly by apical sodium channels and the basolateral NA,K-ATPase. We postulated that dopamine, analogous to its effects in other transporting epithelia, could regulate these sodium transport mechanisms and affect lung liquid clearance. We therefore studied the effects of dopamine on sodium transport and liquid clearance in isolated perfused rat lungs. Instillation of dopamine into the airways caused a dose-dependent increase in liquid clearance from isolated rat lungs of up to 33% above control values at 10(-8) to 10(-4) M concentrations. 10(-6) M amiloride, which selectively inhibits apical sodium channels, decreased basal liquid clearance by 34% but did not inhibit the dopamine-mediated stimulation of lung liquid clearance. Instillation of 10(-4) M amiloride into rat airways, which inhibits other sodium transport mechanisms non-selectively, decreased basal lung liquid clearance by 49% and inhibited the dopamine-mediated stimulation of lung liquid clearance. Perfusion of rat lungs with 5 x 10(-4) M ouabain to specifically inhibit Na,K-ATPase reduced both basal clearance (by 55%) and the dopamine-stimulated increase in lung fluid clearance. Conceivably, the stimulation of lung liquid clearance by dopamine is due to a modulation of Na,K-ATPase in the pulmonary epithelium.

Photolabile amiloride derivatives as cation site probes of the Na,K-ATPase

Treatment of purified canine renal Na,K-ATPase with a range of photoactivatable amiloride derivatives results in inhibition of ATPase activity prior to illumination. Inhibition by amiloride derivatives substituted on a guanidium N could not be prevented by the presence of either K or Na; however, these cations could protect the enzyme against inhibition by derivatives substituted on the 5-position of the pyrazine ring. In the case of 5-(N-ethyl-[2'-methoxy-4'-nitrobenzyl])amiloride (NENMBA), the presence of monovalent cations (Na, K, and Rb) protected the enzyme effectively against inhibition, with concentrations in the millimolar range. ATP did not prevent inhibition; furthermore, native and NENMBA-treated enzyme exhibited normal levels of high affinity [3H]ADP (and hence ATP) binding. The rate of inhibition increased with increasing concentrations of NENMBA. Extensive washing of NENMBA-inhibited enzyme did not restore ATPase activity, showing that NENMBA has an extremely slow off-rate for dissociation from its inhibitory site. Partially inhibited enzyme could be rapidly pelleted and resuspended in NENMBA-free buffer and inhibition was observed to continue, albeit at a somewhat diminished rate, suggesting that NENMBA gains access to its inhibitory site after partitioning into the lipid phase rather than directly from the aqueous solution. Photolysis of NENMBA-inhibited enzyme resulted in covalent incorporation of the reagent into the alpha-subunit of the Na,K-ATPase, as observed by separation of labeled protein on a Laemmli gel and Western analysis using a polyclonal amiloride antibody. Almost all of the covalent labeling could be prevented by the presence of Rb in the incubation and labeling medium. These results suggest that NENMBA inhibits the Na, K-ATPase by disruption of the cation transport domain rather than the catalytic domain of the enzyme and that it promises to be a useful tool for cation site localization.

Stimulation of calcium transport by amiloride in mouse distal convoluted tubule cells

This study examined the mechanism by which amiloride dissociates Na and Ca transport in distal convoluted tubules. Control rates of Na uptake averaged 288 nmol/(min mg protein) and were inhibited 39% by microM amiloride. Amiloride had no effect on Cl uptake. Resting membrane voltage, measured with the voltage-sensitive dye DiOC6 (3), averaged -70 mV. Amiloride hyperpolarized cells in a reversible manner by 18 mV. Control rates of Ca uptake averaged 2.8 nmol/(min mg protein) and increased by 39% in the presence of amiloride. Alterations of intracellular Ca activity were measured in single cells loaded with Fura2-AM. Control intracellular Ca activity averaged 100 nM. Amiloride increased intracellular Ca activity in a concentration-dependent manner to a maximum of 330 nM at microM amiloride. Amiloride analogues ethylisopropyl amiloride (EIPA) and dimethylbenzamil (DMB), which preferentially block Na/H and Na/Ca exchange, respectively, had no effect on Na or Ca influx or on intracellular Ca activity. The dihydropyridine Ca channel blocker nifedipine inhibited amiloride-stimulated Ca uptake and the rise of intracellular Ca activity but had no effect on membrane voltage. It is concluded that amiloride blocks Na entry mediated by Na channels. Inhibition of Na entry results in membrane hyperpolarization, which activates Ca entry by dihydropyridine-sensitive Ca channels.

Mechanisms of intracellular Mg2+ regulation affected by amiloride and ouabain in the guinea-pig taenia caeci

1. The effects of amiloride and ouabain on the regulation of the intracellular, free Mg2+ concentration ([Mg2+]i) were investigated in the taenia isolated from the guinea-pig caecum, using nuclear magnetic resonance (NMR) techniques. 2. [Mg2+]i were mainly estimated from the separation of the alpha- and beta-ATP peaks observed in 31P NMR spectra. In normal (physiological) and nominally Ca(2+)-free solutions, [Mg2+]i was approximately 0.3-0.4 mM. Application of either amiloride or ouabain in Ca(2+)-free solutions significantly increased [Mg2+]i, with only a small change in ATP content. Washout of the drugs reversed the changes in [Mg2+]i. 3. Changes in pHi were estimated from: (1) the chemical shift of phosphoethanolamine, and (2) solving two relational equations of pHi and [Mg2+]i obtained from the beta- and gamma-ATP peaks. Both estimations revealed some intracellular alkalosis during application of these two drugs. After correction for pHi, a significant increase in [Mg2+]i was still obtained 150 min after application of either drug. 4. In the presence of amiloride, simultaneous removal of extracellular Mg2+ and Ca2+ significantly depleted intracellular Mg2+. This result suggests the presence of an amiloride-insensitive (or less sensitive) pathway which passively transports Mg2+ across the plasma membrane. 5. The intracellular Rb+ concentration was monitored as an index of Na(+)-K+ pump activity, using 87Rb NMR. In Ca(2+)-free solutions containing 5 mM Rb+, the intracellular Rb+ concentration was hardly changed by amiloride, but was depleted by additional applications of ouabain. Wash-out of ouabain restored the intracellular Rb+ in the presence of amiloride. 6. These results are consistent with the presence of Na(+)-Mg2+ exchange as an effective Mg(2+)-extruding mechanism in smooth muscle. Although many other factors may cause changes in [Mg2+]i, it seems likely that amiloride directly inhibits the Na(+)-Mg2+ exchanger, whilst ouabain does so indirectly through reduction of the Na+ gradient across the plasma membrane.

Involvement of the Na,K-ATPase in the induction of ion channels by palytoxin

The effects of ouabain, ATP, and vanadate on palytoxin induction of ion channels were examined with the aim of elucidating the role of Na,K-ATPase in palytoxin action. Palytoxin-induced membrane depolarization of crayfish giant axons and single channel currents of frog erythrocytes and mouse neuroblastoma N1E-115 cells were examined using the intracellular microelectrode and patch-clamp techniques. External application of palytoxin in nanomolar concentrations induced depolarization in the crayfish giant axons, and the depolarization was inhibited by pretreatment of the axon with ouabain (10 microM). Internally perfused axons were less sensitive to palytoxin unless ATP (6 mM) was added internally. In patch-clamp experiments, picomolar palytoxin in the patch electrode induced single channels in both cell-attached and inside-out patches of erythrocytes and neuroblastoma cells. The induced channels had a conductance of about 10 pS, reversed near 0 mV in physiological saline solution, and was permeable to Na+, K+, Cs+, and NH4+, but not to choline. Single channel activities induced by palytoxin were inhibited by ouabain (10 microM) and vanadate (1 mM), but promoted by ATP (1 mM). The modulating effects of ouabain, vanadate, and ATP on palytoxin action suggest that the Na,K-ATPase is involved in the induction of single channels by palytoxin. Palytoxin-induced and ouabain-inhibitable single channels were observed in planar lipid bilayer incorporated with purified Na,K-ATPase. The results indicate that an interaction between palytoxin and Na,K-ATPase leads to opening of a 10-pS ion channel. They further raise the possibility that a channel structure may exist in the sodium pump which is uncovered by the action of palytoxin.

Amoxicillin intestinal absorption reduction by amiloride: possible role of the Na(+) -H+ exchanger

Intestinal absorption of beta-lactam antibiotics has been shown to use the dipeptide carrier system. In vitro experiments have established that the efficiency of uptake by enterocytes depends on an inwardly directed proton gradient--dipeptides and beta-lactam antibiotics being cotransported along with hydrogen ion. This gradient is thought to result from the sodium-hydrogen (Na(+)-H+) exchanger located on the brush-border membrane. The aim of the present study was to assess the in vivo relevance of these data in humans by examining the effect of amiloride, a well-known inhibitor of the Na(+) -H+ exchanger, on the bioavailability of amoxicillin in eight healthy volunteers. The results show that amiloride reduces significantly amoxicillin absorption rate (mean time to maximum concentration increases from 1.0 to 1.6 hours, p < 0.05) and absolute bioavailability (by 27%, p < 0.01) and that amiloride-induced inhibition of the intestinal Na(+) -H+ exchange could be associated with an additional inhibitory effect on (Na/K)-ATPase activity. The present data seem to confirm the role of Na(+) -H+ exchange in the uptake of beta-lactams by the intestine and to support the indirect sodium dependence of this carrier system in vivo.

Effect of amiloride on inotropic and toxic actions of ouabain in guinea-pig left atria

The effect of amiloride on the positive inotropic and toxic effects of ouabain in guinea-pig left atria has been studied. In atria driven at 1 Hz, amiloride (0.3 and 0.5 mM) decreased the EC50 but did not affect the maximal tension developed by ouabain. At 0.1 Hz, amiloride did not change either the EC50 or the maximal tension developed by ouabain. Ouabain toxicity (onset of arrhythmias) was not changed by amiloride at either frequency of stimulation. Therefore, amiloride did not antagonize either the positive inotropic or the toxic effect of ouabain. The positive inotropic effect of amiloride has been ascribed to the inhibition of the Na+/Ca2+ exchanger. Since amiloride inhibits also the Na+/H+ exchanger, 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an amiloride derivative which selectively inhibits the Na+/H+ exchange, has been tested to evaluate the role of the Na+/H+ exchange in the amiloride-ouabain interaction. EIPA increased the EC50 values of ouabain and decreased the maximal developed tension by the glycoside in atria driven at 0.1 and 1 Hz, but did not antagonize the toxic response (arrhythmias) of atria to ouabain. It is suggested that the inhibition of Ca2+ exit through the Na+/Ca2+ exchange by amiloride and ouabain may explain the observation that the positive inotropic effects of amiloride and ouabain are additive.

Ba2+ and amiloride uncover or induce a pH-sensitive and a Na+ or non-selective cation conductance in transitional cells of the inner ear

The membrane potential Vm the cytosolic pH (pHi), the transference numbers (t) for K+, Cl- and Na+/non-selective cation (NSC) and the pH-sensitivity of Vm were investigated in transitional cells from the vestibular labyrinth of the gerbil. Vm, pHi, tK+, tCl-, tNa+/NSC, and the pHi sensitivity of Vm were under control conditions were -92 +/- 1 mV (n = 89 cells), pHi 7.13 +/- 0.07 (n = 11 epithelia), 0.87 +/- 0.02 (n = 22), 0.02 +/- 0.01 (n = 19), 0.01 +/- 0.01 (n = 24) and -5 mV/pH unit (n = 13 cells/n = 11 epithelia), respectively. In the presence of 100 mumol/l Ba2+ the corresponding values were: -70 +/- 1 mV (n = 32), pHi 7.16 +/- 0.08 (n = 6), 0.31 +/- 0.05 (n = 4), 0.06 +/- 0.01 (n = 6), 0.20 +/- 0.03 (n = 10) and -16 mV/pH-unit (n = 15/n = 6). In the presence of 500 mumol/l amiloride the corresponding values were: -72 +/- 2 mV (n = 34), pHi 7.00 +/- 0.07 (n = 5), 0.50 +/- 0.04 (n = 6), 0.04 +/- 0.01 (n = 11), 0.28 +/- 0.04 (n = 9) and -26 mV/pH-unit (n = 20/n = 5). In the presence of 20 mmol/l propionate plus amiloride the corresponding values were: -61 +/- 2 mV (n = 27), pHi 6.72 +/- 0.06 (n = 5), 0.30 +/- 0.02 (n = 6), 0.06 +/- 0.01 (n = 5) and 0.40 +/- 0.02 (n = 8), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Generic approaches to obtaining efficacious antigens from vector arthropods

The development of vaccines to control ectoparasites is dependent upon the identification of key parasite antigens. While a rational, pragmatic approach to antigen identification has yielded a successful vaccine candidate from ticks, there may be problems with such an approach when dealing with other ectoparasites. As an alternative approach, the search for vaccine candidates may be facilitated by cloning and expressing parasite genes encoding proteins involved in key physiological roles. A number of criteria may be applied to short-list candidate vaccines, these being; (a) host antibodies should be able to gain access to the parasite antigen; (b) sufficient antibody must gain access to the antigen target; (c) the formation of antibody-antigen complex should disrupt the normal function of the parasite antigen (d) the antigen should share conserved structural/sequence motifs with related, characterised, proteins, thus allowing the use of recombinant DNA methods to clone and express the candidate antigen. We propose three major groups of parasite antigens which may fulfill these criteria; serine proteases, chemoreceptors/ion channels and neuropeptides.

Active Na+ transport and coupled liquid outflow from hydrothoraces of various size

The net rate of liquid flow and Na+ flux across the pleura was determined in anesthetised rabbit during hydrothoraces 0.5 to 5 ml in size, without and with amiloride. In the hydrothoraces with amiloride the net liquid flow and Na+ flux reversed when the volume injected approached zero. This indicates that the active Na+ transport and the consequent liquid absorption occur also under physiological conditions. The difference between the data obtained without and with amiloride provides the net solute-coupled liquid outflow and active Na+ efflux. These parameters increased linearly with the hydrothorax size up to 2 ml (0.39 ml/h and 54 muEq/h, respectively), and then levelled off. The linear relationship allowed their extrapolation to physiological conditions: 0.15 ml/h (0.07 ml.h-1.kg-1) and 21 muEq/h (0.1 muEq.h-1.cm-2). The increase in these parameters with the hydrothorax size seems due to the protein dilution caused by the Ringer injection, because it did not occur if Ringer was added with albumin to keep the protein concentration in the pleural liquid similar to that under physiological conditions.

Amiloride and 5-(N-ethyl-N-isopropyl) amiloride inhibit medium acidification and glucose metabolism by the fission yeast Schizosaccharomyces pombe

We have investigated the mechanism by which amiloride and 5-(N-ethyl-N-isopropyl)amiloride (EIPA) inhibit glucose-stimulated medium acidification in the fission yeast Schizosaccharomyces pombe. The addition of glucose to an unbuffered suspension of cells results in the extrusion of acid. This process was inhibited by diethylstilbestrol (DES), an inhibitor of the H(+)-ATPase (IC50 71 microM), and also by amiloride (IC50 824 microM) and EIPA (IC50 203 microM). The presence of 100 mM NaCl reduced the degree of inhibition observed for amiloride and EIPA, but had no effect on inhibition by DES. N-Methylglucosamine partially protected the cells against the effect of amiloride, but choline chloride did not, suggesting that sodium may be important in the action of amiloride. To establish the site of action of amiloride and EIPA, ATP hydrolysis assays were performed on isolated plasma membranes. H(+)-ATPase activity was inhibited by orthovanadate, but not by amiloride or EIPA. However, both amiloride and EIPA were found to inhibit the incorporation of radioactivity from labelled glucose in S. pombe, with IC50 values of 879 and 272 microM for amiloride and EIPA respectively. Again, 100 mM NaCl was found to reduce the effectiveness of inhibition. Amiloride had no effect on the uptake of 2-deoxyglucose under the same conditions, indicating that amiloride does not inhibit the glucose transporter. We propose that amiloride and EIPA disrupt glucose-induced acidification by inhibiting glucose metabolism.

Intracellular signaling in the regulation of renal Na-K-ATPase. II. Role of eicosanoids

We recently reported a novel intracellular mechanism of renal Na-K-ATPase regulation by agents that increase cell cAMP, which involves protein kinase A-phospholipase A2 and is mediated by one or more arachidonic acid metabolites (Satoh, T., H. T. Cohen, and A. I. Katz. 1992. J. Clin. Invest. 89:1496). The present studies were, therefore, designed to assess the role of eicosanoids in the modulation of Na-K-ATPase activity in the rat cortical collecting duct. The effect of various cAMP agonists (dopamine, fenoldopam, vasopressin, forskolin, and dibutyryl cAMP), which inhibited the pump to a similar extent (approximately 50%), was independent of altered Na entry as it was elicited in the presence of amiloride or nystatin, or when NaCl was replaced with choline Cl. This effect was completely blocked by SKF 525A or ethoxyresorufin, two inhibitors of the cytochrome P450-dependent monooxygenase pathway, or by pretreating the animals with CoCl2, which depletes cytochrome P450. Equimolar concentrations (10(-7) M) of the cyclooxygenase inhibitors indomethacin or meclofenamate caused only a partial inhibition of the cAMP agonists' effect on the pump, whereas nordihydroguaiaretic acid or A 63162, two inhibitors of the lipoxygenase pathway, were without effect. Furthermore, two products of this pathway, leukotriene B4 and leukotriene D4, had no effect on Na-K-ATPase activity, and ICI 198615, a leukotriene receptor antagonist, did not alter pump inhibition by cAMP agonists. Several P450 monoxygenase arachidonic acid metabolites (5,6-epoxyeicosatrienoic acid; 11,12-epoxyeicosatrienoic acid; 11,12-dihydroxyeicosatrienoic acid; and 12(R)-hydroxyeicosatetraenoic acid) as well as PGE2 inhibited the Na:K pump in dose-dependent manner, but the effect of PGE2 was blocked when Na availability was altered, whereas that of 12(R)-HETE remained unchanged. We conclude that the cytochrome P450-monooxygenase pathway of the arachidonic acid cascade plays a major role in the modulation of Na:K pump activity by eicosanoids in the rat cortical collecting duct, and that products of the cyclooxygenase pathway may contribute to pump inhibition indirectly, by decreasing intracellular Na.

Characterization of chicken intestinal brush border membrane Ns/H exchange

1. Na/H exchange is the major pathway for Na uptake in brush border membrane vesicles from chicken small intestine. Hanes-Woolf analysis demonstrated that Na and H competed at the same extravesicular site. The KNa for Na+ at extravesicular pH 6.6 is 35 mM and at pH 7.4, 12 mM. 2. Similar to mammalian intestinal cells, the Na/H exchanger does not appear to have an internal proton modifier site. Varying intravesicular pH from 6.1 to 7.8 stimulates uptake, but a sigmoidal relationship is not observed. 3. The ability of several amiloride analogs to inhibit the exchanger was tested and the inhibitory profile was similar, but not identical to Na/H exchangers in mammalian tissues. The potency series (from most to least potent) is hexamethylamiloride approximately ethylisopropylamiloride > methylisobutylamiloride > dimethylamiloride >> amiloride.

Millimolar amiloride concentrations block K conductance in proximal tubular cells

1. Amiloride, applied at millimolar concentrations, results in the blockade of K+ conductance in amphibian proximal convoluted cells (PCT), fused into giant cells. 2. Amiloride results directly in a blockade of K+ conductance that is not related to inhibition of the Na(+)-H+ antiport, which would lower intracellular pH, adversely affecting K+ conductance. On the contrary, high amiloride concentrations promote entry of this lipophilic base in the cell, leading to higher cell pH. 3. Under voltage clamp conditions, control vs. amiloride, current-voltage curves from PCT fused giant cells intersect at -86.2 +/- 3.4 mV, a value close to the equilibrium potential for potassium. 4. Hexamethylene amiloride, 10(-5) M, irreversibly depolarizes the membrane potential. 5. Barium decreased by 50% the initial slope of realkalinization, following removal of a solution containing NH4Cl, as did amiloride. In addition, these blockers reduced membrane conductance by 40%, suggesting that a fraction of the amiloride-suppressible NH4+ efflux may be conductive. 6. Amiloride does not directly inhibit the Na(+)-K+, ATPase in our preparation, contrary to the prevalent belief. 7. In vivo studies show that amiloride interferes with an apical K+ conductance but it does not alter basolateral K+ conductance.

Inhibitors of Na(+)-H+ exchange block stimulus-provoked pineal melatonin synthesis

In rat pinealocytes, amiloride can modulate adrenergic-stimulated cyclic nucleotide accumulation. In this study, the effect of amiloride on melatonin production was characterized. Addition of 5-(N,N-hexamethylene)amiloride, a potent inhibitor of the Na(+)-H+ antiport, dose dependently inhibited norepinephrine- and isoproterenol-stimulated N-acetyltransferase (NAT) activity and melatonin production. Similar inhibition was also observed when pineal melatonin synthesis was stimulated directly with forskolin or dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP), suggesting that the site of inhibition is distal to cAMP accumulation. Similarities between the inhibitory potencies of amiloride derivatives on the Na(+)-H+ antiport and pineal melatonin synthesis indicate that the observed inhibition on pineal melatonin synthesis by amilorides may be secondary to their actions on the Na(+)-H+ antiport. Further studies also indicate that the inhibitory effect of amilorides was not secondary to its cytotoxic actions and that amilorides had no direct antagonistic action on NAT activity. Our findings, therefore, suggest that, in addition to their effects on cyclic nucleotide accumulation, the Na(+)-H+ antiport also plays an important role in the cAMP-mediated melatonin synthesis in the rat pineal gland.

Mechanisms of acid injury to rabbit esophageal epithelium. Role of basolateral cell membrane acidification

Because mucosal HCl traverses the paracellular pathway before significant damage develops within esophageal epithelium, the effects of mucosal and serosal acidification were compared on Ussing-chambered rabbit esophageal epithelia. Notably serosal, but not mucosal, acidification was associated with cell necrosis, and the latter was accompanied by abolition of potential difference and short-circuit current. The reason for this difference was explored by exposing tissues serosally to ouabain, chloride-free solution, 4-acetamido-4'-isothiocyanatostilbene-2-2'-disulfonic acid (SITS), or amiloride. The results show that serosal acidification, but not ouabainization, is associated with cell necrosis and that cell necrosis induced by serosal acidification can be blocked by SITS and chloride-free solution, but not by amiloride. These findings are compatible with the hypothesis that serosal acidification is more damaging than mucosal acidification because of the greater rate with which hydrogen ions can traverse the basolateral membrane; also, the route for more rapid entry appears to involve a SITS-sensitive, chloride-dependent mechanism (e.g., Cl/HCO3 antiport).

Mechanism of palytoxin-induced Na+ influx into cultured bovine adrenal chromaffin cells: possible involvement of Na+/H+ exchange system

To elucidate the mechanism of palytoxin (PTX)-induced Na+ influx, we examined the effect of amiloride, an inhibitor of Na+/H(+)-antiporter, on PTX-induced Na+ influx into cultured bovine adrenal chromaffin cells in relation to its effects on Ca2+ influx and catecholamine secretion. Amiloride dose-dependently inhibited PTX-induced 22Na+ influx, whereas tetrodotoxin (TTX) had no effect. Amiloride also inhibited PTX-induced Na(+)-dependent 45Ca2+ influx and catecholamine secretion. PTX alone did not significantly affect the intracellular pH, but it decreased in the presence of PTX and amiloride. These results indicate that an amiloride-sensitive Na+/H+ exchange mechanism is probably involved in PTX-induced, TTX-insensitive Na+ influx that triggers Ca2+ influx and catecholamine secretion from the cells.

Effects of pH on Ca2+i, Na+i, and pHi of MDCK cells: Na(+)-Ca2+ and Na(+)-H+ antiporter interactions

The effects of high and low extracellular pH (8.0 and 6.8) on the intracellular concentration of Ca2+, Na+, and H+ were measured in perfused Madin-Darby canine kidney (MDCK) cells cast in agarose gel threads. Cytosolic free Ca2+ (Ca2+i) was measured with aequorin, while intracellular Na+ (Na+i) and H+ (Hi+ or pHi) were determined with the fluorescent indicators SBFI and BCECF, respectively. In addition, H+ secretion was assayed by the pH-stat method, and Na+ or Ca2+ fluxes were measured with 22Na or 45Ca, respectively. H+ secretion was significantly depressed by several experimental conditions that are known to inhibit the Na(+)-H+ antiporter: H+ secretion decreased 44% in the presence of 10(-5) M ethylisopropylamiloride, 49% in Na+o-free media, 44% in the presence of 10(-4) M ouabain, and 32% in the presence of 10(-4) M 8-bromoadenosine 3',5'-cyclic monophosphate. In addition, pHi decreased by 0.2 pH units in Na+o-free media. Finally, recovery from an intracellular acidosis evoked by 20 mM NH4Cl pulse required the presence of extracellular Na+. When the extracellular pH (pHo) was increased from 7.4 to 8.0, H+ secretion increased 58% from 17.5 to 27.7 nmol.min-1.mg protein-1 and Na+ influx increased 48%. As a result, pHi rose from 7.43 to 7.71 and Na+i increased from 15.6 to 19.7 mM. Finally, Ca2+i rose from 120 to 268 nM. These results suggest that the high pHo stimulated the Na(+)-H+ antiporter, and the subsequent rise in Na+i decreased the Na+ electrochemical potential, thereby activating the reverse mode of Na(+)-Ca2+ exchange (Ca2+ influx vs. Na+ efflux) which led to the rise in Ca2+i.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcellular sodium transport in cultured cystic fibrosis human nasal epithelium

Cystic fibrosis (CF) airway epithelia exhibit raised transepithelial Na+ transport rates, as determined by open-circuit isotope fluxes and estimates of the amiloride-sensitive equivalent short-circuit current (Ieq). To study the contribution of apical and basolateral membrane paths to raised Na+ transport in CF, CF nasal epithelial cultures were studied with double-barreled Na(+)-selective microelectrodes and the Ussing chamber technique. Intracellular Na+ activity (acNa) was 24.1 +/- 1.5 mM (n = 36), a value similar to acNa of normal nasal epithelial cells. Reduction of luminal [Na+] to 3 mM abolished Ieq and reduced acNa. Amiloride (10(-4) M) abolished Ieq but increased acNa from 20 +/- 2 to 36 +/- 7 mM (n = 10). Amiloride-induced increase in acNa was not affected by serosal [Na+] reduction but was blocked by preexposure to reduced luminal [Na+]. Amphotericin B increased Ieq during amiloride exposure, indicating that amiloride did not inhibit NA(+)-K(+)-ATPase. Ouabain abolished Ieq and slowly raised acNa. Reduction of serosal [Na+] led to a decrease in acNa that was blocked by bumetanide. It is concluded that 1) CF airway epithelia exhibit an increased apical membrane Na+ permeability, 2) acNa is regulated to a normal level in CF cells despite increased transcellular Na+ fluxes, 3) the abnormal increase in acNa in response to amiloride is dependent on luminal Na+, 4) Na+ is transported across the basolateral membrane by a bumetanide-sensitive cotransport mechanism, and 5) ouabain inhibits the basolateral Na(+)-K(+)-ATPase, causing slow dissipation of the chemical and electrical gradients across the cell membranes.

Sodium transport and intracellular sodium activity in cultured human nasal epithelium

Human airway epithelia are predominantly Na(+)-absorbing epithelia. To investigate the mechanisms for Na+ absorption across airway epithelia, the driving forces and paths for Na+ translocation across each membrane were examined with double-barreled Na(+)-selective microelectrodes in cultured human nasal epithelium (HNE). Under control conditions, intracellular Na+ activity (acNa) was 23 +/- 1 mM (n = 44 preparations, 393 impalements). Amiloride (10(-4) M) hyperpolarized the apical membrane and increased the fractional apical membrane resistance but did not affect acNa. Exposure to Na(+)-free luminal solution induced bioelectric responses similar to amiloride but also reduced acNa to 8 +/- 1 mM. Reduction of luminal Na+ concentration ([Na+]) in the presence of amiloride also reduced acNa without further changes in bioelectric parameters. Reduction of serosal [Na+] decreased aNac, a response blocked by bumetanide (10(-4) M). Ouabain (10(-4) M, serosal) led to a reduction in equivalent short-circuit current (Ieq) and increase in acNa. We conclude that 1) acNa is higher in HNE than in most mammalian epithelial cells, 2) the apical membrane expresses a conductive Na+ path, and 3) the basolateral membrane transports Na+ via the Na(+)-K(+)-adenosinetriphosphatase and a Na(+)-K(+)-2Cl- cotransport system.

Possible role of sodium-hydrogen antiport in acetylcholine-induced relaxation of rat aorta

The decreased extracellular Na concentration (25mM) attenuated vasodilator effect of acetylcholine (ACh) in norepinephrine-treated aortic ring. This attenuation was greater in the low Na medium substituted by Li, which can exchange intracellular H through Na-H antiport, as compared with that substituted by choline, which cannot. 10 microM amiloride canceled the difference between the two low Na mediums. Thus the inhibition of Na-H antiport may counteract the suppressive effect of decreased intracellular Na on ACh vasodilation, suggesting a possible role of Na-H antiport in a release of vasoactive substance(s) from endothelial cells.

Transepithelial acidification by cultures of rabbit proximal tubules grown on filters

Transepithelial acidification in the proximal tubule occurs by the simultaneous actions of the Na(+)-H+ exchanger in the brush border and the basolateral Na(+)-HCO3- cotransporter. The presence of these systems has been demonstrated for cultured cells; however, their contributions to the transepithelial movement of acid equivalents has not been confirmed in monolayers. To examine transepithelial acidification by intact cells, tubules were grown on membrane filters. Confluent cultures developed a transepithelial pH gradient within 6 h by decreasing the pH of medium in the apical chamber (6.66 +/- 0.03) while raising the basolateral pH to 7.40 +/- 0.02. Cells maintained on plastic did not acidify the medium during this time. Amiloride (10-100 microM) inhibited development of the gradient only when placed in the top chamber. 4-Acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS; 10-100 microM), which inhibits basolateral Na(+)-HCO3- cotransport, decreased the gradient only when added to the bottom. These results demonstrate that cultured proximal tubule cells can develop a transepithelial pH gradient and that the polarized distribution of the transport systems is maintained in vitro.

High protein intake accelerates the maturation of Na,K-ATPase in rat renal tubules

We have examined the effect of a high protein intake on renal maturation by studying the Na,K-ATPase activity in proximal convoluted tubules (PCT) and medullary thick ascending limb of Henle (TAL) in kidneys from 16- to 24-day-old rats. The rats received an isocaloric diet containing 21% (normal protein, NP) and 50% (high protein, HP) protein from 16 days of age. HP diet significantly increased the nephron filtration rate after 2 days. There was a proportionate increase in proximal tubular reabsorption. HP diet significantly increased Na,K-ATPase in both PCT and TAL after 4 days, but not after 2 days. Although HP diet caused a similar increase in Na delivery to PCT and TAL, the increase in Na,K-ATPase activity after 8 days was more pronounced in TAL than in PCT. To determine whether stimulation of Na,K-ATPase required the presence of adrenal corticoid hormones, rats adrenalectomized at 16 days of age were studied. HP diet significantly increased Na,K-ATPase in both PCT and TAL from adrenalectomized rats. To determine whether stimulation of Na,K-ATPase in PCT was mediated by increased Na entry via the amiloride-sensitive Na/H exchanger, rats were treated with amiloride via an osmotic minipump from 16 to 20 days of age. Amiloride treatment significantly retarded the normal increase of Na,K-ATPase in NP rats but had no effect on ouabain-insensitive ATPase. HP diet significantly increased PCT Na,K-ATPase, in spite of chronic amiloride treatment.

CONCLUSION

HP intake accelerates maturation of Na,K-ATPase in PCT and TAL regardless of the presence of adrenal corticoid hormone.(ABSTRACT TRUNCATED AT 250 WORDS)

Transepithelial electrical responses to sodium and potassium of nonsensory region of gerbil utricle

Sheets of utricular epithelium from gerbil were mounted in a micro-Ussing chamber. Step changes in [Na] and [K] on each side of the epithelium led to changes in transepithelial potential difference (Vt) and resistance (Rt) which were consistent with the presence of ionic conductances for these two ions. Responses of Vt to 20 mM Rb steps were similar to those to 20 mM K steps. Several common Na and K channel blockers were applied: No evidence for the presence of amiloride-sensitive Na channels was found. At 10(-6) M, amiloride caused no significant changes in Vt but at 10(-3) M it initially decreased or increased Vt when applied from the apical or the basolateral side, respectively. Ba and quinidine each reduced Vt, quinidine more strongly from the basolateral than from the apical side. Tetraethylammonium, another K channel blocker, had no significant effect from either side. These data suggest that in spite of the low value of Rt, the cellular pathway contributes significantly to the voltage response to cation concentration steps in this epithelium. The mode of action of 10(-3) M amiloride and the K channel blockers remains uncertain.