The intracellular localization of amiloride in frog skin

Pyrazine ring-based Na+/H+ exchanger (NHE) inhibitors potently inhibit cancer cell growth in 3D culture, independent of NHE1

The Na+/H+ exchanger-1 (NHE1) supports tumour growth, making NHE1 inhibitors of interest in anticancer therapy, yet their molecular effects are incompletely characterized. Here, we demonstrate that widely used pyrazinoylguanidine-type NHE1 inhibitors potently inhibit growth and survival of cancer cell spheroids, in a manner unrelated to NHE1 inhibition. Cancer and non-cancer cells were grown as 3-dimensional (3D) spheroids and treated with pyrazinoylguanidine-type (amiloride, 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), 5-(N,N-dimethyl)-amiloride (DMA), and 5-(N,N-hexamethylene)-amiloride (HMA)) or benzoylguanidine-type (eniporide, cariporide) NHE1 inhibitors for 2-7 days, followed by analyses of viability, compound accumulation, and stress- and death-associated signalling. EIPA, DMA and HMA dose-dependently reduced breast cancer spheroid viability while cariporide and eniporide had no effect. Although both compound types inhibited NHE1, the toxic effects were NHE1-independent, as inhibitor-induced viability loss was unaffected by NHE1 CRISPR/Cas9 knockout. EIPA and HMA accumulated extensively in spheroids, and this was associated with marked vacuolization, apparent autophagic arrest, ER stress, mitochondrial- and DNA damage and poly-ADP-ribose-polymerase (PARP) cleavage, indicative of severe stress and paraptosis-like cell death. Pyrazinoylguanidine-induced cell death was partially additive to that induced by conventional anticancer therapies and strongly additive to extracellular-signal-regulated-kinase (ERK) pathway inhibition. Thus, in addition to inhibiting NHE1, pyrazinoylguanidines exert potent, NHE1-independent cancer cell death, pointing to a novel relevance for these compounds in anticancer therapy.

Role of apical ion channels in sour taste transduction

Sour taste perception depends primarily on the concentration of H+ in the taste stimulus. Acid stimuli elicit concentration-dependent action potentials in taste cells. Recent patch-clamp studies suggest that protons depolarize taste cells by direct interaction with apically located ion channels. In Necturus maculosus, the voltage-dependent K+ conductance is restricted to the apical membrane of taste cells. The current flows through a variety of K+ channels with unitary conductances ranging from 30 to 175 pS, all of which are blocked directly by citric acid applied to outside-out or perfused cell-attached patches. In contrast, hamster fungiform taste cells appear to utilize the amiloride-sensitive Na+ channel for acid transduction. Amiloride completely inhibits H+ currents elicited by acid stimuli in isolated taste cells, with an inhibition constant similar to that for amiloride-sensitive Na+ currents (Ki = 0.2 microM). Treatment of isolated taste cells with the bioactive peptide arginine-vasopressin results in similar increases in both the amiloride-sensitive Na+ and H+ currents; the effect is mimicked by 8-bromocyclic AMP. These results suggest that H+ can permeate amiloride-sensitive Na+ channels in hamster fungiform taste cells, contributing to the transduction of sour stimuli.

Ca2+-dependent modulation of intracellular Mg2+ concentration with amiloride and KB-R7943 in pig carotid artery

It has long been recognized that magnesium is associated with several important diseases, including diabetes, hypertension, cardiovascular, and cerebrovascular diseases. In the present study, we measured the intracellular free Mg2+ concentration ([Mg2+]i) using 31P nuclear magnetic resonance (NMR) in pig carotid artery smooth muscle. In normal solution, application of amiloride (1 mm) decreased [Mg2+]i by approximately 12% after 100 min. Subsequent washout tended to further decrease [Mg2+]i. In contrast, application of amiloride significantly increased [Mg2+]i (by approximately 13% after 100 min) under Ca2+-free conditions, where passive Mg2+ influx is facilitated. The treatments had little effect on intracellular ATP and pH (pHi). Essentially the same Ca2+-dependent changes in [Mg2+]i were produced with KB-R7943, a selective blocker of reverse mode Na+-Ca2+ exchange. Application of dimethyl amiloride (0.1 mM) in the presence of Ca2+ did not significantly change [Mg2+]i, although it inhibited Na+-H+ exchange at the same concentration. Removal of extracellular Na+ caused a marginal increase in [Mg2+]i after 100-200 min, as seen in intestinal smooth muscle in which Na+-Mg2+ exchange is known to be the primary mechanism of maintaining a low [Mg2+]i against electrochemical equilibrium. In Na+-free solution (containing Ca2+), neither amiloride nor KB-R7943 decreased [Mg2+]i, but they rather increased it. The results suggest that these inhibitory drugs for Na+-Ca2+ exchange directly modulate Na+-Mg2+ exchange in a Ca2+-dependent manner, and consequently produce the paradoxical decrease in [Mg2+]i in the presence of Ca2+.

Altering airway surface liquid volume: inhalation therapy with amiloride and hyperosmotic agents

The thin layer of liquid lining the entire respiratory tract is the first line of defense against the continuous insult of inhaled bacteria and noxious chemicals. Many chronic obstructive diseases of the airway may reflect decreased airway surface liquid, which results from imbalances in ion transport and mucus production. Reduction in the thickness of airway surface liquid leads to reduced mucociliary clearance rates, causing mucus accumulation and infection in the airway. In this chapter, two inhalation therapies to replenish airway surface liquid and enhance mucociliary clearance are discussed: (1) aerosolized hyperosmotic agents; and (2) aerosolized sodium channel blockers. The advantages and disadvantages of each therapy are discussed, as well as future directions for improving airway surface liquid volume by inhalation pharmacotherapy.

Amiloride blocks salt taste transduction of the glossopharyngeal nerve in metamorphosed salamanders

Studies in the last two decades have shown that amiloride-sensitive Na(+) channels play a role in NaCl transduction in rat taste receptors. However, this role is not readily generalized for salt taste transduction in vertebrates, because functional expression of these channels varies across species and also in development in a species. Glossopharyngeal nerve responses to sodium and potassium salts were recorded in larval and metamorphosed salamanders and compared before and after the oral floor was exposed to amiloride, a blocker of Na(+) channels known to be responsible for epithelial ion transport. Pre-exposure to amiloride (100 microM) did not affect salt taste responses in both axolotls (Ambystoma mexicanum) and larval Ezo salamanders (Hynobius retardatus). In contrast, in metamorphosed Ezo salamanders the nerve responses to NaCl were significantly reduced by amiloride. In amphibians amiloride-sensitive components in salt taste transduction seem to develop during metamorphosis.

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.

Cation transport probes: the amiloride series

The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, and of effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. We have reviewed the pharmacology of inhibition of these processes by amiloride an its analogs, as well as the use of amiloride analogs as potential probes for the characterization of ion transport systems.

Amiloride and its analogs as tools in the study of ion transport

Amiloride inhibits most plasma membrane Na+ transport systems. We have reviewed the pharmacology of inhibition of these transporters by amiloride and its analogs. Thorough studies of the Na+ channel, the Na+/H+ exchanger, and the Na+/Ca2+ exchanger, clearly show that appropriate modification of the structure of amiloride will generate analogs with increased affinity and specificity for a particular transport system. Introduction of hydrophobic substituents on the terminal nitrogen of the guanidino moiety enhances activity against the Na+ channel; whereas addition of hydrophobic (or hydrophilic) groups on the 5-amino moiety enhances activity against the Na+/H+ exchanger. Activity against the Na+/Ca2+ exchanger and Ca2+ channel is increased with hydrophobic substituents at either of these sites. Appropriate modification of amiloride has produced analogs that are several hundred-fold more active than amiloride against specific transporters. The availability of radioactive and photoactive amiloride analogs, anti-amiloride antibodies, and analogs coupled to support matrices should prove useful in future studies of amiloride-sensitive transport systems. The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, as well as effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. One must consider whether the effects seen on various cellular processes are direct or due to a cascade of events triggered by an effect on an ion transport system.

Intracellular binding of spin-labeled amiloride: an alternative explanation for amiloride's effects at high concentration

Amiloride, an important inhibitor of Na+ transport and Na+/H+ exchange, has been used in nontransporting tissues to investigate the relationship between ionic fluxes or intracellular pH change and proliferative or synthetic events. Reports that amiloride is permeant and had direct effects on intracellular processes have led us to investigate the possibility that amiloride binds intracellularly to nuclei, mitochondria, and to purified nucleic acids. Using a nitroxide spin-labeled derivative of amiloride (ASp) and electron paramagnetic resonance (EPR) spectroscopy, we have demonstrated that nuclei and mitochondria isolated from trout liver bind significant amounts of ASp especially at the high amiloride concentrations (approximately mM) commonly used to inhibit proliferative events. While the chemical component responsible for ASp binding in these organelles was not identified, native DNA binds significant amounts of ASp whereas single stranded DNA and RNA bind much less. When these observations are taken together with reports of amiloride's direct action on cellular processes, they support the possibility that some of the effects attributed to inhibition of a transport event are caused by amiloride directly.

Embryonic chick allantois: functional isolation and development of sodium transport

By removing the shell membranes from the chorioallantoic membrane, the chorion is damaged, as visualized by electron microscopy, and rendered permeable, as evidenced by penetration of horseradish peroxidase and increased inhibition of the allantoic Na+-K+ pump by ouabain applied on the chorionic side. The short-circuit current (SCC) of this functionally isolated allantoic epithelium is augmented by nystatin, a channel-forming ionophore, when applied to the mucosal surface. Electrical parameters were determined for three age groups between 12 and 19 days of incubation. The SCC approximately doubled from the youngest (12-13 days) to the oldest (18-19 days) groups, whereas the transepithelial resistance (Re) of 700-900 omega X cm2 remained the same. Amiloride, an inhibitor of apical Na+ uptake, inhibited 98-100% of the SCC at 10(-4) M in both 15-16 and 18-19 day epithelia. In the 12- to 13-day preparation 20-25% of the SCC was insensitive to 10(-3) M amiloride. The Ki's for amiloride were similar in all preparations, at about 5 X 10(-7) M. Determination of the Hill coefficients for inhibition revealed a lower value (0.75 +/- 0.03) for the 12-13 day preparation compared with the two older preparations with coefficients not significantly different from unity. Replacing Na+ in the bathing solutions abolished the SCC of 18-19 day epithelia, whereas about 15% of the SCC remained at 12-13 days. Thus, during development, the SCC of the allantoic epithelium increases in magnitude and becomes increasingly (to 100%) amiloride-sensitive and Na+-dependent.

The amiloride-sensitive sodium channel

Net Na+ movement across the apical membrane of high-electrical resistance epithelia is driven by the electrochemical potential energy gradient. This entry pathway is rate limiting for transepithelial transport, occurs via a channel-type mechanism, and is specifically inhibited by the diuretic drug amiloride. This channel is selective for Na+, Li+, and H+, saturates with increasing extracellular Na+ concentration, and is not affected, at least in frog skin epithelium, by changes in apical membrane surface potential. There also appears to be multiple inhibitory regions associated with each Na+ channel. We discuss the possible implications of a voltage-dependent block by amiloride in terms of macroscopic inhibitory phenomena. We describe the use of cultured epithelial systems, in particular, the toad kidney-derived A6 cell line, and the preparation of apical plasma membrane vesicles to study the Na+ entry process. We discuss experiments in which single, amiloride-sensitive channel activity has been detected and summarize current experimental approaches directed at the biochemical identification of this ubiquitous Na+ transport system.

Inhibition by amiloride of chorda tympani responses evoked by monovalent salts

The diuretic, amiloride, is a potent yet reversible inhibitor of passive sodium transport in many epithelia. It has been shown to inhibit sodium transport in dorsal lingual epithelia and to inhibit both psychophysical and neural measures of salt taste. The present results demonstrate that amiloride's action as an inhibitor of integrated whole chorda tympani nerve recordings in the rat is specific for Li and Na salts, displaying little inhibition of neural responses evoked by KCl and RbCl. Amiloride reduces both the phasic and tonic portion of the nerve recording equally. When amiloride inactivates the tonic portion of the nerve response to 250 mM NaCl, only a portion of the response is affected. Complete inactivation does not occur even at high amiloride concentrations. With amiloride flowing constantly over the tongue at 1 microM, 10 microM, or 50 microM a reciprocal plot of stimulus NaCl concentration versus response is non-linear. This result suggests that the inhibition of the NaCl-induced response has both competitive and non-competitive properties. These results support the hypothesis that salt taste is mediated in part by amiloride sensitive Na-channels located in taste receptor cell plasma membranes.

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

Amiloride inhibited the ouabain-sensitive rate of oxygen consumption (QO2) of a suspension of rabbit intact proximal tubules in the presence of different concentrations of extracellular sodium. Measurements of the ouabain-sensitive QO2 in the presence of nystatin, the tissue sodium and potassium contents of the tubules in suspension, and the sodium- and potassium-dependent adenosinetriphosphatase (Na,K-ATPase) activity of lysed tubule membranes indicated that the effect of amiloride was due to a direct inhibition of the Na,K-ATPase activity of the proximal tubule.

Effects of bafilomycin A1 and amiloride on the apical potassium and proton gradients in Drosophila Malpighian tubules studied by X-ray microanalysis and microelectrode measurements

The intracellular distribution of potassium in Malpighian tubules from Drosophila larva was measured by electron probe X-ray microanalysis of freeze-dried cryosections. Application of amiloride alone to the haemolymph space had no effect on the intracellular potassium concentration in the region of intermediate cytoplasm (between the basal region of basal membrane infoldings and the apical brush border), whereas a potassium increase as well as a chloride increase was observed after simultaneous blocking of the potassium conductance of the basal membrane with barium. Injected bafilomycin and amiloride applied in the haemolymph caused an increase of the potassium content in the basal cytoplasm but not in the microvilli. In addition, the intracellular water portion was decreased by bafilomycin. pH measurements in isolated larval anterior tubules with proton-selective microelectrodes showed that bafilomycin added to the bathing solution caused a decrease in intracellular pH. Addition of amiloride had no significant effect on intracellular pH, but the pH of the luminal fluid was decreased within 1 min by 0.5 pH units. The amiloride-induced luminal pH decrease could be inhibited by the metabolic blocker KCN as well as by bafilomycin. Furthermore, removing potassium from the bathing saline caused a slow luminal acidification, which could be blocked by KCN. Our results support the hypothesis of a functionally coupled transport system in the apical membrane consisting of a bafilomycin-sensitive V-ATPase and a K(+)-dependent, amiloride-sensitive K+/H+ exchange system.