Transport Of Other Ions Research Articles

Recent Advances in the Study of Na+/K+-ATPase in Neurodegenerative Diseases

Na+/K+-ATPase (NKA), a large transmembrane protein, is expressed in the plasma membrane of most eukaryotic cells. It maintains resting membrane potential, cell volume and secondary transcellular transport of other ions and neurotransmitters. NKA consumes about half of the ATP molecules in the brain, which makes NKA highly sensitive to energy deficiency. Neurodegenerative diseases (NDDs) are a group of diseases characterized by chronic, progressive and irreversible neuronal loss in specific brain areas. The pathogenesis of NDDs is sophisticated, involving protein misfolding and aggregation, mitochondrial dysfunction and oxidative stress. The protective effect of NKA against NDDs has been emerging gradually in the past few decades. Hence, understanding the role of NKA in NDDs is critical for elucidating the underlying pathophysiology of NDDs and identifying new therapeutic targets. The present review focuses on the recent progress involving different aspects of NKA in cellular homeostasis to present in-depth understanding of this unique protein. Moreover, the essential roles of NKA in NDDs are discussed to provide a platform and bright future for the improvement of clinical research in NDDs.

Assessing transport and retention of nitrate and other materials through the riparian zone and stream channel with simulated precipitation

Abstract Riparian zones, the interfaces between land and stream, perform vital ecosystem functions including transformation and retention of nutrients and sediment moving across the landscape. Although many studies assess transport through and transformation of materials in riparian zones, less is known about the direct influence of precipitation falling on these zones on material retention and transport. Additionally, few experiments can compare riparian retention to stream‐channel retention. We present a novel experimental approach to assess retention of nitrate entering as precipitation in riparian zones and compare riparian retention and movement of nitrate, other ions, sediments to and within the adjacent stream channel. We simulated an intense precipitation event with 15N‐labelled nitrate as a bioactive solute and bromide as an inert tracer. This method extends tracer release approaches applied to streams worldwide and links it to processes at the aquatic/ terrestrial interface. It further allows determination of movement of materials into streams from bankside precipitation. The riparian zone removed or retained a greater proportion of nitrate than the stream relative to bromide; over half the added bromide reached the stream through a few metres of riparian zone, compared to only 0.2% of the added nitrate. Of the 0.2% that reached the stream, 30% of that nitrate was removed or retained by instream processes after travelling 60 downstream. Roughly 10% of the total 15N addition ended up sequestered in the above‐ground portions of the riparian grasses by the end of the growing season, and very little of it was recovered from the soil. We saw little evidence of bulk transport of other ions or sediment from this riparian soil to the stream. Our data are consistent with the concept of high nitrate retention in vegetated riparian zones, even for nitrate falling directly upon them in the form of atmospheric deposition in precipitation.

Design of Sub‐Nanochannels between Graphene Oxide Sheets via Crown Ether Intercalation to Selectively Regulate Cation Permeation

AbstractGraphene‐based membranes are a promising candidate for separating pollutants and ions. In particular, graphene oxide (GO) membranes are widely studied due to their unique nanochannels. The characteristic nanochannels of GO membranes can be manipulated via intercalation of cations, inhibiting the transport of other ions in the diffusion process. To maintain the tailored nanochannel during a pressure‐assisted filtration procedure, it is essential to retain such inserted cations. Here, dibenzo‐18‐Crown‐6 molecules (DB18C6) tightly binding to potassium ions are intercalated into GO nanosheets for preventing the leakage of the potassium ions from the nanochannel in the separation process. The complex between potassium ion and DB18C6 forges sub‐nanochannels between GO nanochannels, controlling the salt rejection rate as well as the permeation of water molecules, and selectively inhibiting the transport of Na+ ions compared to the untreated GO membrane. The as‐prepared GO@Crown composite membranes exhibit excellent NaCl rejection rates (up to 60%), water permeance (3.11–8.86 LMH bar−1), and a Na+ rejection rate (up to 62.5%) as well as an outstanding Na2SO4 rejection rate (up to 88%) in the dead‐end filtration process. Molecular dynamics compute the tunable interlayer spacing of GO@Crown composite membranes and the possible configuration of crown ethers between GO layers, supporting the experimental results.

A novel membrane with ion-recognizable copolymers in graphene-based nanochannels for facilitated transport of potassium ions

Facilitated transport of K+ ions through membranes is of great significance in both science and technology for the efficient and selective separation of potassium ions. Here, a novel membrane with ion-recognizable copolymers in graphene-based nanochannels is developed for facilitated transport of potassium ions. The poly(N-isopropylacrylamide-co-benzo-15-crown-5-acrylamide) (PNB) copolymers are fixed in the lamellate spacings between the partially reduced graphene oxide (prGO) nanosheets. The K+ permeability of the fabricated prGO-PNB membrane remarkably increases at around the lower critical solution temperature (LCST) of the PNB copolymer, because of the dynamic complexation between B15C5 units and K+ ions at around the LCST. The selectivity of K+/Na+ increases with increasing the content of PNB copolymers in the membrane. The proposed membrane shows good stability in water and K+ separation performance. Due to the uniform channels and facilitated transport mechanism of the prGO-PNB membrane, the comprehensive performance of selectivity and permeability of the K+ ions is better than that of previously reported polymer-based nanofiltration membranes, polymer-based facilitated transport membranes and graphene-based membranes. The proposed strategy of this prGO-PNB membrane that combining graphene-based sheets as “skeleton” and substance-recognizable copolymers as “facilitators” is also applicable to fabricate membranes for facilitated transport of other ions or molecules.

Self-assembled highly ordered acid layers in precisely sulfonated polyethylene produce efficient proton transport.

Recent advances in polymer synthesis have allowed remarkable control over chain microstructure and conformation. Capitalizing on such developments, here we create well-controlled chain folding in sulfonated polyethylene, leading to highly uniform hydrated acid layers of subnanometre thickness with high proton conductivity. The linear polyethylene contains sulfonic acid groups pendant to precisely every twenty-first carbon atom that induce tight chain folds to form the hydrated layers, while the methylene segments crystallize. The proton conductivity is on par with Nafion 117, the benchmark for fuel cell membranes. We demonstrate that well-controlled hairpin chain folding can be utilized for proton conductivity within a crystalline polymer structure, and we project that this structure could be adapted for ion transport. This layered polyethylene-based structure is an innovative and versatile design paradigm for functional polymer membranes, opening doors to efficient and selective transport of other ions and small molecules on appropriate selection of functional groups.

Investigation of the effects of pH, aging and scrap tire content on the dissolution behaviors of new scrap tire-concrete mixture structures

Scrap tires represent increasing environmental-pollution problems because they are not biodegradable and require a significantly long time for natural degradation, which is caused by the cross-linked structure of polymeric material and the presence of stabilizers and other additives. Potential use of scrap tires in construction is one of its important application areas and an alternative recovery method. In this study, a series of experiments was conducted to investigate the dissolution behaviors of new concrete-scrap tire mixture structures (brick) under different experimental conditions, such as scrap tire content, aging, and in acidic, basic and natural environments. The obtained results revealed different effects of adding scrap tires, aging and the existence of H+ and OH− on the dissolution of the brick structures. The added sulfuric acid dissolved calcium silicate hydrate, calcium aluminosilicate hydrate and calcium hydroxide of the brick via a naturalization reaction and released more soluble metal ions, particularly calcium, in the solution. Additionally, during acidic corrosion, the porosity significantly increased because calcium was removed from the structure; thus, the transport of other ions was enhanced in the corroded layer by diffusion. However, the addition of sodium hydroxide significantly decreased the solubility of calcium because the calcium silicate hydrate, calcium aluminosilicate hydrate and alkali-silica reaction products precipitated in the presence of calcium. The aging test showed that calcium was substantially dissolved from the brick structure in the first days and slightly slowed down in an acidic medium because the corrosion surface was gradually consumed, which caused the dissolution to occur in the lower part of the corroded region. The increase in scrap tire content increased the surface area and porosity, which resulted in more released calcium and silicon. The study affirmed that the use of granular scrap tires in concrete structures was feasible.

Mathematical modeling of positron emission tomography (PET) data to assess radiofluoride transport in living plants following petiolar administration.

BackgroundIon transport is a fundamental physiological process that can be studied non-invasively in living plants with radiotracer imaging methods. Fluoride is a known phytotoxic pollutant and understanding its transport in plants after leaf absorption is of interest to those in agricultural areas near industrial sources of airborne fluoride. Here we report the novel use of a commercial, high-resolution, animal positron emission tomography (PET) scanner to trace a bolus of [18F]fluoride administered via bisected petioles of Brassica oleracea, an established model species, to simulate whole plant uptake of atmospheric fluoride. This methodology allows for the first time mathematical compartmental modeling of fluoride transport in the living plant. Radiotracer kinetics in the stem were described with a single-parameter free- and trapped-compartment model and mean arrival times at different stem positions were calculated from the free-compartment time-activity curves.ResultsAfter initiation of administration at the bisected leaf stalk, [18F] radioactivity climbed for approximately 10 minutes followed by rapid washout from the stem and equilibration within leaves. Kinetic modeling of transport in the stem yielded a trapping rate of 1.5 +/− 0.3%/min (mean +/− s.d., n = 3), velocity of 2.2 +/− 1.1 cm/min, and trapping fraction of 0.8 +/− 0.5%/cm.ConclusionQuantitative assessment of physiologically meaningful transport parameters of fluoride in living plants is possible using standard positron emission tomography in combination with petiolar radiotracer administration. Movement of free fluoride was observed to be consistent with bulk flow in xylem, namely a rapid and linear change in position with respect to time. Trapping, likely in the apoplast, was observed. Future applications of the methods described here include studies of transport of other ions and molecules of interest in plant physiology.Electronic supplementary materialThe online version of this article (doi:10.1186/s13007-015-0061-y) contains supplementary material, which is available to authorized users.

Effect of cytosolic pH on inward currents reveals structural characteristics of the proton transport cycle in the influenza A protein M2 in cell-free membrane patches of Xenopus oocytes.

Transport activity through the mutant D44A of the M2 proton channel from influenza virus A was measured in excised inside-out macro-patches of Xenopus laevis oocytes at cytosolic pH values of 5.5, 7.5 and 8.2. The current-voltage relationships reveal some peculiarities: 1. “Transinhibition”, i.e., instead of an increase of unidirectional outward current with increasing cytosolic H+ concentration, a decrease of unidirectional inward current was found. 2. Strong inward rectification. 3. Exponential rise of current with negative potentials. In order to interpret these findings in molecular terms, different kinetic models have been tested. The transinhibition basically results from a strong binding of H+ to a site in the pore, presumably His37. This assumption alone already provides inward rectification and exponential rise of the IV curves. However, it results in poor global fits of the IV curves, i.e., good fits were only obtained for cytosolic pH of 8.2, but not for 7.5. Assuming an additional transport step as e.g. caused by a constriction zone at Val27 resulted in a negligible improvement. In contrast, good global fits for cytosolic pH of 7.5 and 8.2 were immediately obtained with a cyclic model. A “recycling step” implies that the protein undergoes conformational changes (assigned to Trp41 and Val27) during transport which have to be reset before the next proton can be transported. The global fit failed at the low currents at pHcyt = 5.5, as expected from the interference of putative transport of other ions besides H+. Alternatively, a regulatory effect of acidic cytosolic pH may be assumed which strongly modifies the rate constants of the transport cycle.

Clarifying the role of (apical) K+ channels in Na+ and Cl− transport

Clarifying the role of (apical) K⁺ channels in Na⁺ and Cl⁻ transport

M1178 Pathogenic and Commensal Escherichia coli Decrease Cyclic-AMP Mediated Ion Secretion in Human Intestinal Epithelial Cells

Background: Enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC respectively) infections in humans cause severe, watery diarrhea characterized by intestinal lesions. Despite much research into E. coli virulence, the effect these pathogens have on enteric ion transport is not well characterized and the underlying mechanisms have not been fully described. Methods: Monolayers of the human intestinal cell line T84 were cultured on semi-permeable membrane supports and exposed to log phase EPEC or EHEC for 2 to 8 hr (MOI=100). They were then mounted in Ussing chambers to measure basal and secretagogue-induced ion transport by monitoring short circuit current (Isc). Whole cell protein expression of the Na+-K+-Cl--cotransporter (NKCC1) was measured by Western blot in extracts of similarly infected T84monolayers.Results: EPEC, but not EHEC, increased the basal ion secretion after 4 hr exposure (control, 1.1±0.1; EPEC, 3.5±0.1; EHEC, 0.8±0.2 μA/cm2). The ∆Isc response to FSK, a cAMP-mediated secretagogue, was significantly (p<0.05) decreased after 4 hr exposure to either bacteria (control, 125.0±4.4; EPEC, 83.2±9.1; EHEC, 100.6±6.3 μA/cm2). However, the Ca2+-mediated response to carbachol was not significantly altered at any time point. Incubation with heat-killed bacteria, sterile culture medium, or basolaterally applied viable bacteria did not reproduce the altered secretory response, indicating that the effect is attachment-dependent and requires apical E. coli. Interestingly, two strains of human commensal bacteria also decreased the response to FSK after 4 or 8 hours (control, 128.1±6.5; C28/2, 4 hr, 96.1±9.0; C1, 8 hr, 82.3±9.1 μA/cm2). The secretory response to 8-Br-cAMP was significantly decreased with exposure to EPEC or EHEC (control, 125.1±2.3; EPEC, 92.4±5.8; EHEC, 105.4±7.1 μA/cm2), suggesting that the alteration is occurring downstream of cAMP-production. However, the residual FSKinduced ion transport seen with cells bathed in chloride-free buffer was not significantly decreased with infection, indicating that transport of other ions was not affected by infection. Total cellular levels of NKCC1, as shown by Western blot, were not changed by EPEC or EHEC. Inhibition of clathrin-coated pit formation using 2 μM phenylarsine oxide prior to bacterial exposure did not affect the decrease in ion secretion, suggesting that the effect was not caused by endocytosis of ion channels. Conclusion: Pathogenic and commensal E. coli decrease cAMP-dependent ion secretion of T84 cells. This effect likely occurs downstream of adenylate cyclase activity and does not involve altered NKCC1 protein expression or ion channel endocytosis.

Monitoring of the Proton Electrochemical Gradient in Reconstituted Vesicles: Quantitative Measurements of Both Transmembrane Potential and Intravesicular pH by Ratiometric Fluorescent Probes

Proteoliposomes carrying reconstituted yeast plasma membrane H(+)-ATPase in their lipid membrane or plasma membrane vesicles are model systems convenient for studying basic electrochemical processes involved in formation of the proton electrochemical gradient (Deltamicro(H) (+)) across the microbial or plant cell membrane. Deltapsi- and pH-sensitive fluorescent probes were used to monitor the gradients formed between inner and outer volume of the reconstituted vesicles. The Deltapsi-sensitive fluorescent ratiometric probe oxonol VI is suitable for quantitative measurements of inside-positive Deltapsi generated by the reconstituted H(+)-ATPase. Its Deltapsi response can be calibrated by the K(+)/valinomycin method and ratiometric mode of fluorescence measurements reduces undesirable artefacts. In situ pH-sensitive fluorescent probe pyranine was used for quantitative measurements of pH inside the proteoliposomes. Calibration of pH-sensitive fluorescence response of pyranine entrapped inside proteoliposomes was performed with several ionophores combined in order to deplete the gradients passively formed across the membrane. Presented model system offers a suitable tool for simultaneous monitoring of both components of the proton electrochemical gradient, Deltapsi and DeltapH. This approach should help in further understanding how their formation is interconnected on biomembranes and even how transport of other ions is combined to it.

Phase Relations and Stability of Magnetoplumbite- and Crichtonite-Series Phases under Upper-Mantle P-T Conditions: an Experimental Study to 15 GPa with Implications for LILE Metasomatism in the Lithospheric Mantle

High-pressure–high-temperature experiments were performed in the range 7–15 GPa and 1300–1600 C to investigate the stability and phase relations of the Kand Ba-dominant members of the crichtonite and magnetoplumbite series of phases in simplified bulk compositions in the systems TiO2–ZrO2–Cr2O3–Fe2O3–BaO–K2O and TiO2–Cr2O3–Fe2O3–BaO–K2O. Both series of phases occur as inclusions in diamond and/or as constituents of metasomatized peridotite mantle xenoliths sampled by kimberlites or alkaline lamprophyres. They can accommodate large ion lithophile elements (LILE) and high field strength elements (HFSE) on a wt % level and, hence, can critically influence the LILE and HFSE budget of a metasomatized peridotite even if present only in trace amounts. The Ba and K end-members of the crichtonite series, lindsleyite and mathiasite, are stable to 11 GPa and 1500–1600 C. Between 11 and 12 GPa, lindsleyite breaks down to form two Ba–Cr-titanates of unknown structure that persist to at least 13 GPa. The highpressure breakdown product of mathiasite is a K–Cr-titanate with an idealized formula KM7O12, where M 1⁄4 Ti, Cr, Mg, Fe. This phase possesses space group P63/m with a 1⁄4 9 175(2) A, c 1⁄4 2 879(1) A, V 1⁄4 209 9(1) A. Towards high temperatures, lindsleyite persists to 1600 C, whereas mathiasite breaks down between 1500 and 1600 C to form a number of complex Ti–Cr-oxides. Ba and K end-members of the magnetoplumbite series, hawthorneite and yimengite, are stable in runs at 7, 10 and 15 GPa between 1300 and 1400 C coexisting with a number of Ti–Cr-oxides. Molar mixtures (1:1) of lindsleyite–mathiasite and hawthorneite–yimengite were studied at 7–10 GPa and 1300– 1400 C, and 9–15 GPa and 1150–1400 C, respectively. In the system lindsleyite–mathiasite, one homogeneous Ba–K phase is stable, which shows a systematic increase in the K/(K þ Ba) ratio with increasing pressure. In the system hawthorneite–yimengite, two coexisting Ba–K phases appear, which are Ba rich and Ba poor, respectively. The data obtained from this study suggest that Baand K-dominant members of the crichtonite and magnetoplumbite series of phases are potentially stable not only throughout the entire subcontinental lithosphere but also under conditions of an average present-day mantle adiabat in the underlying asthenosphere to a depth of up to 450 km. At still higher pressures, both K and Ba may remain stored in alkali titanates that would also be eminently suitable for the transport of other ions with large ionic radii.

The mitochondrial calcium uniporter is a highly selective ion channel.

During intracellular Ca2+ signalling mitochondria accumulate significant amounts of Ca2+ from the cytosol. Mitochondrial Ca2+ uptake controls the rate of energy production, shapes the amplitude and spatio-temporal patterns of intracellular Ca2+ signals, and is instrumental to cell death. This Ca2+ uptake is undertaken by the mitochondrial Ca2+ uniporter (MCU) located in the organelle's inner membrane. The uniporter passes Ca2+ down the electrochemical gradient maintained across this membrane without direct coupling to ATP hydrolysis or transport of other ions. Carriers are characterized by turnover numbers that are typically 1,000-fold lower than ion channels, and until now it has been unclear whether the MCU is a carrier or a channel. By patch-clamping the inner mitochondrial membrane, we identified a previously unknown Ca2+-selective ion channel sensitive to inhibitors of mitochondrial Ca2+ uptake. Our data indicate that this unique channel binds Ca2+ with extremely high affinity (dissociation constant < or =2 nM), enabling high Ca2+ selectivity despite relatively low cytoplasmic Ca2+ concentrations. The channel is inwardly rectifying, making it especially effective for Ca2+ uptake into energized mitochondria. Thus, we conclude that the properties of the current mediated by this novel channel are those of the MCU.

Structural determinants of Ca2+ transport in the Arabidopsis H+/Ca2+ antiporter CAX1.

Ca(2+) levels in plants, fungi, and bacteria are controlled in part by H(+)/Ca(2+) exchangers; however, the relationship between primary sequence and biological activity of these transporters has not been reported. The Arabidopsis H(+)/cation exchangers, CAX1 and CAX2, were identified by their ability to suppress yeast mutants defective in vacuolar Ca(2+) transport. CAX1 has a much higher capacity for Ca(2+) transport than CAX2. An Arabidopsis thaliana homolog of CAX1, CAX3, is 77% identical (93% similar) and, when expressed in yeast, localized to the vacuole but did not suppress yeast mutants defective in vacuolar Ca(2+) transport. Chimeric constructs and site-directed mutagenesis showed that CAX3 could suppress yeast vacuolar Ca(2+) transport mutants if a nine-amino acid region of CAX1 was inserted into CAX3 (CAX3-9). Biochemical analysis in yeast showed CAX3-9 had 36% of the H(+)/Ca(2+) exchange activity as compared with CAX1; however, CAX3-9 and CAX1 appear to differ in their transport of other ions. Exchanging the nine-amino acid region of CAX1 into CAX2 doubled yeast vacuolar Ca(2+) transport but did not appear to alter the transport of other ions. This nine-amino acid region is highly variable among the plant CAX-like transporters. These findings suggest that this region is involved in CAX-mediated Ca(2+) specificity.

Cloning of the 16-kDa V-ATPase proteolipid subunit from the red imported fire ant Solenopsis invicta buren (Hymenoptera: Formicidae).

V-ATPases are ubiquitous proton pumps found in eukaryotes, and are important in regulating the pH of cell compartments and in creating membrane potentials. The V-ATPase creates a proton gradient that is used as an energy source for the transport of other ions. The 16-kDa proteolipid is the proton-translocating subunit c of V-ATPases. Using PCR methods, we have cloned the fire ant 16-kDa subunit c, providing the first molecular characterization of this protein in a social insect. Northern blot analysis revealed three possible different transcripts. The presence of V-ATPases in ant Malpighian tubules had been previously demonstrated, where they provide the proton gradient necessary for the excretion of other ions and the formation of primary urine. The 16-kDa proteolipid is highly conserved among insects, and in ants may be important to the critical processes of diuresis and olfaction as a key component of the V-ATPase. Arch.

Ion transport in conducting polymers doped with electroactive anions examined by EQCM

Transport properties and stability of ionic species in films of conducting polymers polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDT) were examined by electrochemical quartz crystal microgravimetry (EQCM) experiments. The mobility of ions in these polymers were greatly affected by the mode of polymerization with which the polymer film was prepared, as well as by the structure of the ions. When the conducting polymers were doped with macrocyclic anions, phthalocyanine and porphyrin derivatized with sulfonate anionic groups, these anions were not expelled from the PPy or PEDT films by extensive potential cycles. Heteropolytungstate ions have partial mobility in the polymers and were expelled by negative potential sweeps applied to the films that were formed by electropolymerization at a constant potential or constant current. Polymer films formed with alternating potential programs showed drastically diminished permeability to anions. By forming a polymer film with cyclic potential sweeps or square wave potentials, it was possible to examine transport of other ions without complications due to the transport of the initial dopant ions.

Influence of additional ammonium supply on some nutritional aspects in hydroponic rose plants

The influence of an additional supply of ammonium to a nitrate containing hydroponic solution on the mineral nutrition of greenhouse rose plants (Rosa hybrida cv. ‘Lambada’) has been investigated. The effect on NPK uptake, mineral contents in roots and leaves as well as nitrate reductase (NR) and glutamine synthetase (GS) activities were examined. The addition of ammonium in a nutrient solution containing nitrate produced a total nitrogen uptake increase during shoot elongation, while in the absence of ammonium, nitrate uptake was lower during shoot elongation. Adding ammonium to the hydroponics solution caused an increase of potassium uptake while ammonium absorption was constant. Phosphate uptake was always higher in combined nitrate plus ammonium treatment, likewise in this treatment the most relevant effect on plant mineral content was the increase of P concentration in the roots. In both treatments with or without NH+4, free nitrate was notably higher in the roots than in leaves, indicating that nitrate reduction in rose plants takes place mainly in the leaves. NR activity in the leaves was higher when ammonium was present, whereas the root GS activity was similar in both treatments. The influence of ammonium on phosphate uptake and the subsequent effects on transport of other ions and enzymatic activities are discussed.

Losartan antagonism of angiotensin-II-induced potassium secretion across rat colon.

The effect of angiotensin II (ANG II) on potassium transport across the short-circuited rat distal colon was investigated using 86Rb+ as a tracer for unidirectional K+ fluxes. The addition of high concentrations of ANG II (>/=10(-6) M) to the serosal bathing solution had no effect on the mucosal to serosal flux of Rb+, but significantly increased the serosal to mucosal flux and abolished the basal net absorptive Rb+ flux. These ANG-II-induced alterations in Rb+ transport were blocked by the AT1 receptor antagonist losartan and its metabolite EXP3174, which is also known to have AT1 receptor antagonistic activity. In contrast, an AT2 receptor antagonist, PD123319, did not prevent the alterations in colonic Rb+ transport induced by ANG II in vitro. At lower bath concentrations (10(-7) M to 10(-10) M ), ANG II had no measurable effects on Rb+ transport across this tissue but ANG II did have a bimodal effect on short-circuit current (Isc), indicating additional effects on the electrogenic transport of other ions. Dose-dependent reductions in Isc were observed between 10(-7) M (DeltaIsc=1.96+/-0.49 microEq.cm-2.h-1, n=6) and 10(-10) M (DeltaIsc=0.16+/-0.19 microEq.cm-2.h-1, n=7) ANG II, whereas Isc was increased at the higher concentrations (DeltaIsc= 3.36+/-0.52 microEq.cm-2.h-1, n=7, at 10(-4) M). The ANG-II-induced increases and decreases in Isc were both blocked by losartan but not by PD123319. These studies are the first to demonstrate an effect of ANG II on K+ transport across rat colon that is independent of aldosterone and mediated by AT1 receptors.

Two cDNA clones encoding isoforms of the B subunit of the vacuolar ATPase from barley roots.

The vacuolar H+-ATPase of higher plants is a member of the V-ATPase family, which comprises complex, multisubunit ATPases found in a11 eukaryotes. The electrochemical gradient created by the V-ATPase is thought to provide the driving force for the iecondary transport of other ions and metabolites (Taiz, 1992). In barley (Hordeum vulgare L.) roots this enzyme may be involved in the sequestration of Na+ and Ca2+ ions in the vacuole, because the proton gradient produced by the ATPase is used by Na+/H+ and Ca2+/H+ antiports to drive the uptake of Na+ and of Ca2+ (Garbarino and DuPont, 1989; DuPont et ai., 1990). The quatemary structure of the ATPase from barley roots is very similar to that from other organisms, with approximately 10 different subunits (DuPont and Momssey, 1992). Two of the best-characterized subunits of the V-ATPases are the A and B subunits of the large, knob-like head group on the cytoplasmic side of the vacuolar membrane. The A subunit is between 67 and 70 kD, and the B subunit is between 53 and 60 kD in various organisms. Both A and B subunits are present in a stoichiometry of three per enzyme complex and contain ATP binding sites, although the B subunit is not directly involved in ATP hydrolysis (Puopolo et al., 1992). The primary structures of the A and B subunits are highly conserved among widely divergent organisms, making them useful for evolutionary studies. Isoforms of the B subunit have been described for mammals, and the distribution of the isofonns seems to be tissue specific (Puopolo et al., 1992). It was of some interest to leam whether there are also isoforms of the B subunit in plants. Two different cDNA clones for the B subunit of the barley V-ATPase, designated HTBl and HTB2, were selected from a barley root cDNA library. The two clones were very similar to each other, having higher sequence identity to each other than to any other B subunit sequence in the GenBank data base (see Table I for details). Since barley is an inbred diploid organism, it is likely that HTBl and HTB2 are encoded by different genes, rather than alleles of the same gene. The next most similar sequence was that for the Arubidopsis

Avian colonic ion transport: effects of corticosterone and dexamethasone.

1. Corticosterone, a natural corticosteroid hormone in birds, when injected into domestic fowl (Gallus domesticus) (2000 micrograms.kg-1, 4-5 h before experiment) increases both the basal Isc (short-circuit current) and amiloride-sensitive Isc as well as the PD across the colon in vitro. Dexamethasone, a synthetic analogue (650 micrograms.kg-1, 4-5 h before experiment) also increases the basal and amiloride-sensitive Isc as well as PD in these preparations. 2. In marked contrast, longer term injection or infusion of dexamethasone (650 micrograms.kg-1) for 3 or more days caused a decline in basal Isc and PD (the PD often reversed with the serosal side becoming electronegative) and a drop in resistance. However in these preparations, the amiloride-sensitive Isc was significantly elevated which could be accounted for by an increase in net Na flux. 3. No significant change occurs in net flux of Cl or K although unidirectional fluxes in both directions were increased for both ions in birds given dexamethasone for 3 days. 4. A disparity between the basal Isc and the amiloride-sensitive Isc appeared in these preparations from dexamethasone injected birds reflecting the transport of other ions, possibly HCO3- or H+. The possible role of corticosterone in mineral metabolism of birds is discussed.