Low Cation Concentrations Research Articles

Controls on major solutes within the drainage network of a rapidly weathering tropical watershed

Surface water chemistry in the main stem and source points of the Rio Icacos basin (Luquillo Experimental Forest, Puerto Rico) was studied to investigate the factors regulating spatial variability in major solutes in a rapidly weathering landscape. We sampled along the main stem as well as at small source points at high elevation where fresh bedrock is frequently exposed, and at low elevation in the floodplain/colluvial plain of the main stem. Concentrations of silicon, alkalinity, and the sum of base cations were lower at the source points than in the main stem, and were lowest in low‐elevation source points. Calcium and sodium were the dominant cations at all sampling points after sea‐salt correction, reflecting the weathering of plagioclase feldspar throughout the basin. The partial pressure of carbon dioxide (pCO2) tended to be higher, and HCO3− concentrations were lower, in the low‐elevation source points than at other positions in the landscape. When coupled with the relatively low concentrations of Si and base cations, this suggests that the availability of primary reactive minerals, rather than carbonic acid concentrations, limits weathering in these low‐elevation sources. Mechanical denudation appears to enhance chemical weathering rates not only by refreshing reactive mineral surfaces but also by contributing carbon dioxide from the decomposition of organic‐rich material in landslides, which occur frequently. The spatial variability of major solutes appears to depend primarily on the availability of fresh primary reactive minerals, carbon dioxide concentrations, and hydrolysis conditions.

Adsorption of Pb 2+, Cu 2+ and Cd 2+ in FDU-1 silica and FDU-1 silica modified with humic acid

Ordered mesoporous silica with cubic structure, type FDU-1, was synthesized under strong acid media using B-50-6600 poly(ethylene oxide)–poly(butilene oxide)–poly(ethylene oxide) triblock copolymer (EO 39BO 47EO 39) and tetraethyl orthosilicate (TEOS). Humic acid (HA) was modified to the synthesis process at a concentration of 1.5 mmol per gram of SiO 2. Thermogravimetry, small angle X-ray diffraction, nitrogen adsorption and high resolution transmission electron microscopy were used to characterize the samples. The pristine FDU-1 and FDU-1 with incorporated 1.5 mmol of HA were tested for adsorption of Pb 2+, Cu 2+ and Cd 2+ in aqueous solution. Incorporation of humic acid into the FDU-1 silica afforded an adsorbent with strong affinity for Cd 2+, Cu 2+ and Pb 2+ from single ion solutions. Adsorption of Cu 2+ was significantly enhanced after incorporation of humic acid, a fact that can be explained by the formation of complexes with carboxylic and phenolic groups at low concentrations of the metal cation. The results demonstrated the potential applicability of FDU-1 with incorporated HA in the removal of low concentrations of heavy metal cations from aqueous solution, such as wastewaters, after usual precipitation of metal hydroxides in alkaline medium and proper pH conditioning in the range between 6 and 7.

Factors controlling soil water and stream water aluminum concentrations after a clearcut in a forested watershed with calcium-poor soils

The 24 ha Dry Creek watershed in the Catskill Mountains of southeastern New York State USA was clearcut during the winter of 1996–1997. The interactions among acidity, nitrate (NO 3 − ), aluminum (Al), and calcium (Ca2+) in streamwater, soil water, and groundwater were evaluated to determine how they affected the speciation, solubility, and concentrations of Al after the harvest. Watershed soils were characterized by low base saturation, high exchangeable Al concentrations, and low exchangeable base cation concentrations prior to the harvest. Mean streamwater NO 3 − concentration was about 20 μmol l−1 for the 3 years before the harvest, increased sharply after the harvest, and peaked at 1,309 μmol l−1 about 5 months after the harvest. Nitrate and inorganic monomeric aluminum (Alim) export increased by 4−fold during the first year after the harvest. Alim mobilization is of concern because it is toxic to some fish species and can inhibit the uptake of Ca2+ by tree roots. Organic complexation appeared to control Al solubility in the O horizon while ion exchange and possibly equilibrium with imogolite appeared to control Al solubility in the B horizon. Alim and NO 3 − concentrations were strongly correlated in B-horizon soil water after the clearcut (r 2 = 0.96), especially at NO 3 − concentrations greater than 100 μmol l−1. Groundwater entering the stream from perennial springs contained high concentrations of base cations and low concentrations of NO 3 − which mixed with acidic, high Alim soil water and decreased the concentration of Alim in streamwater after the harvest. Five years after the harvest soil water NO 3 − concentrations had dropped below preharvest levels as the demand for nitrogen by regenerating vegetation increased, but groundwater NO 3 − concentrations remained elevated because groundwater has a longer residence time. As a result streamwater NO 3 − concentrations had not fallen below preharvest levels, even during the growing season, 5 years after the harvest because of the contribution of groundwater to the stream. Streamwater NO 3 − and Alim concentrations increased more than reported in previous forest harvesting studies and the recovery was slower likely because the watershed has experienced several decades of acid deposition that has depleted initially base-poor soils of exchangeable base cations and caused long-term acidification of the soil.

The synergistic effect of nisin and lactoferrin on the inhibition of Listeria monocytogenes and Escherichia coli O157:H7

The goal of this study was to determine whether nisin and lactoferrin would act synergistically to inhibit the growth of Listeria monocytogenes and Escherichia coli O157:H7. Lactoferrin and nisin separately or in combination were suspended in peptone yeast glucose broth and following inoculation with L. monocytogenes or E. coli O157:H7 growth inhibition of each pathogen was determined. At 1000 microg ml(-1) lactoferrin L. monocytogenes was effectively inhibited. However, E. coli O157:H7 initially was inhibited and then grew to cell density similar to the control. A combination of 500 microg ml(-1) of lactoferrin and 250 IU ml(-1) of nisin effectively inhibited the growth of E. coli O157:H7, whereas, 250 microg ml(-1) of lactoferrin and 10 IU ml(-1) of nisin were inhibitory to L. monocytogenes. The results suggest that lactoferrin and nisin act synergistically to inhibit the growth of L. monocytogenes and E. coli O157:H7. Natural preservatives that are active against gram-positive and gram-negative pathogens are desirable to the food industry and consumers. This study demonstrates that lactoferrin and nisin work synergistically reducing the levels required independently inhibiting growth of two major foodborne pathogens. Previous reported results indicated a low level of antimicrobial activity; however, this work was not performed in low divalent cation concentration media. It has been suggested that nondivalent cation-limiting medium such as trypticase soy broth (TSB), can reduce or completely eliminate the inhibitory activity. Further knowledge of these interactions can increase the understanding of the antimicrobial activity of lactoferrin. This should make the use of these compounds by industry more attractive.

Influence of cationic molecules on the hairpin to duplex equilibria of self-complementary DNA and RNA oligonucleotides

A self-complementary nucleotide sequence can form both a unimolecular hairpin and a bimolecular duplex. In this study, the secondary structures of the self-complementary DNA and RNA oligonucleotides with different sequences and lengths were investigated under various solution conditions by gel electrophoresis, circular dichroism (CD) and electron paramagnetic resonance (EPR) spectroscopy and a ultraviolet (UV) melting analysis. The DNA sequences tended to adopt a hairpin conformation at low cation concentrations, but a bimolecular duplex was preferentially formed at an elevated cationic strength. On the other hand, fully matched RNA sequences adopted a bimolecular duplex regardless of the cation concentration. The thermal melting experiments indicated a greater change in the melting temperature of the bimolecular duplexes (by ∼20°C) than that of the hairpin (by ∼10°C) by increasing the NaCl concentration from 10 mM to 1 M. Hairpin formations were also observed for the palindrome DNA sequences derived from Escherichia coli, but association of the complementary palindrome sequences was observed when spermine, one of the major cationic molecules in a cell, existed at the physiological concentration. The results indicate the role of cations for shifting the structural equilibrium toward a nucleotide assembly and implicate nucleotide structures in cells.

Measuring Inter-DNA Potentials in Solution

Interactions between short strands of DNA can be tuned from repulsive to attractive by varying solution conditions and have been quantified using small angle x-ray scattering techniques. The effective DNA interaction charge was extracted by fitting the scattering profiles with the generalized one-component method and inter-DNA Yukawa pair potentials. A significant charge is measured at low to moderate monovalent counterion concentrations, resulting in strong inter-DNA repulsion. The charge and repulsion diminish rapidly upon the addition of divalent counterions. An intriguing short range attraction is observed at surprisingly low divalent cation concentrations, approximately 16 mM Mg2+. Quantitative measurements of inter-DNA potentials are essential for improving models of fundamental interactions in biological systems.

Highly compact folding of chromatin induced by cellular cation concentrations. Evidence from atomic force microscopy studies in aqueous solution

We have performed a very extensive investigation of chromatin folding in different buffers over a wide range of ionic conditions similar to those found in eukaryotic cells. Our results show that in the presence of physiological concentrations of monovalent cations and/or low concentrations of divalent cations, small chicken erythrocyte chromatin fragments and chromatin from HeLa cells observed by transmission electron microscopy (TEM) show a compact folding, forming circular bodies of approximately 35 nm in diameter that were found previously in our laboratory in studies performed under very limited conditions. Since TEM images are obtained with dehydrated samples, we have performed atomic force microscopy (AFM) experiments to analyze chromatin structure in the presence of solutions containing different cation concentrations. The highly compact circular structures (in which individual nucleosomes are not visible as separated units) produced by small chromatin fragments in interphase ionic conditions observed by AFM are equivalent to the structures observed by TEM with chromatin samples prepared under the same ionic conditions. We have also carried out experiments of sedimentation and trypsin digestion of chromatin fragments; the results obtained confirm our AFM observations. Our results suggest that the compaction of bulk interphase chromatin in solution at room temperature is considerably higher than that generally considered in current literature. The dense chromatin folding observed in this study is consistent with the requirement of compact chromatin structures as starting elements for the building of metaphase chromosomes, but poses a difficult physical problem for gene expression during interphase.

Use of polyvalent cations to improve the adsorption strength between adsorbed enzymes and supports coated with dextran sulfate

In this paper, we have shown that the presence of low concentrations of trivalent cations during the adsorption of proteins on supports coated with dextran sulfate strongly reinforces the adsorption strength of the proteins, without altering the enzyme activity. This makes much more difficult its desorption (in some cases, the protein remained fully adsorbed under conditions where the conventional immobilized enzymes have been fully desorbed), and in this way, enabling their use in a wider range of conditions. This may be explained if the trivalent cation forms an ionic bridge between the two charges of the sulfate group and the charge of aspartic or glutamic residues. The presence of moderate (e.g., 5–10 mM) of these cations during use may further reinforce the adsorption of the proteins, increasing by a 20–40%, the concentration of NaCl necessary to desorb 50% of the adsorbed proteins. Moreover, the enzymes immobilized under these conditions exhibited an improved stability in thermal and organic solvent inactivations (e.g., penicillin acylase improved its thermal stability by a six-fold factor).

RNA catalysis in model protocell vesicles.

We are engaged in a long-term effort to synthesize chemical systems capable of Darwinian evolution, based on the encapsulation of self-replicating nucleic acids in self-replicating membrane vesicles. Here, we address the issue of the compatibility of these two replicating systems. Fatty acids form vesicles that are able to grow and divide, but vesicles composed solely of fatty acids are incompatible with the folding and activity of most ribozymes, because low concentrations of divalent cations (e.g., Mg2+) cause fatty acids to precipitate. Furthermore, vesicles that grow and divide must be permeable to the cations and substrates required for internal metabolism. We used a mixture of myristoleic acid and its glycerol monoester to construct vesicles that were Mg2+-tolerant and found that Mg2+ cations can permeate the membrane and equilibrate within a few minutes. In vesicles encapsulating a hammerhead ribozyme, the addition of external Mg2+ led to the activation and self-cleavage of the ribozyme molecules. Vesicles composed of these amphiphiles grew spontaneously through osmotically driven competition between vesicles, and further modification of the membrane composition allowed growth following mixed micelle addition. Our results show that membranes made from simple amphiphiles can form vesicles that are stable enough to retain encapsulated RNAs in the presence of divalent cations, yet dynamic enough to grow spontaneously and allow the passage of Mg2+ and mononucleotides without specific macromolecular transporters. This combination of stability and dynamics is critical for building model protocells in the laboratory and may have been important for early cellular evolution.

A Comparative Study of Fe(II) and Fe(III) Interactions with DNA Duplex: Major and Minor Grooves Bindings

The involvement of the Fe cations in autoxidation in cells and tissues is well documented. DNA is a major target in such reaction, and can chelate Fe cation in many ways. The present study was designed to examine the interaction of calf-thymus DNA with Fe(II) and Fe(III), in aqueous solution at pH 6.5 with cation/DNA (P) (P = phosphate) molar ratios (r) of 1:160 to 1:2. Capillary electrophoresis and Fourier transform infrared (FTIR) difference spectroscopic methods were used to determine the cation binding site, the binding constant, helix stability and DNA conformation in Fe-DNA complexes. Structural analysis showed that at low cation concentration (r = 1/80 and 1/40), Fe(II) binds DNA through guanine N-7 and the backbone PO(2) group with specific binding constants of K(G) = 5.40 x 10(4) M(1) and K(P) = 2.40 x 10(4) M(1). At higher cation content, Fe(II) bindings to adenine N-7 and thymine O-2 are included. The Fe(III) cation shows stronger interaction with DNA bases and the backbone phosphate group. At low cation concentration (r = 1:80), Fe(III) binds mainly to the backbone phosphate group, while at higher metal ion content, cation binding to both guanine N-7 atom and the backbone phosphate group is prevailing with specific binding constants of K(G) = 1.36 x 10(5) M(-1) and K(P) = 5.50 x 10(4) M(-1). At r = 1:10, Fe(II) binding causes a minor helix destabilization, whereas Fe(III) induces DNA condensation. No major DNA conformational changes occurred upon iron complexation and DNA remains in the B-family structure.

Modulation of cooperativity in Mycobacterium tuberculosis NADPH‐ferredoxin reductase: Cation‐and pH‐induced alterations in native conformation and destabilization of the NADP+‐binding domain

FprA, a Mycobacterium tuberculosis NADPH-ferredoxin reductase, consists of two structural domains, a FAD-binding and a NADP-binding domain, respectively. For the first time, we demonstrated that native FprA, on thermal treatment underwent partial denaturation with unfolding of only the FAD-binding domain and release of the protein-bound flavin. The NADP-binding domain of this protein is highly resistant to denaturation under these conditions. However, the presence of either 150 mM NaCl or KCl or 10 muM MgCl(2) or CaCl(2) or slightly acidic pH of 6.0 resulted in a highly cooperative and complete thermal unfolding of the protein. Physicochemical investigations showed that the monovalent cations or low concentrations of divalent cations induced compaction of the protein conformation. However, divalent cations at higher concentrations resulted in FAD release leading to stabilization of an enzymatically inactive apoenzyme. Detailed thermal denaturation studies on the native protein and the isolated NADP-binding domain showed that cations and pH 6.0 destabilized only the heat-stable NADP-binding domain. The experimental studies demonstrate that modulation of intramolecular ionic interactions induce significant conformational changes in the NADP-binding domain of FprA, resulting in a substantial increase in the structural cooperativity of the whole molecule. The results presented in this paper are of importance as they demonstrate alterations in the native three-dimensional structure of FprA and cooperativity in protein molecule on slight alteration of pH or modification of ionic interactions in protein.

Two SUR1-specific Histidine Residues Mandatory for Zinc-induced Activation of the Rat KATP Channel

Zinc at micromolar concentrations hyperpolarizes rat pancreatic beta-cells and brain nerve terminals by activating ATP-sensitive potassium channels (KATP). The molecular determinants of this effect were analyzed using insulinoma cell lines and cells transfected with either wild type or mutated KATP subunits. Zinc activated KATP in cells co-expressing rat Kir6.2 and SUR1 subunits, as in insulinoma cell lines. In contrast, zinc exerted an inhibitory action on SUR2A-containing cells. Therefore, SUR1 expression is required for the activating action of zinc, which also depended on extracellular pH and was blocked by diethyl pyrocarbonate, suggesting histidine involvement. The five SUR1-specific extracellular histidine residues were submitted to site-directed mutagenesis. Of them, two histidines (His-326 and His-332) were found to be critical for the activation of KATP by zinc, as confirmed by the double mutation H326A/H332A. In conclusion, zinc activates KATP by binding itself to extracellular His-326 and His-332 of the SUR1 subunit. Thereby zinc could exert a negative control on cell excitability and secretion process of pancreatic beta-and alpha-cells. In fact, we have recently shown that such a mechanism occurs in hippocampal mossy fibers, a brain region characterized, like the pancreas, by an important accumulation of zinc and a high density of SUR1-containing KATP.

Characterization of a native hammerhead ribozyme derived from schistosomes

A recent re-examination of the role of the helices surrounding the conserved core of the hammerhead ribozyme has identified putative loop-loop interactions between stems I and II in native hammerhead sequences. These extended hammerhead sequences are more active at low concentrations of divalent cations than are minimal hammerheads. The loop-loop interactions are proposed to stabilize a more active conformation of the conserved core. Here, a kinetic and thermodynamic characterization of an extended hammerhead sequence derived from Schistosoma mansoni is performed. Biphasic kinetics are observed, suggesting the presence of at least two conformers, one cleaving with a fast rate and the other with a slow rate. Replacing loop II with a poly(U) sequence designed to eliminate the interaction between the two loops results in greatly diminished activity, suggesting that the loop-loop interactions do aid in forming a more active conformation. Previous studies with minimal hammerheads have shown deleterious effects of Rp-phosphorothioate substitutions at the cleavage site and 5' to A9, both of which could be rescued with Cd2+. Here, phosphorothioate modifications at the cleavage site and 5' to A9 were made in the schistosome-derived sequence. In Mg2+, both phosphorothioate substitutions decreased the overall fraction cleaved without significantly affecting the observed rate of cleavage. The addition of Cd2+ rescued cleavage in both cases, suggesting that these are still putative metal binding sites in this native sequence.

Sorption kinetics of strontium in porous hydrous ferric oxide aggregates: I. The Donnan diffusion model

Sorption of ions by hydrous ferric oxide (HFO) often shows a fast initial sorption reaction followed by a much slower sorption process. The second step is diffusion-controlled and can continue for days or months before equilibrium is reached. In this paper, we demonstrate that the diffusion rate may be explained by electrostatic interactions. The internal and external surfaces of HFO are generally positively charged and therefore repel cations. This can result in extremely low cation concentrations in pores, and therefore a significant reduction in pore diffusion rate. The theory is demonstrated here for sorption of Sr 2+ in HFO aggregates. The ion concentrations in the pore space are calculated using a Donnan model and diffusion is calculated from the Donnan concentration and potential gradients. This diffusion model is compared with nonelectrostatic pore diffusion, which does not take electrostatic interactions into account. The Donnan model predicts very low concentrations of Sr 2+ in the pores and diffusion rates that are up to 8000 times lower than predicted with a nonelectrostatic model.

Wheat (Triticum vulgare) chloroplast nuclease ChSI exhibits 5' flap structure-specific endonuclease activity.

The structure-specific ChSI nuclease from wheat (Triticum vulgare) chloroplast stroma has been previously purified and characterized in our laboratory. It is a single-strand-specific DNA and RNA endonuclease. Although the enzyme has been initially characterized and used as a structural probe, its biological function is still unknown. Localization of the ChSI enzyme inside chloroplasts, possessing their own DNA that is generally highly exposed to UV light and often affected by numerous redox reactions and electron transfer processes, might suggest, however, that this enzyme could be involved in DNA repair. The repair of some types of DNA damage has been shown to proceed through branched DNA intermediates which are substrates for the structure-specific DNA endonucleases. Thus we tested the substrate specificity of ChSI endonuclease toward various branched DNAs containing 5' flap, 5' pseudoflap, 3' pseudoflap, or single-stranded bulged structural motifs. It appears that ChSI has a high 5' flap structure-specific endonucleolytic activity. The catalytic efficiency (k(cat)/K(M)) of the enzyme is significantly higher for the 5' flap substrate than for single-stranded DNA. The ChSI 5' flap activity was inhibited by high concentrations of Mg(2+), Mn(2+), Zn(2+), or Ca(2+). However, low concentrations of divalent cations could restore the loss of ChSI activity as a consequence of EDTA pretreatment. In contrast to other known 5' flap nucleases, the chloroplast enzyme ChSI does not possess any 5'-->3' exonuclease activity on double-stranded DNA. Therefore, we conclude that ChSI is a 5' flap structure-specific endonuclease with nucleolytic activity toward single-stranded substrates.

Forest regrowth as the controlling factor of soil nutrient availability 75 years after fire in a deciduous forest of Southern Quebec

The effects of secondary succession on soil nutrient availability following fire in the 1920s was investigated in a hardwood forest of southern Quebec by correlation analyses between soil and solution chemistry, tree species composition, tree growth, litterfall nutrient fluxes and presence of charcoal monitored in six 300 m 2 plots between 1994 and 1998. The results suggests that the pioneer stand of largetooth aspen, paper birch and yellow birch that followed fire enriched the forest floor and upper mineral soil with its K-rich litter, but lowered solution NH4, NO3 and Mg concentrations through its high nutrient demand. High woody biomass primarily observed in the maple-dominated stands was associated with low exchangeable base cation concentrations in the forest floor, suggesting that nutrient immobilisation in trees is also a significant process leading to spatial variation in base cation availability in the forest floor. Finally, charcoal was positively correlated with exchangeable base cations in the forest floor which leads to believe that charcoal has a high affinity for base cations and that it can increase base cation availability decades after forest fire disturbance.

A role for macromolecular crowding effects in the assembly and function of compartments in the nucleus

The mechanisms which cause macromolecules to form discrete compartments within the nucleus are not understood. Here, two ubiquitous compartments, nucleoli, and PML bodies, are shown to disassemble when K562 cell nuclei expand in medium of low monovalent cation concentration; their major proteins dispersed as seen by immunofluorescence and immunoelectron microscopy, and nucleolar transcript elongation fell by ∼85%. These compartments reassembled and nucleolar transcription recovered in the same medium after adding inert, penetrating macromolecules (8 kDa polyethylene glycol (PEG), or 10.5 kDa dextran) to 12% w/v, showing that disassembly was not caused by the low cation concentration. These responses satisfy the criteria for crowding or volume exclusion effects which occur in concentrated mixtures of macromolecules; upon expansion the macromolecular concentration within the nucleus falls, and can be restored by PEG or dextran. These observations, together with evidence of a high concentration of macromolecules in the nucleus (in the range of 100 mg/ml) which must cause strong crowding forces, suggest strongly that these forces play an essential role in driving the formation, and maintaining the function of nuclear compartments. This view is consistent with their dynamic and mobile nature and can provide interpretations of several unexplained observations in nuclear biology.

In-situ generation and analysis of charge transfer materials using an OTTLE cell and resonance Raman scatteringElectronic supplementary information (ESI) available: BPW91/6-31G(d) predicted vibrational frequencies and intensities for the neutral, radical cation and dication species of 1 and 2. See http://www.rsc.org/suppdata/cp/b4/b402015d/

Poly(aryl)amine based charge transfer materials (CTMs) are essential components in a range of present and future technologies, from the Xerox process to display devices based upon light emitting polymers (LEPs). However, there is a lack of detailed understanding regarding the electronic properties of CTMs in their various neutral and oxidized forms. This paper reports the use of an optically transparent thin layer electrochemical (OTTLE) cell in combination with a Raman microprobe system and DFT calculations to provide information on the molecular and electronic structure of the mono- and di-oxidized derivatives of the classic CTM N,N′-diphenyl-N,N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′-diamine (TPD) and the closely related species N,N′-diphenyl-N,N′-bis(2,4-dimethylphenyl)(1,1′-biphenyl)-4,4′-diamine (DMTPD). The resonance Raman scattering profile easily discriminates between the monovalent and divalent cations while DFT calculations permit correlation of the observed vibrational frequencies with localized atomic displacements. The cations are best described in terms of a symmetrical (i.e. fully delocalized) structure. The high sensitivity of the method suggests that it should be appropriate for the observation of low concentrations of the various cations generated from TPD type CTMs during device operation.

Glutamate Release Through Glial Gap-Junction Hemichannels

Ye et al. investigated the role of gap-junction hemichannels in astrocytes and discovered that they mediated glutamate release; regulation of astrocyte hemichannel activity could thus modulate levels of extracellular glutamate in the central nervous system. Abnormal increases in extracellular glutamate, the principal excitatory neurotransmitter in brain, have been implicated in excitotoxic cell death. Under normal conditions, astrocytes accumulate glutamate, which helps to maintain low extracellular concentrations. Gap junctions, formed by aligned hemichannels on adjacent cells, can mediate the passage of molecules between some cell types, including glia. Unopposed hemichannels have been identified in astrocytes; their functions, however, have been unclear. Ye at al. used immunocytochemistry to detect hemichannels in cultured rat hippocampal astrocytes and confirmed their functionality by measuring increased uptake of a fluorescent dye after exposure to nominally divalent cation-free saline (DFCS). Dye permeability was sensitive to gap-junction blockers. The authors monitored changes in extracellular and intracellular glutamate concentration, under different ionic conditions and in the presence or absence of various pharmacological agents, and determined that glutamate efflux in response to DFCS occurred through hemichannels. Increased glutamate efflux was accompanied by a decrease in glutamate uptake, likely secondary to a disruption of ionic gradients associated with hemichannel opening. Removal of divalent cations also elicited gap-junction-blocker-sensitive release of glutamate and other amino acids from mouse optic nerve (a white matter preparation). These data suggest that astrocytes could release glutamate through hemichannels in response to low extracellular divalent cation concentration in vivo and thereby contribute to various neuropathologies.Z.-C. Ye, M. S. Wyeth, S. Baltan-Tekkok, B. R. Ransom, Functional hemichannels in astrocytes: A novel mechanism of glutamate release. J. Neurosci. 23, 3588-3596 (2003). [Abstract] [Full Text]

Glutamate Release Through Glial Gap-Junction Hemichannels

Ye et al. investigated the role of gap-junction hemichannels in astrocytes and discovered that they mediated glutamate release; regulation of astrocyte hemichannel activity could thus modulate levels of extracellular glutamate in the central nervous system. Abnormal increases in extracellular glutamate, the principal excitatory neurotransmitter in brain, have been implicated in excitotoxic cell death. Under normal conditions, astrocytes accumulate glutamate, which helps to maintain low extracellular concentrations. Gap junctions, formed by aligned hemichannels on adjacent cells, can mediate the passage of molecules between some cell types, including glia. Unopposed hemichannels have been identified in astrocytes; their functions, however, have been unclear. Ye at al . used immunocytochemistry to detect hemichannels in cultured rat hippocampal astrocytes and confirmed their functionality by measuring increased uptake of a fluorescent dye after exposure to nominally divalent cation-free saline (DFCS). Dye permeability was sensitive to gap-junction blockers. The authors monitored changes in extracellular and intracellular glutamate concentration, under different ionic conditions and in the presence or absence of various pharmacological agents, and determined that glutamate efflux in response to DFCS occurred through hemichannels. Increased glutamate efflux was accompanied by a decrease in glutamate uptake, likely secondary to a disruption of ionic gradients associated with hemichannel opening. Removal of divalent cations also elicited gap-junction-blocker-sensitive release of glutamate and other amino acids from mouse optic nerve (a white matter preparation). These data suggest that astrocytes could release glutamate through hemichannels in response to low extracellular divalent cation concentration in vivo and thereby contribute to various neuropathologies. Z.-C. Ye, M. S. Wyeth, S. Baltan-Tekkok, B. R. Ransom, Functional hemichannels in astrocytes: A novel mechanism of glutamate release. J. Neurosci. 23 , 3588-3596 (2003). [Abstract] [Full Text]