Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Cations”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,189 records · Page 66Linked to original sources
PubMed1997

Stimulation with cholecystokinin leads to increased ratio between mRNA levels for anionic and cationic trypsinogen in rat pancreas.

An increased ratio between serum levels of immunoreactive anionic and cationic trypsin is a common finding in many forms of pancreatic disease. Experimental studies have shown that increased stimulation with cholecystokinin (CCK) leads to an increase in the ratio between the pancreatic content of anionic and cationic trypsin. To study whether this effect is caused by increased pancreatic synthesis of anionic trypsin in relation to cationic trypsin we studied the levels of mRNA for anionic and cationic trypsinogen in pancreatic extracts from rats exposed to increased CCK levels through chronic subcutaneous administration of CCK. Northern blot and slot blot hybridization techniques were used. The ratio between mRNAs for anionic and cationic trypsin was significantly higher in CCK-treated rats: median level, 2.04 (range, 1.33-4.08) versus a median level of 1.15 (range, 0.97-2.17) in the control group: P < 0.01. These findings support the view that chronic CCK stimulation leads to the increased synthesis of anionic trypsinogen compared to cationic trypsinogen.

Animals↗
PubMed2001

L-arginine effects on Na+ transport in M-1 mouse cortical collecting duct cells--a cationic amino acid absorbing epithelium.

The effect of L-arginine on transepithelial ion transport was examined in cultured M-1 mouse renal cortical collecting duct (CCD) cells using continuous short circuit current (Isc) measurements in HCO3-/CO2 buffered solution. Steady state Isc averaged 73.8 +/- 3.2 microA/cm2 (n = 126) and was reduced by 94 +/- 0.6% (n = 16) by the apical addition of 100 microM amiloride. This confirms that the predominant electrogenic ion transport in M-1 cells is Na+ absorption via the epithelial sodium channel (ENaC). Experiments using the cationic amino acid L-lysine (radiolabeled) as a stable arginine analogue show that the combined activity of an apical system y+ and a basal amino acid transport system y+L are responsible for most cationic amino acid transport across M-1 cells. Together they generate net absorptive cationic amino acid flux. Application of L-arginine (10 mM) either apically or basolaterally induced a transient peak increase in Isc averaging 36.6 +/- 5.4 microA/cm2 (n = 19) and 32.0 +/- 7.2 microA/cm2 (n = 8), respectively. The response was preserved in the absence of bath Cl- (n = 4), but was abolished either in the absence of apical Na+ (n = 4) or by apical addition of 100 microM amiloride (n = 6). L-lysine, which cannot serve as a precursor of NO, caused a response similar to that of L-arginine (n = 4); neither L-NMMA (100 microM; n = 3) nor L-NAME (1 mM; n = 4) (both NO-synthase inhibitors) affected the Isc response to L-arginine. The effects of arginine or lysine were replicated by alkalinization that mimicked the transient alkalinization of the bath solution upon addition of these amino acids. We conclude that in M-1 cells L-arginine stimulates Na+ absorption via a pH-dependent, but NO-independent mechanism. The observed net cationic amino acid absorption will counteract passive cationic amino acid leak into the CCD in the presence of electrogenic Na+ transport, consistent with reports of stimulated expression of Na+ and cationic amino acid transporters by aldosterone.

Amiloride↗
PubMed1976

Effects of cations upon chloroplast membrane subunit. Interactions and excitation energy distribution.

When isolated chloroplasts from mature pea (Pisum sativum) leaves were treated with digitonin under "low salt" conditions, the membranes were extensively solubilized into small subunits (as evidenced by analysis with small pore ultrafilters). From this solubilized preparation, a photochemically inactive chlorophyll - protein complex (chlorophyll alpha/beta ratio, 1.3) was isolated. We suggest that the detergent-derived membrane fragment from mature membranes is a structural complex within the membrane which contains the light-harvesting chlorophyll alpha/beta protein and which acts as a light-harvesting antenna primarily for Photosystem II. Cations dramatically alter the structural interaction of the light-harvesting complex with the photochemically active system II complex. This interaction has been measured by determining the amount of protein-bound chlorophyll beta and Photosystem II activity which can be released into dispersed subunits by digitonin treatment of chloroplast lamellae. When cations are present to cause interaction between the Photosystem II complex and the light-harvesting pigment - protein, the combined complexes pellet as a "heavy" membranous fraction during differential centrifugation of detergent treated lamellae. In the absence of cations, the two complexes dissociate and can be isolated in a "light" submembrane preparation from which the light-harvesting complex can be purified by sucrose gradient centrifugation. Cation effects on excitation energy distribution between Photosystems I and II have been monitored by following Photosystem II fluorescence changes under chloroplast incubation conditions identical to those used for detergent treatment (with the exception of chlorophyll concentration differences and omission of detergents). The cation dependency of the pigment - protein complex and Photosystem II reaction center interactions measured by detergent fractionation, and regulation of excitation energy distribution as measured by fluorescence changes, were identical. We conclude that changes in substructural organization of intact membranes, involving cation induced changes in the interaction of intramembranous subunits, are the primary factors regulating the distribution of excitation energy between Photosystems II and I.

Cell Fractionation↗
PubMed1976

Studies on (Na+ plus K+)-activated ATPase. XXXVII. Stabilization by cations of the enzyme-ouabain complex formed with Mg1+ and inorganic phosphate.

Dissociation of the (Na+ + K+)-ATPase ouabain complex, formed in the presence of Mg2+ and inorganic phosphate (Complex II), is inhibited by Mg2+ (21-45%) and the alkali cations Na+ (25-59%) and K+ (27-75%) when kidney cortex tissue (bovine, rabbit, guinea pig) is the enzyme source. Choline chloride at 200 mM, equivalent to the highest concentration of NaCl tested, does not inhibit. Dissociation of Complex II from brain cortex (bovine, rat, rabbit) or heart muscle (rabbit) is much less inhibited: 0-11% by Na+ and 11-19% by K+. The degree of inhibition is not directly related to the size of the dissociation rate constant (k-) of the various complexes, but rather to the extent of interaction between the cation and ouabain binding sites for these tissues. Inhibition curves for Na+ and K+ are sigmoidal. Half-maximal inhibition for rabbit brain and kidney cortex is at 30-40 mM Na+ and 6-10 mM K+, and the maximally inhibitory concentrations are 50-150 and 15-20 mM, respectively. Maximal inhibition by Na+ or K+ for these tissues is the same. For guinea pig kidney cortex Na+ and K+ are almost equally effective, but 150 mM K+ or 200 mM Na+ are still not saturating, and inhibition curves indicate high- and low-affinity binding sites for the alkali cations. The inhibition curve for Mg2+ is not sigmoidal. In the kidney preparations Mg2+ inhibits half-maximally at 0.4-0.5 mM, maximally at 1-3 mM. Maximal inhibition by Mg2+ is higher than by Na+ or K+ for rabbit kidney cortex and lower for guinea pig kidney cortex. There is no competition or additivity among the cations, indicating the existence of different binding sites for Mg2+ and the alkali cations. Complex II differs in stability in the extent of inhibition, in the dependence of inhibition on the cation concentration and in the absence of antagonism between Na+ and K+, from the ouabain complex formed via phosphorylation by ATP (Complex I). This indicates that the phosphorylation states for the complexes are clearly different.

Adenosine Triphosphatases↗
PubMed1996

Endocytosis and intracellular processing accompanying transfection mediated by cationic liposomes.

Cationic liposomes mediate efficient transfection of mammalian cells, but the manner in which cells internalize and process cationic liposome-DNA complexes has not been well characterized. We exposed several cell types, including human and murine erythroleukemia cells. African green monkey kidney cells (CV-1), isolated rat alveolar type II cells and alveolar macrophages to DNA-cationic liposome complexes containing N-(1-2,3-dioleyloxypropyl)-N,N,N-triethylammonium (DOTMA) and Dioleylphosphatidylethanolamine (DOPE). The morphology of liposome-cell interactions was assessed by electron microscopy. Liposome preparations were complexed to colloidal gold particles or to both plasmid DNA and gold particles. Cells treated with DOTMA liposome-DNA complexes demonstrated endocytosis of the liposome-DNA complexes in coated pits, which were seen in early endosomes, late endosomes, and lysosomes. In isolated alveolar type II cells, the gold-labelled DOTMA lipid apparently mixed with the contents of lamellar bodies. In most cells, gold particles were dispersed throughout the cytoplasmic matrix. In a small proportion of CV-1 and U937 cells, a membrane system resembling the endoplasmic reticulum developed within the nucleus. This novel structure was also present in nuclei after they were isolated from CV-1 cells and then mixed with DOTMA-containing liposomes. Membranes which form after exposure to DOTMA-containing liposomes were 10 nm in thickness as compared to the approx. 8 nm thickness of endogenous cellular membranes. Based on these morphologic observations, we propose that the main route of entry of cationic liposomes into cells is by endocytosis. In some instances, the endosomal compartment releases its cationic liposome-DNA contents into the cytoplasmic matrix. Occasionally, liposomes may enter the nucleus by fusion with the nuclear envelope, creating vesicular and reticular intranuclear membranes. It is not clear at present which, if any of these morphological observations correlates with transfection mediated by cationic liposomes.

Animals↗
PubMed1975

Studies on bursting pacemaker potential activity in molluscan neurons. II. Regulations by divalanet cations.

Identified cells in Aplysia (R15, R2, LPG) and cell 11 in Otala have been used to investigate the effects of divalent cations, temperature, pH and ouabain on neuronal activity. Divalent cations act primarily to regulate the appearance of bursting pacemaker potential (BPP) activity in these cells. These ions are necessary for B generation and will inhibit its appearance at high concentrations (Ca greater than Mg greater that greater than Sr). In addition, Ca is involved in the seasonal modulation of BPP activity in a neurosecretory cell in Otala. Monovalent cations play secondary roles as regulators of BPP activity by competing with divalent cations for the sites involved in the regulation of the (probable) monovalent conductances underlying BPPs. The effects of pH, temperature and ouabain on membrane properties and BPP activity are partly related to their interaction with divalent cations. The results described indicate important roles for divalent cations in the regulation of the expression of BPP activity and its underlying membrane properties both in different nerve cells and in the same cell during dormancy and activity of the snails.

Animals↗
PubMed1984

Electron spin resonance and electron nuclear double resonance studies of cation radicals derived from tocopherol model compounds.

Electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) measurements were performed for the cation radicals obtained from the model compounds of alpha-, beta-, gamma- and delta-tocopherol (vitamin E) by oxidizing the tocopherol precursors in an AlCl3-CH2Cl2 solution. The proton hyperfine coupling constants and g-values were precisely determined. The ENDOR spectra of the cation radicals of alpha-, beta-, gamma- and delta-tocopherol models in CH2Cl2 at -100 degrees C clearly show 10, 6, 6 and 12 different proton hyperfine couplings, respectively. By varying the temperature, the ESR spectra of the alpha- and delta-tocopherol model cations exhibit line-width alternation phenomena characteristic of the hindered rotation of the OH group. However, neither the beta- nor the gamma-tocopherol model cation radical ESR spectra show any sign of an alternating line-width effect. These results are interpreted by assuming that the beta- and gamma- tocopherol model cations are stabilized in the trans and cis conformations, respectively. On the other hand, both the alpha- and delta-tocopherol model cations exist as cis and trans isomers.

Chemical Phenomena↗
PubMed1986

Kinetic, equilibrium and spectroscopic studies on cation association at the active center of acetylcholinesterase: topographic distinction between trimethyl and trimethylammonium sites.

This study examines the importance of electrostatic interactions on ligand association at the active center of acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7). The active-center serine was covalently modified with the dimensionally equivalent isosteric beta-(trimethylammonium)ethyl and 3,3-dimethylbutyl methylphosphonofluoridates. Reactivation of the 3,3-dimethylbutyl methylphosphono-conjugate by the bisquaternary mono-oxime HI-6, after accounting for the capacity for spontaneous reactivation, proceeded at a rate that was 20-fold greater than that for the cationic conjugate. Decidium, a fluorescent bisquaternary ligand that binds with its trimethylammonium moiety within the active center, exhibited affinity for the 3,3-dimethylbutyl conjugate that was within 2-fold that for the native enzyme, but 100-fold greater than for the cationic conjugate. Whereas association of n-alkyl mono- and bisquaternary ligands with the uncharged conjugate was virtually unaltered with respect to the native enzyme, the affinities of edrophonium, phenyltrimethylammonium and N-methylacridinium were reduced 100-fold for the uncharged conjugate relative to native enzyme. These results indicate that the orientations of the 3,3-dimethylbutyl and beta-(trimethylammonium)ethyl moieties with respect to the surface of the enzyme are not equivalent, that modification of the active center does not preclude cation association of active-center-selective ligands, and that aromatic cations associate at an anionic locus which is unique from that at which decidium and the n-alkyl mono- and bisquaternary cations associate. As such, the results point to the presence of a heterogeneity of cation binding sites within a circumscribed distance from the modified serine, and do not sustain the view proposed by Hasan et al. (J. Biol. Chem. 255 (1980) 3898-3904; 256, (1981) 7781-7785) that electrostatic interactions at the active center are subordinate to steric constraints imposed by a dimensionally restricted trimethyl site.

Acetylcholinesterase↗
PubMed1992

Nutrient leaching from conifer needles in relation to foliar apoplast cation exchange capacity.

Limited evidence to date suggests that acidic precipitation promotes leaching of nutrient cations from conifer foliage. In order to evaluate the relative contribution of the apoplast cation exchange complex and symplast nutrient pools to the leached ions, the magnitude of foliar leaching in response to acidic solutions was compared to foliar apoplast cation exchange capacity (CEC) for two conifer tree species (Pseudotsuga menziesii and Picea engelmanii). Leaching potential was determined by immersing detached needles in acidic solutions (pH5.6, 3.1, 2.1) over a range of time intervals (1, 4, 8, 24 h) and foliar apoplast cation exchange capacity was determined by potentiometric titration. Leaching increased with decreasing pH and increasing time of immersion. At pH values 2.1 and 3.1, equivalents of H+ depleted from the acidic solutions approximated equivalents of base cations gained by the solutions. Maximum amounts leached were less than 40 microeq g(-1) dry weight of needles for the combination of calcium, magnesium and potassium. Measured foliar apoplast CEC for these species was approximately 120 microeq g(-1) dry weight of needles. These relative magnitudes indicate that the apoplast rather than the symplast provided the leached ions. Because the apoplast foliar cation exchange capacity appears to be large relative to the rates of H+ diffusion across the cuticle, it may help to insulate the symplast nutrient reservoir from exposure to the H+ of acidic solutions bathing the foliage.

Journal Article↗
PubMed1993

Modelling long-term cation supply in acidified forest stands.

A dynamic soil chemistry model was used to explain the observed decrease in soil base saturation between 1949 and 1984 at three stands in southern Sweden. The results show that acid deposition has caused soil acidification. The model, SAFE (Soil Acidification in Forest Ecosystems), includes the fundamental physical processes such as leaching and accumulation, and chemical processes such as cation exchange, mineral weathering, nutrient uptake and solute equilibrium reactions. The sources and sinks of base cations in the soil system were quantified, showing that weathering, deposition of base cations and depletion of exchangeable base cations supply cations to the soil solution in similar amounts in the upper 1 m during the acidification phase. This demonstrates that budget studies alone cannot be used to distinguish between long-term capacity to resist acidification, represented by weathering, from short-term buffering caused by cation exchange.

Journal Article↗
PubMed2006

The Hofmeister series effect in adsorption of cationic surfactants--theoretical description and experimental results.

Interfacial properties of cationic surfactants show strong dependence on the type of surfactant counterion or on the type of anion of a salt added to the surfactant solution. In the paper, the models of ionic surfactant adsorption that can take into account ionic specific effects are reviewed. Model of ionic surfactant adsorption based on the assumption that the surfactant ions and counterions undergo nonequivalent adsorption within the Stern layer was selected to describe experimental surface tension isotherms of aqueous solutions of a number of cationic surfactants. The experimental isotherms for: n-alkyl trimethylammonium cationic surfactants, namely: C(16)TABr (CTABr or CTAB), C(16)TACl, C(16)TAHSO(4), C(10)TABr and C(12)TABr as well as decyl- and dodecylpyridinium salts with and without various electrolyte anions as Cl(-), Br(-), F(-), I(-), NO(3)(-), ClO(4)(-) and CH(3)COO(-) were described in terms of the model and a good agreement between the theory and experiment was obtained for a wide range of surfactants and added electrolyte concentrations. A very pronounced Hofmeister effect in dependence of surface tension of cationic surfactants on the type of anion was found. Analysing this dependence in terms of the proposed model of ionic surfactant adsorption, strong correlation between "anion surface activity" (the model parameter accounting for ion penetration into the Stern layer), and the ion polarizability was obtained. That suggests that the mechanism related to the dispersive interaction of polarized ion with electric field at interface is responsible for Hofmeister series effects in surface activity of cationic surfactants. The same mechanism was proposed recently to explain the dependence of surface tension increase with electrolyte concentration on anion and cation type.

Journal Article↗
PubMed2006

In vivo gene therapy of type I diabetic mellitus using a cationic emulsion containing an Epstein Barr Virus (EBV) based plasmid vector.

A cationic emulsion containing an insulin expression plasmid was prepared for the treatment of type 1 diabetic mellitus (DM) in vivo. A rat proinsulin-1 gene was inserted to EBV-based plasmid vectors containing CAG promoter. Cationic emulsion composed of DOTAP and squalene was complexed with the plasmid DNA. An intravenous injection of cationic emulsion containing proinsulin gene decreased blood glucose levels for 7 days within normal range. The cationic emulsion exerted more profound effect on blood glucose levels compared to naked DNA. RT-PCR results confirmed that the proinsulin was expressed in several organs containing liver, lung, spleen, and kidney. The refractory response was invoked by multiple injections of naked DNA or cationic emulsion/DNA complex, which was later proven to be an immune response against expressed proinsulin. Therefore, the cationic emulsion showed a promising result as a novel insulin gene therapy vehicle by decreasing blood glucose level for a month.

Animals↗
PubMed2005

Purification and characterization of a cationic peroxidase Cs in Raphanus sativus.

A short distance migrating cationic peroxidase from Korean radish seeds (Raphanus sativus) was detected. Cationic peroxidase Cs was purified to apparent homogeneity and characterized. The molecular mass of the purified cationic peroxidase Cs was estimated to be about 44 kDa on SDS-PAGE. After reconstitution of apoperoxidase Cs with protohemin, the absorption spectra revealed a new peak in the Soret region around 400 nm, which is typical in a classical type III peroxidase family. The optimum pH of peroxidase activity for o-dianisidine oxidation was observed at pH 7.0. Kinetic studies revealed that the reconstituted cationic peroxidase Cs has Km values of 1.18 mM and of 1.27 mM for o-dianisidine and H2O2, respectively. The cationic peroxidase Cs showed the peroxidase activities for native substrates, such as coumaric acid, ferulic acid, and scopoletin. This result suggested that cationic peroxidase Cs plays an important role in plant cell wall formation during seed germination.

Gene Expression Regulation, Developmental↗
PubMed1999

Interaction of cationic liposomes and their DNA complexes with monocytic leukemia cells.

Cationic liposomes complexed with DNA have been used extensively as non-viral vectors for the intracellular delivery of reporter or therapeutic genes in culture and in vivo. We examined the relationship between the characteristics of the lipoplexes, their mode of interaction with monocytic THP-1 cells and their ability to transfect these cells. We determined the size and zeta potential of cationic liposomes (composed of 1,2-dioleoyl-3-(trimethylammonium) propane (DOTAP) and its mixtures with neutral lipids), and lipoplexes at different (+/-) charge ratios. As the (+/-) charge ratio of the lipoplexes decreased to (1/1), a significant reduction in zeta potential and an increase in size was observed. The increase in size resulted from fusion between liposomes promoted by DNA, as demonstrated by a lipid mixing assay, and from aggregation of the complexes. Interaction of liposomes and lipoplexes with THP-1 cells was assessed by monitoring lipid mixing ('fusion') as well as binding and cell association. While no lipid mixing was observed with the 1/2 (+/-) lipid/DNA complexes, lipoplexes with higher (+/-) charge ratios underwent significant fusion in conjunction with extensive cell binding. Liposome binding to cells was dependent on the positive charge of the liposomes, and their fusion could be modulated by the co-lipid. DOTAP/phosphatidylethanolamine (1:1) liposomes fused with THP-1 cells, unlike DOTAP/phosphatidylcholine (1:1) liposomes, although both liposome types bound to the cells to a similar extent. The use of inhibitors of endocytosis indicated that fusion of the cationic liposomes with cells occurred mainly at the plasma membrane level. The presence of serum increased the size of the cationic liposomes, but not that of the lipoplexes. Low concentrations of serum (3%) completely inhibited the fusion of cationic liposomes with cells, while inhibiting binding by only 20%. Our results suggest that binding of cationic liposomes and lipoplexes to cells is governed primarily by electrostatic interactions, whereas their fusion is regulated by the lipid composition and sterically favorable interactions with cell surface molecules. In addition our results indicate no correlation between fusion of the lipoplexes with the plasma membrane and the levels of transfection.

Binding, Competitive↗
PubMed2000

Systemic administration of cationic phosphonolipids/DNA complexes and the relationship between formulation and lung transfection efficiency.

Performances of cationic lipid formulations for intravenous gene delivery to mouse lungs have been previously reported. We report in this study that cationic phosphonolipids, when appropriately formulated, can be good synthetic vectors for gene delivery to lung after intravenous administration. One of our reagents, GLB43, was capable of mediating a 500-fold higher expression in the lungs of mice than could be obtained with free pDNA alone (P=0.018). We demonstrate that the most important parameters for cationic phosphonolipid transfection activity after systemic administration are the chemical structure of the cationic phosphonolipid, the lipid to DNA charge ratio and the inclusion of co-lipid in the formulation. We report using a luciferase reporter gene that transfection activity in vivo 24 h after cationic phosphonolipid systemic administration could not be predicted from in vitro analysis. In contrast to in vitro studies, cationic phosphonolipids including the oleyl acyl chains (GLB43) were more effective than its analogue with the myristyl acyl chains (GLB73). Using pathological analysis of animal livers, we demonstrate that the toxicity level was correlated with the lipoplex formulation and the lipid to DNA ratio.

Animals↗
PubMed1998

Effect of serum components on the physico-chemical properties of cationic lipid/oligonucleotide complexes and on their interactions with cells.

The interactions among serum components and cationic lipid-nucleic acid complexes are central to the understanding of how serum inhibits cellular delivery of oligonucleotides in vitro and in vivo. In this study, we show that several serum proteins, in particular bovine serum albumin (BSA), lipoproteins (HDL and LDL) and macroglobulin, interact with cationic lipid/oligonucleotide complexes, alter the complex diameter and zeta potential (from positive to negative values), and significantly interfere with the ability of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) to deliver phosphorothioate oligonucleotides (ODN) into cells. Serum and BSA do not dissociate the ODN and lipid components, therefore inhibition of delivery cannot be attributed to a displacement of cationic lipid from the ODN. Rather BSA at 2.5 mg/ml, comparable to the amount found in 10% serum, decreases the cell association of ODN by about 5-fold and nuclear uptake of ODN by greater than 20-fold. In contrast, immunoglobulin G, the other major serum component, alters the zeta potential from positive to near neutral, has a modest effect on the diameter of the complex but does not affect cell association or nuclear delivery of the ODN at amounts found in 10% serum. Other molecules found in serum, specifically oleic acid and heparin, displace the ODN from the complex and thus interfere with delivery. This displacement is attenuated by first incubating the complex with BSA. Another manifestation of serum-complex interactions is that ODN significantly and cationic liposomes slightly, activate complement. However, formation of the complex markedly reduces the complement activation of the ODN. Finally, the effect of serum can be partially counteracted by the selection of the helper lipid (DOPE or cholesterol). Inclusion of a helper lipid reduces the effective charge ratio (cationic groups/anionic thioates) required to deliver ODN into cells and permits delivery in the presence of greater percentages of serum in the culture medium. These results support the current view that the binding of serum proteins to the complex is a significant factor in modulating the activity of cationic lipid-ODN complexes in culture and after intravenous administration.

Blood Proteins↗
PubMed1980

Tests of an electrostatic screening hypothesis of the inhibition of neurotransmitter release by cations at the frog neuromuscular junction.

We have investigated an electrostatic screening hypothesis of cationic inhibition of quantal release at the neuromuscular junction of the frog (Rana pipiens). According to this hypothesis, increasing the extracellular concentration of an inhibitory cation reduces the quantal content (m) of the end-plate potential by reducing the ability of negative surface charge to attract Ca2+ to the external surface of the presynaptic membrane. The inhibitory power of various cations should depend only on their net ionic charge and should increase strongly with increasing charge. We have demonstrated, in Ringer's solutions containing modified concentrations of Na+, Ca+, and Mg2+, that at fixed concentrations of Ca2+ and Na+ (a) the dependence of m on [Mg2+]0 is satisfactorily accounted for by electrostatic theory and (b) the dependence of m on the univalent cation concentration of the modified Ringer's solution is satisfactorily predicted from the Mg2+ inhibition of m. (Glucosamine or arginine was used to replace a fraction of the Na+ content of Ringer's solution in the latter experiments.) These results are consistent with electrostatic screening actions of Mg2+ and univalent cations in the inhibition of m. We have also re-examined the inhibition of m caused by the addition to Ringer's solution of two trace concentration divalent cations, Mn2+ and Sr2+. Our data suggest that the inhibition of m by Sr2+ at high quantal contents may also be due to surface charge screening, while the potent inhibitory actions of Mn2+ may be due to its ability to bind negative surface charge.

Animals↗
PubMed2003

Isomerization of cationic azobenzene derivatives in dispersions and films of layered silicates.

Photoisomerization reactions of cationic azobenzene dyes in solutions, dispersions, and films of layered silicates were studied by visible (Vis) spectroscopy. The dyes isomerized reversibly from thermodynamically more stable trans-isomers to cis-isomers when irradiated with ultraviolet (UV) light. Observed trends were compared with the optical changes of the dyes that occurred as a consequence of their adsorption at the silicate surface. Small fractions of the dyes are likely to have isomerized during the adsorption process, even without the UV-light irradiation. The aggregation of the dyes was another reaction taking place at the surface of the silicates. The extent of the UV-light-induced isomerization reactions was reduced for the adsorbed dye cations. The reaction proceeded readily for a dye with monovalent cations. However, the photoisomerization was practically negligible in both dispersions and films of layered silicates for a dye with bivalent cations, whereas the isomerization proceeded in solution. This phenomenon was interpreted in terms of the attractive electrostatic forces between the substrate and the dye cations, which hindered the isomerization reaction. The layer charge of silicates affected the orientation of the dye cations as observed by X-ray diffraction (XRD) measurements. However, the choice of silicate did not significantly affect the fundamental aspects and the described basic trends of the UV-light-induced isomerization reaction.

Journal Article↗