Reduction Of Ionic Strength Research Articles

Effect of urea on solution behavior and heat-induced gelationof chitosan-β-glycerophosphate

In this study, we have investigated the effect of urea on the physicochemical (pH and conductivity) and rheological properties of the chitosan- β-GP system in order to assess the main polymer–polymer interactions at low and high temperature. The pH of the solutions was slightly increased due to the consumption of H + in solution through the hydrolysis of urea. Furthermore, the addition of urea considerably decreased the conductivity, and therefore the ionic strength of the solutions, and this effect was more important at high temperature. It indicated that urea strongly affects polymer–polymer interactions by weakening hydrogen bonds at low temperature, but in addition can hinder hydrophobic effect at high temperature since the reduction of ionic strength results in less screening of electrostatic repulsion between protonated glucosamine groups. At 15 °C, the addition of urea to chitosan- β-GP solutions decreased their elasticity, shortened relaxation times and simplified the relaxation process due to the disruption of hydrogen bonds. Heat-induced gelation of the chitosan system in the presence of urea showed higher gelation temperature ( T gel) in non-isothermal tests and longer gelation time ( t gel) in isothermal conditions. The activation energy for gelation also increased with increasing urea concentration. We concluded that the detrimental effect of urea on the gelation process was mainly related to a decrease in polymer-polymer hydrophobic effect, as shown by the decrease in conductivity.

Conformational changes in dynamin on GTP binding and oligomerization reported by intrinsic and extrinsic fluorescence

The effects of guanine nucleotides on the intrinsic and extrinsic fluorescence properties of dynamin were assessed. The intrinsic Trp (tryptophan) fluorescence spectra of purified recombinant dynamin-1 and -2 were very similar, with a maximum at 332 nm. Collisional quenching by KI was weak (approximately 30%), suggesting that the majority of Trp residues are buried. Binding of guanine nucleotides decreased intrinsic fluorescence by 15-20%. Titration of the effects showed that GTP and GDP bound to a single class of non-interacting sites in dynamin tetramers with apparent dissociation constants (K(d)) values of 5.4 and 7.4 microM (dynamin-1) and 13.2 and 7.1 microM (dynamin-2) respectively. Similar dissociation constant values for both nucleotides were obtained by titrating the quenching of IAEDANS [N-iodoacetyl-N'-(5-sulpho-1-naphthyl)ethylenediamine]-labelled dynamin-2. Despite the similar binding affinities, GTP and GDP result in different conformations of the protein, as revealed by sensitivity to proteinase K fragmentation. Dynamins contain five Trp residues, of which four are in the PH domain (pleckstrin homology domain) and one is in the C-terminal PRD (proline/arginine-rich domain). Guanine nucleotides quenched fluorescence emission from a truncated (DeltaPRD) mutant dynamin-1 to the same extent as in the full-length protein, suggesting conformational coupling between the G (groove)-domain and the PH domain. Efficient resonance energy transfer from PH domain Trp residues to bound mant-GTP [where mant stands for 2'-(3')-O-(N-methylanthraniloyl)] suggests that the G-domain and PH domain are in close proximity (5-6 nm). Promotion of dynamin-2 oligomerization, by reduction in ionic strength or increasing protein concentration, had little effect on intrinsic dynamin fluorescence. However, fluorescence emission from IAEDANS.dynamin-2 showed a significant spectral shift on oligomerization. In addition, energy transfer was observed when oligomerization was promoted in mixtures of IAEDANS.dynamin-2 and 4-(4-dimethylaminophenylazo)benzoic acid-coupled dynamin-2, an effect that was counteracted by GTP but not GDP.

Interactions between laponite and microbial biofilms in porous media: implications for colloid transport and biofilm stability

Quartz sand columns and sand-filled microscope flow cells were used to investigate the transport characteristics of the clay colloid laponite, and a biofilm-forming bacterium, Pseudomonas aeruginosa SG81. Separate experiments were performed with each particle to determine their individual transport characteristics in clean sand columns. In a second set of experiments, bacterial biofilms were formed prior to introduction of the clay colloids. In the independent transport experiments, bacteria and laponite each conformed to known physicochemical principles. A sodium chloride concentration of 7×10−2 M caused complete retention of the laponite within the sand columns. P. aeruginosa SG81 was generally less influenced by ionic strength effects; it showed relatively low mobility at all ionic strengths tested and some (albeit reduced) mobility when introduced to the columns in 1M NaCl, the highest concentration tested, but nevertheless showed reproducible trends. Under conditions favourable to laponite retention and biofilm stability (7×10−2 MNaCl), laponite suspensions were able to remobilise a portion of the attached bacterial biomass. At low ionic strength, the profile of laponite elution was also altered in the presence of a P. aeruginosa biofilm. These observations suggest that while a reduction in ionic strength has a dominant influence on the mobilisation of biological and inorganic colloids, the presence of laponite and biomass can have a distinct influence on the mobility of both types of colloids. Since these events are likely to occur in subsurface environments, our results suggest that colloid–biofilm interactions will have implications for colloid–bound contaminant transport and the remobilisation of pathogens.

Rennet coagulation of skim milk and curd drainage: effect of pH, casein concentration, ionic strength and heat treatment

The effect of the pH of skim milk (6.4 to 6.0), the casein concentration (27 to 36 g .kg -1 ), a reduction in the ionic strength to 0.6 compared to that of unmodified milk and the effect of heat treat- ment on rennet coagulation, gel syneresis and drainage were tested. Coagulation was measured using a formagraph and by viscoelasticimetry, while syneresis was studied using a centrifugation test. Drainage was followed under conditions similar to those of cheesemaking, using either a mesophilic or thermophilic process. The effects of pH and casein concentration on rennet coagulation, gel syneresis and drainage were in accordance with those reported in the literature. Reduction in ionic strength led to shorter coagulation time and firming time. It also led to an increase in gel firmness, gel syneresis, drainage rate, final amount of expelled whey, solids content and hardness of drained curd. Ionic strength appeared to have an interactive effect with pH on coagulation and with the concentration of casein on syneresis and drainage. This effect of the reduction in ionic strength is discussed. rennet coagulation / pH / casein concentration / ionic strength / drainage

Reduction of ionic strength activates single volume-regulated anion channels (VRAC) in endothelial cells

Reduction of ionic strength activates single volume-regulated anion channels (VRAC) in endothelial cells

Effect of Ionic Strength on Length-dependent Ca2+Activation in Skinned Cardiac Muscle

S. H. Smith and F. Fuchs. Effect of Ionic Strength on Length-dependent Ca2+Activation in Skinned Cardiac Muscle. Journal of Molecular and Cellular Cardiology (1999) 31, 2115–2125. The length-dependence of myofilament Ca2+sensitivity is considered to be an important component of the steep force–length relationship in cardiac muscle (Frank–Starling relation). Recent studies suggest that Ca2+sensitivity is a function of the number of strong-binding cross-bridge interactions formed at a given sarcomere length. However, the length-dependent step in the thin filament activation process is still unknown. This study was designed to test the hypothesis that sarcomere length influences the transition of the thin filament from the unattached (blocked) state to the weakly bound (closed) state. This hypothesis was tested by determining the length-dependence of Ca2+sensitivity as a function of ionic strength in skinned bovine ventricular muscle. Previous studies have shown that reduction in ionic strength below a critical level, in the absence of Ca2+, shifts the thin filament to the closed state. In this study normal Ca2+regulation was maintained at low ionic strength but the length-dependence of Ca2+sensitivity and the length-dependence of Ca2+binding were eliminated. These results are consistent with the hypothesis that the transition from the blocked to the closed state is a function of filament geometry as well as Ca2+and ionic strength.

Reduction of ionic strength activates single volume-regulated anion channels (VRAC) in endothelial cells

We have previously shown that a reduction of intracellular ionic strength is involved in the activation of volume-regulated anion channels (VRAC). Here we show in a single-channel study that VRAC can be activated in a cell-attached patch when the cell interior is dialyzed with a solution of decreased ionic strength. For this purpose, bovine pulmonary endothelial (CPAE) cells) were permeabilized with alpha-staphylotoxin (alphaST) which has a molecular weight cut-off size of 2 kDa. If the ionic strength in the bath solution is reduced from 160 mM to 95 mM, single-channel activity is initiated in cell-attached patches sealed before permeabilization. Conductance is outwardly rectifying with approximately 17 pS at negative and 57 pS at positive potentials. Single-channel currents reverse near the calculated equilibrium potential for Cl-. The averaged current shows inactivation at positive potentials. The current is blocked by 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB). An increase in ionic strength reversibly inhibits current activation. It is concluded that a decrease in ionic strength activates single-channel currents through VRAC rather than shifting the set point of a hypothetical volume sensor.

Surface physicochemical properties of Pseudomonas fluorescens and impact on adhesion and transport through porous media

The physiological state of an examined Pseudomonas fluorescens strain had a significant impact on its adhesion to glass surfaces and transport through glass-bead columns. In both batch and column studies collision efficiencies, α, for exponential phase cells were much larger ( α≈2–3) than for stationary or decay phase cells ( α≈0.5–0.7). Centrifugation of exponential phase cells substantially reduced collision efficiencies (α≈0.8). Over the examined range (0.02–0.2 M), ionic strength had no impact on cell attachment. The Lewis acid/base (A/B) character of the cell surface varied with physiological states: exponential phase cells exhibited larger values of the electron–donor parameter of the polar surface tension component, γ S −, than stationary or decay phase cells, resulting in larger calculated cell hydrophilicities. A reduction in exponential phase cell ζ-potential was observed upon centrifugation. Traditional Derjaguin–Landau–Verwey–Overbeek (DLVO) interaction energy profiles (between cells and glass surfaces) indicated energy maxima of the order of 90–130 kT, and secondary energy minima of less than 10 kT. Extended DLVO modeling predicted infinite energy barriers attributable to repulsive A/B interactions, and similar magnitude secondary energy minima. A pseudo-chemical kinetic approach was used to calculate activation energies of adhesion from experimental collision efficiencies. Collision efficiencies were also predicted from a diffusion-governed mass transport model incorporating interacting force fields. Predicted energy barriers underestimated cell collision efficiencies, suggesting that secondary energy minimum interactions governed initial attachment of cells. The partial reversibility of adhesion upon ionic strength reduction supported the secondary minimum interaction hypothesis.

99/01993 Apparatus and process for the regeneration of reversibly deactivated hydrocarbon-synthesis catalysts using slurry rejuvenation with synthesis gas disengagement: Behrmann, W. C. and Leviness, S. C. US 5,817,702 (Cl. 518–700; C07C27/00), 6 Oct 1998, Appl. 850,566, 2 May 1997, 8 pp.

Calanchi are badland-like erosional landforms, common in the Mediterranean region, which form from accelerated erosion processes. The calanchi slopes, naked or differently vegetated, can be considered as formed by different ecological tesserae originated by the mutual interaction of several factors such as erosion, geomorphology, microclimatic conditions, vegetation, ground cover, and pedogenesis. However, information about pedogenesis is rather scarce mainly because the soils developing on calanchi slopes are incessantly disturbed by erosion processes. To understand the role of soil evolution on landslide erosion, we considered three land facets each one made up of four tesserae (T1 to T4), which represented the different steps of soil and vegetal evolution of calanchi. The soil of each tessera was described, sampled by genetic horizons, and the samples were characterized for their physical, mineralogical and chemical properties. Field observations and laboratory data suggest that pedogenesis in the calanchi badlands may progress until a critical threshold. Indeed, advanced plant colonization and solum development improve soil structure, increase soil organic matter, and favor redistribution of nutrients along the profile. The improvement of structure at depth fosters water storage and clay dispersion through soil leaching and reduction of ionic strength of the soil solution, making soil less stable. Depending on the slope gradient, the soil weight acquired during rainfall events may trigger landsliding, mudflows, or collapses that rejuvenate the surface.

99/01996 Characteristics of hollow fly-ash of our country: Ri, J. N. Choson Minjujuui Inmin Konghwaguk Kwahagwon Tongbo, 1998, (1), 33–37. (In Korean)

Calanchi are badland-like erosional landforms, common in the Mediterranean region, which form from accelerated erosion processes. The calanchi slopes, naked or differently vegetated, can be considered as formed by different ecological tesserae originated by the mutual interaction of several factors such as erosion, geomorphology, microclimatic conditions, vegetation, ground cover, and pedogenesis. However, information about pedogenesis is rather scarce mainly because the soils developing on calanchi slopes are incessantly disturbed by erosion processes. To understand the role of soil evolution on landslide erosion, we considered three land facets each one made up of four tesserae (T1 to T4), which represented the different steps of soil and vegetal evolution of calanchi. The soil of each tessera was described, sampled by genetic horizons, and the samples were characterized for their physical, mineralogical and chemical properties. Field observations and laboratory data suggest that pedogenesis in the calanchi badlands may progress until a critical threshold. Indeed, advanced plant colonization and solum development improve soil structure, increase soil organic matter, and favor redistribution of nutrients along the profile. The improvement of structure at depth fosters water storage and clay dispersion through soil leaching and reduction of ionic strength of the soil solution, making soil less stable. Depending on the slope gradient, the soil weight acquired during rainfall events may trigger landsliding, mudflows, or collapses that rejuvenate the surface.

Casein micelle dispersions into water, NaCl and CaCl2: physicochemical characteristics of micelles and rennet coagulation

We have studied the effect of ionic strength from about 0 to 0.12 m on physicochemical characteristics of micelles and on gelation by rennet. Native phosphocaseinate powder was used as it exhibits attractive technological properties. The powder was dissolved at 25 g l−1 casein concentration with 0.2 g l−1 sodium azide into deionised water, NaCl or CaCl2 solutions, at ionic strength from 0 to 0.12 m. The pH of micelle suspensions was corrected to pH 6.6 and, after overnight equilibration at 20°C, pH values were checked and eventually corrected. After re-equilibration, calcium and phosphorus partition, micelle size, centrifuged pellet hydration were determined and κ-casein hydrolysis and gelation of the suspensions were followed after rennet addition. The increase in NaCl concentration of casein micelle suspensions resulted in calcium and phosphorus solubilisation from micelles, and increase in pellet hydration. In the same way, the first-order rate constant of the κ-casein hydrolysis with rennet was reduced, while CMP at the clotting time determined by Formagraph was unchanged. Rennet gelation was consequently retarded by NaCl increase. Suspensions of casein micelles into CaCl2 solutions did not exhibit calcium binding onto the micelles, as in milk, but a small reduction of diffusible phosphorus from 1.55 to 0.24 mm was shown. Pellet hydrations were unchanged. The increase in ionic strength with CaCl2 retarded κ-casein hydrolysis, with an enhanced effect compared to NaCl study. As the CMP at the rennet clotting time was strongly reduced by CaCl2 addition, the effect of CaCl2 increase is typically biphasic, with an initial decrease and a following increase in rennet clotting times. From a technological point of view, reduction of ionic strength of milk led to reduced rennet clotting time. Results are discussed in terms of physicochemical modifications of milk micelles and the effect of ionic strength on enzymic reaction.

Electrostatic Influence on the Kinetics of Ligand Binding to Acetylcholinesterase: DISTINCTIONS BETWEEN ACTIVE CENTER LIGANDS AND FASCICULIN

To explore the role that surface and active center charges play in electrostatic attraction of ligands to the active center gorge of acetylcholinesterase (AChE), and the influence of charge on the reactive orientation of the ligand, we have studied the kinetics of association of cationic and neutral ligands with the active center and peripheral site of AChE. Electrostatic influences were reduced by sequential mutations of six surface anionic residues outside of the active center gorge (Glu-84, Glu-91, Asp-280, Asp-283, Glu-292, and Asp-372) and three residues within the active center gorge (Asp-74 at the rim and Glu-202 and Glu-450 at the base). The peripheral site ligand, fasciculin 2 (FAS2), a peptide of 6.5 kDa with a net charge of +4, shows a marked enhancement of rate of association with reduction in ionic strength, and this ionic strength dependence can be markedly reduced by progressive neutralization of surface and active center gorge anionic residues. By contrast, neutralization of surface residues only has a modest influence on the rate of cationic m-trimethylammoniotrifluoroacetophenone (TFK+) association with the active serine, whereas neutralization of residues in the active center gorge has a marked influence on the rate but with little change in the ionic strength dependence. Brownian dynamics calculations for approach of a small cationic ligand to the entrance of the gorge show the influence of individual charges to be in quantitative accord with that found for the surface residues. Anionic residues in the gorge may help to orient the ligand for reaction or to trap the ligand. Bound FAS2 on AChE not only reduces the rate of TFK+ reaction with the active center but inverts the ionic strength dependence for the cationic TFK+ association with AChE. Hence it appears that TFK+ must traverse an electrostatic barrier at the gorge entry imparted by the bound FAS2 with its net charge of +4.

Role of Protein-Phospholipid Interactions in the Activation of ARF1 by the Guanine Nucleotide Exchange Factor Arno

Arno is a 47-kDa human protein recently identified as a guanine nucleotide exchange factor for ADP ribosylation factor 1 (ARF1) with a central Sec7 domain responsible for the exchange activity and a carboxyl-terminal pleckstrin homology (PH) domain (Chardin, P., Paris, S., Antonny, B., Robineau, S., Béraud-Dufour, S., Jackson, C. L., and Chabre, M. (1996) Nature 384, 481-484). Binding of the PH domain to phosphatidylinositol 4,5-bisphosphate (PIP2) greatly enhances Arno-mediated activation of myristoylated ARF1. We show here that in the absence of phospholipids, Arno promotes nucleotide exchange on [Delta17]ARF1, a soluble mutant of ARF1 lacking the first 17 amino acids. This reaction is unaffected by PIP2, which suggests that the PIP2-PH domain interaction does not directly regulate the catalytic activity of Arno but rather serves to recruit Arno to membranes. Arno catalyzes the release of GDP more efficiently than that of GTP from [Delta17]ARF1, and a stable complex between Arno Sec7 domain and nucleotide-free [Delta17]ARF1 can be isolated. In contrast to [Delta17]ARF1, full-length unmyristoylated ARF1 is not readily activated by Arno in solution. Its activation requires the presence of phospholipids and a reduction of ionic strength and Mg2+ concentration. PIP2 is strongly stimulatory, indicating that binding of Arno to phospholipids is involved, but in addition, electrostatic interactions between phospholipids and the amino-terminal portion of unmyristoylated ARF1GDP seem to be important. We conclude that efficient activation of full-length ARF1 by Arno requires a membrane surface and two distinct protein-phospholipid interactions: one between the PH domain of Arno and PIP2, and the other between amino-terminal cationic residues of ARF1 and anionic phospholipids. The latter interaction is normally induced by insertion of the amino-terminal myristate into the bilayer but can also be artificially facilitated by decreasing Mg2+ and salt concentrations.

Induction of tyrosine phosphorylation and Na+/H+ exchanger activation during shrinkage of human neutrophils.

The ubiquitous isoform of the Na+/H+ exchanger (NHE1) is essential for the regulation of cellular volume. The underlying molecular mechanism, which is poorly understood, was studied in human polymorphonuclear leukocytes (PMN). Suspension of PMN in hypertonic media induced rapid cellular shrinkage and activation of NHE1, which is measurable as a cytosolic alkalinization. Concomitantly, hypertonic stress also induced extensive tyrosine phosphorylation of several proteins. Pretreatment of PMN with genistein, a tyrosine kinase inhibitor, prevented not only the tyrosine phosphorylation in response to a hypertonic shock but also the activation of NHE1. The signal elicited by hyperosmolarity that induces activation of tyrosine kinases and NHE1 was investigated. Methods were devised to change medium osmolarity without altering cell volume and vice versa. Increasing medium and intracellular osmolarity in normovolemic cells failed to activate tyrosine kinases or NHE1. However, shrinkage of cells under iso-osmotic conditions stimulated both tyrosine phosphorylation and NHE1 activity. These findings imply that cells detect alterations in cell size but not changes in osmolarity or ionic strength. The identity of the proteins that were tyrosine-phosphorylated in response to cell shrinkage was also investigated. Unexpectedly, the mitogen-activated protein kinases SAPK, p38, erk1, and erk2 were not detectably phosphorylated or activated. In contrast, the tyrosine kinases p59(fgr) and p56/59(hck) were phosphorylated and activated upon hypertonic challenge. We propose that cells respond to alterations in cell size, but not to changes in osmolarity, with increased tyrosine phosphorylation, which in turn leads to the activation of NHE1. The resulting changes in ion content and cytosolic pH contribute to the restoration of cell volume in shrunken cells.

The Glycosaminoglycan Binding Site Governs Ligand Binding to the Somatomedin B Domain of Vitronectin

The ligand binding functions of vitronectin (Vn) are regulated by its conformational state/degree of multimerization. In the native plasma form of Vn, the C-terminal glycosaminoglycan (GAG) binding domain is believed to be cryptic. Here, evidence is provided that the addition of fucoidan or dextran sulfate to unfractionated plasma results in the formation of covalently and non-covalently stabilized Vn multimers. These multimers express conformationally sensitive antibody epitopes and ligand binding sites located in the N terminus of the Vn molecule. While heparin forms complexes with monomeric plasma Vn and induces conformational changes, a reduction in ionic strength is required for induction of multimerization. In addition, heparin serves as a template for the assembly of type 1 plasminogen activator inhibitor-induced disulfide-linked Vn multimers. These results support a new model for the structure of native Vn. The C-terminal GAG binding domain is predicted to be exposed in the native conformation, whereas the N terminus is cryptic. Ligand binding to the GAG binding site unfolds the N terminus, thereby exposing cryptic ligand binding sites.

Chemical Factors Influencing Colloid-Facilitated Transport of Contaminants in Porous Media

The effects of colloids on the transport of two strongly sorbing solutesa hydrophobic organic compound, phenanthrene, and a metal ion, Ni2+were studied in sand-packed laboratory columns under different pH and ionic strength conditions. Two types of column experiments were performed as follows: (i) sorption/mobilization experiments where the contaminant was first sorbed in the column under conditions where no colloids were released and mobilized under conditions where colloids were released as a result of ionic strength reduction in the influent; and (ii) transport experiments where the contaminant, dissolved or sorbed on colloids, was injected into columns packed with a strongly sorbing porous medium. In the first type of experiment, contaminant mobilization was significant only when all releasable colloids were flushed from the column. In all other cases, although high colloid particle concentrations were encountered, there was no marked effect on total contaminant concentrations. In the second type of ...

Reversibility of shrinkage of mineral acid casein curd as a function of ionic strength, pH and temperature

SummaryMineral acid casein (MAC) curd was subjected to a range of ionic strength, pH and temperature conditions to explore their effects on curd water-holding capacity, particularly the reversibility of shrinkage. The shrinkage of MAC curd on whey removal during the washing stage of casein manufacture could be attributed almost completely to the reduction in ionic strength and not specifically to either calcium or lactose removal. This shrinkage on whey removal during washing could not be reversed by adding whey back to the curd to attain the original ionic environment. In this sense it was irreversible. The pH during formation of MAC curd was more important than later pH adjustments in controlling curd water-holding capacity. The effect of a high precipitation pH in creating a tough, compact curd could not be reversed by subsequently decreasing the pH. However, the effect of a low precipitation pH in creating a soft, open curd could be partly reversed by subsequently increasing the pH. MAC curd shrank by 45% when the temperature was increased from 20 to 80°C. This temperature-dependent shrinkage was virtually fully reversible on cooling. It is suggested that the water-holding capacity of casein curd is governed largely by hydrophobic interactions and that once such interactions have occurred they cannot easily be disrupted by small changes in either pH or ionic strength.

Effects of ionic strength on colloid deposition and release

The deposition and release of colloidal contaminants in groundwaters may be important with respect to the environmental fate of contaminants and the remediation of subsurface contamination. The objective of this paper is to present results of laboratory studies that address the effect of ionic strength on model colloid (polystyrene spheres) deposition and release. Solutions of different ionic strength at near-neutral pH were pumped through a horizontally oriented, 40 cm long column of 0.46 mm silica sand at a velocity of 1 m per day. Pulse inputs of 1.09 μm fluorescent particles were made and the column effluent was continuously monitored with a fluorometer. After complete breakthrough of the pulse, the solution ionic strength was reduced and the subsequent effects on particle release were observed. Deposition results show evidence of the effects of hydrodynamic chromatography and retardation. As expected, deposition increases with increasing ionic strength. The extent of release due to ionic strength reduction depends on the magnitude of the change in ionic strength and the extent of prior deposition.

Effect of Ionic Strength and Outer Surface Charge on the Mechanical Stability of the Erythrocyte Membrane: A Linear Hydrodynamic Analysis

A linear hydrodynamic analysis in normal modes has been developed for a membrane model that simulated the erythrocytes membrane. The characteristic dispersion equation was obtained by solving the linearized dynamic equations. Values of the parameters related to the erythrocyte membrane were used to obtain the critical stability curves as a function of cell surface electric charge and suspending phase ionic strength. According to this model, the mechanical stability of the membrane increases with the decrease of the negative charge value on the outer membrane surface. The conclusion is that the transmembrane-potential and surface-charge changes that follow the reduction in ionic strength were responsible for the stability increase observed in previously reported experiments on heated erythrocytes.

Effects of channel modulation and pH on I sK inhibition by the novel class III antiarrhythmic azimilide (NE-10064)

Inhibition of human I sK channels expressed in Xenopus oocytes by the novel class III antiarrhythmic azimilide was studied under distinct treatments known to increase I sK (hypotonic solution, A23187 and isoproterenol). Azimilide inhibited I sK under all conditions with similar potency. Reduction of ionic strength or pH changes from pH 6.5 to 8.5 did not alter I sK amplitude. However, inhibition of I sK by azimilide was decreased by reduced pH, but not by reduced ionic strength. Further, the apparent affinity of azimilide was increased more than tenfold by increasing pH from 6.5 to 8.5. The data suggest that the neutral form of azimilide, a weak base, inhibits I sK via a lipophilic protein-drug interaction. pH-dependence of azimilide may significantly alter its effects on I sK under distinct pathophysiological conditions (acidosis vs. alkalosis) and in distinct locations (heart vs. kidney).