Changes In Membrane Surface Potential Research Articles

Cation‐Selective Channel Regulated by Anions According to Their Hofmeister Ranking

AbstractSpecificity of small ions, the Hofmeister ranking, is long‐known and has many applications including medicine. Yet it evades consistent theoretical description. Here we study the effect of Hofmeister anions on gramicidin A channels in lipid membranes. Counterintuitively, we find that conductance of this perfectly cation‐selective channel increases about two‐fold in the H2PO4−<Cl−≈Br−≈NO3−<ClO4−<SCN− series. Channel dissociation kinetics show even stronger dependence, with the dwell time increasing about 20‐fold. While the conductance can be quantitatively explained by the changes in membrane surface potential due to exclusion of kosmotropes from (or accumulation of chaotropes at) the surface, the kinetics proved to be more difficult to treat. We estimate the effects of changes in the energetics at the bilayer surfaces on the channel dwell time, concluding that the change would have to be greater than typically observed for the Hofmeister effect outside the context of the lipid bilayer.

Cation-Selective Channel Regulated by Anions According to Their Hofmeister Ranking.

Specificity of small ions, the Hofmeister ranking, is long-known and has many applications including medicine. Yet it evades consistent theoretical description. Here we study the effect of Hofmeister anions on gramicidin A channels in lipid membranes. Counterintuitively, we find that conductance of this perfectly cation-selective channel increases about two-fold in the H2 PO4- <Cl- ≈Br- ≈NO3- <ClO4- <SCN- series. Channel dissociation kinetics show even stronger dependence, with the dwell time increasing about 20-fold. While the conductance can be quantitatively explained by the changes in membrane surface potential due to exclusion of kosmotropes from (or accumulation of chaotropes at) the surface, the kinetics proved to be more difficult to treat. We estimate the effects of changes in the energetics at the bilayer surfaces on the channel dwell time, concluding that the change would have to be greater than typically observed for the Hofmeister effect outside the context of the lipid bilayer.

Cyclodextrins Deliver FA to Membranes Rapidly while Maintaining a Low Concentration of Unbound FA in Water

Cyclodextrins (CDs) have a polar surface and a hydrophobic cavity that can bind and solubilize long chain fatty acids (FA), while remaining soluble in water. In this study we tested the usefulness of CD for acting as a FA donor to model membranes (egg-PC vesicles). We employed i) fluorescein phosphatidylethanolamine (FPE) to detect changes in membrane surface potential when FA anions adsorb to the outer membrane leaflet ii) entrapped pyranine to measure changes in pH after FA diffusion (flip-flop) across the lipid bilayer and iii) ADIFAB to measure the concentration of unbound FA in water. Upon mixing of oleic acid (OA)/CD complexes with vesicles containing either FPE or pyranine, OA rapidly dissociated from CD, bound to the membrane and underwent flip-flop within 1s (online experiments). In the presence of vesicles, CD maintained the concentration of unbound OA in water low, at almost the same levels measured with albumin. In stopped-flow fluorometry, the mixing of OA/CD with vesicles produced a single exponential decrease in the fluorescence of FPE and pyranine (t1/2 < 50 ms). These kinetics reflect the FA dissociation from CD since FA binding to the membrane and flip-flop are very fast (t1/2 < 10 ms). The delivery of OA by CD was faster than those observed with lipid vesicles (t1/2 = 100 ms) and albumin (t1/2 = 200 ms). Thus, CD is an excellent vehicle to FA and deliver FA to membranes very rapidly while maintaining a low concentration of unbound FA.

Calcium-induced aggregation of archaeal bipolar tetraether liposomes derived from the thermoacidophilic archaeon Sulfolobus acidocaldarius.

Previously, we showed that the proton permeability of small unilamellar vesicles (SUVs) composed of polar lipid fraction E (PLFE) from the thermoacidophilic archaeon Sulfolobus acidocaldarius was remarkably low and insensitive to temperature (Komatsu and Chong 1998). In this study, we used photon correlation spectroscopy to investigate the time dependence of PLFE SUV size as a function of Ca2+ concentration. In the absence of Ca2+, vesicle diameter changed little over 6 months. Addition of Ca2+, however, immediately induced formation of vesicle aggregates with an irregular shape, as revealed by confocal fluorescence microscopy. Aggregation was reversible upon addition of EDTA; however, the reversibility varied with temperature as well as incubation time with Ca2+. Freeze-fracture electron microscopy showed that, after a long period of incubation (2 weeks) with Ca2+, the PLFE vesicles had not just aggregated, but had fused or coalesced. The initial rate of vesicle aggregation varied sigmoidally with Ca2+ concentration. At pH 6.6, the threshold calcium concentration (Cr) for vesicle aggregation at 25 and 40 degrees C was 11 and 17 mM, respectively. At pH 3.0, the Cr at 25 degrees C increased to 25 mM. The temperature dependence of Cr may be attributable to changes in membrane surface potential, which was -22.0 and -13.2 mV at 25 and 40 degrees C, respectively, at pH 6.6, as determined by 2-(p-toluidinyl)naphthalene-6-sulfonic acid fluorescence. The variation in surface potential with temperature is discussed in terms of changes in lipid conformation and membrane organization.

Role of membrane surface potential and other factors in the uptake of non‐transferrin‐bound iron by reticulocytes

Reticulocytes suspended in low ionic strength media such as isotonic sucrose solution efficiently take up non-transferrin-bound iron and utilize it for heme synthesis. The present study was undertaken to determine how such media facilitate iron utilization by the cells. The effects of changes in membrane surface potential, membrane permeability, cell size, transmembrane potential difference, oxidation state of the iron, and lipid peroxidation were investigated. Iron uptake to heme, cytosol, and stromal fractions of cells was measured using rabbit reticulocytes incubated with 59Fe-labelled Fe(II) in 0.27 M sucrose, pH 6.5. Suspension of the cells in sucrose led to increased membrane permeability, loss of intracellular K+, decreased cell size, and increased transmembrane potential difference. However, none of these changes could account for the high efficiency of iron uptake which was observed. The large negative membrane surface potential which occurs in sucrose was modified by the addition of mono-, di-, tri-, and polyvalent cations to the solution. This inhibited iron uptake to a degree which for many cations varied with their valency. Other cations (Mn2+, Co2+, Ni2+, Zn2+) were also very potent inhibitors, probably due to direct action on the transport process. Ferricyanide inhibited iron uptake, while ferrocyanide and ascorbate increased the uptake of Fe(III) but not Fe(II). It is concluded that the high negative surface potential of reticulocytes suspended in sucrose solution facilitates iron uptake by aiding the approach of iron to the transport site on the cell membrane. The iron is probably transported into the cell in the ferrous form.

Voltage dependence of two inward currents carried by calcium and barium in the ciliate Stylonychia mytilus.

Two voltage-dependent inward currents in the fresh-water hypotrichous ciliate Stylonychia mytilus have been investigated, using two intracellular micro-electrodes, when either Ca ions or Ba ions are the charge carriers. In cells bathed in Ca-free Ba solution the two inward currents, named current I and current II, could be identified and studied in the absence of outward currents. The two inward currents could also be separated by addition of the plant lectin concanavalin A (0.5 microgram/ml) to the external medium, which resulted in the selective inhibition of current I. When the holding potential was set at values between -45 and -65 mV (normal resting potential is -50 mV), current I was shifted parallel to the holding potential along the voltage axis. This shift was 7.6 mV per 10 mV change in holding potential. The amplitude and voltage relationship of current II was not affected by these changes in the holding potential. The amplitude of current I in Ba solution was maximal when the membrane potential was held at -55 mV; it decreased with higher and lower holding potentials. The rate of activation of current I remained virtually unaffected at holding potentials between -45 and -60 mV, and was somewhat reduced at a holding potential of -65 mV. When the extracellular Ca concentration was varied between 0.1 and 5.0 mM, or when the cells were loaded with EGTA to reduce the intracellular level of ionized Ca, the resting membrane potential and the voltage relationships of both current I and II and of the outward current were shifted along the voltage axis according to the expected changes in membrane surface potential. Double-pulse experiments with varying interval potentials suggested voltage-dependent inactivation of current I and Ca-dependent inactivation of current II. Pre-hyperpolarizing steps of only 1 mV amplitude and 30 ms duration could result in the activation of current I, indicating that the activation voltage of current I closely followed the actual membrane potential. Hence, the same voltage steps elicited similar current I amplitudes with holding potentials between -45 and -60 mV. The results indicate that current II displays voltage properties described for Ca channels in other ciliates and many multicellular preparations, while current I shows an unusual voltage behaviour, which might be regarded as an 'adaptive type of excitation'.(ABSTRACT TRUNCATED AT 400 WORDS)