Addition Of Gramicidin Research Articles

The antimicrobial peptide gramicidin S alters proliferation and inhibits adhesion of L929 cell line fibroblasts

Background: Natural antimicrobial peptides are used in the fight against pathogens resistant to existing synthetic antibiotics. The non-specific mechanism of cytostatic action of antimicrobial peptides, in particular gramicidin S, against bacteria is also effective for damaging the cells of neoplasms. The existence of such a property in a registered antibiotic will indicate its antineoplastic potential and can be used to expand the spectrum of its therapeutic application. Aim of work is to clarify the possible antitumor effect of the antimicrobial peptide gramicidin S. Materials and Methods: Using the methods of confocal laser microscopy and light microscopy, the morphological and functional features of connective tissue cells under the influence of gramicidin S in the concentration range 0.5–50 μg/ml were studied using L929 fibroblasts cell culture. The cell area, nucleus area, and nucleus-to-cytoplasm ratio were determined. To study the migratory and proliferative activity of cells in vitro, the “scratch assay” was used, the confluency of the monolayer of cells was evaluated, morphometric studies were performed, and the relative area of the scratch was measured after 24, 48, and 72 hours. Results: The lytic effect of gramicidin S in a concentration of 50 μg/ml on L929 cells was established, in concentrations of 0.5 μg/ml and 5.0 μg/ml, the antibiotic increases the synthetic activity of cells and stimulates the proliferation of fibroblasts in a monolayer. Cell anisomorphism is more pronounced in the presence of 5.0 μg/ml gramicidin S added to the culturing medium during monolayer formation, while a one-third of the cells in the sample form a population that is morphologically different from other cells in the culture. The addition of gramicidin S at non-lytic concentrations of 0.5 and 5.0 μg/ml to unattached fibroblasts reliably inhibits monolayer formation. Under the influence of 5.0 μg/ml gramicidin S, the rate of monolayer formation is low, even despite the significant content of cells with a high nuclear-cytoplasmic ratio. The kinetics of filling the cell monolayer defect using the “scratch assay” shows that GS in concentrations of 0.5 and 5.0 μg/ml can control the migratory and proliferative properties of L929 cells. Conclusions: The effect of gramicidin S on the morphometric parameters of cells depends on the concentration of the peptide and the cell status in the culture. GS corrupts the adhesive properties of L929 fibroblasts in monolayer cell culture and the rate of cell monolayer formation. Cells at the stage of attachment and monolayer formation were most sensitive to non-lytic concentrations of GS. Inhibition of the adhesive properties of connective tissue cells by gramicidin S is a new non-canonical effect of a known antimicrobial drug, which may indicate the possibility of using gramicidin S as an anti-neoplasm agent.

Ultrafast Fluctuations of High Amplitude Electric Fields in Lipid Membranes.

Understanding the electrostatics of lipid membranes at a molecular level has the potential to offer great insight into the mechanism of voltage-gated processes but has been challenging to study experimentally. In this study, we characterize the equilibrium electric field fluctuations at the interfacial region of lipid bilayers by using a combination of ultrafast time-resolved infrared spectroscopy, molecular dynamics (MD) simulations, and spectral modeling. By monitoring the dynamics of the ester carbonyl stretching vibration in hydrated phosphocholine lipid bilayers, we are able to measure a correlation function for the femtosecond and picosecond fluctuations in the local electric field of the membrane, estimating the standard deviation in these fluctuations as ∼10MV/cm. The addition of gramicidin D at a 1:20 mol ratio with DMPC results in the formation of protein-lipid hydrogen bonds, which alter the dynamics of the ester groups. Using MD simulations, we conclude that ultrafast local field fluctuations exist whether or not water interacts with the ester groups; however, water does accelerate the time-scale of these fluctuations.

Effect of the Structure of Cholesterol‐Based Tethered Bilayer Lipid Membranes on Ionophore Activity

Tethered bilayer lipid membranes (tBLM) are formed on 1) pure tether lipid triethyleneoxythiol cholesterol (EO(3)C) or on 2) mixed self-assembled monolayers (SAMs) of EO(3)C and 6-mercaptohexanol (6MH). While EO(3)C is required to form a tBLM with high resistivity, 6MH dilutes the cholesterol content in the lower leaflet of the bilayer forming ionic reservoirs required for submembrane hydration. Here we show that these ionic reservoirs are required for ion transport through gramicidin or valinomycin, most likely due to the thermodynamic requirements of ions to be solvated once transported through the membrane. Unexpectedly, electrochemical impedance spectroscopy (EIS) shows an increase of capacitance upon addition of gramicidin, while addition of valinomycin decreases the membrane resistance in the presence of K(+) ions. We hypothesise that this is due to previously reported phase separation of EO(3)C and 6MH on the surface. This results in ionic reservoirs on the nanometre scale, which are not fully accounted for by the equivalent circuits used to describe the system.

Fabrication of highly insulating tethered bilayer lipid membrane using yeast cell membrane fractions for measuring ion channel activity

A tethered bilayer lipid membrane (tBLM) was fabricated on a gold electrode using 1,2-dipalmitoyl- sn-glycero-phosphothioethanol as a tethering lipid and the membrane fractions of Saccharomyces pombe yeast cells to deposit the upper leaflet. The membrane fractions were characterized using transmission electron microscopy and dynamic light scattering and found to be similar in size to small unilamellar vesicles of synthetic lipids. The dynamics of membrane-fraction deposition and rupture on the tethering-lipid layer were measured using quartz crystal microgravimetry. The electrochemical properties of the resulting tBLM were characterized using electrical impedance spectroscopy and cyclic voltammetry. The tBLM's electrical resistance was greater than 1 M Ω cm 2 , suggesting a defect-free membrane. The suitability of tBLM produced using membrane fractions for measuring ion-channel activities was shown by a decrease in membrane resistance from 1.6 to 0.43 M Ω cm 2 following addition of gramicidin. The use of membrane fractions to form high-quality tBLM on gold electrodes suggests a new approach to characterize membrane proteins, in which the upper leaflet of the tBLM is deposited, and overexpressed membrane proteins are incorporated, in a single step. This approach would be especially useful for proteins whose activity is lost or altered during extraction, purification, and reconstitution, or whose activities are strongly influenced by the lipid composition of the bilayer.

Estimation of paracellular conductance of primary rat alveolar epithelial cell monolayers.

Freshly isolated rat type II pneumocytes, when grown on permeable tissue culture-treated polycarbonate filters, form confluent alveolar epithelial cell monolayers (RAECM). Cells in RAECM undergo transdifferentiation, exhibiting over time morphological and phenotypic characteristics of type I pneumocytes in vivo. We recently reported that transforming growth factor-beta(1) (TGF-beta(1)) decreases overall monolayer resistance (R(te)) and stimulates short-circuit current in a dose-dependent manner. In this study, we investigated the effects of TGF-beta(1) (50 pM) or 10% newborn bovine serum (NBS) on modulation of paracellular passive ion conductance and its contribution to total passive ion conductance across RAECM. On days 5-7 in culture, tight-junctional resistance (R(tj), kOmegacm(2)) of RAECM, cultured in minimally defined serum-free medium (MDSF) with or without TGF-beta(1) or NBS, was estimated from the relationship between observed transmonolayer voltage and resistance after addition of gramicidin D to apical potassium isethionate Ringer solution under open-circuit conditions. NaCl Ringer solution bathed the basolateral side throughout the experimental period. Results showed that transmonolayer conductance (1/R(te)) and tight-junctional conductance (1/R(tj)) are 0.59 and 0.14 mS/cm(2) for control monolayers in MDSF, 1.59 and 0.38 mS/cm(2) for monolayers exposed to TGF-beta(1), and 0.38 and 0.18 mS/cm(2) for monolayers grown in the presence of NBS. The contributions to total transepithelial ion conductance by the paracellular pathway are estimated to be 23, 23, and 47% for control, TGF-beta(1)-exposed, and newborn bovine serum (NBS)-treated RAECM, respectively.

NADH oxidation and NAD+ reduction catalysed by tightly coupled inside-out vesicles from Paracoccus denitrificans.

Tightly coupled inside-out vesicles were prepared from Paracoccus denitrificans cells (SPP, sub-Paracoccus particles) and characterized kinetically. The rate of NADH oxidation, catalysed by SPP, increases 6-8 times on addition of gramicidin. The vesicles are capable of catalysing Delta micro H+-dependent reverse electron transfer from quinol to NAD+. The kinetic parameters of the NADH-oxidase and the reverse electron transfer carried out by membrane-bound P. denitrificans complex I were estimated and compared with those of the mitochondrial enzyme. The data demonstrate that catalytic properties of the dinucleotide-binding site of the bacterial and mitochondrial complex I are almost identical, pointing out similar organization of the site in mammals and P. denitrificans. Inhibition of the bacterial complex I by a specific inhibitor of Q reduction, rotenone, is very different from that of the mitochondrial enzyme. The inhibitor is capable of suppressing the NADH oxidation reaction only at micromolar concentrations, while the activity of mitochondrial enzyme is suppressed by nanomolar concentrations of rotenone. In contrast to the mitochondrial enzyme, rotenone, even at concentrations as high as 10 micro m, does not inhibit the reverse, Delta micro H+-dependent NAD+-reductase reaction on SPP.

A “Minimal” Sodium Channel Construct Consisting of Ligated S5-P-S6 Segments Forms a Toxin-activatable Ionophore

The large size (six membrane-spanning repeats in each of four domains) and asymmetric architecture of the voltage-dependent Na+ channel has hindered determination of its structure. With the goal of determining the minimum structure of the Na+ channel permeation pathway, we created two stable cell lines expressing the voltage-dependent rat skeletal muscle Na+ channel (micro1) with a polyhistidine tag on the C terminus (muHis) and pore-only micro1 (muPore) channels with S1-S4 in all domains removed. Both constructs were recognized by a Na+ channel-specific antibody on a Western blot. muHis channels exhibited the same functional properties as wild-type micro1. In contrast, muPore channels did not conduct Na+ currents nor did they bind [3H]saxitoxin. Veratridine caused 40 and 54% cell death in muHis- and muPore-expressing cells, respectively. However, veratridine-induced cell death could only be blocked by tetrodotoxin in cells expressing muHis, but not muPore. Furthermore, using a fluorescent Na+ indicator, we measured changes in intracellular Na+ induced by veratridine and a brevotoxin analogue, pumiliotoxin. When calibrated to the maximum signal after addition of gramicidin, the maximal percent increases in fluorescence (deltaF) were 35 and 31% in cells expressing muHis and muPore, respectively. Moreover, in the presence of 1 microm tetrodotoxin, deltaF decreased significantly to 10% in muHis- but not in muPore-expressing cells (43%). In conclusion, S5-P-S6 segments of micro1 channels form a toxin-activable ionophore but do not reconstitute the Na+ channel permeation pathway with full fidelity.

Effect of Gramicidin A on the Dipole Potential of Phospholipid Membranes

The effect of channel-forming peptide gramicidin A on the dipole potential of phospholipid monolayers and bilayers has been studied. Surface pressure and surface potential isotherms of monolayers have been measured with a Langmuir trough equipped with a Wilhelmy balance and a surface potential meter (Kelvin probe). Gramicidin has been shown to shift pressure-area isotherms of phospholipids and to reduce their monolayer surface potentials. Both effects increase with the increase in gramicidin concentration and depend on the kind of phosphatidylcholine used. Application of the dual-wavelength ratiometric fluorescence method using the potential-sensitive dye RH421 has revealed that the addition of gramicidin A to dipalmitoylphosphatidylcholine liposomes leads to a decrease in the fluorescence ratio of RH421. This is similar to the effect of phloretin, which is known to decrease the dipole potential. The comparison of the concentration dependences of the fluorescence ratio for gramicidin and phloretin shows that gramicidin is as potent as phloretin in modifying the membrane dipole potential.

Sequestration of depolarization-induced Ca2+ loads by mitochondria and Ca2+ efflux via mitochondrial Na+/Ca2+ exchanger in bovine adrenal chromaffin cells.

We used fura-2 microfluorometry to investigate the role of mitochondria in regulating the increase in the cytosolic Ca2+ concentration ([Ca]in) and the mechanism(s) underlying the subsequent Ca2+ efflux from mitochondria in bovine adrenal chromaffin cells. The rate of [Ca]in decay during and following stimulation with 100 mM KCl depolarization was markedly increased when the mitochondrial Na+/Ca2+ exchanger was inhibited by clonazepam or CGP-37157(CGP). In contrast, the addition of gramicidin, which increased the cytosolic Na+ concentration, following KCl depolarization caused a secondary increase in [Ca]in. This secondary increase in [Ca]in was prevented by the addition of clonazepam or CGP, and by the removal of external Na+. The subsequent removal of clonazepam or CGP, or the delayed addition of Na+ caused a slow increase in [Ca]in. A protonophore (FCCP) applied following KCl depolarization also caused a robust, secondary increase in [Ca]in, which was insensitive to blocking by clonazepam or CGP. Neither gramicidin nor FCCP altered the [Ca]in decay when applied following stimulation with histamine or caffeine, which mobilized Ca2+ from intracellular stores. These results suggest that the large [Ca]in increase induced by Ca2+ influx, but not by intracellular Ca2+ release, is buffered by mitochondria, and that the mitochondrial Na+/Ca2+ exchanger makes a major contribution to the subsequent Ca2+ efflux from mitochondria.

Nonmediated flip-flop of anionic phospholipids and long-chain amphiphiles in the erythrocyte membrane depends on membrane potential.

The nonmediated inward translocation (flip) of the anionic fluorescent N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)- (NBD-)labeled phospholipid phosphatidylmethanol (PM) from the outer to the inner membrane leaflet of human erythrocytes and vice versa depends on membrane potential. Interestingly, inside-positive potentials due to chloride gradients and the native chloride conductance of the cells resulted in an increase of the flip rates. This flip enhancement could be suppressed by addition of gramicidin D, which increases cation conductance, or 4,4'-diisothiocyanatostilbene-2,2'-disufonate (DIDS), which inhibits anion conductance. Conversely, inside negative potentials established by an outward-directed K+ gradient in the presence of gramicidin on DIDS-treated cells resulted in a decrease of flip rate. Flip rate exhibited an exponential dependence on membrane potential. The opposite effects of the positive and negative potentials were obtained for the outward translocation (flop) from the inner to the outer membrane leaflet. Similar potential dependencies were found for the nonmediated flip of anionic NBD-labeled phosphatidic acid (PA) and 2-(N-decyl)aminonaphthalene-6-sulfonic acid (2,6-DENSA) following blockage of the band-3-mediated component of flip. The membrane potential also influences the stationary distribution of the anionic lipids between the inner and outer leaflets. The distribution is shifted to the inner leaflet by increasingly positive potentials and to the outer leaflet by increasingly negative potentials. It is concluded that nonmediated flip-flop of the anionic phospholipids and the long-chain sulfonate represents electrogenic translocation of the unprotonated charged lipids across the hydrophobic barrier.

Potassium conductance activated during regulatory volume decrease by mudpuppy red blood cells.

The cellular basis of regulatory volume decrease (RVD) by mudpuppy (Necturus maculosus) red blood cells (RBCs) was examined. Volume regulation was inhibited by replacing extracellular Na+ with K+. In contrast, addition of gramicidin (5 microM) to the extracellular medium enhanced RVD. The K(+)-channel blocker quinine (1 mM) also inhibited RVD, and this inhibition was reversed by gramicidin (5 microM). In addition, a 0 Ca(2+)-EGTA Ringer blocked RVD, whereas the Ca2+ ionophore A23187 ( microM) enhanced recovery of cell volume. The stretch-activated ion channel antagonist gadolinium (10 microM) inhibited RVD, and this effect was reversed by A23187 (2 microM). Furthermore, the calmodulin inhibitors pimozide (10 microM) and N-(6-aminohexyl)-5-chloro-1-napthalene-sulfonamide (0.1 mM) blocked RVD, and this inhibition was reversed with gramicidin (5 microM). Consistent with these findings, a K(+)-selective membrane conductance was activated by exposing RBCs to a 0.5x Ringer solution (observed with the whole cell patch clamp technique). This conductance was inhibited by quinine (1 mM), gadolinium (10 microM), and pimozide (10 microM). These results indicate that cell swelling activates a K+ conductance by a Ca(2+)-calmodulin-dependent mechanism and that this channel mediates K+ loss during RVD.

Phase equilibria and molecular packing in the N,N-dimethyldodecylamine oxide/gramicidin D/water system studied by 2H nuclear magnetic resonance spectroscopy

A partial phase diagram of the system N,N-dimethyldodecylamine oxide (DDAO)/water/gramicidin D was determined by 2H-NMR. Both 2H2O and perdeuterated DDAO (DDAO-d31) were studied by solid state NMR techniques. Addition of gramicidin D to the micellar (L1), normal hexagonal (HI) and cubic (I) phases of DDAO induces phase separations, giving two-phase regions, which all contain a lamellar (L alpha) phase. The L alpha phase containing gramicidin is characterized by larger order parameters for DDAO-d31 compared with the corresponding order parameters in the L alpha and HI phases of DDAO-d31/H2O. The L alpha phase may stay in equilibrium with any other phase in the phase diagram. The DDAO exchange between the coexisting phases is slow on the NMR timescale, which is why the recorded NMR spectrum consists of superimposed spectra from the different phases occurring in the sample. Gramicidin D can be solubilized in appreciable quantities only in the lamellar phase of DDAO-d31. Increasing amounts of gramicidin in the liquid crystalline phases result in a continuous increase in the molecular ordering up to about 5 mol% gramicidin, where a plateau is reached. This is consistent with a recent theoretical model describing the influence on the ordering of lipids by a membrane protein with larger hydrophobic thickness than the lipid bilayer. The solvent used for dissolving gramicidin at the incorporation of the peptide in the lipid aggregates has no effect on the 2H-NMR lineshapes of DDAO-d31. It is concluded that gramicidin is solubilized in the L alpha phase and that it always adopts the channel conformation independent of a particular solvent. The channel conformation is also supported by CD studies. In some of the samples, macroscopic orientation of the lipid aggregates is observed. It is concluded that DDAO-d31 in the binary system favors an orientation with the long axis of the hydrocarbon chain perpendicular to the magnetic field, whereas when gramicidin D is present the hydrocarbon chain orients parallel to the magnetic field. This is explained by the fact that gramicidin aligns with its helical axis parallel to the magnetic field, thereby forcing also the DDAO-d31 molecules to obtain such an orientation.

Sugar Transport into Storage Tubers of Stachys sieboldii Miq.: Evidence for Symplastic Unloading and Stachyose Uptake into Storage Vacuoles by an H+‐Antiport Mechanism*

AbstractFollowing assimilation of 14CO2 by leaves of Stachys sieboldii, 14C‐stachyose is translocated into the tubers. Stachyose is accumulated and stored in the vacuoles of the pith parenchyma. Protoplasts and vacuoles were isolated and the uptake of sugars was examined. Uptake of sucrose and sucrosyl oligosaccharides of the raffinose family by protoplasts was very low compared to glucose. Transport parameters for glucose indicated a carrier mediated transport in the lower concentration range which was superimposed by diffusion at higher concentrations (> 10 mM). The very low sugar uptake by protoplasts and the sparse enzyme activities of stachyose synthase in the storage parenchyma as well as acid invertase and α‐galactosidase in the cell walls indicated symplastic unloading of stachyose in the tubers. Experiments on 14C‐stachyose uptake by isolated vacuoles confirmed previous observations by Keller (1992).Isolated vacuoles exhibited ATP and PP hydrolysis and were capable of generating a proton gradient across the tonoplast by a V‐type H+‐ATPase and H+‐PPase. This was demonstrated by fluorescence quenching of quinacrine. Fluorescence could be restored by the addition of gramicidin and partly recovered by the addition of stachyose; mannitol, sorbitol and glucose had no effect. Fluorescence recovery depended on the concentration of stachyose and revealed saturation kinetics (Km = 28 mM). Comparable results have been obtained with tonoplast vesicles by Greutert and Keller (1993).Experimental data presented here provide circumstantial evidence for symplastic unloading of stachyose in the tubers of Stachys sieboldii and demonstrate that the stachyose concentration in the cytoplasm of storage parenchyma cells is kept low by active stachyose transport into the vacuoles. The results suggest a stachyose/H+‐antiport system.

The electrochemical proton potential of Bacillus alcalophilus.

Bacillus alcalophilus strain ATCC 27647 showed usual growth characteristics, when inoculated at pH 10.4. The cells entered the logarithmic phase at pH 10.3, and as growth continued, the pH dropped further to a value of 8.8 in the stationary phase. B. alcalophilus strain DSM 485 showed comparable growth only in the initial phase after the addition to fresh medium. The small initial growth period was succeeded by a long lag phase, where the pH continuously dropped. The cells resumed growth after the pH was about 10.0 and continued to grow accompanied by a further decrease of external pH. The bioenergetic parameters measured in the initial growth phase of the two strains at high pH (10.1-10.3) were nearly the same, i.e. delta pH = +97 to +110 mV, delta psi = -206 to -213 mV and delta microH+ = -109 to -103 mV. The inverted proton gradient of about 1.7-1.9 at high pH decreased, as the external pH dropped during growth. This led to an increase of the proton motive force (delta microH+), although the membrane potential (delta psi) also declined. The ATP/ADP ratio of strain DSM 485 was high (4.5-5.5) at fast growth during the initial and second growth period. The ratio declined to about 1.5 at the end of the lag phase. At the initial growth phase and at the end of the lag phase, the delta microH+ was, however, the same (approximately -106 mV) and considerably lower than in the middle of the second growth period (approximately -140 mV). Fast growth, therefore, correlates with a high ATP/ADP ratio but not necessarily with a high delta microH+. Addition of gramicidin or carbonylcyanide m-chlorophenylhydrazone stopped growth of B. alcalophilus strain DSM 485 at pH 10.3 or 9.5 and gramicidin immediately decreased the internal ATP/ADP ratio from 4.5 to 1.2 at pH 10.3.

Separate K+ and Cl- transport pathways are activated for regulatory volume decrease in jejunal villus cells

We assessed ion transport mechanisms operative during regulatory volume decrease (RVD) in jejunal villus enterocytes, isolated in suspension from guinea pig jejunum and examined with electronic cell sizing. Immediately after reduction of osmolarity (153 mosmol/kg medium) enterocytes swelled, but within 5 min they shrank by 50%. This RVD, which was complete by 20 min, was unaffected by Li+ substitution for Na+ or by Na(+)-free (N-methyl-D-glucose, NMDG+) medium. Passive loss of K+ is required for RVD because both the magnitude and direction of RVD changed when external [K+] varied. Increasing K+ permeability with gramicidin (0.5 microM) accelerated RVD in NMDG+ medium (10.0 +/- 0.8 vs. 6.2 +/- 0.4% min-1, P less than 0.01) suggesting that K+ loss is rate limiting for RVD. Inhibition of K(+)- and Ca2(+)-activated K+ conductance with Ba2+ (5 mM, P less than 0.005), quinine (100 microM, P less than 0.005), or apamin (1 microM, P less than 0.005) prevented RVD. Inhibition of Cl- conductance with 9-anthracenecarboxylic acid (100 microM, P less than 0.005) or dipyridamole (75 microM, P less than 0.005) also prevented RVD. In isotonic HCO3(-)-buffered medium, the addition of gramicidin to cells generated conditions in which anion permeability was rate limiting for cell swelling. This swelling was inhibited 97% by 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). In K(+)-free, HCO3(-)-buffered medium containing DIDS and gramicidin hypotonic swelling resulted in continued (secondary) swelling (rel vol 1.19 +/- 0.01 vs. 1.25 +/- 0.02, P less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Gramicidin A induced fusion of large unilamellar dioleoylphosphatidylcholine vesicles and its relation to the induction of type II nonbilayer structures

The fusogenic properties of gramicidin were investigated by using large unilamellar dioleoylphosphatidylcholine vesicles. It is shown that gramicidin induces aggregation and fusion of these vesicles at peptide to lipid molar ratios exceeding 1/100. Both intervesicle lipid mixing and mixing of aqueous contents were demonstrated. Furthermore, increased static and dynamic light scattering and a broadening of 31P NMR signals occurred concomitant with lipid mixing. Freeze-fracture electron microscopy revealed a moderate vesicle size increase. Lipid mixing is paralleled by changes in membrane permeability: small solutes like carboxyfluorescein and smaller dextrans, FD-4(Mr approximately 4000), rapidly (1-2 min) leak out of the vesicles. However, larger molecules like FD-10 and FD-17 (Mr approximately 9400 and 17,200) are retained in the vesicles for greater than 10 min after addition of gramicidin, thereby making detection of contents mixing during lipid mixing possible. At low lipid concentrations (5 microM), lipid mixing and leakage are time resolved: leakage of CF shows a lag phase of 1-3 min, whereas lipid mixing is immediate and almost reaches completion during this lag phase. It is therefore concluded that leakage, just as contents mixing, occurs subsequent to aggregation and lipid mixing. Although addition of gramicidin at a peptide/lipid molar ratio exceeding 1/50 eventually leads to hexagonal HII phase formation and a loss of vesicle contents, it is concluded that leakage during fusion (1-2 min) is not the result of HII phase formation but is due to local changes in lipid structure caused by precursors of this phase. By making use of gramicidin derivatives and different solvent conformations, it is shown that there is a close parallel between the ability of the peptide to induce the HII phase and its ability to induce intervesicle lipid mixing and leakage. It is suggested that gramicidin-induced fusion and HII phase formation share common intermediates.

Regulation of the imbalance in light excitation between Photosystem II and Photosystem I by cations and by the energized state of the thylakoid membrane

The imbalance in photoactivity between the two photosystems in broken chloroplasts during steady-state electron transport was investigated using modulated chlorophyll a fluorimetry and oxygen evolution. No imbalance in favor of PS II (imbalance term equals zero) was found at low cation concentration (e.g., 10 mM NaCl) where the membranes are unstacked, while some imbalance in favor of PS II (imbalance term about 0.1–0.2) could be observed at ‘high’ cation medium (e.g., 100 mM for univalent, 5 mM for divalent and 100 μM for trivalent cations) where the membranes are stacked. At the high cation concentration the imbalance was particularly noticeable at a pH range 6–7.5 under conditions where the membranes were non-energized, e.g., in the presence of a range of uncouplers and ionophores (an imbalance term of between 0.44 and 1.1). In the absence of uncouplers or ionophores the imbalance term was initially high, decreasing to a low steady-state value during the light-induced energization of the membranes. The increase in the imbalance measured with the addition of gramicidin D was wavelength-dependent, implying changes in the allocation of excitation energy to the photosystems rather than any other mechanism. This effect was reversed at higher pH: At a pH higher than about 8, the imbalance in absence of uncouplers or ionophores was stronger than in their presence. The relation between the state of imbalance and the cross-membrane proton gradient (ΔpH) was not straightforward or simple, as follows: (i) Imbalance was induced by uncouplers and ionophores also at sufficiently low light intensities which produce only very small ΔpH. (ii) Valinomycin (+KCl) had the same effect as uncouplers like gramicidin D, nigericin, NH4Cl and others, although it presumably abolishes membrane potential but not ΔpH. (iii) The effect of gramicidin D and NH4Cl was close to saturation at concentrations which affect ΔpH still minutely. (iv) The same level of large imbalance could be achieved even without uncouplers or ionophores when a much higher cation concentration (approx. 10-fold) than that considered normal to achieve membrane stacking was used. It is therefore concluded that: (a) High cationic levels change the allocation of excitation energy, probably via the screening of negative charges on the thylakoid surface. (b) The effect of uncouplers and ionophores in the presence of high cationic levels most probably reflects, at least in part, the effect on external surface charges, their exposure by membrane energization and their screening by cations.

Volume-activated K+ and Cl- pathways of dissociated epithelial cells (MDCK): role of Ca2+

Osmotic swelling of dissociated Madin-Darby canine kidney (MDCK) cells in NaCl medium is followed by shrinking (regulatory volume decrease, or RVD) or in KCl medium by secondary swelling. The cation ionophore gramicidin has little effect on volumes of isotonic cells but accelerates volume-activated changes in either medium. Immediately after hypotonic exposure, the membrane becomes transiently hyperpolarized followed by depolarization. The depolarization phase is diminished by the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). Swelling is also associated with an almost immediate increase in Ca2+ influx and elevation of cytoplasmic Ca2+ ([Ca2+]i) preceding RVD. In Ca2(+)-free medium, [Ca2+]i rapidly declines to a low level. Osmotic swelling, under these circumstances, is associated with a small transient increase in [Ca2+]i, but RVD or secondary swelling (in KCl) are minimal. Under these conditions, addition of gramicidin or the Ca2(+)-ionophore A23187 induces significant volume changes, although not as large as those found in the presence of Ca2+. Quinine inhibits RVD in the absence of gramicidin, but not in its presence; oligomycin C, DIDS, and trifluoperazine, on the other hand, inhibit in the presence of the ionophore. These findings suggest that in MDCK cells RVD involves activation of distinct conductive K+ and Cl- pathways which allow escape of KCl and osmotically obligated water and that activation of both pathways is associated with elevated [Ca2+]i derived largely from volume activation of a Ca2(+)-influx pathway.

A novel method for measuring membrane conductance changes by a voltage-sensitive optical probe

This study presents a method whose principles enable using a voltage-sensitive optical probe, to quantitatively measure conductivity changes elicited in membrane vesicles and cells. The procedure is based on the fact that the amplitude of the transmembrane potential difference, established across a membrane by an external electric field, is decreased when membrane conductivity is increased upon incorporation of ionophores into the membrane. The method was applied to osmotically swollen thylakoid membranes whose membrane conductivity was changed by the addition of gramicidin or ionomycin. The electric field induced stimulated luminescence from photosystem I (electrophotoluminescence-EPL) was used as a voltagesensitive optical probe. We calculated the induced conductance changes by using a calibrated EPL vs external electric field response curve and measuring the ionophore-mediated attenuation of the EPL signal. The calculated ionophore-unmodified conductance of the thylakoid membrane yields a value of 171 ± 56 nS cm . The value of the membrane conductance, modified by 10 nM gramicidin was found to be 190 ± 56 nS cm . The modified membrane conductance and the membrane conductance changes induced by 1 μM ionomycin in the presence of CaCl 2 were found to be 186 ± 3 nS cm and 15 ± 3 nS cm , respectively.

Effect of gramicidin on the ion permeability of liposomes in the presence of mixed electrolytes

The membrane permeabilities of three anionic species (Cl -, Br - and NO 3 -) and a cation (K + or Na +) entrapped simultaneously within reverse-phase evaporation vesicles (REV) were measured with an ion chromatograph and an atomic absorption spectrophotometer. Anion and cation permeabilities of vesicle membranes conform to first-order kinetics. K + or Na + was scarcely detectable in the liposomes after removing the extravesicular electrolytes by means of a Sephadex column when gramicidin was added at a concentration exceeding 2 dimers of gramicidin per vesicle in the liposomal preparation, whereas the concentration of entrapped anions decreased by approximately first-order kinetics. When gramicidin was added to the liposome solution at an estimated level of 20 dimers per vesicle, only 30-40% of the entrapped K + or Na + permeated rapidly out of the vesicles. Measurement with a potassium ion electrode revealed that rapid release of K + occurs within five minutes after the addition of gramicidin. The K + efflux is partly compensated by H + influx. The membrane permeabilities of entrapped Cl -, Br - and NO 3 - were enhanced 2-3 times compared with those for liposomes without gramicidin.