Electrical measurement of electroneutral fluxes of divalent cations through charged planar phospholipid membranes

Actin protein inside DMPC GUVs and its mechanical response to AC electric fields

Cells are dynamic systems with complex mechanical properties, regulated by the presence of different species of proteins capable to assemble (and disassemble) into filamentous forms as required by different cells functions. Giant unilamellar vesicles (GUVs) of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) are systems frequently used as a simplified model of cells because they offer the possibility of assaying separately different stimuli, which is no possible in living cells. Here we present a study of the effect of acting protein on mechanical properties of GUVs, when the protein is inside the vesicles in either monomeric G-actin or filamentous F-actin. For this, rabbit skeletal muscle G-actin is introduced inside GUVs by the electroformation method. Protein polymerization inside the GUVs is promoted by adding to the solution MgCl₂ and the ion carrier A23187 to allow the transport of Mg⁺² ions into the GUVs. To determine how the presence of actin changes the mechanical properties of GUVs, the vesicles are deformed by the application of an AC electric field in both cases with G-actin and with polymerized F-actin. The changes in shape of the vesicles are characterized by optical microscopy and from them the bending stiffness of the membrane are determined. It is found that G-actin has no appreciable effect on the bending stiffness of DMPC GUVs, but the polymerized actin makes the vesicles more rigid and therefore more resistant to deformations. This result is supported by evidence that actin filaments tend to accumulate near the membrane.

Evidence against formation of A23187 dimers and oligomers in solution: photo-induced degradation of Ionophore A23187

Ionophore A23187 has been proposed to form Ca(2+)- conducting channels that arise from dimers and oligomers of the compound (e.g., Balasubramanian, S. V., and Easwaran, K. R. K. (1989) Biochem. Biophys. Res. Commun. 158, 891-897). To investigate this possibility, the solution behavior of A23187 in chloroform, n-hexane, ethanol, 80% methanol-water, and palmitoyloleoylphosphatidyl choline (POPC) vesicles was investigated using UV-VIS, circular dichroism (CD), and 1H NMR techniques. The concentration dependence of the UV-VIS and CD spectra obtained in freshly prepared chloroform solutions indicates that neutral A23187 (HA) exists as a monomer for ionophore concentrations in the range of 50-1000 microM. The cause of time- and concentration-dependent spectral alterations which gave rise to the dimer/channel hypothesis was also investigated. For solutions of 50-1000 microM A23187 in chloroform, n-hexane, and ethanol stored in the dark, no spectral changes were observed for periods of 2 months. However, solutions in these solvents did show time-dependent spectral changes when exposed to light. In 80% methanol-water or phospholipid vesicles, similar spectral changes were observed, even when the solutions were protected from light. Application of TLC and MS methods indicate that the time-dependent spectral changes reflect degradation of A23187, not dimer or oligomer formation. The degradative processes proceed with half-lives ranging from approximately 75 to > 400 h, and are influenced by several factors, including solvent, exposure to light, ionophore concentration, pH, and the presence of metal ions, EDTA, dissolved oxygen, and a radical inhibitor. The kinetics of Ca2+ transport into Quin-2-loaded POPC vesicles by authentic A23187 give no evidence of a channel mechanism, even following a previous and lengthy coincubation of the ionophore with the vesicles.

Effects of pH conditions on Ca2+ transport catalyzed by ionophores A23187, 4-BrA23187, and ionomycin suggest problems with common applications of these compounds in biological systems

Phospholipid vesicles loaded with Quin-2 and 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) have been used to investigate the effects of pH conditions on Ca2+ transport catalyzed by ionophores A23187, 4-BrA23187, and ionomycin. At an external pH of 7.0, a delta pH (inside basic) of 0.4-0.6 U decreases the rate of Ca2+ transport into the vesicles by severalfold under some conditions. The apparent extent of transport is also decreased. In contrast, raising the pH by 0.4-0.6 U in the absence of a delta pH increases both of these parameters, although by smaller factors. The relatively large effects of a delta pH on the transport properties of Ca2+ ionophores seem to reflect a partial equilibration of the transmembrane ionophore distribution with the H+ concentration gradient across the vesicle membrane. This unequal distribution of ionophore can cause a very slow or incomplete ionophore-dependent equilibration of delta pCa with delta pH. A second factor of less certain origin retards full equilibration of delta pCa when delta pH = 0. These findings call into question several ionophore-based methods that are used to investigate the regulatory activities of Ca2+ and other divalent cations in biological systems. Notable among these are the null-point titration method for determining the concentration of free cations within cells and the use of ionophores plus external cation buffers to calibrate intracellular cation indicators. The present findings also indicate that the transport mode of Ca2+ ionophores is more strictly electroneutral than was thought, based upon previous studies.

Ca2+ transport properties of ionophores A23187, ionomycin, and 4-BrA23187 in a well defined model system

Models for the electroneutral transport of Ca2+ by ionophores A23187, ionomycin, and 4-BrA23187 have been tested in a defined system comprised of 1-palmitoyl-2-oleoyl-sn-glycerophosphatidylcholine vesicles prepared by freeze-thaw extrusion. Quin-2-loaded and CaCl2-loaded vesicles were employed to allow the investigation of transport in both directions. Simultaneous or parallel measurements of H+ transport and membrane potential, respectively, indicate that for any of these ionophores, electrogenic transport events do not exceed 1 in 10,000 when there is no preexisting transmembrane potential. When a potential of approximately 150 mV is imposed across the membrane, transport catalyzed by A23187 remains electroneutral; however, for ionomycin and 4-BrA23187, approximately 10% of transport events may be electrogenic. The defined vesicle system has also been utilized to determine how the rate of Ca2+ transport varies as a function of ionophore and Ca2+ concentration and with the direction of transport. Some aspects of the results are unexpected and should be considered by investigators using ionophores in biological systems. These include the apparent failure of these compounds to fully equilibrate Ca2+ with a high affinity Ca2+ indicator when these species are separated by a membrane, rates of transport that vary markedly with the direction of transport, and extents of transport that are a function of ionophore concentration. At least some of these unexpected behaviors can be explained by a strong influence of delta pH on forward and reverse transport kinetics. In the case of A23187, the data also give some initial insights into the relationship between formation of the transporting species and the entry of this species into the membrane hydrophobic region.

Kinetic properties of cation/H(+)-exchange: calcimycin (A23187)-mediated Ca2+/2H(+)-exchange on the bilayer lipid membrane

The calcimycin (A23187)-mediated electrically silent flux of hydrogen ions coupled with a counter transport of calcium or magnesium ions was measured by the method of local pH changes recording in the unstirred layers near the planar bilayer lipid membrane (BLM). It was shown that: (1) the pH dependence of calcimycin-mediated Ca2+/2H+ exchange had a maximum at pH 7; (2) the apparent Michaelis constant for the alkali earth cations were higher at acidic pH than the corresponding values at alkaline pH; (3) the apparent Michaelis constant for calcium was similar to that for magnesium ions in agreement with calcimycine cation binding constants; (4) the ratio of calcium and magnesium fluxes was independent of pH in the pH range from 5 to 8. (5) the flux was proportional to the calcimycin concentration at pH greater than 6.3 and proportional to the square of the carrier concentration at pH less than 5; (6) the addition of calcium ion chelator EDTA increased the flux significantly. These data were discussed in terms of the model of cation/H(+)-exchange and it was concluded that the dissociation of the cation-carrier complex at the membrane/water interface played an important role in the process of calcimycine operation. The comparison of the kinetic properties of calcimycin with the previously described kinetics of nigericin (Antonenko and Yaguzhinsky (1988) Biol. Membr. (Russian) 5, 718-728) revealed much similarity. On the other hand, a significant difference was found between the mechanism of the nigericin K/Na selectivity and calcimycin Ca/Mg selectivity.

A fluorescence quenching technique for the measurement of paramagnetic ion concentrations at the membrane/water interface. Intrinsic and X537A-mediated cobalt fluxes across lipid bilayer membranes

We have characterized the quenching of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phosphatidylethanolamine by Co2+ in egg phosphatidylcholine (PC) lipid bilayer vesicles. The quenching constant obtained is 59 M-1. We demonstrate one use of this fluorescence quenching technique by measuring intrinsic and X537A-mediated transmembrane Co2+ fluxes in large unilamellar PC vesicles. The intrinsic rate constant for Co2+ flux we measure is 3 X 10(-6) S-1. We confirm that the neutral Co approximately (X537A)2 complex is the main component of the X537A-mediated cobalt flux. Since this method measures the concentration of Co2+ at the site of the fluorophore, it is generally applicable to the measurement of paramagnetic ion concentrations in the region of the membrane/water interface.

The use of phospholipid-impregnated millipore filters for recording nonelectrogenic cation flows in the presence of Men+/nH+ exchangers

It has been shown that with a cation (K+, Na+, Ca2+) concentration gradient on a Millipore filter impregnated with a decane solution of phospholipid, in the presence of a Men+/nH+ exchanger (nigericin, monensin, A23187), addition of a protonophore induces the formation of an electric potential positively charged on the side where the concentration of the cation is lower. The formation of the potential is induced by the hydrogen ion concentration gradient in the filter and in the unstirred layers as a result of the Men+/nH+ exchange. In such a system, with a pH gradient on the filter in the presence of monensin and valinomycin, a potential is generated with the plus on the side of the lower concentration of hydrogen. The effect is the result of the formation of a potassium ion concentration gradient in the unstirred layers in the course of the K+/H+ exchange. It is concluded that phospholipid-impregnated filters can be used for search and identification of electroneutral membrane ionophores of the Men+/nH+ exchanger type.

A new method of the measurement of the electrically neutral fluxes of cations through lipid bilayer membranes induced by Men+/nH+-exchangers

Electrically neutral ionophores (nigericin, monencin) incorporated into a planar bilayer lipid membrane (BLM) bring about hydrogen ion gradient formation in the unstirred layers of BLM if a metal ion gradient on the membrane is prepared. Under these conditions a diffusion potential of a hydrogen ion is generated after addition of a protonophore. Cation selectivity of nigericin, monencin and A23187 has been studied by means of electrical potential measurements in the presence of a protonophore and Men+/nH+-exchangers mentioned above. The data on cation selectivity are in a good agreement with the well known results of the direct measurements of metal ion fluxes. This shows that the effect of generation of the potential on BLM in the presence of a protonophore and a Men+/nH+-exchanger can be used for the estimation of electrically neutral ion fluxes through BLM.