Physical properties of biological membranes determined by the fluorescence of the calcium ionophore A23187

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.

Benzoxazole Alkaloids: Occurrence, Chemistry, and Biology

Benzoxazole alkaloids exhibit a diverse array of structures and interesting biological activities. In spite of the extensive research done on the synthesis and biology, till date there is no concise update on this class of alkaloids. This chapter summarizes the literature on benzoxazole alkaloids till March 2017, which covers their isolation, characterization, possible biosynthetic origins, biological activities, and major synthetic approaches. These alkaloids have been broadly classified in the context of their sources, namely (i) fungal origin, (ii) marine origin, and (iii) plant origin.

Structures and properties of naturally occurring polyether antibiotics

Polyether ionophores represent a large group of natural, biologically active substances produced by Streptomyces spp. They are lipid soluble and able to transport metal cations across cell membranes. Several of polyether ionophores are widely used as growth promoters in veterinary. Polyether antibiotics show a broad spectrum of bioactivity ranging from antibacterial, antifungal, antiparasitic, antiviral, and tumour cell cytotoxicity. Recently, it has been shown that some of these compounds are able to selectively kill cancer stem cells and multidrug-resistant cancer cells. Thus, they are recognized as new potential anticancer drugs. The biological activity of polyether ionophores is strictly connected with their molecular structure; therefore, the purpose of this paper is to present an overview of their formula, molecular structure, and properties.

Methylene blue inhibits function of the 5-HT transporter

BACKGROUND AND PURPOSE

Methylene blue (MB) is commonly employed as a treatment for methaemoglobinaemia, malaria and vasoplegic shock. An increasing number of studies indicate that MB can cause 5-HT toxicity when administered with a 5-HT reuptake inhibitor. MB is a potent inhibitor of monoamine oxidases, but other targets that may contribute to MB toxicity have not been identified. Given the role of the 5-HT transporter (SERT) in the regulation of extracellular 5-HT concentrations, the present study aimed to characterize the effect of MB on SERT.

EXPERIMENTAL APPROACH

Live cell imaging, in conjunction with the fluorescent SERT substrate 4-(4-(dimethylamino)-styryl)-N-methylpyridinium (ASP(+) ), [(3) H]5-HT uptake and whole-cell patch-clamp techniques were employed to examine the effects of MB on SERT function.

KEY RESULTS

In EM4 cells expressing GFP-tagged human SERT (hSERT), MB concentration-dependently inhibited ASP(+) accumulation (IC(50) : 1.4 ± 0.3 µM). A similar effect was observed in N2A cells. Uptake of [(3) H]5-HT was decreased by MB pretreatment. Furthermore, patch-clamp studies in hSERT expressing cells indicated that MB significantly inhibited 5-HT-evoked ion currents. Pretreatment with 8-Br-cGMP did not alter the inhibitory effect of MB on hSERT activity, and intracellular Ca(2+) levels remained unchanged during MB application. Further experiments revealed that ASP(+) binding to cell surface hSERT was reduced after MB treatment. In whole-cell radioligand experiments, exposure to MB (10 µM; 10 min) did not alter surface binding of the SERT ligand [(125) I]RTI-55.

CONCLUSIONS AND IMPLICATIONS

MB modulated SERT function and suggested that SERT may be an additional target upon which MB acts to produce 5-HT toxicity.

The endogenous cannabinoid, anandamide, inhibits dopamine transporter function by a receptor-independent mechanism

The endocannabinoid, anandamide (AEA), modulates the activity of the dopamine transporter (DAT) in heterologous cells and synaptosomal preparations. The cellular mechanisms mediating this effect are unknown. The present studies employed live cell imaging techniques and the fluorescent, high affinity DAT substrate, 4-(4-(dimethylamino)-styryl)-N-methylpyridinium (ASP(+)), to address this issue. AEA addition to EM4 cells expressing yellow fluorescent protein-tagged human DAT (hDAT) produced a concentration-dependent inhibition of ASP(+) accumulation (IC(50): 3.2 +/- 0.8 microM). This effect occurred within 1 min after AEA addition and persisted for 10 min thereafter. Pertussis toxin did not attenuate the effects of AEA suggesting a mechanism independent of G(i)/G(o) coupled receptors. The amidohydrolase inhibitor, phenylmethylsulfonyl fluoride (0.2 mM), failed to alter the AEA-evoked inhibition of ASP(+) accumulation. Methanandamide (10 microM), a metabolically stable analogue of AEA inhibited accumulation but arachidonic acid (10 microM) was without effect suggesting that the effects of AEA are not mediated by its metabolic products. The extent of AEA inhibition of ASP(+) accumulation was not altered in cells pre-treated with 1 microM URB597, a specific and potent fatty acid amide hydrolase inhibitor, and the cyclooxygenase inhibitor, indomethacin (5 microM) Live cell imaging revealed a significant redistribution of hDAT from the membrane to the cytosol in response to AEA treatment (10 microM; 10 min). Similarly biotinylation experiments revealed that the decrease in DAT function was associated with a reduction in hDAT cell surface expression. These results demonstrate that AEA modulates DAT function via a cannabinoid receptor-independent mechanism and suggest that AEA may produces this effect, in part, by modulating DAT trafficking.

Electrical properties of plasma membrane modulate subcellular distribution of K-Ras

K-Ras is a small G-protein, localized mainly at the inner leaflet of the plasma membrane. The membrane targeting signal of this protein consists of a polybasic C-terminal sequence of six contiguous lysines and a farnesylated cysteine. Results from biophysical studies in model systems suggest that hydrophobic and electrostatic interactions are responsible for the membrane binding properties of K-Ras. To test this hypothesis in a cellular system, we first evaluated in vitro the effect of electrolytes on K-Ras membrane binding properties. Results demonstrated the electrical and reversible nature of K-Ras binding to anionic lipids in membranes. We next investigated membrane binding and subcellular distribution of K-Ras after disruption of the electrical properties of the outer and inner leaflets of plasma membrane and ionic gradients through it. Removal of sialic acid from the outer plasma membrane caused a redistribution of K-Ras to recycling endosomes. Inhibition of polyphosphoinositide synthesis at the plasma membrane, by depletion of cellular ATP, resulted in a similar subcellular redistribution of K-Ras. Treatment of cells with ionophores that modify transmembrane potential caused a redistribution of K-Ras to cytoplasm and endomembranes. Ca2+ ionophores, compared to K+ ionophores, caused a much broader redistribution of K-Ras to endomembranes. Taken together, these results reveal the dynamic nature of interactions between K-Ras and cellular membranes, and indicate that subcellular distribution of K-Ras is driven by electrostatic interaction of the polybasic region of the protein with negatively charged membranes.

Comparative effects of carboxylic ionophores on membrane potential and resistance of NG108-15 cells

Comparative analyses were conducted to determine the effects of Na(+) (monensin, MON), K(+) (nigericin, NIG) and Ca(2+) (A23187) selective carboxylic ionophores on differentiated NG108-15 (neuroblastoma X glioma hybrid) cells. Alterations in membrane potential (V(m)), input resistance (Rin) and electrically induced action potential generation were measured using intracellular microelectrode techniques in cells treated with 0.1-30 microM MON and NIG and 0.1-10 microM A23187. Responses to the ionophores were similar in that membrane hyperpolarization and unchanged R(in) predominated with all three compounds. However, significant differences between the ionophores were also detected. MON- and A23187-induced hyperpolarization was generally maintained throughout the 24-min superfusion whereas that produced by NIG diminished with time or was replaced by depolarization. In addition, action potential generation was blocked by NIG, whereas MON had no effect and action potential alterations were evident only with the highest A23187 concentration (10 microM). This study represents the initial comprehensive analysis of the effects of carboxylic ionophores on membrane electrical characteristics of an intact cell system and forms the basis for subsequent work using NG108-15 cells as a model system to evaluate potential therapeutic treatments against the carboxylic ionophores.

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.

Aggregation of lasalocid A in membranes: a fluorescence study

The aggregation behavior of the carboxylic ionophore, lasalocid A, has been studied in egg phosphatidylcholine vesicles by monitoring the intrinsic fluorescence of lasalocid A. Self quenching of lasalocid A fluorescence in vesicles of egg phosphatidylcholine suggests aggregation of lasalocid A. When aggregated lasalocid A is treated with increasing concentrations of lipid, there is an increase in fluorescence due to gradual reduction of self quenching on lateral dilution. This confirms the presence of loosely held non-covalent aggregates of lasalocid A in the membrane. This result is relevant in elucidating the molecular mechanism of cation transport by lasalocid A across membranes.

Ionomycin releases calcium from the sarcoplasmic reticulum and activates Na+/Ca2+ exchange in vascular smooth muscle cells

Ionomycin (1 microM) produced a large spike in cytosolic free Ca2+ [( Ca2+]i). The ionophore had no effect on [Ca2+]i if the sarcoplasmic reticulum had previously been Ca2+ depleted by stimulating neurohormone receptors. Ionomycin markedly increased 45Ca2+ efflux and decreased total cell Ca2+ by 60 to 70% in 1 min. Replacing extracellular Na+ [( Na+]o) with choline or N-methyl-D-glucamine strongly inhibited the effects of ionomycin on 45Ca2+ efflux and total Ca2+. Ionomycin caused similar peak increases in [Ca2+]i in the presence and absence of [Na+]o, but the exponential fall from the peak was faster in the presence of [Na+]o. Dimethylbenzamil, a potent blocker of Na+/Ca2+ exchange in these cells, strongly inhibited the effects of ionomycin on 45Ca2+ efflux and total cell Ca2+. We conclude that the increase in cytosolic free Ca2+ produced by ionomycin may be sufficient to activate the plasma membrane Na+/Ca2+ exchanger which removes Ca2+ from the cytosol and helps restore basal [Ca2+]i.

Aggregation of calcium ionophore (A23187) in phospholipid vesicles

The circular dichroism studies on calcium ionophore, A23187, incorporated in Dipalmitoyl phosphatidyl choline (DPPC) vesicle showed interesting time dependent changes in the CD spectra. Analysis of the data indicated the possible aggregation of the observed dimeric structure of this molecule in non-polar solvents into a stacked dimeric pore in the phospholipid vesicle.

Heterogeneity of lymphocyte calcium metabolism is caused by T cell-specific calcium-sensitive potassium channel and sensitivity of the calcium ATPase pump to membrane potential

Calcium management differs in T and B lymphocytes. [Ca2+]i elevation in response to calcium ionophores is up to 10 times greater in T cells than B cells. There is no difference between them in ionophore uptake. T cells, but not B cells, possess a calcium-sensitive potassium channel which produces membrane hyperpolarization at [Ca2+]i above 200 nM. This alters T cell density providing a rapid and easy method of cell separation. In contrast, B cells depolarize when [Ca2+]i is increased. Isolated B cell membrane vesicle ATP-dependent calcium pump activity is higher than T cell vesicles. Membrane depolarization reduces the [Ca2+]i response to ionomycin, most dramatically in T cells because they are hyperpolarized by increased [Ca2+]i. The most likely basis of this behavior is an effect of membrane potential on lymphocyte membrane calcium pump activity. This mechanism provides an explanation of the inhibitory effect of membrane depolarization on T lymphocyte responses.

Conformational studies of A23187 with mono-, di- and trivalent metal ions by circular dichroism spectroscopy

CD studies carried out on A23187 indicate a solvent-dependent conformation for the free acid. Alkali metal ions were found to bind to the ionophore weakly. Divalent metal ions such as Mg2+, Ca2+, Sr2+, Ba2+ and Co2+ and trivalent lanthanide metal ions like La3+ were found to form predominantly 2:1 (ionophore-metal ion) complexes at low concentrations of metal ions, but both 2:1 and 1:1 complexes were formed with increasing salt concentration. Mg2+ and Co2+ exhibit similar CD behaviour that differs from that observed for the other divalent and lanthanide metal ions. The structure of 2:1 complexes involves two ligand molecules coordinated to the metal ion through the carboxylate oxygen, benzoxazole nitrogen and keto-pyrrole oxygen from each ligand molecule along with one or more solvent molecules. Values of the binding constant were determined for 2:1 complexes of the ionophore with divalent and lanthanide metal ions.

Calcium ionophore-induced changes in HCO3- secretion and Cl- absorption in turtle bladder: relation to action of 3-isobutyl-1-methylxanthine

The calcium ionophore A23187 stimulates luminal alkalinization and inhibits Cl- absorption in short-circuited urinary bladders of postprandial or alkalotic turtles. The ionophore appears to mimic the action of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) by its similar effects on HCO3- secretion and Cl- absorption and by increasing cytosolic cAMP levels of isolated bladder epithelial cells. However, only A23187 (or ionomycin), but not IMBX or cAMP, elevated cytosolic Ca2+ of aequorin- or quin2-loaded cells. Since A23187, but not IBMX or cAMP inhibits luminal acidification, we postulate that cytosolic Ca2+ regulates the acidification process by a cAMP-independent mechanism and controls HCO3- secretion as well as Cl- absorption, at least in part, via cAMP-mediated pathways.

Formation of ion-translocating oligomers by nigericin

At pH 4.0, greater than 10(-7) M nigericin was found capable of conducting net charge transfer across bimolecular lecithin membranes, with a stoichiometry of three uncharged ionophore moieties per cation. At neutral or alkaline pH, nigericin catalyzed the transfer of net charge through dimer forms. In agreement with these results, quantitative analysis of nigericin-potassium complexes formed at pH 4.0 showed a 3:1 ratio, and a 2:1 ratio at neutral or alkaline pH. A 1:1 stoichiometry was observed when the ionophore complex was not transferred from methanol-water to chloroform. Moreover, 1H-NMR spectra of nigericin-cation complexes formed at pH 4.0, displayed clear-cut chemical shift variations different to those observed at neutral or alkaline pH. Thus, it is apparent that acid pH causes a transition from dimeric to trimeric forms of nigericin-cation complexes. The membrane conductance increased up to ten times when negatively charged phosphatidyl glycerol was used, while the conductance decreased in positively charged cetylpyridinium containing membranes at pH 4.0. These results suggest that the nigericin-K+ oligomeric complex is positively charged. In this respect, pKa values around 8.0 were obtained for the nigericin carboxylate group in media of different dielectric constant, indicating that this chemical group is undissociated under these conditions. Moreover, the values for the complex formation constants as well as the delta G values calculated for the dimers and trimers indicated that such ionophore cation oligomeric complexes are thermodynamically stable.

Phospholipid metabolism in human neutrophil subfractions

We describe a procedure for isolating human neutrophil subfractions by sucrose density centrifugation following nitrogen cavitation. Using this procedure we were able to isolate and characterize a cytosolic fraction, two separate plasma membrane-enriched fractions, and specific and azurophilic granule fractions. We used this procedure to examine the subcellular localization of the enzymes and substrates involved in the release of arachidonic acid from cellular phospholipids in response to whole cell stimulation. Whole cells were prelabeled for 2 h with [3H]arachidonic acid and [14C] stearic acid. When prelabeled cells were challenged with calcium ionophore A23187 (2 microM) for 5 min at 37 degrees C, each membrane-associated fraction, including both plasma membrane fractions and specific and azurophilic granule fractions, exhibited deacylation of phosphatidylinositol (PI) and phosphatidylcholine (PC). The specific granule fraction exhibited the greatest proportion of deacylation from PI while the more dense plasma membrane fraction was deacylated to a much lower extent than the other fractions. In terms of mass, the azurophilic granules deacylated the greatest amount of radiolabeled arachidonic acid. Although all membrane fractions may be sources of arachidonic acid to some extent, the azurophilic granule fraction may contain the largest pool of radiolabeled arachidonic acid that is released upon cell stimulation.

Fluorescence energy transfer between ionophore, A23187, and membrane proteins of isolated outer and cytoplasmic membranes of a Gram-negative bacterium

When tryptophan residues of the proteins of outer and cytoplasmic membranes of a moderately halophilic bacterium, Pseudomonas halosaccharolytica ATCC 29423, were excited at the wavelength 270 nm, tryptophan emission was observed at 330 nm. Adding the calcium ionophore, A23187, to both suspensions, the tryptophan emission at 330 nm decreased and the ionophore emission at 430 nm increased. Thus, when the calcium ionophore was increased in both suspensions, the ionophore emission increased with excitation of membrane tryptophan, that is, the fluorescence energy was transferred from tryptophan to the calcium ionophore. Using the Förster equation the critical distance was calculated to be 52 A. As this distance is considerable compared with the diameter of the membrane protein molecules, the ionophore cannot be bound to the membrane proteins. It is probably located in the lipid bilayers.

The stoichiometry of A23187- and X537A-mediated calcium ion transport across lipid bilayers

Initial rates of ionophore-mediated Ca2+ transport across egg phosphatidylcholine bilayers of large unilamellar vesicles were measured using the absorbance change of arsenazo III at 650 nm as an indicator of Ca2+ translocation. A23187 induced the movement of Ca2+ in a 2:1 ionophore: Ca2+ complex, whereas its methyl ester (CH3A23187) and X537A mediated Ca2+ movement in a 1:1 ionophore: Ca2+ complex. The relative potencies of these ionophores in transporting Ca2+ across lipid membranes were A23187 much greater than X537A greater than CH3A23187.

Relationship between the shape and the membrane potential of human red blood cells

Microscopic observations of isotonic suspensions of human red blood cells demonstrate that cell shape is unaltered when the transmembrane electrical potential, or Em, is set in the range -85 to + 10 mV with valinomycin at varied external K+, or Ko X Em was measured with the fluorescent potentiometric indicator, diS-C3(5), as calibrated by a delta pH method. Repeating Glaser's experiments in which echinocytosis was attributed to hyperpolarization, we found that at low ionic strength the pH-dependent effects of amphotericin B appear to be unrelated to Em. The effects of increased intracellular Ca2+, or Cac, on echinocytosis and on Em are separable. With Ca ionophore A23187 half-maximal echinocytosis occurs at greater Cao than that which induces the half-maximal hyperpolarization associated with Ca-induced K+ conductance (Gardos effect). Thus, cells hyperpolarized by increased Cac remain discoidal when Ca is below the threshold for echinocytosis. With A23187 and higher Cao, extensive echinocytosis occurs in cells which are either hyperpolarized or at their resting potential. The Ca-activation curve for echinocytosis is left-shifted by low Ko, a new observation consistent with increased DIDS-sensitive uptake of 45Ca by hyperpolarized cells. These results support the following conclusions: (1) the shape and membrane potential of human red blood cells are independent under the conditions studied; (2) in cells treated with A23187, the Gardos effect facilitates echinocytosis by increasing Cac.

Arachidonic acid metabolism in polymorphonuclear leukocytes from patients with chronic granulomatous disease

The effect of the calcium ionophore A23187 on the release and metabolism of [3H]arachidonic acid was examined in normal polymorphonuclear leukocytes and those obtained from patients with chronic granulomatous disease. The ionophore A23187 which stimulates oxidative metabolism in normal polymorphonuclear leukocytes was ineffective in increasing oxidative metabolism (chemiluminescence) in polymorphonuclear leukocytes from patients with chronic granulomatous disease. However, the ionophore A23187 stimulated the release of [3H]arachidonic acid from chronic granulomatous disease neutrophil phospholipids and stimulated its metabolism into hydroxyeicosatetraenoic acids and leukotrienes.

Influenza A virus-induced polymorphonuclear leukocyte dysfunction

Previous studies have shown that influenza A virus can activate the polymorphonuclear leukocyte (PMN) respiratory burst and that upon subsequent stimulation of the cell there is depressed metabolic function. We examined the mechanism by which influenza virus causes PMN dysfunction by measuring the effect upon the chemiluminescent activity of cells of varying the type of influenza virus used, the period of time that cells were exposed to virus, and the secondary stimulus that was used. The various types of intact influenza virus elicited different amounts of chemiluminescent activity, but when cells were subsequently stimulated with phorbol myristate acetate, each virus caused equivalent depression of the PMN response. Purified glycoproteins incorporated into a liposome structure similarly stimulated the PMN chemiluminescence, yet did not induce PMN dysfunction. Depressed PMN function was noted after as little as 5 min of incubation of cells with virus and occurred to both receptor-dependent (zymosan, N-formylmethionyl-leucyl-phenylalanine, and phorbol myristate acetate) and -independent (calcium ionophore A23187) stimuli.

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

The voltage-sensitive channel-former monazomycin is used as a conductance probe to monitor changes in the trans electrostatic surface potentials of negatively-charged planar phospholipid bilayers. Cis-to-trans electroneutral fluxes of divalent cations mediated by ionophores A23187 and X537A are sensed via the effect of transported divalent cations on the trans surface potentials. Quantitative determinations of neutral Ca2+ and Mg2+ fluxes are made and related to ionophore function.

Conformational analysis of the calcium--A23187 complex at a lipid--water interface

A possible conformation of the complex formed by one calcium ion and two molecules of the ionophore A23187 at a simulated lipid--water interface was predicted by a variant method for conformational analysis. This method takes into account, in addition to the Van der Waals energy, electrostatic interaction, and torsional potential, the alteration of electrostatic forces attributable to changes in dielectric constant at the interface and the transfer energy for each part of the complex as it moves through the lipid-water interface. The most probable conformer was characterized by a two-fold axial symmetry that was maintained during transition to the hydrophobic bulk conformation. Minor changes in the interfacial structure were sufficient to achieve the configuration characteristic of the hydrophobic bulk phase.

Fluorescence study of the divalent cation-transport mechanism of ionophore A23187 in phospholipid membranes

The mechanism for transport of divalent cations across phospholipid bilayers by the ionophore A23187 was investigated. The intrinsic fluorescence of the ionophore was used in equilibrium and rapid-mixing experiments as an indicator of ionophore environment and complexation with divalent cations. The neutral (protonated) form of the ionophore binds strongly to the membrane, with a high quantum yield relative to that in the aqueous phase. The negatively charged form of the ionophore binds somewhat less strongly, with a lower quantum yield, and does not move across the membrane. Complexation of the negatively charged form with divalent cations was measured by the decrease in fluorescence. An apparent rate constant (kapp) for transport of the ionophore across the membrane was determined from the rate of fluorescence changes observed in stopped-flow rapid kinetic experiments. The variation of kapp was studied as a function of pH, temperature, ionophore concentration, membrane lipid composition, and divalent cation concentration and type. Analysis and comparison with equilibrium constants for protonation and complexation show that A23187 and its metal:ionophore complexes bind near the membrane-water interface in the lipid polar-head region. The interfacial reactions occur rapidly, compared with the transmembrane reactions, and are thus in equilibrium during transport. The transport cycle can be described as follows: a 1:1 complex is formed between the membrane bound A23187-(Am-) and the aqueous divalent cation with dissociation constant K1 approximately 4.6 x 10(-4) M. This is in equilibrium with a 1:2 (metal:ionophore) complex (K2 approximately 3.0 x 10(-4) [ionophore/lipid]) that is responsible for transporting the divalent cations across the membrane. The rate constant for translocation of the 1:2 complex is 0.1-0.3 s-1. Dissociation of the complex of the trans side and protonation occur rapidly. The rate constant for translocation of H+ . A23187- is 28 s-1. A theory is presented that is capable of reproducing the kinetic data at any calcium concentration. The cation specificity for ionophore complex transport (kapp), determined at low ionophore concentration for a series of divalent cations, was found to be proportional to the equilibrium constant for 1:1 complexation. The order of ion specificity for these processes was found to be Ca2+ greater than Mg2+ greater Sr2+ greater than Ba2+. Interactions with Na+ were not observed. Maximal values of kapp were observed for vesicles prepared from pure dimyristoyl phosphatidylcholine. Inclusion of phosphatidyl ethanolamine, phosphatidic acid, or dipalmatoyl phosphatidylcholine resulted in lower values of kapp. Calcium transport by A23187 is compared with that of X537A, and it is shown that the former is 67-fold faster. The difference in rates is due to differences in the ability of each ionophore to form a 1:2 complex from a 1:1 complex.

Magnesium buffering in intact human red blood cells measured using the ionophore A23187

1. A method was developed for measuring the cytoplasmic magnesium buffering of intact red cells using the divalent cation selective ionophore A23187. Addition of A23187 to a suspension of red cells induces rapid equilibration of ionized magnesium across the cell membrane. 2. Entry of magnesium into red cells is associated with cell swelling and depolarization of the membrane potential. 3. At an external ionized magnesium concentration of about 0.15 mM corresponding to an internal ionized concentration of 0.4 mM the addition of A23187 did not produce a change in the magnesium content of the cells. This indicates that the normal ionized magnesium concentration inside the oxygenated red cell is about 0.4 mM. 4. The magnesium buffering curve for oxygenated, inosine-fed human red blood cells is adequately described by the existence of three buffer systems of increasing capacity and decreasing affinity. These are 0.15 mM with a Km < 10(-7) M, probably structural magnesium bound within the cell proteins; 1.6 mM with a Km approximately equal to 0.08 mM, mainly ATP and other nucleotides; and about 21-25 mM with a Km approximately equal to 3.6 mM, a major portion of this being organic phosphates. It is suggested that the contribution of 2,3-DPG to the low affinity site involves each phosphate group acting as an independent binding site for magnesium.

Renal magnesium transport and the effects of hypermagnesemia, hypercalcemia, body magnesium stores and parathyroid hormone

Magnesium reabsorption occurs throughout the proximal, loop and distal segments of the nephron. The proximal tubule is less permeable to magnesium than calcium and sodium and most of the filtered load is reclaimed in the thick ascending limb of the loop of Henle. Thus one would expect that factors which regulate magnesium reabsorption should act within this important segment. No single hormone or agent appears to regulate magnesium reabsorption sufficiently to account for urinary changes; rather it appears to be a number of intracellular and extracellular factors acting in concert to effect day to day magnesium homeostasis.

The effect of buffer composition and deoxygenation on the concentration of ionized magnesium inside human red blood cells

1. A method is described in which the concentration of ionized magnesium can be measured in intact red cells. The method uses an equilibrium dialysis technique originally developed by Ferreira & Lew (1976) and Flatman & Lew (1977) where the magnesium permeability of the red cell membrane is increased with the ionophore A23187. 2. The concentration of ionized magnesium in the oxygenated cells was found to be 0.39 mM and was not greatly affected by changes in the composition of the medium. 3. The concentration of ionized magnesium in deoxygenated cells showed more dependence on the composition of the medium. Values of 0.54 and 0.62 mM were found in cells incubated in Tris- and HCO3- buffered media respectively. The difference probably reflects increased competition between chloride and 2,3-diphosphoglycerate for common binding sites on haemoglobin in Tris-buffered cells. 3. Only a small increase of 0.16-0.22 mM was found in the concentration of ionized magnesium when the cells were deoxygenated. These changes are smaller than had been anticipated from estimates of the binding of ATP and 2,3-diphosphoglycerate to oxy- and deoxyhaemoglobin (Bunn, Ransil & Chao, 1971; Berger, Jänig, Gerber, Ruckpaul & Rapoport, 1973; Gerber, Berger, Jänig & Rapoport, 1973) and are unlikely to alter greatly the operation of magnesium-dependent metabolic or transport systems.

Calcium inhibits urinary acidification: effect of the ionophore A23187 on the turtle bladder

The role of intracellular calcium in urinary acidification was studied in the turtle bladder with the use of the ionophore A23187. In the presence of calcium acidification was significantly inhibited in the hemibladders treated with the ionophore as compared to control hemibladders treated with dimethylsulfoxide (the vehicle used to dissolve the ionophore). In the absence of calcium both the ionophore and dimethylsulfoxide failed to cause any change in acidification. The apparent proton motive force and active conductance of H+ were unchanged in dimethylsulfoxide treated hemibladders. In the presence of the ionophore and calcium, the proton motive force and the conductance were significantly decreased when the H+ current was low (less than 30% of control values); when the H+ current was decreased, but not less than 30%, the proton motive force was unchanged. These data provide evidence for an important role of intracellular calcium in the regulation of urinary acidification.

Effects of divalent cation ionophores on the neuron membrane of the crayfish

The effects of divalent cation ionophores, A23187 and X-537A, on the electrical membrane properties were investigated by using the soma membrane of the X-organ of the crayfish. They reduced the amplitude and maximum rate of rise of Ca-action potential in lower concentration. As the concentration increased, a reduction of membrane resistance and hyperpolarization occurred simultaneously. Further increase resulted in membrane depolarization with a further decrease in resistance. The threshold concentration of X537A was 100 times higher than that of A23187. These effects were reversible only when the application period was relatively short, while a longer application resulted in an incomplete reversibility or in no reversibility at all. The ionophore effect was facilitated in high Ca medium and diminished in low Ca medium. In Sr medium, the same effects on the resistance and the membrane potential were barely observable. TEA reduced the effects of A23187 but did not completely inhibit the effects. The Na-cation potential was also reduced by the higher concentration of the ionophore. From these results it is concluded that the divalent cation ionophores. A23187 and X537A, carry divalent cations, Ca ions in a physiological medium, into the neuron soma through the membrane and the consequent increase of the intracellular divalent cations induces K conductance increase and that higher concentration of the ionophore induces the increase in the conductance of the other ion species, such as Na.

The role of electrical fields, ions, and the cortex in the morphogenesis of Acetabularia

Electrophysiological and biochemical aspects of polarity determination and morphogenesis were studied in regenerating Acetabularia. The Ca(++), Mg(++) ionophore, A23187, reversibly inhibits the formation of apical structures (whorls and caps) but does not arrest longitudinal growth. This normal growth correlates with normal electrophysiology as reflected in an apico-basal electrical potential gradient and spontaneous recurrent action potentials which propagate from apex to base. However, the ionophore markedly elevates (32)PO 3 (3-) incorporation into the cortical cytoplasm which is normally low apically and rises to a maximum at the base. A molecular model of membrane-dependent morphogenesis is suggested.

Modulation of 32Pi incorporation into phospholipids of polymorphonuclear leukocytes by ionophore A23187

The effects of ionophore A23187 on the incorporation of 32Pi into phospholipids and on 45Ca2+ uptake and release by polymorphonuclear leukocytes were examined. A23187 increased 32Pi incorporation into phosphatidic acid, phosphatidylglycerol, phosphatidylserine, and the phosphoinositides. It also promoted a rapid burst uptake and release of 45Ca2+ by leukocytes. External Ca2+, but not Mg2+, was required for full stimulation of 32Pi incorporation into phosphatidic acid and the phosphoinositides. In the absence of external Ca2+, the increased radiophosphorus activity of phosphatidic acid, phosphatidylserine and the phosphoinositides was grossly reduced but not eliminated, and the decreased radiophosphorus activity of phosphatidylcholine became pronounced. In addition, the ionophore effect on 32Pi incorporation into leukocyte phospholipids was not abolished by ethyleneglycol bis(beta-amino-ethylether)-N,N'-tetraacetic acid. ATP radiophosphorus activity was also enhanced by the presence of A23187, but the enhancement was much less than that of the acidic phospholipids. Based on these findings, it is suggested that the increased 32Pi incorporation into the acidic phospholipids of leukocytes induced by A23187 was not solely derived from the higher radioactivity of ATP, increased Ca2+ fluxes and perturbation of cellular Ca2+ distribution of leukocytes exposed to A 23187 may trigger part of the altered 32Pi incorporation into phospholipids.

Rate-determining processes in the transport of Pr3+ ions by the ionophore A23187 across phospholipid vesicular membranes. A 1H-MR and theoretical study

Rate constants and activation parameters have been determined for the transport of Pr3+ ions by the ionophore A23187 across dipalmitoyl phosphatidylcholine vesicular membranes. The novel method described depends on the measurement of changes in chemical shift of the 1H-NMR choline head-group signals as Pr3+ is transported from outside to inside the vesicles. The determined rates are directly proportional to A23187 concentration, suggesting that the rate-determining step involves the species [Pr(A23187)](2+). A theoretical analysis of the initial stages of Pr3+ transport leads to the conclusion that diffusion over the image potential barrier is the rate-determing step. Calculation of the form and height of this barrier for the non-equilibrium state gives results which agree well with the experimental activation energy and also correctly predict a two-fold reduction in rate of transport when 7 mol % decane is present in the bilayer.

Stimulation of acetylcholine output from brain slices caused by the ionophores BrX-537A and A23187

1 The effect of two ionophores, BrX-537A (Bromolasolacid) and A 23187, on acetylcholine (ACh) output from brain slices was studied. 2 The slices were prepared from rat cerebral cortex, incubated in Krebs solution containing physostigmine and ACh output determined by bioassay. 3 Both ionophores enhanced ACh output. BrX-537A exerted its maximal effect, a six fold increase, at a concentration of 1.8 micron, while A 23187 caused a three fold increase at a concentration of 58 micron. 4 When the slices were incubated in a Ca-free medium, the effect of A 23187 on ACh output was only reduced, BrX-537A was abolished while that of BrX-537A was also active when disodium edetate (EDTA) was added to the the Ca-free medium. 5 The activity of BrX-537A was not affected by the presence of tetrodotoxin in the incubation medium. 6 The stimulation of ACh output elicited by KCl (25 mM) was increased further by hyoscine, but not by BrX-537A. Hyoscine however had no effect when ACh output was stimulated by BrX-537A. 7 The effect of BrX-537A on ACh output was potentiated by the addition of Mg2+ (9.3 mM) to the incubation medium and was reduced in a Mg-free medium. 8 It is concluded that A 23187 stimulates ACh output by transporting extracellular Ca2+ into cholinergic nerve endings. The effect of BrX-537A does not depend only on Ca2+ but also on other mechanisms.