Phospholipids as ion exchangers: implications for a possible role in biological membrane excitability and anesthesia

Plasma membrane-associated aminopeptidase activities in Chlamydomonas reinhardtii and their biochemical characterization

High aminopeptidase (Apase) activities were found on intact unicellular algae cells. Several lines of evidence strongly indicate that the external Apases on Chlamydomonas reinhardtii (a green alga) cells, characterized in the present study, are plasma membrane-associated proteinases and not secreted in the cell wall or the surrounding medium. This is shown by enzyme activities also detected on a cell wall deficient mutant of C. reinhardtii and by the finding that in assay media and algal conditioned nutrient solutions, respectively, no Apase activities were found after removal of cells. In C. reinhardtii at least two in vivo Apases, one L-leucine-p-nitroanilide and one L-alanine-p-nitroanilide hydrolyzing enzyme (in vivo LeuNAase and AlaNAase, respectively) as well as one in vivo endoproteinase, capable of cleaving carboxybenzoylleucine-p-nitroanilide (CBZLeuNAase), were clearly distinguished by their pH optima for activity and characteristics towards various chemical compounds. In vivo LeuNAase, which cannot unequivocally classified as a metallo- or serine-type proteinase, showed optimum activities between pH 7 and 8.5, stimulation of activity by 1,10-phenanthroline (161%), 2-fold higher activity with L-phenylalanine-p-nitroanilide than with LeuNA and a Km value of 40 microM LeuNA. In vivo AlaNAase favored alkaline pH values, had a Km value of 1.45 mM AlaNA and is probably a metallopeptidase as indicated by 2-fold enhancement of enzyme activity by 5 microM Co2+ and strong inhibition with 1,10-phenanthroline. This enzyme was inhibited completely by a 30 min incubation with 10 microM Hg2+ at room temperature, indicating sensitive SH-groups. In contrast, activity was stimulated 205% by 20 mM iodoacetate in the assay buffer. Both in vivo Apases were efficiently inhibited by 10 mM Pefabloc SC, a serine-type proteinase inhibitor and by two compounds, not yet described as proteinase inhibitors: methyljasmonate, a plant hormone, and dibucaine, a local anestheticum. The latter compound showed the most powerful inhibition on in vivo and in vitro LeuNAase of all reagents tested. From the distribution of Apase activities and characteristics in the cell, it is hypothesized that at least the LeuNAase dissociates easily from the plasma membrane during preparation of cell extracts and binds then unspecifically to various membrane fractions. In conclusion, this is the first report on the existence of external Apase activities on plant cells providing an easy-to-perform, rapid and reliable assay method for these enzymes.

Blockade of tolerance to morphine analgesia by cocaine

Tolerance to morphine analgesia was induced in male Sprague-Dawley rats by s.c. implantation of a morphine base pellet (75 mg) on the first and second day and determining the magnitude of tolerance 72 h after the first implant by s.c. injection of a test dose of morphine (5 mg/kg). Implantation of a cocaine hydrochloride pellet (25 mg), concurrently with morphine pellets or of a cocaine hydrochloride (50 mg) pellet after the development of tolerance, blocked both the development and expression of morphine analgesic tolerance. In morphine-pelleted animals pretreatment for 3 days with desipramine or zimelidine or phenoxybenzamine but not haloperidol produced no significant morphine tolerance. Pretreatment with a combination of desipramine and zimelidine, however, was as effective as cocaine in blocking morphine tolerance. Alpha-Methyl-p-tyrosine methyl ester counteracted the effect of cocaine in blocking morphine tolerance and potentiated the tolerance development. Blockade of morphine tolerance by cocaine was reinforced and facilitated by pretreatment with fenfluramine or p-chlorophenylalanine ethyl ester and to a lesser extent by clonidine and haloperidol. Acute administration of fenfluramine or zimelidine or a combination of desipramine and zimelidine or alpha-methyl-p-tyrosine methyl ester or p-chlorophenylalanine ethyl ester did not significantly affect morphine analgesia. The study suggests an important role of the concomitant depletion of both central noradrenaline and serotonin in the blockade of morphine tolerance by cocaine and stresses the importance of the counter-balancing functional relationship between these two neurotransmitters in the central nervous system.

Local anesthetic-divalent cation binding center interaction

This communication explicitly considers the possibility that local anesthetics interact with divalent cation binding centers, such as chlortetracycline, quin 2, ethyleneglycol bis (B-aminoethyl ether)-N-N,N',N'-tetraacetic acid (EGTA), ethylenediamine tetraacetic acid (EDTA) and ATP. Alterations of local anesthetic fluorescence spectra have been found in the presence of EGTA, EDTA and ATP. On the other hand, the fluorescence of chlortetracycline is enhanced and that of quin 2 is quenched by local anesthetics. The spectrofluorometric evidence presented in this paper clearly indicates that local anesthetics and these divalent cation chelators interact in solution. The fluorescence alterations observed do not derive from parallel changes of their respective absorption spectra, thus, they appear to be due to quantum yield changes. On the basis of the spectral perturbations observed, it is likely that local anesthetics interact with M2+ binding centers via their electron defective aromatic ring. From the association constants obtained in this study, we make an estimation of the free energy of this interaction ranging from -2.8 to -4.0 kcal/mole in the following experimental conditions: pH 7.4 at an ionic strength of 0.1 at 25 degrees. The relevance of these results to define the physical-chemical characteristics of the local anesthetic receptor site is briefly discussed. It is suggested that local anesthetics can bind strongly to Ca2+ and Mg2+ binding centers, provided that a hydrophobic region is located nearby.

pH-dependent influence of membrane-incorporated flunarizine on Ca-binding to phosphatidylserine monolayer membranes

The pH-dependent 45Ca binding to phosphatidylserine monolayers was investigated. Ca binding increased with increasing pH. Between pH 10 and pH 11 a steep increase of Ca binding could be observed. This increase was interpreted to be due to complex Ca binding opposed to ionic binding at low pH. Flunarizine added to the spreading solution of the monolayer dose dependently displaced up to 100% Ca at pH 5 independently of phospholipid packing. At pH 11 less than 20% of Ca could be displaced by flunarizine. Intermediate results were found at pH 7. Flunarizine displaced less Ca from dense than from loosely packed monolayers at pH 7. The results suggest two binding states of flunarizine: ionic binding at low pH and apolar binding at high pH. The latter is much less effective in displacing Ca from phosphatidylserine monolayers. The Ca displacing properties of charged flunarizine may prevent a deleterious phospholipid reorientation within the membrane induced by the intracellular Ca rise during ischemia.

Low affinity binding of taurine to phospholiposomes and cardiac sarcolemma

A sarcolemma-enriched membrane fraction was prepared from the hearts of Sprague-Dawley rats and its ability to bind taurine (0.5-150 mM) was measured. In the absence of cations, the sarcolemma bound a maximum of 661 nmol taurine/mg protein, with a dissociation constant of 19.2 mM and a Hill coefficient of 1.9, indicating positive cooperativity. Scatchard analysis of taurine binding to sarcolemma gave a bell-shaped curve. Neither beta-alanine nor guanidinoethane sulfonate, inhibitors of taurine transport, affected the degree of taurine binding to sarcolemma. However, hypotaurine was an effective antagonist. Equimolar concentrations of Ca2+, Na+ or K+ also reduced taurine binding. Heterogeneous phospholipid vesicles of phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidylserine (18:19:2:1) also bound taurine with positive cooperativity, yielding a bell-shaped Scatchard curve. The affinity of taurine for these mixed phospholipid vesicles was enhanced by the inclusion of cholesterol (50%). Taurine associated in a maximum ratio of 1:1 with homogeneous vesicles of phosphatidylcholine or phosphatidylserine. Vesicles of phosphatidylethanolamine bound taurine in a maximum ratio of 2:1, whereas those of phosphatidylinositol bound insignificant amounts of taurine. These studies demonstrate a low affinity binding to sarcolemma of taurine at concentrations normally present in rat heart. Similar levels of binding were observed in phospholipid vesicles, suggesting that the interaction of taurine with biological membranes involves phospholipids.

Synergistic effects of micromolar concentrations of Zn2+ and Ca2+ on membrane fusion

Resonance Energy Transfer between N-(7-nitro-2,1,3 benzoxadiazol -4 yl) phosphatidyl ethanolamine and N-Lissamine-Rhodamine B sulfonyl) phosphatidyl ethanolamine embedded in two different populations of small unilamellar vesicles made of phosphatidyl serine has been used to study the fusion process induced by Zn2+ and Ca2+. Lipid intermixing demonstrating fusion of liposome membranes can already be observed at 125 and 250 mumol/l of Zn2+. After short time pre-incubations with micromolar concentrations of Zn2+ as low as 150 mumol/l, Ca2+ induces an instantaneous increase of vesicle fusion. The lipid intermixing induced by micromolar concentrations of Ca2+ (250-500 mumol/l) could be increased up to 4 times when pre-incubated with 150 or 200 mumol/l of Zn2+. The effect of 1 mM of Ca2+ alone on lipid intermixing can be mimicked by 150 mumol/l of Zn2+ followed by 500 mumol/l of Ca2+. Our data demonstrate that Zn2+ and Ca2+ act synergistically to affect cation-induced membrane fusion. We suggest that Zn2+ specifically alters the physical state of phospholipid membranes making them more prone to calcium-triggered fusion.

Effect of tetracaine on membrane-bound acetylcholinesterase activity and anilinonaphthalene sulphonate-induced membrane fluorescence

Tetracaine (25-300 microM) reversibly inhibits (in vitro) AChE activity of rat brain synaptosome (4.4-100%) and erythrocyte membrane (3.9-65.2%) in a concentration dependent manner. IC50 values of tetracaine for AChE of synaptosome and erythrocyte membrane are 88 and 200 microM respectively. At sub-inhibitory concentrations (less than or equal to 10 microM) tetracaine activates (8.7-23%) AChE of synaptosome but not of erythrocyte membrane. Lineweaver-Burk plots indicate that tetracaine-induced inhibition of AChE is competitive in nature and Ki value decreases on increasing the concentration (greater than 100 microM) of tetracaine in both synaptosome and erythrocyte membrane. Tetracaine (25-500 microM) produces a concentration dependent increase in 1-anilino-8-naphthalene sulphonate (ANS)-induced relative fluorescence (F470) of both synaptosomal (6.5-102%) and erythrocyte membrane (2.4-53.3%) without shifting their emission maxima (470 nm). Further it is also noted that the quantum yield of F470 of both the membranes increases with the increase of tetracaine concentrations (100-500 microM). These results suggest that the interaction of tetracaine with both the enzyme and its lipid microenvironment may be the cause of inhibition of membrane-bound AChE activity.

Effect of anaesthetics on calcium-induced luminescence of aequorin

The effect of some general and local anaesthetics on the calcium-induced luminescence of aequorin was studied in vitro using a photomultiplier tube and recording technique. Purified aequorin (0.1 microliter) was injected into a 500 micron diameter porous cellulose acetate capillary tube containing 0.5 M KC1, 20 mM phosphate (pH 7.2) and calcium-EGTA buffers. The trapped aequorin was superfused with buffer solutions which sometimes contained anaesthetic (test) solutions. The results showed that some anaesthetics, e.g. urethane, etomidate and lignocaine, increased whereas others, e.g. methohexitone, thiopentone, decreased the light output (luminescence) of aequorin in constant ionized calcium and EGTA buffers. Similar results were produced by some non-anaesthetic drugs, e.g. glycerol, TEA, caffeine, etc. Concentration-response curves for calcium-dependent and -independent luminescence of aequorin showed that anaesthetics variously affected the aequorin response. Some anaesthetics, e.g. lignocaine, increased the maximum response while others, e.g. etomidate, increased the affinity (i.e. decreased EC50s) of aequorin to calcium ions without altering the slope, which remained at about 2. It was concluded that anaesthetics can either excite or depress aequorin luminescence, the effect being dependent on the type and the concentration used.

General anaesthetics: effects on transmitter release

In this report, the physiological effects, observations and events leading to transmitter release in both central nervous system and peripheral synapses are discussed. The presynaptic modulation of transmission at the central nervous system and at the neuromuscular junction was investigated using electrophysiological and neurochemical techniques. It was concluded that various general anaesthetics may affect the presynaptic mechanism of transmission, but these effects were controversial. For example, terminal excitability, which is taken as an index for presynaptic activity, could either be reduced or increased by general anaesthetics. Similar conflicting effects have been reported for the action of general anaesthetics on spontaneous and evoked release of ACh, uptake and release of intracellular Ca2+, and choline transport into the presynaptic nerve terminals.

Effects of ZnCl2 on membrane interactions in myelin of normal and shiverer mice

X-ray diffraction was used to record the effects of metal cations on the structure of peripheral nerve myelin. Acidic saline (pH 5.0) either with or without added metal cations caused myelin to swell by 10-20 A from its native period of 178 A. The X-ray patterns usually showed broad reflections, and higher orders were either weak or unobserved. With added ZnCl2, however, the swollen myelin gave diffraction patterns that retained sharp reflections to approx. 15 A spacing. Alkaline saline (pH 9.7) containing ZnCl2 produced a reduction of the myelin period by approx. 5 A which was at least twice as much as that produced by other metals. To examine the underlying chemical basis for these unique interactions of Zn2+ with myelin, we carried out parallel X-ray experiments on sciatic nerve from the shiverer mutant mouse, which lacks the major myelin basic proteins. Shiverer myelin responded like normal myelin to ZnCl2 in acidic saline; however, in alkaline saline shiverer myelin showed broadened X-ray reflections which indicated disordering of the regularity of the membrane arrays, and additional reflections were recorded which indicated lipid phase separation. This breakdown may come about by the binding of Zn2+ to negatively-charged lipids which could be more exposed due to the absence of myelin basic proteins. Electron density profiles were calculated on the assumption that, except for changes in their packing, the myelin membranes were minimally altered in structure. For both normal and shiverer myelins, treatments under acidic conditions resulted in swelling at the extracellular apposition and a slight narrowing of the cytoplasmic space. This swelling is likely due to adsorption of protons and divalent cations. Interaction between Zn2+ and myelin P0 glycoprotein could preserve an ordered arrangement of the apposed membrane surfaces. Alkaline saline containing ZnCl2 produced compaction at the cytoplasmic apposition in both normal and shiverer myelins possibly through interactions with a portion of P0 glycoprotein which extends into the cytoplasmic space between membranes.

Lipid metabolism in fungi

So far, reviews that have appeared on fungal lipids present data mainly on the lipid composition of these organisms and the influence of lipids on their physiology. These reviews provide little information about the enzymes of lipid metabolism in these organisms and it is assumed, by most workers, that lipid synthesis in all fungi takes place as in Saccharomyces cervesiae, the only fungus in which the complete pathways of phospholipid biosynthesis have been worked out. During the last few years, literature has accumulated on lipid metabolic enzymes of other fungi, as investigators became increasingly interested in this area of research. The present review, after an introduction, will be divided into different sections and each section will deal, comparatively, with various aspects of fungal lipid metabolism and physiology. This review will, therefore, bring out the differences or similarities of lipid metabolism in diverse fungal species.

Isolation of a fraction with Ca2+ ionophore properties from rat liver mitochondria

Isolation of a small protein with properties of a Ca2+ ionophore from calf heart mitochondria has recently been reported [A. Y. Jeng and A. E. Shamoo, 1980, J. Biol. Chem. 255, 6897, 6904]. We have isolated a fraction with similar physical and chemical properties from rat liver mitochondria. In particular, the hepatic preparation is able to bind Ca2+ with high affinity in such a fashion that the resultant complex is soluble in a hydrophobic phase. It will also transport Ca2+ through a stirred organic phase (Pressman cell). Interaction of the liver preparation with Ca2+ is sensitive to inhibitors of mitochondrial Ca2+ uptake. The hepatic preparation contains both protein and lipid components. The phospholipid components were identified and the behavior of a similar mixture of commercially available phospholipids was compared to that of the ionophore fraction from rat liver mitochondria. All of the Ca2+ binding properties of the rat liver preparation could be mimicked by the lipids. In a preliminary experiment, reduction of the phospholipid content of the preparation to less than one lipid phosphate per protein molecule (assuming a molecular weight of 3000 by analogy with the calf heart case) resulted in a protein that was unable to bind Ca2+. We, therefore, suggest that the ability of the preparation to interact with Ca2+ is due to the constituent phospholipids. Measurements of phospholipid-Ca2+ interactions in the model systems and under the conditions of low (microM) Ca2+ and phospholipid concentration utilized here demonstrated an affinity for Ca2+ (Ks approximately 1 microM) and a cation selectivity that have not previously been reported.

Drug-induced surface potential changes of lipid vesicles and the role of calcium

The effects of four different drugs with local anesthetic properties were investigated on the surface potential of phospholipid vesicles composed of electrostatically neutral lipids (phosphatidylcholine), negatively charged lipids (phosphatidylserine) and a mixture of acidic and neutral lipids (soy bean lipids). Propranolol, tetracaine, lidocaine and procaine decrease the negative surface potential of phosphatidylserine and soy bean liposomes and increase that of phosphatidylcholine liposomes. The drugs interact with the liposomes in such a way that the protonated amine groups point towards the polar head groups of the phospholipids and the rest of the molecule is probably incorporated into the hydrophobic core of the lipid bilayer. The same sequence in drug activity normally measured in biological tissues (propranolol greater than tetracaine greater than lidocaine greater than procaine) is found for the surface potential change of the phospholipids. Calcium prevents the binding of the drugs to phosphatidylserine, especially the binding of lidocaine and procaine. Because of its high affinity for negative surface charges, Ca2+ chelates with phosphatidylserine and blocks the incorporation of the drug molecule. Vice versa, when incorporated into the liposomal bilayer, the drug blocks the interaction of calcium. These antagonistic effects are only observed in liposomes made from acidic phospholipids and not in those made from pure electrostatically neutral lipids like phosphatidylcholine.

Interactions between bis(guanylhydrazones) and polyamines in isolated mitochondria

The interactions of naturally occurring polyamines: putrescine, spermidine and spermine, with anticancer bis-guanylhydrazones: methylglyoxal-bis(guanylhydrazone) (MGBG) and 4,4'-diacetyldiphenylurea-bis(guanylhydrazone) (DDUG) were investigated at the level of mitochondrial membrane. The effects of bis-guanylhydrazones on intact rat liver mitochondria were readily prevented or reversed by polyamines and these interactions were also affected by the mitochondrial transmembrane potential. Magnesium cations enhanced the protective action of polyamines. The data indicate that competition exists between the essential anticancer bis(guanylhydrazone) and polyamines for low affinity negatively charged binding sites at the outer surface of inner mitochondrial membrane. The study of drug interactions was extended to the level of isolated tumor mitochondria from rat HTC hepatoma and murine L1210 leukemia cells. A complicated pattern of interactions between the anticancer bis-guanylhydrazones and phenethylbiguanide was obtained.

Effects of divalent cations, trypsin, and phospholipases on the passive permeability to sodium of inside-out vesicles from human red cells

Inside-out vesicles (IOV) were prepared from human red blood cells. Steady-state uptake of 23Na was observed to generally follow an exponential time course with a rate constant of 1.57 +/- 0.09 h-1 (SE). One week of cold storage (0-4 degrees C) increased the rate constant to 2.50 +/- 0.12 h-1 (SE). Mg2+, Ca2+, or Sr2+ decreased the rate of 22Na uptake with no observable differences between the three divalent cations when tested at concentrations of 50 microM. Mg2+ was shown to decrease the rate of 22Na uptake at concentrations as low as 5 microM with maximal effect at 50 to 100 microM. The decrease in rate of 22Na uptake induced by Mg2+ could be enhanced by exposure of IOV to Mg2+ for longer periods of time. Trypsin treatment of OIV increased the rate of uptake of 22Na and was dependent on the concentration of trypsin added between 5 to 25 micrograms/ml (treated for 5 min at 25 degrees C). The ability of Mg2+ (50 microM) to decrease the rate of 22Na uptake was still observed after maximal trypsin treatment. Phospholipase A2 or phospholipase C treatment of IOV increased the rate of 22Na uptake and was dependent on the amount of phospholipase A2 (0.1 to 1.0 units/ml) or phospholipase C (0.25 to 2.5 units/ml) added (treated for 5 min at 25 degrees C). After phospholipase A2 treatment, the observed decrease in the rate of 22Na uptake induced by Mg2+ (50 microM) was generally greater than controls. After phospholipase C treatment, the observed decrease in rate of 22Na uptake induced by Mg2+ (50 microM) was less or absent when compared with controls. Phospholipase C treatment was less effective in preventing the Mg2+ effect the longer IOV were exposed to Mg2+. The results suggest that Mg2+ binds to phospholipid headgroups to reduce Na permeability perhaps by inducing a change in bilayer structure or phospholipid association.

Enhancement of 45Ca2+ binding to acidic lipids by barbiturates, diphenylhydantoin, and ethanol

Effects of barbiturates, diphenylhydantoin, and ethanol on 45Ca2+ binding to acidic lipids have been examined in an organic, solvent-aqueous partition system. Hexobarbital, pentobarbital, and phenobarbital, at concentrations of 0.3 and/or 0.6 mM, enhanced the binding of 45Ca2+ to phosphatidic acid, phosphatidylserine, and sulfatide but not to phosphatidylinositol or cardiolipin. Diphenylhydantoin, 0.3 mM, enhanced 45Ca2+ binding to phosphatidic acid and phosphatidylserine but not to sulfatide. Ethanol at 80 mM did not enhance 45Ca2+ binding to phosphatidic acid, but ethanol decreased the binding to cardiolipin and increased it to sulfatide.

Calcium- and magnesium-induced fusion of mixed phosphatidylserine/phosphatidylcholine vesicles: effect of ion binding

The aggregation, leakage, and fusion of pure PS (phosphatidylserine) and mixed PS/PC (phosphatidylcholine) sonicated vesicles were studied by light scattering, the release of encapsulated carboxyfluorescein, and a new fusion assay which monitors the mixing of the internal compartments of fusing vesicles. On a time scale of 1 min the extent of fusion was considerably greater than leakage. The Ca2+ and Mg2+ concentrations required to induce fusion increased when the PS content of the vesicles was decreased, and/or when the NaCl concentration was increased. Calculations employing a modified Gouy-Chapman equation and experimentally determined intrinsic binding constants of Na+ and Ca2+ to PS were shown to predict correctly the amount of Ca2+ bound in mixed PS/PC vesicles. For vesicles composed of either pure PS or of mixtures with PC in 100 mM NaCl (4:1 and 2:1 PS/PC); the induction of fusion (on a time scale of minutes) occurred when the amount of Ca or Mg bound/PS molecule exceeded 0.35-0.39. The induction of fusion for both pure PS and PS/PC mixed vesicles (with PS exceeding 50%) can be explained by assuming that destabilization of these vesicles requires a critical binding ratio of divalent cations to PS.

Lidocaine-induced reduction in size of experimental myocardial infarction

In anesthetized open chest dogs, the effects of therapeutic doses of lidocaine on myocardial cell respiration, creatine kinase depletion, left ventricular stroke work and cardiac necrosis were assessed. The dogs were subjected to 40 minutes of occlusion of the left anterior descending coronary artery, followed by 5 hours of reperfusion. Group I (12 dogs) had infusion of saline solution; group II (8 dogs) had infusion of 0.2 mg/kg per min of lidocaine (serum level 16.8 +/- 1.4 microgram/ml); group III (5 dogs) had infusion of 0.04 mg/kg per min or lidocaine (serum level 3.6 micrograms/ml). Ischemic regional myocardial blood flow (measured by 9 micrometer spheres of strontium-85) was 6.34 +/- 1.62 ml/100 mg per min in group I, 1.48 +/- 0.59 in group II (p < 0.05) and 1.32 +/- 0.50 in group III (p < 0.05). Oxygen consumed during conversion of adenosine diphosphate to adenosine triphosphate in mitochondria from control and lidocaine-treated ischemic tissue was depressed (p < 0.05) and correlated (r = 0.63) with creatine kinase depletion. Left ventricular stroke work was not significantly different among the three groups. Infarct size (in percent of left ventricular weight) was 12.6 +/- 2.0 for group I, 4.8 +/- 1.2 for group II (p < 0.01) and 4.8 +/- 2.5 for group III (p < 0.05). The data suggest that the reduction of myocardial infarct size by lidocaine was not dependent on enhanced myocardial blood flow and was independent of left ventricular stroke work.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.

Divalent cation-induced interaction of phospholipid vesicle and monolayer membranes

The effects of phospholipid vesicles and divalent cations in the subphase solution on the surface tension of phospholipid monolayer membranes were studied in order to elucidate the nature of the divalent cation-induced vesicle membrane interaction. The monolayers were formed at the air/water interface. Various concentrations of unilamellar phospholipid (phosphatidylserine, phosphatidylcholine and their mixtures) vesicles and divalent cations (Mg2+, Ca2+, Mn2+, etc.) were introduced into the subphase solution of the monolayers. The changes of surface tension of monolayers were measured by the Wilhelmy plate (Teflon) method with respect to divalent ion concentrations and time. When a monolayer of phosphatidylserine and vesicles of phosphatidylserine/phosphatidylcholine (1 : 1) were used, there were critical concentrations of divalent cations to produce a large reduction in surface tension of the monolayer. These concentrations were 16 mM for Mg2+, 7 mM for Sr2+, 6 mM for Ca2+, 3.5 mM for Ba2+ and 1.8 mM for Mn2+. On the other hand, for a phosphatidylcholine monolayer and phosphatidylcholine vesicles, there was no change in surface tension of the monolayer up to 25 mM of any divalent ion used. When a phosphatidylserine monolayer and phosphatidylcholine vesicles were used, the order of divalent ions to effect the large reduction of surface tension was Mn2+ greater than Ca2+ greater than Mg2+ and their critical concentrations were in upon vesicle concentrations as well as the area/molecule of monolayers. For phosphatidylserine monolayers and phosphatidylserine/phosphatidylcholine : 1) vesicles, above the critical concentrations of Mn2+ and Ca2+, the surface tension decreased to a value close to the equilibrium pressure of the monolayers within 0.5 h. This decrease in surface tension of the monolayers is interpreted partly as the consequence of fusion of the vesicles with the monolayer membranes. The order and magnitude of divalent cation concentrations at which phosphatidylserine/phosphatidylcholine (1 : 1) and phosphatidylserine vesicle suspensions showed a large increase in turbidity were similar to those obtained in the above mentioned experiments.

Surface potential of phosphatidylserine monolayers. I. Divalent ion binding effect

Surface potentials of phosphatidylserine monolayers have been measured in the presence of different divalent ion concentrations in order to determine the way in which divalent ions bind to the membrane surface. The association constants for divalent ions (Mg2+, Ca2+ and Mn2+) with the phosphatidylserine membrane have been obtained from the experimental data and simple ion binding theory. The order of divalent ion binding to the membrane is Mn2+ greater than Ca2+ greater than Mg2+. However, none of the divalent ions used completely neutralized the negative charge of phosphatidylserine even at relatively high concentrations. The amounts of the divalent ions bound depended upon the concentration of the monovalent ions present in the subphase. It is suggested that the amounts of bound ions obtained from the use of radioisotope tracer methods may include a considerable contribution from the excess free ions in the double layer region of the phosphatidylserine membrane.

Effects of neomycin on polyphosphoinositides in inner ear tissues and monomolecular films

The effect of neomycin on polyphosphoinositides was studied in vivo and in vitro. In vivo, the incorporation of 32Pi into phosphatidylinositol phosphate and phosphatidylinositol diphosphate was measured in inner ear tissues. Concentrations of neomycin which decreased the electrophysiological response of the chochlea to sound stimulation also decreased labeling of phosphatidylinositol diphosphate. In vitro experiments with brain tissues and polyphosphoinositide extracts indicated a direct interaction between the lipids and neomycin. Neomycin interacts strongly with monomolecular films of polyphosphoinositides. The interaction appears to be complex and is a function of neomycin concentration in the subphase and surface pressure of the film. Condensation of the polyphosphoinositide film is favored at low neomycin concentrations and low film pressures while expansion of the film is favored at high neomycin concentrations and high film pressures. The interactions of neomycin with other negatively charged films (phosphatidyl inositol and phosphatidyl serine) are much weaker, particularly at low neomycin concentrations. The metabolic and physiological consequences of the neomycin/polyphosphoinositide interaction are discussed in regard to the ototoxicity of the drug.

Aspects of the mechanism of action of local anesthetics on the sarcoplasmic reticulum of skeletal muscle

1. The effect was studied of local anesthetics (tetracaine, dibucaine, procaine and xylocaine) on the forward and the backward reactions of the calcium pump of skeletal muscle sarcoplasmic reticulum. 2. The inhibition of the rate of calcium uptake, the rate of calcium-dependent ATP splitting and the rate of calcium-dependent ATP-ADP phosphate exchange by sarcoplasmic reticulum in the presence of the above drugs is at least partially due to the inhibition of the phosphoprotein formation from ATP. 3. The rate of the ADP-induced calcium release from sarcoplasmic reticulum and the rate of ATP synthesis driven by the calcium efflux are inhibited on account of a reduction of the phosphoprotein formation by orthophosphate. 4. The phosphorylation of calcium transport ATPase by either ATP or orthophosphate is diminished by the local anesthetics owing to a reduction in the apparent calcium affinity of sarcoplasmic reticulum emmbranes on the outside and on the inside, respectively. 5. The drug-induced calcium efflux from calcium-preloaded sarcoplasmic reticulum vesicles, a reaction not requiring ADP, is probably not mediated by calcium transport ATPase.

Divalent cations: effects on post-synaptic pharmacology of invertebrate synapses

(1) The effects of divalent cations (Ca++, Mg++, Sr++ and Co++) were studied on the post-synaptic responses of crustacean neuromuscular junctions and identified molluscan neurons to bath and iontophoretic application of putative transmitters. (2) The glutamate response of the crustacean muscle was parabolically dependent on [Ca++]0, while the ACh response of an identified molluscan neuron was inversely dependent on[Ca++]0. Elevated [Ca++]0 depressed both glutamate and ACh depolarizations in a concentration-dependent, reversible manner. Low concentrations of Co++ also depressed both depolarizations in a concentration-dependent, reversible manner. (3) Double-reciprocal plot analyses of the Ca++ and Co++ depressions indicate that these agents were apparently not acting to reduce the affinity of the receptor for the agonist. Elevated concentrations of both Ca++ and Co++ shifted the inversion potential of the ACh response in a hyperpolarizing direction, suggesting a preferential block of the receptor-coupled Na+ conductance. (4) Neither Ca++ nor Co++ depressed Cl- or K+-dependent responses coupled to the putative transmitters GABA, glutamate, dopamine or ACh. (5) The selective inhibition of the ACh and glutamate responses by the general anesthetic pentobarbital was examined as a function of[Ca++]0. Decreasing [Ca++]0 by 5-fold decreased the pentobarbital inhibition by about 50% while increasing [Ca++]0 by 5-fold produced an insignificant increase in the inhibition. (6) The data indicate that divalent cations, like general anesthetics, selectively depress post-synaptic excitatory responses that are primarily Na+-dependent. This selective depression by Ca++ could contribute to its anesthetic and anticonvulsant properties when present in elevated concentrations in the ventricular fluid. The mechanism by which divalent ions and general anesthetics selectively depress receptor-coupled conductances appear to be different: divalent ions preferentially attack the Na+ component while anesthetics block Na+ and K+ conductance equally (possibly by affecting the kinetics of the mechanism).

Phase transitions and fluidity characteristics of lipids and cell membranes

The concept that the liquid crystalline or mesomorphic condition was of importance to biological systems is a relatively old idea. Thus Bernal (1933) when discussing the different types of arrangements of molecules in liquid crystals commented ‘Such structures belong to the liquid crystal as a unit and not to its molecules which may be replaced by others without destroying them and they persist in spite of the complete fluidity of the substance. These are just the properties to be required for a degree of organization between that of the continuous substance, liquid or crystalline solid and even the simplest living cell.’ Stewart (1961) some thirty years later also stated that ‘It is this property – the combination of flow and lability with a preferred and relatively stable molecular orientation – that makes the mesomorphic (i.e. liquid crystal) phase uniquely appropriate to the structure of protoplasm and living tissue.’

Studies on bursting pacemaker potential activity in molluscan neurons. I. Membrane properties and ionic contributions

Bursting pacemaker potential (BPP) activity of identified molluscan neurons has been studied using cells from Aplysia and Otala. The results presented in this paper indicate that (1) a potassium conductance mediates the hyperpolarizing phase of the BPP; (2) the BPP amplitude is directly dependent on [Na+]0; (3) BPP activity requires the presence of divalent cations and is prevented by C02+ and La3+, but not D-600; (4) the apparent increase in membrane resistance during the depolarizing phase of the Bd can be accounted for by the movement of the membrane potential along the non-linear portion of the I-V curve; and (5) non-linear I-V relations and a minimal effective membrane resistance are pre-requisite to BPP generation. Coupled with recent observations on the presence of an inward current in these cells, the results suggest that the mechanisms underlying the BPP are similar to those proposed to describe the myocardial pacemaker potential: the hyperpolarizing phase is due to activation of a potassium conductance which slowly inactivates, resulting in a gradula deplorization until a voltage-dependent inward current is activated which then leads to an increasingly rapid deplorization and initiation of the burst of spikes. It would appear that Na+ may play the major role in carrying the inward current, although a secondary role for divalent cations cannot be discounted.

Effects of internal divalent cations on voltage-clamped squid axons

We have studied the effects of internally applied divalent cations on the ionic currents of voltage-clamped squid giant axons. Internal concentrations of calcium up to 10 mM have little, if any, effect on the time-course, voltage dependence, or magnitude of the ionic currents. This is inconsistent with the notion that an increase in the internal calcium concentration produced by an inward calcium movement with the action potential triggers sodium inactivation or potassium activation. Low internal zinc concentrations ( approximately 1 mM) selectively and reversibly slow the kinetics of the potassium current and reduce peak sodium current by about 40% with little effect on the voltage dependence of the ionic currents. Higher concentrations ( approximately 10 mM) produce a considerable (ca. 90%) nonspecific reversible reduction of the ionic currents. Large hyperpolarizing conditioning pulses reduce the zinc effect. Internal zinc also reversibly depolarizes the axon by 20-30 mV. The effects of internal cobalt, cadmium, and nickel are qualitatively similar to those of zinc: only calcium among the cations tested is without effect.

The effects of diphenylhydantoin and potassium on the biological activity of ouabain in the guinea-pig heart

1. Diphenylhydantoin (DPH) and potassium significantly prevent ouabain intoxication without preventing the inotropic effects of ouabain in the guinea-pig isolated heart.2. The antiarrhythmic effect of DPH and K on ouabain-induced toxicity appears to be related to their ability to reduce ouabain accumulation by the myocardium and thereby prevent the intracellular Na and K changes which lead to the arrhythmic state.