Saturable and unsaturable binding of a volatile anesthetic enflurane with model lipid vesicle membranes

Effects of isoflurane, halothane, and chloroform on the interactions and lateral organization of lipids in the liquid-ordered phase

The first quantitative insight has been obtained into the effects that volatile anesthetics have on the interactions and lateral organization of lipids in model membranes that mimic "lipid rafts". Specifically, nearest-neighbor recogntion measurements, in combination with Monte Carlo simulations, have been used to investigate the action of isoflurane, halothane, and chloroform on the compactness and lateral organization of cholesterol-rich bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in the liquid-ordered (l(o)) phase. All three anesthetics induce a similar weakening of sterol-phospholipid association, corresponding to ca. 30 cal/mol of lipid at clinically relevant concentrations. Monte Carlo lattice simulations show that the lateral organization of the l(o) phase, under such conditions, remains virtually unchanged. In sharp contrast to their action on the l(o) phase, these anesthetics have been found to have a similar strengthening effect on sterol-phospholipid association in the liquid-disordered (l(d)) phase. The possibility of discrete complexes being formed between DPPC and these anesthetics and the biological relevance of these findings are discussed.

Phospholipid complexation of general anesthetics in fluid bilayers

A nearest-neighbor recognition analysis has been performed in cholesterol-rich and cholesterol-poor liposomes derived from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in the presence of varying concentrations of chloroform. This analysis has yielded a fundamentally new, molecular-level view of the interaction of general anesthetics with lipid bilayers, which may be relevant to their biological action; that is, DPPC forms 1:1 complexes with CHCl(3) in both membranes in the fluid bilayer state.

The effect of isoflurane on erythrocyte membranes studied by ATR-FTIR

The effect of isoflurane on erythrocyte membranes has been investigated by means of attenuated total reflection infrared spectroscopy. Infrared spectra were measured on sonicated erythrocyte ghosts layered upon a ZnSe crystal covered with D(2)O saline solutions containing increasing amounts of isoflurane. At clinically relevant anesthetic concentrations and 37 degrees C, significant changes in the structural and dynamic properties of the membrane phospholipid bilayers are observed. Both the acyl chain methylene symmetric and asymmetric stretching modes and the carbonyl ester stretching band displayed frequency shifts interpreted as transitions toward disordered liquid-like structure accompanied by dehydration of the phospholipid polar heads. In turn, no secondary structure-linked changes are observed in the amide I region of membrane proteins. Higher anesthetic concentrations (500-900 microM), resulted in progressive detachment of the multilayers from the ATR crystal and irreversible formation of denatured protein. Polarization studies in correspondence of the acyl lipid methylene stretching bands indicated that isoflurane decreases the dichroic ratio thus inducing disorder in the orientation of the lipid acyl chains.

Interaction of cisplatin with planar model membranes - dose dependent change in electrical characteristics

The drug cisplatin has broad antineoplastic activity against advanced testicular and ovarian cancers, epithelial malignancies, cancers of the head, neck, bladder, oesophagus and lungs. Peripheral neurotoxicity, ototoxicity and nephrotoxicity are its major side effects. The nonspecific action of this drug on the lipid bilayer architecture of membranes has been studied by following the effects produced on the electrical characteristics of model planar bilayer lipid membranes (BLM). The results confirm that the drug has a strong surface interaction with the zwitterionic polar head groups of the amphipathic phospholipids constituting the BLM. The permeability characteristics of cisplatin through the hydrophobic core are limited. Cisplatin does not fluidise the membrane sufficiently to cause its breakdown but creates small ion conducting defects on the membrane bilayer resulting in a marginal increase in ion conductivity. These results indicate that cisplatin exhibits a non-specific action on the lipid bilayer component of the membrane which might be partly responsible for its neurotoxic side effects.

Electrokinetic charge of the anesthetic-induced bR480 and bR380 spectral forms of bacteriorhodopsin

The translational and rotational electrokinetics of the anesthetic-induced spectral transitions bR568-->bR480-->bR380 of bacteriorhodopsin have been investigated. Formation of the bR480 form is associated with an increase of the purple membrane negative electrokinetic charge, while the transformation of bR480 into bR380 is accompanied by a decrease of the membrane negative charge as compared to that of the 480 nm-absorbing form. Removal of anesthetics leads to the back transitions bR480-->bR568 and (in part) bR380-->bR568; however, the electrokinetic charge of the native membranes is not restored. A strong decrease in the electric polarizability and the appearance of a slow polarizability component are also observed in anesthetic-treated membranes. Comparison with the electrokinetic behaviour of partially delipidated membranes and with that of liposomes composed of purple membrane total lipids suggests that: (i) anesthetic molecules partition mainly at the protein/lipid interface inducing irreversible rearrangement of the boundary lipid layer, and (ii) different mode(s) or site(s) of interaction are responsible for the spectral and surface charge effects. The data are compatible with the hypothesis of anesthetics acting through partial dehydration of the membrane surface.

Saturable binding of halothane to rat brain synaptosomes

The hypothesis that volatile anesthetics act directly on or bind specifically to membrane proteins remains controversial. In earlier in situ electron probe microanalysis studies in cardiac muscle we showed preferential partitioning of halothane into mitochondria. To determine whether partitioning represents saturable binding or simple solubility, a photoaffinity labeling method was developed for halothane to examine binding in rat brain synaptosomes. Radioligand binding assays were then used to determine binding parameters for this important inhalational anesthetic. UV-light exposure of synaptosomes incubated with clinical concentrations of [14C]halothane resulted in sufficient labeling to allow characterization of binding sites. Analysis of saturation and competition curves showed that greater than 60% of [14C]halothane photolysis product binding to synaptosomes was specific, with low affinity (Kd = 0.49 +/- 0.16 mM) and high binding site concentration (Bmax = 1.87 +/- 0.75 nmol/mg of protein). Halothane photoaffinity labeling was partially inhibited by isoflurane (20%), chloroform (44%), 2-bromotrifluoroethane (20%), and dichlorotrifluoroethane (20%) but not by ethanol. The Kd measured with this photoaffinity approach is similar to the concentration of halothane required to produce anesthesia in rats.

Interfacial dehydration by anesthetics: an electrocapillary study of surface charge density of adsorbed monolayer

We have proposed that anesthetics destruct the hydration shell of macromolecules irrespective of lipid membranes or proteins. These macromolecular structures are supported by the hydrogen-bonded matrix of water molecules. A loss of this support destabilizes the membranes and proteins. The disordered structures are suboptimal for the assigned biological functions, and anesthesia may ensue. We postulated that the dehydration is prompted mainly by the decrease in the interactions of the surface charges with the water dipole. To prove or disprove the above hypothesis, this study measured the effect of volatile anesthetics (chloroform, halothane, and enflurane) on the surface charge density in adsorbed monolayers by an electrocapillary method. The oil phase was methylisobutylketone (MIBK) with cetyltrimethylammonium chloride (CTAC). The aqueous phase was 0.1 M NaCl. The anesthetics decreased the surface charge density, and the effect paralleled the clinical anesthetic potency. At concentrations that induce surgical stage anesthesia in 50% of the population, these anesthetics reduced the surface charge density by 5%.

Malignant hyperthermia: an altered phospholipid and fatty acid composition in muscle membranes

There is thought to be a genetic defect within the calcium release channel of the sarcoplasmic reticulum in malignant hyperthermia (MH). This primary alteration is hypothesized to influence the function and/or structure of various muscle membrane systems; e.g., to have a direct effect on the composition of the lipid matrix. Therefore, in striated muscle samples, we determined the quantity and fatty acid composition of the various types of membrane phospholipids. German Landrace pigs were classified as normal or susceptible to MH. Total lipid content from longissimus dorsi, semi-membranosus muscle, and heart left ventricular (HLV) samples were extracted with chloroform/methanol and subsequently separated by high performance liquid chromatography. The single phospholipid fractions were collected and, following derivatization, the quantities of individual fatty acids were determined using a capillary gas chromatographic method. In general, samples from the susceptible pigs contained lower absolute amounts of individual phospholipids. The most notable differences occurred in the HLV, where phosphatidylinositol, phosphatidylserine, phosphatidylethanolamine and sphingomyelin were all significantly less (P less than or equal to 0.05). The muscle from the susceptible animals also contained decreased amounts of the polyunsaturated phospholipid-bound fatty acids (P less than or equal to 0.05). These differences in phospholipid and fatty acid concentrations of membranes isolated from swine susceptible to MH may relate to their apparently increased sensitivities to halothane (e.g., fluidizing effects) or elevated temperature.

High pressure and anesthesia: compressibility, molal volume, and partial molal volume of volatile anesthetics

This study was undertaken to provide volume data of volatile anesthetics under high pressure. Molal volumes of liquid halothane, enflurane, and isoflurane and their partial molal volumes in water and in 1-octanol were determined by high-precision solution densitometry at 25.000 +/- 0.0005 degrees C over the pressure range from ambient to 34.56 MPa (341 atm). The isothermal compressibilities of the pure anesthetics and their isothermal partial molal compressibilities at their infinite dilution in water and in 1-octanol have also been calculated at 0.1013 MPa (1 atm).

Infrared spectra of phospholipid membranes: interfacial dehydration by volatile anesthetics and phase transition

Fourier-transform infrared attenuated total reflection (ATR) spectroscopy was used to study the effect of volatile anesthetics on fully hydrated dipalmitoylphosphatidylcholine (DPPC) vesicle membranes. The main phase transition was monitored by the change in the C-H2 asymmetric stretching frequencies of the lipid tails. The surface property was analyzed by the changes in the P = O stretching, (CH3)3-N+ stretching of the hydrophilic head, and C = O stretching of the glycerol skeleton. The partial pressures of those agents that decreased the transition temperature 1.0 C degree were halothane 0.75, enflurane 1.90 and CCl4 0.85 kPa. At a 2:1 lipid/anesthetic mole ratio, the polar anesthetics, halothane and enflurane, increased the ratio of (P = O stretching band area)/((CH3)3-N+ stretching band area) by 26.3% and 21.1%, respectively, whereas apolar CCl4 increased it 10.5%. The water molecules bound to the P = O moiety are apparently replaced by the anesthetic molecules. The deconvoluted C = O spectra showed two peaks: free sn-1 that is closer to the lipid core and hydrogen-bonded sn-2 that is closer to the polar head. Addition of halothane and enflurane, but not CCl4, increased the number of peaks to three. The third peak is free sn-2, formed by disrupting hydrogen-bonding to water. Because the temperature-induced spectral change was limited to C-H2 stretching at the main phase transition, the effects of anesthetics on the lipid membrane structure are not identical to temperature elevation. Among anesthetics, the effects of apolar and polar molecules on the interfacial properties are different.