Model for action of local anaesthetics

Sodium channels in nerve membranes are postulated to be surrounded by lipid molecules in the gel (or crystalline) phase. Addition of local anasethetics triggers a change in the surrounding lipids to the fluid, liquid crystalline phase, allowing the sodium channel to close with resulting local anaesthesia. The experimental evidence for this model is discussed.

Interactions between anesthetics and lipid mixtures. Normal alcohols

The effects of normal alcohols up to 1-dodecanol on phase transitions in phosphatidylcholines and phosphatidylethanolamines have been studied using chlorophyll a as fluorescent probe. With the phosphatidylcholines, alcohols up to octanol cause a lowering of the transition temperature, and a broadening of the transition, whereas for dipalmitoylphosphatidylethanolamine, only a lowering of the transition is observed. The lowering of the phase transition temperature in dipalmitoylphosphatidylcholine by butanol and hexanol is close to that expected for ideal behavior, but the behavior of the longer chain alcohols becomes less ideal. The effects of these alcohols on mixtures of lipids have been studied, and they illustrate the care necessary if these plots of temperatures of onset and completion of gel phase formation are to be called "phase diagrams". The effect of 1 -octanol on mixtures of lipids is to increase the proportion of lipid present in the lipid-crystalline state. In contrast, 1-decanol causes an increase in the phase transition temperature for dimyristoylphosphatidylcholine, although it lowers the transition temperature for dipalmitoylphosphatidylcholine, and 1 -dodecanol raises the transition temperature for both of these phosphatidylcholines, although it lowers that for dipalmitoylphosphatidylethanolamine. Dodecanol appears to behave in these lipid bilayer membranes as a lipid with a phase transition temperature of ca. 55 degrees C. Anesthesia is discussed as a phenomenon of liquidus extension: alcohols up to 1 -octanol increase the proportion of lipid in the liquidus state and result in anesthesia, whereas the longer alcohols do not, and result in catalepsy.

A determination of the mobility gradient in lipid bilayers by 13C nuclear magnetic resonance

The spin-lattice relaxation times of individual 13 C nuclei in phospholipid bilayers have been measured by using both natural-abundance 13 C and selectively 13 C-enriched phospholipids. These relaxation times can be analysed to give values for the diffusion coefficients for motion about individual carbon-carbon bonds, and for motion about the long axis of the hydrocarbon chains in the bilayer. The assumptions involved in this analysis are examined, and the values obtained compared with those for n -alkanes, and with the results of studies of molecular motion in lipid bilayers by other methods. The results indicate that motion about carbon-carbon bonds in phospholipid bilayers is relatively rapid, and emphasize the fluid nature of the hydrocarbon chain region of such bilayers. This picture of the bilayer is compared with that obtained by techniques which measure time-average conformational properties of the hydrocarbon chains in the bilayer.

Motility in normal and filamentous forms of Rhodospirillum rubrum

By suitable choice of medium, Rhodospirillum rubrum has been grown both in normal (length 2 mum) and filamentous (length up to 60 mum) forms. Both forms were highly motile, and negatively-stained preparations showed bipolar flagellated cells, with an average of seven flagella at each pole. Motion consisted of a series of runs and tumbles, the ditribution of run time-lengths being Poissonian. Both forms tumbled in response to dark shock and showed negative chemotaxis to oxygen. The observation that the motility pattern was very similar in normal and filamentous forms makes chemical control of tumbling unlikely and favours a system involving membrane potentials.

Fluorescence studies of chlorophyll a incorporated into lipid mixtures, and the interpretation of "phase" diagrams

The fluorescence of chlorophyll a incorporated into liposomes of mixtures of phosphatidylcholines and phosphatidylethanolamines is reported. Plots of fluorescence intensities against temperature show breaks at characteristic temperatures which can be attributed to the onset and completion of solid phase lipid formation. These temperatures can be plotted to give diagrams analogous to the phase diagrams obtained for macroscopic systems. Complications due to "small-system effects" are discussed, and the experimental diagrams are compared with theoretical phase diagrams calculated for ideal mixing. Introduction of cholesterol leads to a reduction in fluorescence intensity, most readily explained by a 1:1 lipid:cholesterol interaction with exclusion of monomeric, fluorescent, chlorophyll a. Interaction of divalent ions with mixtures of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidylserine leads to exclusion of chlorophyll a from the phosphatidylserine.

Segregation of chlorophyll a incorporated into lipid bilayers

Absorption and fluorescence spectra are reported for chlorophyll a incorporated into a number of aqueous phospholipid dispersions. Absorption spectra show that in dipalmitoylphosphatidylcholine bilayers, monomeric and oligomeric forms of chlorophyll a are present in both the gel and liquid crystalline phases. The formation of aggregates of chlorophyll a is reflected in the fluorescence spectra by a marked concentration quenching. In bilayers conatining small proportions of chlorophyll a, a marked increase in aggregation occurs at the transition temperatures that can be detected calorimetrically. At higher concentrations (greater than 1 chlorophyll:100 lipid), the "pretransition" is abolished in the phosphatidylcholines, and the main transition is broadened, consistent with an orientation for the chlorophyll a with the chlorine ring in the head group region and the phytol chain in the fatty acid chain region of the bilayer. In mixtures of saturated and unsaturated lipids, there is no preferential segregation of the chlorophyll a into the unsaturated lipid.

13C-enriched phosphatidylethanolamines from Escherichia coli

Escherichia coli have been grown using [1-13C]acetate and [2-13C]acetate as the sole carbon source. The 13C NMR spectra of the whole cells and spheroplasts can be readily obtained but give limited information. The 13C NMR spectra of t-e isolated 13C-enriched phosphatidylethanolamines are assigned and analysed to give the biosynthetic pathway for acetate incorporation. This method provides a ready source of 13C-enriched phospholipids.

Reconstitution of a calcium pump using defined membrane components

A (Mg(2+) + Ca(2+))ATPase (ATP phosphohydrolase, EC 3.6.1.3) has been purified from sarcoplasmic reticulum using a single step centrifugation procedure. The preparation is >95% pure by weight and contains only 25-30% of the lipid associated with the enzyme in native sarcoplasmic reticulum. The purified enzyme is unable to accumulate Ca(2+). Using a sedimentation-substitution technique, >98% of the lipid associated with the purified enzyme can be replaced by dioleoyl lecithin without grossly affecting the ATPase activity of the enzyme. The Ca(2+) pump can be restored to this dioleoyl lecithin-substituted enzyme by addition of excess sarcoplasmic reticulum lipids in the presence of cholate. Removal of the cholate by dialysis generates a system which accumulates Ca(2+) at a rate and to a level comparable to native sarcoplasmic reticulum. Significant reconstitution of the Ca(2+) pump can also be achieved using excess dioleoyl lecithin, but since the full expression of the capacity to accumulate Ca(2+) requires the presence of oxalate, these vesicles would appear to be more leaky than those reconstituted with an excess of sarcoplasmic reticulum lipids. Of about 90 lipid molecules which are associated with one molecule of ATPase in native sarcoplasmic reticulum, an average of less than one lipid molecule remains in these reconstituted systems. We have therefore achieved a fully functional Ca(2+) pump containing essentially a single protein and exogenous lipid.