The pharmacology of simple molecules

Odor and chemesthesis from brief exposures to TXIB

An experiment explored ability of subjects to detect vapors of the plasticizer TXIB (2,2,4-trimethyl-1,3-pentanediol diisobutyrate) and ethanol via olfaction and via ocular and nasal chemesthesis, i.e. chemically stimulated feel. Testing, tailored to the sensitivity of each subject, produced psychometric functions for individuals. Olfactory detection of TXIB began at concentrations below 1 ppb (v/v), with 50% correct detection at 1.2 ppb. (Comparable detection for ethanol occurred almost two orders of magnitude higher.) Chemesthetic detection of TXIB began at about 500 ppb, with 50% correct detection at 2.1 ppm for the eye and 4.6 ppm for the nose, both close to saturated vapor concentration. (Comparable detection for ethanol occurred essentially three orders of magnitude higher.) Suggestions that TXIB plays a role in generation of irritative symptoms at concentrations in the range of parts-per-billion need to reckon with a conservatively estimated 200-fold gap between the levels putatively 'responsible' for the symptoms and those even minimally detectable via chemesthesis. Neither the variable of exposure duration nor that of mixing offers a likely explanation. Inclusion of ethanol in the study allowed comparisons pertinent to issues of variability in human chemoreception. An interpretation of the psychometric functions for individuals across materials and perceptual continua led to the conclusion that use of concentration as the metric of detection in olfaction inflates individual differences.

PRACTICAL IMPLICATIONS

This study indicated that the plasticizer TXIB could contribute odor at concentrations in the range of parts-per-billion, but could hardly contribute sensory irritation per se, as alleged in reports of some field studies where TXIB has existed amongst many other organic compounds.

Differential lipid affinity of xenobiotics and natural compounds

Octanol-1/water partitioning currently provides the most widely used model system to simulate both phospholipid target lipids and triglyceride storage lipids. A differentiation between the two lipid classes is now achieved by making use of a water-induced lipid phase separation. Coefficients (K(TG/PC)) for partitioning between trioleoylglycerol (TG) and phosphatidylcholine (PC) were determined for 20 xenobiotics and two biological lipids. K(TG/PC) values are related to KOW through the relationship, log K(TG/PC)=0.33 log KOW-1.078. The present results will allow better predictions on whether drugs and xenobiotics are bioaccumulated, degraded or reach toxicity-related sites. In addition, applications to natural lipophilic compounds and disease-related proteins are discussed.

General anaesthetic actions on ligand-gated ion channels

The molecular mechanisms of general anaesthetics have remained largely obscure since their introduction into clinical practice just over 150 years ago. This review describes the actions of general anaesthetics on mammalian neurotransmitter-gated ion channels. As a result of research during the last several decades, ligand-gated ion channels have emerged as promising molecular targets for the central nervous system effects of general anaesthetics. The last 10 years have witnessed an explosion of studies of anaesthetic modulation of recombinant ligand-gated ion channels, including recent studies which utilize chimeric and mutated receptors to identify regions of ligand-gated ion channels important for the actions of general anaesthetics. Exciting future directions include structural biology and gene-targeting approaches to further the understanding of general anaesthetic molecular mechanisms.

Effect of inhalation anaesthetics on the phase behaviour, permeability and order of phosphatidylcholine bilayers

We have used differential scanning calorimetry and fluorescence anisotropy measurements to investigate the effect of five inhalation anaesthetics of diverse chemical structure (halothane, enflurane, n-pentane, chloroform and diethylether) on the phase behaviour of liposomes prepared from dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC), respectively. The incorporation of these anaesthetics induced a decrease of the phase transition temperature and/or a broadening of the phase transition peak depending on the transverse localisation of the investigated anaesthetic. At high anaesthetic concentrations we observed the disappearance of the pretransition peak and the appearance of a shoulder on the main phase transition peak due to the domain formation of the anaesthetics. An anaesthetic induced carboxyfluorescein efflux from the vesicle lumen was completed within a few minutes after the addition of the anaesthetics, probably resulting from a transient formation of membrane holes. All results are discussed with regard to the physicochemical properties of the anaesthetics applied.

Narcosis due to environmental pollutants in aquatic organisms: residue-based toxicity, mechanisms, and membrane burdens

The well-known correlation between the hydrophobicity of narcotic chemicals and the exposure concentration needed to produce an effect indicates that a lipid phase in the aquatic organism is the most likely target. The molar concentration in aquatic organisms at death is found to be approximately constant for different narcotic chemicals, varying from 2 to 8 mmol/kg organism. Because the proportion of lipid is known, the lethal in vivo membrane burden can be calculated to be 40 to 160 mmol/kg lipid. The exact mechanism underlying narcosis is still unknown. However, disturbance by narcotic chemicals in model membrane systems has been investigated, attention having been paid to disturbance of phospholipids and proteins, and of the interaction between the two groups. Model membrane burdens of different chemicals have been shown to be approximately constant for a particular effect. Different effects are found at different membrane concentrations. In the present review, the toxicity of narcotic chemicals to aquatic organisms is discussed, the possible mechanisms underlying narcosis are reviewed, and a comparison is made between membrane burdens that are lethal in vivo and membrane burdens that cause an effect in in vitro systems.

Hydrophobic organic solvents activate human platelets in vitro

The purpose of this study was to investigate blood platelet function during exposure to the hydrophobic organic solvents toluene, p-xylene and n-hexane. Human blood platelets were exposed for 30 min at 37°C to a saturated atmosphere of p-xylene, toluene or n-hexane. All three solvents, and the aromatics in particular, induced a decrease in the number of single platelets (61-88%) together with an increase in the extracellular levels of ATP plus ADP (45-65% of total) and serotonin (67-100% of total). Passive leakage of [(14)C] adenine-labelled nucleotides from the metabolic pool, due to platelet lysis, was minor or delayed. Electron microscopy of platelets exposed to p-xylene revealed aggregation. The platelets were spherical without pseudopods. Our results indicate that the hydrophobic solvents n-hexane, p-xylene and toluene induce platelet aggregation and dense granule secretion.

Effects of toluene and n-hexane on rat synaptosomal membrane fluidity and integral enzyme activities

The effects of toluene and n-hexane on rat synaptosomal membrane fluidity and the integral enzymes acetylcholinesterase (AChE) and ATPase were studied in vitro. The synaptosome membranes were isolated in Percoll and sucrose gradients. After adding toluene and n-hexane to the incubation mixture (37 degrees) in 2,4,6 and 8 mM concentrations, the fluidity changes were measured by the lateral pyrene diffusion method from Percoll-isolated membranes, and the ATPase and acetylcholinesterase activities were determined from both synaptosome isolations. Addition of toluene caused a linearly correlated increase of the synaptosomal membrane fluidity and a linear decrease of the AChE activity. The ATPase activity did not decrease linearly but dose-dependently. In contrast to the effects of toluene in vitro, addition of n-hexane in the same concentration range had no comparable influence on membrane fluidity nor on the activities of both integral enzymes despite its even higher lipid/water partition coefficient. Toluene increases synaptosomal membrane fluidity and at the same time inhibits the integral enzymes, probably by disturbing the lipid/protein interaction.

The effect of in vitro and in vivo toluene exposure on rat erythrocyte and synaptosome membrane integral enzymes

The mechanism of the anaesthetic effect of toluene on the central nervous system (CNS) was studied by using rat erythrocyte and synaptosome membranes as nerve cell models both in vitro and in vivo. The activities of the membrane-bound integral enzymes acetylcholinesterase (AChE), total adenosine triphosphatase (total ATPase) and magnesium-activated adenosine triphosphatase (Mg2+-ATPase) were determined. A short-term exposure to 2000 p.p.m. of toluene had an inhibitory effect on the enzyme activities studied. The degree of inhibition in erythrocyte membranes in vitro and in vivo, and in synaptosome membranes in vitro were in good correlation. In in vivo conditions, the synaptosome-bound enzymes were, however, significantly more inhibited by toluene, which indicates that membranes in vivo are even more vulnerable to the toxic effects of organic solvents than they are as isolated membranes in vitro. However, our results show that in vitro experiments can be used to predict the toxic nerve cell membrane effects of organic solvents. Toluene caused similar enzyme inhibitions both in neural cell membranes and in erythrocyte membranes. Thus, even peripheral non-excitable cell membranes, like erythrocytes, can be used as nerve cell membrane models in studies on the mechanism of the anaesthesia caused by solvents.