Interaction of local anaesthetics with lipid membranes under inflammatory acidic conditions

The clinical fact that local anaesthetics do not successfully work in the patients with inflammation has been generally interpreted on the basis of inflamed tissue acidification. In order to verify this hypothesis, the interaction of local anaesthetics with lipid membranes was studied by determining the drug-induced changes of membrane physicochemical property (membrane fluidity) at different pH covering inflammatory acidic conditions. At clinically relevant concentrations, lidocaine, procaine, prilocaine and bupivacaine fluidized 1,2-dipalmitoylphosphatidylcholine membranes with the potency decreased with lowering the pH from 7.9 to 5.9. When treated as the aqueous acidic solution (pH 4.0) similar to marketed injection solutions, lidocaine showed more pronounced pH dependence, so the reduction of its membrane-fluidizing effects at acidic pH theoretically correlated to that of its non-ionized membrane-interactive concentrations. Unlike phosphatidylcholine membranes, however, nerve cell model membranes consisting of different phospholipids and cholesterol were fluidized by lidocaine at pH 6.4–6.9 corresponding to the acidity of inflamed tissues. Cationic lidocaine was effective in fluidizing anionic phosphatidylserine and cardiolipin membranes at pH 6.4, but not zwitterionic phospholipid membranes, whereas it was ineffective on any membranes at pH 2.0 where membrane acidic phospholipids were not ionized. Local anaesthetics are considered to form the ion-pairs specifically with counter-ionic phospholipids and act on the membranes of nerve cells even under inflammatory acidic conditions. The drug and membrane interaction causable in inflamed tissue acidification does not support the conventional theory on the local anaesthetic failure associated with inflammation. Received 3 November 2006; revised version accepted 31 January 2007     

Cardiotoxic and antiarrhythmic tertiary amine local anesthetics: sodium channel affinity vs. sodium channel gating

Tertiary amine local anesthetics (LAs) are clinically valuable agents for controlling pain and for treating some cardiac arrhythmias. These drugs inhibit conduction of electrical activity by blocking voltage-gated sodium channels. Interestingly, LAs can influence the conduction of electrical activity in heart muscle without markedly altering normal skeletal muscle activity. This review discusses the interactions between sodium channels and LAs, the methods used to investigate these interactions, and the mechanisms proposed to explain the greater LA sensitivity of cardiac sodium channels as compared with skeletal muscle sodium channels.     

Effect of local anesthetics on electrically excitable bilayer lipid membranes

The effect of three local anesthetics on the action potential of electrically excitable bilayer lipid membranes was studied. The synthetic action potential of bilayers was abolished at drug concentrations roughly identical to those abolishing the action potential in nerves. The threshold depolarizing constant current pulse triggering the synthetic action potential was increased by slightly lower concentrations of these local anesthetics. It is suggested that the anionic channels, which are responsible for the rising phase of the action potential in bilayers, are affected preferentially by these drugs.     

The influence of local anesthetics on molecular organization in phosphatidylethanolamine membranes

The influence of the local anesthetics tetracaine (TTC) and procaine (PRC) on bilayers of specifically deuterated phosphatidylethanolamines (PE) has been studied by 2H and 31P NMR. Dimyristoylphosphatidylethanolamines (DMPE), deuterated at positions 2, 4, and 14 of the sn-2 chain, position 2 of the sn-1 chain, and in the ethanolamine headgroup, were mixed 1:1 with a semisynthetic egg PE and the effect of measured quantities of TTC and PRC on the 2H quadrupole splittings, spin-lattice relaxation times, and 31P chemical shift anisotropy were observed. Experiments were performed at pH 5.5, when the anesthetics are primarily charged, and at pH 9.5, when they are uncharged. Tetracaine was observed to disorder the hydrocarbon region of the bilayer and to induce a conformational change in the PE headgroup. Conversely, procaine had little or no effect on the hydrocarbon region and induced only a small change in the headgroup. These conformational changes and disordering effects, when adjusted for anesthetic partitioning, are essentially independent of the charge on the anesthetic. However, at pH 5.5 and low TTC/PE molar ratios (less than 0.1), the 2H NMR spectra showed two lipid environments--one corresponding to free PE and the other to PE in contact with TTC. Continued addition of TTC resulted in the eventual disappearance of the free PE signal and the corresponding growth of the signal from PE in contact with TTC. At pH 9.5, when TTC is uncharged, only one signal is observed. This indicates that at low pH, when TTC is primarily charged, it has a much slower rate of lateral diffusion in the PE bilayer. In mixtures of PE and phosphatidylserine, a conformational change in the headgroup was noted which was similar to that seen in the pure PE; however, there was no evidence for slow lateral diffusion of the anesthetics. The effects of TTC and PRC on the PE bilayer, when combined with our earlier study of the labeled anesthetics [Kelusky, E.C., and I.C.P. Smith, Biochemistry, 22:6011-6017 (1983)], indicate that TTC penetrates into the hydrocarbon portion of the bilayer whereas PRC sites only in the headgroup region.     

Molecular and physicochemical aspects of local anesthetics acting on nicotinic acetylcholine receptor-containing membranes

Local anesthetics inhibit the ion channel activity of nicotinic acetylcholine receptors in a noncompetitive fashion. This inhibitory action is ascribed to two possible inhibitory mechanisms: an open-channel-blocking mechanism and/or an allosteric process where the drug binds either to the closed channel or to other nonluminal sites, respectively.     

Degradation of ginseng saponins under mild acidic conditions

Ginseng saponins, ginsenosides Rg (1), Re and Rb (1), decomposed under mild acidic conditions to yield prosapogenins. The structures of the prosapogenins were investigated by (13)C-NMR spectroscopy and Rg (1)-prosapogenin II was shown to be a mixture of ginsenoside Rh (1), and its C-20 epimer, produced by hydrolysis followed by epimerization at C-20. Rg (1)-prosapogenin III, the other prosapogenin derived from ginsenoside Rg (1); was a C-25,26 hydrated derivative of Rg (1)-prosapogenin II. Re-prosapogenin II was identified as a mixture of ginsenoside Rg (2) and its C-20 epimer, and Re-prosapogenine III as a C-25,26 hydrated derivative of Re-prosapogenin II.     

On the interaction of local anesthetics with mast cells

The carbanilate local anesthetics carbisocaine, hepatacaine and pentacaine liberate histamine from isolated rat mast cells. Procaine, carticaine, trimecaine, cocaine and butanilicaine were ineffective. Histamine liberation was dose-dependent, followed by calcium displacement from membrane binding sites and occurred without concomitant degranulation. Low temperature and pH-dependent inhibition of histamine liberation indicated a non-specific, membrane perturbing effect of highly liposoluble carbanilate local anesthetics. Conformational changes in the sodium channel on the mast cell membrane induced by carbanilate anesthetics might result in histamine exchange occurring intracellularly.     

Metabolism and excretion of diethylpropion in man under acidic urine conditions

After oral administration, diethylpropion is rapidly and extensively metabolized in man by N-de-ethylation and stereoselective carbonyl reduction. The unchanged drug excreted in acidic urine represents about 2% of the dose, while the total metabolites determined account for some 85%. The excretion curves indicate that the probable contribution of the parent compound to the observed activity is small. The major metabolites, together representing about 70% of the dose, are N-ethylaminopropiophenone, (+)-N-diethylnorpseudoephedrine, (+)-N-ethylnorpseudoephedrine, (-)-norephedrine and (-)-norpseudoephedrine. The other stereoisomers of the three amino-alcohols, and aminopropiophenone, are present in minor amounts.     

Expression and activity of Kdp under acidic conditions in Escherichia coli

Escherichia coli has three major K(+) uptake systems, Trk, Kup, and Kdp, which have been studied extensively at near neutral pH. However, the function of these transporters under acidic conditions is not well understood, although growth and survival under acidic conditions are important for bacterial pathogenesis. In this study, we examined the expression and activity of Kdp under acidic conditions and found that the transport activity of Kdp is decreased at low pH and that the expression of kdp is regulated by the internal K(+) concentration in a pH-independent manner. Consequently, the low activity of Kdp was compensated for by the induction of its elevated expression by low K(+) accumulation via Kdp at acidic pH.     

Diphenylalkylamine calcium antagonists interact with alpha-adrenoceptor binding sites in aortic membranes

Some interactions of calcium antagonists with [3H]prazosin and [3H]yohimbine binding sites were investigated in bovine aorta membranes. Diphenylalkylamines (flunarizine, cinnarizine and bepridil) acted as competitors of the two ligands with Ki values in the microM range. With the exception of verapamil, reference compounds (nifedipine, Bay-K 8644, diltiazem) and the peripheral benzodiazepine receptor antagonist PK 11195 did not displace the ligands. The apparent affinity of the diphenylalkylamines for alpha-adrenoceptor was consistent with the concentrations producing vasodilatation.     

Plasma concentrations and excretion of zipeprol in man under acidic urine conditions

Plasma concentrations of zipeprol, and the urinary excretion of it and two of its basic metabolites, compound II and compound III were examined after oral administration of zipeprol hydrochloride to man. The drug was rapidly and extensively metabolized, and the amount of unchanged drug excreted in the acidic urine varied from 1–5% of the dose; the two basic metabolites accounted for 19–38% of the dose.     

Use of ion-exchange membranes to measure transfer free energies of charged local anesthetics: correlation to anesthetic potency

Ion-selective electrodes, sensitive to local anesthetic cations, were prepared with carboxylated poly(vinyl chloride) (PVC) membranes. Three plasticizers with varying degrees of polarity were used to adjust the hydrophobicity of the membrane. The affinity of the drug to the ion-exchange membrane was measured by the electromotive force of the cation-selective electrodes. The difference in the transfer free energies of the anesthetics for the membrane was estimated in reference to dibucaine. The values correlated to their clinical potencies. By comparing drugs with similar structures, the transfer free energy per methylene moiety linked to the hydrophilic domain was found to be -1.7 kJ.mol-1, and that of Cl linked to the hydrophobic domain was -3.1 kJ.mol-1. Interferences from Na+ and K+ were estimated as the selectivity coefficients against dibucaine. The values were 2.6 x 10(-5) for Na+ and 1.2 x 10(-4) for K+. The ion-exchange membrane appears to mimic the surface properties of cell membranes. These cation-selective electrodes have potential applicability in measuring charged local anesthetic concentrations (activities) in biological materials under limited conditions.     

Development of an albuminous reactive oxygen species assay for photosafety evaluation under experimental biomimetic conditions

The generation of reactive oxygen species (ROS) from an ultraviolet (UV)‐exposed chemical can be an experimental indicator of phototoxic potential. The aim of the present study was to develop a new ROS assay using serum albumin to provide photosafety assessment under experimental biomimetic conditions. To assess assay robustness, a validation study on an albuminous ROS (aROS) assay was conducted with a focus on intra‐ and inter‐day precisions and Z’‐factor reflecting both the assay signal‐to‐noise ratio and variation associated with signal measurements. In the aROS assay on quinine HCl (200 μM), a typical phototoxic drug, both intra‐ and inter‐day precisions (coefficient of variation; CV) were found to be below 4%, and the Z’‐factors for singlet oxygen and superoxide suggested a large separation band between samples and blank signals. To evaluate the prediction capacity, the aROS and ROS assays were applied to 21 phototoxins and 10 non‐phototoxic chemicals. Upon aROS assay on these model chemicals, the individual specificity was 100%, and the positive and negative predictivities were found to be 100% and 81.8%, respectively. The aROS assay can be employed for poorly soluble chemicals for which the ROS assay is unavailable. Comparing the ROS assay data, there seemed to be a photochemical transition of some chemicals in albuminous solution. A molecular interaction between albumin and chemical was also assessed by UV and fluorescent spectroscopic analyses, and the results suggested the limited relationship between the albumin–chemical interaction and the photochemical change. The aROS assay may allow photosafety assessment of new drug entities with a wide range of applicability partly under experimental biomimetic conditions. Copyright © 2013 John Wiley & Sons, Ltd.