Local anesthetic-divalent cation binding center interaction

Mechanism of suppression of insulin signalling with lignocaine

Lignocaine suppresses insulin-stimulated glucose transport into the cells and insulin-stimulated glycogenesis at doses equivalent to that used in the treatment of muscle pain disorder. We evaluated the direct effect of lignocaine on insulin receptor (IR) kinase activity. After lignocaine (40 mM, approximately equivalent to 1%) or an equal volume (100 microl) saline had been injected into the tibialis anterior muscle of rat, insulin (50 mM g-1 body weight) was administered into the portal vein in vivo. Immunoprecipitation and immunoblotting were used to detect insulin-mediated tyrosine phosphorylation of both IR-beta and insulin receptor substrate (IRS)-1, and insulin-stimulated binding of IRS-1 to p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3-K) in the extracted muscle. In the in vitro study, purified IR from rat liver and/or recombinant IRS-1 protein with adenosine triphosphate were incubated with lignocaine (4 or 40 mM). Lignocaine reduced insulin-stimulated tyrosine phosphorylation of IR-beta to 12.6+/-5.7% (P<0.001), and IRS-1 to 32.1+/-18.8% (P<0.01), and also reduced insulin-stimulated binding of IRS-1 to p85 to 27.4+/-12.7% (P<0.001) relative to control (100%) in muscle in vivo. The in vitro study revealed that lignocaine directly inhibited both basal and insulin-stimulated tyrosine phosphorylation of IR. These results indicate that clinically used doses of lignocaine inhibit insulin signalling in skeletal muscle. The inhibitory effect of lignocaine on tyrosine kinase activity of the IR underlies the suppression of insulin signalling with lignocaine.

Conformational features of charged dibucaine in solution as investigated by 13C- and 1H-NMR

Conformational features of charged dibucaine in [2H6]DMSO were elucidated by measuring 13C and 1H spin-lattice relaxation rates and 1H-(1H) and 13C-(1H) nuclear Overhauser effects. The reorientational correlation time of the aromatic moiety was evaluated at 0.16 ns at room temperature and side chains were observed to display segmental motion. Relevant distances were calculated by isolating dipolar interaction terms of 1H-1H or 13C-1H pairs. The 'preferred' conformation in solution was shown to present several analogies, but also some differences, with the structures obtained by solid state experiments, energy calculations and LIS data.

Induction of apoptotic cell death in a neuroblastoma cell line by dibucaine

Dibucaine, a local anesthetic known to interact with cell membranes, induced apoptosis in SK-N-MC human neuroblastoma cells in a dose-dependent manner. Apoptosis was demonstrated by direct visualization of morphological nuclear changes using a DAPI staining technique and confirmed by the production of characteristic ladder patterns of DNA fragmentation on gel electrophoresis. At concentrations which induced apoptosis, dibucaine significantly altered membrane fluidity, indicating that fluidity may be a major target for the cytotoxic action of dibucaine. Also, dibucaine increased intracellular calcium levels more effectively in calcium-containing Krebs-Ringer buffer than in calcium-free Krebs-Ringer buffer. Removal of extracellular calcium or addition of antioxidants or protein synthesis inhibitor effectively blocked dibucaine-induced apoptosis. These results suggest that membrane damage, intracellular calcium levels, and oxygen free radicals may be involved in the apoptosis induced by dibucaine.

Unfolding and trypsin inactivation studies reveal a conformation drift of glucose-6-phosphate dehydrogenase upon binding of NADP

Binding of NADP to glucose-6-phosphate dehydrogenase (G6PD) from Dicentrarchus labrax liver has stabilized its native structure against thermal inactivation, guanidine hydrochloride unfolding and inactivation by tryptic digestion. The time-course of G6PD inactivation by guanidine hydrochloride in the presence of NADP has provided experimental evidence in favor of a conformational drift upon NADP binding to the bass enzyme. Based on the inactivation patterns obtained when the enzyme was treated with guanidine hydrochloride and trypsin, it is proposed that the enzyme conformation induced upon NADP binding is in slow equilibrium with the conformation stabilized in the absence of NADP. FPLC studies have shown that micromolar concentrations of NADP induced oligomerization of G6PD. In addition, the different K0.5 values of NADP binding to the enzyme, ranging from 1-2 microM (from trypsin inactivation) to 90 microM (from titration of the intrinsic fluorescence), suggest a step-wise binding of NADP to the oligomer, with negative cooperativity in the saturation process.

The local anesthetic tetracaine destabilizes membrane structure by interaction with polar headgroups of phospholipids

The effect of the local anesthetic tetracaine at less than 10 mM on the water permeability of the phospholipid membrane was examined using liposomes composed of various molar ratios of negatively charged cardiolipin to electrically neutral phosphatidylcholine by monitoring their osmotic shrinkage in hypertonic glucose solution at 30 degrees C. The concentration of tetracaine causing the maximum velocity of shrinkage of liposomes increased with increase in the molar ratio of cardiolipin. Tetracaine increased the zeta-potential of the negatively charged liposomal membrane toward the positive side due to the binding of its cationic form to the negatively charged polar headgroups in the membrane. The maximum velocity of water permeation induced by osmotic shock was observed at essentially the same tetracaine concentration giving a zeta-potential of the liposomal membrane of 0 mV. These concentrations were not affected by change in the sort of acyl-chain of phospholipids in the liposomes when their negative charges were the same. These results suggests that the membrane integrity is governed mainly by the electrical charge of phospholipid polar headgroups when phospholipid bilayers are in the highly fluid state, and that positively charged tetracaine molecules neutralize the negative surface charge, lowering the barrier for water permeation through phospholipid bilayers.

Kinetic characterization of the normal and procaine-perturbed reaction cycles of the sarcoplasmic reticulum calcium pump

We investigated the effect of the local anesthetic procaine on the activity of the calcium pump protein of sarcoplasmic reticulum (SR) vesicles. Procaine slowed down the rate of calcium uptake by SR vesicles without enhancing the vesicles' passive permeability. This slowing of the unidirectional pumping rate was reflected by the inhibition of the maximal rate of the transport-coupled Ca(2+)-ATPase activity. The inhibition was dependent on Mg2+ concentration; at optimal (i.e. low) concentrations of magnesium, half-maximal inhibition occurred with procaine concentrations close to 15-20 mM. Inhibition of ATPase was not mediated by a change in the properties of the bulk lipid phase. Procaine moderately reduced the true affinity of ATPase for ATP, whereas equilibrium binding of calcium to ATPase in the absence of ATP was virtually not modified by procaine. In fast-kinetics studies, we explored the various intermediate steps in the ATPase catalytic cycle, in order to determine which of them were targets for inhibition by procaine. We found that procaine slowed down ATPase dephosphorylation, an effect which is at least partly responsible for the observed inhibition of overall ATPase activity. In contrast, procaine accelerated the calcium-induced transconformation of unphosphorylated ATPase in the absence of ATP, and altered neither the rate of the Ca(2+)-dependent phosphorylation of ATPase, nor the rate of the dissociation of Ca2+ from phosphorylated ATPase towards the SR lumen, a critical step, the rate of which was measured by a novel fast-filtration method. These results are discussed with respect to the possible site(s) of binding of this amphiphile on the ATPase, and in relation to the contribution of individual steps in the catalytic cycle to the rate limitation of unperturbed SR ATPase activity.

Hemin and hemeprotein bleaching during linoleic acid oxidation by lipoxygenases

Hemin and hemoglobin are bleached by lipoxygenases, type 1 (from soybean) or type 2 (from platelets), during linoleic acid oxidation. This process has been found to be related to the inhibition of the lipoxygenase activity, measured as hydroperoxide generation and to produce oxodienes as well. All these parameters have been determined simultaneously from measurements of the absorbance at 234, 285, 375 and 410 nm to detect hydroperoxides, oxodienes, hemin and hemoglobin, respectively, using a diode array spectrophotometer. The inhibition of lipoxygenase activity by these pigments has been found to be competitive with linoleic acid, showing an increase of 4-7-fold of the Km value of linoleic acid in the presence of concentrations of hemin and hemoglobin as low as 0.2 and 0.02 microM, respectively, for the case of platelet lipoxygenase activity. The concentrations of hemin and of hemoglobin producing the inhibition of 50% of lipoxygenase activity are: 0.25 and 0.02 microM for the platelet isoenzyme, and 1.4 and 0.18 microM for the soybean isoenzyme, respectively. From the quenching of the intrinsic fluorescence of soybean lipoxygenase activity by hemin, we have obtained a dissociation constant of hemin-soybean lipoxygenase of 0.5 microM. The results obtained in this paper for the cooxidation process of hemin and hemoglobin by lipoxygenase can be rationalized in terms of hemin binding at or near to the catalytic center, resulting in a lesser binding of linoleic acid and an enhanced release of radicals, and pigment bleaching by radicals and lipid hydroperoxides.

Modulation of calcium fluxes across synaptosomal plasma membrane by local anesthetics

We have studied the effects of local anesthetics (dibucaine, tetracaine, lidocaine, and procaine) on calcium fluxes through the plasma membrane of synaptosomes. All these local anesthetics inhibit the ATP-dependent calcium uptake by inverted plasma membrane vesicles at concentrations close to those that promote an effective blockade of the action potential. The values obtained for the K0.5 of inhibition of calcium uptake are the following: 23 microM (dibucaine), 0.44 mM (lidocaine), 1.5 mM (procaine), and 0.8 mM (tetracaine). There is a good correlation between these K0.5 values and the concentrations of the local anesthetics that inhibit the Ca2(+)-dependent Mg2(+)-ATPase of these membranes. In addition, except for procaine, these local anesthetics stimulate severalfold the Ca2+ outflow via the Na+/Ca2+ exchange in these membranes. This effect, however, is observed at concentrations slightly higher than those that effectively inhibit the ATP-dependent Ca2+ uptake, e.g., 80-700 microM dibucaine, 2-10 mM lidocaine, and 1-3 mM tetracaine. The results suggest that the Ca2+ buffering of neuronal cytosol is altered by these anesthetics at pharmacological concentrations.

Modulation of the Ca2+, Mg2(+)-ATPase activity of synaptosomal plasma membrane by the local anesthetics dibucaine and lidocaine

It has been previously shown that local anesthetics inhibit the total Ca2+, Mg2(+)-ATPase activity of synaptosomal plasma membranes. We have carried out kinetic studies to quantify the effects of these drugs on the different Ca2(+)-dependent and Mg2(+)-dependent ATPase activities of these membranes. As a result we have found that this inhibition is not altered by washing the membranes with EDTA or EGTA. We have also found that the Ca2(+)-dependent ATPase activity is not significantly inhibited in the concentration range of these local anesthetics and under the experimental conditions used in this study. The inhibition of the Mg2(+)-dependent ATPase activities of these membranes was found to be of a noncompetitive type with respect to the substrate ATP-Mg2+, did not significantly shift the Ca2+ dependence of the Ca2+, Mg2(+)-ATPase activity, and occurred in a concentration range of local anesthetics that does not significantly alter the order parameter (fluidity) of these membranes. Modulation of this activity by the changes of the membrane potential that are associated with the adsorption of local anesthetics on the synaptosomal plasma membrane is unlikely, on the basis of the weak effect of membrane potential changes on the Ca2+,Mg2(+)-ATPase activity. It is suggested that the local anesthetics lidocaine and dibucaine inhibit the Ca2+, Mg2(+)-ATPase of the synaptosomal plasma membrane by disruption of the lipid annulus.