Effects of local anesthetics on the passive permeability of sarcoplasmic reticulum vesicles to Ca2+ and Mg2+

Influence of N-dodecyl-N,N-dimethylamine N-oxide on the activity of sarcoplasmic reticulum Ca2+-transporting ATPase reconstituted into diacylphosphatidylcholine vesicles: Effects of bilayer physical parameters

Sarcoplasmic reticulum Ca-transporting ATPase (EC 3.6.1.38) was isolated from rabbit white muscle, purified and reconstituted into vesicles of synthetic diacylphosphatidylcholines with monounsaturated acyl chains using the cholate dilution method. In fluid bilayers at 37 degrees C, the specific activity of ATPase displays a maximum (31.5+/-0.8 IU/mg) for dioleoylphosphatidylcholine (diC18:1PC) and decreases progressively for both shorter and longer acyl chain lengths. Besides the hydrophobic mismatch between protein and lipid bilayer, changes in the bilayer hydration and lateral interactions detected by small angle neutron scattering (SANS) can contribute to this acyl chain length dependence. When reconstituted into dierucoylphosphatidylcholine (diC22:1PC), the zwitterionic surfactant N-dodecyl-N,N-dimethylamine N-oxide (C12NO) stimulates the ATPase activity from 14.2+/-0.6 to 32.5+/-0.8 IU/mg in the range of molar ratios C12NO:diC22:1PC=0/1.2. In dilauroylphosphatidylcholines (diC12:0PC) and diC18:1PC, the effect of C12NO is twofold-the ATPase activity is stimulated at low and inhibited at high C12NO concentrations. In diC18:1PC, it is observed an increase of activity induced by C12NO in the range of molar ratios C12NO:diC18:1PC< or =1.3 in bilayers, where the bilayer thickness estimated by SANS decreases by 0.4+/-0.1 nm. In this range, the 31P-NMR chemical shift anisotropy increases indicating an effect of C12NO on the orientation of the phosphatidylcholine dipole N(+)-P- accompanied by a variation of the local membrane dipole potential. A decrease of the ATPase activity is observed in the range of molar ratios C12NO:diC18:1PC=1.3/2.5, where mixed tubular micelles are detected by SANS in C12NO+diC18:1PC mixtures. It is concluded that besides hydrophobic thickness changes, the changes in dipole potential and curvature frustration of the bilayer could contribute as well to C12NO effects on Ca(2+)-ATPase activity.

Hypothalamic hypophyseal inhibitory factor (HHIF) increases intrasynaptosomal free calcium concentration

We have isolated from bovine hypothalamic and pituitary tissues a sodium pump inhibitor that is structurally different from ouabain. By mass spectrometric analysis, this purified factor revealed a single unique molecular ion with an accurate mass of 412.277 and a mass spectra different from that of ouabain. It has been previously shown that this factor inhibits the Ca2+, Mg(2+)-ATPase of the plasma membrane of synaptosomes. Because Ca2+ plays a major role in cellular excitability, we carried out a systematic study of the effects of this inhibitor on the Ca2+ transport processes across the plasma membrane of synaptosomes: We measured ATP-dependent calcium uptake, Na(+)-Ca2+ exchange, and passive permeability using 45Ca2+ and Millipore filtration, chlortetracycline fluorescence, and light-scattering, respectively. This factor inhibits the Na+, K(+)-ATPase activity of the synaptosomal plasma membrane vesicles in the same range of concentrations that produced an increase of intrasynaptosomal free calcium, with nearly the same K0.5 value. In addition, in this concentration range, this factor stimulated 10- to 11-fold the passive flux of Ca2+ and 2.5- to 3-fold the Ca2+ influx via the Na(+)-Ca2+ exchange in these membranes with respect to control values. Measurements of fluorescence anisotropy showed that in this concentration range, the inhibitor did not significantly change the order parameter (fluidity) of these membranes. These results suggest that besides its known inhibition of the sodium pump, this factor could play a role in the control of Ca2+ homeostasis by direct modulation of transport systems implicated in the control of intracellular calcium.

Effect of haloperidol on the sarcoplasmic reticulum Ca-dependent adenosine triphosphatase

Several effects of the neuroleptic agent haloperidol on the sarcoplasmic reticulum (SR) Ca-dependent adenosine triphosphatase (Ca-ATPase) and Ca transport are described. Haloperidol inhibits the Ca-ATPase activity in the presence of calcimycin. The effect depends on the conditions of preexposure of the membranes to the drug: the inhibition increases with the preincubation time; Ca and Mg protect the enzyme against the effect of the drug. The inhibitory effect of haloperidol decreases upon increasing [Ca2+], at constant [Mg], and disappears at 20 mM [Mg] for any [Ca2+], and at 0.5 mM [Ca2+] for any [Mg2+]. Haloperidol also inhibits phosphorylation of the enzyme by Pi, and ATP-dependent Ca2+ uptake, in both cases with apparent Ki = 0.10-0.15 mM, and increases the rate of Ca efflux from preloaded vesicles in this concentration range. The results suggest that haloperidol interacts with the catalytic site, interfering with the effect of the divalent catalytic cation, but not at other steps of the enzymatic cycle, where Mg2+ and Ca2+ are also activators. They are consistent with a reaction model where haloperidol interacts with the E2 conformers of the enzyme, with lower affinity for the phosphoenzyme than for the dephospho species. The inhibition of Ca uptake by SR vesicles is ascribed to an increased Ca2+ permeability rather than to the inhibition of the Ca-ATPase, which requires higher concentrations of the drug.

Interaction of alkanols and local anesthetics with spin-labeled Ca(2+)-ATPase of sarcoplasmic reticulum vesicles

Alkanols and tertiary amine derivative local anesthetics modify the activity of Ca(2+)-ATPase. In order to investigate the primary binding sites, associated to the functional changes, sarcoplasmic reticulum (SR) Ca(2+)-ATPase was labeled with maleimide derivative spin labels which bind covalently to SH groups of cysteine residues and allow to probe the regions of the protein close to those residues. The EPR measurements showed motional constraints induced by drug-treatment which indicate changes in the enzyme dynamics and structure. n-Alkanols are shown to affect some of the protein-bound labels by restricting their motion. There is, however, no correlation between the functional effects and the observed motional restriction, in the sense that concentrations of the different alcohols leading to the same functional effects do not induce the same degree of restriction. Dibucaine and tetracaine at functional relevant concentrations also restrict the movement of protein bound labels. But, in this case, correlation between spectral changes and functional effects is observed.

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 sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase by pentobarbital

The dependence of the (Ca2+ + Mg2+)-ATPase activity of sarcoplasmic reticulum vesicles upon the concentration of pentobarbital shows a biphasic pattern. Concentrations of pentobarbital ranging from 2 to 8 mM produce a slight stimulation, approximately 20-30%, of the ATPase activity of sarcoplasmic reticulum vesicles made leaky to Ca2+, whereas pentobarbital concentrations above 10 mM strongly inhibit the activity. The purified ATPase shows a higher sensitivity to pentobarbital, namely 3-4-fold shift towards lower values of the K0.5 value of inhibition by this drug. These effects of pentobarbital are observed over a wide range of ATP concentrations. In addition, this drug shifts the Ca2+ dependence of the (Ca2+ + Mg2+)-ATPase activity towards higher values of free Ca2+ concentrations and increases several-fold the passive permeability to Ca2+ of the sarcoplasmic reticulum membranes. At the concentrations of pentobarbital that inhibit this enzyme in the sarcoplasmic reticulum membrane, pentobarbital does not significantly alter the order parameter of these membranes as monitored with diphenylhexatriene, whereas the temperature of denaturation of the (Ca2+ + Mg2+)-ATPase is decreased by 4-5 C degrees, thus, indicating that the conformation of the ATPase is altered. The effects of pentobarbital on the intensity of the fluorescence of fluorescein-labeled (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum also support the hypothesis of a conformational change in the enzyme induced by millimolar concentrations of this drug. It is concluded that the inhibition of the sarcoplasmic reticulum ATPase by pentobarbital is a consequence of its binding to hydrophobic binding sites in this enzyme.

Inhibitory effect of local anesthetics on Na+/H+ antiporter in brush border membrane-reconstituted vesicles

The effects of three local anesthetics, lidocaine, dibucaine, and tetracaine, on Na+/H+ antiporter activity were examined in brush border membrane-reconstituted vesicles. Lidocaine at 10 microM inhibited H+ efflux in the presence of an inward Na+ gradient, suggesting that this anesthetic specifically inhibits the Na+/H+ antiporter. On the other hand, dibucaine and tetracaine decreased H+ efflux even in the absence of a Na+ gradient.

Local anesthetic-divalent cation binding center interaction

This communication explicitly considers the possibility that local anesthetics interact with divalent cation binding centers, such as chlortetracycline, quin 2, ethyleneglycol bis (B-aminoethyl ether)-N-N,N',N'-tetraacetic acid (EGTA), ethylenediamine tetraacetic acid (EDTA) and ATP. Alterations of local anesthetic fluorescence spectra have been found in the presence of EGTA, EDTA and ATP. On the other hand, the fluorescence of chlortetracycline is enhanced and that of quin 2 is quenched by local anesthetics. The spectrofluorometric evidence presented in this paper clearly indicates that local anesthetics and these divalent cation chelators interact in solution. The fluorescence alterations observed do not derive from parallel changes of their respective absorption spectra, thus, they appear to be due to quantum yield changes. On the basis of the spectral perturbations observed, it is likely that local anesthetics interact with M2+ binding centers via their electron defective aromatic ring. From the association constants obtained in this study, we make an estimation of the free energy of this interaction ranging from -2.8 to -4.0 kcal/mole in the following experimental conditions: pH 7.4 at an ionic strength of 0.1 at 25 degrees. The relevance of these results to define the physical-chemical characteristics of the local anesthetic receptor site is briefly discussed. It is suggested that local anesthetics can bind strongly to Ca2+ and Mg2+ binding centers, provided that a hydrophobic region is located nearby.

Interaction of the local anesthetics dibucaine and tetracaine with sarcoplasmic reticulum membranes. Differential scanning calorimetry and fluorescence studies

The local anesthetics dibucaine and tetracaine inhibit the (Ca2+ + Mg2+)-ATPase from skeletal muscle sarcoplasmic reticulum [DeBoland, A. R., Jilka, R. L., & Martonosi, A. N. (1975) J. Biol. Chem. 250, 7501-7510; Suko, J., Winkler, F., Scharinger, B., & Hellmann, G. (1976) Biochim. Biophys. Acta 443, 571-586]. We have carried out differential scanning calorimetry and fluorescence measurements to study the interaction of these drugs with sarcoplasmic reticulum membranes and with purified (Ca2+ + Mg2+)-ATPase. The temperature range of denaturation of the (Ca2+ + Mg2+)-ATPase in the sarcoplasmic reticulum membrane, determined from our scanning calorimetry experiments, is ca. 45-55 degrees C and for the purified enzyme ca. 40-50 degrees C. Millimolar concentrations of dibucaine and tetracaine, and ethanol at concentrations higher than 1% v/v, lower a few degrees (degrees C) the denaturation temperature of the (Ca2+ + Mg2+)-ATPase. Other local anesthetics reported to have no effect on the ATPase activity, such as lidocaine and procaine, did not significantly alter the differential scanning calorimetry pattern of these membranes up to a concentration of 10 mM. The order parameter of the sarcoplasmic reticulum membranes, calculated from measurements of the polarization of the fluorescence of diphenylhexatriene, is not significantly altered at the local anesthetic concentrations that shift the denaturation temperature of the (Ca2+ + Mg2+)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Partitioning of local anesthetics into membranes: surface charge effects monitored by the phospholipid head-group

The binding of the charged form of two local anesthetics, dibucaine and etidocaine, to bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was measured simultaneously with ultraviolet spectroscopy and deuterium magnetic resonance. Because of their amphiphilic molecular structure, both drugs intercalate between the lipid molecules, increasing the surface area and imparting a positive electric charge onto the membrane. The ultraviolet (UV) binding isotherms were therefore analyzed in terms of a model which specifically took into account the bilayer expansion as well as the charge-induced concentration variations near the membrane surface. By formulating a quantitative expression for the change in surface area upon drug intercalation and combining it with the Gouy-Chapman theory, the binding of charged dibucaine and etidocaine to the lipid membrane was best described by a partition equilibrium, with surface partition coefficients of 660 +/- 80 M-1 and 11 +/- 2 M-1 for dibucaine and etidocaine, respectively (pH 5.5, 0.1 M NaCl/50 mM buffer). Deuterium magnetic resonance demonstrated further that the binding of drug changed the head-group conformation of the lipid molecules. Invoking the intercalation model, a linear variation of the deuterium quadrupole splittings of the choline segments with the surface charge density was observed, suggesting that the phosphocholine head-group may act as a 'molecular electrometer' with respect to surface charges.