Passive Ion Permeability Research Articles

Taurine administration prevents the intestine from the damage induced by beta-lactoglobulin sensitization in a murine model of food allergy

BackgroundAllergy to cow's milk proteins has often been associated with dysfunction of the intestinal mucosa caused by chronic inflammation in infants. This study evaluated the protective effect of taurine on intestinal damage induced by beta-lactoglobulin (β-Lg) in Balb/c mice used as an animal model of allergy to cow's milk proteins. MethodsBalb/c mice were treated with taurine administered orally by gavage (3mmol/kg/day) or intraperitoneally (100mg/kg/day) for two weeks, then sensitized intraperitoneally with β-Lg. The electrophysiological parameters: active ion transport of chloride (Short-circuit current: Isc) and the passive ion permeability (Conductance: G) were measured ex vivo in Ussing chamber by intestine challenge with β-Lg. Histological study was used to assess gut inflammation. Serum levels of TNF-α and IL-6 were measured. Serum IgG and IgE anti-β-Lg were determined by ELISA. ResultsCompared with sensitized mice, β-Lg challenge of intestinal epithelium of taurine-pre-treated mice in Ussing chamber did not influence the intensity of Isc, nor produce any changes in the G, reflecting a reduction in the secretory response and epithelial permeability. Histological and morphometric analysis showed that taurine reduced the intestinal damage and limited intestine retraction caused by β-Lg sensitization. No statistically significant difference in the serum levels of TNF-α or IL-6 was found after oral or intraperitoneal administration of taurine. Treatment with taurine significantly decreased the IgG (p<0.001) and IgE anti β-Lg levels (p<0.05). ConclusionsThese results have for the first time provided evidence that pre-treatment with taurine appears to prevent intestinal damage induced by β-Lg.

Supplementation with β-carotene or vitamin E protects against increase in anaphylactic response in β-lactoglobulin-sensitized Balb/c mice: ex vivo study

Balb/c mice fed 0 mg, 20 mg of β-carotene (BC) or 50 mg of vitamin E (Vit E) per Kg of diet for 7 weeks were immunized intraperitoneally with β-Lactoglobulin (β-Lg). Specific IgG and IgE anti β-Lg were determined in the mouse sera by an enzyme-linked immunosorbent assay (ELISA), and the jejunum was used for assessment of local anaphylactic responses ex vivo in Ussing chambers. Short-circuit current (Isc), a measure of active ion transport, and epithelial conductance (G), a measure of passive ion permeability, were measured for each tissue. Specimens were also processed for examination by histological analysis. Compared with sensitized mice (unsupplemented mice), those fed BC or Vit E had lower β-Lg-specific IgG (p < 0.001 and p < 0.0001, respectively) and IgE levels (p < 0.02 and p < 0.01, respectively). A secretory response reflecting local intestinal anaphylaxis was evident in sensitized mice, as indicated by an increase in Isc (p < 0.009). Sensitized mice demonstrated also an enhanced epithelial permeability as measured by conductance (p < 0.03). BC supplementation resulted in a significant decrease in secretory response (p < 0.02) with no effect on the epithelial permeability. Interestingly, the secretory response and epithelial permeability were significantly reduced by supplementation with Vit E (p < 0.02 and p < 0.009, respectively). These findings provide evidence for the first time that both BC and Vit E regulate epithelial ion transport, but it should be noted that only Vit E-supplemented mice showed a decreased tissue conductance, indicating protection from β-Lg-induced epithelial permeability increase. Analysis of histological sections revealed that BC or Vit E treatment clearly reduced the microscopic lesions caused by β-Lg sensitization.

Dose Effects of Apical versus Basolateral Zinc Supplementation on Epithelial Resistance, Viability, and Metallothionein Expression in Two Intestinal Epithelial Cell Lines.

Zinc supplementation is used to reduce diarrhea incidence in piglets and it has been shown in vitro that the antisecretory effects are maximal after basolateral zinc application. To examine whether the application site and dose of zinc also influence passive ion permeability and viability, porcine (IPEC-J2) and human (Caco-2) intestinal epithelial cells were treated with increasing zinc concentrations (0-200μM) at either the apical or basolateral side. Transepithelial electrical resistance and viability decreased and expression of metallothionein and the efflux zinc transporter 1 increased most prominently when zinc was added in high concentrations at the basolateral side of IPEC-J2 cells. Zinc transporter 4, a zinc importer, was not affected. Heat shock protein 70 mRNA expression increased only after basolateral addition of 200μM zinc in IPEC-J2 cells. Thus, zinc can elicit toxic effects especially when added at the basolateral side, with IPEC-J2 cells being more susceptible than Caco-2 cells.

Monitoring Proton Flux Quantitatively; Influenza Proton Channel A/M2

An improved methodology for monitoring proton translocation across membranes is presented, along with results from the Influenza A virus proton channel A/M2.We have constructed a liposome-production instrument which creates proteoliposomes from a lipid/detergent/protein mixture by gradually adding hydrophobic beads while continuously monitoring sample turbidity. The method is fast and reproducible, and facilitates enhanced control of key protein reconstitution parameters. It also enables synchronous measurements of protein-mediated ion flow and passive permeability across the bilayer. Two novel pH-sensors are presented: Glu3 and TCHP. These porphyrin-based probes are membrane-impermeable, do not interact with biological complexes, have physiologically appropriate pK, and display high extinction coefficients. Glu3 is also ratiometric in emission.H+, K+ and Na+ permeabilities were determined in liposomes of different lipid and cholesterol composition. The effect of detergent/lipid and lipid/protein ratios on ion permeability was systematically investigated. The proton channel A/M2, key to Influenza A virus propagation and an antiviral drug target, was successfully reconstituted. The proton translocation rate was determined to 8.3 protons per second and A/M2 tetramer. We also found that the presence of protein in the bilayer enhanced the passive ion permeability. Lowering the protein/lipid ratio minimized this effect, and prolonged the measurement window of proton movements to several minutes, and thus resulted in the most reliable data.Preliminary data from membrane-spanning subunits of respiratory Complex I are also presented.

Charge-to-substrate ratio during organic cation uptake by rat OCT2 is voltage dependent and altered by exchange of glutamate 448 with glutamine

Uptake of substrate and electric charge was measured simultaneously in voltage-clamped Xenopus laevis oocytes expressing rat organic cation transporter 2 (rOCT2). At 0 mV, saturating substrate concentrations induced uptake of more positive elementary charges than monovalent organic cations, with charge-to-substrate ratios of 1.5 for guanidinium(+), 3.5 for tetraethylammonium(+), and 4.0 for 1-methyl-4-phenylpyridinium(+). At negative holding potentials, the charge-to-substrate ratios decreased toward unity. At 0 mV, charge-to-substrate ratios higher than unity were observed at different extracellular pH and after replacement of extracellular Na(+), K(+), Ca(2+), Mg(2+), and/or Cl(-). Charge-to-substrate ratios were not influenced by intracellular succinate(2-) or glutarate(2-). The effects of membrane potential and ion substitution strongly suggest that the surplus of transported positive charge is not generated by passive ion permeabilities. Rather, we hypothetize that small cations are taken up together with organic cation substrates whereas the outward reorientation of the empty transporter is electroneutral. Nonselective cotransport of small cations was supported by the three-dimensional structures of rOCT2 in its inward-facing and outward-facing conformations, which we determined by homology modeling based on known corresponding structures of H(+)-lactose permease of E. coli, and by functional analysis of OCT mutants. In our model, the innermost cavity of the outward-open binding cleft is negatively charged by Glu448 and Asp475, whereas the inward-open innermost cavity is electroneutral, containing Asp379, Asp475, Lys215, and Arg440. Substitution of Glu448 by glutamine reduced the charge-to-TEA(+) ratio at 0 mV to unity. The observed charge excess associated with organic cation uptake into depolarized cells may contribute to tubular damage in renal failure.

Structural and functional changes in the membrane and membrane skeleton of red blood cells induced by peroxynitrite

The changes in passive ion permeability of the red blood cell membrane after peroxynitrite action (3 µM–3 mM) have been studied by biophysical (using radioisotopes of rubidium, sodium and sulphur (sulphate)) and electrophysiological methods. The enhancement of passive membrane permeability to cations (potassium and sodium ions) and the inhibition of anion flux through the anion exchanger in peroxynitrite-treated red blood cells were revealed. In patch-clamp experiments the whole-cell conductance after peroxynitrite (80 μM) treatment of red blood cells increased 3–3.5-fold with a shift in the reversal potential from − 7.0 ± 1.5 mV to − 4.3 ± 0.9 mV ( n = 7, p = 0.005). The addition of cobalt and nickel ions to red blood cell suspensions before peroxynitrite treatment had no effect on the peroxynitrite-induced cation flux but zinc ions in the same condition decreased cation flux about 2-fold. Using atomic force microscopy methods we revealed an increase in red blood cell membrane stiffness and the membrane skeleton complexity after peroxynitrite action. We conclude that the peroxynitrite-induced water and ion imbalance and reorganization in membrane structure lead to crenation of red blood cells.

Allometric dependence of the life span of mammal erythrocytes on thermal stability and sphingomyelin content of plasma membranes

Thermal stability of erythrocyte membrane is a measure for its ability to maintain permeability barrier at deleterious conditions. Hence, it could impact the resistance of erythrocytes against detrimental factors in circulation. In this study the thermostability of erythrocyte membranes was expressed by the temperature, T(go), at which the transmembrane gradient of ion concentration rapidly dissipated during transient heating. T(go) is the inducing temperature of the membrane transition that activated passive ion permeability at hyperthermia causing thermal hemolysis. A good allometric correlation of T(go) to the resistance against thermal hemolysis and the life span of erythrocytes were found for 13 mammals; sheep, cow, goat, dog, horse, man, rabbit, pig, cat, hamster, guinea pig, rat, and mouse. For the same group, the values of T(go) were strictly related to the sphingomyelin content of erythrocyte membranes. The residual ion permeability, P, was temperature activated from 38 to 57 degrees C with activation energy of 250+/-15 kJ/mol that strongly differed from that below 37 degrees C. The projected value of P at 37 degrees C was about half that of residual physiological permeability for Na+ and K+ that build ground for possible explanation of the life span vs membrane thermostability allometric correlation.

Impact of thermohaemolysis-associated membrane alteration on the passive ion permeability and life-span of erythrocytes

Suspending erythrocytes in medium containing sucrose prevented heat-induced lysis at its early stage. This allowed determination of the thermohaemolysis-related ion permeability by measuring the initial rate of the stipulated shrinkage of erythrocytes. Thus, correspondingly, the coefficient P of the ion permeability was calculated for heated human erythrocytes using ouabain-pretreated cells in 37–45°C range and intact cells in 50–58°C range. The values obtained for P obeyed a straight line Arrhenius plot over the entire 37–58°C range suggesting that the ion permeability was activated by a single mechanism earlier identified as relevant to thermohaemolysis. At the 37–58°C range, the activation energy of the P was 250±15 kJ/mol which was markedly different from the value of 56 kJ/mol known for the 10–37°C range. For erythrocytes from five mammals, similar temperature dependencies of the P were obtained over 45–60°C range. For erythrocytes from all species, excluding horse, the P, extrapolated at 37°C, had a value comparable with the known coefficient of the passive, ouabain-insensitive cation permeability at 37°C. For ouabain-treated human erythrocytes at 37°C, the period of thermohaemolysis-related shrinkage in sucrose containing media was found to be about six times shorter than the life-span of intact cells which substantiated the role of the active transport in balancing the thermohaemolysis-related diffusion of ions at 37°C. Consequently, the thermal resistance of erythrocytes, which was earlier related to their sphingomyelin content, was now found also to be in good correlation with their life-span in the circulation of 11 mammals.

Phospholipid subclass specific alterations in the passive ion permeability of membrane bilayers: separation of enthalpic and entropic contributions to transbilayer ion flux.

Alterations in phospholipid class, subclass, and individual molecular species contribute to the diversity of biologic membranes, but their effects on membrane passive ion permeability have not been systematically studied. Herein, we developed a simple and efficient fluorescence technique based upon the loss of valinomycin-inducible membrane potential to characterize the passive flux of ions across phospholipid bilayers. Detailed kinetic characterization of ion flux across membrane bilayers composed of discrete chemical entities demonstrated that the class, subclass, and individual molecular species of each phospholipid have substantive effects on membrane passive ion permeability properties. Increasing the degree of unsaturation in either the sn-1 or sn-2 aliphatic chains in phosphatidylcholine markedly enhanced transmembrane ion flux, with over 10-fold differences in the first-order rate constant manifested in molecular species containing four double bonds in comparison to those possessing three double bonds (e.g., kapp = 0.0014 min-1 for 1-octadec-9'-enoyl-2-octadec-9', 12'-dienoyl-sn-glycero-3-phosphocholine (18:1-18:2 phosphatidylcholine) while kapp = 0.021 min-1 for 1,2-dioctadec-9', 12'-dienoyl-sn-glycero-3-phosphocholine (18:2-18:2 phosphatidylcholine)). Moreover, although the apparent first-order rate constants for transmembrane ion flux in vesicles composed of phosphatidylcholine or plasmanylcholine containing palmitate at the sn-1 position and arachidonate at the sn-2 position were similar (kapp = 0.04 min-1 at 22 degreesC for both), the kapp for corresponding vesicles composed of plasmenylcholine was 20-fold less (kapp = 0.002 min-1 at 22 degreesC). Examination of the temperature dependence of passive membrane ion permeability demonstrated that altered ion flux across membranes composed of choline glycerophospholipids was primarily due to entropic effects without substantial changes in the activation energy for ion translocation. For example, Ea = 19.7 +/- 0.5 and 20.7 +/- 0.6 kcal.mol-1 for 1-hexadecanoyl-2-eicosa-5',8',11', 14'-tetraenoyl-sn-glycero-3-phosphocholine (16:0-20:4 phosphatidylcholine) and 1-O-(Z)-hexadec-1'-enyl-2-eicosa-5',8',11', 14'-tetraenoyl-sn-glycero-3-phosphocholine (16:0-20:4 plasmenylcholine), respectively, while their difference in the entropies of activation (DeltaS) was 4.3 +/- 0.5 cal.mol-1.K-1. Collectively, these results identify substantial differences in the membrane passive ion permeability properties of phospholipid classes, subclasses, and molecular species present in biologic membranes of eukaryotic cells and identify entropic alterations as an important contributor to these differences.

The effect of bilayer composition on calcium ion transport facilitated by fluid shear stress

Passive calcium ion permeability across liposome bilayers is increased during exposure to fluid shear forces attainable in the mammalian vasculature. In this study, liposomes prepared from three different lipid mixtures (phosphatidylcholine alone; phosphatidylcholine and cholesterol; a mixture of anionic and cationic phospholipids plus cholesterol) are exposed to uniform shear stress in a rotational viscometer. Liposome permeability to calcium ion is estimated from continuous measurement of free intraliposome calcium ion concentration using a fluorescence technique. Calcium ion permeability in the absence of fluid force and susceptibility to shear-induced permeability modulation are positively correlated with estimated bilayer compressibility. Fluid shear forces are presumed to influence bilayer packing and modulate defect formation in proportion to bilayer compressibility. Bilayer defects produced by fluid forces may increase liposome permeability.

The permeability and the effect of acyl-chain length for phospholipid bilayers containing cholesterol: theory and experiment

The model of Cruzeiro-Hansson et al. (Biochim. Biophys. Acta (1989) 979, 166–1176) for lipid-cholesterol bilayers at low cholesterol concentrations is used to predict the thermodynamic properties and the passive ion permeability of lipid bilayers as a function of acyl-chain length and cholesterol concentration. Numerical simulations based on the Monte Carlo method are used to determine the equilibrium state of the system near the main gel-fluid phase transition. The permeability is calculated using an ansatz which relates the passive permeability to the amount of interfaces formed in the bilayer when cholesterol is present. The model predicts at low cholesterol contents an increase in the membrane permeability in the transition region both for increasing cholesterol concentration and for decreasing chain length at a given value of the reduced temperature. This is in contrast to the case of lipid bilayers containing high cholesterol concentrations where the cholesterol strongly surpresses the permeability. Experimental results for the Na + permeability of C 15PC and DPPC (C 16PC) bilayers containing cholesterol are presented which confirm the theoretical predictions at low cholesterol concentrations.

Density fluctuations in saturated phospholipid bilayers increase as the acyl-chain length decreases

A systematic computer simulation study is conducted for a model of the main phase transition of fully hydrated saturated diacyl phosphatidylcholine bilayers (DMPC, DPPC, and DSPC). With particular focus on the fluctuation effects on the thermal properties in the transition region, the study yields data for the specific heat, the lateral compressibility, and the lipid-domain size distribution. Via a simple model assumption the transmembrane passive ion permeability is derived from the lipid-domain interfacial measure. A comparative analysis of the various data shows, in agreement with a number of experiments, that the lateral density fluctuations and hence the response functions increase as the acyl-chain length is decreased.

Intrinsic molecules in lipid membranes change the lipid-domain interfacial area: cholesterol at domain interfaces

A theoretical analysis of the effects of intrinsic molecules on the lateral density fluctuations in lipid bilayer membranes is carried out by means of computer simulations on a microscopic interaction model of the gel-to-fluid chain-melting phase transition. The inhomogeneous equilibrium structures of gel and fluid domains, which in previous work (Cruzerio-Hansson, L. and Mouritsen, O.G. (1988) Biochim. Biophys. Acta 944, 63–72) were shown to characterize the transition region of pure lipid membranes, are here shown to be enhanced by intrinsic molecules such as cholesterol. Cholesterol is found to increase the interfacial area and to accumulate in the interfaces. The interfacial area, the average cluster size, the lateral compressibility, and the membrane area are calculated as functions of temperature and cholesterol concentration. It is shown that the enhancement by cholesterol of the lateral density fluctuations and the lipid-domain interfacial area is most pronounced away from the transition temperature. The implications of the results are discussed in relation to passive ion permeability and function of interfacially active enzymes such as phospholipase.

Application of a molecular enzyme kinetic model for aging cells and tissues

According to the membrane hypothesis of aging (MHA), cellular senescence is attributable to a life-long, cross-linking action of oxygen-free radicals in the cell plasma membrane, resulting in a continuous decrease of the passive ion permeabilities. The consequent increase in the intracellular potassium content is accompanied by a considerable condensation of the intracellular mass (i.e., by loss of water). MHA suggested that an age-dependent increase in the physical density of the intracellular mass can underly the well-known age-dependent decreases of the macromolecular synthetic processes, the enzymic turnover rates, etc. MHA was partly based on a molecular enzyme kinetic model (MEKM) suggesting that environmental factors can substantially influence the enzyme catalysis and regulation through collisional coupling. However, the possible quantitative ranges of alterations in enzyme activities have not been estimated. This paper concludes, using principal features of the two models, that known age-dependent changes in the membrane lipid fluidity and intracellular density may result in even a 10-fold overall decrease in the enzyme activities (characterized by k cat and k −1) during the life.

Passive ion permeability of lipid membranes modelled via lipid-domain interfacial area

A microscopic interaction model of the gel-to-fluid chain-melting phase transition of fully hydrated lipid bilayer membranes is used as a basis for modelling the temperature dependence of passive transmembrane permeability of small ions, e.g. Na +. Computer simulation of the model shows that the phase transition is accompanied by strong lateral density fluctuations which manifest themselves in the formation of inhomogeneous equilibrium structures of coexisting gel and fluid domains. The interfaces of these domains are found to be dominated by intermediate lipid-chain conformations. The interfacial area is shown to have a pronounced peak at the phase transition. By imposing a simple model for ion diffusion through membranes which assigns a high relative permeation rate to the domain interfaces, the interfacial area is then identified as a membrane property which has the proper temperature variation to account for the peculiar experimental observation of a strongly enhanced passive ion permeability at the phase transition. The excellent agreement with the experimental data for Na +-permeation, taken together with recent experimental results for the phase transition kinetics, provides new insight into the microphysical mechanism of reversibel electric breakdown. This insight indicates that there is no need for aqueous pore-formation to explain the experimental observation of a dramatic increase in ion conductance subsequent to electric pulses.

Influence of Constant Current Iontophoresis on the Impedance and Passive Na+ Permeability of Excised Nude Mouse Skin

The impedance of excised nude mouse skin was determined over the frequency range 0.2–2500Hz. Impedance versus frequency plots were obtained for skin which had undergone progressive hydration over a period of 8 h, and for similarly hydrated skin which, during the hydration period, was also exposed to a current density of 0.16mA cm2 for 1h. The parallel frequency‐dependent skin resistance and capacitance were then calculated from the impedance data. The skin resistance, at 0.2 Hz, for skin exposed to the iontophoretic current decreased by a factor of ∼5 compared with that observed for skin which had undergone hydration, implying that the current had altered the ion conducting pathways of the skin. In addition, the Na− flux was measured and shown to be linearly correlated (r = 0.99) with the inverse of the impedance of the skin at 0.2Hz. This implies that the low frequency impedance of the skin is a measure of the passive ion permeability of the skin, and that this technique can be used as a noninvasive way to assess the relative effects of different types of iontophoretic current on the skin.

Paracellular ionic and transcellular water diffusions across rabbit corneal endothelium.

1. Corneal endothelial cell membrane electrical resistance is estimated at about 400 M omega by using intracellular micro-electrodes. 2. If all ion diffusion across the endothelium were transcellular, electrical resistance of the endothelial monolayer would be about 1300 omega cm2, as there are about 3000 cells mm-2. 3. Measured endothelial monolayer resistance is 12.7 +/- 0.8 omega cm2 (mean +/- S.E., n = 6), which indicates that about 99% of ion diffusion does not cross the transcellular pathway, but must pass through the paracellular route. Arguments are presented which suggest that the proportion may be even higher. 4. Endothelial passive ion permeabilities are about the same as corneal stromal passive ion permeabilities. 5. Stromal water diffusional permeability is about the same as stromal ion permeability, after allowance is made for free diffusion coefficients. In contrast, endothelial water diffusional permeability is so high as to be unmeasurable. 6. It is concluded that ions diffuse across the corneal endothelium through the paracellular route, and that water diffuses across the endothelium mainly through the transcellular route.

Are membrane microwave effects related to a critical phase transition?

We present a model that can explain reported nonthermal effects in biological membranes that occur when they are exposed to very low intensity electromagnetic fields in the microwave portion of the spectrum. One argument is that the external field drives the membrane closer to a critical point, and that critical fluctuations begin to dominate the behavior of the system. In an alternative argument, the membrane is initially close enough to its critical point for the system to be highly susceptible to specific external perturbations. Thus continuous, i.e., critical, phase transitions serve to mediate the field–membrane coupling via the greatly enhanced susceptibility of the system when it is near its critical point. We also discuss a specific example related to the experimentally observed anomaly in passive sodium ion permeability at the phase transition in the erythrocyte membrane when exposed to microwaves, and we propose additional experiments that will serve to test our hypotheses.

Optical study of active ion transport in lipid vesicles containing reconstituted Na,K-ATPase.

A fluorescence method is described for the measurement of ATP-driven ion fluxes in lipid vesicles containing purified Na,K-ATPase. The membrane voltage of enzyme containing vesicles was measured by using a voltage-sensitive indocyanine dye. By addition of valinomycin the vesicle membrane is made selectively permeable to K+ so that the membrane voltage approaches the Nernst potential for K+. With constant external K+ concentration, the time course of internal K+ concentration can be continuously measured as change of the fluorescence signal after activation of the pump. The optical method has a higher time resolution than tracer-flux experiments and allows an accurate determination of initial flux rates. From the temperature dependence of active K+ transport its activation energy was determined to be 115 kJ/mol. ATP-stimulated electrogenic pumping can be measured as fast fluorescence change when the membrane conductance is low (i.e., at low or zero valinomycin concentration). In accordance with expectation, the amplitude of the fast signal change increases with decreasing passive ion permeability of the vesicle membrane. The resolution of the charge movement is so high that a few pump turnovers can be easily detected.

Interaction of charged amphiphiles with Na+-Ca2+ exchange in cardiac sarcolemmal vesicles.

We have investigated the interaction of several charged amphiphiles with the Na+-Ca2+ exchange mechanism in a highly purified preparation of canine cardiac sarcolemmal vesicles. In all cases, the hydrophobic part of the molecule was an unbranched alkyl group. All anionic lauryl derivatives stimulated (by up to 100%) the initial rate of Na+-Ca2+ exchange in the order lauryl sulfate greater than dodecyl sulfonate greater than lauric acid. All cationic lauryl derivatives (dodecylamine, dodecyltrimethylamine, laurylcholine) were potent inhibitors of Na+-Ca2+ exchange (approximately 50% at 20 microM amphiphile). The effects of the charged amphiphiles on Na+-Ca2+ exchange were not secondary to altered passive ion permeabilities or to altered membrane surface potential. The anionic compound lauryl sulfate stimulated sarcolemmal Na+-Ca2+ exchange activity by increasing the apparent affinity of the exchanger for Ca2+. In contrast, cationic dodecylamine did not change the apparent Km (Ca2+) and acted as a noncompetitive inhibitor of Na+-Ca2+ exchange. The effectiveness of the amphiphiles could be varied by altering the length of the alkyl chain. The more hydrophobic the molecule (i.e. the longer the alkyl chain), the more potent was the stimulation or inhibition of Na+-Ca2+ exchange. This implies that the amphiphiles most probably become embedded in the membrane lipid bilayer to exert effects on Na+-Ca2+ exchange. The Na+-Ca2+ exchange mechanism is more sensitive to the charged amphiphiles than are other sarcolemmal transport mechanisms. We have previously suggested (Philipson, K. D., and Nishimoto, A. Y. (1984) J. Biol. Chem. 259, 16-19) that negatively charged phospholipids could stimulate Na+-Ca2+ exchange activity. We propose that the charged amphiphiles modulate Na+-Ca2+ exchange activity by acting as phospholipid analogues. The amphiphiles are useful tools for studying the interaction of the Na+-Ca2+ exchange mechanism with the lipid bilayer.