Impermeant potential-sensitive oxonol dyes: I. Evidence for an "on-off" mechanism

Membrane potential mediates the cellular binding of nanoparticles

The use of nanoparticles for cellular therapeutic or sensing applications requires nanoparticles to bind, or adhere, to the cell surface. While nanoparticle parameters such as size, shape, charge, and composition are important factors in cellular binding, the cell itself must also be considered. All cells have an electrical potential across the plasma membrane driven by an ion gradient. Under standard conditions the ion gradient will result in a -10 to -100 mV potential across the membrane with a net negative charge on the cytosolic face. Using a combination of flow cytometry and fluorescence microscopy experiments and dissipative particle dynamics simulations, we have found that a decrease in membrane potential leads to decreased cellular binding of anionic nanoparticles. The decreased cellular binding of anionic nanoparticles is a general phenomenon, independent of depolarization method, nanoparticle composition, and cell type. Increased membrane potential reverses this trend resulting in increased binding of anionic nanoparticles. The cellular binding of cationic nanoparticles is minimally affected by membrane potential due to the interaction of cationic nanoparticles with cell surface proteins. The influence of membrane potential on the cellular binding of nanoparticles is especially important when considering the use of nanoparticles in the treatment or detection of diseases, such as cancer, in which the membrane potential is decreased.

Mechanism of response of potential-sensitive dyes studied by time-resolved fluorescence

The mechanism of response of two potential-sensitive dyes, diOC(2)(5) (3,3'-diethyloxadicarbocyanine iodide) and oxonol V (bis-[3-phenyl-5-oxoisoxazol-4-yl]pentamethine oxonol), were studied by using steady-state and time-resolved fluorescence techniques. The lipid concentration dependence of the Deltapsi (membrane potential)-induced change in total fluorescence intensity was quite different for these two dyes. Time-resolved fluorescence measurements showed that the fluorescence decay of these dyes in membranes could be resolved into at least three exponentials. Deltapsi-induced changes in the levels of these three populations were also measured under a variety of conditions. In the case of diOC(2)(5) an inside negative Deltapsi increased the levels of the bound forms. This shows that diOC(2)(5) responds to Deltapsi mainly by an "on-off" mechanism whereby Deltapsi perturbs the membrane-water partition coefficient of the dye. The Deltapsi-induced changes approached zero when the dye was totally membrane bound. In contrast, the Deltapsi-induced response of oxonol V increased with increased membrane binding. An inside negative Deltapsi decreased the level of the bound form with a longer lifetime. This shows that the mechanism of response of oxonol V is a Deltapsi-induced shift in the equilibrium between bound forms of the dye.

Plasma membrane biophysical properties linked to the antiproliferative effect of interferon-alpha

The relationship of plasma membrane biophysical properties to the anti-proliferative effect of interferon-alpha (IFN-alpha) was investigated in Daudi lymphoblasts cell lines with sensitivity to growth inhibition, parallel clonal variants selected for resistance, and one revertant subclone. Lateral mobility of surface differentiation antigens (I2, CD19, CD20, and sIgM-kappa) were measured by fluorescence recovery after photobleaching (FRAP). The mean diffusion coefficients, D, values for two clones of IFN-alpha resistant Daudi cells were significantly higher (D = 8.1-11 x 10(-10) cm2/sec) than for parental sensitive cells (D = 4.9-7.4 x 10(-10) cm2/sec). Microviscosity of the plasma membranes were probed by electron spin resonance (ESR) spectrometry. These results also indicate a greater degree of molecular motional freedom in resistant cells. Treatment of sensitive lymphoblasts with IFN-alpha (100-400 U/10(6) cells) for 5-30 min consistently increased mean values of D and the degree of spin-probe motional freedom, whereas no significant differences were detected in resistant cells. The effect of IFN-alpha on the membrane potential (Em) of Daudi cells was quantitated by flow cytometry using a voltage-sensitive oxonol dye. Membrane potential of all clones was similar (-50 to -56 mV). Treatment with IFN-alpha for 8-10 min caused hyperpolarization in the sensitive cells (deltaEm up to 45 mV), but only minimal hyperpolarization in the resistant ones (deltaEm up to 7 mV). We concluded that sensitivity to IFN-alpha and treatment with IFN-alpha are related to the biophysical status of plasma membranes.

Volume-Sensitive Chloride Channels Do Not Mediate Activation-Induced Chloride Efflux in Human Neutrophils

Many agents that activate neutrophils, enabling them to adhere to venular walls at sites of inflammation, cause a rapid Cl(-) efflux. This Cl(-) efflux and the increase in the number and affinity of beta(2) integrin surface adhesion molecules (up-regulation) are all inhibited by ethacrynic acid and certain aminomethyl phenols. The effectiveness of the latter compounds correlates with their inhibition of swelling-activated Cl(-) channels (I(Clvol)), suggesting that I(Clvol) mediates the activator-induced Cl(-) efflux. To test this hypothesis, we used whole-cell patch clamp in hypotonic media to examine the effects of inhibitors of up-regulation on I(Clvol) in neutrophils and promyelocytic leukemic HL-60 cells. Both the channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid and [3-methyl-1-p-sulfophenyl-5-pyrazolone-(4)]-[1,3-dibutylbarbituric acid]-pentamethine oxonol (WW781), a nonpenetrating oxonol, inhibited I(Clvol) at concentrations similar to those that inhibit beta(2) integrin up-regulation. However, ethacrynic acid, at the same concentration that inhibits activator-induced Cl(-) efflux and up-regulation, had no effect on I(Clvol) and swelling-activated Cl(-) efflux, providing evidence against the involvement of I(Clvol) in the activator-induced Cl(-) efflux.

The noncompetitive inhibitor WW781 senses changes in erythrocyte anion exchanger (AE1) transport site conformation and substrate binding

WW781 binds reversibly to red blood cell AE1 and inhibits anion exchange by a two-step mechanism, in which an initial complex (complex 1) is rapidly formed, and then there is a slower equilibration to form a second complex (complex 2) with a lower free energy. According to the ping-pong kinetic model, AE1 can exist in forms with the anion transport site facing either inward or outward, and the transition between these forms is greatly facilitated by binding of a transportable substrate such as Cl(-). Both the rapid initial binding of WW781 and the formation of complex 2 are strongly affected by the conformation of AE1, such that the forms with the transport site facing outward have higher affinity than those with the transport site facing inward. In addition, binding of Cl(-) seems to raise the free energy of complex 2 relative to complex 1, thereby reducing the equilibrium binding affinity, but Cl(-) does not compete directly with WW781. The WW781 binding site, therefore, reveals a part of the AE1 structure that is sensitive to Cl(-) binding and to transport site orientation, in addition to the disulfonic stilbene binding site. The relationship of the inhibitory potency of WW781 under different conditions to the affinities for the different forms of AE1 provides information on the possible asymmetric distributions of unloaded and Cl(-)-loaded transport sites that are consistent with the ping-pong model, and supports the conclusion from flux and nuclear magnetic resonance data that both the unloaded and Cl(-)-loaded sites are very asymmetrically distributed, with far more sites facing the cytoplasm than the outside medium. This asymmetry, together with the ability of WW781 to recruit toward the forms with outward-facing sites, implies that WW781 may be useful for changing the conformation of AE1 in studies of structure-function relationships.

Merocyanine 540 as a fluorescence indicator for molecular packing stress at the onset of lamellar-hexagonal transition of phosphatidylethanolamine bilayers

The fluorescence of Merocyanine 540 (MC 540) is sensitive to the molecular packing of membrane lipids. Therefore, the fluorescence of MC 540 is expected to be sensitive to the curvature-related packing stress at the onset of the lamellar-hexagonal phase transition. We measured the fluorescence intensity of MC 540 when the temperatures of lipid bilayers approached their lamellar-hexagonal phase transitions. The fluorescence of MC 540 in the presence of egg and dioleoylphosphatidylethanolamine bilayers increased at the respective lamellar-hexagonal phase transitions of these lipids. Furthermore, increases in fluorescence intensity were also observed at temperatures just below their phase transitions. The enhanced fluorescence was not due to the specific interaction of the dye with the ethanolamine headgroup, because no such increase was observed when the probe was exposed to phosphatidylethanolamines which do not form hexagonal phase within the range of applied temperature. In addition, when the temperature of the lamellar-hexagonal phase transition was shifted, by the addition of a small amount of phosphatidylcholine, the dependence of the fluorescence intensity on temperature was modified accordingly. We postulate that the change of MC 540 fluorescence intensity at temperatures approaching the lamellar-hexagonal phase transition reflects changes in the partition of MC 540 into the fluid lipid phase. The change in partition is influenced by the curvature stress in bilayers at temperatures just below the lamellar-hexagonal phase transition.

Nonmediated flip-flop of anionic phospholipids and long-chain amphiphiles in the erythrocyte membrane depends on membrane potential

The nonmediated inward translocation (flip) of the anionic fluorescent N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)- (NBD-)labeled phospholipid phosphatidylmethanol (PM) from the outer to the inner membrane leaflet of human erythrocytes and vice versa depends on membrane potential. Interestingly, inside-positive potentials due to chloride gradients and the native chloride conductance of the cells resulted in an increase of the flip rates. This flip enhancement could be suppressed by addition of gramicidin D, which increases cation conductance, or 4,4'-diisothiocyanatostilbene-2,2'-disufonate (DIDS), which inhibits anion conductance. Conversely, inside negative potentials established by an outward-directed K+ gradient in the presence of gramicidin on DIDS-treated cells resulted in a decrease of flip rate. Flip rate exhibited an exponential dependence on membrane potential. The opposite effects of the positive and negative potentials were obtained for the outward translocation (flop) from the inner to the outer membrane leaflet. Similar potential dependencies were found for the nonmediated flip of anionic NBD-labeled phosphatidic acid (PA) and 2-(N-decyl)aminonaphthalene-6-sulfonic acid (2,6-DENSA) following blockage of the band-3-mediated component of flip. The membrane potential also influences the stationary distribution of the anionic lipids between the inner and outer leaflets. The distribution is shifted to the inner leaflet by increasingly positive potentials and to the outer leaflet by increasingly negative potentials. It is concluded that nonmediated flip-flop of the anionic phospholipids and the long-chain sulfonate represents electrogenic translocation of the unprotonated charged lipids across the hydrophobic barrier.

Use of voltage-sensitive dyes and optical recordings in the central nervous system

Understanding the spatio-temporal features of the information processing occurring in any complex neural structure requires the monitoring and analysis of the activity in populations of neurons. Electrophysiological and other mapping techniques have provided important insights into the function of neural circuits and neural populations in many systems. However, there remain limitations with these approaches. Therefore, complementary techniques which permit the monitoring of the spatio-temporal activity in neuronal populations are of continued interest. One promising approach to monitor the electrical activity in populations of neurons or on multiple sites of a single neuron is with voltage-sensitive dyes coupled with optical recording techniques. This review concentrates on the use of voltage-sensitive dyes and optical imaging as tools to study the activity in neuronal populations in the central nervous system. Focusing on 'fast' voltage-sensitive dyes first, several technical issues and developments in optical imaging will be reviewed. These will include more recent developments in voltage-sensitive dyes as well as newer developments in optical recording technology. Second, studies using voltage-sensitive dyes to investigate information processing questions in the central nervous system and in the invertebrate nervous system will be reviewed. Some emphasis will be placed on the cerebellum, but the major goal is to survey how voltage-sensitive dyes and optical recordings have been utilized in the central nervous system. The review will include optical studies on the visual, auditory, olfactory, somatosensory, auditory, hippocampal and brainstem systems, as well as single cell studies addressing information processing questions. Discussion of the intrinsic optical signals is also included. The review attempts to show how voltage-sensitive dyes and optical recordings can be used to obtain high spatial and temporal resolution monitoring of neuronal activity.

Anomalous response of oxonol-V to membrane potential in mitochondrial proton pumps

The response of the fluorescent membrane potential probe oxonol-V (bis[3-phenyl 5 oxoisoxazol-4-yl]pentamethine oxonol) in submitochondrial particles (SMP) was dependent upon whether the potential (inside positive) was generated by active proton pumps or by valinomycin-aided passive K+ influx. The fluorescence intensity showed a decrease in the former case and an increase in the latter situation. This anomalous behavior was not observed with other similar anionic probes. Gradual inhibition of proton pumping activity showed that the difference in the response of oxonol-V is not due to possible difference in the magnitude of membrane potential generated in these two situations. In the presence of membrane permeant anions such as TPB- (tetraphenyl boron) or chlorate, the direction of response of oxonol-V fluorescence was the same in both situations. Time-resolved fluorescence of the oxonol-V-SMP system showed three populations: one free form (fluorescence lifetime approximately 60 ps) and two SMP-bound forms (lifetimes of 0.45 ns and 1.4 ns). A fourth population was created during the action of proton pumps. The shorter lifetime (approximately 250 ps) of this new bound form suggest this population to be an aggregated form. This population was absent during the action of proton pumps in the presence of TPB- or chlorate. These results suggest the creation of a charge separated state during the action of proton pumps. The decrease in fluorescence intensity could be the result of aggregation of oxonol-V around the positive end of a proton pump existing in a dipole or charge separated state.

Fluorescence lifetime imaging of intracellular calcium in COS cells using Quin-2

We describe the first fluorescence lifetime images of cells. To demonstrate this new capability we measured intracellular images of Ca2+ in COS cells based on the Ca(2+)-dependent fluorescence lifetime of Quin-2. Apparent fluorescence lifetimes were measured by the phase-modulation method using a gain-modulated image intensifier and a slow-scan CCD camera. We describe methods to correct the images for photobleaching during acquisition of the data, and to correct for the position-dependent response of the image intensifier. The phase angle Quin-2 images were found to yield lower than expected Ca2+ concentrations, which appears to be the result of the formation of fluorescent photoproducts by Quin-2. Fluorescence lifetime imaging (FLIM) does not require wavelength-radiometric probes and appears to provide new opportunities for chemical imaging of cells.

WW-781, a potent reversible inhibitor of red cell Cl- flux, binds to band 3 by a two-step mechanism

WW-781 ([3-methyl-1-p-sulfophenyl-5-pyrazolone-(4)]-[1,3-dibutylbarbit uric acid]-pentamethine oxonol), a fluorescent dye that has been used for measuring membrane potentials by optical methods, inhibits human red blood cell Cl- exchange, which is mediated by the membrane protein known as band 3 or capnophorin. The inhibition is slowly reversible upon removal of WW-781 from the medium, with a half time of approximately 4.7 min in 150 mM Cl- medium at 0 degrees C. The mechanism of inhibition by WW-781 involves a two-step binding reaction. WW-781 binds rapidly to band 3 to form an initial complex, which can also rapidly dissociate. Formation of this initial complex is followed by the much slower formation of a second complex (with a rate constant of approximately 1.1 min-1), probably involving a protein conformational change, through which WW-781 is more tightly bound to band 3. At low concentrations, WW-781 inhibits Cl- exchange with a stoichiometry of 1 WW-781 molecule per band 3 monomer, suggesting that under these conditions the binding of WW-781 is highly selective for the band 3 protein.

Synchronized repolarization after defibrillation shocks. A possible component of the defibrillation process demonstrated by optical recordings in rabbit heart

BACKGROUND

It is currently believed that defibrillation shocks act primarily by stimulating excitable myocardium to abolish wave fronts. Recent studies have shown that shocks applied during pacing not only stimulate excitable myocardium but also prolong the depolarization and refractoriness of myocardium already in a depolarized state. This study investigates the effects of shocks on fibrillation action potentials.

METHODS AND RESULTS

Recordings of membrane action potentials free of shock artifact were obtained using the voltage-sensitive dye WW781 during defibrillation of isolated rabbit hearts. These records showed that the shocks caused an additional phase of depolarization beginning with an initial rapid depolarization of the optical signal followed by a slow phase of repolarization. This occurred throughout all phases of the fibrillation action potential from just after completion of the upstroke to a time of near maximal repolarization. Defibrillation shocks, however, had the additional effect of causing the myocardium to repolarize at a constant time after the shock regardless of its prior electrical activity--the constant repolarization time response. This effect was not dependent on the presence of D600 (methoxyverapamil) or continuous coronary perfusion. It was accompanied by a similar constancy in the return of myocardial excitability. Recordings taken from multiple adjacent recording sites also showed a constant repolarization time among them.

CONCLUSIONS

A simple model of reentry is used to illustrate how the constant repolarization response, in addition to wave front termination and refractoriness extension, could play a role in the successful termination of fibrillation by electrical shock.

The use of the potential-sensitive fluorescent probe bisoxonol in mast cells

The regulation of the plasma membrane potential of rat peritoneal mast cells at the resting state and during activation was investigated using bisoxonol as a potential-sensitive fluorescent dye. Fluorescence microphotography showed that this negatively charged probe was not only present in the plasma membrane, but was also distributed in the cytoplasm. The intracellular localization of bisoxonol was confirmed by conducting experiments which showed that bisoxonol fluorescence was not enhanced in ATP-permeabilized mast cells. Rotenone (10(-7) M) and oligomycin (10(-6) M) did not change the fluorescence of bisoxonol showing, therefore, mitochondrial depolarization was not recorded with bisoxonol and suggesting that bisoxonol may represent a useful probe to study plasma membrane potential changes in the absence of exocytosis. We showed that, in non-stimulated mast cells, the blockade of the sodium pump enhanced the fluorescence of bisoxonol as did gramicidin a non selective ionophore used to fully depolarize the cells. High concentration of potassium (30 mM) as well as different ionic channel blockers did not significantly change the fluorescence intensity of bisoxonol, suggesting that ionic channel permeabilities were not involved in maintaining the resting plasma membrane potential of mast cells. Mast cells stimulated by compound 48/80 completely lost the fluorescence, shown by fluorescence microphotography, suggesting that exocytotic phenomena might induce a dye redistribution which is not only due to changes in the plasma membrane potential. In mast cells pretreated with pertussis toxin, which blocks mast cell-exocytosis, compound 48/80 induced a delayed (2 min) decrease of bisoxonol fluorescence which was shown to be dependent on the activity of the sodium pump. Considering that bisoxonol is a useful potential-sensitive probe in exocytosis-deprived mast cells, our results suggest that the sodium pump is mainly involved in the changes of plasma membrane potential of mast cells.

Optical recordings in the rabbit heart show that defibrillation strength shocks prolong the duration of depolarization and the refractory period

The present data were obtained using the technique of optical recording with the voltage-sensitive dye WW781. This technique, unlike electrical methods, was able to provide uninterrupted recordings free of artifacts during defibrillation shocks. Optical recordings were made from sites on the ventricular epicardium of perfused rabbit hearts during electrical pacing. Continuous recordings of the electrophysiological responses of an intact heart to defibrillation threshold-strength shocks were made. It was shown that these shocks were able to stimulate normal-appearing action potentials in nonrefractory myocardium. A new and unexpected finding was that defibrillation threshold-strength shocks were also able to evoke a sustained, depolarizing response from myocardium already undergoing an action potential. This prolonged the time that the myocardium remained in the depolarized state. Prolongation of the depolarized state was accompanied by an equal prolongation of the refractory period. There was no indication that this depolarizing shock response was due to damage of the myocardium by the shock, to heterogeneous electrical responses in the optical recording area, or to the methods used in this study. It is hypothesized that these shocks were able to elicit a new action potential in already depolarized myocardium by hyperpolarizing portions of the myocardium's cellular membranes and, in so doing, to reactivate the fast sodium current. This effect, if prevalent in a fibrillating ventricle, could play a role in the defribillation process by effectively resynchronizing electrical activity.

Evidence for the interaction of mast cell-degranulating peptide with pertussis toxin-sensitive G proteins in mast cells

K(+)-channel blocker properties have been reported for mast cell-degranulating peptide (MCD) in the central nervous system, but its action mechanism in mast cells remains unknown. We studied the effect of MCD on the membrane potential of rat peritoneal mast cells using the fluorescent probe bis-oxonol. Unexpectedly, MCD induced a decrease in bis-oxonol fluorescence, in a rapid and then a slower phase, suggesting hyperpolarization of mast cells. Other K(+)-channel blockers, tetraethylammonium and 4-aminopyridine, did not significantly modify the bis-oxonol fluorescence and did not alter the effect of MCD. The late phase of bis-oxonol fluorescence decrease was inhibited by ouabain and by potassium deprivation, whereas histamine release was not affected. The first phase of putative hyperpolarization induced by MCD coincided with histamine release and with the generation of inositol polyphosphates. Prior treatment of the cells with pertussis toxin inhibited these effects of MCD. MCD stimulated the GTPase activity of purified G proteins (G0/Gi) in a concentration-dependent manner. These results indicate that the effect of MCD on mast cells is unrelated to K+ channels but that it is relevant to the activation of pertussis toxin-sensitive G proteins leading to the activation of phospholipase C. A direct interaction of MCD with G proteins is proposed, which, unlike mastoparan, does not require positive cooperativity.

Response of the electrochromic dye, merocyanine 540, to membrane potential in rat liver mitochondria

Merocyanine binds extensively to rat liver mitochondria in spite of the presence of a sulfonic acid group which would suggest only limited penetration through the membrane. Passive binding shows both tight and weak binding components and is dependent on salt concentration and ionic strength in accord with the Gouy-Chapman theory. The binding of merocyanine to mitochondria is accompanied by both a fluorescence enhancement and a spectral shift. Induction of an electrical field by either respiration or K+ diffusion potential results in a partial reversal of the spectral shift seen on dye binding. At low temperature, the merocyanine spectral response to an electrical field is biphasic, consisting of a fast phase with a t1/2 of less than 1 sec at 15 degrees C and a slower phase which may vary considerably in rate and extent with conditions. The spectral shift during the two phases appears similar, but differ in sensitivity to ionic strength and temperature. The spectral shift during the fast phase at 15 degrees C indicates that the major component is a decrease in bound monomer and an increase in the aqueous dimer, indicating an "on-off" mechanism. It is suggested that the fast and slow phases of the merocyanine response may be due to two different populations of dye, possibly located at the outer and inner surfaces, respectively, of the mitochondrial membrane. The electrophoretic movement of the dye located in the membrane interior would result in the temperature-sensitive slow phase response. Demonstration of the proportionality of the fast phase response to the magnitude of the membrane potential suggests the usefulness of merocyanine in studies with mitochondrial systems.

A circulating factor(s) mediates cell depolarization in hemorrhagic shock

Cell depolarization in hemorrhagic shock has been attributed to hypoperfusion, but the mechanism remains unclear. Suspensions of single cell lines loaded with the potential-sensitive fluorescent dye bis-(1,3-dibutylbarbiturioc acid) trimethine oxonal (DIBAC) and exposed for 30 minutes to rat plasma drawn either before or after hemorrhagic shock (bled 20 mL/kg: mean arterial blood pressure less than 40 mmHg) were studied. Plasma drawn after, but not before, hemorrhage led to partial depolarization regardless of cell type (rat H9C2 skeletal muscle, A-10 smooth muscle, C-9 liver, adrenal, kidney, red blood cell [RBC], white blood cell [WBC]) or species (cat, dog, pig RBC; cat WBC; mouse C2C12 skeletal muscle; and human intestinal smooth muscle [HISM]). Dialysis did not remove the factor(s), suggesting a molecular weight of more than 10,000 daltons. The factor appeared within 5 minutes of shock. The depolarization amplitude increased as a function of plasma concentration and demonstrated saturation kinetics indicating specific receptor binding. Cells were equivalently oxygenated, excluding hypoperfusion as a necessary condition for depolarization. Tumor necrosis factor or platelet activating factor alone or in combination were not effective in this system. Stable measurements can be obtained with this noninvasive system that avoids cell injury consequent to cell impalement with electrodes. This system provides a sensitive in vitro bioassay that should permit identification of the plasma factors mediating cell depolarization, as well as definition of the responsible intracellular mechanisms.

Two mechanisms by which fluorescent oxonols indicate membrane potential in human red blood cells

Optical potentiometric indicators have been used to monitor the transmembrane electrical potential (Em) of many cells and organelles. A better understanding of the mechanisms of dye response is needed for the design of dyes with improved responses and for unambiguous interpretation of experimental results. This paper describes the responses to delta Em of 20 impermeant oxonols in human red blood cells. Most of the oxonols interacted with valinomycin, but not with gramicidin. The fluorescence of 15 oxonols decreased with hyperpolarization, consistent with an "on-off" mechanism, whereas five oxonols unexpectedly showed potential-dependent increases in fluorescence at less than 2 microM [dye]. Binding curves were determined for two dyes (WW781, negative response and RGA451, positive response) at 1 mM [K]o (membrane hyperpolarized with gramicidin) and at 90 mM [K]o (delta Em = 0 with gramicidin). Both dyes showed potential-dependent decreases in binding. Changes in the fluorescence of cell suspensions correlated with changes in [dye]bound for WW781, in accordance with the "on-off" mechanism, but not for RGA451. Large positive fluorescence changes (greater than 30%) dependent on Em were observed between 0.1 and 1.0 microM RGA451. A model is suggested in which RGA451 moves between two states of different quantum efficiencies within the membrane.

A nonlinear electrostatic potential change in the T-system of skeletal muscle detected under passive recording conditions using potentiometric dyes

Voltage-sensing dyes were used to examine the electrical behavior of the T-system under passive recording conditions similar to those commonly used to detect charge movement. These conditions are designed to eliminate all ionic currents and render the T-system potential linear with respect to the command potential applied at the surface membrane. However, we found an unexpected nonlinearity in the relationship between the dye signal from the T-system and the applied clamp potential. An additional voltage- and time-dependent optical signal appears over the same depolarizing range of potentials where change movement and mechanical activation occur. This nonlinearity is not associated with unblocked ionic currents and cannot be attributed to lack of voltage clamp control of the T-system, which appears to be good under these conditions. We propose that a local electrostatic potential change occurs in the T-system upon depolarization. An electrostatic potential would not be expected to extend beyond molecular distances of the membrane and therefore would be sensed by a charged dye in the membrane but not by the voltage clamp, which responds solely to the potential of the bulk solution. Results obtained with different dyes suggest that the location of the phenomena giving rise to the extra absorbance change is either intramembrane or at the inner surface of the T-system membrane.

A stopped-flow kinetic study of the interaction of potential-sensitive oxonol dyes with lipid vesicles

The interaction of the dyes oxonol V and oxonol VI with unilamellar dioleoylphosphatidylcholine vesicles was investigated using a fluorescence stopped-flow technique. On mixing with the vesicles, both dyes exhibit an increase in their fluorescence, which occurs in two phases. According to the dependence of the reciprocal relaxation time on vesicle concentration, the rapid phase appears to be due to a second-order binding of the dye to the lipid membrane, which is very close to being diffusion-controlled. The slow phase is almost independent of vesicle concentration, and it is suggested that this may be due to a change in dye conformation or position within the membrane, possibly diffusion across the membrane to the internal monolayer. The response times of the dyes to a rapid jump in the membrane potential has also been investigated. Oxonol VI was found to respond to the potential change in less than 1 s, whereas oxonol required several minutes. This has been attributed to lower mobility of oxonol V within the lipid membrane.

Impermeant potential-sensitive oxonol dyes: II. The dependence of the absorption signal on the length of alkyl substituents attached to the dye

We have measured the potential-dependent light absorption changes of 43 impermeant oxonol dyes with an oxidized cholesterol bilayer lipid membrane system. The size of the signal is strongly dependent on the chain length of alkyl groups attached to the chromophore. Dye molecules with intermediate chain lengths give the largest signals. To better understand the dependence of the absorbance signal on alkyl chain length, a simple equilibrium thermodynamic analysis has been derived. The analysis uses the free energy of dye binding to the membrane and the "on-off" model (E.B. George et al., J. Membrane Biol., 103:245-253, 1988a) for the potential-sensing mechanism. In this model, a population of dye molecules in nonpolar membrane binding sites is in a potential-dependent equilibrium with a second population of dye that resides in an unstirred layer adjacent to the membrane. Dye in the unstirred layer is in a separate equilibrium with dye in the bulk bathing solution. The equilibrium binding theory predicts a "sigmoidally shaped" increase in signal with increasing alkyl chain length, even for very nonpolar dyes. We suggest that aggregation of the more hydrophobic dyes in the membrane bathing solution may be responsible for their low signals, which are not predicted by the theory.

Impermeant potential-sensitive oxonol dyes: III. The dependence of the absorption signal on membrane potential

We have measured potential-dependent changes in the absorption of light by oxidized cholesterol bilayer lipid membranes in the presence of impermeant oxonol dyes. The magnitude of the absorption signal increased linearly with the size of potential steps over a range of 500 mV. The signal also increased when the offset voltage of the pulse train was increased from -150 to +150 mV. The data are consistent with the "on-off" mechanism proposed by E. B. George et al. (J. Membrane Biol. 103:245-253, 1988) in which the probe undergoes potential-dependent movement between a binding site in the membrane and an aqueous region just off the surface of the membrane. An equilibrium thermodynamic analysis of the experimental data indicates that the negatively charged oxonol chromophore senses only 5-10% of the total membrane potential difference across the membrane when it is driven into a nonpolar binding site on the membrane.