Kinetics of the potential-sensitive extrinsic probe oxonol VI in beef heart submitochondrial particles

Interferometric excitation fluorescence lifetime imaging microscopy

Fluorescence lifetime imaging microscopy (FLIM) is a well-established technique with numerous imaging applications. Yet, one of the limitations of FLIM is that it only provides information about the emitting state. Here, we present an extension of FLIM by interferometric measurement of fluorescence excitation spectra. Interferometric Excitation Fluorescence Lifetime Imaging Microscopy (ixFLIM) reports on the correlation of the excitation spectra and emission lifetime, providing the correlation between the ground-state absorption and excited-state emission. As such, it extends the applicability of FLIM and removes some of its limitations. We introduce ixFLIM on progressively more complex systems, directly compare it to standard FLIM, and apply it to quantitative resonance energy transfer imaging from a single measurement.

Ca2+ sensitivity of synaptic vesicle dopamine, gamma-aminobutyric acid, and glutamate transport systems

The effect of Ca2+ on the uptake of neurotransmitters by synaptic vesicles was investigated in a synaptic vesicle enriched fraction isolated from sheep brain cortex. We observed that dopamine uptake, which is driven at expenses of the proton concentration gradient generated across the membrane by the H+-ATPase activity, is strongly inhibited (70%) by 500 microM Ca2+. Conversely, glutamate uptake, which essentially requires the electrical potential in the presence of low Cl- concentrations, is not affected by Ca2+, even when the proton concentration gradient greatly contributes for the proton electrochemical gradient. These observations were checked by adding Ca2+ to dopamine or glutamate loaded vesicles, which promoted dopamine release, whereas glutamate remained inside the vesicles. Furthermore, similar effects were obtained by adding 150 microM Zn2+ that, like Ca2+, dissipates the proton concentration gradient by exchanging with H+. With respect to gamma-aminobutyric acid transport, which utilizes either the proton concentration gradient or the electrical potential as energy sources, we observed that Ca2+ or Zn2+ do not induce great alterations in the gamma-aminobutyric acid accumulation by synaptic vesicles. These results clarify the nature of the energy source for accumulation of main neurotransmitters and suggest that stressing concentrations of Ca2+ or Zn2+ inhibit the proton concentration gradient-dependent neurotransmitter accumulation by inducing H+ pump uncoupling rather than by interacting with the neurotransmitter transporter molecules.

Synaptic vesicle Ca2+/H+ antiport: dependence on the proton electrochemical gradient

Synaptic vesicles isolated from sheep brain cortex accumulate Ca2+ by a mechanism of secondary active transport associated to the H(+)-pump activity. The process can be visualized either by measuring Ca(2+)-induced H+ release or DeltapH-dependent Ca2+ accumulation. We observed that the amount of Ca2+ taken up by the vesicles increases with the magnitude of the DeltapH across the membrane, particularly at Ca2+ concentrations (approximately 500 microM) found optimal for the antiporter activity. Similarly, H+ release induced by Ca2+ increased with the magnitude of DeltapH. However, above 60% DeltapH (high H(+)-pump activity), the net H+ release from the vesicles decreased as the pump-mediated H+ influx exceeded the Ca(2+)-induced H+ efflux. We also observed that the Ca2+/H+ antiport activity depends, essentially, on the DeltapH component of the electrochemical gradient (approximately 3 nmol Ca2+ taken up/mg protein), although the Deltaphi component may also support some Ca2+ accumulation by the vesicles (approximately 1 nmol/mg protein) in the absence of DeltapH. Both Ca(2+)-induced H+ release and DeltapH-dependent Ca2+ uptake could be driven by an artificially imposed proton motive force. Under normal conditions (H+ pump-induced DeltapH), the electrochemical gradient dependence of Ca2+ uptake by the vesicles was checked by inhibition of the process with specific inhibitors (bafilomycin A(1), ergocryptin, folymicin, DCCD) of the H(+)-pump activity. These results indicate that synaptic vesicles Ca2+/H+ antiport is indirectly linked to ATP hydrolysis and it is essentially dependent on the chemical component (DeltapH) of the electrochemical gradient generated by the H(+)-pump activity.

A theoretical description of non-steady-state diffusion of hydrophobic ions across lipid vesicle membranes including effects of ion-ion interactions in the aqueous phase

A theoretical model of hydrophobic ion diffusion across vesicular membranes is presented, which is based upon activated rate theory. The model is applicable to the sudden addition of hydrophobic ions to a vesicle suspension, for example in a stopped-flow experiment. The time course of diffusion is calculated by numerical integration of differential rate equations for the ion concentrations and electrical potential differences across the membrane. The model utilizes the three-capacitor model of the membrane and an extended Debye-Hückel theory, taking into account non-neutrality on each side of the membrane. At low ionic strengths good agreement is found between the infinite time diffusion potential and the equilibrium Nernst potential. At large excess of inert electrolyte discrepancies are found, but under such conditions the membrane potential is negligible due to screening.

The behavior of a fast-responding barbituric acid potential-sensitive molecular probe in bovine heart submitochondrial particles

The barbituric acid probe diBa-C2-(5) responds to the formation of a membrane potential (delta psi) in bovine heart submitochondrial particles (SMP) by a CCCP-reversible, 5-7 nm red shift of the probe absorption spectrum. This shift can be enhanced by the addition of nigericin, an observation that indicates that the probe is specifically sensitive to delta psi. Probe-SMP binding analyses indicate that, relative to the resting state, the ratio of the dye dissociation constant to the maximum number of binding sites decreases by a factor of 30 when delta psi is generated. This observation suggests that the origin of the potential-dependent shift of the probe absorption spectrum is increased occupancy of the SMP membrane by diBa-C2-(5). The time course of the ATP-induced diBa-C2-(5) spectral shift in SMP was complete in nominally 0.2 s and could be described by a single-exponential rate equation. There was no evidence for a slower-phase signal when the data collection time period was increased to 250 s. The apparent first-order rate constants obtained from the single exponential analyses of the barbituric acid ATP-generated signal, however, were a linear function of probe concentration at fixed SMP membrane concentration. The resulting second-order rate constant obtained by linear regression was nominally 1 x 10(7) M (dye)-1 s-1; this value is two to three orders of magnitude higher than that of a number of other well-established probes of delta psi in mitochondrial preparations. Based on the invariance of the kinetics of the oxidation of cytochromes c and c1 by ATP-driven reversed electron transport in the presence and absence of the probe, diBa-C2-(5) does not appear to permeate the SMP membrane on a time scale of milliseconds to several minutes.

Electrogenic and electroneutral transport modes of renal Na/K ATPase reconstituted into proteoliposomes

This paper describes measurements of electrical potentials generated by renal Na/K-ATPase reconstituted into proteoliposomes, utilizing the anionic dye, oxonol VI. Calibration of absorption changes with imposed diffusion potentials allows estimation of absolute values of electrogenic potentials. ATP-dependent Nacyt/Kexc exchange in K-loaded vesicles generates large potentials, up to 250 mV. By comparing initial rates or steady-state potentials with ATP-dependent 22Na fluxes in different conditions, it is possible to infer whether coupling ratios are constant or variable. For concentrations of Nacyt (2-50 mM) and ATP (1-1000 microM) and pH's (6.5-8.5), the classical 3Nacyt/2Kexc coupling ratio is maintained. However, at low Nacyt concentrations (less than 0.8 mM), the coupling ratio is apparently less than 3Nacyt/2Kexc. ATP-dependent Nacyt/congenerexc exchange in vesicles loaded with Rb, Cs, Li and Na is electrogenic. In this mode congeners, including Naexc, act as Kexc surrogates in an electrogenic 3Nacyt/2congenerexc exchange. (ATP + Pi)-dependent Kcyt/Kexc exchange in K-loaded vesicles is electroneutral. ATP-dependent "uncoupled" Na flux into Na- and K-free vesicles is electroneutral at pH 6.5-7.0 but becomes progressively electrogenic as the pH is raised to 8.5. The 22Na flux shows no anion specificity. We propose that "uncoupled" Na flux is an electroneutral 3Nacyt/3Hexc exchange at pH 6.5-7.0 but at higher pH's the coupling ratio changes progressively, reaching 3Na/no ions at pH 8.5. Slow passive pump-mediated net K uptake into Na- and K-free vesicles is electroneutral, and may also involve Kcyt/Hexc exchange. We propose the general hypothesis that coupling ratios are fixed when cation transport sites are saturated, but at low concentrations of transported cations, e.g., Nacyt in Na/K exchange and Hexc in "uncoupled" Na flux, coupling ratios may change.

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.

Oxonol VI as an optical indicator for membrane potentials in lipid vesicles

Experiments with large unilamellar dioleoylphosphatidylcholine vesicles were carried out in order to study the effect of membrane potential on the fluorescence of Oxonol VI. A partition equilibrium of dye between membrane and water was found to exist with a partition coefficient gamma identical to c lipid/c water of about 19,000 (at zero voltage). In the presence of an inside-positive membrane potential, the negatively charged dye accumulates in the intravesicular aqueous space according to a Nernst equilibrium. This leads to an increased adsorption of dye to the inner lipid monolayer and to a concomitant increase of fluorescence. The fluorescence change can be calibrated as a function of transmembrane voltage by generating a potassium diffusion potential in the presence of valinomycin. The intrinsic fluorescence of the membrane-bound dye is not affected by voltage; the whole influence of voltage on the fluorescence results from voltage-dependent partitioning of the dye between water and membrane. The voltage dependence of the apparent partition coefficient can be quantitatively described by a three-capacitor model in which the dye is assumed to bind to adsorption planes located on the hydrocarbon side of the membrane/solution interface. Oxonol VI was found to be suitable for detecting changes of membrane potential associated with the activity of the (Na+ + K+)-ATPase in reconstituted vesicles. When ATP is added to the external medium, pump molecules with the ATP-binding side facing outward become activated; this results in a translocation of net positive charge towards the vesicle interior. Under this condition, fluorescence changes corresponding to (inside-positive) potentials of up to 150-200 mV are observed. After the build-up of the membrane potential, a quasi-stationary state is reached in which the pump current is compensated by a back-flow of charge through passive conductance pathways.

Membrane potential and cation content of osteoblast-like cells (UMR 106) assessed by fluorescent dyes

A number of cellular functions have recently been associated with alterations of the membrane potential in non-excitable cells. To assess the electrophysiologic regulation of osteoblast function, a method for measuring the membrane potential (Em) of a rat osteogenic sarcoma cell line (UMR 106) by the voltage-sensitive oxonol dye di-BA-C4(3) was developed. The fluorescent signal of di-BA-C4(3) was calibrated through a null point method using the protonophore FCCP. At null point, Em is equivalent to H+ equilibrium potential, and may be calculated by the Nernst equation. Intracellular pH (pHi) changes induced by the protonophore were monitored using BCECF, a pH-sensitive fluorescent probe. In the presence of FCCP, intracellular pH was found to be linearly correlated to extracellular pH (pHo). Therefore, the value of pHi at null point was extrapolated as well. With this technique, we estimated the plasma membrane potential of the "putative" rat osteoblasts (UMR 106) as -28.3 +/- 4.0 mV (n = 10). This method corrected the 16% overestimation of Em derived from the assumption that pHi does not change during the calibration procedure, as described in previous studies employing pH null point techniques. With null point methods, using BCECF and the carboxylic ionophores nigericin and monensin, intracellular concentrations of potassium and sodium were also measured and found to be 125 +/- 0.7 mM (n = 3) and 24 +/- 5.3 mM (n = 3), respectively. Although the Em of UMR 106 cells was dependent on extracellular potassium concentration, these cells did not behave as a potassium electrode. The sodium/potassium permeability ratio, calculated by the Goldman equation, was estimated at 0.317. This high membrane permeability to sodium may contribute to the genesis of the low plasma membrane potential of UMR 106 cells.

The interaction of potential-sensitive molecular probes with dimyristoylphosphatidylcholine vesicles investigated by 31P-NMR and electron microscopy

The effect of a number of commonly employed potential-sensitive molecular probes on the 31P-NMR properties of dimyristoylphosphatidylcholine vesicles at two field strengths has been investigated in order to obtain information on the location and effect of these probes on the membrane bilayer. In comparison to the control dye-free vesicle spectrum, the probes diS-C3-(5) and diS-C4-(5), when added to a vesicle suspension, cause a substantial broadening of the 31P resonance with no detectable chemical shift within an uncertainty of +/- 0.05 ppm at 24 MHz. The spin-lattice and spin-spin relaxation times are also reduced when the cyanines are present by well over 20% relative to those of the control vesicle preparation. The addition of anionic probes, including several oxonol derivatives and merocyanine 540, causes no chemical shift, line broadening, or changes in the relaxation times. Possible explanations for the failure of the anionic probes to alter the vesicle 31P-NMR properties include charge repulsion between these dyes and the phosphate group that prevents the probes from penetrating the bilayer to a depth sufficient to alter the local motion of the phosphate moiety. The 31P resonance broadening and reduction in the relaxation times caused by the two cyanines is at least in part due to an increase in vesicle size as judged by electron microscopy measurements, although an inhibition of the local phosphate motion as well cannot be completely eliminated. The cyanine-mediated increase in vesicle size appears to be due to an irreversible vesicle-fusion process possibly initiated by the screening of surface charge by these probes. The implications of these observations in relation to functional energy-transducing preparations is discussed.

Elicitor stimulation of the defense response in cultured plant cells monitored by fluorescent dyes

Addition of fungal elicitors to plant cells in suspension is known to stimulate biochemical changes in the plant cell leading to production of defense compounds. In this paper we demonstrate that introduction of elicitors from the pathogenic fungus Verticillium dahliae to cultured cotton, tobacco, or soybean cells leads to a rapid, dramatic change in the fluorescence of several membrane-associated potentiometric or pH-sensitive dyes. The fluorescence transitions occur abruptly following a brief (0 to 10 min) lag period in apparently most cells of the suspension simultaneously. Furthermore, both the length of the lag period and the rate of the subsequent fluorescence change were shown to be highly dependent on elicitor concentration. When the crude elicitor extract was separated by gel filtration chromatography into several active fractions, the ability of each fraction to stimulate phytoalexin production in the cotton cell suspension was found to correlate directly with the rate of the fluorescence decrease in the fluorescence assay. Because the assay is rapid, simple to perform, quantitative, and reproducible, it represents an attractive alternative to the more cumbersome and perhaps less quantitative elicitor assays currently in use. The fact that membrane-potential-sensitive dyes of different structure respond to elicitation of plant cells similarly further suggests, but does not prove, that asymmetric ion fluxes into or out of the plant cell are involved in the initial events of elicitor signal transduction.

Interaction of the extrinsic potential-sensitive molecular probe diS-C3-(5) with pigeon heart mitochondria under equilibrium and time-resolved conditions

Some aspects of the interaction of the extrinsic, potential-sensitive, molecular probe diS-C3-(5) with pigeon heart mitochondria are reported in this paper. Binding studies based on fluorimetry indicate that the ratio of the dissociation constant to the maximum number of binding sites, KD/n, is larger for succinate-containing mitochondria than that for cyanide-inhibited preparations. These observations suggest that the basis of the energy-dependent diS-C3-(5) optical signals is the ejection of the probe from the mitochondrial membrane. A more detailed analysis indicated that the major change in the binding parameters is a reduction in the maximum number of binding sites, n, when a charge gradient is formed at the expense of substrate. Using rapid mixing techniques, the time course of the passive association of diS-C3-(5) with mitochondria, that of the glutamate- and ATP-dependent optical signals, and the effect of this probe on the rate at which the energy-dependent cytochrome c oxidase Soret band shift signal develops have been monitored. Retardation the ATP-dependent cytochrome c oxidase Soret band shift signal suggests that the probe readily permeates the mitochondrial membrane. The first-order rate law that the glutamate-dependent signal obeys suggests that the rate-limiting step in the development of this signal is the dissociation of the dye from the mitochondrial membrane or the permeation of this membrane by the probe. The faster phase of the ATP-induced signal likely reflects the initial transfer of dye from the bulk aqueous phase followed by a slower probe permeation process that obeys a first-order rate law. This probe appears to distribute across the mitochondrial membrane in accordance with the transmembrane potential as judged by its effect on the ATP-dependent cytochrome c oxidase Soret band shift signal. DiS-C3-(5) also appears to inhibit the NADH dehydrogenase.

An electron transfer dependent membrane potential in chromaffin-vesicle ghosts

Adrenal medullary chromaffin-vesicle membranes contain a transmembrane electron carrier that may provide reducing equivalents for intravesicular dopamine beta-hydroxylase in vivo. This electron transfer system can generate a membrane potential (inside positive) across resealed chromaffin-vesicle membranes (ghosts) by passing electrons from an internal electron donor to an external electron acceptor. Both ascorbic acid and isoascorbic acid are suitable electron donors. As an electron acceptor, ferricyanide elicits a transient increase in membrane potential at physiological temperatures. A stable membrane potential can be produced by coupling the chromaffin-vesicle electron-transfer system to cytochrome oxidase by using cytochrome c. The membrane potential is generated by transferring electrons from the internal electron donor to cytochrome c. Cytochrome c is then reoxidized by cytochrome oxidase. In this coupled system, the rate of electron transfer can be measured as the rate of oxygen consumption. The chromaffin-vesicle electron-transfer system reduces cytochrome c relatively slowly, but the rate is greatly accelerated by low concentrations of ferrocyanide. Accordingly, stable electron transfer dependent membrane potentials require cytochrome c, oxygen, and ferrocyanide. They are abolished by the cytochrome oxidase inhibitor cyanide. This membrane potential drives reserpine-sensitive norepinephrine transport, confirming the location of the electron-transfer system in the chromaffin-vesicle membrane. This also demonstrates the potential usefulness of the electron transfer driven membrane potential for studying energy-linked processes in this membrane.

The interaction of the potential-sensitive molecular probe merocyanine 540 with phosphorylating beef heart submitochondrial particles under equilibrium and time-resolved conditions

The interaction of the potential-sensitive extrinsic molecular probe merocyanine 540 ( M540 ) with phosphorylating submitochondrial particles has been investigated under equilibrium and time-resolved conditions. The addition of ATP to a M540 -membrane suspension produces oligomycin and CCCP-sensitive spectral changes with absolute maxima near 490, 530, and 565 nm; a 1- to 2-nm red shift of the dye absorption spectrum is also evident in the longer-wavelength region of the spectrum. In fixed-wavelength work, the energy-dependent optical signals were increased by the addition of nigericin and NH4Cl, and could be subsequently restored to the control level by valinomycin or KSCN, respectively. These observations suggest that M540 is specifically sensitive to the membrane-potential portion of the electrochemical gradient presumably present in the submitochondrial particle system in the presence of substrate. Binding analyses based on the Langmuir adsorption isotherm and the direct linear method indicate that the M540 dissociation constant is decreased by the presence of ATP with little or no change in the maximum number of binding sites. The M540 dissociation constant was markedly decreased when 0.1 M NaCl was present in the medium, suggesting that the association of this probe with the membrane may be subject to considerable surface charge repulsion. Results from the binding analyses indicate that the origin of the energy-dependent spectral changes may be an enhanced association of M540 with the submitochondrial particle membrane resulting from the transfer of dye from the aqueous phase to membrane-binding sites. The time course of the NADH-, ATP-, or succinate-induced signal developed slowly, on a time scale of tens of seconds, and follows a second-order rate law, suggesting that the rate-limiting step in the development of the ATP-induced M540 signal may be the transfer of dye from the aqueous phase to membrane-binding sites. The enhanced passive binding of M540 to the submitochondrial particle membrane in the presence of NaCl reduces the concentration of free dye apparently available to redistribute to the membrane when substrate is present, with a concomitant reduction in the observed pseudo-first-order and the second-order rate constants. If the effective free dye concentration is estimated from binding data and used in the plot from which the latter rate constant is obtained, the value of this constant compares favorably with the obtained in the absence of the electrolyte.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of Cd2+ on ATP-driven membrane potential in beef heart mitochondrial H+-ATPase: a study using the voltage-sensitive probe oxonol VI

Beef heart mitochondrial H+-ATPase (F1-F0) vesicles were prepared by lysolecithin extraction of ETPH. ATP-driven membrane potential was monitored indirectly by following absorbance changes of the potential-sensitive dye oxonol VI. The steady-state potential was discharged by oligomycin and/or Cd2+ (a dithiol reagent). At 13 degrees C, the agents appeared to act synergistically; at 24 degrees C the data were equivocal. When Cd2+ was added before energization, the membrane potential was markedly attenuated. Both effects of Cd2+ were inhibited by dithiothreitol. The activation energy for oligomycin-sensitive ATPase exhibited a discontinuity at 16 degrees C. However, the temperature dependence of the rate of potential discharge by oligomycin showed no such discontinuity. The results are discussed in terms of the involvement of thiol groups in proton translocation and the thermotropic behavior of the membrane vesicles.

Isolation of a highly active H+-ATPase from beef heart mitochondria

The lysolecithin extraction procedure originally described by Sadler et al. (1974) has been modified to yield a H+-ATPase with high levels of Pi-ATP exchange activity (400-600 nmol x min-1 x mg-1). This activity is further enhanced (1400-1600 nmol x min-1 x mg-1) following sucrose density gradient centrifugation in the presence of asolectin. This enhancement results in part from a lipid-dependent activation and in part from removal of inactive complexes. The H+ translocating activity of the complex has been determined spectrophotometrically using binding of oxonol VI as an indicator of membrane potential. Pi-ATP exchange, ATP hydrolysis, and oxonol binding are sensitive to energy-transfer inhibitors (oligomycin, rutamycin) and/or uncouplers (DNP, FCCP).

The behavior of the fluorescence lifetime and polarization of oxonol potential-sensitive extrinsic probes in solution and in beef heart submitochondrial particles

The fluorescence polarization and lifetime of the extrinsic potential-sensitive probes oxonols V and VI have been investigated both for the dyes free in aqueous and ethanol solutions and in the presence of beef heart submitochondrial particles under resting and energy-transducing conditions. The emission lifetime of the dyes appears to be inversely related to the solvent dielectric constant and increases as the solvent is changed from an aqueous medium to ethanol to the biological membrane. The fluorescence decay curve becomes biphasic in the presence of the membrane preparation and consists of a faster decaying component, the lifetime of which is the same as that of the probe in aqueous solution and of a slower decaying component. The longer lived component suffers an uncoupler-sensitive decrease in lifetime when ATP is added to the medium. The decrease in lifetime of the longer lived species is accompanied by large depolarizations of the dye fluorescence. These observations are consistent with a redistribution-type mechanism for the energy-dependent spectral changes involving the movement of probe from the aqueous phase to the membrane vesicles. The rotational relaxation time of oxonols V and VI is increased by over an order of magnitude when these dyes associate with the membrane. This observation is consistent with a previously developed model for the location of the dyes in the bilayer in which the side chains serve as anchors, preventing the rapid tumbling of the probe in the membrane.

The measurement of membrane potential using optical indicators

Conclusions

Optical methods have become established as a major experimental protocol for following membrane potential. They can provide a rapid, continuous record of the potential and have a very wide applicability. However, when used to make quantitative assertions about membrane potential, optical methods have a number of weaknesses. Even the most reliable calibration procedures depend on accurate evaluation of a small number, namely the internal ion concentration, in a large background, that is total ion levels. However, a consensus seems to be emerging that the plasma membrane potential of non-excitable cells nevertheless has considerable magnitude: typical values are −60 mV for lymphocytes (Rink et al., 1980), −20 to −100 mV, depending on metabolic load, for Ehrlich ascites tumour cells (Philo & Eddy, 1978; but see also Smith & Robinson, 1980), and −66 to −86 mV for neutrophils (Tatham et al., 1980). In our own experiments using monolayer cultures of cells grown to confluence (Bashford et al., 1981) the potential across the plasma membrane is of the order of −100 mV (see Fig. 2). Membrane potentials of similar magnitude have been found using ion-distribution methods and microelectrodes in neuroblastoma cells and lymphocytes (Deutsch et al., 1979a,b). In the latter studies ions of different charge were used to provide upper and lower estimates of the potential, the presumed effects of binding being very different for anions and cations. A similar approach, in this case the use of optical indicators of different charge, has been taken by Rink et al. (1980), and this would seem to be one way in which to diminish the uncertainties involved in dye calibration. Unfortunately many anions, particularly oxonols, form complexes with valinomycin (Lavie & Sonenberg, 1980; Rink et al., 1980), although we have found no evidence for such a complex with bis isoxazolone oxonols (J.C. Smith and C.L. Bashford, unpublished observations). It is apparent that calibration procedures not dependent on valinomycin should be sought in order to establish optical methods as a quantitative approach to the study of membrane potential.