Monitoring membrane potentials in Ehrlich ascites tumor cells by means of a fluorescent dye

Purine nucleobase transport in bloodstream forms of Trypanosoma brucei is mediated by two novel transporters

The mechanism and inhibitor sensitivity of hypoxanthine transport by bloodstream forms of Trypanosoma brucei brucei was investigated. The dose response curve for the inhibition of hypoxanthine transport (1 microM) by guanosine was biphasic; approximately 90% of transport activity was inhibited with a Ki value of 10.8 +/- 1.8 microM, but 10% of the activity remained insensitive to concentrations as high as 2 mM. These two components of hypoxanthine transport are defined as guanosine-sensitive (H2) and guanosine-insensitive (H3). Hypoxanthine influx by both components was saturable, but there was a marked difference in their Km values (123 +/- 15 nM and 4.7 +/- 0.9 microM for H2 and H3, respectively) although the Vmax values (1.1 +/- 0.2 and 1.1 +/- 0.1 pmol (10[7] cells)[-1] s[-1], n = 3) were similar. Hypoxanthine uptake via the H2 carrier was inhibited by purine bases and analogues as well as by some pyrimidine bases and one nucleoside (guanosine), whereas the H3 transporter was sensitive only to inhibition by purine nucleobases. H2-mediated hypoxanthine uptake was inhibited by ionophores, ion exchangers and the potential H+-ATPase inhibitors, N,N'-dicyclohexylcarbodiimide (DCCD) and N-ethylmaleimide (NEM). Measurements of the intracellular pH and membrane potential of bloodstream trypanosomes in the presence and absence of these agents established a linear correlation between protonmotive force and rate of [3H]hypoxanthine (30 nM) uptake. We conclude that hypoxanthine transport in bloodstream forms of T. b. brucei occurs by two transport systems with different affinities and substrate specificities, one of which, H2, appears to function as a H+-/hypoxanthine symporter.

Hypoxanthine uptake through a purine-selective nucleobase transporter in Trypanosoma brucei brucei procyclic cells is driven by protonmotive force

The mechanism of purine nucleobase transport in procyclic cells of the protozoan parasite Trypanosoma brucei brucei was investigated. Hypoxanthine uptake at 22 degrees C was rapid and saturable, exhibiting an apparent Km of 9.3 +/- 2.0 microM and a Vmax of 4.5 +/- 0.8 pmol x (10(7) cells)(-1) x s(-1). All the natural purine nucleobases tested (Ki 1.8-7.2 microM), as well as the purine analogues oxypurinol and allopurinol, inhibited hypoxanthine influx in a manner consistent with the presence of a single high-affinity carrier. Nucleosides and pyrimidine nucleobases had little or no effect on hypoxanthine influx. The uptake process was independent of extracellular sodium, but inhibited by ionophores inducing cytosolic acidification (carbonyl cyanide chlorophenylhydrazone, nigericin, valinomycin) or membrane depolarisation (gramicidin) as well as by the adenosine triphosphatase inhibitors N-ethylmaleimide and N,N'-dicyclohexylcarbodiimide. Using the fluorescent dyes bisoxonol and 2',7'-bis-(carboxyethyl)-5,6-carboxy-fluorescein to determine membrane potential and intracellular pH (pHi), the rate of hypoxanthine uptake was shown to be directly proportional to the protonmotive force. Similarly, under alkaline extracellular conditions hypoxanthine uptake was reversibly inhibited alongside a reduction in protonmotive force. In addition, hypoxanthine accelerated the rate of pH, recovery to pH 7 after base-loading with NH4Cl, indicative of a proton influx concurrent with hypoxanthine transport. Finally, after pretreatment of cells with N-ethylmaleimide, hypoxanthine induced a slow membrane depolarisation, demonstrating that hypoxanthine transport is electrogenic. These data show that hypoxanthine uptake in T. b. brucei procyclic cells is dependent on the protonmotive force, and are consistent with a nucleobase/H+-symporter model for this transporter.

Chloride accumulation in freshly isolated Ehrlich ascites tumor cells: the role of the Na/K/2Cl cotransporter

When Ehrlich ascites tumor cells are removed from the peritoneal cavity and incubated in a saline solution, cells lose water, sodium, lactate and hydrogen ions and gain chloride. The gain of intracellular chloride exceeds that predicted from passive distribution. As chloride has been purported to play a role in volume regulation, it was of interest to identify factors responsible for controlling or maintaining intracellular chloride out of electrochemical equilibrium in Ehrlich cells. The results demonstrate that chloride accumulation in freshly isolated Ehrlich cells is sensitive to bumetanide, low extracellular K+ and low extracellular Na+, and is insensitive to DIDS. We conclude that chloride accumulation occurs due to the activity of the Na/K/2Cl cotransporter.

Membrane potential of rat hepatoma cells in culture: influence of factors affecting amino acid transport

The effect has been studied of various media, hormones and of amino acids on the membrane potential of rat hepatoma cells in culture measured by microelectrode impalement. Cells in Eagle's minimal essential medium plus 5% serum had a value which varied daily from about 5-8 mV, inside negative. The membrane potential of rat hepatocytes was measured to be 8.7 +/- 0.2 mV, inside negative. The membrane potential of the hepatoma cells was decreased by insulin and increased by glucagon. Membrane potential was unaffected by change of medium to Hanks' or Earle's balanced salt solutions or deprivation of serum. It was, however, reduced in cells in phosphate-buffered saline and by reduction of pH. The former effect was shown to be due to the higher [Na+] of phosphate-buffered saline as opposed to the other media. Addition of alanine, glycine, serine, proline and methylaminoisobutyrate all reduced membrane potential by 2-3 mV. Smaller decreases were seen with methionine, leucine and phenylalanine, but none with glutamine, threonine, BCH (2-aminonorborane-2-carboxylic acid) and D-alanine. The results are compared with the effects of similar conditions on aminoisobutyrate uptake. Whilst there was a correlation under some conditions there was not under others. It is concluded that for the hepatoma cells factors additional to the membrane potential must exert some influence on the capacity for amino acid transport.

Small lipid-soluble cations are not membrane voltage probes for Neurospora or Saccharomyces

Small lipid-soluble cations, such as tetraphenylphosphonium (TPP+) and tetraphenylarsonium (TPA+) are frequently used as probes of membrane voltage (delta psi, or Vm) for small animal cells, organelles, and vesicles. Because much controversy has accompanied corresponding measurements on 'walled' eukaryotic cells (plants, fungi), we studied their transport and relation to Vm in the large-celled fungus Neurospora crassa-where Vm can readily be determined with microelectrodes-as well as in the most commonly used model eukaryotic cell, the yeast Saccharomyces cerevisiae. We found no reasonable conditions under which the distribution of TPP+ or TPA+, between the cytoplasm (i) and extracellular solution (o), can serve to estimate Vm, even roughly, in either of these organisms. When applied at probe concentrations (i.e., < or = 100 microM, which did not depolarize the cells nor deplete ATP), TPP+ stabilized at ratios (i/o) below 30 in both organisms. That would imply apparent Vm values positive to -90 mV, in the face of directly measured Vm values (in Neurospora) negative to -180 mV. When applied at moderate or high concentrations (1-30 mM), TPP+ and TPA+ induced several phases of depolarization and changes of membrane resistance (Rm), as well as depletion of cytoplasmic energy stores. Only the first phase depolarization, occurring within the perfusion-turnover time and accompanied by a nearly proportionate decline of Rm, could have resulted from TPP+ or TPA+ currents per se. And the implied currents were small. Repeated testing, furthermore, greatly reduced the depolarizing effects of these lipid-soluble ions, implicating an active cellular response to decrease membrane permeability.

Regulation of taurine transport in Ehrlich ascites tumor cells

Taurine influx is inhibited and taurine efflux accelerated when the cell membrane of Ehrlich ascites tumor cells is depolarized. Taurine influx is inhibited at acid pH partly due to the concomitant depolarization of the cell membrane partly due to a reduced availability of negatively charged free carrier. These results are in agreement with a 2Na,1C1,1taurine cotransport system which is sensitive to the membrane potential due to a negatively charged empty carrier. Taurine efflux from Ehrlich cells is stimulated by addition of LTD4 and by swelling in hypotonic medium. Cell swelling in hypotonic medium is known to result in stimulation of the leukotriene synthesis and depolarization of the cell membrane. The taurine efflux, activated by cell swelling, is dramatically reduced when the phospholipase A2 is inhibited indirectly by addition of the anti-calmodulin drug pimozide, or directly by addition of RO 31-4639. The inhibition is in both cases lifted by addition of LTD4. The swelling-induced taurine efflux is also inhibited by addition of the 5-lipoxygenase inhibitors ETH 615-139 and NDGA. It is concluded that the swelling-induced activation of the taurine leak pathway involves a release of arachidonic acid from the membrane phospholipids and an increased oxidation of arachidonic acid into leukotrienes via the 5-lipoxygenase pathway. LTD4 seems to act as a second messenger for the swelling induced activation of the taurine leak pathway either directly or indirectly via its activation of the Cl- channels, i.e., via a depolarization of the cell membrane.

Assessment of membrane potential changes using the carbocyanine dye, diS-C3-(5): synchronous excitation spectroscopy studies

The fluorescence of the voltage sensitive dye, diS-C3-(5), has been analyzed by means of synchronous excitation spectroscopy. Using this rather rare fluorescence technique we have been able to distinguish between the slightly shifted spectra of diS-C3-(5) fluorescence from cells and from the supernatant. It has been found that diS-C3-(5) fluorescence in the supernatant can be selectively monitored at lambda exc = 630 nm and lambda em = 650 nm, while the cell associated fluorescence can be observed at lambda exc = 690 nm and lambda em = 710 nm. A modified theory for the diS-C3-(5) fluorescence response to the membrane potential is presented, according to which a linear relationship exists between the logarithmic increment of the dye fluorescence intensity in the supernatant, 1n I/I degrees, and the underlying change in the plasma membrane potential, delta psi p = psi p - psi p degrees. The theory has been tested on human myeloid leukemia cells (line ML-1) in which membrane potential changes were induced by valinomycin clamping in various K+ gradients. It has been demonstrated that the membrane potential change, delta psi p, can be measured on an absolute scale.

Membrane potential, anion and cation conductances in Ehrlich ascites tumor cells

The fluorescence intensity of the dye 1,1'-dipropylox-adicarbocyanine (DiOC3-(5] has been measured in suspensions of Ehrlich ascites tumor cells in an attempt to monitor their membrane potential (Vm) under different ionic conditions, after treatment with cation ionophores and after hypotonic cell swelling. Calibration is performed with gramicidin in Na+-free K-/choline-media, i.e., standard medium in which NaCl is replaced by KCl and cholineCl and where the sum of potassium and choline is kept constant at 155 mM. Calibration by the valinomycin "null point" procedure described by Laris et al. (Laris, P.C., Pershadsingh, A., Johnstone, R.M., 1976, Biochim, Biophys. Acta 436:475-488) is shown to be valid only in the presence of the Cl- -channel blocker indacrinone (MK196). Distribution of the lipophilic anion SCN- as an indirect estimation of the membrane potential is found not to be applicable for the fast changes in Vm reported in this paper. Incubation with DiOC3-(5) for 5 min is demonstrated to reduce the Cl permeability by 26 +/- 5% and the NO3- permeability by 15 +/- 2%, while no significant effect of the probe could be demonstrated on the K+ permeability. Values for Vm, corrected for the inhibitory effect of the dye on the anion conductance, are estimated at -61 +/- 1 mV in isotonic standard NaCl medium, -78 +/- 3 mV in isotonic Na+-free choline medium and -46 +/- 1 mV in isotonic NaNO3 medium. The cell membrane is depolarized by addition of the K+ channel inhibitor quinine and it is hyperpolarized when the cells are suspended in Na+-free choline medium, indicating that Vm is generated partly by potassium and partly by sodium diffusion. Ehrlich cells have previously been shown to be more permeable to nitrate than to chloride. Substituting NO3- for all cellular and extracellular Cl- leads to a depolarization of the membrane, demonstrating that Vm is also generated by the anions and that anions are above equilibrium. Taking the previously demonstrated single-file behavior of the K+ channels into consideration, the membrane conductances in Ehrlich cells are estimated at 10.4 microS/cm2 for K+, 3.0 microS/cm2 for Na+, 0.6 microS/cm2 for Cl- and 8.7 microS/cm2 for NO3-. Addition of the Ca2+-ionophore A23187 results in net loss of KCl and a hyperpolarization of the membrane, indicating that the K+ permeability exceeds the Cl- permeability also after the addition of A23187. The K+ and Cl- conductances in A23187-treated Ehrlich cells are estimated at 134 and 30 microS/cm2, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Validation of the use of the lipophilic thiocyanate anion for the determination of membrane potential in Ehrlich ascites tumor cells

The utility of the lipophilic anion thiocyanate (SCN-) as a probe for the indirect estimation of the cell membrane potential (Vm) in Ehrlich ascites tumor cells has been evaluated by comparison to direct electrophysiological measurements. SCN accumulation is consistent with first-order uptake into a single, kinetically-identifiable cellular compartment, achieving steady-state distribution in 20-30 min at 22 degrees C. The steady state distribution ratio ([SCN-]e/[SCN-]e) in physiological saline is 0.44 +/- 0.02. Treatment of the cells with propranolol (0.13 mM), an activator of Ca2+ dependent K+ channels, reduces the steady-state distribution ratio to 0.19 +/- 0.02. Conversely, treatment with BaCl2 (10 mM), an antagonist of the pathway, increases the SCN- distribution ratio to 0.62 +/- 0.01. The equilibrium potentials (VSCN) calculated under these conditions are virtually identical to direct electrophysiological measurements of the Vm made under the same conditions. The effect of varying extracellular [K+] ([K+]e) in the presence of constant [Na+]e = 100 mM has also been tested. In control cells, elevation of [K+]e from 6 to 60 mM reduces VSCN from -20.6 +/- 1.0 to -13.2 +/- 1.2 mV. Again, microelectrode measurements give excellent quantitative agreement. Propranolol increases the sensitivity of the cells to varying [K+]e, so that a 10-fold elevation reduces VSCN by approximately 31 mV. BaCl2 greatly reduces this response: a 10-fold elevation in [K+]e yielding only a 4-mV reduction in VSCN. It is concluded that the membrane potential of Ehrlich cells can be estimated accurately from SCN- distribution measurements.

Electrical properties of Ehrlich ascites tumor cells

The cell membrane potential (PD) of Ehrlich ascites tumor cells was measured continuously at 37 degrees C with conventional microelectrodes during rapid alterations of extracellular fluid composition. At extracellular electrolyte composition mimicking the in vivo situation PD is -56.7 +/- 0.7 mV and the apparent membrane resistance is 62.2 +/- 2.2 M omega. Increasing extracellular potassium concentration from 5.4 to 20.0 mmol/l depolarizes the cell membrane by +18.4 +/- 0.5 mV. Thus, the transference number for potassium (tk, apparent slope potassium conductance over slope membrane conductance) is 0.53 +/- 0.01. A significant correlation is observed between tk and PD: tk = -(0.014 +/- 0.001) [1/mV] X PD [mV] -(0.243 +/- 0.051). 0.7 mmol/l barium depolarizes the cell membrane by +28.2 +/- 0.7 mV, increases the apparent membrane resistance by a factor of 2.6 +/- 0.1 and abolishes the apparent potassium conductance. Reduction of extracellular sodium concentration from 141 to 21 mmol/l depolarizes the cell membrane by +3.1 +/- 1.3 mV. Similarly, 0.1 mmol/l amiloride depolarizes the cell membrane by +3.3 +/- 0.7 mV. Reduction of extracellular chloride concentration from 128 to 67 mmol/l hyperpolarizes the cell membrane by -2.5 +/- 0.2 mV. 1 mmol/l anthracene-9-COOH does not significantly alter PD. Temporary omission of glucose from the extracellular fluid has no appreciable effect on PD. In conclusion, PD of Ehrlich ascites tumor cells is in the range of other mammalian epithelial cells and is generated mainly by potassium diffusion, while the conductances to sodium and chloride appear to be small.

Energetics of Na+-dependent amino acid co-transport in Ehrlich ascites tumor cells

The energy available from the Na+ electrochemical potential gradient (delta mu Na) has been evaluated in Ehrlich ascites tumor cells during accumulation of 2-aminoisobutyric acid. Cells were incubated in media of varying [Na+] (25-154 mM) in the presence of 0.25 mM 2-aminoisobutyric acid to establish maximum steady-state accumulation of the amino acid. Membrane potential (Vm) and intracellular Na+ activity (aNa) were estimated using standard electrophysiological techniques. In physiological saline ([Na+] = 154 mM) aNa is 4.4 +/- 0.6 mM, giving an apparent Na+ activity coefficient (gamma app) in the cytoplasm of 0.17 +/- 0.02. Vm under these conditions is -20.8 +/- 2.1 mV. From these values, delta mu Na = 9.9 +/- 0.8 kJ/mol. Concomitant determinations of 2-aminoisobutyric acid (AIB) accumulation show an energy requirement (delta mu AIB) of 8.5 +/- 0.5 kJ/mol. Stepwise reductions in extracellular [Na+] give parallel reductions in aNa, Vm and 2-aminoisobutyric acid accumulation. However, under all conditions tested the energy available from the Na+ electrochemical potential gradient exceeds that needed to drive 2-aminoisobutyric acid uptake. The effects of 2-aminoisobutyric acid on Vm have also been determined. Addition of AIB (10 mM) to steady-state cells leads to membrane depolarization (resting Vm = -22.1 +/- 1.3 mV; plus AIB Vm = -16.2 +/- 1.2 mV) within 1 min. Subsequent repolarization of the membrane to resting levels occurs within 10 min. The repolarization phase is blocked in the presence of ouabain (2 mM). The results establish that the energy available from the Na+ gradient is sufficient to serve as a source for 2-aminoisobutyric acid accumulation.

Intracellular Na+, K+ and Cl- activities in Ehrlich ascites tumor cells

Ehrlich ascites tumor cell membrane potential (Vm) and intracellular Na+, K+ and Cl- activities were measured under steady-state conditions in normal saline medium (Na+ = 154, K+ = 6, Cl- 150 mequiv./l). Membrane potential was estimated to be -23.3 +/- 0.8 mV using glass microelectrodes. Intracellular ion activities were estimated with similar glass electrodes rendered ion-selective by incorporation of ion-specific ionophores. Measurements of Vm and ion-activity differences were made in the same populations of cells. Under these conditions the intracellular Na+, K+ and Cl- activities are 4.6 +/- 0.5; 68.3 +/- 8.0; and 43.6 +/- 2.1 mequiv./l, respectively. The apparent activity coefficients for Na+ and K+ are 0.18 +/- 0.02 and 0.41 +/- 0.05 respectively. These are significantly lower than the activity coefficients expected for the ions in physiological salt solutions (0.71 and 0.73, respectively). The activity coefficient for intracellular Cl- (0.67 +/- 0.03), however, is close to that of the medium (0.73), and the transmembrane electrochemical potential difference for Cl- is not different from zero. The results establish that the energy available from the Na+ electrochemical gradient is much greater than previously estimated from chemical measurements.

Differential effects of transmembrane potential on two Na+-dependent transport systems for neutral amino acids

The effects of changes of membrane potential on amino acid transport through systems A, ASC and L was investigated in the Ehrlich cell and the human erythrocyte. Changes of membrane potential were produced by incubating cells whose K+ permeability had been increased, either by valinomycin or by activation of Ca2+-dependent K+ channels, in medium containing different K+ concentrations. The changes in membrane potential were followed by measuring the distribution ratio reached by lipophilic indicators. Transport through Na+-dependent system A was sensitive to the membrane potential, the rate of amino acid uptake increasing 2.2-3.1-times for each 60 mV-hyperpolarization. The Na+-dependent system ASC was insensitive to membrane potential. The Na+-independent system L was not directly affected by membrane potential, but the steady-state accumulation of system L substrates was increased by hyperpolarization.

Inhibition of neutrophil activation by p-bromophenacyl bromide and its effects on phospholipase A2

In an effort to elucidate the nature of the inhibitory effects of p-bromophenacyl bromide (pBPB) on neutrophil stimulation, we have examined its effects on several stages of stimulus-response coupling. Pretreatment of rat neutrophils with pBPB resulted in a dose- and time-dependent irreversible inhibition of both N-formylmethionyl-leucylphenylalanine (fMet-Leu-Phe)-induced lysosomal enzyme release and change in transmembrane potential. Inhibition of the biological responses to the chemotactic peptide fMet-Leu-Phe was not due to receptor inactivation since fMet-Leu-[3H]-Phe binding to the formyl peptide receptor was not significantly altered by pBPB pretreatment. Inhibition by pBPB of phorbol myristate acetate (PMA)-induced changes in transmembrane potential and the generation of superoxide (0-2) was also observed. pBPB treatment appeared to inhibit activation of the NADPH oxidase without a direct effect on the oxidase itself. This inhibitory effect was not accompanied by cell death or decrease in cellular titratable sulphydryl groups (at least at doses less than 20 microM). There was, however, significant inhibition of a membranous fraction of fMet-Leu-Phe-induced phospholipase A2 activity by pretreatment with 10 microM pBPB, although total cellular phospholipase A2 was only minimally (less than 20% inhibition) affected. These data would indicate that pBPB inhibits an early event associated with stimulus-response coupling in rat polymorphonuclear leukocytes (i.e. change in transmembrane potential). The inhibitory effects of pBPB may be secondary to the inhibition of a critical membranous fraction of cell bound phospholipase A2 activity or its activation, necessary for the initiation of cell activation.

Influence of Ca2+ on the plasma membrane potential and electrogenic uptake of glycine by myeloma cells. Involvement of a Ca2+-activated K+ channel

The involvement of Ca2+-activated K+ channels in the regulation of the plasma membrane potential and electrogenic uptake of glycine in SP 2/0-AG14 lymphocytes was investigated using the potentiometric indicator 3,3'-diethylthiodicarbocyanine iodide. The resting membrane potential was estimated to be -57 +/- 6 mV (n = 4), a value similar to that of normal lymphocytes. The magnitude of the membrane potential and the electrogenic uptake of glycine were dependent on the extracellular K+ concentration, [K+]o, and were significantly enhanced by exogenous calcium. The apparent Vmax of Na+-dependent glycine uptake was doubled in the presence of calcium, whereas the K0.5 was not affected. Ouabain had no influence on the membrane potential under the conditions employed. Additional criteria used to demonstrate the presence of Ca2+-activated K+ channels included the following: (1) addition of EGTA to calcium supplemented cells elicited a rapid depolarization of the membrane potential that was dependent on [K+]o; (2) the calmodulin antagonist, trifluoperazine, depolarized the membrane potential in a dose-dependent and saturable manner with an IC50 of 9.4 microM; and (3) cells treated with the Ca2+-activated K+ channel antagonist, quinine, demonstrated an elevated membrane potential and depressed electrogenic glycine uptake. Results from the present study provide evidence for Ca2+-activated K+ channels in SP 2/0-AG14 lymphocytes, and that their involvement regulates the plasma membrane potential and thereby the electrogenic uptake of Na+-dependent amino acids.

Lymphocyte membrane potential and Ca2+-sensitive potassium channels described by oxonol dye fluorescence measurements

A method is described for quantitative measurement of lymphocyte transmembrane electrical potential difference (psi) by flow cytometric recording of the oxonol dye fluorescence of single cells. Both the simultaneous collection and analysis of multiple optical parameters and the use of a negatively charged oxonol probe allowed more accurate measurement of psi than may be obtained by bulk cell suspension techniques employing cationic voltage indicators. Mouse spleen and human blood lymphocyte psi was calculated to be -70 mV. T and B lymphocytes maintain a constant psi as extracellular K+ is varied from 2 to 10 mM and the deviation from K+ equilibrium potentials (EK) is shown to result from Na+ permeability. At [K+]o values greater than 10 mM, lymphocytes behave as K+ electrodes. Examination of lymphocyte subsets showed that hyperpolarization induced by the Ca2+ ionophore A23187 occurs only in T cells. This response was identified as activation of a Ca2+-sensitive K+ channel by pharmacologic manipulations. Hence, T cells depolarized by 4-aminopyridine (4-AP, 10 mM) were observed to return to resting psi by A23187-induced elevation of [Ca2+]i. Cells depolarized by quinine (100 microM) were unaffected by A23187. The Ca2+-activated channel does not contribute to resting psi in T cells since it may be selectively blocked by quinine (20 microM) or modulated by calmodulin antagonists (5 microM trifluperazine) without affecting resting psi.

Membrane voltage, resistance, and channel switching in isolated mouse fibroblasts (L cells): a patch-electrode analysis

The whole-cell patch-electrode technique of Fenwick, Marty & Neher (1982) has been applied to single suspension-cultured mouse fibroblasts. Seals in the range of 10-50 G omega were obtained without special cleaning of the cell membranes. Rupture of the membrane patch inside the electrode was accompanied by a shift of measured potential into the range -10 to -25 mV, but in most cases with little change in the recorded resistance. The latter fact implied that the absolute resistance of the cell membrane must be in the same range as the seal resistance and the recorded potential is a poor measure of actual cell membrane potential. Steady-state current-voltage curves (range -160 mV to +80 mV) were generated before and after rupture of the membrane patch, and the difference between these gave (zero-current) membrane potentials of -50 to -75 mV, which represents a leak-corrected estimate of the true cell-membrane potential. The associated slope conductivity of the cell membrane was 5-15 microS/cm2 (assumed smooth-sphere geometry, cells 13-15 microns in diameter) and was K+-dominated. With 0.1 mM (or more) free Ca2+ filling the patch electrode, membrane potentials in the range -60 to -85 mV were observed following patch rupture, with associated slope conductivities of 200-400 microS/cm2, also K+-dominated. Similar voltages and conductivities were observed at the peak of pulse-induced 'hyperpolarizing activation' (Nelson, Peacock, & Minna, 1972), and the two phenomena probably reflect the behaviour of Ca2+-activated K+ channels. Both the pulse-induced conductance and the Ca2+-activated conductance spontaneously decayed, the latter over periods of 5-15 min following patch rupture. Sr2+, Ba2+, and Co2+ could also activate the putative K+ channels, but only Sr2+ really mimicked Ca2+. Co2+ and Ba2+ activated with a delay of several minutes following patch rupture, and deactivated quickly with a small decrease of conductance and a large decrease of membrane potential. Evidently, Co2+ and Ba2+ affect channel specificity as well as channel opening and closing kinetics.

Oxonol dyes as monitors of membrane potential: the effect of viruses and toxins on the plasma membrane potential of animal cells in monolayer culture and in suspension

Optical indicators of the cationic, cyanine and anionic oxonol classes were used to evaluate the plasma membrane potential of animal cells in suspension and in monolayer culture. The optical signals were calibrated by using diffusion potentials either of K+ (in the presence of valinomycin) or of H+ (in the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone; FCCP); both classes of dye gave similar values of plasma membrane potential, in the range -40 to -90 mV for different cell types. Addition of haemolytic Sendai virus or Staphylococcus aureus alpha-toxin depolarizes cells and causes them to leak monovalent cations; these effects are antagonized by extracellular Ca2+. Cells infected with vesicular stomatitis or Semliki Forest virus become depolarized during an infectious cycle; infection with other viruses was without affect on plasma membrane potential.

Calcium-induced heterogeneous changes in membrane potential detected by flow cytofluorimetry

Ionophore-induced changes in the cell-associated fluorescence of samples of approx. 50000 individual murine L1210 leukemia cells which had been incubated with the voltage-sensitive dye 3,3'-dihexyloctacarbocyanine iodide (DiOC6(3] were monitored by flow cytometry. The K+ ionophore valinomycin (1 microM) produced homogeneous changes in the fluorescence of the entire population, the magnitude of which was dependent upon the concentration of extracellular K+. These changes allowed the estimation of the potassium equilibrium potential of the cells, by the null-point method, to be -11.9 mV. The Ca2+ ionophore A23187 (500 nM) produced heterogeneous changes in fluorescence, with populations of both hyperpolarised and depolarised cells. In addition, the depolarised population underwent an apparent size change, with a reduction in cell volume. This heterogeneity of response resulted in a minimal change in the median fluorescence value for the whole population, which suggests that it would not have been detectable by methods dependent upon net population-averaged changes in fluorescence. Removal of extracellular Na+ or preincubation of cells with amiloride (500 microM) effectively eliminated the depolarised population. Removal of extracellular K+ increased the hyperpolarised population. These findings provide evidence for the presence of Ca2+-induced Na+ exchange and Ca2+-induced K+ efflux mechanisms in these cells which may be expressed simultaneously in the cell population.

Plasma membrane potential of Lettré cells does not depend on cation gradients but on pumps

The plasma membrane potential of Lettré cells has been determined with the optical indicator oxonol-V and found to be -57 mV at 37 degrees C (range -20 to -80 mV depending on the physiological condition of the cells). Increasing extracellular K+ does not depolarize cells: even in the presence of 155 mM K+ the potential is -41 mV; membrane potential is also insensitive to the chemical gradient of Na+, Mg2+, Ca2+ or Cl-. Ouabain depolarizes the cells; H+ efflux from cells is stimulated by extracellular Na+. We propose that in Lettré cells the plasma membrane potential is generated by electrogenic cation pumps. The balancing fluxes of Na+ and K+ are mainly through electroneutral cation exchanges (Na+/K+ and Na+/H+) and the magnitude of the potential is limited by organic anion leaks. Such a mechanism may operate in other biological membranes also.

KCl loss and cell shrinkage in the Ehrlich ascites tumor cell induced by hypotonic media, 2-deoxyglucose and propranolol

Ehrlich ascites tumor cells lose KCl and shrink after swelling in hypotonic media and in response to the addition of 2-deoxyglucose, propranolol, or the Ca2+ ionophore, A23187, plus Ca2+ in isotonic media. All of these treatments activate cell shrinkage via a pathway with the following characteristics: (1) the KCl loss responsible for cell shrinkage does not alter the membrane potential; (2) NO3(-) does not substitute for Cl-; (3) the net KCl movements are not inhibited by quinine or DIDS; and (4) early in this study furosemide was effective in inhibiting cell shrinkage but this sensitivity was subsequently lost. This evidence suggests that the KCl loss in these cells occurs via a cotransport mechanism. In addition, hypotonic media and the other agents used here stimulate a Cl(-) - Cl(-) exchange, a net loss of K+ and a net gain of Na+ which are not responsible for cell shrinkage. The Ehrlich cell also appears to have a Ca2+-activated, quinine-sensitive K+ conductive pathway but this pathway is not part of the mechanism by which these cells regulate their volume following swelling or shrink in isotonic media in response to 2-deoxyglucose or propranolol. Shrinkage by the loss of K+ through the Ca2+ stimulated pathway appears to be limited by Cl- conductive movements; for when NO3(-), an anion demonstrated here to have a higher conductive movement than Cl-, is substituted for Cl-, the cells will shrink when the Ca2+-stimulated K+ pathway is activated.

The use of a cyanine dye in measuring membrane potential in yeast

An attempt was made to use 3,3'-dipropylthiacarbocyanine as a membrane potential probe in yeast by following both its fluorescence changes and its uptake by the cells under different conditions. It was found that the uptake of the dye into the cytoplasmic compartment was translated into an increased fluorescence, and the uptake by the mitochondria produced a quenching of the fluorescence. The experiments to measure uptake showed that a large amount of the dye was taken up by the cells under "deenergized" conditions. The uptake of the cyanine, however, was significantly reduced by the omission of the substrate, by deenergization of the mitochondria, or by the addition of K+, but not by Na+. This cyanine seems to be a good, qualitative indicator of the potential of the plasma membrane and of the mitochondria of the cells, with a faster response than those probes used before in yeast.

The use of fluorescent dyes to measure membrane potentials: a critique

Under controlled conditions, fluorescent cyanine dyes can be used to measure membrane potentials of cell suspensions. Similar changes in membrane potential can be followed both with fluorescent dyes and electrophysiological probes in response to changes in the ion composition of the medium. Recent reports that attempt to abrogate the use of the cyanine dyes in measurements of the membrane potential are misleading.

The use of fluorescent dyes to measure membrane potentials: a response

The use of fluorescent cyanine dyes to estimate membrane potential in cell suspensions has been considered. Several problems related tot he application of the dyes have been reviewed. These problems include: 1) alteration of the membrane potential (Em) and factors involved in establishing Em by the dyes themselves, 2) the effects of altered energy metabolism on the fluorescent response of the dyes and on Em, and 3) calibration of dye fluorescence. Recent reports that advocate the use of the fluorescent dyes are misleading.

Stimulation of Na+ -dependent amino acid uptake by activation of the Ca2+ -dependent K+ channel in the Ehrlich ascites tumor cell

The activation of Ca2+ -dependent K+ channel by propranolol or by ascorbate-phenazine methosulphate stimulates Na+ -dependent transport of alpha-aminoisobutyric acid. This stimulation arises from a membrane hyperpolarization due to the specific increase of membrane K+ conductance. The same treatment does not modify the Na+ -independent uptake of the norbornane amino acid.

Influence of glucose on Ehrlich cell volume, ion transport, and membrane potential

Incubating Ehrlich ascites tumor cells with 10 mM glucose at room temperature resulted in the following changes. The cells shrank, reaching a minimum volume after 1 h. The decrease in cell volume was 50-90% inhibited by 1 mM furosemide. The mmol K+ and Cl-/mg dry wt decreased, and mmol Na+/mg dry wt increased over the 1 h incubation. The net loss of KCl was inhibited by 1 mM furosemide. Immediately after the addition of glucose, the influx of 86Rb sensitive to ouabain decreased, whereas the influx sensitive to furosemide increased. The total influx of 86Rb with glucose was similar to that of controls. The effluxes of 86Rb and 36Cl increased immediately after the addition of glucose. These effluxes did not increase, however, in the presence of 1 mM furosemide. Initially the ouabain-sensitive Na+ efflux was not changed with glucose, but the ouabain-insensitive Na+ efflux decreased. Furosemide (1 mM) did not influence Na+ efflux. With time the ouabain-sensitive Na+ efflux increased as cellular Na+ levels rose so that at 1 h the ouabain-sensitive Na+ efflux from glucose-treated cells was 2.5-3 times that of control cells. The potential difference across the membrane gradually became more negative by approximately 25 mV, reaching a maximum after 1 h. The hyperpolarization was reversed by 1 mM ouabain. The changes in ionic fluxes on the addition of glucose are compared with changes in ionic fluxes seen during volume regulation.

Ca2+-dependent K+ transport in the Ehrlich ascites tumor cell

The possible presence and properties of the Ca2+-dependent K+ channel have been investigated in the Ehrlich ascites tumor cell. The treatment with ionophore A23187 + CA2+, propranolol or the electron donor system ascorbate-phenazine methosulphate, all of which activate that transport system in the human erythrocyte, produces in the Ehrlich cell a net loss of K+ (balanced by the uptake of Na+) and a stimulation of both the influx and the efflux of 86Rb. These effects were antagonized by quinine, a known inhibitor of the Ca2+-dependent K+ channel in other cell systems, and by the addition of EGTA to the incubation medium. Ouabain did not have an inhibitory effect. These results suggests that the Ehrlich cell possesses a Ca2+-dependent K+ channel whose characteristics are similar to those described in other cell systems.

Fluorescence changes of rhodamine 6G associated with changes in membrane potential in synaptosomes

The intensity of rhodamine 6G fluorescence was found to be a useful scale for measuring the membrane potential in synaptosomes. The fluorescence of rhodamine 6G in synaptosomal suspensions increases with depolarization in the synaptosomes induced by the replacement of cations in the medium or by the addition of agents known to depolarize the membrane potential. Considering the character of the dye, we have derived an equation which gives the relation between the fluorescence intensity of the dye and the membrane potential. The change in membrane potential (diffusion potential) of synaptosomes was calculated using the equation. The calculated membrane potential was proportional to the logarithm of the K+ concentration above 20 mM, and the slope of membrane potential against log [K+] was about 52 mV per decade of concentration. The permeability ratio (Px/Pk; the ratio of the permeability constants of a given cation, X+, and K+) was estimated from the calculated membrane potential.

Quantitation of corneal endothelial potentials using a carbocyanine dye

The carbocyanine dye, diS-C3-(5) was used to quantitate the plasma membrane potential of the bullfrog corneal endothelium. It was shown that valinomycin hyperpolarized the endothelial cell and that in the presence of the ionophore the membrane potential largely reflected the K+ equilibrium potential. Using calibration curves constructed by changing medium K+ concentration in the presence of valinomycin, and nigericin and ouabain to abolish ion gradients and electrogenic pump activity, the cell membrane potential was calculated to be 28.6 +/- 4.2 mV. The major source of this potential was a K+ diffusion potential, and the membrane Na+ conductance reduced the cell potential to less than the apparent K+ equilibrium potential of 51.5 +/- 5.1 mV. About 20% of the cell potential could be ascribed to the rheogenic (Na+ + K+)-ATPase.

Membrane potentials in respiring and respiration-deficient yeasts monitored by a fluorescent dye

Changes in fluorescence of 3,3'-dipropylthiodicarbocyanine iodide which had been equilibrated with suspensions of the wild-type yeast Saccharomyces cerevisiae and of respiration-deficient mutants were followed. The changes have been attributed to changes of yeast membrane potentials, since the fluorescence with wild-type yeast could be affected in a predictable manner by uncouplers and the pore-forming agent nystatin. As in other systems, a rise of steady-state fluorescence was ascribed to depolarization and a drop of the fluorescence to hyperpolarization. (1) A considerable rise in steady-state fluorescence was brought about by addition of antimycin A or some other mitochondrial inhibitors to respiring cells. A major part of the composite membrane potential monitored in intact yeast cells appeared to be represented by the membrane potential of mitochondria. (2) Addition of D-glucose and of other substrates of hexokinase, including non-metabolizable 2-deoxy-D-glucose, induced a two-phase response of fluorescence, indicating transient depolarization followed by repolarization. Such a response was not elicited by other sugars which had been reported to be transported into the cells by a glucose carrier or by D-galactose in galactose-adapted cells. The depolarization was explained by electrogenic ATP exit from mitochondria to replenish the ATP consumed in the Hexokinase reaction and the repolarization by subsequent activation of respiration. (3) In non-respiring cells only a drop in fluorescence was induced by glucose and this was ascribed to an ATP-dependent polarization of the plasma membrane. (4) Steady-state fluorescence in suspensions of respiration-deficient mutants, lacking cytochrome a, cytochrome b, or both, was high an remained unaffected by uncouplers and nystatin. This indicates that membranes of the mutants may have been entirely depolarized. A partial polarization, apparently restricted to the plasma membrane, could be achieved by glucose addition.

Specific requirement for inorganic phosphate for induction of bilayer membrane conductance by the cationic uncoupler carbocyanine dye

The trinucleous divalent cationic cyanine dye triS-C4(5) was shown to be an uncoupler of oxidative phosphorylation in mitochondria only in reaction medium containing inorganic phosphate (Pi). This dye also induced marked increase in the electrical conductance of a phospholipid bilayer membrane in bathing solution containing Pi, but not in solution containing Tris-HCl buffer without Pi. Time-dependent fluctuation of the electrical current across the bilayer membrane was observed in the presence of triS-C4(5) only in bathing solution containing Pi. This fluctuation could be due to perturbation of the bilayer membrane structure induced by the cooperative action of the cyanine dye and Pi, and this perturbation should be directly related to their effects in increasing membrane conductance and also causing uncoupling in mitochondria.

Intercellular recognition: quantitation of initial binding events

The hypothesis that intercellular adhesion can be subdivided into two separable phenomena--an initial recognition event and a subsequent stabilization--is supported by the use of a cell binding assay that provides a quantitative measure of intercellular binding strengths. Radioactive single cells are brought into contact with cell monolayers at 4 degrees C in sealed compartments. The compartments are inverted and a centrifugal force is then applied to dislodge the probe cells from the monolayers. By varying the speed of centrifugation, the force maintaining associations between embryonic chicken neural retina cells was determined to be on the order of 10(-5) dyne. Topographic specificities of single neural retina cells for retinal monolayers from various regions of the retina were detected with this assay and corresponded to those observed in more traditional assays at 37 degrees C. Also observed were two time- and temperature-dependent stabilization processes in which the force required for dislodgment increased. One of the stabilization processes was sensitive to dinitrophenol and was inactive at 4 degrees C; the second was still active in metabolically blocked cells. The metabolic-dependent process resulted in interactions at least 13 times as strong as the initial binding. The metabolic-independent process resulted in about a 2-fold increase in binding strength and had a temperature dependence similar to that of membrane diffusional phenomena.

Effect of monovalent cation ionophores on lymphocyte cellular metabolism

The effect of valinomycin, nigericin and gramicidin on the cellular O2 consumption and on ATP content has been investigation. It has been found that while valinomycin and nigericin interfere with mitochondrial functions, gramicidin D does not show any appreciable effect. These results are explained in terms of the differing abilities of ionophores to redistribute among intracellular membranes.

The accumulation of amino acids by mouse ascites-tumour cells. Dependence on but lack of equilibrium with the sodium-ion electrochemical gradient

1. The fluorescent dye 3,3'-dipropyloxadicarbocyanine was used to show that the tumour cells absorbed 2-aminoisobutyrate, glycine, L-leucine and L-isoleucine and certain other amino acids electrogenically. The Km values with respect to amino acid concentration ([A]o), obtained from the fluorescence assays, varied through the above series from 0.8 to 26 mM, with Vmax. fairly constant. 2. Similar Km values described the uptake of the 14C-labelled amino acids in five instances where this was measured. 3. Each amino acid lowered the membrane potential (E) by 10-20 mV when its cellular concentration ([A]i) had reached a steady value and [A]o was 10mM. In these experiments energy metabolism was maintained by glycolysis, 2,4-dinitrophenol was present and cellular respiration was inhibited. The corresponding net flow of amino acid through the Na+ symport was deduced by making use of the fact that the depolarization an amino acid initially caused was roughly proportional to the net influx of amino acid itself. 4. The steady-state depolarization was attributed to the presence of a leak pathway for the amino acid with a rate coefficient PA. As assayed in the absence of Na+, PA was about 5-fold larger for isoleucine than for glycine. 5. Direct estimates of Vmax./PA were similar to those inferred from the extent of depolarization in the steady state and [A]i. 6. A mathematical model was used to predict [A]i/[A]o in term of the measured values of [Na]o, [Na]i, E, Km and Vmax./PA. The predicted and observed values agreed fairly well when [A]o was 1 mM or 10 mM. 7. [A]i/[A]o varied from about 2.5 for 10 mM-isoleucine to 30 for 1 mM-2-aminoisobutyrate when delta microNa, expressed as a ratio, was ostensibly in the range 19-43. 8. The concentration of 2-aminoisobutyrate from a 0.1 mM solution in the presence or absence of ouabain was consistent with the model, whereas the concentration of isoleucine from a 0.1 mM solution exceeded the predicted values 2-5-fold. 9. The tumour cells concentrated 2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid by a non-electrogenic mechanism, with which isoleucine may also interact.

Na+-dependent transport of tricarboxylic acid cycle intermediates by renal brush border membranes. Effects on fluorescence of a potential-sensitive cyanine dye

The effect of the transport of tricarboxylic acid cycle intermediates on the membrane potential of renal brush border vesicles was studied using fluorescence of the cyanine dye, 3,3'-dipropylthiadicarbocyanine iodide. The behavior of the dye in the preparation was established with valinomycin-induced K+-diffusion potentials; increases in fluorescence were associated with depolarizing conditions. Addition of 1 mm succinate or citrate to membrane/dye suspensions produced transient increases in fluorescence, indicative of a depolarizing event(s) associated with the transport of these substrates. The transient response in fluorescence was Na+ dependent, of greater magnitude under Na+-gradient as compared to Na+-equilibrium conditions, and was a saturable function of substrate concentration. The specificity of the fluorescence response was identical to that obtained from studies of the competitive inhibition of succinate transport by tricarboxylic acid cycle intermediates and analogs We conclude that the major tricarboxylic acid cycle intermediates are transported via a common Na+-dependent transport system in renal brush border membranes.

Amino Acid Transport and stimulation by substrates in the absence of a Na2+ electrochemical potential gradient

Uptake of alpha-aminoisobutyric acid (AIB) was examined in Ehrlich ascites tumor cells treated with the cation-exchange ionophore nigericin (20 microgram/ml). Membrane voltages were measured using the voltage-sensitive dye diethyloxadicarbocyanine (DOCC). In normal phosphate-buffered media, nigericin changed the distribution ratios of Na+ and K+ (the ratio of intra- to extracellular concentrations) nearly to unity, but AIB was still accumulated to a distribution ratio of approximately 9.0. When all but 40 mM Na+ in the medium was replaced by choline, nigericin resulted in K+ loss and Na+ gain and both cation distribution ratios approached 2.8-3.4, as would be expected if both ions were distributing near electrochemical equilibrium with a membrane voltage in the range of -28 to -33 mV. This conclusion was supported by the observation that the addition of 5 X 10(-7) M valinomycin to the nigericin-treated cell suspension produced no change in DOCC absorbance. In spite of the apparent zero electrochemical potential gradients for Na+ and K+, AIB was accumulated to a distribution ratio of 5.4 in the low-Na+ medium. Addition of 0.1 mM oubain or 50 microM vanadate did not alter the extent of AIB accumulation as would have been expected if a large component of the membrane voltage were due to electrogenic operation of the (Na+ + K+)-ATPase. Addition of lactate, pyruvate or glucose increased the AIB distribution ratios to 11.9, 9.4 and 15.3, respectively. The effect of glucose could be explained, at least in part, by an enhanced Na+ electrochemical potential gradient. However, neither lactate nor pyruvate produced any change either in membrane voltage or the intracellular Na+ concentration. Therefore, these results confirm the existence of a metabolic energy source which is coupled to AIB accumulation and operates in addition to the Na+ co-transport mechanism, and which is augmented by metabolic substrates such as lactate and pyruvate.