Single-file diffusion through the Ca2+-activated K+ channel of human red cells

Determinants of cation transport selectivity: Equilibrium binding and transport kinetics

The crystal structures of channels and transporters reveal the chemical nature of ion-binding sites and, thereby, constrain mechanistic models for their transport processes. However, these structures, in and of themselves, do not reveal equilibrium selectivity or transport preferences, which can be discerned only from various functional assays. In this Review, I explore the relationship between cation transport protein structures, equilibrium binding measurements, and ion transport selectivity. The primary focus is on K(+)-selective channels and nonselective cation channels because they have been extensively studied both functionally and structurally, but the principles discussed are relevant to other transport proteins and molecules.

Equilibrium selectivity alone does not create K+-selective ion conduction in K+ channels

Potassium (K(+)) channels are selective for K(+) over Na(+) ions during their transport across membranes. We and others have previously shown that tetrameric K(+) channels are primarily occupied by K(+) ions in their selectivity filters under physiological conditions, demonstrating the channel's intrinsic equilibrium preference for K(+) ions. Based on this observation, we hypothesize that the preference for K(+) ions over Na(+) ions in the filter determines its selectivity during ion conduction. Here, we ask whether non-selective cation channels, which share an overall structure and similar individual ion-binding sites with K(+) channels, have an ion preference at equilibrium. The variants of the non-selective Bacillus cereus NaK cation channel we examine are all selective for K(+) over Na(+) ions at equilibrium. Thus, the detailed architecture of the K(+) channel selectivity filter, and not only its equilibrium ion preference, is fundamental to the generation of selectivity during ion conduction.

Preferential binding of K+ ions in the selectivity filter at equilibrium explains high selectivity of K+ channels

K⁺ channels exhibit strong selectivity for K⁺ ions over Na⁺ ions based on electrophysiology experiments that measure ions competing for passage through the channel. During this conduction process, multiple ions interact within the region of the channel called the selectivity filter. Ion selectivity may arise from an equilibrium preference for K⁺ ions within the selectivity filter or from a kinetic mechanism whereby Na⁺ ions are precluded from entering the selectivity filter. Here, we measure the equilibrium affinity and selectivity of K⁺ and Na⁺ ions binding to two different K⁺ channels, KcsA and MthK, using isothermal titration calorimetry. Both channels exhibit a large preference for K⁺ over Na⁺ ions at equilibrium, in line with electrophysiology recordings of reversal potentials and Ba²⁺ block experiments used to measure the selectivity of the external-most ion-binding sites. These results suggest that the high selectivity observed during ion conduction can originate from a strong equilibrium preference for K⁺ ions in the selectivity filter, and that K⁺ selectivity is an intrinsic property of the filter. We hypothesize that the equilibrium preference for K⁺ ions originates in part through the optimal spacing between sites to accommodate multiple K⁺ ions within the selectivity filter.

K+ binding in the G-loop and water cavity facilitates Ba2+ movement in the Kir2.1 channel

K+ are selectively coordinated in the selectivity filter and concerted K+ and water movements in this region ensure high conduction rates in K+ channels. In channels with long pores many K+ binding sites are located intracellular to the selectivity filter (inner vestibule), but their contribution to permeation has not been well studied. We investigated this phenomenon by slowing the ion permeation process via blocking inwardly rectifying Kir2.1 channels with Ba2+ in the selectivity filter and observing the effect of K+ in the inner vestibule on Ba2+ exit. The dose-response effect of the intracellular K+ concentration ([K+]i) on Ba2+ exit was recorded with and without intracellular polyamines, which compete with K+ for binding sites. Ba2+ exit was facilitated by the cooperative binding of at least three K+. Site-directed mutagenesis studies suggest that K+ interacting with Ba2+ bound in the selectivity filter were located in the region between selectivity filter and cytoplasmic pore, i.e. the water cavity and G-loop. One of the K+ binding sites was located at residue D172 and another was possibly at M301. This study provides functional evidence for the three K+ binding sites in the inner vestibule previously identified by crystal structure study.

The Occupancy of Ions in the K+ Selectivity Filter: Charge Balance and Coupling of Ion Binding to a Protein Conformational Change Underlie High Conduction Rates

Potassium ions diffuse across the cell membrane in a single file through the narrow selectivity filter of potassium channels. The crystal structure of the KcsA K+ channel revealed the chemical structure of the selectivity filter, which contains four binding sites for K+. In this study, we used Tl+ in place of K+ to address the question of how many ions bind within the filter at a given time, i.e. what is the absolute ion occupancy? By refining the Tl+ structure against data to 1.9A resolution with an anomalous signal, we determined the absolute occupancy of Tl+. Then, by comparing the electron density of Tl+ with that of K+, Rb+ and Cs+, we estimated the absolute occupancy of these three ions. We further analyzed how the ion occupancy affects the conformation of the selectivity filter by analyzing the structure of KcsA at different concentrations of Tl+. Our results indicate that the average occupancy for each site in the selectivity filter is about 0.63 for Tl+ and 0.53 for K+. For K+, Rb+ and Cs+, the total number of ions contained within four sites in the selectivity filter is about two. At low concentrations of permeant ion, the number of ions drops to one in association with a conformational change in the selectivity filter. We conclude that electrostatic balance and coupling of ion binding to a protein conformational change underlie high conduction rates in the setting of high selectivity.

Energetic optimization of ion conduction rate by the K+ selectivity filter

The K+ selectivity filter catalyses the dehydration, transfer and rehydration of a K+ ion in about ten nanoseconds. This physical process is central to the production of electrical signals in biology. Here we show how nearly diffusion-limited rates are achieved, by analysing ion conduction and the corresponding crystallographic ion distribution in the selectivity filter of the KcsA K+ channel. Measurements with K+ and its slightly larger analogue, Rb+, lead us to conclude that the selectivity filter usually contains two K+ ions separated by one water molecule. The two ions move in a concerted fashion between two configurations, K+-water-K+-water (1,3 configuration) and water-K+-water-K+ (2,4 configuration), until a third ion enters, displacing the ion on the opposite side of the queue. For K+, the energy difference between the 1,3 and 2,4 configurations is close to zero, the condition of maximum conduction rate. The energetic balance between these configurations is a clear example of evolutionary optimization of protein function.

Differences in the actions of some blockers of the calcium-activated potassium permeability in mammalian red cells

1. The actions of some inhibitors of the Ca2+-activated K+ permeability in mammalian red cells have been compared. 2. Block of the permeability was assessed from the reduction in the net loss of K+ that followed the application of the Ca2+ ionophore A23187 (2 microM) to rabbit red cells suspended at a haematocrit of 1% in a low potassium solution ([K]0 0.12-0.17 mM) at 37 degrees C. Net movement of K+ was measured using a K+-sensitive electrode placed in the suspension. 3. The concentrations (microM +/- s.d.) of the compounds tested causing 50% inhibition of K+ loss were: quinine, 37 +/- 3; cetiedil, 26 +/- 1; the cetiedil congeners UCL 1269, UCL 1274 and UCL 1495, approximately 150, 8.2 +/- 0.1, 0.92 +/- 0.03 respectively; clotrimazole, 1.2 +/- 0.1; nitrendipine, 3.6 +/- 0.5 and charybdotoxin, 0.015 +/- 0.002. 4. The characteristics of the block suggested that compounds could be placed in two groups. For one set (quinine, cetiedil, and the UCL congeners), the concentration-inhibition curves were steeper (Hill coefficient, nH, > or = 2.7) than for the other (clotrimazole, nitrendipine, charybdotoxin) for which nH approximately 1. 5. Compounds in the first set alone became less active on raising the concentration of K+ in the external solution to 5.4 mM. 6. The rate of K+ loss induced by A23187 slowed in the presence of high concentrations of cetiedil and its analogues, suggesting a use-dependent component to the inhibitory action. This was not seen with clotrimazole. 7. The blocking action of the cetiedil analogue UCL 1274 could not be overcome by an increase in external Ca2+ and its potency was unaltered when K+ loss was induced by the application of Pb2+ (10 microM) rather than by A23187. 8. These results, taken with the findings of others, suggest that agents that block the red cell Ca2+-activated K+ permeability can be placed in two groups with different mechanisms of action. The differences can be explained by supposing that clotrimazole and charybdotoxin act at the outer face of the channel whereas cetiedil and its congeners may block within it, either at or near the K+ binding site that determines the flow of K+.

Different mechanisms of Ca2+ transport in NMDA and Ca2+-permeable AMPA glutamate receptor channels

The channel of the glutamate N-methyl-D-aspartate receptor (NMDAR) transports Ca2+ approximately four times more efficiently than that of Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPAR). To investigate the basis of this difference in these glutamate receptors (GluRs), we measured the ratio of Cs+ efflux and Ca2+ influx in recombinant NMDAR and Ca2+-permeable AMPAR channels expressed in human embryonic kidney 293 (HEK 293) cells over a wide voltage range. At any one potential, this biionic flux ratio was measured by quantifying the total charge and the charge carried by Ca2+ using whole-cell currents and fluorometric techniques (dye overload) with Cs+ internally and Ca2+ externally (1.8 or 10 mM) as the only permeant ions. In AMPAR channels, composed of either GluR-A(Q) or GluR-B(Q) subunits, the biionic flux ratio had a biionic flux-ratio exponent of 1, consistent with the prediction of the Goldman-Hodgkin-Katz current equation. In contrast, for NMDAR channels composed of NR1 and NR2A subunits, the biionic flux-ratio exponent was approximately 2, indicating a deviation from Goldman-Hodgkin-Katz. Consistent with these results, in NMDAR channels under biionic conditions with high external Ca2+ and Cs+ as the reference ions, Ca2+ permeability (PCa/PCs) was concentration dependent, being highest around physiological concentrations (1-1.8 mM; PCa/PCs approximately 6.1) and reduced at both higher (110 mM; PCa/PCs approximately 2.6) and lower (0.18 mM; PCa/PCs approximately 2.2) concentrations. PCa/PCs in AMPAR channels was not concentration dependent, being around 1.65 in 0.3-110 mM Ca2+. In AMPAR and NMDAR channels, the Q/R/N site is a critical determinant of Ca2+ permeability. However, mutant AMPAR channels, which had an asparagine substituted at the Q/R site, also showed a biionic flux-ratio exponent of 1 and concentration-independent permeability ratios, indicating that the difference in Ca2+ transport is not due to the amino acid residue located at the Q/R/N site. We suggest that the difference in Ca2+ transport properties between the glutamate receptor subtypes reflects that the pore of NMDAR channels has multiple sites for Ca2+, whereas that of AMPAR channels only a single site.

Nonindependent K+ movement through the pore in IRK1 potassium channels

We measured unidirectional K+ in- and efflux through an inward rectifier K channel (IRK1) expressed in Xenopus oocytes. The ratio of these unidirectional fluxes differed significantly from expectations based on independent ion movement. In an extracellular solution with a K+ concentration of 25 mM, the data were described by a Ussing flux-ratio exponent, n', of approximately 2.2 and was constant over a voltage range from -50 to -25 mV. This result indicates that the pore of IRK1 channels may be simultaneously occupied by at least three ions. The IRK1 n' value of 2.2 is significantly smaller than the value of 3.5 obtained for Shaker K channels under identical conditions. To determine if other permeation properties that reflect multi-ion behavior differed between these two channel types, we measured the conductance (at 0 mV) of single IRK1 channels as a function of symmetrical K+ concentration. The conductance could be fit by a saturating hyperbola with a half-saturation K+ activity of 40 mM, substantially less than the reported value of 300 mM for Shaker K channels. We investigated the ability of simple permeation models based on absolute reaction rate theory to simulate IRK1 current-voltage, conductance, and flux-ratio data. Certain classes of four-barrier, three-site permeation models are inconsistent with the data, but models with high lateral barriers and a deep central well were able to account for the flux-ratio and single channel data. We conclude that while the pore in IRK1 and Shaker channels share important similarities, including K+ selectivity and multi-ion occupancy, they differ in other properties, including the sensitivity of pore conductance to K+ concentration, and may differ in the number of K+ ions that can simultaneously occupy the pore: IRK1 channels may contain three ions, but the pore in Shaker channels can accommodate four or more ions.

Characterization of voltage-dependent calcium influx in human erythrocytes by fura-2

Thus far, the methods used to determine erythrocyte Ca2+ influx have not allowed the assessment of the kinetics of ion uptake. To overcome this drawback, we studied a new method, using the fluorescent Ca2+-chelator fura-2, which directly quantifies intracellular Ca2+ changes in human erythrocytes. This method has the advantage over previous techniques that it monitors continuously cellular Ca2+ levels. The Ca2+ influx is modulated by cellular membrane potential in the presence of a transmembrane Ca2+ concentration gradient and exhibits a first slow increase of the intracellular Ca2+ concentration, followed, after the reachment of a threshold value of 125 +/- 13 nM Ca2+, by a faster increase until a plateau is reached. The influx rate is inhibited by dihydropyridines in the micromolar range. These findings support the hypothesis that erythrocyte Ca2+ influx is mediated by a carrier similar to the slow Ca2+ channels and is dependent on membrane depolarization.

Unidirectional K+ fluxes through recombinant Shaker potassium channels expressed in single Xenopus oocytes

We describe a method to evaluate the ratio of ionic fluxes through recombinant channels expressed in a single Xenopus oocyte. A potassium channel encoded by the Drosophila Shaker gene tested by this method exhibited flux ratios far from those expected for independent ion movement. At a fixed extracellular concentration of 25 mM K+, this channel showed single-file diffusion with an Ussing flux-ratio exponent, n', of 3.4 at a membrane potential of -30 mV. There was an apparent, small voltage dependence of this parameter with n' values of 2.4 at -15 and -5 mV. These results indicate that the pore in these channels can simultaneously accommodate at least four K+ ions. If each of these K+ ions is in contact with two water molecules, the minimum length of the pore is 24 A.

Orientation independence of single-vacancy and single-ion permeability ratios

Single-vacancy models have been proposed as open channel permeation mechanisms for K+ channels. Single-ion models have been used to describe permeation through Na+ channels. This paper demonstrates that these models have a distinctive symmetry property. Their permeability ratios, measured under biionic conditions, are independent of channel orientation when the reversal potential is zero. This symmetry is a property of general m-site single-vacancy channels, m-site shaking-stack channels, as well as m-site single-ion channels. An experimental finding that the permeability ratios of a channel did not have this symmetry would provide evidence that a single-vacancy or single-ion model is an incorrect or incomplete description of permeation.

Conduction properties of the cloned Shaker K+ channel

The conduction properties of the cloned Shaker K+ channel were studied using electrophysiological techniques. Single channel conductance increases in a sublinear manner with symmetric increases in K+ activity, reaching saturation by 0.6 M K+. The Shaker K+ channel is highly selective among monovalent cations; under bi-ionic conditions, its selectivity sequence is K+ > Rb+ > NH+4 > Cs+ > Na+, whereas, by relative conductance in symmetric solutions, it is K+ > NH+4 > Rb+ > Cs+. In Cs+ solutions, single channel currents were too small to be measured directly, so nonstationary fluctuation analysis was used to determine the unitary Cs+ conductance. The single channel conductance displays an anomalous molefraction effect in symmetric mixtures of K+ and NH+4, suggesting that the conducting pore is occupied by multiple ions simultaneously.

Flux, coupling, and selectivity in ionic channels of one conformation

Ions crossing biological membranes are described as a concentration of charge flowing through a selective open channel of one conformation and analyzed by a combination of Poisson and Nernst-Planck equations and boundary conditions, called the PNP theory for short. The ion fluxes in this theory interact much as ion fluxes interact in biological channels and mediated transporters, provided the theoretical channel contains permanent charge and has selectivity created by (electro-chemical) resistance at its ends. Interaction occurs because the flux of different ionic species depends on the same electric field. That electric field is a variable, changing with experimental conditions because the screening (i.e., shielding) of the permanent charge within the channel changes with experimental conditions. For example, the screening of charge and the shape of the electric field depend on the concentration of all ionic species on both sides of the channel. As experimental interventions vary the screening, the electric field varies, and thus the flux of each ionic species varies conjointly, and is, in that sense, coupled. Interdependence and interaction are the rule, independence is the exception, in this channel.

Correlated ion flux through parallel pores: application to channel subconductance states

Many ion channels that normally gate fully open or shut have recently been observed occasionally to display well-defined subconductance states with conductances much less than those of the fully open channel. One model of this behavior is a channel consisting of several parallel pores with a strong correlation between the flux in each pore such that, normally, they all conduct together but, under special circumstances, the pores may transfer to a state in which only some of them conduct. This paper introduces a general technique for modeling correlated pores, and explores in detail by computer simulation a particular model based upon electric interaction between the pores. Correlation is obtained when the transient electric field of ions passing through the pores acts upon a common set of ionizable residues of the channel protein, causing transient changes in their effective pK and hence in their charged state. The computed properties of such a correlated parallel pore channel with single occupation of each pore are derived and compared to those predicted for a single pore that can contain more than one ion at a time and also to those predicted for a model pore with fluctuating barriers. Experiments that could distinguish between the present and previous models are listed.

Shaking stack model of ion conduction through the Ca(2+)-activated K+ channel

Motivated by the results of Neyton and Miller (1988. J. Gen. Physiol. 92:549-586), suggesting that the Ca(2+)-activated K+ channel has four high affinity ion binding sites, we propose a physically attractive variant of the single-vacancy conduction mechanism for this channel. Simple analytical expressions for conductance, current, flux ratio exponent, and reversal potential under bi-ionic conditions are found. A set of conductance data are analyzed to determine a realistic range of parameter values. Using these, we find qualitative agreement with a variety of experimental results previously reported in the literature. The exquisite selectivity of the Ca(2+)-activated K+ channel may be explained as a consequence of the concerted motion of the "stack" in the proposed mechanism.

A paradox concerning ion permeation of the delayed rectifier potassium ion channel in squid giant axons

1. The fully activated current-voltage relation (I-V) of the delayed rectifier potassium ion channel in squid giant axons has a non-linear dependence upon the driving force, V-EK, as I have previously demonstrated, where V is membrane potential and EK is the equilibrium potential for potassium ions. 2. The non-linearity of the I-V relation and its dependence upon external potassium ion concentration are both well described, phenomenologically, by the Goldman-Hodgkin-Katz (GHK) flux equation, as I have also previously demonstrated. As illustrated below, this result can be modelled using the Eyring rate theory of single-file diffusion of ions through a channel in the low-occupancy limit of the theory. 3. The GHK equation analysis and the low-occupancy limit of the Eyring rate theory are both consistent with the independence principle for movement of ions through the channel, which is at odds with tracer flux ratio results from the delayed rectifier, published elsewhere. Those results suggest that the channel is multiply occupied by two, or perhaps three, ions. 4. The resolution of this paradox is provided by a triple-binding site, multiple-occupancy model in which only one vacancy, at most, is allowed in the channel. This model predicts current-voltage relations which are consistent with the data (and with the phenomenological prediction of the GHK flux equation). The model is also consistent, approximately, with the tracer flux ratio results.

1990: annus mirabilis of potassium channels

Voltage-gated potassium channels make up a large molecular family of integral membrane proteins that are fundamentally involved in the generation of bioelectric signals such as nerve impulses. These proteins span the cell membrane, forming potassium-selective pores that are rapidly switched open or closed by changes in membrane voltage. After the cloning of the first potassium channel over 3 years ago, recombinant DNA manipulation of potassium channel genes is now leading to a molecular understanding of potassium channel behavior. During the past year, functional domains responsible for channel gating and potassium selectivity have been identified, and detailed structural pictures underlying these functions are beginning to emerge.

Ca2(+)-activated K+ channel from human erythrocyte membranes: single channel rectification and selectivity

The Ca(+)-activated K+ channel of the human red cell membrane was characterized with respect to rectification and selectivity using the patch-clamp technique. In inside-out patches exposed to symmetric solutions of K+, Rb+, and NH+4, respectively, inward rectifying i-V curves were obtained. The zero current conductances were: K+ (23.5 pS +/- 3.2) greater than NH+4 (14.2 pS +/- 1.2) greater than Rb+ (11.4 pS +/- 1.8). With low extracellular K+ concentrations (substitution with Na+) the current fluctuations reversed close to the Nernst potential for the K ion and the rectification as well as the i-V slopes decreased. With mixed intracellular solutions of K+ and Na+ enhanced rectification were observed due to a Na+ block of outward currents. From bi-ionic reversal potentials the following permeability sequence (PK/PX was calculated: K+ (1.0) greater than Rb+ (1.4 +/- 0.1) greater than NH+4 (8.5 +/- 1.3) greater than Li+ (greater than 50); Na+ (greater than 110); Cs+ (much greater than 5). Li+, Na+, and Cs+ were not found to carry any current, and only minimum values of the permeability ratios were estimated. Tl+ was permeant, but the permeability and conductance were difficult to quantify, since with this ion the single channel activity was extremely low and the channels seemed to inactivate. The inward rectification in symmetric solutions indicate an asymmetric open channel structure, and the different selectivity sequences based on conductances and permeabilities reflect inter-ionic interactions in the permeation process.

The gating of human red cell Ca2(+)-activated K(+)-channels is strongly affected by the permeant cation species

Using inside-out patches, the effect of various permeant cations on the gating behaviour of the human red cell Ca2(+)-activated K(+)-channel was examined. For symmetric solutions the dwell time histograms indicated two shut and two open states. Mean open times as well as the open-state probability were affected by the permeant cation species: Rb+ stabilised the channel in the open configuration, whereas NH4+ had a destabilising effect. Intermediate stability was obtained in K+ solutions. Bi-ionic experiments indicated that the gating was influenced by the ion species occupying the channel, rather than by ions bound to external modifier sites.

A simple model for multi-ion permeation. Single-vacancy conduction in a simple pore model

Recent experimental evidence suggests that certain membrane channels operate in a nearly ion-saturated state. We therefore consider a "single-vacancy" model of ion permeation: if a channel has n conducting sites, it will contain either n or n-1 ions. Simple analytical expressions for the current, conductance, and reversal potential under bi-ionic conditions are derived. The results are compared with those of single ion models and recent experiments on Ca2(+)-activated K+ channels.

Nifedipine inhibits calcium-activated K transport in human erythrocytes

The effect of nifedipine on K transport across human erythrocytes was investigated. Nifedipine had no effect on K influx mediated by the Na-K pump, Na-K-2Cl cotransport, or the passive residual K flux. However, it inhibited the K and water loss from ATP-depleted cells in the presence of external Ca (Cao). Similar inhibition of Ca-activated K [K(Ca)] efflux was observed in fresh cells exposed to Cao and A23187 or ionomycin. The inhibition was observed even when nifedipine was added after initiation of the K(Ca) efflux and was not readily reversed by washing cells with drug-free media. When K(Ca) efflux was plotted as a function of external free Ca, nifedipine reduced the maximum K(Ca) efflux but had no effect on the Ca concentration required for half-maximum K(Ca) efflux. The inhibition of K(Ca) efflux by nifedipine was not consequent to its effect on conductive Cl permeability, because valinomycin-induced K efflux in Cl media was enhanced rather than reduced by nifedipine and because the inhibition was also seen with SCN, a nonlimiting anion. Nifedipine inhibited the K(Ca) efflux with a dissociation constant (Kd) of 4 microM. The inhibitory capacity of nifedipine was reduced by increasing external K. Nifedipine reduced not only the basic conductance but also the zero-current K conductance with a Kd of 23 microM. Other Ca-channel blockers, such as verapamil and diltiazem, did not inhibit K(Ca) efflux, but other dihydropyridines, including BAY K 8644, a Ca-channel agonist, were effective in inhibiting K(Ca) efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane sidedness and the interaction of H+ and K+ on Ca2(+)-activated K+ transport in human red blood cells

The sided effects of H+ on Ca2(+)-stimulated K+ transport (the Gardos channel) were studied in human red blood cells. Cells were loaded with Ca2+ during energy depletion with the internal pH adjusted to desired levels prior to treatment with the anion-exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which inhibits pH equilibration across the membrane. This treatment provides a "pH clamp" whereby the internal and external H+ (H+i and H+o) concentrations can be varied separately. Channel activity was evaluated by measuring either net K+ loss or unidirectional 42K+ efflux from cells where SO2(-4) replaced Cl- on both sides of the membrane. When pHi was set at 7.4, decreasing pHo from values of 8.0 to 5.0 inhibited K+ efflux. This effect of H+o could be overcome by increasing K+o at all values of pHo. In addition, this effect of K+o could be separated from its effects on altering the membrane potential, indicating an interaction between K+o and H+o on the channel. A similar interaction was shown to occur between H+i and K+i. K+o is known to be required for activation of Ca2(+)-stimulated K+ transport, since the channel in cells preincubated in the absence of K+o (prior to exposure to Ca+i) becomes refractory to subsequent activation by Ca2+i and K+o. We found that H+o would not substitute for K+o in this regard nor would H+o inhibit the protective effect of K+o; in addition, H+ was not transported inward in exchange for K+i. Thus it would appear that there are two external sites where K+o interacts with the channel. One site is antagonized by H+o, whereas the second site is required for channel activation independent of H+ in the range studied. The inside of the channel would have, by an analogous argument, at least one site where K+i and H+i interact.

Reconstitution of a calcium-activated potassium channel in basolateral membranes of rabbit colonocytes into planar lipid bilayers

A highly enriched preparation of basolateral membrane vesicles was isolated from rabbit distal colon surface epithelial cells employing the method described by Wiener, Turnheim and van Os (Weiner, H., Turnheim, K., van Os, C.H. (1989) J. Membrane Biol. 110:147-162) and incorporated into planar lipid bilayers. With very few exceptions, the channel activity observed was that of a high conductance. Ca2+-activated K+ channel. This channel is highly selective for K+ over Na+ and Cl-, displays voltage-gating similar to "maxi" K(Ca) channels found in other cell membranes, and kinetic analyses are consistent with the notion that K+ diffusion through the channel involves either the binding of a single K+ ion to a site within the channel or "single-filing" ("multi-ion occupancy"). Channel activity is inhibited by the venom from the scorpion Leiurus quinquestriatus, Ba2+, quinine, and trifluoperazine. The possible role of this channel in the function of these cells is discussed.

Voltage-activated cation transport in human erythrocytes

We report here the effects of membrane potential on the permeability of the human erythrocyte to Na, K, and Ca. Membrane potential was changed either by varying the K concentration gradient in the presence of valinomycin or by varying the concentration gradient of the permeant anion nitrate in the presence of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. When the membrane potential was changed from inside negative (-10 mV) to inside positive (greater than 40 mV), influx, efflux, and net flux of Na and K increased. Marked net cation loss and cell shrinkage occurred in the absence of a chemical gradient for Na and K. This voltage-dependent increase in Na and K conductance is partially inhibited by 10 microM ruthenium red and persists when the membrane potential is returned to -10 mV after transient exposure to inside-positive potentials. A similar voltage-dependent behavior was found for Ca influx. The voltage-activated Ca influx is almost completely inhibited by 10 microM ruthenium red.

Ca2+-activated K+ conductance of the human red cell membrane: voltage-dependent Na+ block of outward-going currents

Human red cells were prepared with various cellular Na+ and K+ concentrations at a constant sum of 156 mM. At maximal activation of the K+ conductance. gK(Ca). the net efflux of K+ was determined as a function of the cellular Na+ and K+ concentrations and the membrane potential. Vm, at a fixed [K+]ex of approximately 3.5 mM. Vm was only varied from (Vm-EK) approximately equal to 25 mV and upwards, that is, outside the range of potentials with a steep inward rectifying voltage dependence (Stampe & Vestergaard-Bogind, 1988). gK(Ca) as a function of cellular Na+ and K+ concentrations at Vm = -40.0 and 40 mV indicated a competitive, voltage-dependent block of the outward current conductance by cellular Na+. Since the present Ca2+-activated K+ channels have been shown to be of the multi-ion type, the experimental data from each set of Na+ and K+ concentrations were fitted separately to a Boltzmann-type equation, assuming that the outward current conductance in the absence of cellular Na+ is independent of voltage. The equivalent valence determined in this way was a function of the cellular Na+ concentration increasing from 0.5 to 1.5 as this concentration increased from 11 to 101 mM. Data from a previous study of voltage dependence as a function of the degree of Ca2+ activation of the channel could be accounted for in this way as well. It is therefore suggested that the voltage dependence of gK(Ca) for outward currents at (Vm-Ek) greater than 25 mV reflects a voltage-dependent Na+ block of the Ca2+-activated K+ channels.

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)

Halothane inhibits hyperpolarization and potassium channels in human red blood cells

The effect of halothane on the Ca2+-sensitive K+ channel in human erythrocytes has been investigated. The red cells were suspended in buffer-free salt solutions containing Ca2+ or 45Ca2+. The protonophore CCCP was added to bring about a rapid equilibration of protons across the plasma membrane. After addition of the divalent cation ionophore A23187, the cells took up Ca2+ and this caused the K+ channels to open. When the medium contained 1 mM K+, the addition of A23187 induced a transient hyperpolarization of the cells, as monitored by measurement of the pH of the medium. The cellular pH, being buffered by haemoglobin, was virtually constant. Halothane reversibly inhibited hyperpolarization and limited the release of cellular K+ in a dose-dependent way, but did not inhibit the Ca2+-transporting properties of A23187. No stimulatory effects of halothane were observed even at low halothane concentrations. In conclusion, halothane reversibly inhibits the Ca2+-sensitive K+ channel in human erythrocytes with an ED50 of about 0.5 mM.

Ca2+-activated K+ conductance of human red cell membranes exhibits two different types of voltage dependence

The voltage dependence for outward-going current of the Ca-activated K+ conductance (gK(Ca] of the human red cell membrane has been examined over a wide range of membrane potentials (Vm at constant values of [K+]ex, [K+]c and pHc, the intact cells being preloaded to different concentrations of ionized calcium. Outward-current conductances were calculated from initial net effluxes of K+ and the corresponding (Vm - EK) values. The basic conductance, defined as the outward-current conductance at (Vm - EK) greater than or equal to 20 mV and [K+]ex greater than or equal to 3 mM (B. Vestergaard-Bogind, P. Stampe and P. Christophersen, J. Membrane Biol. 95:121-130, 1987) was found to be a function of cellular ionized Ca. At all degrees of Ca activation gK(Ca) was an apparently linear function of voltage (Vm range -40 to +70 mV), the absolute level as well as the slope decreasing with decreasing activation. In a simple two-state model the constant voltage dependence can, at the different degrees of Ca activation, be accounted for by a Boltzmann-type equilibrium function with an equivalent valence of approximately 0.4, assuming chemical equilibrium at Vm = 0 mV. Alternatively, the phenomenon might be explained by a voltage-dependent block of the outward current by an intracellular ion. Superimposed upon the basic conductance is the apparently independent inward-rectifying steep voltage function with an equivalent valence of approximately 5 and chemical equilibrium at the given EK value.

The effect of ATP, intracellular calcium and the anion exchange inhibitor DIDS on conductive anion fluxes across the human red cell membrane

The influence of ATP depletion, the intracellular ionized Ca-concentration, anion substitution and DIDS on the conductive anion fluxes across the human red cell membrane has been examined. Under physiological or near physiological conditions it is not possible to observe conductive anion fluxes across the erythrocyte membrane in that anions totally dominate the membrane conductance. Consequently anions are at electro-chemical equilibrium and the netflux is zero. However, conductive anion fluxes can be induced by raising the potassium conductance, either by addition of valinomycin, or by triggering the native calcium activated potassium channel by addition of the Ca2+ ionophore A23187 to cells suspended in a calcium containing medium. The interpretation of data from experiments with valinomycin induced netfluxes has normally been done according to a constant field model, and the results have consequently been given as permeabilities. Since it has been demonstrated recently, that these cation pathways do not conform to a constant field scheme (Bennekou, P. and Christophersen, P. (1986) J. Membr. Biol. 93, 221-227 and Vestergaard-Bogind, B., Stampe, P. and Christophersen, P. (1985) J. Membr. Biol. 88, 67-75), it has been chosen, instead of permeabilities, to calculate the ion conductances from net efflux data, using an independent estimate of the membrane potential. The main result reported, is that only one component is found for the conductive anion fluxes in the presence of DIDS using the latter theoretical framework, whereas a sizeable DIDS-insensitive component is found when the constant field analysis is used. Furthermore it is found that ATP and intracellular calcium do not influence the anion conductances.

Voltage dependence of the Ca2+-activated K+ conductance of human red cell membranes is strongly dependent on the extracellular K+ concentration

The conductance of the Ca2+-activated K+ channel (gK(Ca)) of the human red cell membrane was studied as a function of membrane potential (Vm) and extracellular K+ concentration ([K+]ex). ATP-depleted cells, with fixed values of cellular K+ (145 mM) and pH (approximately 7.1), and preloaded with approximately 27 microM ionized Ca were transferred, with open K+ channels, to buffer-free salt solutions with given K+ concentrations. Outward-current conductances were calculated from initial net effluxes of K+, corresponding Vm, monitored by CCCP-mediated electrochemical equilibration of protons between a buffer-free extracellular and the heavily buffered cellular phases, and Nernst equilibrium potentials of K ions (EK) determined at the peak of hyperpolarization. Zero-current conductances were calculated from unidirectional effluxes of 42K at (Vm-EK) approximately equal to 0, using a single-file flux ratio exponent of 2.7. Within a [K+]ex range of 5.5 to 60 mM and at (Vm-EK) greater than or equal to 20 mV a basic conductance, which was independent of [K+]ex, was found. It had a small voltage dependence, varying linearly from 45 to 70 microS/cm2 between 0 and -100 mV. As (Vm-EK) decreased from 20 towards zero mV gK(Ca) increased hyperbolically from the basic value towards a zero-current value of 165 microS/cm2. The zero-current conductance was not significantly dependent on [K+]ex (30 to 156 mM) corresponding to Vm (-50 mV to 0). A further increase in gK(Ca) symmetrically around EK is suggested as (Vm-EK) becomes positive. Increasing the extracellular K+ concentration from zero and up to approximately 3 mM resulted in an increase in gK(Ca) from approximately 50 to approximately 70 microS/cm2. Since the driving force (Vm-EK) was larger than 20 mV within this range of [K+]ex this was probably a specific K+ activation of gK(Ca).

IN CONCLUSION

The Ca2+-activated K+ channel of the human red cell membrane is an inward rectifier showing the characteristic voltage dependence of this type of channel.

Delayed activation of calcium pump during transient increases in cellular Ca2+ concentration and K+ conductance in hyperpolarizing human red cells

The net Ca2+ influx was increased in human red cells in suspension by adding moderate concentrations of the Ca2+ ionophore A23187, and due to the increased cellular Ca2+ concentration [( Ca]i) the K+ channels opened (the 'Gardos effect'). At low K+ concentration and with the protonophore CCCP in the buffer-free medium the cells hyperpolarized and the extracellular pH (pH0) increased, enhancing the A23187-mediated net Ca2+ influx. This elicited a prolonged response, viz. a primary transient increase of pH0 and [Ca]i followed by one or more spontaneous pH0 and [Ca]i transients. We explored the pump-mediated Ca2+ efflux by blocking the A23187-mediated Ca2+ flux with CoCl2 at appropriate times during the prolonged response. The Ca2+ pumping was higher during the descendent than during the ascendent phase of the primary transient at equal values of [Ca]i. The data were analyzed using a mathematical model that accounts for the prolonged oscillatory response, including pH0 and [Ca]i. In conclusion, the activation of the Ca2+ pump is delayed due to slow binding of cellular calmodulin, which is a hysteretic response to a rapid increase of the cellular Ca2+ concentration. This mechanism may be important for generation and execution of transient signals in other types of cell.

Single-file diffusion through K+ channels in frog skin epithelium

The ratio between the unidirectional fluxes of K+ across the frog skin with K-permeable outer membranes was determined in the absence of Na+ in the apical solutions. The experiments were performed under presteady-state conditions to be able to separate the flux ratio for K+ through the cells from contributions to the fluxes through extracellular leaks. The cellular flux ratio deviated strongly from the value calculated from the flux ratio for electrodiffusion. The experiments can be explained if the passive K transport through the epithelial cells proceeds through specific channels by single-file diffusion with a flux ratio exponent of about 2.5.

Separate, Ca2+-activated K+ and Cl- transport pathways in Ehrlich ascites tumor cells

The net loss of KCl observed in Ehrlich ascites cells during regulatory volume decrease (RVD) following hypotonic exposure involves activation of separate conductive K+ and Cl- transport pathways. RVD is accelerated when a parallel K+ transport pathway is provided by addition of gramicidin, indicating that the K+ conductance is rate limiting. Addition of ionophore A23187 plus Ca2+ also activates separate K+ and Cl- transport pathways, resulting in a hyperpolarization of the cell membrane. A calculation shows that the K+ and Cl- conductance is increased 14- and 10-fold, respectively. Gramicidin fails to accelerate the A23187-induced cell shrinkage, indicating that the Cl- conductance is rate limiting. An A23187-induced activation of 42K and 36Cl tracer fluxes is directly demonstrated. RVD and the A23187-induced cell shrinkage both are: inhibited by quinine which blocks the Ca2+-activated K+ channel, unaffected by substitution of NO-3 or SCN- for Cl-, and inhibited by the anti-calmodulin drug pimozide. When the K+ channel is blocked by quinine but bypassed by addition of gramicidin, the rate of cell shrinkage can be used to monitor the Cl- conductance. The Cl- conductance is increased about 60-fold during RVD. The volume-induced activation of the Cl- transport pathway is transient, with inactivation within about 10 min. The activation induced by ionophore A23187 in Ca2+-free media (probably by release of Ca2+ from internal stores) is also transient, whereas the activation is persistent in Ca2+-containing media. In the latter case, addition of excess EGTA is followed by inactivation of the Cl- transport pathway. These findings suggest that a transient increase in free cytosolic Ca2+ may account for the transient activation of the Cl- transport pathway. The activated anion transport pathway is unselective, carrying both Cl-, Br-, NO-3, and SCN-. The anti-calmodulin drug pimozide blocks the volume- or A23187-induced Cl- transport pathway and also blocks the activation of the K+ transport pathway. This is demonstrated directly by 42K flux experiments and indirectly in media where the dominating anion (SCN-) has a high ground permeability. A comparison of the A23187-induced K+ conductance estimated from 42K flux measurements at high external K+, and from net K+ flux measurements suggests single-file behavior of the Ca2+-activated K+ channel. The number of Ca2+-activated K+ channels is estimated at about 100 per cell.

Analysis of the all or nothing behaviour of Ca-dependent K channels in one-step inside-out vesicles from human red cell membranes

The all or nothing behaviour of Ca2+-dependent K+ channels has been analyzed in one-step inside-out vesicles. There is a threshold for Ca2+ below which the K+ channels remain silent, and which ranges between the 10(-6) and 10(-8) M for different vesicles under the experimental conditions tested, in the absence of Mg2+. The increase of Ca2+ concentration within this range recruits a larger fraction of the vesicles to the active (permeable to 86Rb+) state. The apparent rate of 86Rb+ transport through each individual channel was found to increase, however, with Ca2+ concentration. This finding is not an artefact due to size heterogeneity of the vesicle population, and it is consistent with the variations of the mean open time of the channels with Ca2+ concentration reported previously in patch-clamp experiments. The electron donor system ascorbate + phenazine-methosulphate increases the rate of 86Rb+ transport through the channels whereas oxidized cytochrome c has the opposite effect.