The poplar K+ channel KPT1 is associated with K+ uptake during stomatal opening and bud development

To gain insights into the performance of poplar guard cells, we have measured stomatal conductance and aperture, guard cell K+ content and K+-channel activity of the guard cell plasma membrane in intact poplar leaves. In contrast to Arabidopsis, broad bean and tobacco grown under same conditions, poplar stomata operated just in the dynamic range - any change in conductance altered the rate of photosynthesis. In response to light, CO2 and abscisic acid (ABA), the stomatal opening velocity was two to five times faster than that measured for Arabidopsis thaliana, Nicotiana tabacum and Vicia faba. When stomata opened, the K+ content of guard cells increased almost twofold, indicating that the very fast stomatal opening in this species is mediated via potassium uptake. Following impalement of single guard cells embedded in their natural environment of intact leaves with triple-barrelled microelectrodes, time-dependent inward and outward-rectifying K+-channel-mediated currents of large amplitude were recorded. To analyse the molecular nature of genes encoding guard cell K+-uptake channels, we cloned K+-transporter Populustremula (KPT)1 and functionally expressed this potassium channel in a K+-uptake-deficient Escherichia coli mutant. In addition to guard cells, this K+-transporter gene was expressed in buds, where the KPT1 gene activity strongly correlated with bud break. Thus, KPT1 represents one of only few poplar genes associated with bud flush.     

Voltage-Dependent K-Channel in Protoplasmic Droplets of Chara corallina: A Single Channel Patch Clamp Study

Passive transport of potassium through the plasma membrane of a protoplasmic droplet isolated from large internodal cells of Chara corallina Klein ex Willd., em, R.D.W. has been investigated using the patchclamp technique. When the membrane is hyperpolarized the conductance of a single K⁺-channel is of the order of magnitude of 100 picoSiemens and is reduced by tetraethylammonium chloride. Its open time is voltage dependent. This voltage-dependent K⁺-channel displays rectifying properties. The channel density is about 0.1 channel per square micrometer of membrane. When the membrane is depolarized the conductance of a single channel is of the order of magnitude of 30 picoSiemens and is insensitive to tetraethylammonium chloride. These results suggest that K⁺-channels are incorporated in the plasma membrane during membranogenesis of a protoplasmic droplet. They constitute further evidence for the existence of voltage-dependent K⁺-channels in plant cells.     

Inward-rectifier potassium channels in basolateral membranes of frog skin epithelium

Inward-rectifier K channel: using macroscopic voltage clamp and single- channel patch clamp techniques we have identified the K+ channel responsible for potassium recycling across basolateral membranes (BLM) of principal cells in intact epithelia isolated from frog skin. The spontaneously active K+ channel is an inward rectifier (Kir) and is the major component of macroscopic conductance of intact cells. The current- voltage relationship of BLM in intact cells of isolated epithelia, mounted in miniature Ussing chambers (bathed on apical and basolateral sides in normal amphibian Ringer solution), showed pronounced inward rectification which was K(+)-dependent and inhibited by Ba2+, H+, and quinidine. A 15-pS Kir channel was the only type of K(+)-selective channel found in BLM in cell-attached membrane patches bathed in physiological solutions. Although the channel behaves as an inward rectifier, it conducts outward current (K+ exit from the cell) with a very high open probability (Po = 0.74-1.0) at membrane potentials less negative than the Nernst potential for K+. The Kir channel was transformed to a pure inward rectifier (no outward current) in cell- attached membranes when the patch pipette contained 120 mM KCl Ringer solution (normal NaCl Ringer in bath). Inward rectification is caused by Mg2+ block of outward current and the single-channel current-voltage relation was linear when Mg2+ was removed from the cytosolic side. Whole-cell current-voltage relations of isolated principal cells were also inwardly rectified. Power density spectra of ensemble current noise could be fit by a single Lorentzian function, which displayed a K dependence indicative of spontaneously fluctuating Kir channels. Conclusions: under physiological ionic gradients, a 15-pS inward- rectifier K+ channel generates the resting BLM conductance in principal cells and recycles potassium in parallel with the Na+/K+ ATPase pump.     

Changes in autorhythmic heart frequency elicited by redox agents

In isolated frog heart it was established that methylene-blue (MB, an oxidizing agent) decreased, while ascorbate (ASC, a reducing agent) increased the frequency of autorhythmic heart contractions. After MB treatment, in parallel with this phenomenon, the extracellular K+ concentration [K+]o showed a slow increase, but following ASC application a slow decrease occurred. Since these correlations are in good accordance with the idea that the pacemaking ability of heart, among other properties, depends on the voltage and time-dependent decrease in potassium conductance following the spike, changes in [K+]o might be one mechanism by which oxidizing and reducing agents modulate heart frequencies. On the basis of the effect of insulin (INS) and K-strophantoside (STR) on these modulatory influences, it is presumed that the changes in slow delta [K+]o transients might result, at least partly, from the effect of redox agents on the active transport system. In light of the increase in passive K+ fluxes after oxidant treatment and the decrease in this parameter following reductant treatment an effect of redox agents on the characteristics of the K+-channel is also postulated.     

Reconstitution of highly purified saxitoxin-sensitive Na+-channels into planar lipid bilayers.

Highly purified Na+-channels isolated from rat brain have been reconstituted into virtually solvent-free planar lipid bilayer membranes. Two different types of electrically excitable channels were detected in the absence of any neurotoxins. The activity of both channels was blocked by saxitoxin. The first channel type is highly selective for Na+ over K+ (approximately 10:1), it shows a bursting behavior, a conductance of 25 pS in Na+-Ringer and undergoes continuous opening and closing events for periods of minutes within a defined range of negative membranes voltages. The second channel type has a conductance of 150 pS and a lower selectivity for Na+ and K+ (2.2:1); only a few opening and closing events are observed with this channel after one voltage jump. The latter type of channel is also found with highly purified Na+-channel from Electrophorus electricus electroplax. A qualitative analysis of the physicochemical and pharmacological properties of the high conductance channel has been carried out. Channel properties are affected not only by saxitoxin but also by a scorpion (Centruroides suffusus suffusus) toxin and a sea anemone (Anemonia sulcata) toxin both known to be selective for the Na+-channel. The spontaneous transformation of the large conductance channel type into the small one has been considered; the two channel types may represent the expression of activity of different conformational states of the same protein.     

Voltage and temperature dependence of single K+ channels isolated from canine cardiac sarcoplasmic reticulum.

The temperature and voltage dependence of gating and conductance of sarcoplasmic reticulum K+ channels (S-R K+) isolated from adult canine hearts were studied using the reconstituted bilayer technique. Fusion of vesicles from this preparation frequently resulted in the incorporation of a single channel. Only bilayers into which a single S-R K+ channel had fused were studied. The three conductance states of the channel, fully open (O2), substate conductance (O1), and closed (C) were studied as a function of voltage (-50 to +50 mV) and temperature (16 to 37 degrees C). Permeation through the O1 state showed the same temperature dependence as the O2 state corresponding to an enthalpy of permeation of 4.1-4.2 kcal/mol, which is similar to that for K+ diffusion through water. As expected, increased temperature increased the frequency of gating transitions and shortened the average dwell time spent in any conductance state. Over the range of 25 to 37 degrees C, the average dwell time spent in the O1, O2, and C states decreased by 44 +/- 11, 36 +/- 13, and 78 +/- 7% (n = 3 to 4 channels), respectively. The ratio of probabilities between the various conductance states was not strongly temperature sensitive. Analysis of the voltage dependence of this channel was carried out at 37 degrees C and revealed that the dwell times of the O1 and O2 states were voltage insensitive and the probability ratio (PO2:PO1) was approximately 7 and was voltage insensitive.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fused cells of frog proximal tubule: I. Basic membrane properties

Summary Patch-clamp techniques were used to study a K channel in the cell membrane of MDCK cells. This cell line derives from the kidney of a normal dog, presumably from the distal nephron, a region involved in potassium secretion. The cells were cultured in confluent monolayers and approached from the apical side. The K channel we describe is Ca²⁺ and voltage activated, has a conductance of 221±7 pS, and can be inhibited by 10mm tetraethylammonium and by 1mm quinidine, but not by 4-aminopyridine, nor by 1mm Ba²⁺ added to the outer side. Using the whole-cell configuration, we find that most of the cationic conductance of the membrane is constituted by a K-specific one (maximum K conductance 32.1±3.9 nSvs. a leak conductance of 1.01±0.17 nS). Comparisons of the maximum K conductance with that of a single K channel indicates that an MDCK cell has an average of 145 such channels. The membrane capacity is 24.5±1.4 pF.     

Computer simulation for studying calcium dependent abnormalities in firing mechanism of molluscan neurones

Computer modelling technique is proposed to assist in physiological research on invertebrate neuronal membranes. The firing mechanism of a single patch of invertebrate neuronal membrane has been studied in dependence on maximum Ca++ conductance. The calculations are based on modification of Hodgkin-Huxley's data completed by a straight line approximation between experimental points of the kinetic parameters of Ca++ current and early transient potassium current. The time course of conductance changes is assumed to be proportional to m2h for Ca++ current. Three distinct potassium currents are involved into the model, viz. transient potassium current, delayed potassium current and Ca++-dependent potassium current. The modified Euler method run on a digital computer has been used for numerical integration of kinetic equations. Significant effects of Ca++ conductance on spike broadening, plateau development and spike afterhyperpolarization are represented. In the range of small Ca++ conductance an infinite spontaneous activity can be triggered by a short (suprathreshold) current pulse which may be considered a model of pacemaker activity. Plateau development resulting from potassium blocking or decreasing potassium equilibrium is facilitated by Ca++ conductance in the range of greater Ca++ conductance. The effects of voltage sensitivity of the coupling coefficient describing the current of Ca++-dependent K+ channels were studied and compared to the voltage independent case. The coupling coefficient seems to be a crucial factor in broadening the range of Ca++ conductance responsible for pacemaker activity. For greater values of Ca++ conductance, a decrease of the coupling coefficient leads to a transition from prolonged bursting to interruption of burst activity by burst-afterhyperpolarization. The blocking effect of 4-aminopyridine on fast outward current has been studied by the model which has a practical significance considering that aminopyridine is known as a convulsive agent. We suppose that it is reasonable to study the convulsive effects of aminopyridine by the model based on the kinetics of the isolated neuronal membrane. The model may help in understanding the ionic background underlying abnormal network activity during epileptic discharges of mammalian neurones.     

Whole-cell currents in single and confluent M-1 mouse cortical collecting duct cells

M-1 cells, derived from a microdissected cortical collecting duct of a transgenic mouse, grown to confluence on a permeable support, develop a lumen-negative amiloride-sensitive transepithelial potential, reabsorb sodium, and secrete potassium. Electron micrographs show morphological features typical of principal cells in vivo. Using the patch clamp technique distinct differences are detected in whole-cell membrane current and voltage (Vm) between single M-1 cells 24 h after seeding vs cells grown to confluence. (a) Under control conditions (pipette: KCl- Ringer; bath: NaCl-Ringer) Vm averages -42.7 +/- 3.4 mV in single cells vs -16.8 +/- 4.1 mV in confluent cells. Whole-cell conductance (Gcell) in confluent cells is 2.6 times higher than in single cells. Cell capacitance values are not significantly different in single vs confluent M-1 cells, arguing against electrical coupling of confluent M- 1 cells. (b) In confluent cells, 10(-4)-10(-5) M amiloride hyperpolarizes Vm to -39.7 +/- 3.0 mV and the amiloride-sensitive fractional conductance of 0.31 shows a sodium to potassium selectivity ratio of approximately 15. In contrast, single cells express no significant amiloride-sensitive conductance. (c) In single M-1 cells, Gcell is dominated by an inwardly rectifying K-conductance, as exposure to high bath K causes a large depolarization and doubling of Gcell. The barium-sensitive fraction of Gcell in symmetrical KCl-Ringer is 0.49 and voltage dependent. (d) In contrast, neither high K nor barium in the apical bath affect confluent M-1 cells, showing that confluent cells lack a significant apical K conductance. (e) Application of 500 microM glibenclamide reduces whole-cell currents in both single and confluent M-1 cells with a glibenclamide-sensitive fractional conductance of 0.71 and 0.83 in single and confluent cells, respectively. Glibenclamide inhibition occurs slower in confluent M-1 cells than in single cells, suggesting a basolateral action of this lipophilic drug on ATP-sensitive basolateral K channels in M-1 cells. (f) A component of the whole-cell conductance in M-1 cells appears as a deactivating outward current during large depolarizing voltage pulses and is abolished by extracellular chloride removal. The deactivating chloride current averages 103.6 +/- 16.1 pA/cell, comprises 24% of the outward current, and decays with a time constant of 179 +/- 13 ms. The outward to inward conductance ratio obtained from deactivating currents and tail currents is 2.4, indicating an outwardly rectifying chloride conductance.     

Maxi K+ channels and their relationship to the apical membrane conductance in Necturus gallbladder epithelium

Using the patch-clamp technique, we have identified large-conductance (maxi) K+ channels in the apical membrane of Necturus gallbladder epithelium, and in dissociated gallbladder epithelial cells. These channels are more than tenfold selective for K+ over Na+, and exhibit unitary conductance of approximately 200 pS in symmetric 100 mM KCl. They are activated by elevation of internal Ca2+ levels and membrane depolarization. The properties of these channels could account for the previously observed voltage and Ca2+ sensitivities of the macroscopic apical membrane conductance (Ga). Ga was determined as a function of apical membrane voltage, using intracellular microelectrode techniques. Its value was 180 microS/cm2 at the control membrane voltage of -68 mV, and increased steeply with membrane depolarization, reaching 650 microS/cm2 at -25 mV. We have related maxi K+ channel properties and Ga quantitatively, relying on the premise that at any apical membrane voltage Ga comprises a leakage conductance and a conductance due to maxi K+ channels. Comparison between Ga and maxi K+ channels reveals that the latter are present at a surface density of 0.09/microns 2, are open approximately 15% of the time under control conditions, and account for 17% of control Ga. Depolarizing the apical membrane voltage leads to a steep increase in channel steady-state open probability. When correlated with patch-clamp studies examining the Ca2+ and voltage dependencies of single maxi K+ channels, results from intracellular microelectrode experiments indicate that maxi K+ channel activity in situ is higher than predicted from the measured apical membrane voltage and estimated bulk cytosolic Ca2+ activity. Mechanisms that could account for this finding are proposed.     

Mutations in the pore regions of the yeast K+ channel YKC1 affect gating by extracellular K+.

The product of the Saccharomyces cerevisiae K+-channel gene YKC1 includes two pore-loop sequences that are thought to form the hydrophilic lining of the pore. Gating of the channel is promoted by membrane depolarization and is regulated by extracellular K+ concentration ([K+]o) both in the yeast and when expressed in Xenopus oocytes. Analysis of the wild-type current now shows that: (i) [K+]o suppresses a very slowly relaxing component, accelerating activation; (ii) [K+]o slows deactivation in a dose-dependent fashion; and (iii) Rb+, Cs+ and, to a lesser extent, Na+ substitute for K+ in its action on gating. We have identified single residues, L293 and A428, at equivalent positions within the two pore loops that affect the [K+]o sensitivity. Substitution of these residues gave channels with reduced sensitivity to [K+]o in macroscopic current kinetics and voltage dependence, but had only minor effects on selectivity among alkali cations in gating and on single-channel conductance. In some mutants, activation was slowed sufficiently to confer a sigmoidicity to current rise at low [K+]o. The results indicate that these residues are involved in [K+]o sensing. Their situation close to the permeation pathway points to an interaction between gating and permeation.     

Calcium-sensitive potassium channel inhibitors antagonize genistein- and daidzein-induced arterial relaxation in vitro

Estradiol-17beta relaxes rabbit coronary artery rings via large conductance Ca2+-activated K+-channels (K(Ca)). Genistein and daidzein are plant-derived estrogen-like compounds. The aim of the present study was to investigate whether potassium channels participate in the genistein- and daidzein-induced arterial relaxation like they do in the case of estradiol-17beta. Endothelium-denuded superior mesenteric arterial rings from non-pregnant Wistar female rats were used. At a concentration of 10 microM, estradiol-17beta, genistein and daidzein relaxed noradrenaline precontracted arterial rings, (58 +/- 4%, 45 +/- 5% and 31 +/- 3%, respectively; (n=6-8)). Genistein- and daidzein-induced relaxations were inhibited both by iberiotoxin (1-10 nM) and charybdotoxin (30 nM), the antagonists of large conductance Ca2+-activated K+-channels (K(Ca)). Estradiol-17beta-induced relaxation was reduced by iberiotoxin (30 nM). Estradiol-17beta- and daidzein-induced relaxations were also decreased by apamin (0.1-0.3 microM), an antagonist of small conductance Ca2+-activated K+-channels. The antagonists of voltage-dependent K+-channels (K(V)) (4-aminopyridine), ATP-sensitive K+-channels (K(ATP)) (glibenclamide), or inward rectifier K+-channels (KIR) (barium) had no effect on the relaxation responses of any of the compounds studied. Estrogen receptor antagonist tamoxifen did not inhibit the relaxations. In conclusion, in the noradrenaline precontracted rat mesenteric arteries, the relaxations caused by estradiol-17beta, genistein and daidzein were antagonized by large and small conductance K(Ca)-channel inhibitors, suggesting the role of these channels as one of the relaxation mechanisms.     

Blue light activates potassium-efflux channels in flexor cells from Samanea saman motor organs via two mechanisms

Light-induced leaflet movement of Samanea saman depends on the regulation of membrane transporters in motor cells. Blue light (BL) stimulates leaflet opening by inducing K(+) release from the flexor motor cells. To elucidate the mechanism of K(+)-efflux (K(D))-channel regulation by light, flexor motor cell protoplasts were patch-clamped in a cell-attached configuration during varying illumination. Depolarization elicited outward currents through single open K(D) channels. Changes in cell membrane potential (E(M)) were estimated by applying voltage ramps and tracking the change of the apparent reversal potential of K(D)-channel current. BL shifted E(M) in a positive direction (i.e. depolarized the cell) by about 10 mV. Subsequent red light pulse followed by darkness shifted E(M) oppositely (i.e. hyperpolarized the cell). The BL-induced shifts of E(M) were not observed in cells pretreated with a hydrogen-pump inhibitor, suggesting a contribution by hydrogen-pump to the shift. BL also increased K(D)-channel activity in a voltage-independent manner as reflected in the increase of the mean net steady-state patch conductance at a depolarization of 40 mV relative to the apparent reversal potential (G(@40)). G(@40) increased by approximately 12 pS without a change of the single-channel conductance, possibly by increasing the probability of channel opening. Subsequent red-light and darkness reversed the change in G(@40). Thus, K(+) efflux, a determining factor for the cell-volume decrease of flexor cells, is regulated by BL in a dual manner via membrane potential and by an independent signaling pathway.     

Properties of membrane ion conductances evoked by hormonal stimulation of guinea-pig and rabbit isolated hepatocytes

Membrane conductance changes evoked in isolated guinea-pig or rabbit hepatocytes by hormonal stimulation were studied with the whole-cell patch clamp technique. In Cl-containing solutions, noradrenaline (NA), ATP or angiotensin II (AII) evoked an increase of conductance to both K (GK) and Cl (GCl) ions. Activation of GK occurred after a delay of several seconds and was sustained in the presence of hormone. Activation of GCl was transient, lasting several seconds, and arose either at the same time or shortly after the increase in GK. Conductances showed an initial rapid rise and slow oscillatory changes during maintained hormone application. The NA-induced current reversed at -19 mV in Cl solutions, between the equilibrium potentials for chloride (ECl = 0 mV) and potassium ions (EK = -85 mV), and at -75 mV, near EK, in Cl-free solution. In both conditions whole-cell current-voltage curves were linear in the range -100 mV to +40 mV. The conductance increase produced by NA to Cl- ions was about 50 nS, that to K+ ions was 6 nS. The potassium conductance increase was abolished by the polypeptide toxin apamin (50 nM). An increase in membrane current noise was associated with NA-evoked outward K+ current and blocked by apamin. Spectral analysis gave estimates of the elementary K channel conductance of 1.7 pS. Power spectra were fitted by two Lorentzian components, with average half-power frequencies of 2 and 190 Hz. These results are discussed in relation to the single-channel properties and indicate that the open probability of K channels during the NA response is high. In Cl solutions, with apamin to block the K conductance, no increase in current noise was detected during the large Cl conductance evoked by NA. This suggests either that Cl channels are of very low unitary conductance (less than 1 pS) or that Cl transport is due to a membrane carrier. The complex time-course of hormonally evoked conductances is not due to the properties of ion conductances per se but probably to underlying changes of intracellular second-messenger concentration.     

Calcium uptake in rat liver mitochondria accompanied by activation of ATP-dependent potassium channel

The influence of potassium ions on calcium uptake in rat liver mitochondria is studied. It is shown that an increase in K+ and Ca2+ concentrations in the incubation medium leads to a decrease in calcium uptake in mitochondria together with a simultaneous increase in potassium uptake due to the potential-dependent transport of K+ in the mitochondrial matrix. Both effects are more pronounced in the presence of an ATP-dependent K+-channel (K+(ATP)-channel) opener, diazoxide (Dz). Activation of the K+(ATP)-channel by Dz alters the functional state of mitochondria and leads to an increase in the respiration rate in state 2 and a decrease in the oxygen uptake and the rate of ATP synthesis in state 3. The effect of Dz on oxygen consumption in state 3 is mimicked by valinomycin, but it is opposite to that of the classical protonophore uncoupler CCCP. It is concluded that the potential-dependent uptake of potassium is closely coupled to calcium transport and is an important parameter of energy coupling responsible for complex changes in oxygen consumption and Ca2+-transport properties of mitochondria.     

A selective filter model of the potassium channel

A model based on analogy between filter and crown-ether molecule in a two-phase system organic solvent-water is proposed for potassium channel selective filter. The selectivity of K+-channel can be quantitatively described by this model.     

Effect of conditioning potential on potassium current kinetics in the frog node.

The kinetics of potassium conductance changes were determined in the voltage clamped frog node (Rana esculenta), as a function of conditioning prepotential. The conditioning potential duration varied from 1 to 50 ms and the amplitude between -60 and +130 mV (relative to rest). The conductance kinetics were determined at a single test potential of +20 mV (depolarization) by means of the slope of log [ninfinity - nt] vs. time relationship which defines the time constant of the process (tau). The values of tau, after conditioning hyperpolarizations, were around 5 ms, up to 10 times greater than values obtained following a strong depolarization. The tau vs. pre-potential curve was sigmoid in shape. These differences were only slightly dependent on [K+]0 or conditioning pulse duration. The steady-state current values were also found to be a function of conditioning potential. After conditioning hyperpolarizations, the log [ninfinity - nt] vs. time curve could not be fitted by a single exponent regardless of the power of n chosen. The prepotential dependency of potassium current kinetics is inconsistent with the Hodgkin-Huxley axon model where the conductance parameters are assumed to be in either one of two possible states, and where the rate of transfer from one state to the other follows first order kinetics. In contrast the described kinetics may be consistent with complex multistate potassium "channel" models or membranes consisting of a number of types of channels.     

Purification of a K(+)-channel protein of sarcoplasmic reticulum by assaying the channel activity in the planar lipid bilayer system

A K(+)-channel protein of the sarcoplasmic reticulum (SR) was purified by assaying the channel activity in a planar lipid bilayer system. The light fraction of SR vesicles was solubilized in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and fractionated by an anion-exchange chromatography and followed by gel filtration chromatography and affinity chromatography with concanavalin A. All fractions in each steps were mixed with asolectin solubilized in CHAPS and reconstituted into vesicles by dialysis. The channel activity of each fraction was assayed after the reconstituted vesicles had been fused into a planar lipid bilayer. The final fraction which showed the K(+)-channel activity contained only 100 kDa protein in a silver-stained gel after SDS-PAGE and an anti-Ca(2+)-ATPase antibody did not recognize the protein. The characteristics of the K(+)-channel were identical to those observed in native SR vesicles when using the same method. The channel showed a single-channel conductance of 120 pS in 0.1 M KCl and marked voltage dependence. The channel did not permeate Ca2+ and Cl- and was blocked by neomycin B.     

Conditioning prepulses and kinetics of potassium conductance in the frog node

Summary The kinetics of potassium conductance were analyzed in response to voltage-clamp steps with holding potential (–75 mV) as initial condition and after a positive prepulse to-wards +45 mV of 10-msec duration. As the potassium reversal potentialE _K altered during potassium current flow, a method to obtain the conductance independent ofE _K was used. Conductance kinetics at 15°C were analyzed according to the Hodgkin-Huxley (HH) model. The time constant of potassium activation, with holding potential as initial condition, is a monotonous decreasing function of membrane potential. Its value ofca. 9 msec at –50 mV decreases to 1 msec at +30 mV. Changes inE _K did not affect the voltage dependency of this time constant. The time constant of potassium deactivation, i.e. the off-response following a 10-msec prepulse towards +45 mV, shows a completely different voltage dependency. At a membrane potential of –90 mV it is approximately 2 msec and gradually increases for more positive voltages towards a maximum value of about 6 msec, that is reached between –5 and 0 mV. At still larger values of membrane voltage this time constant starts to fall again. It is concluded that a HH-model, as applied for a single population of potassium channels, has to be rejected. Computer simulations indicate that an extension to two populations of independent potassium channels, each with HH-kinetics, is also inconsistent with the observed results.     

Two novel cardiac atrial K+ channels, IK.AA and IK.PC

Two K(+)-selective channels in neonatal rat atrial cells activated by lipophilic compounds have been characterized in detail. The arachidonic acid-stimulated channel (IK.AA) had a slope conductance of 124 +/- 17 pS at +30 mV in symmetrical 140 mM potassium and a mean open time of approximately 1 ms, and was relatively voltage independent. IK.AA activity was reversibly increased by lowering pH to 6.0. Arachidonic acid was most effective in activating this channel, although a number of lipophilic compounds resulted in activation. Surprisingly, choline, a polar molecule, also activated the channel. A second K+ channel was activated by 10 microM phosphatidylcholine applied to the intracellular surface of inside-out atrial patches. This channel (IK.PC) had a slope conductance of 60 +/- 6 pS at +40 mV and a mean open time of approximately 0.6 ms, and was also relatively voltage independent. Fatty acids are probably monomeric in the membrane under the conditions of our recording; thus detergent effects are unlikely. Since a number of compounds including fatty acids and prostaglandins activated these two channels, an indirect, channel-specific mechanism may account for activation of these two cardiac K+ channels.