The antipsychotic drugs sertindole and pimozide block erg3, a human brain K(+) channel

The antipsychotic drugs sertindole and pimozide are known to prolong the QT interval on the electrocardiogram via a high affinity block of the cardiac K(+) channel known as HERG (human ether-a-go-go-related gene; erg1). We wished to test whether these drugs also displayed high affinity for the related neuronal K(+) channel erg3. The cDNA encoding erg3 channel was cloned from a human brain library. Northern analysis confirmed that the channel was localized to brain relative to other tissues including heart, liver and lung. Within the brain, erg3 was expressed in higher amounts in the frontal lobe and cerebellum relative to the temporal, parietal and occipital lobes. Transient expression of erg3 in Chinese hamster ovary cells produced outwardly directed K(+) currents that activated at approximately -50 mV and produced a large transient component at positive membrane potentials. Inward tail currents measured at -100 mV were blocked in a dose-dependent fashion by sertindole resulting in an IC(50) value of 43 nM. Significant inhibition was observed at concentrations as low as 3 nM. Block of erg3 by sertindole also displayed a positive voltage-dependence. Pimozide blocked erg3 channel currents with an IC(50) of 103 nM and significant inhibition was noted at concentrations of 10 nM and higher. We conclude that erg3 can be blocked by certain antipsychotic drugs like sertindole and pimozide. Inhibition of erg3 or related K(+) channels in the brain may contribute to the efficacy/side effect profiles of some antipsychotic drugs.     

Time course of TEA(+)-induced anomalous rectification in squid giant axons

Changes in the voltage clamp currents of squid giant axons wrought by low axoplasmic TEA+ (tetraethylammonium chloride) concentrations (0.3 mM and above) are described. They are: (a) For positive steps from the resting potential in sea water, the K+ current increases, decreases, then increases, instead of increasing monotonically. (b) For positive steps from the resting potential in 440 mM external K+, the current has an exponentially decaying component, whose decay rate increases with axoplasmic [TEA+]. The control currents increase monotonically. (c) For negative steps from the resting potential in 440 mM external K+, the current record has a peak followed by a decay that is slow relative to the control. The control record decreases monotonically. Qualitatively these findings can be described by a simple kinetic model, from which, with one assumption, it is possible to calculate the rate at which K+ ions move through the K+ channels. An interesting conclusion from (c) is that the channels cannot be closed by the normal voltage-sensitive mechanism (described by Hodgkin and Huxley) until they are free of TEA+.     

Cloning and functional characterization of the smooth muscle ether-a-go-go-related gene K+ channel. Potential role of a conserved amino acid substitution in the S4 region

The human ether-a-go-go-related gene (HERG) product forms the pore-forming subunit of the delayed rectifier K(+) channel in the heart. Unlike the cardiac isoform, the erg K(+) channels in native smooth muscle demonstrate gating properties consistent with a role in maintaining resting potential. We have cloned the smooth muscle isoform of HERG, denoted as erg1-sm, from human and rabbit colon. erg1-sm is truncated by 101 amino acids in the C terminus due to a single nucleotide deletion in the 14th exon. Sequence alignment against HERG showed a substitution of alanine for valine in the S4 domain. When expressed in Xenopus oocytes, erg1-sm currents had much faster activation and deactivation kinetics compared with HERG. Step depolarization positive to -20 mV consistently produced a transient outward component. The threshold for activation of erg1-sm was -60 mV and steady-state conductance was approximately 10-fold greater than HERG near the resting potential of smooth muscle. Site-directed mutagenesis of alanine to valine in the S4 region of erg1-sm converted many of the properties to that of the cardiac HERG, including shifts in the voltage dependence of activation and slowing of deactivation. These studies define the functional role of a novel isoform of the ether-a-go-go-related gene K(+) channel in smooth muscle.     

Voltage-gated potassium channels in brown fat cells

We studied the membrane currents of isolated cultured brown fat cells from neonatal rats using whole-cell and single-channel voltage-clamp recording. All brown fat cells that were recorded from had voltage-gated K currents as their predominant membrane current. No inward currents were seen in these experiments. The K currents of brown fat cells resemble the delayed rectifier currents of nerve and muscle cells. The channels were highly selective for K+, showing a 58-mV change in reversal potential for a 10-fold change in the external [K+]. Their selectivity was typical for K channels, with relative permeabilities of K+ greater than Rb+ greater than NH+4 much greater than Cs+, Na+. The K currents in brown adipocytes activated with a sigmoidal delay after depolarizations to membrane potentials positive to -50 mV. Activation was half maximal at a potential of -28 mV and did not require the presence of significant concentrations of internal calcium. Maximal voltage-activated K conductance averaged 20 nS in high external K+ solutions. The K currents inactivated slowly with sustained depolarization with time constants for the inactivation process on the order of hundreds of milliseconds to tens of seconds. The K channels had an average single-channel conductance of 9 pS and a channel density of approximately 1,000 channels/cell. The K current was blocked by tetraethylammonium or 4-aminopyridine with half maximal block occurring at concentrations of 1-2 mM for either blocker. K currents were unaffected by two blockers of Ca2+-activated K channels, charybdotoxin and apamin. Bath-applied norepinephrine did not affect the K currents or other membrane currents under our experimental conditions. These properties of the K channels indicate that they could produce an increase in the K+ permeability of the brown fat cell membrane during the depolarization that accompanies norepinephrine-stimulated thermogenesis, but that they do not contribute directly to the norepinephrine-induced depolarization.     

Vasa recta pericytes express a strong inward rectifier K+ conductance

Strong inward rectifier potassium channels are expressed by some vascular smooth muscle cells and facilitate K+-induced hyperpolarization. Using whole cell patch clamp of isolated descending vasa recta (DVR), we tested whether strong inward rectifier K+ currents are present in smooth muscle and pericytes. Increasing extracellular K+ from 5 to 50 and 140 mmol/l induced inward rectifying currents. Those currents were Ba2+ sensitive and reversed at the K+ equilibrium potential imposed by the electrode and extracellular buffers. Ba2+ binding constants in symmetrical K+ varied between 0.24 and 24 micromol/l at -150 and -20 mV, respectively. Ba2+ blockade was time and voltage dependent. Extracellular Cs+ also blocked the inward currents with binding constants between 268 and 4,938 micromol/l at -150 and -50 mV, respectively. Ba2+ (30 micromol/l) and ouabain (1 mmol/l) depolarized pericytes by an average of 11 and 24 mV, respectively. Elevation of extracellular K+ from 5 to 10 mmol/l hyperpolarized pericytes by 6 mV. That hyperpolarization was reversed by Ba2+ (30 micromol/l). We conclude that strong inward rectifier K+ channels and Na+-K+-ATPase contribute to resting potential and that KIR channels can mediate K+-induced hyperpolarization of DVR pericytes.     

Regulation of the conductance and resting potential by extracellular K+ in frog taste receptor cells

The effect of extracellular K+ on membrane currents was investigated by the patch clamp and fast perfusion techniques in frog (Rana temporaria) taste receptor cells (TRCs). When added to the bath, K+ increased the TRC conductance. The integral current and current fluctuations depended on the K+ concentration (2.5-90 mM) in the manner which suggested extracellular K+ to serve as a ligand activating ionic channels (potassium-activated (PA) channels). The influence of different ions on the PA current reversal potential indicated that the responsible channels are mainly permeable to K+ and H+. Relative permeabilities were estimated as P(H):P(K) = 3600:1. With 110 mM KCl in the patch pipette and 110 mM NaCl in the bath, isolated TRCs exhibited the resting potentials from -75 to -65 mV. When raised from 2.5 to 110 mM, extracellular K+ intensively depolarized TRCs. Membrane potential vs. K+ concentration displayed a slope of about 41 mV per logarithmic unit. This indicates that the K+ permeability of the TRC membrane dominates the other in setting the potential. With 10 mM K+ in the bath, the PA channels were the major contributor to setting the TRC resting potential. External K+ markedly increased the sensitivity of isolated TRCs to bath solution pH due to the activation of the PA channels suggesting their role in sour transduction.     

Inwardly rectifying potassium channels (Kir) in central nervous system glia: a special role for Kir4.1 in glial functions

Glia in the central nervous system (CNS) express diverse inward rectifying potassium channels (Kir). The major function of Kir is in establishing the high potassium (K+) selectivity of the glial cell membrane and strongly negative resting membrane potential (RMP), which are characteristic physiological properties of glia. The classical property of Kir is that K+ flows inwards when the RMP is negative to the equilibrium potential for K+ (E(K)), but at more positive potentials outward currents are inhibited. This provides the driving force for glial uptake of K+ released during neuronal activity, by the processes of "K+ spatial buffering" and "K+ siphoning", considered a key function of astrocytes, the main glial cell type in the CNS. Glia express multiple Kir channel subtypes, which are likely to have distinct functional roles related to their differences in conductance, and sensitivity to intracellular and extracellular factors, including pH, ATP, G-proteins, neurotransmitters and hormones. A feature of CNS glia is their specific expression of the Kir4.1 subtype, which is a major K+ conductance in glial cell membranes and has a key role in setting the glial RMP. It is proposed that Kir4.1 have a primary function in K+ regulation, both as homomeric channels and as heteromeric channels by co-assembly with Kir5.1 and probably Kir2.0 subtypes. Significantly, Kir4.1 are also expressed by oligodendrocytes, the myelin-forming cells of the CNS, and the genetic ablation of Kir4.1 results in severe hypomyelination. Hence, Kir, and in particular Kir4.1, are key regulators of glial functions, which in turn determine neuronal excitability and axonal conduction.     

胞外钙对豚鼠耳蜗Deiters细胞钾电流的调制

为观察胞外钙对豚鼠耳蜗单个离体Deiters细胞钾电流的调控作用并探讨其机制,实验记录了Deiters细胞在正常细胞外液和无钙外液中的全细胞钾电流(whole cell K^ currents,IK),并分析了其电生理学特性的改变。结果观察到,Deiters细胞与在正常细胞外液中相比,在祛除细胞外液中的Ca^2 后Ik电流幅值明显增加,弦电导值亦明显增加,但其平衡电位未明显改变。在无钙外液中Ik电流的反转电位向超极化方向明显移位,更接近于按照Ner-nst方程得出的K^ 理论平衡电位;而且其稳态激活曲线亦向超极化方向明显移位,但其激活趋势与正常相比无明显改变。此外,观察了Deiters细胞中钙抑制性钾电流的电流-电压关系和电导-电压关系,发现两者均呈“S”形,提示此钙抑制性钾电流可能存在2种不同的钾电导成分。由此,推测可能有两种机制参与胞外钙对Deiters细胞钾电流的调控：(1)Deiters细胞中的Ik通道可能存在一个Ca^2 敏感结构域,胞外Ca^2 可能通过改变此结构域而对Ik电流产生调制;(2)Deiters细胞中可能存在一种新型的双相门控性钾通道或钾通道耦联型受体或是一种新型的钾通道亚型,祛除胞外Ca^2 可激活此新型钾电导而对L电流产生调制。由此推测,在听觉形成过程中,胞外钙浓度下降可以对Deiters细胞的全细胞钾电流产生调制,从而更有利于Deiters细胞内K^ 外流,进而有效地缓冲外毛细胞周围的K^ 浓度：而且还可以使Deiters细胞产生更快的复极化并有利于维持其静息状态。     

K+ conductance activated during regulatory volume decrease. The channels in Ehrlich cells and their possible molecular counterpart

K+ currents activated by hypotonic cell swelling have been studied in Ehrlich ascites tumour cells by the whole-cell recording mode of the patch-clamp technique. K+ together with Cl- currents developed in the absence of added intracellular Ca2+ and with strong buffering of internal Ca2+ in experiments conducted at 37 degrees C. Manipulation of the extracellular medium with other cations suggests a selectivity sequence of K+ > Rb+ > NH4+ > or = Na+ approximately equals Li+ approximately equals Cs+. The current-voltage relationship of the volume-sensitive K+ current was well fitted with the Goldman-Hodgkin-Katz current equation between -130 and 20 mV at both physiological and high K+ extracellular solutions. The class III antiarrhytmic drug clofilium blocked the volume-sensitive K+ current in a voltage-independent manner. Clofilium was also found to be a strong inhibitor of the regulatory volume decrease (RVD) response of Ehrlich cells. The leukotriene D4 (LTD4) can activate the same current in isotonicity, consistent with a role for this compound in the signalling process of volume regulation. It is suggested that K+ channels activated by cell swelling belong to the so-called background K+ channel group. These are voltage-independent channels which underlie the resting potential of many cells and have recently been identified as belonging to a family of K+ channels with two pore domains in tandem (2P-4TM). Preliminary experiments show the presence of the TASK-2 channel, a member of the 2P-4TM family inhibited by acid extracellular pH, in Ehrlich cells and suggest that it might underlie the swelling-induced K+ current.     

Voltage-sensitive potassium channels in Limulus ventral photoreceptors

The steady-state slope conductance of Limulus ventral photoreceptors increases markedly when the membrane is depolarized from rest. The ionic basis of this rectification has been examined with a voltage-clamp technique. Tail currents that occur when membrane potential is repolarized after having been depolarized have been identified. The tail currents reverse direction at a voltage that becomes more positive when Ko is increased. Rectification is reduced by extracellular 4-aminopyridine and by intracellular injection of tetra-ethyl-ammonium (TEA). These results indicate that the membrane rectification around resting potential is due primarily to voltage-sensitive K+ channels. The increase in gK caused by depolarization is not mediated by a voltage-dependent rise in in Cai++, since intracellular injection of Ca++ causes a decrease rather than an increase in slope conductance. TEA can be used to examine the functional role of the K+ channels because it blocks them without substantially affecting the light-activated Na+ conductance. The effect of TEA on response-intensity curves shows that the K+ channels serve to compress the voltage range of receptor potentials.     

Selectivity and gating of the type L potassium channel in mouse lymphocytes

Type l voltage-gated K+ channels in murine lymphocytes were studied under voltage clamp in cell-attached patches and in the whole-cell configuration. The kinetics of activation of whole-cell currents during depolarizing pulses could be fit by a single exponential after an initial delay. Deactivation upon repolarization of both macroscopic and microscopic currents was mono-exponential, except in Rb-Ringer or Cs-Ringer solution in which tail currents often displayed "hooks," wherein the current first increased or remained constant before decaying. In some cells type l currents were contaminated by a small component due to type n K+ channels, which deactivate approximately 10 times slower than type l channels. Both macroscopic and single channel currents could be dissected either kinetically or pharmacologically into these two K+ channel types. The ionic selectivity and conductance of type l channels were studied by varying the internal and external permeant ion. With 160 mM K+ in the cell, the relative permeability calculated from the reversal potential with the Goldman-Hodgkin-Katz equation was K+ (identical to 1.0) greater than Rb+ (0.76) greater than NH4+ = Cs+ (0.12) much greater than Na+ (less than 0.004). Measured 30 mV negative to the reversal potential, the relative conductance sequence was quite different: NH4+ (1.5) greater than K+ (identical to 1.0) greater than Rb+ (0.5) greater than Cs+ (0.06) much greater than Na+, Li+, TMA+ (unmeasurable). Single channel current rectification resembled that of the whole-cell instantaneous I-V relation. Anomalous mole-fraction dependence of the relative permeability PNH4/PK was observed in NH4(+)-K+ mixtures, indicating that the type l K+ channel is a multi-ion pore. Compared with other K+ channels, lymphocyte type l K+ channels are most similar to "g12" channels in myelinated nerve.     

A whole-cell and single-channel study of the voltage-dependent outward potassium current in avian hepatocytes

Voltage-dependent membrane currents were studied in dissociated hepatocytes from chick, using the patch-clamp technique. All cells had voltage-dependent outward K+ currents; in 10% of the cells, a fast, transient, tetrodotoxin-sensitive Na+ current was identified. None of the cells had voltage-dependent inward Ca2+ currents. The K+ current activated at a membrane potential of about -10 mV, had a sigmoidal time course, and did not inactivate in 500 ms. The maximum outward conductance was 6.6 +/- 2.4 nS in 18 cells. The reversal potential, estimated from tail current measurements, shifted by 50 mV per 10-fold increase in the external K+ concentration. The current traces were fitted by n2 kinetics with voltage-dependent time constants. Omitting Ca2+ from the external bath or buffering the internal Ca2+ with EGTA did not alter the outward current, which shows that Ca2+-activated K+ currents were not present. 1-5 mM 4-aminopyridine, 0.5-2 mM BaCl2, and 0.1-1 mM CdCl2 reversibly inhibited the current. The block caused by Ba was voltage dependent. Single-channel currents were recorded in cell-attached and outside-out patches. The mean unitary conductance was 7 pS, and the channels displayed bursting kinetics. Thus, avian hepatocytes have a single type of K+ channel belonging to the delayed rectifier class of K+ channels.     

Evidence for protein phosphatase 1 and 2A regulation of K+ channels in two types of leaf cells.

Ion channels control ion fluxes across membranes, membrane potential, and signal transduction between and within cells. Protein kinases and phosphatases are important regulators involved in stimulus-response coupling in eukaryotic organisms. We have identified in extracts of Vicia faba leaf cells protein phosphatase activities inhibited by okadaic acid (OA) and calyculin A (CA), two inhibitors of protein phosphatases 1 and 2A. Using whole-cell patch-clamp techniques, we have demonstrated that inward K+ currents in guard cells are inhibited by nanomolar concentrations of OA or CA, whereas outward K+ currents are not affected. However, the same inhibitors enhance the magnitude of outward K+ currents in mesophyll cells. A phosphatase antagonist, adenosine-5'-O-(3-thiotriphosphate), has an effect similar to OA and CA on outward K+ currents in mesophyll cells. Our findings suggest that protein phosphatases 1 and/or 2A play different physiological roles in modulating the activity of K+ channels in mesophyll cells and guard cells.     

Steady-state currents through voltage-dependent, dihydropyridine-sensitive Ca2+ channels in GH3 pituitary cells.

Ca2+ influx via voltage-dependent Ca2+ channels is known to be elicited during action potentials but possibly also occurs at the resting potential. The steady-state current through voltage-dependent Ca2+ channels and its role for the electrical activity was, therefore, investigated in pituitary GH3 cells. Applying the recently developed 'nystatin-modification' of the patch-clamp technique, most GH3 cells (18 out of 23 cells) fired spontaneous action potentials from a baseline membrane potential of 43.7 +/- 4.6 mV (mean +/- s.d., n = 23). The frequency of action potentials was stimulated about twofold by Bay K 8644 (100 nM), a Ca(2+)-channel stimulator, and action potentials were completely suppressed by the Ca(2+)-channel blocker PN 200-110 (100 nM). Voltage clamping GH3 cells at fixed potentials for several minutes and with 1 mM Ba2+ as divalent charge carrier, we observed steady-state Ca(2+)-channel currents that were dihydropyridine-sensitive and displayed a U-shaped current-voltage relation. The results strongly suggest that the observed long lasting, dihydropyridine-sensitive Ca(2+)-channel currents provide a steady-state conductivity for Ca2+ at the resting potential and are essential for the generation of action potentials in GH3 pituitary cells.     

Biophysical Properties of Human Medulloblastoma Cells

Medulloblastoma is a pediatric high-grade cerebellar malignancy derived from neuronal precursors. Although electrophysiologic characteristics of cerebellar granule neurons at all stages of cell development have been well described, such characterization has not been reported for medulloblastoma. In this study we attempt to characterize important electrophysiologic features of medulloblastoma that may distinguish it from the surrounding cerebellum. Using patient-derived cell lines and tumor tissues, we show that medulloblastoma cells have no inward Na+ current or transient K+ current involved in action potential generation and propagation, typically seen in granule neurons. Expression and function of calcium-activated, large-conductance K+ channels are diminished in medulloblastoma, judged by electrophysiology and Western analysis. The resting membrane potential of medulloblastoma cells in culture is quite depolarized compared to granule neurons. Interestingly, medulloblastoma cells express small, fast-inactivating calcium currents consistent with T-type calcium channels, but these channels are activated only from hyperpolarized potentials, which are unlikely to occur. Additionally, a background acid-sensitive K+ current is present with features characteristic of TASK1 or TASK3 channels, such as inhibition by ruthenium red. Western analysis confirms expression of TASK1 and TASK3. In describing the electrophysiologic characteristics of medulloblastoma, one can see features that resemble other high-grade malignancies as opposed to normal cerebellar granule neurons. This supports the notion that the malignant phenotype of medulloblastoma is characterized by unique changes in ion channel expression.     

On the resting potential of isolated frog sympathetic neurons

One of the oldest questions of electrophysiology, the origin of the resting potential, has yet to be answered satisfactorily for most cells. Isolated frog sympathetic neurons, studied with whole-cell recording, generally have resting potentials of approximately -75 mV with an input resistance of approximately 300 M omega. These properties are not expected from the M-type K+ current (IM) or from other ionic currents previously described in these cells. In the -60 to -110 M mV voltage region, at least three currents are present: an inwardly rectifying current (IQ), a resting current with little voltage sensitivity carried at least in part by K+, and a (Na+,K+)ATPase pump current. The resting K+ current, not IM or IQ is the primary ionic current near the resting potential under these conditions. The electrogenic pump contributes an additional approximately 10 mV of hyperpolarization.     

Ion channels in the plasma membrane of protoplasts from the halophytic angiosperm Zostera muelleri

Patch clamp studies show that there may be as many as seven different channel types in the plasma membrane of protoplasts derived from young leaves of the halophytic angiosperm Zostera muelleri. In whole-cell preparations, both outward and inward rectifying currents that activate in a timeand voltage-dependent manner are observed as the membrane is either depolarized or hyperpolarized. Current voltage plots of the tail currents indicate that both currents are carried by K⁺. The channels responsible for the outward currents have a unit conductance of approximately 70 pS and are five times more permeable to K⁺ than to Na⁺. In outside-out patches we have identified a stretch-activated channel with a conductance of 100 pS and a channel that inwardly rectifies with a conductance of 6 pS. The reversal potentials of these channels indicate a significant permeability to K⁺. In addition, the plasma membrane contains a much larger K⁺ channel with a conductance of 300 pS. Single channel recordings also indicate the existence of two Cl^– channels, with conductances of 20 and 80 pS with distinct substates. The membrane potential difference of perfused protoplasts showed rapid action potentials of up to 50 mV from the resting level. The frequency of these action potentials increased as the external osmolarity was decreased. The action potentials disappeared with the addition of Gd³⁺, an effect that is reversible upon washout.We would like to thank K. Morris and D. McKenzie for technical assistance and the Australian Research Council for financial support.     

The Ca2+-activated K+ channel and its functional roles in smooth muscle cells of guinea pig taenia coli

Currents through single potassium channels were studied in cell-attached or inside-out patches from collagenase-dispersed smooth muscle cells of the guinea pig taenia coli. Under conditions mimicking the physiological state with [K+]i = 135 mM: [K+]o = 5.4 mM, three distinct types of K+ channel were identified with conductances around 0 mV of 147, 94, and 63 pS. The activities of the 94- and 63-pS channel were observed infrequently. The 147-pS channel was most abundant. It has a reversal potential of approximately -75 mV. It is sensitive to [Ca2+]i and to membrane potential. At -30 mV, the probability of a channel being open is at a minimum. At more positive voltages, the probability follows Boltzman distribution. A 10-fold change in [Ca2+]i causes a 25-mV negative shift of the voltage where half of the channels are open; an 11.3-mV change in membrane potential produces an e-fold increase in the probability of the channel being open when P is low. At voltages between -30 and -50 mV, the open probability increases in an anomalous manner because of a large decrease of the channel closed time without much change in the channel open time. This anomalous activity may play a regulatory role in maintaining the resting potential. The histograms of channel open and closed time fit well, respectively, with single and double exponential distributions. Upon step depolarizations by 100-ms pulses, the 147-pS channel opens with a brief delay. The delay shortens and both the number of open channels and the open time increase with increasing positivity of the potential. The averaged currents during the step depolarizations closely resemble the delayed rectifying outward K+ currents in whole-cell recordings.     

A new scheme of symbiosis: ligand- and voltage-gated anion channels in plants and animals.

Anion channels in the plasma membrane of both plant and animal cells participate in a number of important cellular functions such as volume regulation, trans-epithelial transport, stabilization of the membrane potential and excitability. Only very recently attention has turned to the presence of anion channels in higher plant cells. A dominant theme among recent discoveries is the role of Ca2+ in activating or modulating channel current involved in signal transduction. The major anion channel of stomatal guard cell protoplasts is a 32-40 pS channel which is highly selective for anions, in particular NO3-, Cl- and malate. These channels are characterized by a steep voltage dependence. Anion release is elicited upon depolarization and restricted to a narrow voltage span of -100 mV to the reversal potential of anions. During prolonged activation the current slowly inactivates. A rise in cytoplasmic calcium in the presence of nucleotides evokes activation of the anion channels. Following activation they catalyse anion currents 10-20 times higher than in the inactivated state thereby shifting the resting potential of the guard cell from a K(+)-conducting to an anion-conducting state. Patch-clamp studies have also revealed that growth hormones directly affect voltage-dependent activity of the anion channel in a dose-dependent manner. Auxin binding resulted in a shift of the activation potential towards the resting potential. Auxin-dependent gating of the anion channel is side- and hormone-specific. Its action is also channel-specific as K+ channels coexisting in the same membrane patch were insensitive to this ligand.(ABSTRACT TRUNCATED AT 250 WORDS)