大脑皮层神经元上ATP激活的多电导单通道

用膜片箝技术的细胞贴附式（ｃｅｌｌ—ａｔｔａｃｈｅｄ）和内面向外式（ｉｎｓｉｄｅ—ｏｕｔ）,在机械分离的新生大鼠的大脑皮层神经元上,记录到ＡＴＰ激活的单通道电流。按电流幅度可以分为三类,其电导分别为２０ｐＳ、３２ｐＳ和５８ｐＳ,各占全部开放事件数的１２％、６７％和２１％,其中３２ｐＳ为优势电导。三种电导状态之间可直接转换。３２ｐＳ和５８ｐＳ以及３２ｐＳ和２０ｐＳ之间的转换最为常见。结果表明：新生ＳＤ大鼠的大脑皮层神经元上存在ＡＴＰ激活的多电导单通道。     

Epithelial sodium channels: characterization by using the patch-clamp technique

The patch-clamp technique was used to resolve currents through individual Na-selective ion channels in the apical membrane of the rat cortical collecting tubule. The channels had a single unit conductance of 5 pS under control conditions (cell-attached patches, room temperature, 140 mM NaCl in the pipette). They appeared to be highly selective for Na, as K conduction through them was not measurable in inside-out patches. The channels underwent spontaneous transitions between open and closed states, both states being long-lived. At physiological temperature (37C) the conductance increased to 9 pS and the spontaneous transitions became more rapid. In the presence of amiloride on the luminal side of the membrane, the channel flickered rapidly between open and blocked states. The size of the current transitions did not change. This channel activity was observed only in rats that were fed a low-Na diet to elevate aldosterone secretion. In addition to mineralocorticoid control, the activity of the channels in inside-out patches were modulated by the pH on the cytoplasmic side of the membrane. Alkalinization from pH 6.4 to 7.4 increased the probability of channels' being open by eightfold. Changes in Ca concentration on the cytoplasmic side of the membrane did not directly affect the channels. However, addition of ionomycin, a Ca ionophore, to the bath resulted in a decrease in channel activity measured in cell-attached patches. This suggests that high cytoplasmic Ca may indirectly down-regulate Na channels in this tissue.     

Memory is a property of an ion channels pool: ion channels formed by Staphylococcus aureus alpha-toxin.

The short-time depolarization effects on the integral conductance induced by S. aureus alpha-toxin (ST) in planar lipid bilayer membranes has been studied. Ion channels formed by ST were found to have several potential-induced nonconductance (closed) states. The transitions of ion channels between the states are only through one conductance state. The transition of ST-channels from closed to open state is induced by membrane depolarization. The amplitude current after a series of voltage pulses is a function of pulse number, and is effectively independent of the time interval between the neighbouring pulses. Therefore, a membrane which contains a pool of ion channels "remembers" its previous existence. A simple model can be used to explain this phenomenon.     

Proteolipid of adenosinetriphosphatase from yeast mitochondria forms proton-selective channels in planar lipid bilayers

Proteolipid isolated from yeast mitochondrial adenosinetriphosphatase by butanol extraction is reincorporated into lipid vesicles from which planar membranes are formed. The proteolipid permits electric conductance through the membrane. This conductance occurs through membrane channels which are highly selective for protons. Proton channels in the membrane are directly observed at high proton concentrations in the aqueous phases. Channels open and close independently from each other; their open-state conductances and lifetimes are monodisperse but influenced by the applied voltage (12 pS and 3 s, respectively, at pH 2.2 and 100 mV). Proton channels do not occur in single proteolipid molecules; the conducting structure consists of at least two polypeptide chains since channels form in a (reversible) bimolecular reaction of nonconducting forms of proteolipid. The number of proton channels at a constant proteolipid concentration changes in sharp transitions and by orders of magnitudes upon critical changes of membrane composition and pH. These transitions are caused by transitions of proteolipid organization in the membrane from a dispersed state (equilibrium between channel-forming "dimers" and a large pool of "monomers") to a state of almost complete aggregation of proteolipid which stabilizes large proton-conducting structures (probably associates of channel-forming dimers). This self-association of isolated proteolipid into structures containing proton-selective channels suggests that the six proteolipids in the adenosinetriphosphatase complex exist as a self-associating entity containing most likely three proton channels.     

Modification of a voltage-gated K+ channel from sarcoplasmic reticulum by a pronase-derived specific endopeptidase

AK+ -selective membrane conductance channel from rabbit sarcoplasmic reticulum (SR) is studied in an artificial planar phospholipid bilayer. Membranes containing many such channels display voltage-dependent conductance, which is well described by a two-state conformational equilibrium with a free energy term linearly dependent on applied voltage. Pronase-derived alkaline proteinase b (APb), when added to the side of the membrane opposite to the SR vesicles (trans side), reduces the voltage dependence of the K+ conductance. Single-channel fluctuation experiments show that after APb treatment, the channel is still able to undergo transitions between its open and closed states, but that the probability of forming the open state is only slightly voltage-dependent. In terms of the conformational model, the enzyme's primary effect is to reduce the effective gating charge of the opening process by over 80%; a second effect of APb is to reduce the internal free energy of opening from +1.2 to +0.4 kcal/mol. The kinetics of APb action are strongly voltage-dependent, so as to indicate that the enzyme can react only with the channel's open state. The results imply that the channel contains a highly charged polypeptide region which moves in the direction perpendicular to the membrane plane when transitions between the open and closed states occur. A lysine or arginine residue in this region becomes exposed to the trans aqueous solution when the channel is in its open conformation.     

The Nature of the Negative Resistance in Bimolecular Lipid Membranes Containing Excitability-Inducing Material

When sufficiently small amounts of excitability-inducing material (EIM) are added to a bimolecular lipid membrane, the conductance is limited to a few discrete levels and changes abruptly from one level to another. From our study of these fluctuations, we have concluded that the EIM-doped bilayer contains ion-conducting channels capable of undergoing transitions between two states of different conductance. The difference in current between the "open" and "closed" states is directly proportional to the applied membrane potential, and corresponds to a conductance of about 3 x 10^-10 ohm^-1. The fraction of the total number of channels that is open varies from unity to zero as a function of potential. The voltage-dependent opening and closing of channels explains the negative resistance observed for bimolecular lipid membranes treated with greater amounts of EIM.     

Discrete conductance fluctuations in lipid bilayer protein membranes

Discrete fluctuations in conductance of lipid bilayer membranes may be observed during the initial stages of membrane interaction with EIM ("excitability inducing material"), during destruction of the EIM conductance by proteolysis, and during the potential-dependent transitions between low and high conductance states in the "excitable" membranes. The discrete conductance steps observed during the initial reaction of EIM with the lipid membranes are remarkably uniform, even in membranes of widely varying lipid composition. They range only from 2 to 6 x 10_-10 ohm_-1 and average 4 x 10_-10 ohm_-1. Steps found during destruction of the EIM conductance by proteolysis are somewhat smaller. The transition between high conductance and low conductance states may involve steps as small as 0.5 x 10_-10 ohm_-1. These phenomena are consistent with the formation of a stable protein bridge across the lipid membrane to provide a polar channel for the transport of cations. T6he uniform conductance fluctuations observed during the formation of these macromolecular channels may indicate that the ions in a conductive channel, in its open state, are largely protected from the influence of the polar groups of the membrane lipids. Potential-dependent changes in conductance may be due to configurational or positional changes in the protein channel. Differences in lipid-lipid and lipid-macromolecule interactions may account for the variations in switching kinetics in various membrane systems.     

Purification and patch clamp analysis of a 40-pS channel from rat liver mitochondria.

Patch clamp analysis of membranes reconstituted with a fraction isolated from detergent-solubilized mitochondrial membranes by affinity chromatography on immobilized quinine earlier indicated the presence of two classes of ion channels, of about 40- and 140-pS conductance in medium including 150 mM KCl. Now a 57-kDa constituent of the quinine-affinity column eluate has been identified as the 40-pS channel. Protein fractions derived from the quinine-affinity column eluate by preparative isoelectric focusing with a Rotofor cell have been reconstituted into phospholipid vesicle membranes by detergent dialysis, and vesicles have been enlarged for patch clamping by dehydration and rehydration. Voltage clamp analysis has been carried out on excised patches bathed symmetrically in buffered medium containing 150 mM KCl and 100 microM CaCl2. Patches of membrane incorporating the 57-kDa protein exhibit 40-pS conductance transitions. The magnitude of conductance transitions is similar when Na+ replaces K+ in the bathing medium, indicating little selectivity of the 40-pS channel for K+ relative to Na+. Another fraction derived from the quinine-affinity column eluate is found to contain the larger channel, now estimated to have an average conductance of about 130 pS. Patches of control membrane prepared in the same way but without protein exhibit no channel activity.     

Ligand-Dependent Opening of the Multiple AMPA Receptor Conductance States: A Concerted Model

Modulation of the properties of AMPA receptors at the post-synaptic membrane is one of the main suggested mechanisms underlying fast synaptic transmission in the central nervous system of vertebrates. Electrophysiological recordings of single channels stimulated with agonists showed that both recombinant and native AMPA receptors visit multiple conductance states in an agonist concentration dependent manner. We propose an allosteric model of the multiple conductance states based on concerted conformational transitions of the four subunits, as an iris diaphragm. Our model predicts that the thermodynamic behaviour of the conductance states upon full and partial agonist stimulations can be described with increased affinity of receptors as they progress to higher conductance states. The model also predicts the existence of AMPA receptors in non-liganded conductive substates. However, the probability of spontaneous openings decreases with increasing conductances. Finally, we predict that the large conductance states are stabilized within the rise phase of a whole-cell EPSC in glutamatergic hippocampal neurons. Our model provides a mechanistic link between ligand concentration and conductance states that can explain thermodynamic and kinetic features of AMPA receptor gating.     

Multiple conductance levels in rat heart inner mitochondrial membranes studied by patch clamping.

The behavior of the mitochondrial inner membrane multiple conductance channel (MCC) which has a peak conductance of 1-1.5 nS has been examined in rat heart mitochondria. MCC can display several unique characteristics: (a) prolonged open and closed times on the order of seconds to minutes, (b) a voltage dependence in which MCC opens (negative potential) or closes (positive potential) generally in steps, (c) a response to inhibitors such as amiodarone in steps corresponding at least approximately to those in (b), (d) a 'free-running mode' in which the current level rapidly fluctuates between a minimum of nine conductance levels but with a preferred occupation of the 0.5-0.7 nS levels, and (e) very large transitions (1-1.5 nS) resolved at 4 kHz bandwidth as single events with variable mean open time.     

Multiple conductances in the large K+ channel from Chara corallina shown by a transient analysis method.

The large conductance K+ channel in the tonoplast of Chara corallina has subconductance states (substates). We describe a method that detects substates by monitoring the time derivative of channel current. Substates near to the full conductance tend to have long durations and high probabilities, while those of smaller amplitude occur with less probability and short duration. The substate pattern is similar in cell-attached, inside-out and outside-out patches over a range of temperatures. The pattern changes at high Ca2+ concentration (10 mol m-3) on the cytoplasmic face of inside-out patches. One substate at approximately 50% of the full conductance is characterized by a high frequency of transitions from the full conductance level. This midstate conductance is not a constant proportion of the full conductance but changes as a function of membrane potential difference (p.d.) showing strong inward rectification. We suggest that the channel is a single pore that can change conformation and/or charge profile to give different conductances. The mean durations of the full conductance level and the midstate decrease as the membrane p.d. becomes more negative. Programs for analysis of channel kinetics based on an half-amplitude detection criterion are shown to be unsuitable for analysis of the K+ channel.     

Properties of amphotericin B channels in a lipid bilayer

Properties of individual ionic channels formed by polyene antibiotic Amphotericin B were studied on brain phospholipid membranes containing cholesterol. The ionic channels have a closed state and an open one (with conductance of about 6.5 pS in 2 M KCl). The conductance value of an open channel is independent of cholesterol concentration in the membrane and of pH in the range from 3.5 to 8.0. The voltage-current characteristics of a single channel are superlinear. Zero current potential value in the case of different KCl concentrations in the two solutions indicates preferential but not ideal anionic selectivity of a single channel. Channel conductivity grows as the electrolyte concentration is increased and tends to a limiting value at high concentrations. A simple model having only one site for an ion was shown to represent satisfactorily an open channel behaviour under different conditions. An individual ionic channel performs a large number of transitions between the open and closed states during its life-time of several minutes. Rate constants of these transitions depend on the kind and concentration of salt in aqueous solutions. The switching system functioning is not influenced by an ion situated inside the pore.     

A new algorithm for idealizing single ion channel data containing multiple unknown conductance levels.

A new algorithm is presented for idealizing single channel data containing any number of conductance levels. The number of levels and their amplitudes do not have to be known a priori. No assumption has to be made about the behavior of the channel, other than that transitions between conductance levels are fast. The algorithm is relatively insensitive to the complexity of the underlying single channel behavior. Idealization may be reliable with signal-to-noise ratios as low as 3.5. The idealization algorithm uses a slope detector to localize transitions between levels and a relative amplitude criterion to remove spurious transitions. After estimating the number of conductances and their amplitudes, conductance states can be assigned to the idealized levels. In addition to improving the quality of the idealization, this "interpretation" allows a statistical analysis of individual (sub)conductance states.     

Gating transitions in bacterial ion channels measured at 3 microns resolution

Ion channels of high conductance (>200 pS) are widespread among prokaryotes and eukaryotes. Two examples, the Escherichia coli mechanosensitive ion channels Ec-MscS and Ec-MscL, pass currents of 125-300 pA. To resolve temporal details of conductance transitions, a patch-clamp setup was optimized for low-noise recordings at a time resolution of 3 microns (10-20 times faster than usual). Analyses of the high-resolution recordings confirm that Ec-MscL visits many subconductance states and show that most of the intersubstate transitions occur more slowly than the effective resolution of 3 micros. There is a clear trend toward longer transition times for the larger transitions. In Ec-MscS recordings, the majority of the observed full conductance transitions are also composite. We detected a short-lived (approximately 20 microns) Ec-MscS substate at 2/3 of full conductance; transitions between 2/3 and full conductance did not show fine structure and had a time course limited by the achieved resolution. Opening and closing transitions in MscS are symmetrical and are not preceded or followed by smaller, rapid currents ("anticipations" or "regrets"). Compared with other, lower-conductance channels, these measurements may detect unusually early states in the transitions from fully closed to fully open. Increased temporal resolution at the single-molecule level reveals that some elementary steps of structural transitions are composite and follow several alternative pathways, while others still escape resolution. High-bandwidth, low-noise single-channel measurements may provide details about state transitions in other high-conductance channels; and similar procedures may also be applied to channel- and nanopore-based single-molecule DNA measurements.     

Excitability, instability and phase transitions in squid axon membrane under internal perfusion with dilute salt solutions

Using giant axons of squid, Doryteuthis, available in Hokkaido, Japan, it was shown that axons internally perfused with a dilute sodium salt solution undergo an abrupt transition from a resting to a depolarized state on addition of KCl to an external medium containing CaCl₂. Under internal perfusion with a dilute solution of sodium or cesium salt, it was possible to induce abrupt transitions between the two (i.e., resting and depolarized) states of the membrane by changing the temperature. “Giant fluctuations” in the state of the axon membrane were demonstrated at and near the critical points of the axon membrane. These findings are interpreted as supporting the view that an abrupt change in the membrane potential and conductance is an electrochemical manifestation of a phase transition of the membrane macromolecules.     

Thermodynamic and kinetic studies of the gating behavior of a K+- selective channel from the sarcoplasmic reticulum membrane

A voltage-dependent, K+-selective ionic channel from sarcoplasmic reticulum of rabbit skeletal muscle has been studied in a planar phospholipid bilayer membrane. The purpose [corrected] of this work is to study the mechanism by which the channel undergoes transitions between its conducting and nonconducting states. Thermodynamic studies show that the "open" and "closed" states of the channel exist in a voltage-dependent equilibrium, and that the channel displays only a single open state; the channel conductance is 120 pmho in 0.1 M K+. The channel's gating process follows single exponential kinetics at all voltages tested, and the individual opening and closing rate constants are exponentially dependent on voltage. The individual rate constants may also be determined from a stochastic analysis of channel fluctuations among multiple conductance levels. Neither the thermodynamic nor the kinetic parameters of gating depend on the absolute concentration of channels in the bilayer. The results are taken as evidence that the channel gates by an unusually simple two-state conformational mechanism in which the equivalent of 1.1 net charges are moved across the membrane during the formation of the open channel.     

Hidden Markov model analysis of intermediate gating steps associated with the pore gate of shaker potassium channels.

Cooperativity among the four subunits helps give rise to the remarkable voltage sensitivity of Shaker potassium channels, whose open probability changes tenfold for a 5-mV change in membrane potential. The cooperativity in these channels is thought to arise from a concerted structural transition as the final step in opening the channel. Recordings of single-channel ionic currents from certain other channel types, as well as our previous recordings from T442S mutant Shaker channels, however, display intermediate conductance levels in addition to the fully open and closed states. These sublevels might represent stepwise, rather than concerted, transitions in the final steps of channel activation. Here, we report a similar fine structure in the closing transitions of Shaker channels lacking the mutation. Describing the deactivation time course with hidden Markov models, we find that two subconductance levels are rapidly traversed during most closing transitions of chimeric, high conductance Shaker channels. The lifetimes of these levels are voltage-dependent, with maximal values of 52 and 22 micros at -100 mV, and the voltage dependences of transitions among these states suggest that they arise from equivalent conformational changes occurring in individual subunits. At least one subconductance level is found to be traversed in normal conductance Shaker channels. We speculate that voltage-dependent conformational changes in the subunits give rise to changes in a "pore gate" associated with the selectivity filter region of the channel, producing the subconductance states. As a control for the hidden Markov analysis, we applied the same procedures to recordings of the recovery from N-type inactivation in Shaker channels. These transitions are found to be instantaneous in comparison.     

Identification and functional characterization of a dual GABA/taurine transporter in the bullfrog retinal pigment epithelium

Intracellular microelectrodes, fluorescence imaging, and radiotracer flux techniques were used to investigate the physiological response of the retinal pigment epithelium (RPE) to the major retinal inhibitory neurotransmitter, gamma-aminobutyric acid (GABA). GABA is released tonically in the dark by amphibian horizontal cells, but is not taken up by the nearby Muller cells. Addition of GABA to the apical bath produced voltage responses in the bullfrog RPE that were not blocked nor mimicked by any of the major GABA-receptor antagonists or agonists. Nipecotic acid, a substrate for GABA transport, inhibited the voltage effects of GABA. GABA and nipecotic acid also inhibited the voltage effects of taurine, suggesting that the previously characterized beta- alanine sensitive taurine carrier also takes up GABA. The voltage responses of GABA, taurine, nipecotic acid, and beta-alanine all showed first-order saturable kinetics with the following Km's: GABA (Km = 160 microM), beta-alanine (Km = 250 microM), nipecotic acid (Km = 420 microM), and taurine (Km = 850 microM). This low affinity GABA transporter is dependent on external Na, partially dependent on external Cl, and is stimulated in low [K]o, which approximates subretinal space [K]o during light onset. Apical GABA also produced a significant conductance increase at the basolateral membrane. These GABA-induced conductance changes were blocked by basal Ba2+, suggesting that GABA decreased basolateral membrane K conductance. In addition, the apical membrane Na/K ATPase was stimulated in the presence of GABA. A model for the interaction between the GABA transporter, the Na/K ATPase, and the basolateral membrane K conductance accounts for the electrical effects of GABA. Net apical-to-basal flux of [3H]-GABA was also observed in radioactive flux experiments. The present study shows that a high capacity GABA uptake mechanism with unique pharmacological properties is located at the RPE apical membrane and could play an important role in the removal of GABA from the subretinal space (SRS). This transporter could also coordinate the activities of GABA and taurine in the SRS after transitions between light and dark.     

Interactions between intrinsic membrane protein and electric field. An approach to studying nerve excitability

We have approached the problem of nerve excitability through three questions: (a) What is the diagram for a channel? That is, what conformational states can the protein assume, and what transitions between these conformations are permitted? (b) What is the channel conductance associated with each conformation the channel can assume? (c) How do the rates for conformational transition depend upon membrane potential? These three questions arise from a standard statistical mechanical treatment of a nerve membrane containing several classes of identical, independent channels. Gating of channels, in this view, is associated with conformational changes of the channel protein, and it is assumed these conformations are distinct. The precise formulation of these questions is presented in terms of the theoretical treatment, and the approaches we have taken to answer the questions are indicated. Our present results indicate: Transition rates should depend exponentially on membrane potential over a limited voltage range, but probably will show a more complex dependence for extremes of the range; channels probably can take on only two conductances, open and shut, but more complicated situations are not entirely excluded; the diagram for a channel cannot be determined from standard voltage clamp data alone, but by studying gating currents and conductance fluctuations, it should be possible to select between alternative plausible physical mechanisms.     

The effect of prymnesin on the electric conductivity of thin lipid membranes

Summary A proteolipidic toxin, prymnesin, when added to the aqueous solutions around thin lipid membranes causes a marked increase in membrane conductance. The toxin-treated membrane is cation-permselective. The extent of cation permselectivity is dependent upon ionic strength of the aqueous solutions in a fashion similar to the dependence of cation permselectivity of a cation exchanger containing about 100mm of fixed negative sites. Dose-response relationship studies reveal a linear relation between log prymnesin concentration and log membrane conductance. The slope of the curve is around 3 if the toxin is applied to one side of the membrane and is around 7 if the toxin is applied to both sides of the membrane. The membrane treated with toxin on one side only is clearly asymmetric in its properties. These characteristics are expressed by an asymmetric current-voltage relationship, and by asymmetric sensitivity of membrane conductance to pH and to salt concentration. The conductance of the toxin-treated membrane is inversely proportional to temperature. It is suggested that aggregates of toxin moieties assemble in the membrane to form negatively charged aqueous pores. There is roughly a good correlation between the increase in membrane conductance and the increase in membrane permeability to urea if both were attributed to the formation of aqueous channels in the membrane.