Phloretin affects the fast potassium channels in frog nerve fibres

The effect of phloretin (20-100 M), a dipolar organic compound, on the voltage clamp currents of the frog node of Ranvier has been investigated. The Na currents are simply reduced in size but not otherwise affected. Phloretin has no effect on the slow 4-aminopyridine-resistant K channels. However, the voltage dependence and time course of the fast K conductance (g _K) is markedly altered. The g _K(E) curve, determined by measuring fast tail currents at different pulse potentials, normally exhibits a bend at –50 mV indicating the existence of two types of fats K channels. Phloretin shifts the g _K (E) curve to more positive potentials, reduces its slope and its maximum and abolishes the distinction between the two tpyes of fast K channels. The effect becomes more pronounced with time. Phloretin also markedly slows the opening of the fast K channels, but has much less effect on the closing. Opening can be accelerated again by a long depolarizing prepulse which presumably removes part of the phloretin block. It is concluded that phloretin selectively affects the fast K channels of the nodal membrane. The results are compared with similar observations on the squid giant axon. Offprint requests to: H. Meves     

Pharmacological and kinetic analysis of K channel gating currents

We have measured gating currents from the squid giant axon using solutions that preserve functional K channels and with experimental conditions that minimize Na channel contributions to these currents. Two pharmacological agents were used to identify a component of gating current that is associated with K channels. Low concentrations of internal Zn2+ that considerably slow K channel ionic currents with no effect on Na channel currents altered the component of gating current associated with K channels. At low concentrations (10-50 microM) the small, organic, dipolar molecule phloretin has several reported specific effects on K channels: it reduces K channel conductance, shifts the relationship between channel conductance and membrane voltage (Vm) to more positive potentials, and reduces the voltage dependence of the conductance-Vm relation. The K channel gating charge movements were altered in an analogous manner by 10 microM phloretin. We also measured the dominant time constants of the K channel ionic and gating currents. These time constants were similar over part of the accessible voltage range, but at potentials between -40 and 0 mV the gating current time constants were two to three times faster than the corresponding ionic current values. These features of K channel function can be reproduced by a simple kinetic model in which the channel is considered to consist of two, two-state, nonidentical subunits.     

Effects of Density and Gating of Delayed-Rectifier Potassium Channels on Resting Membrane Potential and its Fluctuations

The aim of this study is to evaluate directly, using a reduced experimental system, the nature of interactions between voltage-gated potassium channels and the resting membrane potential. Xenopus oocytes were injected with various concentrations of cRNA coding for a delayed-rectifier potassium channel Shaker-IR. The effects of the density and kinetics of the expressed channels on resting membrane potential is explored in isolated (``inside-out') patches. The channel density is given in terms of maximal conductance (G _max), measured from the maximal slope of the I-V curve under voltage clamp conditions. The capacitance of the experimental setup is approximately 1 pF. At high channel densities (G _max > 10 pA/mV) the mean membrane potential is stabilized at approximately −60 mV. This resting membrane potential is more than 35 mV positive to the reversal potential for potassium ions under the same experimental conditions. Analyses of voltage clamp experiments indicate that at high channel densities the mean membrane potential is determined by the rates of channel activation and deactivation, but is not affected by the rates involved in the process of slow (C-type) inactivation. In contrast, at lower channel densities membrane potential is very unstable, and its mean value and amplitude of fluctuations are strongly affected by the process of slow (C-type) inactivation. Received: 21 March 1996/Revised: 6 August 1996     

Concentration-jump analysis of voltage-dependent conductances activated by glutamate and kainate in neurons of the avian cochlear nucleus.

We have examined the mechanisms underlying the voltage sensitivity of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors in voltage-clamped outside-out patches and whole cells taken from the nucleus magnocellularis of the chick. Responses to either glutamate or kainate had outwardly rectifying current-voltage relations. The rate and extent of desensitization during prolonged exposure to agonist, and the rate of deactivation after brief exposure to agonist, decreased at positive potentials, suggesting that a kinetic transition was sensitive to membrane potential. Voltage dependence of the peak conductance and of the deactivation kinetics persisted when desensitization was reduced with aniracetam or blocked with cyclothiazide. Furthermore, the rate of recovery from desensitization to glutamate was not voltage dependent. Upon reduction of extracellular divalent cation concentration, kainate-evoked currents increased but preserved rectifying current-voltage relations. Rectification was strongest at lower kainate concentrations. Surprisingly, nonstationary variance analysis of desensitizing responses to glutamate or of the current deactivation after kainate removal revealed an increase in the mean single-channel conductance with more positive membrane potentials. These data indicate that the rectification of the peak response to a high agonist concentration reflects an increase in channel conductance, whereas rectification of steady-state current is dominated by voltage-sensitive channel kinetics.     

Single K+ channels in endocrine cells dispersed from the cricket (Gryllus bimaculatus) corpora allata

Single channel currents were recorded from cell-attached patches of endocrine cells of the adult male cricket corpora allata. Three distinct types of K+ channels were identified; a weak inward rectifier (Type 1), a strong inward rectifier (Type 2) and a weak outward rectifier (Type 3). The type 1 channel had a slope conductance of 191 +/- 9 pS (n = 4) at negative membrane potentials (Vm) and 101 +/- 6 pS (n = 6) at positive Vm. In addition, the channel showed fast open-closed kinetics at negative Vm and slow open-closed kinetics at positive Vm. The open probability (Po) of this channel was strongly voltage-dependent at positive Vm, but less voltage-dependent at negative Vm. The reversal potential was not modified significantly by the substitution of gluconate for external Cl- but was modified after N-methyl-D-glucamine (NMDG+) was substituted for external K+, according to the Nernst equation for a K+-selective channel. The type 2 channel had a slope conductance of 44 +/- 2 pS (n = 5) at negative Vm, but no detectable outward current was observed at positive Vm. This channel showed very slow open-closed kinetics at negative Vm and its Po was not voltage-dependent. The type 3 channel had a limit conductance of 55 +/- 12 pS (n = 3) at negative Vm and 88 +/- 10 pS (n = 3) at positive Vm. This channel showed slow open-closed kinetics at negative Vm and fast open-closed kinetics at positive Vm. The Po for the channel was voltage-dependent at positive Vm but was voltage-independent at negative Vm. These three types of K+ channels may be important for the control of the resting membrane potential, and may thus participate in the regulation of Ca2+ influx and juvenile hormone secretion in corpora allata cells.     

Inwardly rectifying single-channel and whole cell K+ currents in rat ventricular myocytes

Summary The voltage-dependent properties of inwardly rectifying potassium channels were studied in adult and neonatal rat ventricular myocytes using patch voltage-clamp techniques. Inward rectification was pronounced in the single-channel currentvoltage relation and outward currents were not detected at potentials positive to the calculated reversal potential for potassium (E _k). Single-channel currents having at least three different conductances were observed and the middle one was predominant. Its single-channel conductance was nonlinear ranging from 20 to 40 pS. Its open-time distribution was fit by a single exponential and the time constants decreased markedly with hyperpolarization fromE _k. The distribution of the closed times required at least two exponentials for fitting, and their taus were related to the bursting behavior displayed at negative potentials. The steady-state probability of being open (P _o) for this channel was determined from the single-channel records; in symmetrical isotonic K solutionsP _o was 0.73 at –60 mV, but fell to 0.18 at –100 mV. The smaller conductance was about one-half the usual value and the open times were greatly prolonged. The large conductance was about 50 percent greater than the usual value and the open times were very brief. TheP _o(V) relation, the kinetics and the conductance of the predominant channel account for most of the whole cell inwardly rectifying current. The kinetics suggest that an intrinsic K⁺-dependent mechanism may control the gating, and the conductance of this channel. In the steady state, the opening and closing probabilities for the two smaller channels were not independent of each other, suggesting the possibility of a sub-conductance state or cooperativity between different channels.     

Potassium current and the effect of cesium on this current during anomalous rectification of the egg cell membrane of a starfish

The kinetics of the membrane current during the anomalous or inward- going rectification of the K current in the egg cell membrane of the starfish Mediaster aequalis were analyzed by voltage clamp. The rectification has instantaneous and time-dependent components. The time- dependent increase in the K conductance for the negative voltage pulse as well as the decrease in the conductance for the positive pulse follows first-order kinetics. The steady-state conductance increases as the membrane potential becomes more negative and reaches the saturation value at about -40 mV more negative than the K equilibrium potential, V(K). The entire K conductance can be expressed by g(K).n; g g(K) represents the component for the time-independent conductance which depends on V-V(K) and [K+]o, and n is a dimensionless number (1 is greater than or equal to n is greater than or equal to 0) and determined by two rate constants which depend only on V-V(K). Cs+ does not carry any significant current through the K channel but blocks the channel at low concentration in the external medium. The blocking effect increases as the membrane potential is made more negative and the potential-dependent blocking by the external Cs+ also has instantaneous and time-dependent components.     

The membrane potential of Arabidopsis thaliana guard cells; depolarizations induced by apoplastic acidification

The apoplastic pH of guard cells probably acidifies in response to light, since light induces proton extrusion by both guard cells and epidermal leaf cells. From the data presented here, it is concluded that these apoplastic pH changes will affect K⁺ fluxes in guard cells of Arabidopsis thaliana (L.) Heynh. Guard cells of this species were impaled with double-barrelled microelectrodes, to measure the membrane potential (E_m) and the plasma-membrane conductance. Guard cells were found to exhibit two states with respect to their E_m, a depolarized and a hyperpolarized state. Apoplastic acidification depolarized E_m in both states, though the origin of the depolarization differed for each state. In the depolarized state, the change in E_m was the result of a combined pH effect on instantaneously activating conductances and on the slow outward rectifying K⁺ channel (s-ORC). At a more acidic apoplastic pH, the current through instantaneously activated conductances became more inwardly directed, while the maximum conductance of s-ORC decreased. The effect on s-ORC was accompanied by an acceleration of activation and deactivation of the channel. Experiments with acid loading of guard cells indicated that the effect on s-ORC was due to a lowered intracellular pH, caused by apoplastic acidification. In the hyperpolarized state, the pH-induced depolarization was due to a direct effect of the apoplastic pH on the inward rectifying K⁺ channel. Acidification shifted the threshold potential of the channel to more positive values. This effect was accompanied by a decrease in activation times and an increase of deactivation times, of the channel. From the changes in E_m and membrane conductance, the expected effect of acidification on K⁺ fluxes was calculated. It was concluded that apoplastic acidification will increase the K⁺-efflux in the depolarized state and reduce the K⁺-influx in the hyperpolarized state. Received: 28 April 1997 / Accepted: 10 November 1997     

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.     

Properties and modulation of alpha human atrial natriuretic peptide (alpha-hANP)-formed ion channels

Using the lipid bilayer technique we have optimized recording conditions and confirmed that alpha human atrial natriuretic peptide [alpha-hANP(1-28)] forms single ion channels. The single channel currents recorded in 250/50 mM KCl cis/trans chambers show that the ANP-formed channels were heterogeneous, and differed in their conductance, kinetic, and pharmacological properties. The ANP-formed single channels were grouped as: (i) H202- and Ba2+-sensitive channel with fast kinetics; the nonlinear current-voltage (I-V) relationship of this channel had a reversal potential (Erev) of -28.2 mV, which is close to the equilibrium potential for K+ (EK = -35 mV) and a maximal slope conductance (gmax) of 68 pS at positive potentials. Sequential ionic substitution (KCl, K gluconate and choline Cl) of the cis solution suggests that the current was carried by cations. The fast channel had three modes (spike mode, burst mode, and open mode) that differed in their kinetics but not in their conductance properties. (ii) A large conductance channel possessing several subconductance levels that showed time-dependent inactivation at positive and negative membrane potentials (Vm). The inactivation ratio of the current at the end of the voltage step (Iss) to the initial current (Ii) activated immediately after the voltage step, (Iss/Ii), was voltage dependent and described by a bell-shaped curve. The maximal current-voltage (I-V) relationship of this channel, which had an Erev of +17.2 mV, was nonlinear and the value of gmax was 273 pS at negative voltages. (iii) A transiently-activated channel: the nonlinear I-V relationship of this channel had an Erev of -29.8 mV and the value of gmax was 160 pS at positive voltages. We propose that the voltage-dependence of the ionic currents and the kinetic parameters of these channel types indicate that if they were formed in vivo and activated by cytosolic factors they could change the membrane potential and the electrolyte homeostasis of the cell.     

Single-Channel Analysis of a Delayed Rectifier K+ Channel in Xenopus Myelinated Nerve

Single-channel properties of a delayed rectifier voltage-gated K⁺ channel (I-type) were investigated in peripheral myelinated axons from Xenopus laevis. Channels activated between −60 and −40 mV with a potential of half-maximal activation, E₅₀, at −47.5 mV. Averaged single-channel currents activated with a time delay at all membrane potentials tested. Time to half-maximal activation decreased from 80 to 1.6 msec between −60 and +40 mV. The channel inactivated monoexponentially with a time constant of 10.9 sec at −40 mV. The time constant of deactivation was 126 msec at −80 mV and 16.9 msec at −110 mV. In symmetrical 105 mm K⁺, the single-channel conductance (γ) was 22 and 13 pS at negative and positive membrane potentials, respectively, at 13–15°C. In Na⁺-rich solution with 2.5 mm extracellular K⁺γ was 7 pS and the reversal potential was negative to −80 mV, indicating a high selectivity for K⁺ over Na⁺. γ depended on extracellular K⁺ concentration (K _D = 19.6 mm) and temperature (Q ₁₀= 1.45). External tetraethylammonium (TEA) reduced the apparent single-channel current amplitude at all potentials tested with a half-maximal inhibiting concentration (IC₅₀) of 0.6 mm. Open probability of the channel, but not single-channel current amplitude was decreased by extracellular dendrotoxin (DTX, IC₅₀= 6.8 nm) and mast cell degranulating peptide (MCDP, IC₅₀= 41.9 nm). In Ringer solution the membrane potential of macroscopic I-channel patches was about −65 mV and depolarized under TEA and DTX. It is concluded that besides their activation during action potentials, I-channels may also stabilize the resting membrane potential. Received: 2 June 1995/Revised: 13 October 1995     

Current Oscillations Under Voltage-Clamp Conditions: An Interplay of Series Resistance and Negative Slope Conductance

Using the patch-clamp technique, we observed profound oscillations of the whole-vacuole outward current across the tonoplast of Mesembryanthemum crystallinum L. (common ice plant). These current oscillations showed a clear voltage dependence and appeared at membrane potentials more positive than 90–100 mV. This paper describes the oscillations in terms of two separate mechanisms. First, the Mesembryanthemum vacuolar membrane shows a negative slope conductance at membrane potentials more positive than 100–120 mV. The fact that the oscillations and the negative slope conductance show a similar threshold potential suggests that (part of) the same mechanism is involved in both phenomena. The second mechanism involved is the voltage drop across the series resistance. As a result, the potential actually experienced by the vacuolar membrane deviates from the command potential defined by the patch-clamp amplifier. This deviation depends in an Ohmic manner on the current magnitude. We suggest that the interplay of the negative slope conductance and the voltage drop across the series resistance can cause a positive feedback which is responsible for the current oscillations. Received: 30 April 1999/Revised: 9 September     

Possible reduction of surface charge by a mutation inParamecium tetraurelia

Summary Under voltage clamp, a mutant ofParamecium tetraurelia (teaB) shows a shift in the positive direction of the voltage sensitivity of the Ca conductance and the depolarization inactivation curve by 10 mV with no change in the total conductance. This effect can be mimicked in the wild type by the addition of external Ca²⁺ or Mg²⁺. The mutation also shifts the resting potential and the voltage sensitivities of the delayed rectification (depolarization-sensitive) K conductance and the anomalous rectification (hyperpolarization-sensitive) K conductance in the positive direction to a similar extent. This systematic shift of channel voltage sensitivities is best explained by the reduction of the surface negative charges of the membrane due to the mutation.     

Patch clamp analysis of the dominant plasma membrane K+ channel in root cell protoplasts of Plantago media L. Its significance for the P and K state

Ion channels in the plasma membrane of root cell protoplasts of Plantago media L. were studied with the patch clamp technique in the cell-attached patch and outside-out patch configuration. An outward rectifying potassium channel was dominantly present in the plasma membrane. It appears responsible for the diffusional part, dominated by the K⁺ diffusion potential, of the cell membrane potential, in vivo. This channel is activated at potentials near to and more positive than the K⁺ diffusion potential. The dependence of this ion channel on K⁺ activity and voltage has been characterized. The current-voltage relationships of the open channel at various K⁺ concentrations are described by a four-state model. The membrane potential of intact protoplasts appears either dominated by the K⁺ diffusion potential, the protoplast is then said to be in the K state, or by the pump potential generated by the plasma membrane-bound proton pump/H⁺ ATPase, the P state. An experimental procedure is described to determine in cell-attached patch mode the state of the protoplast, either K or P state.Institution paper no.: ECOTRANS publication no. 45.     

Inward and outward currents of native and cloned K(ATP) channels (Kir6.2/SUR1) share single-channel kinetic properties

BackgroundThe ATP-sensitive K⁺ (K(ATP)) channel is found in a variety of tissues extending from the heart and vascular smooth muscles to the endocrine pancreas and brain. Common to all K(ATP) channels is the pore-forming subunit Kir6.x, a member of the family of small inwardly rectifying K⁺ channels, and the regulatory subunit sulfonylurea receptor (SURx). In insulin secreting β-cells in the endocrine part of the pancreas, where the channel is best studied, the K(ATP) channel consists of Kir6.2 and SUR1. Under physiological conditions, the K(ATP) channel current flow is outward at membrane potentials more positive than the K⁺ equilibrium potential around ?80 mV. However, K(ATP) channel kinetics have been extensively investigated for inward currents and the single-channel kinetic model is based on this type of recording, whereas only a limited amount of work has focused on outward current kinetics.MethodsWe have estimated the kinetic properties of both native and cloned K(ATP) channels under varying ionic gradients and membrane potentials using the patch-clamp technique.ResultsAnalyses of outward currents in K(ATP) and cloned Kir6.2ΔC26 channels, alone or co-expressed with SUR1, show openings that are not grouped in bursts as seen for inward currents. Burst duration for inward current corresponds well to open time for outward current.ConclusionsOutward K(ATP) channel currents are not grouped in bursts regardless of membrane potential, and channel open time for outward currents corresponds to burst duration for inward currents.     

Effects of the dipolar form of phloretin on potassium conductance in squid giant axons.

The effects of phloretin on membrane ionic conductances have been studied in the giant axon of the squid, Loligo pealei. Phloretin reversibly suppresses the potassium and sodium conductances and modifies their dependence on membrane potential (Em). Its effects on the potassium conductance (GK) are much greater than on the sodium conductance; no effects on sodium inactivation are observed. Internal perfusion of phloretin produces both greater shifts in GK(Em) and greater reductions maximum GK than does external perfusion; the effect of simultaneous internal and external perfusion is little greater than that of internal perfusion alone. Lowering the internal pH, which favors the presence of the neutral species of weakly acidic phloretin (pKa 7.4), potentiates the actions of internally perfused phloretin. Other organic cations with dipole moments similar to phloretin's have little effect on either potassium or sodium conductances in squid axons. These results can be explained by either of two mechanisms; on postulates a phloretin "receptor" near the voltage sensor component of the potassium channel which is accessible to drug molecules applied at either the outer or inner membrane surface and is much more sensitive to the neutral than the negatively charged form of the drug. The other mechanism proposes that neutral phloretin molecules are dispersed in an ordered array in the membrane interior, producing a diffuse dipole field which modifies potassium channel gating. Different experimental results support these two mechanisms, and neither hypothesis can be disproven.     

Effects of phlorizin and phloretin on passive and dynamic electrical properties in muscle membrane

The effects of phlorizin and phloretin on the cable properties were investigated in frog sartorius muscle by conventional cable analysis. Actions of phloretin on voltage-dependent ionic conductances were also studied by analysis of the phase plane trajectories. Both drugs evoked a significant decrease in specific membrane resistance (Rm) in chloride-containing Ringer's solution. The linear membrane capacitance increased by about 30%. On the contrary, in the presence of the non-penetrating anion, glutamate, a slight increase in Rm was induced by phlorizin. It is suggested that these drugs may increase the chloride conductance in the muscle membrane. Under the effect of phloretin the resting membrane potential remained unchanged but the amplitude of the action potential was lowered and the rate of repolarization was significantly reduced. The rate of depolarization during the "foot" of the action potential and the conduction velocity calculated from the rate constant of depolarization decreased. The maximum Na conductance was not altered by phloretin but K conductance was reduced. The time constant (tau K) reflecting the kinetic properties of K conductance was increased about seven-fold. It is suggested that great importance may be attributed to the dipole properties of these drugs in the actions presented above.     

Phosphorylation modulates potassium conductance and gating current of perfused giant axons of squid

The presence of internal Mg-ATP produced a number of changes in the K conductance of perfused giant axons of squid. For holding potentials between -40 and -50 mV, steady-state K conductance increased for depolarizations to potentials more positive than approximately -15 mV and decreased for smaller depolarizations. The voltage dependencies of both steady-state activation and inactivation also appears shifted toward more positive potentials. Gating kinetics were affected by internal ATP, with the activation time constant slowed and the characteristic delay in K conductance markedly enhanced. The rate of deactivation also was hastened during perfusion with ATP. Internal ATP affected potassium channel gating currents in similar ways. The voltage dependence of gating charge movement was shifted toward more positive potentials and the time constants of ON and OFF gating current also were slowed and hastened, respectively, in the presence of ATP. These effects of ATP on the K conductance occurred when no exogenous protein kinases were added to the internal solution and persisted even after removing ATP from the internal perfusate. Perfusion with a solution containing exogenous alkaline phosphatase reversed the effects of ATP. These results provide further evidence that the effects of ATP on the K conductance are a consequence of a phosphorylation reaction mediated by a kinase present and active in perfused axons. Phosphorylation appears to alter the K conductance of squid giant axons via a minimum of two mechanisms. First, the voltage dependence of gating parameters are shifted toward positive potentials. Second, there is an increase in the number of functional closed states and/or a decrease in the rates of transition between these states of the K channels.     

Slo3 K+ channels: voltage and pH dependence of macroscopic currents

The mouse Slo3 gene (KCNMA3) encodes a K(+) channel that is regulated by changes in cytosolic pH. Like Slo1 subunits responsible for the Ca(2+) and voltage-activated BK-type channel, the Slo3 alpha subunit contains a pore module with homology to voltage-gated K(+) channels and also an extensive cytosolic C terminus thought to be responsible for ligand dependence. For the Slo3 K(+) channel, increases in cytosolic pH promote channel activation, but very little is known about many fundamental properties of Slo3 currents. Here we define the dependence of macroscopic conductance on voltage and pH and, in particular, examine Slo3 conductance activated at negative potentials. Using this information, the ability of a Horrigan-Aldrich-type of general allosteric model to account for Slo3 gating is examined. Finally, the pH and voltage dependence of Slo3 activation and deactivation kinetics is reported. The results indicate that Slo3 differs from Slo1 in several important ways. The limiting conductance activated at the most positive potentials exhibits a pH-dependent maximum, suggesting differences in the limiting open probability at different pH. Furthermore, over a 600 mV range of voltages (-300 to +300 mV), Slo3 conductance shifts only about two to three orders of magnitude, and the limiting conductance at negative potentials is relatively voltage independent compared to Slo1. Within the context of the Horrigan-Aldrich model, these results indicate that the intrinsic voltage dependence (z(L)) of the Slo3 closed-open equilibrium and the coupling (D) between voltage sensor movement are less than in Slo1. The kinetic behavior of Slo3 currents also differs markedly from Slo1. Both activation and deactivation are best described by two exponential components, both of which are only weakly voltage dependent. Qualitatively, the properties of the two kinetic components in the activation time course suggest that increases in pH increase the fraction of more rapidly opening channels.     

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.