The Conditions for Initiating "All-or-Nothing" Repolarization in Cardiac Muscle

Solutions have been computed for the point polarization of an infinite cable-like membrane obeying the equations used to reproduce the Purkinje fiber action potential (Noble, 1960, 1962a) in order to determine the conditions for initiating all-or-nothing repolarization during the action potential plateau. It was found that all-or-nothing repolarization would not be obtainable during the first half of the action potential in spite of the fact that the membrane current-voltage relations contain regions of negative conductance. At the point at which the all-or-nothing response is first obtained, the computed threshold is large and repolarization almost back to the resting potential would be required in order to initiate the response. The results are discussed in relation to the experimental evidence at present available on repolarization in heart muscle.     

The Ventral Photoreceptor Cells of Limulus : III. A voltage-clamp study

In the dark, the ventral photoreceptor of Limulus exhibits time-variant currents under voltage-clamp conditions; that is, if the membrane potential of the cell is clamped to a depolarized value there is an initial large outward current which slowly declines to a steady level. The current-voltage relation of the cell in the dark is nonlinear. The only ion tested which has any effect on the current-voltage relation is potassium; high potassium shifts the reversal potential towards zero and introduces a negative slope-conductance region. When the cell is illuminated under voltage-clamp conditions, an additional current, the light-induced current, flows across the cell membrane. The time course of this current mimics the time course of the light response (receptor potential) in the unclamped cell; namely, an initial transient phase is followed by a steady-state phase. The amplitude of the peak transient current can be as large as 60 times the amplitude of the steady-state current, while in the unclamped cell the amplitude of the peak transient voltage never exceeds 4 times the amplitude of the steady-state voltage. The current-voltage relations of the additional light-induced current obtained for different instants of time are also nonlinear, but differ from the current-voltage relations of the dark current. The ions tested which have the greatest effect on the light-induced current are sodium and calcium; low sodium decreases the current, while low calcium increases the current. The data strongly support the hypothesis that two systems of electric current exist in the membrane. Thus the total ionic current which flows in the membrane is accounted for as the sum of a dark current and a light-induced current.     

Nonlinear current-voltage relationships in cultured macrophages

Intracellular recordings of cultured mouse thioglycolate-induced peritoneal exudate macrophages reveal that these cells can exhibit two different types of electrophysiological properties characterized by differences in their current-voltage relationships and their resting membrane potentials. The majority of cells had low resting membrane potentials (-20 to -40 mV) and displayed current-voltage relationships that were linear for inward-going current pulses and rectifying for outward-going pulses. Small depolarizing transients, occurring either spontaneously or induced by current pulses, were seen in some cells with low resting membrane potentials. A second smaller group of cells exhibited more hyperpolarized resting membrane potentials (-60 to -90 mV) and S-shaped current-voltage relationships associated with a high- resistance transitional region. Cells with S-shaped current-voltage relationships sometimes exhibited two stable states of membrane potential on either side of the high-resistance transitional region. These data indicate that macrophages exhibit complex electrophysiological properties often associated with excitable cells.     

Inward current generated by Na-Ca exchange during the action potential in single atrial cells of the rabbit.

To investigate the underlying ionic mechanism of the late plateau phase of the action potential in rabbit atrium the whole-cell patch-clamp technique with intracellular perfusion was used. We recorded the inward current during repolarizations following a brief 2 ms depolarizing pulse to +40 mV from a holding potential of between -70 and -80 mV. The development of this current coincides with the onset of the late plateau phase of the action potential. Peak activation of the current occurs about 10 ms from the beginning of the depolarizing pulse, and it decays spontaneously with a slow timecourse. Its voltage dependency from -40 mV to +40 mV shows very steep activation (-40 to -20 mV) and shows almost the same maximum magnitude between -10 mV and +40 mV. This behaviour is quite different from that of the calcium current. The inward current and the late plateau phase of the action potential were both abolished by the application of 5 mM EGTA, 1 microM ryanodine and by reducing the Na+ gradient. The fully activated current-voltage relation of the inward current was plotted as the difference current before and after treatment with Ryanodine, Diltiazem, 20 mM Na+ inside or 30% Na+ outside and shows an exponential voltage dependence with the largest magnitude of the current occurring at negative potentials. The current-voltage (I-V) curve was well fitted by the Na-Ca exchange equation, i = A exp (-(1 - r)EF/RT). The results suggest that the inward current contributes to the generation of the late plateau phase of the rabbit atrial action potential, and is activated by intracellular calcium released from the sarcoplasmic reticulum. Sarcoplasmic reticulum calcium release appears to be triggered both by the membrane voltage and by the calcium current. It is concluded that the inward current is generated by Na-Ca exchange.     

The current-voltage relationships of liposomes and mitochondria.

Current-voltage relationships were determined for various membrane systems. We show that phospholipid and mitochondrial membranes exhibit linear relations between H+ flux and pH gradients. These membranes, however, exhibited non-linear relationships when the applied voltage was a membrane potential. The current-voltage relationship approximated to an exponential function. This relationship was found to be linearized when the membranes were treated with an electrogenic proton ionophore. The incorporation of cytochrome c oxidase (EC 1.9.3.1) was found to have no effect on the current-voltage characteristics of the phospholipid membranes. When a membrane potential of more than 140 mV was imposed across vesicular and mitochondrial membranes, they exhibited reversible di-electric breakdown. This phenomenon was correlated with the requirement of a permeant ion for the experimental demonstration of proton translocation by so-called 'proton pumps'.     

MEMBRANE POTENTIAL OF THE SQUID GIANT AXON DURING CURRENT FLOW

The squid giant axon was placed in a shallow narrow trough and current was sent in at two electrodes in opposite sides of the trough and out at a third electrode several centimeters away. The potential difference across the membrane was measured between an inside fine capillary electrode with its tip in the axoplasm between the pair of polarizing electrodes, and an outside capillary electrode with its tip flush with the surface of one polarizing electrode. The initial transient was roughly exponential at the anode make and damped oscillatory at the sub-threshold cathode make with the action potential arising from the first maximum when threshold was reached. The constant change of membrane potential, after the initial transient, was measured as a function of the total polarizing current and from these data the membrane potential is obtained as a function of the membrane current density. The absolute value of the resting membrane resistance approached at low polarizing currents is about 23 ohm cm.². This low value is considered to be a result of the puncture of the axon. The membrane was found to be an excellent rectifier with a ratio of about one hundred between the high resistance at the anode and the low resistance at the cathode for the current range investigated. On the assumption that the membrane conductance is a measure of its ion permeability, these experiments show an increase of ion permeability under a cathode and a decrease under an anode.     

Modulation of electrical activity of neuron RPa2 ofHelix pomatia

Neuron RPa2 ofHelix pomatia can generate rhythmic (beating) or periodic (bursting) activity. A spontaneous switch from beating to bursting activity takes place in the course of tens of minutes. Similar changes in electrical activity can be induced by the addition of the water-soluble fraction obtained from a homogenate of snail ganglia to the experimental chamber. Artificial polarization of the membrane of neuron RPa2 by asteady inward current leads to an increase in the duration of intervals between bursts and to a decrease in the number of action potentials in the burst. With an increase in amplitude of the polarizing current, action potential generation ceases completely, but generation of waves of membrane potential persists. If the voltage on the neuron membrane is clamped, periodic fluctuations of membrane current disappear. It is suggested that action potential generation by neurons RPa2 is determined by the properties of the potential-dependent conductance of its membrane, i.e., that it is endogenous in origin and can be regulated by compounds acting on the membrane. These compounds, secreted by other neurons, resemble neurotransmitters or neurohormones.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 4, pp. 406–412, July–August, 1981.     

Blocking of Ca-dependent outward current in molluscan neuron somatic membrane by raised intracellular pH

The action of a raised intracellular pH (pH_i) on transmembrane ionic currents was investigated on isolated unidentified neurons ofHelix pomatia under intracellular dialysis and membrane voltage clamping conditions. With a rise in pH_i from 7.3 to 9.0 and in the simultaneous presence of an inward calcium current, the outward potassium current was considerably reduced and the current-voltage characteristic curve was shifted toward more positive membrane potential values. The inward calcium current was practically unchanged in this case. If, however, the calcium current was inhibited by the action of cadmium ions, no decrease in the outward current was observed, only a shift of the I_K(V) curve toward more positive values of membrane potential. It is suggested that an increase in pH_i selectively blocks the Ca-dependent component of the outward potassium current.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 4, pp. 426–430, July–August, 1982.     

Influence of intercellular clefts on potential and current distribution in a multifiber preparation.

A theoretical model is presented for current and voltage clamp of multifiber bundles in a double sucrose gap. Attention is focused on methodological errors introduced by the intercellular cleft resistance. The bundle is approximated by a continuous geometry. Voltage distribution, as a function of radial distance and time, is defined by a parabolic partial differential equation which is specified for different membrane characteristics. Assuming a linear membrane, analytical solutions are given for current step and voltage step conditions. The theoretical relations (based on Bessel functions) may be used to calculate membrane conductance and capacity from experimental clamp data. The case of a nonlinear membrane with standard Hodgkin-Huxley kinetics for excitatory Na current is treated assuming maximum Na conductances (gNa) of 120, 10, and 1 mmho/cm2. Numerical simulations are presented for potential and current distribution in a bundle of 60 microns diameter during depolarizing voltage steps. Adequate voltage control is restricted to the peripheral fibers of the bundle whereas the membrane potential of the inner fibers deviates from the command level during early inward current, tending to the Na equilibrium potential. In the peak current-voltage diagram the loss of voltage control is reflected by an increased steepness of the negative region and a decreased slope conductance of the positive region. With gNa = 120 mmho/cm2, the positive slope conductance is approximately 25% of the slope expected from ideal space clamping. With the lower values of gNa, the slope conductance ratio is in the order of 50%. Implications of the results for an experimental voltage clamp analysis of early inward current on multifiber preparations are discussed.     

Voltage- and time-dependent action of histrionicotoxin on the endplate current of the frog muscle

Histrionicotoxin, a toxin isolated from skin secretions of a Colombian arrow poison frog, Dendrobates histrionicus, decreased the amplitude and time-course of the endplate current, and altered the voltage dependence of the half-decay time. In addition, the toxin produced a characteristic nonlinearity in the current-voltage relationship of the endplate current when 3-s voltage conditioning steps were used. Reduction in time of the conditioning steps to 10 ms made the current-voltage relationship linear. The decrease in peak amplitude of the endplate current (epc) produced by histrionicotoxin measured during long hyperpolarizing conditioning steps was fitted by a single exponential function. The calculated rate constants ranged from 0.03 to 0.14 s-1 and varied with membrane potential at hyperpolarizing levels. The voltage- and time-dependent action of histrionicotoxin does not require an initial activation of receptors by acetylcholine (ACh). The characteristic of the current-voltage relationship can be accounted for by the observed voltage and time dependency of the attenuation of the endplate current amplitude in the presence of histrionicotoxin during long conditioning steps. These effects of histrionicotoxin on the peak amplitude, and on the voltage and time dependence of the epc were concentration-dependent and slowly reversible upon washing out the toxin. Thus, the voltage- and time-dependent action of histrionicotoxin at the endplate is related to an increase in the affinity between the toxin and the ACh receptor-ionic channel complex. This increase in affinity is postulated to be due to a conformational change of the macromolecule in the presence of histrionicotoxin which is demonstrated to be relatively slow, i.e., on the order of tens of seconds.     

Nonselective cation channels in intact human T lymphocytes.

Nonselective cation channels were found in single channel recordings from cell-attached patches on human T lymphocytes. These channels were active under conditions that should lead to cell swelling (hypotonic bath solutions with NaCl or KCl); however, a definite dependence of activity on cell swelling has not been proven. Under these conditions similar channels were found in 20 of 23 patches from 11 different blood donors. The current-voltage relation was approximately linear for outward current (11-14 pS) and inwardly rectifying (to 23 pS) when the intact cells were depolarized with high KCl in the bath. The voltage dependence of channel activity is consistent with closing at hyperpolarized membrane potentials (Vm less than or equal to -50 mV) and block of open channels at strongly depolarized membrane potentials (Vm greater than 0 mV). Reversal potentials under all ionic gradients tested are consistent with a channel that is poorly selective between Na+ and K+ ions. Active channels in cell-attached patches were rapidly blocked by bath addition of the membrane-permeant inhibitor quinine. Channels that were active in cell-attached became quiescent after patch excision; however, two patches remained active long enough to obtain current-voltage relations. These were linear with a slope conductance for outward current of 8-11 pS. Because of the clustering of single-channel openings, detailed voltage dependence of kinetics and probability of opening were not studied.     

Repolarization currents in embryonic chick atrial heart cell aggregates.

Outward membrane currents in aggregates of atrial cells prepared from 7-12-d-old chick embryonic hearts were measured with the two microelectrode voltage-clamp technique. Two outward current components, Ix1 and Ix2, were found in the plateau potential range of the action potential. The Ix1 component is activated between -50 and -20 mV; the Ix2 component is activated between -15 and +20 mV. The Ix1 component inwardly rectifies, whereas Ix2 has an approximately linear current-voltage relation. These preparations lack a time-dependent pacemaker current component, even though they beat spontaneously with an interbeat interval of approximately 1 s. A mathematical model of electrical activity is described based on our measurements of time-dependent outward current, and measurements in the literature of inward current components.     

Ionic mechanisms of the transmembrane current evoked by injection of cyclic amp into identified Helix pomatia neurons

Ionic mechanisms of the transmembrane current evoked by injection of cyclic AMP into identified neurons ofHelix pomatia were investigated by the voltage clamp method. Injection of cyclic AMP into neurons RPa3, LPa2, LPa3, and LPl1 was shown to cause the development of a two-component transmembrane (cyclic AMP) current. The current-voltage characteristic curve of the early component is linear in the region from –40 to –90 mV; the reversal potential of the early component, determined by extrapolation, lies between –5 and +20 mV; the current-voltage characteristic curve of the late component also is linear and has a reversal potential between –55 and –60 mV. A decrease in the sodium concentration in the external medium from 100 to 25 mM led to a decrease in amplitude of the cyclic AMP current and to a shift of the reversal potential for the early component by 30–32 mV toward hyperpolarization. It is suggested that the early component of the cyclic AMP current in neurons RPa3, LPa2, LPa3, and LPl1 is associated with an increase in permeability of the neuron membrane chiefly for sodium ions, whereas the late component is correspondingly connected with permeability for potassium ions. Injection of cyclic AMP also caused the appearance of a transmembrane inward current in neuron LPa8, but it was independent of the holding potential and was unaccompanied by any change in membrane permeability. It is suggested that this current may be due to a change in the activity of the electrogenic ion pump.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 5, pp. 526–532, September–October, 1980.     

Reaction of tetraethylammonium with the open and closed conformations of the acetylcholine receptor ionic channel complex

The effect of tetraethylammonium (TEA) bromide on the neurally and iontophoretically evoked endplate current (EPC) of frog sartorius muscle was investigated using voltage-clamp and noise analysis techniques, and its binding to the acetylcholine (ACh) receptor ionic channel complex was determined on the electric organ of Torpedo ocellata. TEA (250-500 microM) produced an initial enhancement followed by a slow decline in the amplitude of the endplate potential and EPC, but caused only depression in the amplitude of the miniature endplate potential and current. In normal ringer's solution, the EPC current-voltage relationship was approximately linear, and the decay phase varied exponentially with membrane potential. Upon addition of 50-100 microM TEA, the current-voltage relationship became markedly nonlinear at hyperpolarized command potentials, and with 250-2000 microM TEA, there was an initial linear segment, an intermediate nonlinear segment, and a region of negative conductance. The onset of nonlinearity was dose-dependent, undergoing a 50 mV shift for a 10-fold increase in TEA concentration. The EPC decay phase was shortened by TEA at hyperpolarized but not depolarized potentials, and remained a single expotential function of time at all concentrations and membrane potentials examined. These actions of TEA were found to be independent of the sequence of polarizations, the length of the conditioning pulse, and the level of the initial holding potential. TEA shifted the power spectrum of ACh noise to higher frequencies and produced a significant depression of single channel conductance. The shortening in the mean channel lifetime agreed closely with the decrease in the EPC decay time constant. At the concentrations tested, TEA did not alter the EPC reversal potential, nor the resting membrane potential, and had little effect on the action potential duration. TEA inhibited the binding of both [3H] ACh (Ki = 200 microM) and [3H]perhydrohistrionicotoxin (Ki = 280 microM) to receptor-rich membranes from the electric organ of Torpedo ocellata, and inhibited the carbamylcholine-activated 22Na+ efflux from these microsacs. It is suggested that TEA reacts with the nicotinic ACh-receptor as well as its ion channel; the voltage-dependent actions are associated with blockade of the ion channel. The results are compatible with a kinetic model in which TEA first binds to the closed conformation of the receptor-ionicchannel complex to produce a voltage-depdndent depression of endplate conductance and sudsequently to its open conformation, giving rise to the shortening in the EPC decay and mean channel lifetime.     

Changes in intracellular pH affect calcium currents in Paramecium caudatum

The relation between intracellular pH and membrane excitability was studied in the holotrich ciliate Paramecium caudatum. Intracellular pH (pHi) was measured with recessed-tip ion-sensitive microelectrodes (Thomas 1974) and electrical properties were examined by current stimulation and conventional two-electrode voltage clamp. Under normal conditions the resting pHi of Paramecium was 6.80 +/- 0.05. Intracellular alkalinization enhanced the early Ca current, while internal acidification depressed the Ca current. Both effects occurred in a voltage-independent manner. The late outward current was relatively unaffected by these alterations. Results obtained with replacement of extracellular Ca2+ by Ba2+ also support a direct effect of pHi on current through the Ca channel. Intracellular alkalinization to pH 7.15 converted graded, quasi-regenerative Ca responses elicited by injected current pulses into all-or-none action potentials. This change to all-or-none behaviour is presumed to be due to the increase in Ca current and a consequent change in the balance of inward and outward currents. Extracellular pH changes had little effect on pHi, resting membrane potential or the current-voltage relations. The intracellular pH was also independent of shifts in membrane potential. The results are consistent with a model in which Ca channel permeability is blocked by intracellular protonation of a single titratable site having an apparent dissociation constant of 6.2.     

Voltage noise influences action potential duration in cardiac myocytes

Stochastic gating of ion channels introduces noise to membrane currents in cardiac muscle cells (myocytes). Since membrane currents drive membrane potential, noise thereby influences action potential duration (APD) in myocytes. To assess the influence of noise on APD, membrane potential is in this study formulated as a stochastic process known as a diffusion process, which describes both the current-voltage relationship and voltage noise. In this framework, the response of APD voltage noise and the dependence of response on the shape of the current-voltage relationship can be characterized analytically. We find that in response to an increase in noise level, action potential in a canine ventricular myocytes is typically prolonged and that distribution of APDs becomes more skewed towards long APDs, which may lead to an increased frequency of early after-depolarization formation. This is a novel mechanism by which voltage noise may influence APD. The results are in good agreement with those obtained from more biophysically-detailed mathematical models, and increased voltage noise (due to gating noise) may partially underlie an increased incidence of early after-depolarizations in heart failure.     

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.     

Characterization of the inward-rectifying potassium current in cat ventricular myocytes

Whole-cell membrane currents were measured in isolated cat ventricular myocytes using a suction-electrode voltage-clamp technique. An inward-rectifying current was identified that exhibited a time-dependent activation. The peak current appeared to have a linear voltage dependence at membrane potentials negative to the reversal potential. Inward current was sensitive to K channel blockers. In addition, varying the extracellular K+ concentration caused changes in the reversal potential and slope conductance expected for a K+ current. The voltage dependence of the chord conductance exhibited a sigmoidal relationship, increasing at more negative membrane potentials. Increasing the extracellular K+ concentration increased the maximal level of conductance and caused a shift in the relationship that was directly proportional to the change in reversal potential. Activation of the current followed a monoexponential time course, and the time constant of activation exhibited a monoexponential dependence on membrane potential. Increasing the extracellular K+ concentration caused a shift of this relationship that was directly proportional to the change in reversal potential. Inactivation of inward current became evident at more negative potentials, resulting in a negative slope region of the steady state current-voltage relationship between -140 and -180 mV. Steady state inactivation exhibited a sigmoidal voltage dependence, and recovery from inactivation followed a monoexponential time course. Removing extracellular Na+ caused a decrease in the slope of the steady state current-voltage relationship at potentials negative to -140 mV, as well as a decrease of the conductance of inward current. It was concluded that this current was IK1, the inward-rectifying K+ current found in multicellular cardiac preparations. The K+ and voltage sensitivity of IK1 activation resembled that found for the inward-rectifying K+ currents in frog skeletal muscle and various egg cell preparations. Inactivation of IK1 in isolated ventricular myocytes was viewed as being the result of two processes: the first involves a voltage-dependent change in conductance; the second involves depletion of K+ from extracellular spaces. The voltage-dependent component of inactivation was associated with the presence of extracellular Na+.     

Modeling the membrane potential generation of bacteriorhodopsin

The archaeon Halobacterium salinarum can grow phototrophically with only light as its energy source. It uses the retinal containing and light-driven proton pump bacteriorhodopsin to enhance the membrane potential which drives the ATP synthase. Therefore, a model of the membrane potential generation of bacteriorhodopsin is of central importance to the development of a mathematical model of the bioenergetics of H. salinarum. To measure the current produced by bacteriorhodopsin at different light intensities and clamped voltages, we expressed the gene in Xenopus laevis oocytes. We present current-voltage measurements and a mathematical model of the current-voltage relationship of bacteriorhodopsin and its generation of the membrane potential. The model consists of three intermediate states, the BR, L, and M states, and comparisons between model predictions and experimental data show that the L to M reaction must be inhibited by the membrane potential. The model is not able to fit the current-voltage measurements when only the M to BR phase is membrane potential dependent, while it is able to do so when either only the L to M reaction or both reactions (L to M and M to BR) are membrane potential dependent. We also show that a decay term is necessary for modeling the rate of change of the membrane potential.     

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.