Showing papers on "Conductance published in 1986"

Four-terminal phase-coherent conductance.

Abstract: A conductance formula for a sample of arbitrary shape with four terminals is derived to describe transport in the limit where carriers can traverse the sample without suffering phase-destroying events. The Onsager-Casimir symmetry relations are deduced. Experiments measure an off-diagonal Onsager coefficient and the magnetoconductance of such a sample is asymmetric even in the presence of an Aharonov-Bohm flux only. Symmetry relations between conductance measurements which exchange the role of current and voltage leads are predicted.

Aharonov-Bohm effect in normal metal quantum coherence and transport

Abstract: We review some of the recent surprising theoretical and experimental results obtained on the transport properties of small disordered metal samples. Even in the presence of disorder, the quantum mechanical interference of electron wavefunctions can still be observed. The Aharonov-Bohm effect is a particularly clear demonstration of this. In doubly connected structures (such as loops of wire) threaded by a magnetic flux, the electrical conductance oscillates because of the Aharonov-Bohm effect. In fact, because the electron trajectories are diffusive (i.e. random walks), even a lone wire (a singly connected structure) will exhibit a random pattern of conductance fluctuations as a function of the magnetic field because of the same interference effects. All that is required for the observation of these interferences is that the electrons retain ‘phase memory’ duing the period of transit through the sample. The length over which memory is maintained (the phase coherence length) can be much larger tha...

Ionic channels formed by Staphylococcus aureus alpha-toxin: Voltage-dependent inhibition by divalent and trivalent cations

Abstract: The interaction of Staphylococcus aureus alpha-toxin with planar lipid membranes results in the formation of ionic channels whose conductance can be directly measured in voltage-clamp experiments. Single-channel conductance depends linearly on the solution conductivity suggesting that the pores are filled with aqueous solution; a rough diameter of 11.4 +/- 0.4 A can be estimated for the pore. The conductance depends asymmetrically on voltage and it is slightly anion selective at pH 7.0, which implies that the channels are asymmetrically oriented into the bilayer and that ion motion is restricted at least in a region of the pore. The pores are usually open in a KCl solution but undergo a dose- and voltage-dependent inactivation in the presence of di- and trivalent cations, which is mediated by open-closed fluctuations at the single-channel level. Hill plots indicate that each channel can bind two to three inactivating cations. The inhibiting efficiency follows the sequence Zn2+ greater than Tb3+ greater than Ca2+ greater than Mg2+ greater than Ba2+, suggesting that carboxyl groups of the protein may be involved in the binding step. A voltage-gated inactivation mechanism is proposed which involves the binding of two polyvalent cations to the channel, one in the open and one in the closed configuration, and which can explain voltage, dose and time dependence of the inactivation.

Single cyclic GMP-activated channel activity in excised patches of rod outer segment membrane.

Abstract: The plasma membrane of retinal rod outer segments contains a cyclic GMP-activated conductance1–6 which appears to be the light-sensitive conductance involved in phototransduction7. Recently, it has been found that this conductance is partially blocked by Mg2+ (refs 3, 8, 9) and Ca2+ (refs 3, 4, 8–11) at physiological concentrations, thus possibly accounting for the absence of observable single-channel activity in excised membrane patches1,3,8 and for the unusually small apparent unit conductance deduced from noise measurements on intact cells4,10,12,13. We now report that, as expected from this idea, single cGMP-activated channel activity can be detected from an excised rod membrane patch in the absence of divalent cations. The most prominent unitary current had a mean conductance of ∼25 pS. Both individual channel openings (mean open time ∼1 ms) and short bursts of openings (mean burst duration of about a few milliseconds) were observed. In addition, there were smaller events which probably represented other states of the conductance. The mean current increased with the third power of cGMP concentration, suggesting that there are at least three cGMP-binding sites on the channel molecule. With 0.2 mM Mg2+ in the cGMP-containing solution, a flickering block of the open channel was observed; the effect of Ca2+ was similar. The results resolve a puzzle about the light-sensitive conductance by demonstrating that it is an aqueous pore rather than a carrier.

Cyclic GMP-sensitive conductance of retinal rods consists of aqueous pores

Abstract: The surface membrane of retinal rod and cone outer segments contains a cation-selective conductance which is activated by 3',5'-cyclic guanosine monophosphate (cGMP). Reduction of this conductance by a light-induced decrease in the cytoplasmic concentration of cGMP appears to generate the electrical response to light, but little is known about the molecular nature of the conductance. The estimated unitary conductance is so small that ion transport might occur via either a carrier or a pore mechanism. Here we report recordings of cGMP-activated single-channel currents from excised rod outer segment patches bathed in solutions low in divalent cations. Two elementary conductances, of approximately 24 and 8 pS, were observed. These conductances are too large to be accounted for by carrier transport, indicating that the cGMP-activated conductance consists of aqueous pores. The dependence of the channel activation on the concentration of cGMP suggests that opening of the pore is triggered by cooperative binding of at least three cGMP molecules.

High potassium conductance in astrocyte endfeet

Abstract: The distribution of potassium conductance over the surface of freshly dissociated salamander astrocytes was determined by monitoring cell depolarizations evoked by focal increases in the extracellular potassium concentration. The specific potassium conductance of the endfoot processes of these cells was approximately tenfold higher than the conductance of other cell regions. This dramatically nonuniform conductance distribution may play an important role in the regulation of extracellular potassium levels by glia in the brain.

Active Transmission Channels and Universal Conductance Fluctuations

Abstract: The transport through a segment of a disordered system is determined by the eigenvalues of a large random matrix. The effectively independent active transmission channels are associated with these eigenvalues which are closest to unity. A decreasing number of those survives when the system's length increases. They determine the conductance and its fluctuations, which are found to be independent, within broad limits, of the size, disorder and nature of the system. This universality is due to the strong correlations in the spectra of large random matrices, providing a new insight on and generalizing the extremely interesting recent results of Altschuler, Lee and Stone.

Bulk electron traps in zinc oxide varistors

Abstract: Three types of commercial zinc oxide varistor materials were examined using admittance spectroscopy between 30 and 350 K. Maxima in the ac conductance at frequencies from 1 to 100 kHz are observed. The maxima are interpreted as arising from electron traps located within the depletion regions of double Schottky barriers at ZnO‐ZnO grain boundaries. Two traps are observed in each material and are likely to be from common origin. The trap energies are found to be 0.17 and 0.33 eV below the conduction‐band edge. The traps are likely to be associated with native defects in ZnO and may influence device characteristics such as voltage overshoot.

Determination of gap state density in polycrystalline silicon by field‐effect conductance

Abstract: We have obtained the density of states (DOS) in polycrystalline silicon from the analysis of the field‐effect conductance. The DOS exhibits a U‐shaped distribution with an exponential band tail. The method is very sensitive and accounts for the effect of film morphology and differences in device processing (e.g., post‐hydrogenation).

A large anion-selective channel has seven conductance levels

Abstract: Ion channels have generally been found to have two predominant conductance levels thought to be associated with 'open' and 'closed' states, but intermediate (subconductance) states have also been reported. We have now found that a large conductance, anion-selective channel in pulmonary alveolar epithelial cells can adopt any of six open levels of conductance that are integer multiples of 60-70 pS. The channel is usually either fully open or fully closed. The frequencies of the different conductance levels are inconsistent with the notion that there are six independent channels. We suggest that the channel consists of six conducting pathways in parallel, 'co-channels', with a shared gating mechanism that can synchronously render all of them non-conducting. Other channels with lower maximum conductance may operate in a similar way but multiple conductance levels would not easily be detected because of a less favourable signal-to-noise ratio.

Electrical properties of the light-sensitive conductance of rods of the salamander Ambystoma tigrinum.

Abstract: The light-sensitive conductance of isolated rods from the retina of the tiger salamander was studied using a voltage-clamp method. The membrane current of the outer segment was collected with a suction electrode while the internal voltage was measured and controlled with a pair of intracellular electrodes. Saturating light blocked the outer segment current at all potentials, the residual conductance usually becoming less than 20 pS. This suggests that light-sensitive channels comprise the main ionic conductance in the surface membrane of the outer segment. Current-voltage relations determined 10-40 ms after changing the voltage showed outward-going rectification, the outward current increasing e-fold for a depolarization of 11-14 mV. The reversal potential of the light-sensitive current was estimated as 5 +/- 4 mV. This is consistent with other evidence indicating that the channel is not exclusively permeable to Na. Applying steady light, lowering external Ca, or changing the intracellular voltage to a new steady level scaled the light-sensitive current without altering the reversal potential or the form of the rectification. This suggests that all three manipulations change the number of channels in the conducting state without changing the ionic concentration gradients or the mechanism of permeation through an 'open' channel. Hyperpolarizing voltage steps slowly increased the light-sensitive current and depolarizing steps reduced it. A gating variable Y expressing the fractional activation of the light-sensitive conductance in the steady state was derived from the ratio of the instantaneous and steady-state currents. Y declined at voltages positive to -100 mV and usually reached a minimum near 0 mV, with a secondary rise positive to 0 mV. Around the dark voltage Y changed e-fold in roughly 25 mV. The voltage-dependent gating in (6). appeared to involve two delays similar in magnitude to those of the four principal delays in the rod's response to a dim flash. Steady background light shortened the time-scale of gating and flash responses to a similar degree. Clamping the voltage at the dark level had little effect on the photocurrent evoked by a flash. The small, delayed effect actually observed is explained by the slow voltage-dependent gating of the light-sensitive conductance. Hyperpolarization had little effect on the kinetics of the response to a flash, but depolarization slowed the response, causing it to reach a larger, later peak. Depolarization also prolonged the blockage of the light-sensitive current after a saturating flash.(ABSTRACT TRUNCATED AT 400 WORDS)

A patch-clamp study of histamine-secreting cells.

Abstract: The ionic conductances in rat basophilic leukemia cells (RBL-2H3) and rat peritoneal mast cells were investigated using the patch-clamp technique. These two cell types were found to have different electrophysiological properties in the resting state. The only significant conductance of RBL-2H3 cells was a K+-selective inward rectifier. The single channel conductance at room temperature increased from 2-3 pS at 2.8 mM external K+ to 26 pS at 130 mM K+. This conductance, which appeared to determine the resting potential, could be blocked by Na+ and Ba2+ in a voltage-dependent manner. Rat peritoneal mast cells had a whole-cell conductance of only 10-30 pS, and the resting potential was close to zero. Sometimes discrete openings of channels were observed in the whole-cell configuration. When the Ca2+ concentration on the cytoplasmic side of the membrane was elevated, two types of channels with poor ion specificity appeared. A cation channel, observed at a Ca2+ concentration of approximately 1 microM, had a unit conductance of 30 pS. The other channel, activated at several hundred micromolar Ca2+, was anion selective and had a unit conductance of approximately 380 pS in normal Ringer solution and a bell-shaped voltage dependence. Antigenic stimulation did not cause significant changes in the ionic conductances in either cell type, which suggests that these cells use a mechanism different from ionic currents in stimulus-secretion coupling.

Voltage-dependent channel formation by rods of helical polypeptides.

Abstract: The voltage-dependence of channel formation by alamethicin and its natural analogues can be described by a dipole flip-flop gating model, based on electric field-induced transbilayer orientational movements of single molecules. These field-induced changes in orientation result from the large permanent dipole moment of alamethicin, which adopts α-helical conformation in hydrophobic medium. It was, therefore, supposed that the only structural requirement for voltage-dependent formation of alamethicin-type channels might be a rigid lipophilic helical segment of minimum length. In order to test this hypothesis we synthesized a family of lipophilic polypeptides—Boc-(Ala-Aib-Ala-Aib-Ala) n -OMe,n=1–4—which adopt α-helical conformation forn=2–4 and studied their interaction with planar lipid bilayers. Surprisingly, despite their large difference in chain length, all four polypeptides showed qualitatively similar behavior. At low field strength of the membrane electric field these polypeptides induce a significant, almost voltage-independent increase of the bilayer conductivity. At high field strength, however, a strongly voltage-dependent conductance increase occurs similar to that observed with alamethicin. It results from the opening of a multitude of ion translocating channels within the membrane phase. The steady-state voltage-dependent conductance depends on the 8th–9th power of polypeptide concentration and involves the transfer of 4–5 formal elementary charges. From the power dependences on polypeptide concentration and applied voltage of the time constants in voltage-jump current-relaxation experiments, it is concluded that channels could be formed from preexisting dodecamer aggregates by the simultaneous reorientation of six formal elementary charges. Channels exhibit large conductance values of several nS, which become larger towards shorter polypeptide chain length. A mean channel diameter of 19 A is estimated corresponding roughly to the lumen diameter of a barrel comprised of 10 α-helical staves. Similar to experiments with the N-terminal Boc-derivative of alamethicin we did not observe the burst sequence of nonintegral conductance steps typical of natural (N-terminal Ac-Aib)-alamethicin. Saturation in current/voltage curves as well as current inactivation in voltage-jump current-relaxation experiments are found. This may be understood by assuming that channels are generated as dodecamers but, while reaching the steady state, reduce their size to that of an octamer or nonamer. We conclude that the overall behavior of these synthetic polypeptides is very similar to that of alamethicin. They exhibit the same concentration and voltage-dependences but lack the stabilizing principle of resolved channel states characteristic of alamethicin.

Direct observation of ensemble averaging of the Aharonov-Bohm effect in normal-metal loops.

Abstract: Aharonov-Bohm magnetoconductance oscillations have been measured in series arrays of 1, 3, 10, and 30 submicron-diameter Ag loops. At constant temperature, the amplitude of the h/e oscillations is observed to decrease as the square root of number of loops, while the amplitude of h/2e conductance oscillations, measured in the same samples, is independent of the number of series loops. This is direct confirmation of the ensemble averaging properties of h/e oscillations in multiloop systems. The amplitude of the h/e oscillations is in good agreement with recent calculations.

Insulation of the conduction pathway of muscle transverse tubule calcium channels from the surface charge of bilayer phospholipid.

Abstract: Functional calcium channels present in purified skeletal muscle transverse tubules were inserted into planar phospholipid bilayers composed of the neutral lipid phosphatidylethanolamine (PE), the negatively charged lipid phosphatidylserine (PS), and mixtures of both. The lengthening of the mean open time and stabilization of single channel fluctuations under constant holding potentials was accomplished by the use of the agonist Bay K8644. It was found that the barium current carried through the channel saturates as a function of the BaCl2 concentration at a maximum current of 0.6 pA (at a holding potential of 0 mV) and a half-saturation value of 40 mM. Under saturation, the slope conductance of the channel is 20 pS at voltages more negative than -50 mV and 13 pS at a holding potential of 0 mV. At barium concentrations above and below the half-saturation point, the open channel currents were independent of the bilayer mole fraction of PS from XPS = 0 (pure PE) to XPS = 1.0 (pure PS). It is shown that in the absence of barium, the calcium channel transports sodium or potassium ions (P Na/PK = 1.4) at saturating rates higher than those for barium alone. The sodium conductance in pure PE bilayers saturates as a function of NaCl concentration, following a curve that can be described as a rectangular hyperbola with a half-saturation value of 200 mM and a maximum conductance of 68 pS (slope conductance at a holding potential of 0 mV). In pure PS bilayers, the sodium conductance is about twice that measured in PE at concentrations below 100 mM NaCl. The maximum channel conductance at high ionic strength is unaffected by the lipid charge. This effect at low ionic strength was analyzed according to J. Bell and C. Miller (1984. Biophysical Journal. 45:279-287) and interpreted as if the conduction pathway of the calcium channel were separated from the bilayer lipid by approximately 20 A. This distance thereby effectively insulates the ion entry to the channel from the bulk of the bilayer lipid surface charge. Current vs. voltage curves measured in NaCl in pure PE and pure PS show that similarly small surface charge effects are present in both inward and outward currents. This suggests that the same conduction insulation is present at both ends of the calcium channel.

Comparison of the light-sensitive and cyclic GMP-sensitive conductances of the rod photoreceptor: noise characteristics

Abstract: Patch-clamp recordings were made from outer segments of single rod photoreceptors isolated from toad retina. Cyclic GMP (cGMP) and its hydrolysis-resistant analog, 8-bromo-cGMP, increased the conductance of excised membrane patches when applied to the intracellular face of the membrane but not when applied to the extracellular face. The rectification properties of the cGMP-dependent conductance depended on the concentration of divalent cations bathing the intra- and extracellular membrane faces. Current flow was favored from the side of the membrane, either internal or external, that was exposed to bathing fluid free of divalent cations. The power spectral density of cGMP- induced noise was fitted by a sum of 2 Lorentzian components, with corner frequencies differing approximately 15-fold on average. This suggests that channel gating is controlled by 2 kinetic components. To compare the cGMP-sensitive conductance with the light-sensitive conductance of the rod, cell-attached patch-clamp recordings were made from intact, dark-adapted rods, and noise arising from the light- sensitive conductance was analyzed. The power spectrum of the light- sensitive noise also exhibited 2 Lorentzian components similar to those of the cGMP-sensitive conductance in excised patches. These results are consistent with the idea that cGMP opens the light-sensitive channel of the rod outer segment.

Analysis of ac Surface Photovoltages in a Depleted Oxidized p-Type Silicon Wafer

Abstract: The majority carrier conductance due to the hole flow towards the surface of a wafer from the bulk has been formulated following the half-sided junction model previously reported An empirical equation for the carrier drift velocities in very high electric fields has been proposed for the formulation Besides the conductance, the depletion layer capacitance, interface trap capacitance and conductance are found to be responsible for ac surface photovoltages in the depletion case The majority carrier conductance can explain the formerly observed conductance of 27 S/m2 in a 76-mm-diameter 10-mΩm oxidized p-type Si wafer Analysis of the formerly reported surface photovoltages reveals a surface potential, fixed oxide charge density, hole capture cross section and interface trap density of 032 V, 17 mC/m2, 20×10-20 m2 and 20×1016 m-2 eV1 respectively

Regional specialization of the membrane of retinal glial cells and its importance to K+ spatial buffering.

Abstract: Electrophysiological experiments, obtained primarily from dissociated salamander cells, demonstrate that the K+ conductance of Muller cells is distributed in a highly nonuniform manner over the cell surface. A large fraction of the total cell conductance is localized to that portion of the endfoot process that faces the vitreous humor. Along the remainder of the cell, specific K+ conductance is larger in the outer plexiform layer than in neighboring regions. High-endfoot conductance directs K+ spatial buffering currents preferentially through the endfoot process, leading to an efficient form of spatial buffering termed K+ siphoning. Preliminary experiments suggest that the endfeet of astrocytes also have high K+ conductance.

Computer simulation for the conductance of a contact interface. II

Abstract: With the remarkable increase in the performance of computers, techniques of computer simulation are widely applied. A computer simulation based on the finite element method and Monte Carlo method is applied to the problem of contact, and the conductance of a global model for contact is studied. The model consists of two unit cubes and a random interface between the cubes. More specifically, the two identical uniform conducting cubes are in contact across a pair of adjacent faces through a layer of thickness 2d subdivided into N x N small subsquares of which a randomly selected area fraction f has the same conductivity as the cubes (a clean-contact state), while the remaining area.fraction (1 - f) represents zero conductivity. The conductance of this global model for contact is computed and compared with previously published results. Particularly, when N, d, and f are changed, the behavior of the global conductance is investigated and discussed. It is clearly shown that because of extreme current concentration through the contact area the conductance does not decrease until f becomes close to zero.

The proton channel, CF0, in thylakoid membranes: Only a low proportion of CF1‐lacking CF0 is active with a high unit conductance (169 fS)

Abstract: We investigated the conductance of pea thylakoid membranes and their capacity for photophosphorylation as function of the extraction of chloroplast coupling factor CF1. The degree of extraction was varied via the incubation time in EDTA-containing hypo-osmolar medium and was measured by rocket electroimmunodiffusion. The conductance of thylakoid membranes was measured by flash kinetic spectrophotometry. The time course of extraction followed the time course of thylakoid swelling. Contrary to expectation increasing loss of CF1 did not primarily increase the velocity of proton efflux from each vesicle. Instead proton-tight vesicles were converted to leaky ones, which lost phosphorylating activity. Two subpopulations occurred, although both types of vesicles, leaky and proton-tight ones, were CF1-depleted to a similar degree. This implied that only a small fraction of CF1-lacking CF0 was functional as a proton channel. Tight vesicles had no functional channels while leaky ones had at least one. We determined the proportion of tight vesicles in three independent ways: via the residual phosphorylation activity, via measurements of proton efflux and via measurements of the electric relaxation across the membrane. The results obtained were identical. A statistical evaluation of the data led us to the following conclusions. (a) EDTA treatment produced vesicles containing approximately 105 chlorophyll molecules, equivalent to a total of approximately 100 CF0CF1 per vesicle. (b) Even at the highest degree of extraction (75% of total CF1 extracted) only 2.5 out of 75 exposed CF0 per vesicle were proton-conducting. (c) The unit conductance of one open CF0 channel was 169 ± 18 fS at pH 7.5 and room temperature. At an electrical driving force of 100 mV this was equivalent to the passage of approximately 105 protons/s. The most important consequence of this relatively high unit conductance was that a single open CF0 channel was capable of dissipating the protonmotive force of one vesicle, thereby deactivating the whole remaining catalytic capacity of this vesicle.

Large cation-selective pores from rat liver peroxisomal membranes incorporated to planar lipid bilayers

Abstract: Fusion of a highly purified fraction of rat liver peroxisomal membranes to planar lipid bilayers incorporates large, cation-selective voltage-dependent pores. TheP K/P Cl ratio of these pores, estimated in KCl gradients, is close to 4. The pores display several conductance states and spend most of the time open at voltages near 0 mV, closing at more positive and negative voltages. At voltages near 0 mV the most frequent open state has a conductance of 2.4 nS in 0.3m KCl. At voltages more positive and more negative than 10 mV the most frequent open state displays a conductance of 1.2 nS in 0.3m KCl. With these results pore diameters of 3 and 1.5 nm, respectively, can be estimated. We suggest that these pores might account for the unusually high permeability of peroxisomes to low molecular weight solutes. Fusion also incorporates a perfectly anion-selective, two-open states channel with conductances of 50 and 100 pS in 0.1m KCl.

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

Abstract: 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.

Capacitance and conductance deep level transient spectroscopy in field‐effect transistors

Abstract: An analysis of conductance transients in field‐effect transistors for small values of drain‐source voltage is presented which enables absolute values of trap concentration to be evaluated. The relationships use parameters which can be easily measured as distinct from the estimated values of mobility profiles used in previously published calculations. Excellent quantitative agreement between capacitance and conductance results on large area gallium arsenide field‐effect transistors has been obtained. In addition, conductance deep level transient studies have demonstrated that the method of measurement and analysis can be used for micron and submicron devices which are much too small for capacitive measurements.

Ion channels activated by light in Limulus ventral photoreceptors

Abstract: The light-activated conductance of Limulus ventral photoreceptors was studied using the patch-clamp technique. Channels (40 pS) were observed whose probability of opening was greatly increased by light. In some cells the latency of channel activation was nearly the same as that of the macroscopic response, while in other cells the channel latency was much greater. Like the macroscopic conductance, channel activity was reduced by light adaptation but enhanced by the intracellular injection of the calcium chelator EGTA. The latter observation indicates that channel activation was not a secondary result of the light-induced rise in intracellular calcium. A two-microelectrode voltage-clamp method was used to measure the voltage dependence of the light-activated macroscopic conductance. It was found that this conductance is constant over a wide voltage range more negative than zero, but it increases markedly at positive voltages. The single channel currents measured over this same voltage range show that the single channel conductance is independent of voltage, but that channel gating properties are dependent on voltage. Both the mean channel open time and the opening rate increase at positive voltages. These properties change in a manner consistent with the voltage dependence of the macroscopic conductance. The broad range of similarities between the macroscopic and single channel currents supports the conclusion that the 40-pS channel that we have observed is the principal channel underlying the response to light in these photoreceptors.