Single potassium channels in membranes of isolated mesophyll barley vacuoles

Summary Voltage-dependent K channels could be identified in on-cell and excised patch-clamp records on membranes of isolated plant cell vacuoles. The current through a membrane patch is dominated by a channel population with a conductance of about 121 pS in symmetrical 250mm KCl solution. The single channel adopts at least two conducting levels the 121-pS state being most frequently observed. The channel shows outward rectification, representing a cation flux into the vacuoles. The rectification appears to be caused by a vanishing open probability and a short channel lifetime at hyperpolarizing voltages. A selectivity ratio of potassium over sodium of about 6 was derived as an estimate. Occasionally, an additional population of K channels with a single-channel conductance of approximately 18 pS is observed. This channel type exhibits outward rectification as well.     

Ion channels in human endothelial cells.

Ion channels were studied in human endothelial cells from umbilical cord by the patch clamp technique in the cell attached mode. Four different types of ion channels were recorded: i) potassium channel current that rectifies at positive potentials in symmetrical potassium solutions (inward rectifier); ii) low-conductance non-selective cation channel with a permeability ratio K:Na:Ca = 1:0.9:0.2; iii) high-conductance cation-selective channel that is about 100 times more permeable for calcium than for sodium or potassium; iv) high-conductance potassium channel with a permeability ratio K:Na = 1:0.05. The extrapolated reversal potential of the inwardly rectifying current was near to the potassium equilibrium potential. The slope conductance decreased from 27 pS in isotonic KCl solution to 7 pS with 5.4 mmol/l KCl and 140 mmol/l NaCl in the pipette but 140 mmol/l KCl in the bath. The low-conductance non-selective cation channel showed a single-channel conductance of 26 pS with 140 mmol/l Na outside, 28 pS with 140 mmol/l K outside, and rectified in inward direction in the presence of Ca (60 mmol/l Ca, 70 mmol/l Na, 2.7 mmol/l K in the pipette) at negative potentials. The current could be observed with either chloride or aspartate as anion. The high-conductance non-selective channel did not discriminate between Na and K. The single-channel conductance was about 50 pS. The extrapolated reversal potential was more positive than +40 mV (140 K or 140 Na with 5 Ca outside). Both the 26 and 50 pS channel showed a run-down, and they rapidly disappeared in excised patches. The high-conductance potassium channel with a single-channel conductance of 170 pS was observed only rarely. It reversed near the expected potassium equilibrium potential. The 26 pS channel could be stimulated with histamine and thrombin from outside in the cell-attached mode. Both the 26 pS as well as the 50 pS channel can mediate calcium flux into the endothelial cell.     

Evidence for a small anion-selective channel in the cell wall of Mycobacterium bovis BCG besides a wide cation-selective pore.

Two channels were observed in extracts of whole Mycobacterium bovis BCG cells using organic solvents and detergents. The channels derived from organic solvent treatment had a single-channel conductance of about 4.0 nS in 1 M KCl in lipid bilayer membranes with properties similar to those of the channels discovered previously in Mycobacterium smegmatis and Mycobacterium chelonae. The channel was in its open configuration only at low transmembrane potentials. At higher voltages it switched to closed states that were almost impermeable for ions. Lipid bilayer experiments in the presence of detergent extracts of whole cells revealed another channel with a single-channel conductance of only 780 pS in 1 M KCl. Our results indicate that the mycolic acid layer of M. bovis BCG contains two channels, one is cation-selective and its permeability properties can be finely controlled by cell wall asymmetry or potentials. The other one is anion-selective, has a rather small single-channel conductance and is voltage-insensitive. The concentration of channel-forming proteins in the cell wall seems to be small, which is in agreement with the low cell wall permeability for hydrophilic solutes.     

Regulation of apical K and Na channels and Na/K pumps in rat cortical collecting tubule by dietary K

The patch-clamp technique was used to study the properties and the density of conducting K and Na channels in the apical membrane of rat cortical collecting tubule. The predominant K channel observed in cell- attached patches (SK channels) had an outward single-channel conductance (with LiCl in the pipette) of 10 pS. The inward conductance (with KCl in the pipette) was 42 pS. The channel had a high open probability that increased with depolarization. Kinetic analysis indicated the presence of a single open state and two closed states. Increasing K intake by maintaining animals on a high K diet for 12-16 d increased the number of SK channels per patch by threefold (0.7- 2.0/patch) over control levels. In addition, conducting Na-selective channels, which were not observed in control animals, were seen at low density (0.5/patch). These channels had properties similar to those observed when the animals were on a low Na diet, except that the mean open probability (0.84) was higher. In other experiments, the whole- cell patch clamp technique was used to measure Na channel activity (as amiloride-sensitive current, INa) and Na pump activity (as ouabain- sensitive current, Ipump). In animals on a high K diet, INa was greater than in controls but much less than in rats on a low Na diet. Ipump was greater after K loading than in controls or Na-depleted animals. These K diet-dependent effects were not accompanied by a significant increase in plasma aldosterone concentrations. To further investigate the relationship between K channel activity and mineralocorticoids, rats were maintained on a low Na diet to increase endogenous aldosterone secretion. Under these conditions, no increase in SK channel density was observed, although there was a large increase in the number of Na channels (to 2.7/patch). Aldosterone was also administered exogenously through osmotic minipumps. As with the low Na diet, there was no change in the density of conducting SK channels, although Na channel activity was induced. These results suggest that SK channels, Na channels and Na/K pumps are regulated during changes in K intake by factors other than aldosterone.     

Inwardly rectifying whole-cell and single-channel K currents in the murine macrophage cell line J774.1

Summary Inward currents in the murine macrophage-like cell line J774.1 were studied using the whole-cell and cell-attached variations of the patch-clamp technique. When cells were bathed in Na Hanks' (KCl=4.5mm, NaCl=145mm), and the electrode contained Na-free K Hanks' (KCl=145mm) single-channel currents were observed at potentials below –40 mV which showed inward rectification, were K-selective, and were blocked by 2.5mm Ba in the pipette. Single-channel conductance was 29 pS, and was proportional to the square root of [K] _o . Channels manifested complex kinetics, with multiple open and closed states. The steady-state open probability of the channel was voltage dependent, and declined from 0.9 to 0.45 between –40 and –140 mV. When hyperpolarizing voltage pulses were repetitively applied in the cell-attached patch mode, averaged single-channel currents showed inactivation. Inactivation of inwardly rectifying whole-cell current was measured in Na Hanks' and in two types of Na-free Hanks': one with a normal K concentration (4.5mm) and the other containing 145mm K. Inactivation was shown to have Na-dependent and Na-independent components. Properties of single-channel current were found to be sufficient to account for the behavior of the macroscopic current, except that single-channel current showed a greater degree of Na-independent inactivation than whole-cell current.     

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.     

Characterization of a calcium-activated potassium channel in human fibroblasts

The cell-attached and inside-out patch clamp techniques were used to record single-channel currents from human epidermal fibroblasts. A large-conductance channel (320 pS in symmetric 140 mM KCl) with high potassium selectivity was observed in many patches, particularly those located at the borders of the cells. The channel exhibited both voltage and calcium sensitivity and, therefore, was regarded as a variety of the large-conductance calcium-activated potassium channels reported in many preparations. Probability density functions, fitted to histograms of open and closed time durations at 35 degrees C, usually displayed a minimum of one open state and two closed states. However, kinetic analysis by the fractal method suggested more complicated behavior, particularly for the closed condition. It was not uncommon to observe several channels in one patch. This was distinguishable from the presence of subconductances, which were also observed. Although this channel could have many roles, it seems likely to mediate the calcium-activated conductance that underlies the hyperpolarizing response of fibroblasts to mechanical, electrical, or chemical stimuli.     

Channel formation by salmon and human calcitonin in black lipid membranes.

In this study we investigated the interaction of salmon and human calcitonin (Ct) with artificial lipid bilayer membranes. Both peptides were able to form either transient or permanent channels in the model membranes. The channels formed by salmon Ct at concentration (125 nM) had, on average, a single-channel conductance of 0.58 +/- 0.04 nS in 1M KCl (+10 mV), which is voltage-dependent at lower voltages. Human Ct forms at the same concentration channels with a much lower probability, and high voltages of up to +150 mV were needed to initiate channel formation. The estimated single-channel conductance formed under these conditions was approximately 0.0119 +/- 0.0003 nS in 1 M KCl. Both salmon and human Ct channels were found to be permeable to calcium ions. The possibility is discussed that the superior therapeutic effect of salmon Ct as a tool to treat bone disorders, including Paget disease, osteoporosis, and hypercalcemia of malignancy, rather than human Ct is related to the lack of the fibrillating property of salmon Ct. Preliminary data indicate that also eel and porcine Ct and carbocalcitonin form channels in model membranes.     

The cell wall porin of Nocardia farcinica: biochemical identification of the channel-forming protein and biophysical characterization of the channel properties

A channel-forming protein was identified in cell wall extracts of the Gram-positive, strictly aerobic bacterium Nocardia farcinica . The cell wall porin was purified to homogeneity and had an apparent molecular mass of about 87 kDa on tricine-containing SDS–PAGE. When the 87 kDa protein was boiled for a longer time in sodium dodecylsulphate (SDS) it dissociated into two subunits with molecular masses of about 19 and 23 kDa. The 87 kDa form of the protein was able to increase the specific conductance of artificial lipid bilayer membranes from phosphatidylcholine (PC) phosphatidylserine (PS) mixtures by the formation of ion-permeable channels. The channels had on average a single-channel conductance of 3.0 nS in 1 M KCl, 10 mM Tris-HCl, pH 8, and were found to be cation selective. Asymmetric addition of the cell wall porin to lipid bilayer membranes resulted in an asymmetric voltage dependence. The single-channel conductance was only moderately dependent on the bulk aqueous KCl concentration, which indicated point charge effects on the channel properties. The analysis of the single-channel conductance data in different salt solutions using the Renkin correction factor, and the effect of negative charges on channel conductance suggested that the diameter of the cell wall porin is about 1.4–1.6 nm. Channel-forming properties of the cell wall porin of N. farcinica were compared with those of mycobacteria and corynebacteria. The cell wall porins of these members of the order Actinomycetales share common features because they form large and water-filled channels that contain negative point charges.     

Cav1.4 encodes a calcium channel with low open probability and unitary conductance

When transiently expressed in tsA-201 cells, Ca(v)1.4 calcium channels support only modest whole-cell currents with unusually slow voltage-dependent inactivation kinetics. To examine the basis for this unique behavior we used cell-attached patch single-channel recordings using 100 mM external barium as the charge carrier to determine the single-channel properties of Ca(v)1.4 and to compare them to those of the Ca(v)1.2. Ca(v)1.4 channel openings occurred infrequently and were of brief duration. Moreover, openings occurred throughout the duration of the test depolarization, indicating that the slow inactivation kinetics observed at the whole-cell level are caused by sustained channel activity. Ca(v)1.4 and Ca(v)1.2 channels displayed similar latencies to first opening. Because of the rare occurrence of events, the probability of opening could not be precisely determined but was estimated to be <0.015 over a voltage range of -20 to +20 mV. The single-channel conductance of Ca(v)1.4 channels was approximately 4 pS compared with approximately 20 pS for Ca(v)1.2 under the same experimental conditions. Additionally, in the absence of divalent cations, Ca(v)1.4 channels pass cesium ions with a single-channel conductance of approximately 21 pS. Although Ca(v)1.2 opening events were best described kinetically with two open time constants, Ca(v)1.4 open times were best described by a single time constant. BayK8644 slightly enhanced the single-channel conductance in addition to increasing the open time constant for Ca(v)1.4 channels by approximately 45% without, however, causing the appearance of an additional slower gating mode. Overall, our data indicate that single Ca(v)1.4 channels support only minute amounts of calcium entry, suggesting that large numbers of these channels are needed to allow for significant whole-cell current activity, and providing a mechanism to reduce noise in the visual system.     

Single-channel recordings from two types of amiloride-sensitive epithelial Na+ channels

We report here the first evidence in intact epithelial cells of unit conductance events from amiloride-sensitive Na+ channels. The events were observed when patch-clamp recordings were made from the apical surface of cultured epithelial kidney cells (A6). Two types of channels were observed: one with a high selectivity to Na+ and one with relatively low selectivity. The characteristics of the low-selectivity channel are as follows: single-channel conductance ranged between 7 and 10 pS (mean = 8.4 +/- 1.3), the current-voltage (I-V) relationship displayed little if any nonlinearity over a range of +/- 80 mV (with respect to the patch pipette) and the channel Na+/K+ selectivity was approximately 3-4:1. Amiloride, a cationic blocker of the channel, reduced channel mean open time and increased channel mean closed times as the voltage of the cell interior was made more negative. Amiloride induced channel flickering at increased negative potentials (intracellular potential with respect to the patch) but did not alter the single-channel conductance or the I-V relationship from that observed in control patches. The characteristics of the high-selectivity channel are: a single-channel conductance of 1-3 pS (mean = 2.8 +/- 1.2), the current-voltage relationship is markedly nonlinear with a Na+/K+ selectivity greater than 20:1. The mean open and closed times for the two types of channels are quite different, the high-selectivity channel being open only about 10% of the time while the low-selectivity channel is open about 30% of the time.     

Ion channels on synaptic vesicle membranes studied by planar lipid bilayer method.

An anion selective channel and three types of cation selective channels were found in planar lipid bilayers incorporating synaptic vesicles from rat brains. In asymmetric KCl solutions (cis: 300 mM/trans: 150 mM), the anion selective channel showed a single-channel conductance of 94 pS and was inactivated by negative voltages and by 4-acetoamido-4'-isothiocyanostilbene-2,2'-disulfonic acid disodium salt (SITS). In the same solution, single-channel conductances of three types of cation selective channels were 250 pS (Type 1), 248 pS (Type 2), and 213 pS (Type 3), respectively. These channels resembled one another in single-channel conductances but were different in gating behaviors. Type 1 channel, which was most frequently observed, had a remarkable subconducting state (175 pS). Type 2 channel had a flickering state that increased as the potential became more positive, and a long inactive state that increased as the potentials were more negative. Type 3 channel, which was also sensitive to the potentials, had the open-channel probability increased as the potential became more positive.     

Two types of outward K+ channel currents in early embryonic chick ventricular myocytes

Outward K+ currents were recorded from 3-day-old embryonic chick ventricular myocytes using the patch clamp method. Two types of macroscopic outward currents were observed, one with rapid activation and de-activation time courses, and the other displaying a slower activation and long-duration tail currents. A time-dependent inactivation at positive potentials was a feature of the rapidly-activating current, allowing resolution of an early outward current. Single K+ channel currents were recorded using the outside-out patch technique. Two classes of K+ channels, which may contribute to the macroscopic currents, were differentiated on the basis of their conductances and kinetics. One class (ca 20 pS conductance) showed a rapid activation upon depolarization, and the other class (ca 60 pS) had a more delayed activation. A time-dependent inactivation of the rapid-activating, single-channel K+ current was also recorded. The two types of K+ channels contribute outward current during the plateau and promote the repolarization of the action potential, and the slowly de-activating K+ current may also be involved in the electrogenesis of automaticity observed in some of these cells.     

The cell wall porin of the gram-positive bacterium Nocardia asteroides forms cation-selective channels that exhibit asymmetric voltage dependence

Detergent-solubilized cell wall extracts of the gram-positive, strictly aerobic bacterium Nocardia asteroides contain channel-forming activity as judged from reconstitution experiments using lipid bilayer membranes. The cell wall porin was identified as a protein with an apparent molecular mass of about 84 kDa based on SDS-PAGE. The porin was purified to homogeneity using preparative SDS-PAGE. The 84-kDa protein was no longer observed after heating in SDS buffer. The presumed dissociation products were not observed on SDS-polyacrylamide gels. The cell wall porin increased the specific conductance of artificial lipid bilayer membranes from phosphatidylcholine/phosphatidylserine mixtures by the formation of cation-selective channels, which had an average single-channel conductance of 3.0 nS in 1 M KCl. The single-channel conductance was only moderately dependent on the bulk aqueous KCl concentration, which indicated negative point charge effects on the channel properties. The analysis of the concentration dependence of the single-channel conductance using the effect of negative charges on channel conductance suggested that the diameter of the cell wall channel is about 1.4 nm. Asymmetric addition of the cell wall porin to lipid bilayer membranes resulted in an asymmetric voltage dependence. The cell wall channel switched into substates, when the cis side of the membrane, the side of the addition of the protein, had negative polarity. Positive potentials at the cis side had no influence on the conductance of the cell wall channel. Received: 23 September 1998 / Accepted: 9 December 1998     

The role of specific surface loop regions in determining the function of the imipenem-specific pore protein OprD of Pseudomonas aeruginosa.

Pseudomonas aeruginosa OprD is a specific porin which facilitates the uptake of basic amino acids and imipenem across the outer membrane. In this study, we examined the effects of deletions in six of the proposed eight surface loops of OprD on the in vivo and in vitro functions of this protein. Native OprD formed very small channels in planar lipid bilayers, with an average single-channel conductance in 1.0 M KCl of 20 pS. When large numbers of OprD channels were incorporated into lipid bilayer membranes, addition of increasing concentrations of imipenem to the bathing solutions resulted in a progressive blocking of the membrane conductance of KCl, indicating the presence of a specific binding site(s) for imipenem in the OprD channel. From these experiments, the concentration of imipenem value of resulting in 50% inhibition of the initial conductance was calculated as approximately 0.6 microM. In contrast, no decrease in channel conductance was observed for the OprDdeltaL2 channel upon addition of up to 2.4 microM imipenem, confirming that external loop 2 was involved in imipenem binding. Deletion of four to eight amino acids from loops 1 and 6 had no effect on antibiotic susceptibility, whereas deletion of eight amino acids from loops 5, 7, and 8 resulted in supersusceptibility to beta-lactams, quinolones, chloramphenicol, and tetracycline. Planar lipid bilayer analysis indicated that the OprDdeltaL5 channel had a 33-fold increase in single-channel conductance in 1 M KCl but had retained its imipenem binding site. The disposition of these loop regions in the interior of the OprD channel is discussed.     

TcdA1 of Photorhabdus luminescens: Electrophysiological Analysis of Pore Formation and Effector Binding

Tc toxins are widely distributed among different gram-negative and gram-positive bacteria, where they act as pathogenicity factors. The toxins are composed of different components that form oligomers for biological activity. Lipid bilayer experiments were performed with the TcdA1 component of the Tc toxin from Photorhabdus luminescens, which preferentially kills insects by actin polymerization. TcdA1 was able to increase the specific conductance of artificial lipid bilayer membranes by the formation of ion-permeable channels. The channels had on average a single-channel conductance of 125 pS in 150 mM KCl and were found to be cation selective. The single-channel conductance of the TcdA1-channels was only moderately dependent on the bulk aqueous KCl concentration, which indicated point-charge effects on the channel properties. Experiments to study the voltage dependence of the TcdA1 channel demonstrated that it is reconstituted in a fully oriented way when it is added to only one side of the lipid bilayer membrane. A combination of biologically active components (TccC3) and a possible chaperone (TcdB2) blocked the TcdA1-mediated conductance efficiently in a dose-dependent manner when they were added to the cis side of the membrane. The half-saturation constant for binding of TcdB2-TccC3 to TcdA1 is in the low nanomolar range.     

Bordetella pertussis major outer membrane porin protein forms small, anion-selective channels in lipid bilayer membranes.

The major outer membrane protein of molecular weight 40,000 (the 40K protein) of a virulent isolate of Bordetella pertussis was purified to apparent homogeneity. The purified protein formed an oligomer band (of apparent molecular weight 90,000) on sodium dodecyl sulfate-polyacrylamide gels after solubilization at low temperatures. The porin function of this protein was characterized by the black lipid bilayer method. The 40K protein formed channels smaller than all other constitutive major outer membrane porins studied to date. The average single-channel conductance in 1 M KCl was 0.56 nS. This was less than a third of the conductance previously observed for Escherichia coli porins. Zero-current potential measurements made of the porin to determine its ion selectivity revealed the porin to be more than 100-fold selective for anions over cations. The single-channel conductance was measured as a function of salt concentration. The data could be fitted to a Lineweaver-Burk plot suggesting an anion binding site with a Kd of 1.17 M Cl- and a maximum possible conductance through the channel of 1.28 nS.     

haemolysin of Escherichia coli: Comparison of pore-forming properties between chromosome and plasmid-encoded haemolysins

Abstract Lipid bilayer experiments were performed with chromosome-encoded haemolysin of Escherichia coli . The addition of the toxin to the aqueous phase bathing lipid bilayer membranes of asolectin resulted in the formation of transient ion-permeable channels with two states at small transmembrane voltages. One is prestate (single-channel conductance 40 pS in 0.15 M KCl) of the open state, which had a single-channel conductance of 420 pS in 0.15 M KCl and a mean lifetime of 30 s. Membranes formed of pure lipids were rather inactive targets for this haemolysin. Experiments with different salts suggested that the haemolysin channel was highly cation-selective at neutral pH. The mobility sequence of the cations in the channel was similar if not identical to their mobility sequence in the aqueous phase. The single-channel data were consistent with a wide, water-filled channel with an estimated minimal diameter of about 1 nm. The pore-forming properties of chromosome-encoded haemolysin were compared with those of plasmid-encoded haemolysin. Both toxins share common features, oligomerize probably to form pores in lipid bilayer membranes. Both types of haemolysin channels have similar properties but different lifetimes.     

Ion channels activated by osmotic and mechanical stress in membranes of opossum kidney cells

Summary For patch-clamp measurements cultured kidney (OK) cells were exposed to osmotic and mechanical stress. Superfusion of a cell in whole cell configuration with hypotonic media (190 mOsm) evokes strong depolarization, which is reversible by returning to the isotonic bath medium. In the cell-attached configuration the exposure to hypotonic media evokes up to six ion channels of homogeneous single-channel properties in the membrane patch. Subsequently, the channels became activated after a time lag of a few seconds. At an applied membrane potential of 0 mV, the corresponding membrane current is directed inward and shows a transient behavior in the time range of minutes. In the same membrane patch these ion channels can be activated by application of negative hydrostatic pressure. The channel has a single-channel conductance of about 22 pS and is permeable to Na⁺ and K⁺ as well as to Cl^–. It is suggested that volume regulation involves mechanoreceptor-operated ion channels.     

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.