Conduction and selectivity in potassium channels

A large anion-selective channel has seven conductance levels

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.

Kinase C activator 1,2-oleoylacetylglycerol attenuates voltage-dependent calcium current in sensory neurons

The diacylglycerol analogue 1,2-oleoylacetylglycerol (OAG) and the phorbol ester 12-deoxyphorbol 13-isobutyrate (DPB) were tested for their effects on the voltage-dependent calcium (Ca) current in embryonic chicken dorsal root ganglion neurons in vitro. OAG (0.6-60 microM) and DPB (0.01-50 microM) produced reversible decreases in Ca current. Neither drug affected resting membrane conductance, the voltage-dependent potassium current, or the Ca current-voltage relationship. The concentrations of OAG and DPB that reduced Ca current correlate well with those concentrations that have been shown, in other systems, to activate protein kinase C-dependent phosphorylation. The time course for OAG action on Ca current is also consistent with an involvement of kinase C. Incubation of dorsal root ganglion cells in 60 microM OAG prevented further reductions in Ca current by either 50 microM DPB or 10 microM norepinephrine, a known modulator of the voltage-dependent Ca channel in these cells. This evidence suggests that protein kinase C may play a role in modulating Ca channel function.

Intracellular calcium regulates basolateral potassium channels in a chloride-secreting epithelium

The two individual cell membranes of epithelia are functionally coupled, so that changes in apical membrane conductance are paralleled by changes in basolateral K+ conductance. However, the signal that regulates basolateral K+ conductance, thereby coupling the two membranes, is unknown. We tested the hypothesis that the cellular calcium concentration, [Ca2+]c, may regulate basolateral K+ conductance in canine tracheal epithelium, a Cl- -secreting epithelium that shows marked membrane coupling. Three findings support the hypothesis. First, the intracellular Ca2+ antagonist 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) attenuated the secretory response. Second, the secretagogue epinephrine increased [Ca2+]c, as measured with quin-2. Third, we found a K+ channel that was activated by Ca2+ on the cytosolic side of the membrane. Thus, cytosolic Ca2+ regulates the basolateral K+ conductance and may be the signal responsible for functional coupling of the two cell membranes.

Hormonal activation of single K+ channels via internal messenger in isolated pancreatic acinar cells

The mechanism underlying hormonal activation of potassium channels was investigated in pig pancreatic acinar cells by patch-clamp single-channel and whole-cell current recordings. It was shown directly that a peptide hormone belonging to the cholecystokinin-gastrin family, CCK5, can activate single voltage-sensitive potassium channels which can be blocked by tetraethylammonium. The single-channel currents were recorded from electrically isolated cell-attached membrane patches to which the hormone had no access and the activation must therefore involve an intracellular messenger. The hormonal response requires external Ca2+ in the isolated membrane-patch area indicating that calcium gating is not directly linked to hormone-receptor interaction.

Single-channel activity of bilayers derived from sea urchin sperm plasma membranes at the tip of a patch-clamp electrode

Changes in the plasma membrane permeability of echinoderm sperm play a fundamental role in the acrosome reaction. During the reaction there is an increase in intracellular Ca2+ and Na+ and an efflux of H+ and K+. We have formed bilayers at the tip of patch pipets from a mixture of lipid vesicles and sea urchin sperm plasma membranes (12-50 microgram protein/ml). We observed three types of K+ channels (conductances: 22, 46, and 82 pS), two of which are partially blocked by TEA, and one Cl- channel (148 pS). The presence of K+ channels in sperm plasma membranes is consistent with the inhibition by TEA of the acrosome reaction in whole sperm and the membrane potential change that occurs during the reaction.

Electrophysiological studies of sodium cotransport in epithelia: toward a cellular model

During the past two decades, microelectrophysiological studies of small intestine and renal proximal tubule employing conventional as well as ion-selective microelectrodes have contributed significantly to our understanding of the nature of Na-coupled entry processes at the apical membrane as well as the overall workings of the simple model illustrated in FIGURE 1. These studies have unequivocally established the rheogenic and conductive nature of the Na-coupled sugar and amino-acid entry processes across the apical membrane of small intestine (and renal proximal tubule) and have, in addition, disclosed that the properties of the basolateral membrane respond to an increase in Na-coupled solute entry with an increase in the ability of the Na-K pump to extrude Na with little or no change in (Na)c32 and a parallel increase in the conductance of that barrier to K. Although these responses may be "triggered" by cell swelling, it is unclear how a cell "recognizes" minimal swelling and how this recognition, in turn, culminates in the observed changes in basolateral membrane pump-leak properties. Clearly, these findings have brought us to the interfaces between cell physiology and cell and molecular biology and have raised a number of intriguing questions that focus on the more global question: How do epithelial cells work?

Single channel recordings obtained from basolateral membranes of isolated rabbit enterocytes

Epithelial cells isolated by hyaluronidase incubation from rabbit small intestine were used to explore the presence of ionic channels by the patch-clamp method. Recordings were made from cell-attached or excised patches of basolateral membrane. Evidence was obtained for the presence of at least two kinds of channels conducting potassium currents. One of these can be shown to be activated by Ca2+.

Interactions of amiloride and other blocking cations with the apical Na channel in the toad urinary bladder

A simple model of the action of amiloride to block apical Na channels in the toad urinary bladder was tested. According to the model, the positively charged form of the drug binds to a site in the lumen of the channel within the electric field of the membrane. In agreement with the predictions of the model: (1) The voltage dependence of amiloride block was consistent with the assumption of a single amiloride binding site, at which about 15% of the transmembrane voltage is sensed, over a voltage range of +/- 160 mV. (2) The time course of the development of voltage dependence was consistent with that predicted from the rate constants for amiloride binding previously determined. (3) The ability of organic cations to mimic the action of amiloride showed a size dependence implying a restriction of access to the binding site, with an effective diameter of about 5 angstroms. In a fourth test, divalent cations (Ca, Mg, Ba and Sr) were found to block Na channels with a complex voltage dependence, suggesting that these ions interact with two or more sites, at least one of which may be within the lumen of the pore.

K+ and Cl- conductances in the apical membrane from secreting oxyntic cells are concurrently inhibited by divalent cations

This study concerns the properties of rapid K+ and Cl transport pathways that are present in the (H+ + K+)-ATPase membrane from stimulated, and secreting, gastric oxyntic cells. Ion permeabilities in the isolated stimulation-associated vesicles were monitored via the rates of H+ efflux under conditions of exclusive H+/K+ counterflux or H+ - Cl co-efflux, as well as by comparison of equilibration rates for 86Rb and 36Cl under conditions of equilibrium exchange and unidirectional salt flux. These latter studies suggest that Rb+ and Cl pathways are conductive and independent. In spite of the functional independence of the ion pathways, several divalent cations inhibit Rb+ and Cl isotopic exchange as well as the H+ efflux that is dependent on either K+ or anion (Cl, SCN, NO2) fluxes. Zn2+ is the more potent inhibitor, reducing by 50% the sensitive component of K+, Cl, and NO2 fluxes at about 20 microM; Mn2+ has a similar effect at 200 microM. Ni2+ and Co2+ were roughly equipotent to Mn2+ while Mg2+ and Ca2+ had no inhibitory effect. These results suggest that the stimulation-induced permeabilities, while functioning independently, may be physically linked, i.e., residing within a single entity. In similar studies carried out in (H+ + K+)-ATPase vesicles obtained from nonstimulated cells, no vestiges of sensitivity to the inhibitory divalent cations could be detected. The implications of these findings for the physiology of the oxyntic cell in the context of a model for membrane fusion are discussed.

Effect of phospholipid surface charge on the conductance and gating of a Ca2+-activated K+ channel in planar lipid bilayers

A Ca-activated, K-selective channel from plasma membrane of rat skeletal muscle was studied in artificial lipid bilayers formed from either phosphatidylethanolamine (PE) or phosphatidylserine (PS). In PE, the single-channel conductance exhibited a complex dependence on symmetrical K+ concentration that could not be described by simple Michaelis-Menten saturation. At low K+ concentrations the channel conductance was higher in PS membranes, but approached the same conductance observed in PE above 0.4 m KCl. At the same Ca2+ concentration and voltage, the probability of channel opening was significantly greater in PS than PE. The differences in the conduction and gating, observed in the two lipids, can be explained by the negative surface charge of PS compared to the neutral PE membrane. Model calculations of the expected concentrations of K+ and Ca2+ at various distances from a PS membrane surface, using Gouy-Chapman-Stern theory, suggest that the K+-conduction and Ca2+-activation sites sense a similar fraction of the surface potential, equivalent to the local electrostatic potential at a distance of 9 A from the surface.

Delayed rectification in the calf cardiac Purkinje fiber. Evidence for multiple state kinetics

We have investigated the delayed rectifier current (Ix) in the calf cardiac Purkinje fiber using a conventional two-microelectrode voltage clamp arrangement. The deactivation of Ix was monitored by studying decaying current tails after the application of depolarizing voltage prepulses. The reversal potential (Vrev) of these Ix tails was measured as a function of prepulse magnitude and duration to test for possible permeant ion accumulation- or depletion-induced changes in Vrev. We found that prepulse-induced changes in Vrev were less than 5 mV, provided that prepulse durations were less than or equal to 3.5 s and magnitudes were less than or equal to +35 mV. We kept voltage pulse structures within these limits for the remainder of the experiments in this study. We studied the sensitivity of Vrev to variation in extracellular K+. The reversal potential for Ix is well described by a Goldman-Hodgkin-Katz relation for a channel permeable to Na+ and K+ with PNa/PK = 0.02. The deactivation of Ix was always found to be biexponential and the two components shared a common reversal potential. These results suggest that it is not necessary to postulate the existence of two populations of channels to account for the time course of the Ix tails. Rather, our results can quantitatively be reproduced by a model in which the Ix channel can exist in three (two closed, one open) conformational states connected by voltage dependent rate constants.

On the potassium conductance increased by opioids in rat locus coeruleus neurones

Intracellular recordings were made from locus coeruleus neurones in slices cut from rat pons and superfused in vitro. Membrane currents were recorded with a single-electrode switch-clamp amplifier. Opioids, enkephalin analogues or morphine, caused a concentration-dependent potassium current, which had a maximum value of about 300 pA at -60 mV. The opioid-sensitive potassium conductance was independent of membrane potential between -60 and -130 mV, but became less as the membrane potential was changed from -60 to -30 mV. The opioid outward current was reduced by quinine (100 microM-1 mM) and barium (30 microM-2 mM), but not by 4-aminopyridine (100 microM-1 mM) or tetraethylammonium (10 mM). A potassium current with similar properties flowed for several seconds after a burst of action potentials; this appeared to result from calcium entering the neurone during the action potentials. The alpha 2-adrenoceptor agonists noradrenaline and clonidine caused a concentration-dependent potassium conductance increase which had the same maximum value as that caused by opioids in the same neurones. Experiments in which an opioid and an alpha 2-adrenoceptor agonist were superfused together indicated that the same potassium conductance is increased by both agonists.

Modulation of single Ca2+-dependent K+-channel activity by protein phosphorylation

There is considerable evidence that cyclic AMP can modulate the electrical activity of excitable cells and that protein phosphorylation by the catalytic subunit (CS) of cAMP-dependent protein kinase is a necessary step in these modulatory effects. In analogy to alterations in enzyme activities following phosphorylation, it seems possible that direct phosphorylation of ion-channel proteins may alter their gating properties, giving rise to the observe changes in electrical activity. However, the results obtained so far do not indicate whether it is ion channels themselves that are phosphorylated, or whether phosphorylation is simply an early step in some cascade of events which leads ultimately to modulation of channel activity. The development of single-channel recording techniques has provided a way to investigate this question. Here we describe effects of CS on the activity of individual CA2+-dependent K+ channels from the nervous system of the land snail Helix measured in isolated membrane patches and in artificial phospholipid bilayers. The results demonstrate that cAMP-dependent protein phosphorylation produces long-lasting changes in the activity of individual channels, and indicate that the relevant phosphorylation site is closely associated with the channel.

Quinine inhibits Ca2+-independent K+ channels whereas tetraethylammonium inhibits Ca2+-activated K+ channels in insulin-secreting cells

The effects of quinine and tetraethylammonium (TEA) on single-channel K+ currents recorded from excised membrane patches of the insulin-secreting cell line RINm5F were investigated. When 100 microM quinine was applied to the external membrane surface K+ current flow through inward rectifier channels was abolished, while a separate voltage-activated high-conductance K+ channel was not significantly affected. On the other hand, 2 mM TEA abolished current flow through voltage-activated high-conductance K+ channels without influencing the inward rectifier K+ channel. Quinine is therefore not a specific inhibitor of Ca2+-activated K+ channels, but instead a good blocker of the Ca2+-independent K+ inward rectifier channel whereas TEA specifically inhibits the high-conductance voltage-activated K+ channel which is also Ca2+-activated.

ATP-sensitive inward rectifier and voltage- and calcium-activated K+ channels in cultured pancreatic islet cells

K+ channels in cultured rat pancreatic islet cells have been studied using patch-clamp single-channel recording techniques in cell-attached and excised inside-out and outside-out membrane patches. Three different K+-selective channels have been found. Two inward rectifier K+ channels with slope conductances of about 4 and 17 pS recorded under quasi-physiological cation gradients (Na+ outside, K+ inside) and maximal conductances recorded in symmetrical K+-rich solutions of about 30 and 75 pS, respectively. A voltage- and calcium-activated K+ channel was recorded with a slope conductance of about 90 pS under the same conditions and a maximal conductance recorded in symmetrical K+-rich solutions of about 250 pS. Single-channel current recording in the cell-attached conformation revealed a continuous low level of activity in an apparently small number of both the inward rectifier K+ channels. But when membrane patches were excised from the intact cell a much larger number of inward rectifier K+ channels became transiently activated before showing an irreversible decline. In excised patches opening and closing of both the inward rectifier K+ channels were unaffected by voltage, internal Ca2+ or externally applied tetraethylammonium (TEA) but the probability of opening of both inward rectifier K+ channels was reduced by internally applied 1-5 mM adenosine-5'-triphosphate (ATP). The large K+ channel was not operational in cell-attached membrane patches, but in excised patches it could be activated at negative membrane potentials by 10(-7) to 10(-6) M internal Ca2+ and blocked by 5-10 mM external TEA.

Ba2+ uptake and the inhibition by Ba2+ of K+ flux into rat liver mitochondria

Rapid uptake of Ba2+ by respiring rat liver mitochondria is accompanied by a transient stimulation of respiration. Following accumulation of Ba2+, e.g. at a concentration of 120 nmol per mg protein, the mitochondria exhibit reduced rates of state 3 and uncoupler-stimulated respiration. ADP-stimulated respiration is inhibited at a lower concentration of Ba2+ than is required to affect uncoupler-stimulated respiration, suggesting a distinct effect of Ba2+ on mechanisms involved in synthesis of ATP. Ba2+, which has an ionic radius similar to that of K+, inhibits unidirectional K+ flux into respiring rat liver mitochondria. This effect on K+ influx is observable at concentrations of Ba2+, e.g. 23 to 37 nmol per mg protein, which cause no significant change in state 4 or uncoupler-stimulated respiration. The accumulated Ba2+ decreases the measured Vmax of K+ influx, while having little effect on the apparent Km for K+. The inhibition of K+ influx by Ba2+ is seen in the presence and absence of mersalyl, an activator of K+ influx. In contrast, under the conditions studied, Ba2+ has no apparent effect on the rate of unidirectional K+ efflux. These data are consistent with the idea that K+ may enter and leave mitochondria via separate mechanisms.

Properties of adenosine-triphosphate-regulated potassium channels in guinea-pig ventricular cells

A class of K channels in cardiac muscle is reversibly blocked by intracellular adenosine 5'-triphosphate (ATP). The characteristics of this K channel were studied by recording single-channel currents in ventricular cells isolated enzymatically from guinea-pig heart. The reversal potential of single-channel currents agreed well with the K equilibrium potential. Blockers of other K channels, such as tetraethylammonium and 4-aminopyridine, decreased the mean open time of the channel. The chord conductance increased as the 0.24th power of the K concentration on the outer surface of the membrane, and showed a marked inward-going rectification on strong depolarizations. The degree of rectification was larger with increasing Na concentration on the inner side of the membrane. The kinetics of the channel were almost voltage independent, but depended on the concentration of intracellular ATP. The conductance of the channel was not affected by ATP. When channel kinetics were examined in the presence of ATP, the distribution of open times and closed times was fitted well with a sum of two exponential components. When ATP concentration was increased, the time constants obtained from the open-time histogram decreased and those from the closed-time histogram increased, resulting in a decrease of the open-state probability. The channel was blocked by ATP, adenosine 5'-diphosphate,5'-adenylylimidodiphosphate, guanosine 5'-triphosphate and uridine 5'-triphosphate, but not by adenosine 5'-monophosphate, creatine phosphate, creatine or adenosine. Plots of the open-state probability versus the ATP concentration revealed Michaelis-Menten saturation kinetics with strong co-operativity of multiple receptor sites (Hill coefficient 3-4, concentration of half-saturation 0.5 mM). It was concluded that this K channel has three or four receptor sites selective for triphosphate nucleotide on the inner surface of the membrane, and that the channel is blocked through the binding of agonists to the receptors.

Action of tetraethylammonium on calcium-activated potassium channels in pig pancreatic acinar cells studied by patch-clamp single-channel and whole-cell current recording

The effects of tetraethylammonium ions on currents through high-conductance voltage- and Ca2+-activated K+ channels have been studied with the help of patch-clamp single-channel and whole-cell current recording on pig pancreatic acinar cells. In excised outside-out membrane patches TEA (1 to 2 mM) added to the bath solution virtually abolishes unitary current activity except at very positive membrane potentials when unitary currents corresponding to a markedly reduced conductance are observed. TEA in a lower concentration (0.2 mM) markedly reduces the open-state probability and causes some reduction of the single-channel conductance. In inside-out membrane patches bath application of TEA in concentrations up to 2 mM has no effect on single-channel currents. At a higher concentration (10 mM) slight reductions in single-channel conductance occur. In whole-cell current recording experiments TEA (1 to 2 mM) added to the bath solution completely suppresses the outward currents associated with depolarizing voltage jumps to membrane potentials of 0 mV and blocks the major part (70 to 90%) of the outward currents even at very positive membrane potentials (30 to 40 mV). In contrast TEA (2 mM) added to the cell interior (pipette solution) has no effect on the outward K+ current. Our results demonstrate that TEA in low concentrations (1 to 2 mM) acts specifically on the outside of the plasma membrane to block current through the high-conductance Ca2+- and voltage-activated K+ channels.

The effect of phenylalanine on the electrical properties of proximal tubule cells in the frog kidney

The present study was designed to elucidate the effects of sodium-coupled transport on the electrical properties of proximal tubule cells in the isolated perfused frog kidney. Cable analysis techniques have been employed to determine the resistance of the luminal and peritubular cell membranes in parallel (Rm) and the apparent ratio of the luminal over the peritubular cell membrane resistance (VDR). Furthermore, the sensitivity of the potential difference across the peritubular cell membrane (PDpt) to 6-fold increases of peritubular potassium concentration (delta PDk) was taken as a measure of the relative potassium conductance of this membrane. In the absence of luminal phenylalanine, PDpt amounts to -60 +/- 1 mV (n = 90), Rm to 36 +/- 3 k omega cm (n = 22), VDR to 1.81 +/- 0.14 (n = 20), and delta PDk to 15.0 +/- 0.9 mV (n = 25). The application of 10 mmol/l phenylalanine replacing 10 mmol/l raffinose leads to a rapid (within 30 s) depolarisation of PDpt to 50 +/- 5% of its control value and to a delayed (within 12 min) recovery to 95 +/- 5% of control. The rapid depolarisation is associated with a decline of Rm and VDR, indicating a decrease mainly of the luminal cell membrane resistance. During recovery of PDpt there is a parallel increase of VDR and a further decline of Rm pointing to a decline of the basolateral cell membrane resistance. Delta PDk is decreased during rapid depolarisation but increases again during the recovery phase. Thus, phenylalanine initially decreases but then increases above control the apparent potassium conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Ouabain decreases apparent potassium-conductance in proximal tubules of the amphibian kidney

According to a previous study from this laboratory, the electrochemical gradient for potassium across the peritubular cell membrane of proximal tubules in the isolated perfused frog kidney increases following the application of ouabain. In order to test, if this phenomenon were due to a decrease of potassium conductance, the effects of ouabain on cell membrane resistances and the sensitivity of the peritubular cell membrane potential difference (PDpt) to step changes of peritubular potassium and bicarbonate concentration were studied. In the absence of ouabain, PDpt averaged -60 +/- 3 mV (n = 25). A step increase of peritubular potassium concentration from 3 to 18 mmol/l (pH 8.07) depolarizes PDpt (delta PDk) by +24 +/- mV (n = 8). An increase of bicarbonate from 20 to 40 mmol/l (pH 8.07) hyperpolarizes PDpt (delta PDb) by -2.8 +/- 0.4 mV (n = 9). The resistance of the luminal and peritubular cell membranes in parallel (Rm) amounts to 45 +/- 9 k omega cm (tubule length) (n = 4) and the voltage divider ratio (VDR) to 1.4 +/- 0.2 (n = 7). The resistance of the cellular cable (cellular core, Rc) approaches 131 +/- 37 M omega/cm (n = 4). Peritubular application of 0.1 mmol/l ouabain leads to a gradual decline of PDpt (t1/2 approx. 30 min), to an increase of Rm, a decrease of delta PDk and an increase of delta PDb. VDR and Rc are not changed significantly. The data point to a functional link between the sodium/potassium ATPase and the potassium conductance of the peritubular cell membrane.

An ion's view of the potassium channel. The structure of the permeation pathway as sensed by a variety of blocking ions

We have studied the block of potassium channels in voltage-clamped squid giant axons by nine organic and alkali cations, in order to learn how the channel selects among entering ions. When added to the internal solution, all of the ions blocked the channels, with inside-positive voltages enhancing the block. Cesium blocked the channels from the outside as well, with inside-negative voltages favoring block. We compared the depths to which different ions entered the channel by estimating the "apparent electrical distance" to the blocking site. Simulations with a three-barrier, double-occupancy model showed that the "apparent electrical distance," expressed as a fraction of the total transmembrane voltage, appears to be less than the actual value if the blocking ion can pass completely through the channel. These calculations strengthen our conclusion that sodium and cesium block at sites further into the channel than those occupied by lithium and the organic blockers. Our results, considered together with earlier work, demonstrate that the depth to which an ion can readily penetrate into the potassium channel depends both on its size and on the specific chemical groups on its molecular surface. The addition of hydroxyl groups to alkyl chains on a quaternary ammonium ion can both decrease the strength of binding and allow deeper penetration into the channel. For alkali cations, the degree of hydration is probably crucial in determining how far an ion penetrates. Lithium, the most strongly hydrated, appeared not to penetrate as far as sodium and cesium. Our data suggest that there are, minimally, four ion binding sites in the permeation pathway of the potassium channel, with simultaneous occupancy of at least two.

Clonal sublines of rat neurotumor RT4 and cell differentiation. V. Comparison of Na+ influx, Rb+ efflux, and action potential among stem-cell, neuronal, and glial cell types

A multipotential stem-cell-type cell line (RT4-AC) isolated from a rat peripheral neurotumor differentiates in culture into two neuronal-type cells (RT4-B and RT4-E) or into a glial-type cell (RT4-D). The neuronal classification of RT4-B and RT4-E cells is based on their positive response to veratridine in the tetrodotoxin-sensitive Na+-influx and Rb+-efflux assays and on the action potential observed upon hyperpolarized stimulation. In addition, these neuronal cell types do not synthesize two glial proteins, S100 protein (S100P) and glial fibrillary acidic protein (GFAP). The glial classification of RT4-D is based on the syntheses of S100P and GFAP. Additionally, RT4-D does not display veratridine-activated Na+ influx and Rb+ efflux nor action potential. The stem cell type, RT4-AC, expresses both neuronal and glial properties to a lesser degree. In the neuronal-type cell lines of the RT4 family (RT4-B and RT4-E), the large veratridine-activated Na+ influx can further be stimulated by scorpion toxin. The Na+ influx of the stem cell (RT4-AC), however, is only slightly stimulated by veratridine alone, but greatly stimulated by the addition of veratridine and scorpion toxin. These observations suggest that a progressive differentiation of voltage-dependent Na+ channels may have occurred by the cell-type conversion from the stem cell type to the neuronal cell types. The exact nature of the change in Na+ channels is currently not known.

Single voltage-dependent and outward rectifying K+-channels in isolated rat heart cells

Studies on single K+-channel currents recorded from isolated rat heart muscle cells, in which early repolarization is known to be exceptionally fast, are reported here. A K+-channel which is blocked by TEA (tetraethylammonium) from the inside only has been found. The total open time of the channel, measured in steady-state after activation, indicated outward rectifying properties. The single channel conductance increases with depolarization from 25 pS at -70 mV to 75 pS at + 70 mV. Selectivity of the channel has also been measured and it was found that only Rb+ and K+ can permeate the channel, whereas the permeability (P) for Li+, Na+, Cl-, Mg2+, and Ca2+ is less than 0.05 times PK+. Ba2+ and CS+ block the channel activity. These results clearly demonstrate the existence of K+-selective outward rectifying conductance pathways in rat ventricular myocytes.

Quinidine blockage of K+ channels in the basolateral membrane of larval bullfrog skin

The skin of frog larvae (Rana catesbeiana) was used to study the characteristics of basolateral K+ channels with fluctuation analysis. K2SO4 and Na2SO4 Ringer's were used as mucosal and serosal solution, respectively. After addition of Nystatin (138 U/ml) the transepithelial conductance and short-circuit current (Isc) increased considerably. Most of Isc was carried by K+, moving from the mucosal to the serosal side. This current could be depressed by quinidine, added to both compartments or to mucosal side only. Fluctuation analysis showed that quinidine induced a Lorentzian component in the power density spectrum. Assuming pseudo-first order kinetics for the channel occlusion by quinidine the current through the open K+ channel and channel density were calculated: iK = 0.22 pA, M = 7.7 channels/microns2.

Influence of lead ions on cation permeability in human red cell ghosts

Intracellular Pb2+ ions can replace Ca2+ ions in stimulating the Ca-dependent K permeability of human red blood cells. In metabolically depleted resealed ghosts, the threshold for stimulation of 86Rb efflux by internal Pb2+ is around 5 X 10(-10) M, and stimulation is half-maximal at about 2 X 10(-9) M, and maximal at 10(-8) M Pb2+. There is no effect on 22Na efflux in this concentration range. 86Rb efflux is antagonized by internal Mg2+ ions, and by the channel-blocking drugs quinidine and diS-C2(5), as observed for the Ca-dependent K permeability in red cells. In ghosts containing EDTA, which prevents any internal effects of Pb2+ ions, external Pb2+ increases both 22Na and 86Rb permeability when its concentration exceeds 6 X 10(-7) M. This effect is seemingly unrelated to the Ca-dependent K permeability. This work makes extensive use of Pb2+ ion buffers, and gives information about their preparation and properties.

Single-file diffusion through the Ca2+-activated K+ channel of human red cells

The ratio between the unidirectional fluxes through the Ca2+-activated K+-specific ion channel of the human red cell membrane has been determined as a function of the driving force (Vm-EK). Net effluxes and 42K influxes were determined during an initial period of approximately 90 sec on cells which had been depleted of ATP and loaded with Ca. The cells were suspended in buffer-free salt solutions in the presence of 20 microM of the protonophore CCCP, monitoring in this way changes in membrane potential as changes in extracellular pH. (Vm-EK) was varied at constant EK by varying the Nernst potential and the conductance of the anion and the conductance of the potassium ion. In another series of experiments EK was varied by suspending cells in salt solutions with different K+ concentrations. At high extracellular K+ concentrations both of the unidirectional fluxes were determined as 42K in- and effluxes in pairs of parallel experiments. Within a range of (Vm-EK) of -6 to 90 mV the ratio between the unidirectional fluxes deviated strongly from the values predicted by Ussing's flux ratio equation. The Ca2+-activated K+ channel of the human red cell membrane showed single-file diffusion with a flux ratio exponent n of 2.7. The magnitude of n was independent of the driving force (Vm-EK), independent of Vm and independent of the conductance gK.

High-conductance K+ channel in pancreatic islet cells can be activated and inactivated by internal calcium

The Ca2+-activated K+ channel in rat pancreatic islet cells has been studied using patch-clamp single-channel current recording in excised inside-out and outside-out membrane patches. In membrane patches exposed to quasi-physiological cation gradients (Na+ outside, K+ inside) large outward current steps were observed when the membrane was depolarized. The single-channel current voltage (I/V) relationship showed outward rectification and the null potential was more negative than -40 mV. In symmetrical K+-rich solutions the single-channel I/V relationship was linear, the null potential was 0 mV and the single-channel conductance was about 250 pS. Membrane depolarization evoked channel opening also when the inside of the membrane was exposed to a Ca2+-free solution containing 2mM EGTA, but large positive membrane potentials (70 to 80 mV) were required in order to obtain open-state probabilities (P) above 0.1. Raising the free Ca2+ concentration in contact with the membrane inside ( [Ca2+]i) to 1.5 X 10(-7) M had little effect on the relationship between membrane potential and P. When [Ca2+]i was increased to 3 X 10(-7) M and 6 X 10(-7) M smaller potential changes were required to open the channels. Increasing [Ca2+]i further to 8 X 10(-7) M again activated the channels, but the relationship between membrane potential and P was complex. Changing the membrane potential from -50 mV to +20 mV increased P from near 0 to 0.6 but further polarization to +50 mV decreased P to about 0.2. The pattern of voltage activation and inactivation was even more pronounced at [Ca2+]i = 1 and 2 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

The mechanism of action of Ba2+ and TEA on single Ca2+-activated K+ -channels in arterial and intestinal smooth muscle cell membranes

The interaction of Ba2+ and TEA with Ca2+-activated K+ channels was studied in isolated membrane patches of cells from longitudinal jejunal smooth muscle of rabbit and from guinea-pig small mesenteric artery (100 micron external diameter). Ba2+ applied from the inside of the membrane did not reduce unit current, except at high concentrations, but channels failed to open for long periods (s). This effect became much stronger when the potential gradient was in a direction driving Ba2+ into the channel and was reduced by increasing K+ ion concentration on the outside of the membrane. These results are consistent with Ba2+ entering the open channel and blocking at a site most of the way through the channel bore. In contrast, TEA and procaine dose-dependently reduced unit current amplitude at all patch potentials and slightly increased mean open time. Their effects were not detectably voltage-dependent and could be explained by TEA and procaine blocking the open channel with a timecourse that was faster than the frequency response of the recording system. The lack of appreciable voltage-dependence suggests that TEA and procaine bind to a site near to the inner mouth of the channel.

Charybdotoxin, a protein inhibitor of single Ca2+-activated K+ channels from mammalian skeletal muscle

The recent development of techniques for recording currents through single ionic channels has led to the identification of a K+-specific channel that is activated by cytoplasmic Ca2+. The channel has complex properties, being activated by depolarizing voltages and having a voltage-sensitivity that is modulated by cytoplasmic Ca2+ levels. The conduction behaviour of the channel is also unusual, its high ionic selectivity being displayed simultaneously with a very high unitary conductance. Very little is known about the biochemistry of this channel, largely due to the lack of a suitable ligand for use as a biochemical probe for the channel. We describe here a protein inhibitor of single Ca2+-activated K+ channels of mammalian skeletal muscle. This inhibitor, a minor component of the venom of the Israeli scorpion, Leiurus quinquestriatus, reversibly blocks the large Ca2+-activated K+ channel in a simple biomolecular reaction. We have partially purified the active component, a basic protein of relative molecular mass (Mr) approximately 7,000.

Catecholamine inhibition of calcium action potentials in rat locus coeruleus neurones

Intracellular recordings were made from neurones in the nucleus locus coeruleus in a slice of tissue cut from the rat pons. Clonidine (100 nM-10 microM), noradrenaline (10 microM-1 mM) and adrenaline (10 microM-1 mM) all reduced the duration of the spontaneously occurring action potential of the neurones. This effect was also observed on the action potential in the presence of tetrodotoxin, which results from calcium entering the cell. These concentrations of clonidine, noradrenaline and adrenaline always hyperpolarized the membrane. This hyperpolarization was prevented by two procedures which block potassium currents--intracellular caesium and extracellular barium. In conditions of potassium current blockade, noradrenaline (100 microM-1 mM) and adrenaline (20 microM-1 mM) shortened the calcium action potential but clonidine was ineffective even at 10 microM. Adrenaline and noradrenaline also suppressed inward calcium and barium currents measured under voltage clamp. This action of noradrenaline and adrenaline was not prevented by yohimbine (10 microM), propranolol (20 microM) or prazosin (1 microM); it was reduced by a concentration of phentolamine about 100 times higher than its Ke for alpha 2-adrenoceptors on locus coeruleus neurones. It is concluded that noradrenaline and adrenaline can directly inhibit calcium action potentials in locus coeruleus neurones when applied in high concentrations, but that this does not involve an alpha 2-adrenoceptor.

Basolateral membrane responses to transport modifiers in the frog skin epithelium

Application of transepithelial square voltage pulses to the frog skin leads to responses in the transepithelial current and intracellular potential which include transient components. Determinations at 600 ms allow for meaningful estimates of basolateral membrane responses to transport modifiers. Oxytocin produced a large and sustained increase in the amiloride-inhibitable short circuit current (Im) which was accompanied by a large increase of both apical and basolateral membrane conductance (ga and gb, respectively). While Im and ga increased nearly simultaneously, gb started to increase several minutes after the increase in the two other parameters. Insulin also increased Im, ga and gb. As with oxytocin, the increases in Im and ga often preceded the changes in gb. Ouabain reduced Im and ga. The effects on gb were more complex, since sometimes the inhibition of Im was first accompanied by an increase followed by a decrease while in other instances only minor changes in conductance could be observed. The currently available information regarding the control of cytoplasmic [Ca2+] and the effects of Ca2+ on cell membrane properties are used to construct a model in which changes in cytoplasmic [Ca2+] account for the observed behavior of the basolateral membrane.

Single chloride-selective channels active at resting membrane potentials in cultured rat skeletal muscle

The patch-clamp technique was used to characterize channels that could contribute to the resting Cl-conductance in the surface membrane of cultured rat skeletal muscle. Two Cl- -selective channels, in addition to the Cl- -selective channel of large conductance described previously (Blatz and Magleby, 1983), were observed. One of these channels had fast kinetics and a conductance of 45 +/- 1.8 pS (SE) in symmetrical 100 mM KCl. The other had slow kinetics and a conductance of 61 +/- 2.4 pS. The channel with fast kinetics typically closed within 1 ms after opening and flickered between the open and shut states. The channel with slow kinetics typically closed within 10 ms after opening and displayed less flickering. Both channels were active in excised patches of membrane held at potentials similar to resting membrane potentials in intact cells, and both were open a greater percentage of time with depolarization. Under conditions of high ion concentrations, both channels exhibited nonideal selectivity for Cl- over K+ with the permeability ratio PK/PCl of 0.15-0.2. Additional experiments on the fast Cl- channel indicated that its activity decreased with lowered pHi and that SO2-4 and CH3SO-4 were ineffective charge carriers. These findings, plus the observation that the fast Cl- channel was also active in membrane patches on intact cells, suggest that the fast Cl- channel provides a molecular basis for at least some of the resting Cl- conductance. The extent to which the slow Cl- channel contributes is less clear as it was typically active only after excised patches of membrane had been exposed to high concentrations of KCl at the inner membrane surface.

Morphological properties and membrane channels of the growth cones induced in PC12 cells by nerve growth factor

Large growth cones were produced in vitro by nerve growth factor (NGF) treatment of multinucleate cells produced by chemical fusion of cells of the neuron-like clone PC12. These endings were studied both at the light microscopic and ultrastructural levels. The activity of ionic channels at growth cones was recorded with intracellular microelectrodes, patch recording of single channels, and whole cone recording from mechanically isolated growth cones. Morphologically, these large growth cones were characterized by the presence of microspikes and filopodia, by the presence of actin demonstrated immunohistochemically, and by the presence of catecholamine fluorescence. At the ultrastructural level they contained a broad spectrum of organelles with a distribution characteristic of neuronal growth cones, including dense core vesicles, abundant smooth membrane cisternae, microtubules, and a filamentous network. The presence of channels capable of generating action potentials was revealed by intracellular microelectrode recording from the growth cone in the presence of locally applied tetraethylammonium (TEA). TEA appeared to block outward current channels that could effectively shunt inward current activated by depolarization. Action potentials elicited by depolarizing current in the presence of TEA could be blocked reversibly by Cd2+, a specific blocker of Ca channels. These action potentials were often followed by a long after-hyperpolarization lasting hundreds of milliseconds. This after-hyperpolarization was similar to that recorded in the cell body of PC12 cells where it appears to be mediated by Ca-activated K current. Single channel recording from outside-out excised patches of membrane from the growth cones perfused with KF revealed the presence of voltage sensitive Na channels, Ca-activated K channels, and K channels resembling delayed rectifier K channels. Macroscopic currents recorded from mechanically isolated growth cones in the "whole cone" configuration showed rapid inward currents at potentials greater than or equal to -40 mV, followed by delayed outward currents at more positive potentials, a finding providing additional evidence for the presence of Na and K channels in growth cones.

Effects of apamin, quinine and neuromuscular blockers on calcium-activated potassium channels in guinea-pig hepatocytes

The bee venom peptide, apamin, has been radiolabelled with 125I, the monoiodinated derivative purified, and its binding to intact guinea-pig liver cells studied. At 37 degrees C 125I-monoiodoapamin associated with, and dissociated from, guinea-pig hepatocytes remarkably rapidly. The association and dissociation rate constants were 1.4 X 10(8) M-1 s-1 and 0.035 s-1 respectively. Equilibrium binding studies demonstrated a saturable binding component compatible with 1:1 binding to a single class of site and having an equilibrium dissociation constant (KL) of 390 pM. The maximal binding capacity was 1.1 fmol mg-1 dry wt. of tissue. Unlabelled apamin displaced bound 125I-monoiodoapamin with a KI of 380 pM, which is consistent with the concentration of apamin required to inhibit Ca2+-activated K+ permeability (PK(Ca) ) in these cells. Inhibitable binding of 125I-monoiodoapamin to rat hepatocytes was much less than to guinea-pig hepatocytes and could not be reliably quantified. Neither was there any discernible inhibitable binding to human erythrocytes. This is in keeping with the reported lack of apamin-sensitive Ca2+-activated K+ channels in these cell types. Various agents were tested for their ability to inhibit monoiodoapamin binding to, and Ca2+-mediated K+ efflux from, guinea-pig hepatocytes. All compounds tested which inhibited binding also blocked K+ efflux at similar concentrations. TEA and quinine affected hepatocytes only at high concentration (KI = 5.8 and 0.51 mM respectively). 9-aminoacridine, quinacrine and chloroquine were slightly more effective (KI = 70-180 microM). By far the most active compounds (apart from apamin) were the neuromuscular blocking agents; tubocurarine, pancuronium and atracurium (KI = 7.5, 6.8 and 4.5 microM respectively). Gallamine was slightly less effective (KI = 14 microM) and decamethonium and hexamethonium much less so (KI = 620 and 760 microM respectively). 3,4-diaminopyridine, alpha-bungarotoxin and tetrodotoxin were among several compounds which showed little or no affinity for apamin binding sites or inhibition of K+ efflux in guinea-pig hepatocytes. The saturable binding of 125I-monoiodoapamin to guinea-pig hepatocytes corresponds to about 1700 sites per cell. Assuming, tentatively, that binding sites correspond to channels the rate of K+ loss observed following agonist action can readily be explained if these channels have unitary conductances in the range reported for PK(Ca) in other tissues.

Inhibitory synaptic potentials resulting from alpha 2-adrenoceptor activation in guinea-pig submucous plexus neurones

Intracellular recordings were obtained from neurones of the guinea-pig submucous plexus. Inhibitory synaptic potentials (i.p.s.p.s) were compared with hyperpolarizations evoked by brief, local applications of noradrenaline and by superfusion with adrenoceptor agonists. Hyperpolarizing potentials elicited by brief applications of noradrenaline were similar to the i.p.s.p. in latency of onset, amplitude, time course, conductance increase, reversal potential and ionic dependence. Both responses were blocked by low concentrations of Ba2+ and quinine. 6-hydroxydopamine selectively and irreversibly abolished the i.p.s.p. and resulted in a complete loss of catecholamine fluorescent nerve fibres in the submucous plexus. The alpha 2-adrenoceptor antagonists, phentolamine, yohimbine and RX781094, reversibly blocked the i.p.s.p. and the noradrenaline hyperpolarization. Prazosin, propranolol, atropine and naloxone had no effect on these responses. Superfusion with noradrenaline and clonidine produced dose-dependent membrane hyperpolarizations. Noradrenaline and clonidine dose-hyperpolarization curves were shifted to the right in a parallel fashion by alpha 2-adrenoceptor antagonists. Determination of the dissociation equilibrium constants for phentolamine, yohimbine and RX781094 showed that the hyperpolarization produced by noradrenaline perfusion is due to alpha 2-adrenoceptor activation. It is concluded that the release of noradrenaline from sympathetic nerves activates post-synaptic alpha 2-adrenoceptors, resulting in the K+ conductance increase which underlies the i.p.s.p. in submucous plexus neurones.

Ion and membrane changes in the brain during anoxia

Anoxia has two main effects on the brain, a rapid, reversible loss of function and permanent damage when the period of anoxia exceeds a critical length of time. The initial loss of function is related to a K+-conductance increase of the nerve membrane, leading to reduction of membrane resistance and hyperpolarization. After a few minutes, a non-selective increase of membrane permeability mediates rapid transfer of ions between the intra- and extracellular spaces. The subsequent rise of intracellular Ca2+ concentration may be responsible for the nerve cell death.

Physiological role of apical potassium ion channels in frog skin

The K+ permeability of the apical membrane of frog skin (Rana temporaria) was analysed by recording the short-circuit current and its fluctuations in the presence of a mucosa-to-serosa-oriented K+ concentration gradient. Loading of the animals with KCl resulted in an augmentation of the Ba2+-blockade component of the short-circuit current and the plateau value of the K+-dependent relaxation noise. Poisoning of active transport and exposing both sides of the epithelium to KCl Ringer solution caused an increase of the K+ current and its fluctuations recorded after restoring the inward-oriented K+ gradient. Serosal quinidine (5 X 10(-4) M), which is thought to increase intracellular Ca2+ activity, depressed the K+ current and the relaxation noise. This effect was completely reversible. Removal of Na+ from the serosal solution, which is known to result in an elevation of intracellular Ca2+ by abolishing the driving force for the Na+/Ca2+ exchanger, also reduced the K+ current and the Lorentzian plateau. Both parameters returned to their control values after restoring the Na+ gradient across the basolateral membranes. It is concluded from these experiments that the apical K+ permeability is controlled by factors which depend on the intracellular K+ and Ca2+ concentration and that the apical K+ channels may constitute a pathway for K+ secretion.

Voltage-dependent kinetics of an anionic channel of large unit conductance in macrophages and myotube membranes

Using the patch-clamp technique single-channel parameters and kinetic properties of an anionic channel are studied in cell-attached and excised membrane patches from peritoneal macrophages of mouse and cultured chicken myotubes. The channel has a unit conductance of about 340 pS with a Q10 of 1.3. In addition a subconductance state of about 210 pS is frequently adopted. The selectivity ratio of PCl/PNa is about 5. In excised membrane patches the activation of the channel appears to be independent of Ca either in the cytoplasmic or the extracellular medium. The channel induced current fluctuations appear in a burst like pattern. At least three non-conducting channel states could be distinguished kinetically. The mean lifetime of one of these states exhibits a strikingly steep voltage dependence which could be correlated to the mean shut interval between consecutive bursts. A similar steep voltage dependence was found for the mean lifetimes of bursts. The burst kinetic shows an about bell-shaped dependence on voltage. The results suggest that the burst kinetic and the kinetic within bursts are regulated by independent voltage sensitive mechanisms. The burst kinetic was analyzed by ensemble averages of voltage-jump current relaxations performed on the single channel level. A model of two voltage-sensitive gates is proposed for a description of the burst kinetic.

Calcium-dependent after-potentials in visceral afferent neurones of the rabbit

Intracellular recordings were made from neurones in nodose ganglia excised from rabbits. In C neurones, 1-60 action potentials were followed by an after-hyperpolarization with a peak amplitude of 16 mV and a time constant of decay ranging from 3 to 10 s. In A neurones, the action potentials were followed only by a brief (up to 50 ms) after-hyperpolarization. The after-hyperpolarization was associated with an increase in the membrane conductance to potassium ions; it reversed polarity at the potassium equilibrium potential. The increase in conductance following the action potentials was blocked by removal of calcium ions, or addition of cobalt to the extracellular solution. Intracellular injection of ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) abolished the after-hyperpolarization; intracellular injection of calcium mimicked the after-hyperpolarization. It is concluded that calcium entry during the action potential leads to a long-lasting increase in potassium conductance in visceral afferent C neurones.

Control of K+ conductance by cholecystokinin and Ca2+ in single pancreatic acinar cells studied by the patch-clamp technique

The effect of cholecystokinin (CCK) and internal Ca2+ on outward K+ current in isolated pig pancreatic acinar cells has been investigated using the patch-clamp method for whole-cell current recording under voltage-clamp conditions. CCK (2 X 10(-10) M) applied to the bath evoked a marked increase in the outward K+ current associated with depolarizing voltage steps, and this effect was fully reversible and acutely dependent on the presence of external Ca2+. When strongly buffered Ca2+-EGTA solutions were used inside the cells CCK failed to evoke an effect. Increasing the internal Ca2+ concentration [( Ca2+]i) from 5 X 10(-8) M to 10(-7) and 5 X 10(-7) M mimicked the effect of CCK. It would appear therefore that CCK controls K+ conductance in the acinar cells via changes in the internal free ionized Ca2+ concentration.

Electrophysiological properties of cellular and paracellular conductive pathways of the rabbit cortical collecting duct

Microelectrode techniques were applied to the rabbit isolated perfused cortical collecting duct to provide an initial quantitation and characterization of the cell membrane and tight junction conductances. Initial studies demonstrated that the fractional resistance (ratio of the resistance of the apical cell membrane to the sum of the resistances of the apical and basolateral membranes) was usually independent of the point along the tubule of microelectrode impalement--implicating little cell-to-cell coupling--supporting the application of quantitative techniques to the cortical collecting duct. It was demonstrated that in the presence of amiloride, either reduction in the luminal pH or the addition of barium to the perfusate selectively reduced the apical membrane potassium conductance. From the changes in Gte and fractional resistance upon reducing the luminal pH or addition of barium to the perfusate, the transepithelial, apical membrane, basolateral membrane and tight junction conductances were estimated to be 9.3, 6.7, 8.1 and 6.0 mS cm-2, respectively. Ninety to ninety-five percent of the apical membrane conductance reflected the barium-sensitive potassium conductance in the presence of amiloride with an estimated potassium permeability of 1.1 X 10(-4) cm sec-1. Reduction in the perfusate pH to 4.0 caused a 70% decrease in the apical membrane potassium conductance, implying a blocking site with an acidic group having a pKa near 4.4. It is concluded that both the transcellular and paracellular pathways of the cortical collecting tubule have high ionic conductances, and that the apical membrane conductance primarily reflects a high potassium conductance. Furthermore, both reduction in the perfusate pH and addition of barium to the perfusate selectively block the apical potassium channels, although the site of inhibition likely differs since the two ions display markedly different voltage-dependent blocks of the channel.

Quinine blocks a calcium-activated potassium conductance in mammalian enteric neurones

Quinine (100 microM) abolished the slow calcium-dependent afterhyperpolarization which occurs after an action potential in some neurones of the guinea-pig myenteric and submucous plexus. This occurred without any effect on the amplitude or time course of the action potential itself, or on the faster calcium-independent afterhyperpolarization. Tetraethylammonium did not reduce the slow afterhyperpolarization. Quinine also abolished the hyperpolarization which was evoked by intracellular injection of calcium ions.