Permeation in potassium channels: implications for channel structure

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

Site-specific mutations in a minimal voltage-dependent K+ channel alter ion selectivity and open-channel block

MinK is a small membrane protein of 130 amino acids with a single potential membrane-spanning alpha-helical domain. Its expression in Xenopus oocytes induces voltage-dependent, K(+)-selective channels. Using site-directed mutagenesis of a synthetic gene, we have identified residues in the hydrophobic region of minK that influence both ion selectivity and open-channel block. Single amino acid changes increase the channel's relative permeability for NH4+ and Cs+ without affecting its ability to exclude Na+ and Li+. Blockade by two common K+ channel pore blockers, tetraethylammonium and Cs+, was also modified. These results suggest that an ion selectivity region and binding sites for the pore blockers within the conduction pathway have been modified. We conclude that the gene encoding minK is a structural gene for a K+ channel protein.

Hypertonicity in fused Madin-Darby canine kidney cells: transient rise in NaHCO3 followed by sustained KCl accumulation

We investigated mechanisms of regulatory volume increase in fused Madin-Darby canine kidney (MDCK) cells, a cell line originally derived from renal collecting duct. The intracellular ion concentrations as well as the concentration of the volume marker tetramethylammonium+ were measured by means of ion-selective microelectrodes. Application of hypertonic Ringer bicarbonate solution (+150 mmol/l mannitol) resulted in cell shrinkage to 84 +/- 2% of the initial cell volume (shrinkage expected for an ideal osmometer = 66%), indicating a significant regulatory volume increase. During the first 90 s of the hypertonic stress, a transient increase in intracellular Na+ and HCO3- concentrations was observed. It was followed by a sustained increase in intracellular K+ and Cl- concentrations. Ouabain (0.1 mmol/l) as well as amiloride (1 mmol/l) reduced K+ accumulation significantly, whereas the H+/K(+)-ATPase inhibitor SCH 28080 had no effect. Hypertonic stress hyperpolarized the cell membrane potential by 19 +/- 2 mV, owing to the decrease of the ratio of Cl- conductance to K+ conductance of the cell membrane. We conclude: (a) acute hypertonic stress activates Na+/H+ exchange in MDCK cells; (b) transient alteration of intracellular Na+ and pH stimulates Na+/K(+)-ATPase and Cl-/HCO3- exchange, exchange, both leading to the sustained intracellular accumulation of KCl; (c) a high intracellular KCl concentration is maintained by the partial reversion of the Cl-/K+ conductance ratio of the plasma membrane.

1990: annus mirabilis of potassium channels

Voltage-gated potassium channels make up a large molecular family of integral membrane proteins that are fundamentally involved in the generation of bioelectric signals such as nerve impulses. These proteins span the cell membrane, forming potassium-selective pores that are rapidly switched open or closed by changes in membrane voltage. After the cloning of the first potassium channel over 3 years ago, recombinant DNA manipulation of potassium channel genes is now leading to a molecular understanding of potassium channel behavior. During the past year, functional domains responsible for channel gating and potassium selectivity have been identified, and detailed structural pictures underlying these functions are beginning to emerge.

Functional characterization of a minimal K+ channel expressed from a synthetic gene

A gene for a slowly activating, voltage-dependent K(+) -selective channel was designed and synthesized on the basis of its known amino acid sequence. The synthetic gene was cloned into a transcription vector, and in vitro transcribed mRNA was injected into Xenopus oocytes for electrophysiological assay of the resulting ionic currents. The currents are voltage-dependent and highly selective for K+ over Na+. The selectivity among monovalent cations follows a familiar K(+)- channel sequence: K+ greater than Rb+ greater than NH4+ greater than Cs+ much greater than Na+, Li+. The currents are inhibited by Ba2+, Cs+, and tetraethylammonium (TEA), common pore blockers of K+ channels. Open-channel blockade by Cs+ (but not by Ba2+ or TEA) depends on applied voltage. The major inhibitory effect of Ba2+ is to alter channel gating by favoring the closed state; this effect is specific for Ba2+ and is relieved by external K+. The results argue that although the polypeptide expressed is very small for a eukaryotic ion channel, 130 amino acid residues in length, the ionic currents observed are indeed mediated by a genuine K(+) -channel protein. This synthetic gene is therefore well suited for a molecular analysis of the basic mechanisms of K(+) -channel function.

Whole-cell and single-channel currents across the plasmalemma of corn shoot suspension cells

Whole-cell sealed-on pipettes have been used to measure electrical properties of the plasmalemma surrounding protoplasts isolated from Black Mexican sweet corn shoot cells from suspension culture. In these protoplasts the membrane resting potential (Vm) was found to be -59 +/- 23 mV (n = 23) in 1 mM Ko+. The mean Vm became more negative as [K+]o decreased, but was more positive than the K+ equilibrium potential. There was no evidence of electrogenic pump activity. We describe four features of the current-voltage characteristic of the plasmalemma of these protoplasts which show voltage-gated channel activity. Depolarization of the whole-cell membrane from the resting potential activates time- and voltage-dependent outward current through K(+)-selective channels. A local minimum in the outward current-voltage curve near Vm = 150 mV suggests that these currents are mediated by two populations of K(+)-selective channels. The absence of this minimum in the presence of verapamil suggests that the activation of one channel population depends on the influx of Ca2+ into the cytoplasm. We identify unitary currents from two K(+)-selective channel populations (40 and 125 pS) which open when the membrane is depolarized; it is possible that these mediate the outward whole-cell current. Hyperpolarization of the membrane from the resting potential produces time- and voltage-dependent inward whole-cell current. Current activation is fast and follows an exponential time course. The current saturates and in some cases decreases at membrane potentials more negative than -175 mV. This current is conducted by poorly selective K+ channels, where PCl/PK = 0.43 +/- 0.15. We describe a low conductance (20 pS) channel population of unknown selectivity which opens when the membrane is hyperpolarized. It is possible that these channels mediate inward whole-cell current. When the membrane is hyperpolarized to potentials more negative than -250 mV large, irregular inward current is activated. A third type of inward whole-cell current is briefly described. This activates slowly and with a U-shaped current-voltage curve over the range of membrane potentials -90 less than Vm less than 0 mV.

Mutations affecting internal TEA blockade identify the probable pore-forming region of a K+ channel

The active site of voltage-activated potassium channels is a transmembrane aqueous pore that permits ions to permeate the cell membrane in a rapid yet highly selective manner. A useful probe for the pore of potassium-selective channels is the organic ion tetraethylammonium (TEA), which binds with millimolar affinity to the intracellular opening of the pore and blocks potassium current. In the potassium channel encoded by the Drosophila Shaker gene, an amino acid residue that specifically affects the affinity for intracellular TEA has now been identified by site-directed mutagenesis. This residue is in the middle of a conserved stretch of 18 amino acids that separates two locations that are both near the external opening of the pore. These findings suggest that this conserved region is intimately involved in the formation of the ion conduction pore of voltage-activated potassium channels. Further, a stretch of only eight amino acid residues must traverse 80 percent of the transmembrane electric potential difference.

Blockade of voltage-dependent 42K efflux from rat brain synaptosome by minaprine and tetrahydroaminoacridine

The effect of minaprine (3-(2-morpholinoethylamino)-4-methyl-6-phenylpyridazine) on the K+ channels was studied by means of 42K efflux from rat brain synaptosomes, comparing the effects of 4-aminopyridine and 9-amino-1,2,3,4-tetrahydroacridine (THA). 42K efflux from rat brain synaptosomes was classified into five components: a resting component (R), a rapidly inactivating, voltage-dependent component (T), a slowly inactivating, voltage-dependent component (S) and a voltage-dependent, Ca(2+)-dependent component which is divided into a fast phase (CT) and a slower phase (CS). 4-Aminopyridine selectively inhibited 42K efflux of component T. THA blocked both S and T components. The inhibitory effect of THA on the 42K efflux of component S was quite pronounced compared with that of component T. Minaprine inhibited the 42K efflux of components S and T but the inhibitory effect on component S was observed with a lower dose of minaprine than that needed for the effect on component T. Minaprine had no effect on the Ca(2+)-dependent component while THA blocked component CT. 42K efflux of the resting component was not changed by minaprine, THA or 4-aminopyridine. These results suggest that minaprine blocks Ca2+ independent voltage-dependent K+ channel is involved in the pharmacological actions of minaprine.

Solute inaccessible aqueous volume changes during opening of the potassium channel of the squid giant axon

We have applied solutions with varying osmotic pressures symmetrically to the inside and outside of perfused, TTX-treated, giant axons. The potassium conductance G decreased with increasing osmotic stress, but there was no effect on either the shape or the position of the voltage-current curve. One must distinguish three possible actions of the osmotic agent: osmotic stress, channel blocking, and lowered solution conductivity. To do so, we compared results obtained working with pairs of internal and external solutions of either (a) equal osmotic stress, (b) equal conductivity, or (c) the same blocking agent. There was the same change in G irrespective of the type of stressing species (sorbitol or sucrose); this provides some evidence against a blocking mechanism. The conductivity of the external solution had a small effect on K currents; internal solution conductivity had none. A change in series resistance of the Schwann cell layer could account for the small effect of external solution conductivity. The primary cause of G depression appears, then, to be the applied osmotic stress. Using this result, we have developed models in which the channel has a transition between closed states under voltage control but osmotically insensitive and a closed/open step that is voltage-independent but osmotically sensitive. We have assumed that the conductance of this open state does not change with osmotic stress. In this way, we estimate that an additional 1,350 +/- 200 A3 or 40-50 molecules of solute-inaccessible water appear to associate with the average delayed rectifier potassium channel of the squid axon when it opens.

Energy transduction in the bacterial flagellar motor. Effects of load and pH

The effect of load and pH on the relation between proton potential and flagellar rotation has been studied in cells of a smooth-swimming Streptococcus strain. The driving potential, speeds of free-swimming bacteria, and rotation rates of bacteria tethered to glass by a single flagellum were measured. The relation between rotation rate of tethered bacteria and potential was remarkably linear up to nearly -200 mV. The relation between swimming speed and potential exhibited both saturation and threshold, as previously observed in other species. The form of these relations depended on pH. The equivalence of the electrical and chemical potential components of the proton potential in enabling swimming depended on the voltage. Our observations may be most simply accommodated by a kinetic scheme that links transmembrane proton transits to a tightly coupled work cycle. The properties of this scheme were elucidated by computer simulations of the experimental plots. These simulations indicated that the protonable groups that participate in the rate limiting reactions have a fractional electrical distance between three-fourths to all of the way toward the cytoplasm with a corresponding mean proton binding affinity of 10(-7.3)-10(-7.0) M, respectively.

Tansley Review No. 21 Plant ion channels: whole-cell and single channel studies

Ion channels are proteins which catalyse rapid, passive, electrogenic uniport of ions through pores spanning an otherwise poorly permeable lipid bilayer. Among other processes, fluxes through ion channels are responsible for action potentials - large, transient changes in membrane potential which have been known of in plants for over 100 years. Much disparate information on ion channels in plant cells has accumulated over the past few years. In an attempt to synthesize these data, the properties of at least 18 different ion channels are collated in this review. Channels are initially classified according to ion selectivity (Ca²⁺ , Cl^- , K⁺ and H⁺ ); then gating characteristics (i.e. control of opening and closing), unitary conductance and pharmacology are used to distinguish further different sub-types of channels. To provide a background for this overview, the fundamental properties which define ion channels in animal cells, namely conduction, selectivity and gating, are described. Appropriate techniques for the study of ion channels are also assessed. The review concludes with a discussion on the role of ion channels in plant cells, although any comment on functions beyond turgor regulation and general statements about signalling remains largely speculative. The study of ion channels in plant cells is still at an early stage and it is hoped that this review will provide a framework upon which further work in both algae and vascular plants can be based. CONTENTS Summary 305 I. Introduction: plant electrophysiology 306 II. A general description of ion channels 306 III. Ion channels in plants 310 IV. Ca²⁺ channels 313 V. Cl^- channels 315 VI. K⁺ channels in the plasma membrane 318 VII. K⁺ channels in the tonoplast 322 VIII. Channels in thylakoids 324 IX. H⁺ channels 324 X. Functions of channels 325 XI. Conclusions 328 Acknowledgements 328 References 329.

Ca2+ channels from the sea urchin sperm plasma membrane

Ca2+ influx across the sea urchin sperm plasma membrane is a necessary step during the egg jelly-induced acrosome reaction. There is pharmacological evidence for the involvement of Ca2+ channels in this influx, but their presence has not been directly demonstrated because of the small size of this cell. Sea urchin sperm Ca2+ channels are being studied by fusing isolated plasma membranes into planar lipid bilayers. With this strategy, a Ca2+ channel has been detected with the following characteristics: (a) the channel exhibits a high mainstate conductance (gamma MS) of 172 pS in 50 mM CaCl2 solutions with voltage-dependent decaying to smaller conductance states at negative Em; (b) the channel is blocked by millimolar concentrations of Cd2+, Co2+, and La3+, which also inhibit the egg jelly-induced acrosome reaction; (c) the gamma MS conductance sequence for the tested divalent cations is the following: Ba2+ greater than Sr2+ greater than Ca2+; and (d) the channel discriminates poorly for divalent over monovalent cations (PCa/PNa = 5.9). The sperm Ca2+ channel gamma MS rectifies in symmetrical 10 mM CaCl2, having a maximal slope conductance value of 94 pS at +100 mV applied to the cis side of the bilayer. Under these conditions, a different single-channel activity of lesser conductance became apparent above the gamma MS current at positive membrane potentials. Also in 10 mM Ca2+ solutions, Mg2+ permeates through the main channel when added to the cis side with a PCa/PMg = 2.9, while it blocks when added to the trans side. In 50 mM Ca2+ solutions, the gamma MS open probability has values of 1.0 at voltages more positive than -40 mV and decreases at more negatives potentials, following a Boltzmann function with an E0.5 = -72 mV and an apparent gating charge value of 3.9. These results describe a novel Ca2(+)-selective channel, and suggest that the main channel works as a single multipore assembly.

Potassium channel currents in intact stomatal guard cells: rapid enhancement by abscisic acid

Evidence of a role for abscisic acid (ABA) in signalling conditions of water stress and promoting stomatal closure is convincing, but past studies have left few clues as to its molecular mechanism(s) of action; arguments centred on changes in H(+)-pump activity and membrane potential, especially, remain ambiguous without the fundamental support of a rigorous electrophysiological analysis. The present study explores the response to ABA of K(+) channels at the membrane of intact guard cells of Vicia faba L. Membrane potentials were recorded before and during exposures to ABA, and whole-cell currents were measured at intervals throughout to quantitate the steady-state and time-dependent characteristics of the K(+) channels. On adding 10 μM ABA in the presence of 0.1, 3 or 10 mM extracellular K(+), the free-running membrane potential (V m) shifted negative-going (-)4-7 mV in the first 5 min of exposure, with no consistent effect thereafter. Voltage-clamp measurements, however, revealed that the K(+)-channel current rose to between 1.84- and 3.41-fold of the controls in the steady-state with a mean halftime of 1.1 ± 0.1 min. Comparable changes in current return via the leak were also evident and accounted for the minimal response in V m. Calculated at V m, the K(+) currents translated to an average 2.65-fold rise in K(+) efflux with ABA. Abscisic acid was not observed to alter either K(+)-current activation or deactivation.These results are consistent with an ABA-evoked mobilization of K(+) channels or channel conductance, rather than a direct effect of the phytohormone on K(+)-channel gating. The data discount notions that large swings in membrane voltage are a prerequisite to controlling guard-cell K(+) flux. Instead, thev highlight a rise in membrane capacity for K(+) flux, dependent on concerted modulations of K(+)-channel and leak currents, and sufficiently rapid to account generally for the onset of K(+) loss from guard cells and stomatal closure in ABA.

Potassium channel toxins

Many venom toxins interfere with ion channel function. Toxins, as specific, high affinity ligands, have played an important part in purifying and characterizing many ion channel proteins. Our knowledge of potassium ion channel structure is meager because until recently, no specific potassium channel toxins were known, or identified as such. This review summarizes the sudden explosion of research on potassium channel toxins that has occurred in recent years. Toxins are discussed in terms of their structure, physiological and pharmacological properties, and the characterization of toxin binding sites on different subtypes of potassium ion channels.

Characterization of a voltage-gated K+ channel that accelerates the rod response to dim light

In this study a K+ current, IKx, in isolated salamander rod photoreceptors was characterized and its role in shaping small photovoltages was examined. IKx is a standing outward current of about 40 pA at -30 mV that deactivates slowly when the cell is hyperpolarized (tau max = 0.25 s). The voltage and time dependence of IKx are similar to that of M-current, but IKx can be distinguished from M-current because it is not suppressed by acetylcholine and is "blocked" by external Ba2+ in a surprising manner: the activation range of IKx is shifted strongly in the positive direction. Using current-clamp recordings and a computer simulation of the photo-response, we show that IKx figures prominently in setting the dark resting potential and accelerates the voltage response to small photocurrents.

Calcium-dependent potassium channel in Paramecium studied under patch clamp

We have studied a class of Ca2+i-dependent K channels in inside-out excised membrane patches from Paramecium under patch clamp. single channels had a conductance of 72 +/- 9.0 pS in a solution containing 100 mM K+. The channels were selective for K+ over Rb+ with the permeability ratio of 1: 0.56, and over Na+, Cs+ or NH+4 with a ratio 1: less than 0.1. The channel activity was dependent on Ca2+i, which was applied to the cytoplasmic side; the Ca2+i concentration for the half maximal activation was 2 microM. The Hill coefficient for the Ca2+i dependence of the channel activity was 2.58, indicating that more than two Ca2+i bindings are necessary for full activation. Unlike most Ca2+i-dependent K channels in other organisms, the channels in Paramecium were slightly more active upon hyperpolarization than upon depolarization. The voltage dependence was fitted to a Boltzmann curve with 41.2 mV per e-fold change in channel activity. While a high Ca2+i concentration activated the channels, it also irreversibly reduced the channel activity over time. The decay of channel activity occurred faster at higher Ca2+i concentrations. Quaternary ammonium ions suppressed ion passage through the channel; more highly alkylated quaternary ammonium ions were more efficient in blocking. Ba2+i and Ca2+i were relatively ineffective in blockage. it was concluded that these Ca2+i-dependent K channels in Paramecium are different from the previously described Ca2+i-dependent K channels, and are perhaps of a novel class.

Alpha-receptor and cholinergic receptor-linked changes in cytosolic Ca2+ and membrane potential in primary rat astrocytes

Both phenylephrine and carbachol caused a sustained increase in Ca2+ influx and intracellular free Ca2+ of primary astrocytes as measured with 45Ca2+ and fura-2. The responses to phenylephrine and carbachol were additive, suggesting that they use different releasable pools of Ca2+. If extracellular Ca2+ was removed by EGTA only a transient rise in cytosolic Ca2+ was seen upon application of the agonists. Both compounds caused depolarization of the astrocyte membrane as determined with the optical probe 3,3-diethylthiadicarboxyamineiodide. Activation of protein kinase C with 12-tetradecanoylphorbol myristate acetate (TPA) or the diacylglycerol analogue dioctanoylglycerol (DiC8) also depolarized the cells. A prior activation of protein kinase C with TPA or DiC8 abolished the depolarizing effect of phenylephrine suggesting that they act through the same mediators. If the cells were made ideally permeable to K+ with the ionophore valinomycin, or the K+ channels had been blocked with Ba2+, neither TPA nor phenylephrine had any significant effect on the membrane potential. Neither TPA nor phenylephrine had any effect on the 86Rb+ equilibrium potential across the cell membrane. The results suggest that the depolarizing effect of these substances could be through a blocking of K+ channels.

Ca2+-activated K+ channel in rat pancreatic islet B cells: permeation, gating and blockade by cations

Activation of Ca2+-dependent K+ conductance has long been postulated to contribute to the cyclical pauses in glucose-induced electrical activity of pancreatic islet B cells. Here we have examined the gating, permeation and blockade by cations of a large-conductance, Ca2+-activated K+ channel in these cells. This channel shares many features with BK (or maxi-K+) Ca2+-activated K+ channels in other cells. (1) Its 'permeability' selectivity sequence is PT1+: PK+: PRb+: PNH4+: PNa+, Li+, Cs+ = 1.3:1.0:0.5:0.17: less than 0.05. Permeant, as well as impermeant, cations reduce channel conductance. (2) Its conductance saturates at 325-350 pS with bath KCl greater than 400 mM (144 mM KCl pipette). (3) It shows asymmetric blockade by tetraethylammonium ion (TEA) and Na+. (4) It is sensitive to Ca2+i over the range 5 nM-100 microM; over the range 50-200 nM, channel activity varies as [Ca2+ free]1-2. (5) It is sensitive to internal pH over the range 6.85-7.35, but the decrease in channel activity seen with reduced pHi may be partially compensated by the increase in free Ca2+ concentration which occurs on acidification of buffered Ca2+/EGTA solutions.

Electrorheological effects and gating of membrane channels

A hypothesis is presented on the gating of ion channels. This is considered as a consequence, in part, of a large increase in viscosity of the water in the "vestibule" region of the channel in the high field present when the channel is not conducting. This part of gating amounts to "melting" of the high viscosity part of the water upon release of the field. The resulting model accounts qualitatively for a number of phenomena in the literature, including the steepness of the voltage dependence of gating, the slowing of gating upon substitution of D2O for H2O, and the pressure dependence of the gating kinetics. The viscosity increase with field is well known in the literature; several forms of electroviscous effects, a viscoelectric effect, and a generalized electrorheological effect have been described. This model appears closest to an electrorheological effect in which boundary water out to a few molecular diameters is structured in the presence of a high field, while the boundary (here, protein) moves. The size of the channel entrance is small enough for this effect to prevent conductivity. The remainder of the gating current, which occurs at more polarized potentials, is attributed to protein motion. Some consequences of the model are discussed. Qualitative comparison with published data is included.

Stretch-inactivated ion channels coexist with stretch-activated ion channels

Stretch-activated ion channels of animal, plant, bacterial, and fungal cells are implicated in mechanotransduction and osmoregulation. A new class of channel has now been described that is stretch-inactivated. These channels occur in neurons, where they coexist with stretch-activated channels. Both channels are potassium selective. The differing stretch sensitivities of the two channels minimize potassium conductance over an intermediate range of tension, with the consequence that, over this same range, voltage-gated calcium channels are most readily opened. Thus, by setting the relation between membrane tension and transmembrane calcium fluxes, stretch-sensitive potassium channels may participate in the control of calcium-dependent motility in differentiating, regenerating, or migrating neurons.

86Rubidium release from cultured primary astrocytes: effects of excitatory and inhibitory amino acids

The effects of high K+, glutamate and its analogue, kainate, on K+ release were studied in primary astrocyte cultures prepared from newborn rat brains using 86Rb+ as a tracer for K+. An increase in 86Rb+ release was observed when the extracellular K+ concentration was elevated (10-40 mM). Glutamate and kainate stimulated the release in a dose-dependent manner, 100 microM concentrations being about as equally effective as high K+ (40 mM). Both compounds also caused an increase in the absorbance of the cyanine dye, 3,3'-diethylthiadicarbocyanine, indicating depolarization of the membrane. No significant Na+-dependent uptake of [3H]kainate occurred in the cells, thus excluding the possibility that depolarization was due to electrogenic uptake of amino acid into the cells. GABA and taurine significantly depressed the high K+- and glutamate-induced 86Rb+ release. Taurine itself caused a small increase in 86Rb+ release and the membrane was depolarized, judging from the increase in the absorbance of the cyanine dye, 3,3'-diethylthiadicarbocyanine. No effect of taurine was observed when the Cl- concentration was reduced in the experimental medium. The results suggest that cultured astrocytes respond by membrane depolarization to high external K+ and to glutamate and kainate. The degree of this depolarization can be modified by the inhibitory amino acids GABA, taurine and glycine, the effect of taurine probably being mediated by an increase in Cl- conductance across the cell membrane. The role of functional receptors for amino acid transmitters and the effects observed are discussed.

Isomeric yohimbine alkaloids block calcium-activated K+ channels in medullary thick ascending limb cells of rabbit kidney

The alpha2-adrenergic antagonist yohimbine (YOH) and the closely related isomers corynanthine (COR) and rauwolscine (RAU) caused brief interruptions in current characteristic of a fast blocker Ca2+-activated K+ channels in cultured medullary thick ascending limb (MTAL) cells. The apparent dissociation constants (Kapp) for COR, YOH, and RAU, respectively, at the intracellular face of the channel in the presence of 200 mM K+ are 45 +/- 1, 98 +/- 2, and 310 +/- 33 microM. The Kapp for COR on the extracellular side also in the presence of 200 mM K+ was much greater at 1.6 +/- 0.17 mM. Increasing K+ on the same side as the blocker relieves the blocking reaction. The Kapp for the alkaloids varies with K+ in a manner quantitatively consistent with K+ and the alkaloids competing for a common binding site. Finally, blocking by the charged form of these alkaloids is voltage dependent with changes in Kapp of 86 +/- 7 and 94 +/- 6 microM per e-fold change in voltage for blockers applied either from the inside or outside. The alkaloids block at an electrical distance similar to tetraethylammonium, suggesting that the site within the channel pore of these molecules may be similar.

Ion and ion channel involvement in alpha-melanocyte-stimulating hormone secretion from superfused slices of rat hypothalamus

Release of alpha-melanocyte-stimulating hormone (alpha-MSH) from 250 micron frontal slices of rat hypothalamus superfused at 37 degrees C with oxygenated artificial cerebrospinal fluid (ACSF) was quantified in freeze-dried samples of ACSF by radioimmunoassay. Significant reproducible increases in alpha-MSH release were caused by 40-70 mM K+ in ACSF, maximum release being caused by 50 mM K+. Fifty mM K+-stimulated alpha-MSH release was abolished in the absence of Ca2+ from ACSF and by the presence of 10(-6) M tetrodotoxin. Tetrapentylammonium ions, 10(-4) M and 10(-3) M, stimulated dose-dependent increases in alpha-MSH release. The data support a putative neurotransmitter/neuromodulator role for alpha-MSH in the CNS.

Charybdotoxin block of Shaker K+ channels suggests that different types of K+ channels share common structural features

Charybdotoxin (CTX), a 37 amino acid protein isolated from the venom of L. quinquestriatus, is a high-affinity blocker of various Ca2(+)-activated K+ channels. CTX also blocks Drosophila Shaker (Sh) clone H4 transient K+ currents expressed in Xenopus oocytes with similar affinity (Kd = 3.6 nM). CTX blocks both the open and the closed states of Sh channels with no apparent change in gating behavior. In addition, the block is enhanced as the ionic strength is lowered. These properties are identical to those of CTX block of Ca(+)-activated K+ channels, and these results suggest that the external pore openings of these two functionally dissimilar K+ channels may share common structural features.

The effect of K+ and glutamate receptor agonists on the membrane potential of suspensions of primary cultures of rat astrocytes as measured with a cyanine dye, DiS-C2-(5)

The cyanine dye DiS-C2-(5) was used to investigate the effect of K+ and glutamate receptor agonists on the membrane potential of whole populations of primary rat astrocytes in suspension. Increasing the external K+ concentration from 5 to 40 mM caused a depolarization of the cells. Ba2+ blocked the response to K+, whereas 4-aminopyridine had no effect on the depolarization. The effect of added external K+ was enhanced by the addition of the neutral K+ ionophore valinomycin. This supports the view that the membrane potential of primary astrocytes is dependent of the K+ gradient, and suggests that the membrane is not ideally permeable to K+ ions. Glutamate caused a depolarization of the cells which was not affected by Ba2+. In the presence of veratridine and ouabain no effect of glutamate was seen. The cells were also depolarized by the glutamate receptor agonists quisqualate, kainate and N-methyl-D-aspartate (NMDA). The response to kainate was blocked by kynurenate, which also diminished the depolarization caused by glutamate. NMDA was effective when added after kainate. The effect of the glutamate receptor agonists tested was generally smaller than that of glutamate itself, and a prior addition of one of the agonists diminished the response to glutamate. The results obtained suggest that cyanine dyes are well suited for investigating the behavior of whole populations of cultured primary astrocytes.

An emerging pharmacology of peptide toxins targeted against potassium channels

Voltage-dependent ion channels are a difficult class of proteins to approach biochemically. Many such channels are present at low density in relevant tissues and exist as multiple subtypes that can be distinguished electrophysiologically. In particular, K channels appear to be a diverse family of proteins characterized by many different conductance properties, gating behaviors and regulatory phenomena. Fortunately, specific peptide toxins for K channels are present in the venoms of insects, scorpions, snakes and possibly other species. The available sequences of these peptides define several different families of toxins. Electrophysiological and radioligand binding studies suggest that these toxins can be used to distinguish subclasses of K channels that share similar toxin binding sites. The growing databank of sequence homologies for both toxins and channels is, in essence, a codebook for identifying common elements of structure and function. The continuing development of toxins as biochemical probes should help to uncover the molecular basis and physiological significance of K-channel diversity.

Glucose deprivation depolarizes plasma membrane of cultured astrocytes and collapses transmembrane potassium and glutamate gradients

Primary cultures of astrocytes were used to investigate the effects of glucose deprivation on plasma membrane potential, on the respiration and on the energy status of these cells. Plasma membrane potential, as monitored with a cyanine dye, 3,3'-diethylthiadicarbocyanine, hyperpolarized by about 100% when glucose was added to substrate-deprived cells. The effect of glucose was prevented by iodoacetate or ouabain. In the absence of glucose, cellular adenosine triphosphate/adenosine diphosphate ratio was extensively reduced and pyruvate was unable either to restore energy status or to hyperpolarize the plasma membrane of astrocytes, although it was the preferential substrate for mitochondria within the cells. Glucose deprivation and inhibition of glycolysis or respiration in the presence of glucose caused dramatic decrease in transmembrane potassium ion and L-glutamate gradients. The gradients were not restored in the presence of pyruvate. Thus, aerobic glycolysis, rather than oxidation of pyruvate, is required to maintain maximal plasma membrane potential, adenosine triphosphate/adenosine diphosphate ratios as well as K+ and L-glutamate gradients. This evidence, together with the unresponsiveness of astrocyte respiration to ouabain, indicates a functional dissociation between energy dissipation at the plasma membrane and mitochondrial synthesis of adenosine triphosphate. The results are discussed with regard to the vulnerability of glia at low levels of blood glucose and the contribution of glial dysfunction to development of hypoglycaemic encephalopathy.

Role of calcium-activated potassium channels in transmitter release at the squid giant synapse

1. Several compounds known to block Ca2+-activated K+ channels were microinjected into squid 'giant' presynaptic terminals to test the hypothesis that these channels mediate Ca2+-dependent neurotransmitter release. 2. Injection of tetrapentylammonium, nonyl-triethylammonium and decamethonium all reversibly blocked transmission evoked by presynaptic action potentials. 3. All three of these compounds blocked presynaptic Ca2+ channels. The actions of tetrapentylammonium on presynaptic Ca2+ influx were examined in detail and found to be quantitatively sufficient to account for the ability of this compound to inhibit transmitter release. 4. Injection of Ba2+, another agent known to block Ca2+-activated K+ channels, also reversibly blocked evoked transmitter release. Ba2+ simultaneously enhanced basal (asynchronous) transmitter release and thus may be decreasing evoked release by depleting transmitter quanta available for release. 5. None of these results provide any support for the hypothesis that Ca2+-activated K+ channels mediate Ca2+-dependent release of transmitter at the squid synapse. However, our results have identified a new class of compounds that block Ca2+ channels from their cytoplasmic surface.