Aminoglycoside blockade of Ca2+-activated K+ channel from rat brain synaptosomal membranes incorporated into planar bilayers

Summary Ca²⁺-activated K⁺ channels from rat brain synaptosomal membranes were incorporated into planar lipid bilayers, and the effects of aminoglycoside antibiotics on the single channel conductance (258±13 pS at 100mm K⁺) were investigated. Aminoglycosides reduced the single channel conductance from the cis (cytoplasmic) side in a dose- and voltage-dependent manner. Voltage dependence of the blockade indicated an interaction between positively charged amino residues of aminoglycoside antibiotics and a binding site located within the electric field of the ion-conducting pathway. The order of blocking potency was consistent with that of the number of amino residues of aminoglycosides (neomycin (6)>dibekacin (5)>ribostamycin (4)=kanamycin (4)), while the electrical distance (z=0.46–0.49) of the binding site kept almost constant for each drug. Thesezs were almost the same with those (0.46–0.51) of alkyldiamine blockers with two amino residues (total net charge of +2) and approximately twice of those (0.25–0.26) of alkylmonoamine blockers (total net charge of +1). Assuming that amino residues of aminoglycosides and alkylamines shared the same binding site located at 25% voltage drop from the cytoplasmic surface of the channel, the site would have to be at least large enough to accommodate one diamino sugar residue of the aminoglycoside in order to simultaneously interact with two positively charged amino groups. Dose- and voltage-dependent blockade of the channel by gallamine, an extremely bulky trivalent organic cation, supported the picture that the channel has a wide mouth on the cytoplasmic side and its pore region, where voltage drop occurs, may also be quite wide and nonselective, suddenly tapering to a constriction where most charged cations block the channel by occluding the K⁺-conducting pathway.     

Activation and conductance properties of ryanodine-sensitive calcium channels from brain microsomal membranes incorporated into planar lipid bilayers

Summary Rat brain microsomal membranes were found to contain high-affinity binding sites for the alkaloid ryanodine (k _d 3nm.B _max 0.6 pmol per mg protein). Exposure of planar lipid bilayers to microsomal membrane vesicles resulted in the incorporation, apparently by bilayer-vesicle fusion, of at least two types of ion channel. These were selective for Cl^– and Ca²⁺, respectively. The reconstituted Ca²⁺ channels were functionally modified by 1 m ryanodine, which induced a nearly permanently open subconductance state. Unmodified Ca²⁺ channels had a slope conductance of almost 100 pS in 54mm CaHEPES and a Ca²⁺/TRIS⁺ permeability ratio of 11.0. They also conducted other divalent cations (Ba²⁺>Ca²⁺>Sr²⁺>Mg²⁺) and were markedly activated by ATP and its nonhydrolysable derivative AMPPCP (1mm). Inositol 1,4,5-trisphosphate (1–10 m) partially activated the same channels by increasing their opening rate. Brain microsomes therefore contain ryanodine-sensitive Ca²⁺ channels, sharing some of the characteristics of Ca²⁺ channels from striated but not smooth muscle sarcoplasmic reticulum. Evidence is presented to suggest they were incorporated into bilayers following the fusion of endoplasmic reticulum membrane vesicles, and their sensitivity to inositol trisphosphate may be consistent with a role in Ca²⁺ release from internal membrane stores.     

Sensitivity of high-conductance potassium channels in synaptosomal membranes from the rat brain to intracellular pH

High-conductance potassium channels have been studied in inside-out patches excised from proteoliposomes reconstituted from giant liposomes and rat brain synaptosomes. Acid pH in the medium reduced single channel current amplitude and increased the mean open probability and the frequency of channel opening. This was accompanied by a shortening of the open time constant at positive potential and by shortening of the longer closed time constant. The decrease of channel amplitude, the increase of the open probability and the decrease in the longer closed time constant can be explained by neutralization of negative charges of the membrane and by a decrease in the surface membrane potential which mimics membrane depolarization. The shortening of the mean open time is apparently due to a channel blockade by protons. Correspondence to: H. Zemková     

Properties of single- and double-barreled Cl channels of shark rectal gland in planar bilayers

Chloride channels from the apical plasma membrane fraction of rectal gland of Squalus acanthias were characterized by incorporation into planar bilayers in the presence of cAMP-PK/ATP. In a total of 80 bilayer preparations, 21 Cl-selective channels were observed as single channels and 13 as pairs. This was a significantly greater number of double Cl channels than expected from a binomial distribution. The double Cl channels were divided into two groups based on kinetic and voltage-dependent behavior. One group had properties identical to the single channels (gb1) while the other was consistent with a double-barreled channel (gb2) with coordinated activity between proto-channels. The single-channel slope conductances of gb1 and gb2 from -60 to +20 mV with a 250/70 mM KCl gradient were 41 and 75 pS, respectively. With symmetrical 250 mM KCl, the I-V relation of gb1 showed outward rectification with 47.8 +/- 6.6 pS at cis negative potentials and 68.9 +/- 6.1 pS at cis positive potentials. gb1 was open from 70 to 95% at all electrochemical potentials from -80 to +40 mV. gb2 was steeply voltage dependent between -80 and -20 mV. Both gb1 and gb2 were insensitive to Ca (from 100 nm to 1 microM), blocked by 0.1 mM DIDS and highly selective for chloride. These data suggest that double-barreled Cl channels are related to the family of small, outwardly rectifying Cl channels of epithelial membranes.     

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

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

Toad bladder amiloride-sensitive channels reconstituted into planar lipid bilayers

In the present study we used established methods to obtain apical membrane vesicles from the toad urinary bladder and incorporated these membrane fragments to solvent-free planar lipid bilayer membranes. This resulted in the appearance of a macroscopic conductance highly sensitive to the diuretic amiloride added to the cis side. The blockage is voltage dependent and well described by a model which assumes that the drug binds to sites in the channel lumen. This binding site is localized at about 15% of the electric field across the membrane. The apparent inhibition constant (K(0)) is equal to 0.98 microM. Ca2+, in the micromolar range on the cis side, is a potent blocker of this conductance. The effect of the divalent has a complex voltage dependence and is modulated by pH. At the unitary level we have found two distinct amiloride-blockable channels with conductances of 160 pS (more frequent) and 120 pS. In the absence of the drug the mean open time is around 0.5 sec for both channels and is not dependent on voltage. The channels are cation selective (PNa/PCl = 15) and poorly discriminate between Na+ and K+ (PNa/PK = 2). Amiloride decreases the lifetime in the open state of both channels and also the conductance of the 160-pS channel.     

Properties of an anion-selective channel from rat colonic enterocyte plasma membranes reconstituted into planar phospholipid bilayers

Summary Vesicles derived from epithelial cells of the colonic mucosa of the rat were fused to planar phospholipid bilayer membranes, revealing spontaneously switching anion-conducting channels of 50 pS conductance (at-30 mV with 200mm Cl^– each side). The equilibrium selectivity series was I^– (1.7)/Br^– (1.3)/Cl^– (1.0)/F^– (0.4)/HCO ₃ ^– (0.4)/Na (<0.11.). Only one dominant open-state conductance could be resolved, which responded linearly to Cl^– concentrations up to 600mm. The singlechannel current-voltage curve was weakly rectifying with symmetrical solutions. When 50 mV were exceeded at the highconductance branch of the curve, switching was arrested in the closed state. At more moderate voltages (±40 mV) kinetics were dominated by one open state of about 35-msec lifetime and two closed states of about 2 and 9-msec lifetime. Of these, the more stable closed state occurred less often. At these voltages one additional closed state of significantly longer lifetime (>0.5 sec) was observed.     

Purification of B-50 by 2-mercaptoethanol extraction from rat brain synaptosomal plasma membranes

Several methods have been described previously for the purification of the nervous-tissue specific protein kinase C substrate B-50 (GAP-43). In this paper we present a new purification method for B-50 from rat brain which employs 2-mercaptoethanol to release the protein from isolated synaptosomal plasma membranes. Most likely, 2-mercaptoethanol reduces disulfide bonds involved in the linkage of B-50 to the membrane. After washing the membranes with 100 mM NaCl to detach loosely bound proteins, B-50 is the major protein (and the only protein kinase C substrate) released by 0.5% 2-mercaptoethanol treatment. Further purification to apparent homogeneity is achieved by affinity chromatography on calmodulin sepharose. B-50 binds to calmodulin in the absence of calcium and specifically elutes from the column with 3 mM calcium. The procedures described is simple, rapid and highly suitable for large scale purification of B-50 from rat brain.     

Voltage-dependent block by saxitoxin of sodium channels incorporated into planar lipid bilayers.

We have previously studied single, voltage-dependent, saxitoxin-(STX) blockable sodium channels from rat brain in planar lipid bilayers, and found that channel block by STX was voltage-dependent. Here we describe the effect of voltage on the degree of block and on the kinetics of the blocking reaction. From their voltage dependence and kinetics, it was possible to distinguish single-channel current fluctuations due to blocking and unblocking of the channels by STX from those caused by intrinsic channel gating. The use of batrachotoxin (BTX) to inhibit sodium-channel inactivation allowed recordings of stationary fluctuations over extended periods of time. In a range of membrane potentials where the channels were open greater than 98% of the time, STX block was voltage-dependent, provided sufficient time was allowed to reach a steady state. Hyperpolarizing potentials favored block. Both association (blocking) and dissociation (unblocking) rate constants were voltage-dependent. The equilibrium dissociation constants computed from the association and dissociation rate constants for STX block were about the same as those determined from the steady-state fractional reduction in current. The steepness of the voltage dependence was consistent with the divalent toxin sensing 30-40% of the transmembrane potential.     

Voltage dependence of liver gap-junction channels reconstituted into liposomes and incorporated into planar bilayers.

The voltage dependence of rat liver gap junctions was investigated using non-denaturing solubilization and reconstitution of gap-junction protein into proteoliposomes in controlled conditions of connexon aggregation. The presence of liver connexin 32 in reconstituted proteoliposomes was checked with specific antibodies. The proteoliposomes were inserted into planar lipid bilayers by fusion. The single-channel conductance was voltage independent, and its magnitude was 700-1900 pS in 1 M NaCl, as expected from other reports, assuming that conductance is linear with ion activity. The channels were open at zero voltage and completely closed above 40 mV in either direction. This steep voltage dependence corresponded to an open/closed-state voltage difference of 19 mV and to 3.5 gating charges moving through the field. When several channels were inserted into the bilayer, a large fraction of the membrane conductance became voltage insensitive. These results show that the isolated channel units are highly voltage dependent and are consistent with the assumption that aggregated connexons interact through links which prevent voltage-sensitive conformational changes.     

Anion channel forming activity from the plant pathogenic bacteriumClavibacter michiganense ssp. nebraskense

Summary The plant pathogenic bacteriumClavibacter michiganense ssp. nebraskense secretes an anion channel forming activity (CFA) into the culture fluid. The CFA inserts spontaneously into planar lipid membranes when culture fluid of this species is added to the aqueous phase of the bilayer chamber. The channels formed are highly anion selective. The conductance decreases for larger anions (Cl^–>SCN^–>SO ₄ ^2– ) and is practically zero for gluconate. The channels show a unique voltage dependence : (i) The single-channel conductance increases linearly with voltage up to 200 mV saturating at 250 mV with 25±1 pS (300mm KCl). The channel is closed at negative voltage relative to the side of insertion (diode-typeI–V curve). (ii) The average number of open channels also increases with voltage. The Poisson distribution of channel numbers indicates independent opening of the channels.Channel activity can be abolished by protease treatment of the planar bilayer. The channels can be blocked by indanyloxyacetic acid (IAA-94) and by pH>10. The CFA was purified yielding one major band on the SDS gel with a relative molecular mass of 65,000. The putative involvement of the CFA in the toxicity of this plant pathogen is discussed and compared to other toxins like colicins and to the diphtheria toxin group.     

Potassium channels from the plasma membrane of rye roots characterized following incorporation into planar lipid bilayers

Plasma membrane was purified from roots of rye (Secale cereale L. cv. Rheidol) by aqueous-polymer two-phase partitioning and incorporated into planar bilayers of 1-palmitoyl-2-oleoyl phosphatidylethanolamine by stirring with an osmotic gradient. Since plasmamembrane vesicles were predominantly oriented with their cytoplasmic face internal, when fused to the bilayer the cytoplasmic side of channels faced the trans chamber. In asymmetrical (cis:trans) 280100 mM KCl, five distinct K⁺-selective channels were detected with mean chord-conductances (between +30 and -30 mV; volyages cis with respect to trans) of 500 pS, 194 pS, 49 pS, 21 pS and 10 pS. The frequencies of incorporation of these K⁺ channels into the bilayer were 48, 21, 50, 10 and 9%, in the order given (data from 159 bilayers). Only the 49 pS channel was characterized further in this paper, but the remarkable diversity of K⁺ channels found in this preparation is noteworthy and is the subject of further study. In symmetrical KCl solutions, the 49 pS channel exhibited non-ohmic unitary-current/voltage relationships. The chord-conductance (between +30 and-30 mV) of the channel in symmetrical 100 mM KCl was 39 pS. The unitary current was greater at positive voltages than at corresponding negative voltages and showed considerable rectification with increasing positive and negative voltages. This would represent inward rectification in vivo. Gating of the channel was not voltage-dependent and the channel was open for approx. 80% of the time. Presumably this is not the case in vivo, but we are at present uncertain of the in vivo controls of channel gating. The distribution of channel-open times could be approximated by the sum of two negative exponential functions, yielding two open-state time constants (_o, the apparent mean lifetime of the channel-open state) of 1.0 ms and 5.7 s. The distribution of channel-closed times was best approximated by the sum of three negative exponential functions, yielding time constants (_c, the apparent mean lifetime of the channel-closed state) of 1.1 ms, 51 ms and 11 s. This indicates at least a five-state kinetic model for the activity of the channel. The selectivity of the 49 pS channel, determined from both reversal potentials under biionic conditions (100 mM KCl100 mM cation chloride) and from conductance measurements in symmetrical 100 mM cation chloride, was Rb⁺ K⁺ > Cs⁺ > Na⁺ > Li⁺ > tetraethylammonium (TEA⁺). The 49 pS channel was reversibly inhibited by quinine (1 mM) but TEA⁺ (10 mM), Ba²⁺ (3 mM), Ca²⁺ (1 mM), 4-aminopyridine (1 mM) and charybdotoxin (3 M) were without effect when applied to the extracellular (cis) surface.Abbreviations and Symbols GHK Goldman-Hodgkin-Katz - I/V current/voltage - PEG polyethyleneglycol - P_o probability o f the channel being open - TEA⁺ tetraethylammonium - _c apparent mean lifetime of the channel-closed state - _o apparent mean lifetime of the channel-open state P.J.W. was supported by a grant from the Science and Engineering Research Council Membrane Initiative (GR/F 33971) to Professor E.A.C. MacRobbie and M.T. by the Glaxo Junior Research Fellowship at Churchill College, Cambridge. We thank Dr. D.T. Cooke (AFRC, Long Ashton Research Station, University of Bristol, UK) and Ms. J. Marshall (University of York, UK) for their advice and assistance with the aqueous-polymer two-phase partitioning of plasma membrane from rye roots, Mr. J. Banfield and Miss P. Parmar (University of Cambridge, UK) for technical assistance and Professor E.A.C. MacRobbie, Dr. G. Thiel (University of Cambridge, UK), Dr. M.R. Blatt (Wye College, University of London, UK), Dr. D. Sanders and Dr. E. Johannes (University of York, UK) for helpful discussions.     

Calmodulin selectively modulates the guanylate cyclase activity by repressing the lipid phase separation temperature in the inner half of the bilayer of rat brain synaptosomal plasma membranes

The association of [¹²⁵I-]calmodulin with rat brain synaptosomal plasma membranes, when incubated for 1 h at 25° in the presence or in absence of 20 M Ca²⁺, follows a sigmoid path with a Hill coefficient h=1.79±0.12 and h=1.72±0.11, respectively. The total association of calmodulin with the membrane increased approx. 60%–80% at all the range of calmodulin concentrations used in the presence of 20 M Ca²⁺. A three fold increase of guanylate cyclase activity was shown in the presence of low concentrations of calmodulin (up to 10 mM); higher concentrations (up to 40 mM) however, led to a progressive inhibition of the enzyme activity with respect to maximal stimulation. Calmodulin increased the lipid fluidity of synaptosomal plasma membranes labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH), as indicated by the steady-state fluorescence anisotropy [(r_o/r)-1]^–1. Arrhenius-type plots of [(r_o/r)-1]^–1 indicated that the lipid separation of the membrane at 22.7±1.2° was perturbed by calmodulin such that the temperature was reduced to 16.3±0.9° and 15.5±0.8° in the absence or in the presence of 20 M Ca²⁺. Arrhenius plots of guanylate cyclase and acetylcholinesterase activities exhibited brak points at 25.7±1.4° and 22.3±1.0° in control synaptosomal plasma membranes, respectively. The break point for the guanylate cyclase was reduced to 16.3±0.9° in calmodulin treated synaptosomal plasma membranes whereas that of acetylcholinesterase remained unaffected (21.1±0.9°). The allosteric properties of guanylate cyclase by Mn-GTP (as reflected by changes in the Hill coefficient) were modulated by calmodulin while those of acetylcholinesterase by fluoride (F^–) were not altered. We propose that calmodulin achieves these effects through asymmetric perturbations of the membrane lipid structure and that increase in membrane fluidity of the inner leaflet of the membrane induced by calmodulin may be an early key event to the process of neurotransmitter release.     

A chloride-permeable channel from Phaseolus vulgaris roots incorporated into planar lipid bilayers

Ion channels are key participants in physiological processes of plant cells. Here, we report the first characterization of a high conductance, Cl(-)-permeable channel, present in enriched fractions of plasma membranes of bean root cells. The Cl(-) channel was incorporated into planar lipid bilayers and its activity was recorded under voltage clamp conditions. The channel is voltage-dependent, excludes the passage of cations (K(+), Na(+), and Ca(2+)), and is inhibited by micromolar concentrations of Zn(2+). The Cl(-) conductance here characterized represents a previously undescribed channel of plant cells.     

An anion channel of sarcoplasmic reticulum incorporated into planar lipid bilayers: Single-channel behavior and conductance properties

Summary An anion channel of sarcoplasmic reticulum vesicle has been incorporated into planar lipid bilayers by means of a fusion method and its basic properties were investigated. Analysis of fusion processes suggested that one SR vesicle contained approximately one anion channel. The conductance of this channel has several substates and shows a flickering behavior. The occupation probability of each substate was voltage dependent, which induced an inward rectification of macroscopic currents. Further, the anion channel was found to have the following properties. (1) The single-channel conductance is about 200 pS at 100mm Cl^–. (2) The channel does not select among monovalent anions but SO ₄ ^2– hardly permeates through the channel. (3) SO ₄ ^2– added to thecis side (the side to which SR vesicles were added) inhibits Cl^– current competitively in a voltage-dependent manner. (4) An analysis of this voltage dependence suggests that the binding site of SO ₄ ^2– is located at about 36% of the way across the channel from thecis entrance.