Conduction and block by organic cations in a K+-selective channel from sarcoplasmic reticulum incorporated into planar phospholipid bilayers

Sweetening K-channels: what sugar taught us about permeation and gating

Because they enable for the modification of both viscosity and osmolarity, sugars have been used as a biophysical probe of voltage-gated K-channels for a while. Viscosity variations made it possible to measure the pore sizes in large and small conductance K-channels using techniques similar to those used in the 1980s to study the gramicidin A channel. These analyses led to the finding that the size of the internal mouth appears to be the primary cause of the conductance differences between Shaker-like channels and large conductance BK-channels. As an osmotic agent, adding sugar unilaterally causes streaming potentials that indicate H₂O/K⁺ cotransport across the BK-channel pore. Osmotic experiments on Shaker K-channels suggest that the pore gate operation and the slow inactivation displace comparable amounts of water. Functionally isolated voltage sensors allow estimation of individual osmotic work for each voltage sensing charge during voltage-activation, reporting dramatic internal and external remodeling of the Voltage Sensing Domain´s solvent exposed surfaces. Remarkably, each charge of the VSD appears to take a unique trajectory. Thus, manipulation of viscosity and osmolarity, together with 3D structures, brings in solid grounds to harmonize function and structure in membrane proteins such as K-channels and, in a wider scope, other structurally dynamic proteins.

Enhanced activity of multiple TRIC-B channels: an endoplasmic reticulum/sarcoplasmic reticulum mechanism to boost counterion currents

KEY POINTS

There are two subtypes of trimeric intracellular cation (TRIC) channels but their distinct single-channel properties and physiological regulation have not been characterized. We examined the differences in function between native skeletal muscle sarcoplasmic reticulum (SR) K+ -channels from wild-type (WT) mice (where TRIC-A is the principal subtype) and from Tric-a knockout (KO) mice that only express TRIC-B. We find that lone SR K+ -channels from Tric-a KO mice have a lower open probability and gate more frequently in subconducting states than channels from WT mice but, unlike channels from WT mice, multiple channels gate with high open probability with a more than six-fold increase in activity when four channels are present in the bilayer. No evidence was found for a direct gating interaction between ryanodine receptor and SR K+ -channels in Tric-a KO SR, suggesting that TRIC-B-TRIC-B interactions are highly specific and may be important for meeting counterion requirements during excitation-contraction coupling in tissues where TRIC-A is sparse or absent.

ABSTRACT

The trimeric intracellular cation channels, TRIC-A and TRIC-B, represent two subtypes of sarcoplasmic reticulum (SR) K+ -channel but their individual functional roles are unknown. We therefore compared the biophysical properties of SR K+ -channels derived from the skeletal muscle of wild-type (WT) or Tric-a knockout (KO) mice. Because TRIC-A is the major TRIC-subtype in skeletal muscle, WT SR will predominantly contain TRIC-A channels, whereas Tric-a KO SR will only contain TRIC-B channels. When lone SR K+ -channels were incorporated into bilayers, the open probability (Po) of channels from Tric-a KO mice was markedly lower than that of channels from WT mice; gating was characterized by shorter opening bursts and more frequent brief subconductance openings. However, unlike channels from WT mice, the Po of SR K+ -channels from Tric-a KO mice increased as increasing channel numbers were present in the bilayer, driving the channels into long sojourns in the fully open state. When co-incorporated into bilayers, ryanodine receptor channels did not directly affect the gating of SR K+ -channels, nor did the presence or absence of SR K+ -channels influence ryanodine receptor activity. We suggest that because of high expression levels in striated muscle, TRIC-A produces most of the counterion flux required during excitation-contraction coupling. TRIC-B, in contrast, is sparsely expressed in most cells and, although lone TRIC-B channels exhibit low Po, the high Po levels reached by multiple TRIC-B channels may provide a compensatory mechanism to rapidly restore K+ gradients and charge differences across the SR of tissues containing few TRIC-A channels.

Trimeric intracellular cation channels and sarcoplasmic/endoplasmic reticulum calcium homeostasis

Trimeric intracellular cation channels (TRIC) represents a novel class of trimeric intracellular cation channels. Two TRIC isoforms have been identified in both the human and the mouse genomes: TRIC-A, a subtype predominantly expressed in the sarcoplasmic reticulum (SR) of muscle cells, and TRIC-B, a ubiquitous subtype expressed in the endoplasmic reticulum (ER) of all tissues. Genetic ablation of either TRIC-A or TRIC-B leads to compromised K(+) permeation and Ca(2+) release across the SR/ER membrane, supporting the hypothesis that TRIC channels provide a counter balancing K(+) flux that reduces SR/ER membrane depolarization for maintenance of the electrochemical gradient that drives SR/ER Ca(2+) release. TRIC-A and TRIC-B seem to have differential functions in Ca(2+) signaling in excitable and nonexcitable cells. Tric-a(-/-) mice display defective Ca(2+) sparks and spontaneous transient outward currents in arterial smooth muscle and develop hypertension, in addition to skeletal muscle dysfunction. Knockout of TRIC-B results in abnormal IP3 receptor-mediated Ca(2+) release in airway epithelial cells, respiratory defects, and neonatal lethality. Double knockout mice lacking both TRIC-A and TRIC-B show embryonic lethality as a result of cardiac arrest. Such an aggravated lethality indicates that TRIC-A and TRIC-B share complementary physiological functions in Ca(2+) signaling in embryonic cardiomyocytes. Tric-a(-/-) and Tric-b(+/-) mice are viable and susceptible to stress-induced heart failure. Recent evidence suggests that TRIC-A directly modulates the function of the cardiac ryanodine receptor 2 Ca(2+) release channel, which in turn controls store-overload-induced Ca(2+) release from the SR. Thus, the TRIC channels, in addition to providing a countercurrent for SR/ER Ca(2+) release, may also function as accessory proteins that directly modulate the ryanodine receptor/IP3 receptor channel functions.

Effects of monovalent cations on Ca2+ uptake by skeletal and cardiac muscle sarcoplasmic reticulum

Ca(2+) transport by the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA) is sensitive to monovalent cations. Possible K(+) binding sites have been identified in both the cytoplasmic P-domain and the transmembrane transport-domain of the protein. We measured Ca(2+) transport into SR vesicles and SERCA ATPase activity in the presence of different monovalent cations. We found that the effects of monovalent cations on Ca(2+) transport correlated in most cases with their direct effects on SERCA. Choline(+), however, inhibited uptake to a greater extent than could be accounted for by its direct effect on SERCA suggesting a possible effect of choline on compensatory charge movement during Ca(2+) transport. Of the monovalent cations tested, only Cs(+) significantly affected the Hill coefficient of Ca(2+) transport (n(H)). An increase in n(H) from approximately 2 in K(+) to approximately 3 in Cs(+) was seen in all of the forms of SERCA examined. The effects of Cs(+) on the maximum velocity of Ca(2+) uptake were also different for different forms of SERCA but these differences could not be attributed to differences in the putative K(+) binding sites of the different forms of the protein.

Immuno-proteomic approach to excitation--contraction coupling in skeletal and cardiac muscle: molecular insights revealed by the mitsugumins

A comprehensive understanding of excitation-contraction (E-C) coupling in skeletal and cardiac muscle requires that all the major components of the Ca(2+) release machinery be resolved. We utilized a unique immuno-proteomic approach to generate a monoclonal antibody library that targets proteins localized to the skeletal muscle triad junction, which provides a structural context to allow efficient E-C coupling. Screening of this library has identified several mitsugumins (MG); proteins that can be localized to the triad junction in mammalian skeletal muscle. Many of these proteins, including MG29 and junctophilin, are important components in maintaining the structural integrity of the triad junction. Other triad proteins, such as calumin, play a more direct role in regulation of muscle Ca(2+) homeostasis. We have recently identified a family of trimeric intracellular cation-selective (TRIC) channels that allow for K(+) movement into the endoplasmic or sarcoplasmic reticulum to counter a portion of the transient negative charge produced by Ca(2+) release into the cytosol. Further study of TRIC channel function and other novel mitsugumins will increase our understanding of E-C coupling and Ca(2+) homoeostasis in muscle physiology and pathophysiology.

Rapid intracellular TEA block of the KcsA potassium channel

Intracellular tetraethylammonium (TEA) inhibition was studied at the single-channel level in the KcsA potassium channel reconstituted in planar lipid bilayers. TEA acts as a fast blocker (resulting in decreased current amplitude) with an affinity in the 75 mM range even at high bandwidth. Studies over a wide voltage range reveal that TEA block has a complex voltage-dependence that also depends on the ionic conditions. These observations are examined in the context of permeation models to extend our understanding of the coupling between permeant ions and TEA blockade.

Effects of intracellular magnesium on Kv1.5 and Kv2.1 potassium channels

We characterized the effects of intracellular Mg(2+) (Mg(2+) (i)) on potassium currents mediated by the Kv1.5 and Kv2.1 channels expressed in Xenopus oocytes. Increase in Mg(2+) (i) caused a voltage-dependent block of the current amplitude, apparent acceleration of the current kinetics (explained by a corresponding shift in the steady-state activation) and leftward shifts in activation and inactivation dependencies for both channels. The voltage-dependent block was more potent for Kv2.1 [dissociation constant at 0 mV, K(d)(0), was approximately 70 mM and the electric distance of the Mg(2+) binding site, delta, was 0.2] than for the Kv1.5 channel [K(d)(0) approximately 40 mM and delta = 0.1]. Similar shifts in the voltage-dependent parameters for both channels were described by the Gouy-Chapman formalism with the negative charge density of 1 e(-)/100 A(2). Additionally, Mg(2+) (i) selectively reduced a non-inactivating current and increased the accumulation of inactivation of the Kv1.5, but not the Kv2.1 channel. A potential functional role of the differential effects of Mg(2+) (i) on the Kv channels is discussed.

Site of action of fatty acids and other charged lipids on BKCa channels from arterial smooth muscle cells

Fatty acids and other negatively charged single-chain lipids increase large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel activity, whereas sphingosine and other positively charged single-chain lipids suppress activity. Because these molecules are effective on both inside-out and outside-out patches and because they can flip across the bilayer, the location of their site of action is unclear. To identify the site of action of charged lipids on this channel, we used two compounds that are unlikely to flip across the lipid bilayer. Palmitoyl coenzyme A (PCoA) was used to identify the site of action of negatively charged lipids, and a positively charged myristoylated pentapeptide (myr-KPRPK) was used to investigate the site of action of positively charged lipids. The effect of these compounds on channel activity was studied in excised patches using patch-clamp techniques. In "normal" ionic strength solutions and in experiments where high-ionic strength solutions were used to shield membrane surface charge, PCoA increased channel activity only when applied to outside-out patches, suggesting that the site of action of negatively charged lipids is located on the outer surface of the membrane. A decrease in activity, similar to that of other positively charged lipids, was observed only when myr-KPRPK was applied to outside-out patches, suggesting that positively charged lipids suppress activity by also acting on the outer membrane surface. Some channel blockade effects of myr-KPRPK and KPRPK are also described. The sidedness of action suggests that modulation of channel activity by single-chain lipids can occur by their interaction with the channel protein.

Sarcoplasmic reticulum K(+) channels from human and sheep atrial cells display a specific electro-pharmacological profile

It has recently been proposed that the Ca(2+) uptake by the SR is inhibited by blocking Cl(-) and/or K(+) movements across this intracellular membrane. We have characterised the functional and pharmacological profile of the SR K(+) channel derived from human and sheep atrial cells. Mammalian atrial SR preparations were subjected to [(3)H]-ryanodine binding assays, SDS-PAGE analysis and channel protein reconstitution into planar lipid bilayers. Assessment of [(3)H]-ryanodine binding on the SR Ca(2+) release channel revealed that it was inhibited by both Ruthenium Red and Mg(2+) with IC(50) values of 4.11 microM and 9.12 m M, respectively. In crude populations as well as in all SR-enriched fractions, activity of K(+) selective channels was recorded. This channel displayed a high conductance value of 193 and 185 pS for human and sheep preparations respectively. Gating and conducting behaviours of this channel were unaffected by the addition of up to 5m M 4-Aminopyridine (4-AP), 100 n M Iberiotoxin (IbTX), 10 microM E-4031 and 30 microM amiodarone. However, 100n M Dendrotoxin (gamma-DTX) largely increase the occurrence of the SR K(+) channel subconducting states without an effect on the main unitary conductance. These results demonstrate that the SR K(+) channel, present in all mammalian atrial SR membranes tested (as assessed by [(3)H]-ryanodine binding and its typical inhibition by ruthenium red and the magnesium), displays different properties than those classically described for cardiac sarcolemmal K(+) channels. Despite the fact that the biophysical properties of the SR K(+) channel are well known, its molecular identity remains to be ascertained.

Gating and conductance properties of BK channels are modulated by the S9-S10 tail domain of the alpha subunit. A study of mSlo1 and mSlo3 wild-type and chimeric channels

The COOH-terminal S9-S10 tail domain of large conductance Ca(2+)-activated K(+) (BK) channels is a major determinant of Ca(2+) sensitivity (Schreiber, M., A. Wei, A. Yuan, J. Gaut, M. Saito, and L. Salkoff. 1999. Nat. Neurosci. 2:416-421). To investigate whether the tail domain also modulates Ca(2+)-independent properties of BK channels, we explored the functional differences between the BK channel mSlo1 and another member of the Slo family, mSlo3 (Schreiber, M., A. Yuan, and L. Salkoff. 1998. J. Biol. Chem. 273:3509-3516). Compared with mSlo1 channels, mSlo3 channels showed little Ca(2+) sensitivity, and the mean open time, burst duration, gaps between bursts, and single-channel conductance of mSlo3 channels were only 32, 22, 41, and 37% of that for mSlo1 channels, respectively. To examine which channel properties arise from the tail domain, we coexpressed the core of mSlo1 with either the tail domain of mSlo1 or the tail domain of mSlo3 channels, and studied the single-channel currents. Replacing the mSlo1 tail with the mSlo3 tail resulted in the following: increased open probability in the absence of Ca(2+); reduced the Ca(2+) sensitivity greatly by allowing only partial activation by Ca(2+) and by reducing the Hill coefficient for Ca(2+) activation; decreased the voltage dependence approximately 28%; decreased the mean open time two- to threefold; decreased the mean burst duration three- to ninefold; decreased the single-channel conductance approximately 14%; decreased the K(d) for block by TEA(i) approximately 30%; did not change the minimal numbers of three to four open and five to seven closed states entered during gating; and did not change the major features of the dependency between adjacent interval durations. These observations support a modular construction of the BK channel in which the tail domain modulates the gating kinetics and conductance properties of the voltage-dependent core domain, in addition to determining most of the high affinity Ca(2+) sensitivity.

Phosphate ion channels in sarcoplasmic reticulum of rabbit skeletal muscle

1. Phosphate ions (P(i)) enter intracellular Ca2+ stores and precipitate Ca2+. Since transport pathways for P(i) across the membrane of intracellular calcium stores have not been identified and anion channels could provide such a pathway, we have examined the P(i) conductance of single anion channels from the sarcoplasmic reticulum (SR) of rabbit skeletal muscle using the lipid bilayer technique. 2. Two anion channels in skeletal muscle SR, the small conductance (SCl) and big conductance (BCl) chloride channels, were both found to have a P(i) conductance of 10 pS in 50 mM P(i). The SCl channel is a divalent anion channel which can pass HPO4(2-) as well as SO4(2-) (60 pS in 100 mM free SO4(2-)). The BCl channel is primarily a monovalent anion channel. The SCl and BCl channels are permeable to a number of small monovalent anions, showing minor selectivity between Cl-, I- and Br- (Cl- > I- > Br-) and relative impermeability to cations and large polyatomic anions (Cs+, Na+, choline+, Tris+, Hepes- and CH3O3S-). 3. The P(i) conductance of SCl and BCl channels suggests that both channel types could sustain the observed P(i) fluxes across the SR membrane. Comparison of the blocking effects of the phosphonocarboxylic acids, ATP and DIDS, on the anion channels with their effects on P(i) transport suggests that the SCl channel is the more likely candidate for the SR P(i) transport mechanism. 4. The SCl channel, with previously unknown function, provides a regulated pathway for P(i) across the SR membrane which would promote P(i) entry and thereby changes in the rapidly releasable Ca2+ store during onset and recovery from muscle fatigue. Anion channels may provide a pathway for P(i) movement into and out of Ca2+ stores in general.

Structure-function study on a de novo synthetic hydrophobic ion channel

Ion conduction properties of a de novo synthesized channel, formed from cyclic octa-peptides consisting of four alternate L-alanine (Ala) and N'-acylated 3-aminobenzoic acid (Aba) moieties, were studied in bilayer membranes. The single-channel conductance was 9 pS in symmetrical 500 mM KCl. The channel favored permeation of cations over anions with a permeability ratio (PCl-/PK+) of 0.15. The selectivity sequence among monovalent cations based on permeability ratio (PX+/PK+) fell into an order: NH4+(1.4) > Cs+(1. 1) >/= K+(1.0) > Na+(0.4) >> Li+(0). The conductance-activity relationship of the channel in K+ solutions followed simple Michaelis-Menten kinetics with a half-maximal saturating activity of 8 mM and a maximal conductance of 9 pS. The permeability ratio PNa+/PK+ remained constant ( approximately 0.40) under biionic concentrations from 10 to 500 mM. These results suggests that the channel is a one-ion channel. The pore diameter probed by a set of organic cations was approximately 6 A. The single-channel current was blocked by Ca2+ in a dose-dependent manner that followed a single-site titration curve with a voltage-dependent dissociation constant of 0.6 mM at 100 mV. The electric distance of the binding site for Ca2+ was 0.07 from both entrances of the channel, indicating the presence of two symmetrical binding sites in each vicinity of the channel entrance. Correlations between conduction properties and structural aspects of the channel are discussed in terms of a three-barrier and two-binding-site (3B2S) model of Eyring rate theory. All available structural information supported an idea that the channel was formed from a tail-to-tail associated dimer of the molecule, the pore of which was lined with hydrophobic acyl chains. This is the first report to have made a systematic analysis of ion permeation through a hydrophobic pore.

Potassium Channel Blockers Inhibit Adoptive Transfer of Experimental Allergic Encephalomyelitis by Myelin-Basic-Protein-Stimulated Rat T Lymphocytes

Agents which block T cell K(+) currents can prohibit both proliferative and effector cell functions in T cells activated by mitogens or phorbol esters. This study examined the effects of some of these blocking agents on the immune responsiveness of guinea pig myelin basic protein (GPMBP)-reactive Lewis rat T lymphocytes, which are capable of mediating the adoptive transfer of experimental allergic encephalomyelitis (EAE), an accepted animal model for multiple sclerosis. Both the proliferative functions (DNA synthesis and cell blastogenesis) and the EAE transfer activities of GPMBP-reactive lymphocytes were examined following GPMBP-induced activation in the presence of agents shown to block the outwardly rectifying K(+) current in these cells. At concentrations which completely inhibited DNA synthesis, as measured by [(3)H]thymidine incorporation, and cell blastogenesis, tetraethylammonium (TEA), 4-aminopyridine (4-AP) and methoxyverapamil (D60) completely blocked the subsequent adoptive transfer of EAE into naive syngeneic Lewis rats. The concentrations at which these blockers produced a 50% reduction in DNA synthesis were estimated to be 16, 1.6 and 32 &mgr;M for TEA, 4-AP and D-600, respectively, which were roughly equivalent to the EC(50) to block the K(+) current. Apamine, a potent Ca(2+)-activated K(+) channel blocker, at a concentration several orders of magnitude higher than is necessary to block Ca(2+)-activated K(+) channels, reduced the maximal K(+) conductance in GPMBP-reactive T cell K(+) channels by about 20%, but did not alter either [H(3)H]thymidine incorporation or the adoptive transfer of EAE. These results indicate that delayed rectifier K(+) channel blockers may prevent the activation of GPMBP-reactive T cells, thus prohibiting encephalitogenic effector cell functions. Copyright 1997 S. Karger AG, Basel

Block of the cGMP-gated cation channel of catfish rod and cone photoreceptors by organic cations

Tetraalkylammonium compounds and other organic cations were used to probe the structure of the internal and external mouths of the pore of cGMP-gated cation channels from rod and cone photoreceptors. Both rod and cone channels were blocked by tetramethyl- through tetrapentylammonium from the intracellular side in a voltage-dependent fashion at millimolar to micromolar concentrations. The dissociation constant at 0 mV (KD(O)) decreased monotonically with increasing carbon chain length from approximately 80 mM (TMA) to approximately 80 microM (TPeA), where the dissociation constant in rod channels is approximately 50% that of cone channels. N-Methyl-D-glucamine and the buffer Tris also blocked the cone channel in a voltage-dependent fashion at millimolar concentrations, but with lower affinity than similarly sized tetraalkylammonium blockers. Block by tetrahexylammonium (THxA) was voltage-independent, suggesting that the diameter of the intracellular mouth of these channels is less than the size of THxA but larger than TPeA. The location of the binding site for intracellular blockers was approximately 40% across the voltage-drop from the intracellular side. The addition of one carbon to each of the alkyl side chains increased the binding energy by approximately 4 kJ mol-1, consistent with hydrophobic interactions between the blocker and the pore. Cone, but not rod, channels were blocked by millimolar concentrations of extracellular TMA. The location of the extracellular binding site was approximately 13% of the voltage drop from the extracellular side. In cone channels, the two blocker binding sites flank the location of the cation binding site proposed previously.

ORK1, a potassium-selective leak channel with two pore domains cloned from Drosophila melanogaster by expression in Saccharomyces cerevisiae

A K+ channel gene has been cloned from Drosophila melanogaster by complementation in Saccharomyces cerevisiae cells defective for K+ uptake. Naturally expressed in the neuromuscular tissues of adult flies, this gene confers K+ transport capacity on yeast cells when heterologously expressed. In Xenopus laevis oocytes, expression yields an ungated K(+)-selective current whose attributes resemble the "leak" conductance thought to mediate the resting potential of vertebrate myelinated neurons but whose molecular nature has long remained elusive. The predicted protein has two pore (P) domains and four membrane-spanning helices and is a member of a newly recognized K+ channel family. Expression of the channel in flies and yeast cells makes feasible studies of structure and in vivo function using genetic approaches that are not possible in higher animals.

Subconductance block of single mechanosensitive ion channels in skeletal muscle fibers by aminoglycoside antibiotics

The activity of single mechanosensitive channels was recorded from cell-attached patches on acutely isolated skeletal muscle fibers from the mouse. The experiments were designed to investigate the mechanism of channel block produced by externally applied aminoglycoside antibiotics. Neomycin and other aminoglycosides reduced the amplitude of the single-channel current at negative membrane potentials. The block was concentration-dependent, with a half-maximal concentration of approximately 200 microM. At high drug concentrations, however, block was incomplete with roughly one third of the current remaining unblocked. Neomycin also caused the channel to fluctuate between the open state and a subconductance level that was also roughly one third the amplitude of the fully open level. An analysis of the kinetics of the subconductance fluctuations was consistent with a bimolecular reaction between an aminoglycoside molecule and the open channel (kon = approximately 1 x 10(6) M-1s-1 and koff = approximately 400 s-1 at -60 mV). Increasing the external pH reduced both the rapid block of the open channel and the frequency of the subconductance fluctuations, as if both blocking actions were produced by a single active drug species with a pKa = approximately 7.5. The results are interpreted in terms of a mechanism in which an aminoglycoside molecule partially occludes ion flow through the channel pore.

Conducting and voltage-dependent behaviors of potassium ion channels reconstituted from diaphragm sarcoplasmic reticulum: comparison with the cardiac isoform

Sarcoplasmic reticulum (SR) K+ channels from canine diaphragm were studied upon fusion of longitudinal and junctional membrane vesicles into planar lipid bilayers (PLB). The large-conductance cation selective channel (gamma(max) = 250 pS; Km = 33 mM) displays long-lasting open events which are much more frequent at positive than at negative voltages. A major subconducting state about 45% of the fully-open state current amplitude was occasionally observed at all voltages. The voltage-dependence of the open probability displays a sigmoid relationship that was fitted by the Boltzmann equation and expressed in terms of thermodynamic parameters, namely the free energy (delta Gi) and the effective gating charge (Zs): delta Gi = 0.27 kcal/mol and Zs = -1.19 in 250 mM potassium gluconate (K-gluconate). Kinetic analyses also confirmed the voltage-dependent gating behavior of this channel, and indicate the implication of at least two open and three closed states. The diaphragm SR K+ channel shares several biophysical properties with the cardiac isoform: g = 180 pS, delta Gi = 0.75 kcal/mol, Zs = -1.45 in 150 mM K-gluconate, and a similar sigmoid P(o)/voltage relationship. Little is known about the regulation of the diaphragm and cardiac SR K+ channels. The conductance and gating of these channels were not influenced by physiological concentrations of Ca2+ (0.1 microM-1 mM) or Mg2+ (0.25-1 mM), as well as by cGMP (25-100 microM), lemakalim (1-100 microM), glyburide (up to 10 microM) or charybdotoxin (45-200 nM), added either to the cis or to the trans chamber. The apparent lack of biochemical or pharmacological modulation of these channels implies that they are not related to any of the well characterized surface membrane K+ channels. On the other hand, their voltage sensitivity strongly suggests that their activity could be modulated by putative changes in SR membrane potential that might occur during calcium fluxes.

Quaternary ammonium ion blockade of IK in nerve axons revisited. Open channel block vs. state independent block

The mechanism of blockade of the delayed rectifier potassium ion channel in squid giant axons by intracellular quaternary ammonium ions (QA) appears to be remarkably sensitive to the structure of the blocker. TEA, propyltriethyl-ammonium (C3), and propyltetraethylammonium (TAA-C3) all fail to alter the deactivation, or "tail" current time course following membrane depolarization, even with relatively large concentrations of the blockers, whereas butyltriethylammonium (C4), butyltetraethylammonium (TAA-C4), and pentytriethyammonium (C5) clearly do have such an effect. The relative electrical distance of blockade for all of these ions is approximately 0.25-0.3 from the inner surface of the membrane. The observations concerning TEA, C3, and TAA-C3 suggest that these ions can block the channel in either its open or its closed state. The results with C4, TAA-C4, and C5 are consistent with the open channel block model. Moreover, the sensitivity of block mechanism to the structure of the blocker suggests that the gate is located close to the QA ion binding site and that TEA, C3, and TAA-C3 do not interfere with channel gating, whereas C4, TAA-C4, C5, and ions having a longer hydrophobic "tail" than C5 do have such an effect. The parameters of block obtained for all QA ions investigated were unaffected by changes in the extracellular potassium ion concentration.

Ion selectivity of colicin E1: III. Anion permeability

The antibiotic protein colicin E1 forms ion channels in planar lipid bilayers that are capable of conducting monovalent organic cations having mean diameters of at least 9 A. Polyvalent organic cations appear to be completely impermeant, regardless of size. All permeant ions, whether large or small, positively or negatively charged, are conducted by this channel at very slow rates. We have examined the permeability of colicin E1 channels to anionic probes having a variety of sizes, shapes, and charge distributions. In contrast to the behavior of cations, polyvalent as well as monovalent organic anions were found to permeate the colicin E1 channel. Inorganic sulfate was able to permeate the channel only when the pH was 4 or less, conditions under which the colicin E1 protein is predominantly in an anion-preferring conformational state. The less selective state(s) of the colicin E1 channel, observed when the pH was 5 or greater, was not permeable to inorganic sulfate. The sulfate salt of the impermeant cation Bis-T6 (N,N,N',N'-tetramethyl-1,6-hexanediamine) had no effect on the single channel conductance of colicin E1 channels exposed to solutions containing 1 M NaCl at pH 5. The complete lack of blocking activity by either of these two impermeant ions indicates that both are excluded from the channel lumen. These results are consistent with our hypothesis that there is but a single location in the lumen of the colicin E1 channel where positively charged groups can be effectively hydrated. This site may coincide with the location of the energetic barrier which impedes the movement of anions.

Dimensions of the narrow portion of a recombinant NMDA receptor channel

Glutamate-activated single-channel and ensemble currents were recorded from Xenopus laevis oocytes and HEK 293 cells expressing a recombinant NMDA receptor, assembled from NR1 and NR2A subunits. Cesium was the main charge carrier, and organic cations were used to determine the presence of vestibules of this channel and to estimate its pore diameter. The large organic cations tris-(hydroxymethyl)-aminomethane (Tris), N-methyl-glucamine (NMG), arginine (NMG), arginine (Arg), choline, and tetramethylammonium (TMA), when added in millimolar concentrations to the extracellular or cytoplasmic side, produced a voltage-dependent blockade of single-channel Cs+ currents. These molecules behaved as impermeant ions that only partially traverse the channel from either side. The smaller cations trimethylammonium (TriMA) and dimethylammonium (DMA) produced a small and nearly voltage-independent reduction in current amplitude, suggesting that they are permeant. In biionic experiments with Cs+ as the reference ion, the large blocking cations NMG, Arg, Tris, TMA, choline, hexamethonium (Hme), triethylammonium (TriEA), and tetraethylammonium (TEA) showed no measurable permeability. TriMA and smaller ammonium derivatives were permeant. Both the permeability and single-channel conductance of organic cations, relative to Cs+, decreased as the ion size increased. The results suggest that the NMDA receptor has extracellular and cytoplasmic mouths that can accommodate large cations up to 7.3 A in mean diameter. The narrow portion of the pore is estimated to have a mean diameter of 5.5 A.

Monovalent amidiniums block calcium channels in chick sensory neurons

The effect of amidiniums on high-threshold Ca2+ channel currents (ICa) was studied in chick dorsal root ganglion neurons. Guanidinium reduced ICa in a dose-dependent fashion. The block was relieved by increasing the concentration of the permeant ions, Ba2+ or Ca2+, suggesting a competition for a common binding site within the channel. Formamidinium and methyl-guanidinium suppressed ICa with similar potencies, whereas L-arginine had no effect. A neutral amidine, urea, increased ICa. In Ca(2+)-free solutions guanidinium and Na+ permeated through the Ca2+ channel equally well. Structure-activity relationship obtained for blocking efficacies of different amidiniums are used to discuss possible configurations of the selectivity filter in the Ca2+ channel.

O2 occlusion and cyanide induced immediate relaxation and contraction of murine skeletal muscle

The acute changes of muscle tone and membrane current upon occlusion of oxygenation (O2 occlusion) were studied in vitro in mouse diaphragms. O2 occlusion immediately produced a contraction and a relaxation, respectively, in ryanodine- and high K(+)-contracted muscles while a biphasic change (an initial decrease then a late increase) of muscle tone was produced in muscles contracted with caffeine. The O2 occlusion effects were reversed after reoxygenation. CN- produced similar acute changes of muscle tone and abolished O2 occlusion effects. The O2 occlusion-induced relaxation in high K+ medium was converted into a contraction by 3,4-diaminopyridine and by low Cl- Tyrode's. O2 occlusion induced a small outward current and membrane hyperpolarization at a rate slower than the changes of muscle tone. Glybenclamide inhibited all of the changes induced by O2 occlusion. It is possible that the K+ and Cl- permeabilities of sarcoplasmic reticulum are highly sensitive to hypoxic challenge and related to the immediate changes of muscle tone after O2 occlusion.

Trinitrophenyl-ATP blocks colonic Cl- channels in planar phospholipid bilayers. Evidence for two nucleotide binding sites

Outwardly rectifying 30-50-pS Cl- channels mediate cell volume regulation and transepithelial transport. Several recent reports indicate that rectifying Cl- channels are blocked after addition of ATP to the extracellular bath (Alton, E. W. F. W., S. D. Manning, P. J. Schlatter, D. M. Geddes, and A. J. Williams. 1991. Journal of Physiology. 443:137-159; Paulmichl, M., Y. Li, K. Wickman, M. Ackerman, E. Peralta, and D. Clapham. 1992. Nature. 356:238-241). Therefore, we decided to conduct a more detailed study of the ATP binding site using a higher affinity probe. We tested the ATP derivative, 2',3',O-(2,4,6-trinitrocyclohexadienylidene) adenosine 5'-triphosphate (TNP-ATP), which has a high affinity for certain nucleotide binding sites. Here we report that TNP-ATP blocked colonic Cl- channels when added to either bath and that blockade was consistent with the closed-open-blocked kinetic model. The TNP-ATP concentration required for a 50% decrease in open probability was 0.27 microM from the extracellular (cis) side and 20 microM from the cytoplasmic (trans) side. Comparison of the off rate constants revealed that TNP-ATP remained bound 28 times longer when added to the extracellular side compared with the cytoplasmic side. We performed competition studies to determine if TNP-ATP binds to the same sites as ATP. Addition of ATP to the same bath containing TNP-ATP reduced channel amplitude and increased the time the channel spent in the open and fast-blocked states (i.e., burst duration). This is the result expected if TNP-ATP and ATP compete for block, presumably by binding to common sites. In contrast, addition of ATP to the bath opposite to the side containing TNP-ATP reduced amplitude but did not alter burst duration. This is the result expected if opposite-sided TNP-ATP and ATP bind to different sites. In summary, we have identified an ATP derivative that has a nearly 10-fold higher affinity for reconstituted rectifying colonic Cl- channels than any previously reported blocker (Singh, A. K., G. B. Afink, C. J. Venglarik, R. Wang, and R. J. Bridges. 1991. American Journal of Physiology. 260 [Cell Physiology. 30]:C51-C63). Thus, TNP-ATP should be useful in future studies of ion channel nucleotide binding sites and possibly in preliminary steps of ion channel protein purification. In addition, we have obtained good evidence that there are at least two nucleotide binding sites located on opposite sides of the colonic Cl- channel and that occupancy of either site produces a blocked state.

BisG10, a K+ channel blocker, affects the calcium release channel from skeletal muscle sarcoplasmic reticulum

The action of bisG10, a potent K+ channel inhibitor, was tested on the Ca2+ release from isolated sarcoplasmic reticulum vesicles of rabbit skeletal muscle. Using a rapid filtration technique, we found that the drug inhibited Ca(2+)-induced Ca2+ release elicited in the presence of extravesicular K+ as counter-ion. This inhibition was not reversed by the addition of valinomycin and still occurred when Cl- was used as co-ion, indicating that not only K+ channels are involved in the inhibiting effect. We found that bisG10 decreased the binding of ryanodine to sarcoplasmic reticulum vesicles, showing that bisG10 is able to block the sarcoplasmic reticulum Ca2+ release channel.

Ultrastructure of sarcoballs on the surface of skinned amphibian skeletal muscle fibres

The formation of sarcoballs on the surface of skinned fibres from semitendinosus muscles of Xenopus laevis, and the sarcoplasmic reticulum content of the structures, have been studied using conventional electron microscopic techniques and immunoelectron microscopy. Examination of the fibres showed many membrane-bound blebs projecting from the surface in areas where vesicles of internal membranes (including sarcoplasmic reticulum, T-tubules and mitochondria) were clustered in interfilament spaces. The blebs varied in size from 1 micron to 150 microns and those with diameters > 10 microns are referred to as sarcoballs. Small blebs were often seen in close association with each other and might have fused during sarcoball formation. The interior of the sarcoball was filled with foam-like material made up of vesicles with diameters of 100 nm to 1.0 microns. The sarcoplasmic reticulum membrane content of the sarcoballs was evaluated using two monoclonal antibodies, one to the Ca2+ ATPase of the sarcoplasmic reticulum and the second to ryanodine receptor calcium release channels in the junctional-face membrane. The antibodies bound to some components of the surface and interior of the sarcoball, but not to mitochondrial-like structures and tubular vesicles. The results show that a large component of the sarcoball and its surface is derived from sarcoplasmic reticulum and suggest that mitochondria and T-tubules might also contribute membranes to the structures. Our hypothesis is that (a) blebs bud out from the surface of the skinned fibre following fusion of internal vesicles that are extruded along interfilament channels during unrestrained contractures, (b) blebs grow into sarcoballs by additional fusion of internal membrane vesicles and fusion of adjacent blebs, and (c) the sarcoball is a foam-like structure composed of bathing medium and membrane lipid (containing membrane proteins).

Ion selectivity of colicin E1: II. Permeability to organic cations

Channels formed by colicin E1 in planar lipid bilayers have large diameters and conduct both cations and anions. The rates at which ions are transported, however, are relatively slow, and the relative anion-to-cation selectivity is modulated over a wide range by the pH of the bathing solutions. We have examined the permeability of these channels to cationic probes having a variety of sizes, shapes, and charge distributions. All of the monovalent probes were found to be permeant, establishing a minimum diameter at the narrowest part of the pore of approximately 9 A. In contrast to this behavior, all of the polyvalent organic cations were shown to be impermeant. This simple exclusionary rule is interpreted as evidence that, when steric restrictions require partial dehydration of an ion, the structure of the channel is able to provide a substitute electrostatic environment for only one charged group at time.

Block of the sheep cardiac sarcoplasmic reticulum Ca(2+)-release channel by tetra-alkyl ammonium cations

The purified ryanodine receptor channel of the sheep cardiac muscle sarcoplasmic reticulum (SR) membrane functions as a calcium-activated cation-selective channel under voltage-clamp conditions following reconstitution into planar phospholipid bilayers. We have investigated the effects of the tetra-alkyl ammonium (TAA) cations, (CnH2n+1)4N+ and the trimethyl ammonium cations, ethyltrimethyl ammonium and propyltrimethyl ammonium, on potassium conductance through the receptor channel. Small TAA cations (n = 1-3) and the trimethyl ammonium derivatives act as asymmetric, voltage-dependent blockers of potassium current. Quantitative analysis of the voltage dependence of block indicates that the conduction pathway of the sheep cardiac SR ryanodine receptor channel contains two distinct sites for the interaction of these small organic cations. Sites are located at approximately 50% for tetramethyl ammonium (TMA+) and 90% for tetraethyl ammonium (TEA+) and tetrapropyl ammonium (TPrA+) of the voltage drop across the channel from the cytosolic face of the protein. The chemical substitution of an ethyl or propyl group for one of the methyl groups in TMA+ increases the voltage dependence of block to a level similar to that of TEA+ and TPrA+. The zero-voltage dissociation constant (Kb(0)) falls with the increasing number of methyl and methylene groups for those blockers acting 90% of the way across the voltage drop. This is interpreted as suggesting a hydrophobic binding site at this point in the conduction pathway. The degree of block increases as the concentration of small TAA cations is raised. The concentration dependence of tetraethyl ammonium block indicates that the cation interacts with a single site within the conduction pathway with a Km of 9.8 +/- 1.7 mM (mean +/- SD) at 40 mV. Larger TAA cations (n = 4-5) do not induce voltage-dependent block of potassium current of the form seen with the smaller TAA cations. These data support the contention that the sheep cardiac SR ryanodine receptor channel may be occupied by at most one ion at a time and suggest that a large proportion of the voltage drop falls over a relatively wide region of the conduction pathway.

Reappraisal of the role of sodium ions in excitation-contraction coupling in frog twitch muscle

Tetanic and twitch tension were recorded on isolated frog twitch fibres under experimental conditions modifying the influx of sodium ions. In current clamp conditions replacing Li+ for Na+ did not modify the electrical activity but drastically decreased the plateau of tetanic tension. In voltage clamp conditions replacing Li+ for Na+ did not modify the inward currents but induced a marked decrease of the plateau of the tetanic tension for depolarizations between the activation threshold and the reversal potential of sodium current. Under veratridine treatment, during tetanic depolarization, a slow inward sodium (or lithium) current developed. This induced a parallel increase of the tetanic tension which was much more pronounced in sodium than in lithium containing solution. The twitch tension obtained during short depolarization was increased by greater than 100% during veratridine treatment with a sizeable decrease (40%) of the delay between the end of depolarization and the beginning of tension. All these results could be reproduced in calcium-free solution. Our data confirm that the entry of sodium ions (and to a lesser extent of lithium ions) is able to modulate the release of calcium from the sarcoplasmic reticulum (SR). We discuss these results in terms of a model where sodium ions entering the compartment between the tubular membrane and the SR junctional membrane carry counter charges through the SR K+ channels and help to maintain the SR Ca2+ release. This could occur in particular during a physiological tetanic contraction where the junctional compartment is probably filled with Na+ ions and depleted of K+ ions.

Purification of a K(+)-channel protein of sarcoplasmic reticulum by assaying the channel activity in the planar lipid bilayer system

A K(+)-channel protein of the sarcoplasmic reticulum (SR) was purified by assaying the channel activity in a planar lipid bilayer system. The light fraction of SR vesicles was solubilized in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and fractionated by an anion-exchange chromatography and followed by gel filtration chromatography and affinity chromatography with concanavalin A. All fractions in each steps were mixed with asolectin solubilized in CHAPS and reconstituted into vesicles by dialysis. The channel activity of each fraction was assayed after the reconstituted vesicles had been fused into a planar lipid bilayer. The final fraction which showed the K(+)-channel activity contained only 100 kDa protein in a silver-stained gel after SDS-PAGE and an anti-Ca(2+)-ATPase antibody did not recognize the protein. The characteristics of the K(+)-channel were identical to those observed in native SR vesicles when using the same method. The channel showed a single-channel conductance of 120 pS in 0.1 M KCl and marked voltage dependence. The channel did not permeate Ca2+ and Cl- and was blocked by neomycin B.

Monovalent cation conductance in the ryanodine receptor-channel of sheep cardiac muscle sarcoplasmic reticulum

1. The ryanodine receptor protein of sheep cardiac muscle sarcoplasmic reticulum membranes functions as a ligand-regulated ion channel following solubilization with the zwitterionic detergent CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1- propane sulphonate); purification by density gradient centrifugation, reconstitution into proteo-liposomes and incorporation into planar phospholipid bilayers. 2. In the absence of divalent cations, measurable conductance is observed with the group 1a cations and with some larger organic cations. In symmetric 210 mM solutions the following conductance sequence was determined: K+ greater than Rb+ = NH4+ greater than Na+ = Cs+ greater than Li+ much greater than Tris+. 3. Other organic cations, e.g. TEA+, do not produce measurable current under these conditions. 4. Single-channel conductance saturates with increasing ionic activities of K+, Na+ and Li+. Saturation curves are described by Michaelis-Menten kinetic schemes with the following values of maximal conductance and apparent dissociation constant: K+ 900 pS, 19.9 mM; Na+ 516 pS, 17.8 mM; Li+ 248 pS, 9.1 mM. 5. The channel displays only minor differences in permeability amongst the group 1a cations. Relative permeability, monitored under bi-ionic conditions, yields the following sequence: Na+, 1.15 greater than K+, 1.00 = Li+, 0.99 greater than Rb+, 0.87 greater than Cs+, 0.61. Under similar conditions the permeability ratio of NH4+ to K+ was found to be 1.32 and that for Tris+ to K+ was 0.22. 6. The K+ conductance is reduced by low concentrations of the impermeant cation TEA+. Block appears as a smooth reduction in single-channel current amplitude and the degree of block is dependent upon applied voltage. These observations are consistent with a single-site blocking scheme in which TEA+ has access to a site within the voltage drop of the channel from only the cytosolic face of the channel protein and interacts with a site located approximately 90% of the electrical distance across the channel. The zero-voltage dissociation constant for TEA+ block is 50 mM. 7. Single-channel conductance measurements in mixtures of K(+)-Na+ and K(+)-Li+ reveal no anomalous behaviour as the mole fraction of the ions is varied. 8. With monovalent cations as permeant species, the sheep cardiac sarcoplasmic reticulum ryanodine receptor protein functions as a poorly selective, ligand-regulated channel. Under the conditions described here the channel functions as a single-ion pore. It is proposed that discrimination is largely dependent upon the strength of interaction of the permeant ion with a binding site in the conduction pathway.

Effects of Mg2+ and ATP on the phosphate transporter of sarcoplasmic reticulum

The extra uptake of Ca2+ by vesicles of sarcoplasmic reticulum (SR) observed in the presence of Pi, attributable to transport of Pi by the Pi-transporter, has been studied. It has been shown that the Pi transporter is stimulated by ATP. Single channel conductance measurements have shown that the Cl- channel in the SR membrane is impermeable to Pi. It is suggested that the transporter could be an ion antiporter system. Studies of uptake as a function of pH and Mg2+ concentration suggest that transport of MgHPO4 and H2PO-4 are faster than transport of HPO2-4. For oxalate and pyrophosphate, Mg2+ binding inhibits transport. It is suggested that protonation of lysine residue(s) at the anion binding site increase the rate of transport.

Potassium channels in aortic microsomes: conductance, selectivity, barium-induced blockage and subconductance states

The activity of potassium channels of canine aortic sarcoplasmic reticulum was measured using the planar lipid bilayer-fusion technique. The channels have a conductance of 208 pS (400/100 mM K+ in cis/trans solutions) and potassium-to-sodium permeability ratio of 7.7 Ba2+ ions produced two main effects: one is the interruption of channel currents for tens to hundreds of milliseconds in a voltage-dependent manner, and the other is the appearance of a second conductance level with amplitude about 60% of the main level.

Divalent cation selectivity for external block of voltage-dependent Na+ channels prolonged by batrachotoxin. Zn2+ induces discrete substates in cardiac Na+ channels

The mechanism of block of voltage-dependent Na+ channels by extracellular divalent cations was investigated in a quantitative comparison of two distinct Na+ channel subtypes incorporated into planar bilayers in the presence of batrachotoxin. External Ca2+ and other divalent cations induced a fast voltage-dependent block observed as a reduction in unitary current for tetrodotoxin-sensitive Na+ channels of rat skeletal muscle and tetrodotoxin-insensitive Na+ channels of canine heart ventricular muscle. Using a simple model of voltage-dependent binding to a single site, these two distinct Na+ channel subtypes exhibited virtually the same affinity and voltage dependence for fast block by Ca2+ and a number of other divalent cations. This group of divalent cations exhibited an affinity sequence of Co congruent to Ni greater than Mn greater than Ca greater than Mg greater than Sr greater than Ba, following an inverse correlation between binding affinity and ionic radius. The voltage dependence of fast Ca2+ block was essentially independent of CaCl2 concentration; however, at constant voltage the Ca2+ concentration dependence of fast block deviated from a Langmuir isotherm in the manner expected for an effect of negative surface charge. Titration curves for fast Ca2+ block were fit to a simplified model based on a single Ca2+ binding site and the Gouy-Chapman theory of surface charge. This model gave similar estimates of negative surface charge density in the vicinity of the Ca2+ blocking site for muscle and heart Na+ channels. In contrast to other divalent cations listed above, Cd2+ and Zn2+ are more potent blockers of heart Na+ channels than muscle Na+ channels. Cd2+ induced a fast, voltage-dependent block in both Na+ channel subtypes with a 46-fold higher affinity at 0 mV for heart (KB = 0.37 mM) vs. muscle (KB = 17 mM). Zn2+ induced a fast, voltage-dependent block of muscle Na+ channels with low affinity (KB = 7.5 mM at 0 mV). In contrast, micromolar Zn2+ induced brief closures of heart Na+ channels that were resolved as discrete substate events at the single-channel level with an apparent blocking affinity of KB = 0.067 mM at 0 mV, or 110-fold higher affinity for Zn2+ compared with the muscle channel. High-affinity block of the heart channel by Cd2+ and Zn2+ exhibited approximately the same voltage dependence (e-fold per 60 mV) as low affinity block of the muscle subtype (e-fold per 54 mV), suggesting that the block occurs at structurally analogous sites in the two Na+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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 100 mM 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) greater than dibekacin (5) greater than ribostamycin (4) = kanamycin (4], while the electrical distance (z delta = 0.46-0.49) of the binding site kept almost constant for each drug. These z delta s were almost the same with those (0.46-0.51) of alkyl-diamine 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.

Ion channels in the nuclear envelope

Cell nuclei are capable of partitioning a wide variety of molecules from the cytosol, including macromolecules such as proteins and RNA, and smaller peptides, amino acids, sugars and Na+ and K+ ions, all of which can be accumulated in or excluded from the nuclear domain. There are two mechanisms behind this compartmentalization: selective retention of freely diffusible molecules, and selective entry through the nuclear envelope. It is generally accepted that the nuclear envelope restricts only the larger molecules. Here we apply the patch-clamp technique to isolated murine pronuclei and show that the nuclear envelope contains K(+)-selective channels which have multiple conductance states, the maximal conductance being 200 pS. These channels, which contribute to the nuclear membrane potential, may be important in balancing the charge carried by the movement of macromolecules in and out of the nucleus.

Effects of tetracaine and procaine on skinned muscle fibres depend on free calcium

The local anaesthetics, tetracaine and procaine have previously been found to block, induce or potentiate Ca2+ release from the sarcoplasmic reticulum (SR) of skeletal muscle depending on the preparation, experimental conditions and design. We now show that low concentrations of tetracaine and procaine block SR Ca2+ release whereas high concentrations induce release from the SR of amphibian and mammalian skinned fibres. Both actions depend on pCa, such that a shift in pCa can alter their effect from blocking to releasing Ca2+. In skinned fibres with Ca2+-loaded SR, tetracaine (1 mM) produced a tonic contraction with a time to half-peak of 15-20 s and a magnitude reaching 80% of maximum force. Ca2+ release by tetracaine or procaine occurred at pCa less than or equal to 6.5 and was not blocked by Ruthenium Red (RR) (25 mM). This action of tetracaine was attributed to SR Ca2+ release rather than to a displacement of bound Ca2+ because fibres lacking a functional SR due to pre-treatment with quercetin (100 mM), A 23187 (100 micrograms ml-1) or Triton X-100 (1%) did not contract after additions of tetracaine. Lower concentrations of tetracaine (0.5 mM) and procaine (less than or equal to 10 mM) blocked contractions due to caffeine (at pCa greater than or equal to 6.73), sulphydryl oxidizing agents, or Ca2+-induced Ca2+ release (CICR). The inhibition of CICR as a function of pCa was difficult to measure quantitatively since lowering pCa to elicit CICR twitches was sufficient to initiate tetracaine-induced tonic contractions. Experiments with isolated SR vesicles showed that 1 mM tetracaine inhibited CICR, over a wide range of pCa but 3-5 mM tetracaine induced rapid Ca2+ release. The opposite effects of tetracaine and procaine depend mostly on their concentration in SR vesicles and/or pCa in skinned fibres. Blockade of release seems to occur via the CICR pathway, and induction of release through an increase in SR membrane permeability.

Single chloride-selective channel from cardiac sarcoplasmic reticulum studied in planar lipid bilayers

The behavior of single Cl- channel was studied by fusing isolated canine cardiac sarcoplasmic reticulum (SR) vesicles into planar lipid bilayers. The channel exhibited unitary conductance of 55 pS (in 260 mM Cl-) and steady-state activation. Subconductance states were observed. Open probability was dependent on holding potentials (-60 to +60 mV) and displayed a bell-shaped relationship, with probability values ranging from 0.2 to 0.8 with a maximum at -10 mV. Channel activity was irreversibly inhibited by DIDS, a stilbene derivative. Time analysis revealed the presence of one time constant for the full open state and three time constants for the closed states. The open and the longer closed time constants were found to be voltage dependent. The behavior of the channel was not affected by changing Ca2+ and Mg2+ concentrations in both chambers, nor by adding millimolar adenosine triphosphate, or by changing the pH from 7.4 to 6.8. The presence of sulfate anions decreased the unit current amplitude, but did not affect the open probability. These results reveal that at the unitary level the cardiac SR anion-selective channel has distinctive as well as similar electrical properties characteristic of other types of Cl- channels.

Conduction and blocking properties of a predominantly anion-selective channel from human platelet surface membrane reconstituted into planar phospholipid bilayers

We have investigated the basic properties of a predominantly anion-selective channel derived from highly purified human platelet surface membrane. Single channels have been reconstituted into planar phospholipid bilayers by fusion of membrane vesicles and recorded under voltage-clamp conditions. The channel is found to have the following properties: (i) Channel activity occurs in bursts of openings separated by long closed periods. (ii) The current-voltage relationship is nonlinear. Channel current is seen to rectify, with less current flowing at positive than at negative voltages. Rectification may be due to asymmetric block by HEPES/Tris buffers. In 450 mM KCl, 5 mM HEPES/Tris, pH 7.2, the single channel conductance at -40 mV is approximately 160 pS and at +40 mV is approximately 90 pS. (iii) The conductance-concentration relationship follows a simple saturation curve. Half maximal conductance is achieved at a concentration of approximately 1000 mM KCl, and the curve saturates at a conductance of approximately 500 pS. (iv) Reversal potentials interpreted in terms of the Goldman-Hodgkin-Katz equation indicate a Cl: K permeability ratio of 4:1. (v) The channel accepts all of the halides as well as a number of other anions. The following sequence of relative anion permeabilities (in the presence of K+) is obtained: F- less than acetate- less than gluconate- less than Cl- less than Br- less than I- less than NO3- less tha SCN-.(vi) Cations as large as TEA+ are permeant. (vii) Current through the channel is blocked in the presence of DIDS, SITS and ATP, but not by Zn2+.

Stilbene disulfonate blockade of colonic secretory Cl- channels in planar lipid bilayers

We studied blockade by 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS) of a secretory Cl- channel from colonic enterocyte plasma membrane vesicles incorporated into planar lipid bilayer membranes. Except for intermittent long-lived closed periods (100 ms to several min), the control channel open probability (Po) was greater than 90%. DNDS, added to the cis or vesicle-containing side, which corresponds to the outer membrane side of the channel, caused a dramatic increase in the number of current transitions from the open-to-closed state. DNDS caused a concentration-dependent decrease in Po with a maximum inhibition of 95 +/- 2.0% and a half-maximal inhibitory concentration of 3.3 +/- 1.4 microM. DNDS added to the trans side of the channel had no effect on either the single-channel conductance or kinetic behavior of the channel. Kinetic analysis revealed that DNDS blockade from the cis side could be explained by a linear, closed-open-blocked, kinetic scheme. The estimated DNDS block rate constants were kon = 3.2 X 10(7) M-1.s-1 and koff = 52 s-1, yielding an equilibrium dissociation constant (KD) of 2.1 +/- 0.38 microM, similar to the Ki for inhibition of Po. The effects of DNDS were fully reversible after perfusion of the cis compartment with DNDS-free solution. In contrast, the covalently reactive 4,4'-diisothiocyano-substituted stilbene disulfonate caused an irreversible blockade of the Cl- channel.

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.

Tetramine: occurrence in marine organisms and pharmacology

The occurrence of tetramethylammonium ion (TMA) in marine organisms is reviewed. The pharmacological action of TMA is also discussed, with special emphasis on the sign and symptoms experienced by oral poisonings. It is concluded that the major manifestations of TMA poisonings may be attributed to interactions with the autonomous nervous system. Fatal intoxications are due to neuromuscular blockade.

Potassium channel of cardiac sarcoplasmic reticulum is a multi-ion channel

We have characterized mechanisms of ionic permeation in the K channel of canine cardiac sarcoplasmic reticulum (SR K channel). Ionic selectivity, as measured by relative permeabilities, followed Eisenman sequence l, a low field strength sequence. Slope conductance measured in symmetrical solutions across the bilayer followed Eisenman sequence V. In all cases, the selectivity characteristics of the prominent subconductance state (O1) were similar to those of the main-state (O2). Further, our studies have revealed that this channel differs in three significant ways from the highly characterized SR K channel of skeletal muscle. First, the ratio of permeabilities Cs+ to K+ was a complex function of ion concentration. Second, the concentration dependence of conductance was not well described by the Michaelis-Menten formalism. Instead, we modeled the observed relations using a more general approach based on classical rate theory. Third, mole fraction experiments (Cs+ with K+) demonstrated a prominent anomalous effect. Certain of our Cs+ data required the Eyring rate theory approach for adequate interpretation. We adopted a symmetrical energy profile incorporating ion-ion interaction and thereby accounted for much of the data. We conclude that the canine cardiac SR K channel is significantly different from that of skeletal muscle, and it may accommodate more than one ion at a time.

Physiological role and selectivity of the in situ potassium channel of the sarcoplasmic reticulum in skinned frog skeletal muscle fibers

The role of K+ as a counterion during Ca2+ release from the sarcoplasmic reticulum (SR) has been investigated. An optical technique using the Ca2+-sensitive dye antipyrylazo III monitored Ca2+ release from skinned (sarcolemma removed) muscle fibers of the frog. Skinned fibers were used since the removal of the sarcolemma allows direct access to the SR membrane. Releases were stimulated by caffeine, which activates Ca2+ release directly by binding to a receptor on the SR. Two different methods were used to decrease the SR K+ conductance so that its effect on Ca2+ release could be assessed: (a) the SR K+ channel blocker, 1,10-bis-quanidino-n-decane (bisG10) was used to eliminate current pathways and (b) substitution of the impermeant ion choline for K+ was used to decrease charge carriers. Both bisG10 and choline substitution caused a concentration-dependent decrease in the Ca2+ release rate. Therefore we conclude that K+ is an important counterion for Ca2+ during its release from the SR. The selectivity of the in situ SR K+ channel to several monovalent cations was determined by substituting them for K+ and comparing their effect on Ca2+ release. The substituted ions were expected to affect Ca2+ release in proportion to their ability to support a counterion flux, which is, in turn, a function of their relative conductance through the SR K+ channel. The selectivity sequence determined by these experiments was K+ = Rb+ = Na+ greater than Cs+ greater than Li+ greater than choline.