Voltage-dependent calcium channels from Paramecium cilia incorporated into planar lipid bilayers

The principal conductance in Giardia lamblia trophozoites possesses functional properties similar to the mammalian ClC-2 current

The human intestinal pathogen Giardia lamblia is a flagellated unicellular protozoan with pronounced medical and biological relevance. However, the basic physiology of Giardia trophozoites has been sparsely studied, especially the electrical and ionic properties of their cellular membrane which are virtually unknown. In this study, we were able to record and characterize the macroscopic ionic currents of Giardia trophozoite membrane by electrophysiological methods of the patch clamp technique. Giardia trophozoites showed a high current density (∼600 pA/pF at -140 mV) that was activated upon hyperpolarization. This current was carried by a chloride-selective channel (I Cl-G) and it was the most important determinant of the membrane potential in Giardia trophozoites. Moreover, this conductance was able to carry other halide anions and the sequence of permeability was Br(-) > Cl(-) ≈ I(-) ≫ F(-). Besides the voltage-dependent inward-rectifying nature of I Cl-G, its activation and deactivation kinetics were comparable to those observed in ClC-2 channels. Extracellular pH modified the voltage-dependent properties of I Cl-G, shifting the activation curve from a V 1/2 = -79 ± 1 mV (pH 7.4) to -93 ± 2 mV (pH 8.4) and -112 ± 2 mV (pH 5.4). Furthermore, the maximal amplitude of I Cl-G measured at -100 mV showed dependence to external pH in a bell-shaped fashion reported only for ClC-2 channels. Therefore, our results suggest that I Cl-G possesses several functional properties similar to the mammalian ClC-2 channels.

Ion channels in microbes

Studies of ion channels have for long been dominated by the animalcentric, if not anthropocentric, view of physiology. The structures and activities of ion channels had, however, evolved long before the appearance of complex multicellular organisms on earth. The diversity of ion channels existing in cellular membranes of prokaryotes is a good example. Although at first it may appear as a paradox that most of what we know about the structure of eukaryotic ion channels is based on the structure of bacterial channels, this should not be surprising given the evolutionary relatedness of all living organisms and suitability of microbial cells for structural studies of biological macromolecules in a laboratory environment. Genome sequences of the human as well as various microbial, plant, and animal organisms unambiguously established the evolutionary links, whereas crystallographic studies of the structures of major types of ion channels published over the last decade clearly demonstrated the advantage of using microbes as experimental organisms. The purpose of this review is not only to provide an account of acquired knowledge on microbial ion channels but also to show that the study of microbes and their ion channels may also hold a key to solving unresolved molecular mysteries in the future.

Characterization of single channel currents from primary cilia of renal epithelial cells

The primary cilium is a ubiquitous, non-motile microtubular organelle lacking the central pair of microtubules found in motile cilia. Primary cilia are surrounded by a membrane, which has a unique complement of membrane proteins, and may thus be functionally different from the plasma membrane. The function of the primary cilium remains largely unknown. However, primary cilia have important sensory transducer properties, including the response of renal epithelial cells to fluid flow or mechanical stimulation. Recently, renal cystic diseases have been associated with dysfunctional ciliary proteins. Although the sensory properties of renal epithelial primary cilia may be associated with functional channel activity in the organelle, information in this regard is still lacking. This may be related to the inherent difficulties in assessing electrical activity in this rather small and narrow organelle. In the present study, we provide the first direct electrophysiological evidence for the presence of single channel currents from isolated primary cilia of LLC-PK1 renal epithelial cells. Several channel phenotypes were observed, and addition of vasopressin increased cation channel activity, which suggests the regulation, by the cAMP pathway of ciliary conductance. Ion channel reconstitution of ciliary versus plasma membranes indicated a much higher channel density in cilia. At least three channel proteins, polycystin-2, TRPC1, and interestingly, the alpha-epithelial sodium channel, were immunodetected in this organelle. Ion channel activity in the primary cilium of renal cells may be an important component of its role as a sensory transducer.

Hypo-osmotic or Ca2+-rich external conditions trigger extra contractile vacuole complex generation in Paramecium multimicronucleatum

The freshwater ciliated protozoan, Paramecium multimicronucleatum, usually possesses two contractile vacuole complexes (CVCs). The number of CVCs in a single cell, however, may vary from 1 to 7. We found that the number of cells that have more than two CVCs increased after the cells were exposed to a hypo-osmotic or a high Ca2+ condition. It is assumed that the biological significance of this increase in the number of CVCs is to enhance the cell's ability to eliminate excess water or Ca2+ from the cytosol. An extra CVC was either generated de novo in the posterior region of the cell or, when in the anterior region, by binary fission of the anterior CVC. Generation of these extra CVCs was not inhibited by aphidicolin, a potent inhibitor of DNA synthesis in the micronuclei of Paramecium, even though normal duplication of the CVC that accompanies normal cell division was completely inhibited by this inhibitor. These results suggest that generation of extra CVCs is controlled by a hypothetical regulatory mechanism that is activated either by a hypo-osmotic or by a Ca2+-rich condition and that differs from the regulatory mechanism that governs normal CVC duplication during cell division.

Mechanisms of pyrethroid neurotoxicity: implications for cumulative risk assessment

The Food Quality Protection Act (FQPA) of 1996 requires the United States Environmental Protection Agency to consider the cumulative effects of exposure to pesticides having a 'common mechanism of toxicity.' This paper reviews the information available on the acute neurotoxicity and mechanisms of toxic action of pyrethroid insecticides in mammals from the perspective of the 'common mechanism' statute of the FQPA. The principal effects of pyrethroids as a class are various signs of excitatory neurotoxicity. Historically, pyrethroids were grouped into two subclasses (Types I and II) based on chemical structure and the production of either the T (tremor) or CS (choreoathetosis with salivation) intoxication syndrome following intravenous or intracerebral administration to rodents. Although this classification system is widely employed, it has several shortcomings for the identification of common toxic effects. In particular, it does not reflect the diversity of intoxication signs found following oral administration of various pyrethroids. Pyrethroids act in vitro on a variety of putative biochemical and physiological target sites, four of which merit consideration as sites of toxic action. Voltage-sensitive sodium channels, the sites of insecticidal action, are also important target sites in mammals. Unlike insects, mammals have multiple sodium channel isoforms that vary in their biophysical and pharmacological properties, including their differential sensitivity to pyrethroids. Pyrethroids also act on some isoforms of voltage-sensitive calcium and chloride channels, and these effects may contribute to the toxicity of some compounds. Effects on peripheral-type benzodiazepine receptors are unlikely to be a principal cause of pyrethroid intoxication but may contribute to or enhance convulsions caused by actions at other target sites. In contrast, other putative target sites that have been identified in vitro do not appear to play a major role in pyrethroid intoxication. The diverse toxic actions and pharmacological effects of pyrethroids suggest that simple additivity models based on combined actions at a single target are not appropriate to assess the risks of cumulative exposure to multiple pyrethroids.

Contribution of the plasmalemma to Ca2+ homeostasis in hair cells

Calcium influx through transduction channels and efflux via plasmalemmal Ca(2+)-ATPases (PMCAs) are known to contribute to calcium homeostasis and modulate sensory transduction in vertebrate hair cells. To examine the relative contributions of apical and basolateral pathways, we analyzed the calcium dynamics in solitary ciliated and deciliated guinea pig type I and type II vestibular hair cells. Whole-cell patch-clamp recordings demonstrated that these cells had resting potentials near -70 mV and could be depolarized by 10-20 mV by superfusion with high potassium. Fura-2 measurements indicated that ciliated type II cells and deciliated cells of either type had low basal [Ca(2+)](i), near approximately 90 nm, and superfusion with high potassium led to transient calcium increases that were diminished in the presence of Ca(2+) channel blockers. In contrast, measurements of type I ciliated cells, hair cells with large calyceal afferents, were associated with a higher basal [Ca(2+)](i) of approximately 170 nm. High-potassium superfusion of these cells induced a paradoxical decrease in [Ca(2+)](i) that was augmented in the presence of Ca(2+) channel blockers. Optical localization of dihydropyridine binding to the kinocilium suggests that they contain L-type calcium channels, and as a result apical calcium influx includes a contribution from voltage-dependent ion channels in addition to entry via transduction channels localized to the stereocilia. Eosin block of PMCA significantly altered both [Ca(2+)](i) baseline and transient responses only in ciliated cells suggesting that, in agreement with immunohistochemical studies, PMCA is primarily localized to the bundles.

A Ca(2+)- and voltage-modulated flagellar ion channel is a component of the mechanoshock response in the unicellular green alga Spermatozopsis similis

In flagellate green algae, behavioral responses to photo- and mechanoshock are induced by different external stimuli within 10-15 ms. In the accompanying changes in flagella beat, Ca(2+) has important regulatory roles. Although the axonemal Ca(2+) responsive elements are well characterized, analyses of flagellar channels involved in Ca(2+) signalling as well as other ion channels at the single-channel level were not yet conducted in green algae. To gain a further understanding of these important signaling elements in movement responses, intact flagella of Spermatozopsis similis were isolated and characterized and the solubilized flagellar membrane proteins were reconstituted into liposomes. We observed three types of channel activity, two of which were weakly anion and cation-selective and in the high-conductance regime typical for porin-like solute channels. The dominating channel activity was a voltage dependent, rectifying, low conductance (Lambda=80 pS in 50 mM KCl) cation-selective channel modulated by, and highly permeable to, Ca(2+) ions (SFC1: Spermatozopsis flagellar cation channel 1). Depolarizations necessary to activate SFC1 probably only occur in vivo during avoidance reactions of this alga. Ca(2+)-activation of SFC1 points to a direct link to Ca(2+)-mediated signaling pathway(s) in the flagella. Both the response to mechanoshock and SFC1 activity were inhibited by Gd(3+) and Ba(2+), thus supporting our assumption that SFC1 represents a major flagellar ion channel involved in this green algal avoidance reaction.

A calcium-activated, large conductance and non-selective cation channel in Paramecium cell

A non-selective cation channel was found in mutant Paramecium cells (K115). This cell had been selected as a resistant mutant in a high-K+ solution. In patch clamp studies of these cells in the inside-out configuration, this channel was activated by bath applications of elevated Ca2+ concentrations. The channels became very active when the Ca2+ concentration was above 3.2 microM. The channel was also activated by depolarization. The voltage dependency was steep upon depolarization, whereas upon hyperpolarization the channel activity barely changed. This channel had poor selectivity for monovalent alkali cations. Using the Goldman-Hodgkin-Katz equation for the reversal potential, the permeability ratios with respect to K+ for Na+, Rb+, Cs+ and Li+ were nearly 1. Although the permeability ratios were similar for each cation, the single channel conductances differed. The single channel conductances were 467 pS with K+ as the charge carrier, 406 pS with Na+, 397 pS with Rb+, 253 pS with Cs+ and 198 pS with Li+ upon depolarization in 100 mM cation solutions. A similar calcium-activated large conductance channel was observed in the wild-type (G3) Paramecium cells but was very rare.

Protein kinase C induced calcium influx and sustained enhancement of ciliary beating by extracellular ATP

The major purpose of this work was to determine protein kinase C (PKC) influence on ciliary beat frequency (CBF) and to assess participation of PKC in purinergic ciliary stimulation. The experiments were performed by simultaneous measurement of [Ca2+]i and CBF on tissue culture of frog esophagus epithelium. The PKC activators TPA and DiC8 produced significant elevation of [Ca2+]i and strong frequency enhancement. The calcium elevation was inhibited by lowering the extracellular calcium level, or by La3+, but was unaffected by verapamil and the phospholipase C inhibitor U-73122, suggesting that Ca2+ influx was via non-voltage-operated calcium channels. The inhibition of [Ca2+]i elevation resulted in corresponding inhibition of CBF enhancement. The effect of TPA was blocked by the selective PKC inhibitors chelerythrine, calphostin C, and GF109203X, and by the enzyme downregulation. The downregulation of PKC, or the enzyme inhibitors did not affect the immediate response to extracellular ATP but caused rapid decay of initially stimulated [Ca2+]i and CBF to the basal level. These results suggest that PKC produces CBF enhancement via activation of calcium influx through non-voltage-operated calcium channels. This calcium influx seems to be responsible for the duration of ciliary stimulation produced by the extracellular ATP.

Characterisation of the voltage-activated calcium current in the marine ciliate Euplotes vannus

We have isolated the early Ca current (ICa) from the whole cell current that activates upon depolarisations in the marine ciliate Euplotes vannus. The peak of ICa activated within 4.2 ms at depolarisations to 5 mV with an amplitude of 2.5 +/- 0.35 nA and was reduced to 1.0 +/- 0.14 nA (n = 5) when the extracellular Ca concentration was changed from 10 to 1 mM. The voltage-dependent activation curve was steeper and shifted to more negative values when external Ca2+ was replaced by Ba2+. The early inward current inactivated with a double-exponential time course including a fast and a slow component, and no inactivation was recorded with Ba2+. The time constants for the recovery from inactivation varied between 44 and 153 ms according to the depolarisation-dependent Ca influx. At the common resting potential of -25 mV, ICa was not steady-state inactivated; ICa half-inactivated at -14.5 mV, and totally inactivated at -5 mV. ICa was inhibited by 10 mM extracellular Cd2+. The peptides omega-conotoxin-GVIA (20 microM), omega-conotoxin-MVIIC (600 nM), omega-agatoxin-IVA (60 nM) and calciseptine (900 nM) did not block ICa. The benzothiazepine-derivative diltiazem (100 microM) and the dihydropiridine nifedipine (100 microM) inhibited 51% and 33% of ICa, respectively. The naphthalene sulfonamide W7 reduced ICa with an inhibition coefficient of 33 microM.

Functional reconstitution of ion channels from Paramecium cortex into artificial liposomes

Toward isolating channel proteins from Paramecium, we have explored the possibility of functionally reconstituting ion channels in an artificial system. Proteins from Paramecium cortex reconstituted with soybean azolectin retained several channels whose activities were readily registered under patch clamp. The most commonly encountered activities were three: (i) a 71-pS cation channel that opens at all voltages unless di- or trivalent cations were added to close them, (ii) a 40 pS monovalent cation channel, and (iii) a large-conductance channel that prefers anions and exhibits many subconductance states. These channels survived mild detergent treatments without observable functional alterations. The possible origin of these channels from internal membranes, the possible role of 71-pS channel in internal Ca2+ release, and the prospects of their purification are discussed.

Cortical ultrastructure and chemoreception in ciliated protists (Ciliophora)

The ciliated protists (ciliates) offer a unique opportunity to explore the relationship between chemoreception and cell structure. Ciliates resemble chemosensory neurons in their responses to stimuli and presence of cilia. Ciliates have highly patterned surfaces that should permit precise localization of chemoreceptors in relation to effector organelles. Furthermore, ciliates are easy to grow and to manipulate genetically; they can also be readily studied biochemically and by electrophysiological techniques. This review contains a comparative description of the ultrastructural features of the ciliate cell surface relevant to chemoreception, examines the structural features of putative chemoreceptive cilia, and provides a summary of the electron microscopic information available so far bearing on chemoreceptive aspects of swimming, feeding, excretion, endocytosis, and sexual responses of ciliates. The electron microscopic identification and localization of specific chemoreceptive macromolecules and organelles at the molecular level have not yet been achieved in ciliates. These await the development of specific probes for chemoreceptor and transduction macromolecules. Nevertheless, the electron microscope has provided a wealth of information about the surface features of ciliates where chemoreception is believed to take place. Such morphological information will prove essential to a complete understanding of reception and transduction at the molecular level. In the ciliates, major questions to be answered relate to the apportionment of chemoreceptive functions between the cilia and cell soma, the global distribution of receptors in relation to the anterior-posterior, dorsal-ventral, and left-right axes of the cell, and the relationship of receptors to ultrastructural components of the cell coat, cell membrane, and cytoskeleton.

Regulation of adenylyl cyclase from Paramecium by an intrinsic potassium conductance

Hyperpolarization of the cell membrane of Paramecium stimulates adenosine 3',5'-monophosphate (cAMP) formation. Manipulations of the K+ resting conductance of the ciliate by adaptation in different buffers affected excitability of the cAMP generating system. Blockade of K+ channels inhibited hyperpolarization-stimulated cAMP formation. A mutant of Paramecium that is unable to control its K+ resting conductance had a defect in cAMP formation. Purified adenylyl cyclase, when incorporated into an artificial lipid bilayer membrane, revealed properties of a voltage-independent K+ channel. This indicates that the adenylyl cyclase of Paramecium has a secondary function as carrier of the K+ resting conductance. A hyperpolarization-activated K+ efflux appears to directly regulate adenylyl cyclase activity in vivo.

Voltage-gated Ca entry in isolated bovine capillary endothelial cells: evidence of a new type of BAY K 8644-sensitive channel

Isolated bovine capillary endothelial cells have been examined for voltage-dependent Ca entry. All cells displayed a low threshold activity, with the main characteristics of a T-type transient current, when examined using whole-cell recording for activation and inactivation and cell-attached conditions or inside-out patches for the elementary conductance (8 pS). 25% of the cells displayed an additional sustained current in 5 mM CaCl2 above -40 mV, which was enhanced by application of BAY K 8644, but almost insensitive to superfusion with nicardipine. Two types of channels (2.8 and 21 pS, in 110 mM BaCl2) were shown to have a BAY K 8644 sensitivity. The large conductance channels were L-type channels. The smaller events were elicited at more hyperpolarized potentials (by some 30 mV). Their mean open time was 16 ms in control conditions. In presence of BAY K 8644, additional long open times were observed (up to 100 ms as compared to 7.8 ms for the time constants of the slow mode of the L-type channel). We refer to these channels as SB channels: of small conductance and sensitive to BAY K 8644. In the presence of nicardipine, SB channels are not noticeably modified, in contrast to the L-type openings which are abolished. Also, SB open times are close to control values when nicardipine is added after a BAY K 8644 application. We suggest that, at physiological concentrations of divalent ions, an SB-type activity is elicited above -40 mV which generates the low threshold sustained current.

Variety of Ca(2+)-permeable channels in human carcinoma A431 cells

Patch-clamp methods were used to search for and characterize channels that mediate calcium influx through the plasma membrane of human carcinoma A431 cells. Here we present four Ca(2+)-permeable channel types referred to as SG, G, 1 and BI. With 105 mM Ca2+ as the charge carrier, at 30-33 degrees C their mean unitary conductances (in pS) are: 1.3 (SG), 2.4 (G), 3.7 (I) and 12.8 (BI). SG and G channels are activated by nonhydrolyzable analogues of guanosine 5-triphosphate (GTP) applied to the inside of the membrane, suggesting an involvement of G-proteins in the control of their activity. I and BI channels are activated by inositol 1,4,5-trisphosphate (InsP3). G, I, BI and possibly SG channels are activated from the extracellular side of the membrane by epidermal growth factor (EGF) and histamine. It is assumed that all identified Ca2+ channels take part in the generation of the agonist-induced intracellular Ca2+ signal. The variety of Ca-channel types seems to be necessary to tune cell responses according to the respective type and level of an external signal, on the one hand, and to the functional state of the cell, on the other.

The influence of Ca2+ antagonists on the ciliary activity of the guinea pig trachea

The ciliary activity of the guinea pig tracheal respiratory epithelium was measured using a photoelectric device. Fourteen animals were administered 75 mg diltiazem/kg body weight 2 days, 1 day and 4 h prior to the investigation. Thirteen animals served as controls. In the pretreated group, the mean ciliary stroke frequency (13.4 +/- 3.0 Hz) was significantly lower (P = 0.02) than in the control group (16.6 +/- 3.7 Hz). The application of aerosolized 0.3 mmol/l nifedipine solution on the tracheal preparations of the control animals also significantly reduced the stroke frequency (P = 0.02). The ciliary activity never dropped beyond a frequency of about 8 Hz. The most probable interpretation of these results is a modulating effect of Ca2+ antagonists on ciliary activity by blocking voltage-gated calcium channels.

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.

Paramecium calcium channels are blocked by a family of calmodulin antagonists

Although the voltage-sensitive Ca channel present in Paramecium has been subjected to detailed physiological and genetic analysis, no organic ligands have been described that block this channel with high affinity and that ultimately can be used to identify channel components. Based on a previous observation that the naphthalene sulfonamide calmodulin antagonist W-7 can block Paramecium Ca channels at high concentration, we have synthesized analogs of W-7 that block these channels at concentrations of less than 1 microM. The effectiveness of these compounds was tested both by a sensitive behavioral assay and on Ca channels that had been incorporated into planar lipid bilayers. Despite the fact that these compounds are effective Paramecium calmodulin antagonists, two independent lines of evidence suggest that W-7 and its analogs block the Ca channel by a mechanism that is independent of their action on calmodulin. In addition, the sensitivity to W-7 or dihydropyridines of Ca channels present in a number of eukaryotic phyla has been used to identify similarities in Ca channels from widely diverse organisms. It appears that the pharmacological specificity provides a means to group Ca channels.

Cardiac calcium channels in planar lipid bilayers. L-type channels and calcium-permeable channels open at negative membrane potentials

Planar lipid bilayer recordings were used to study Ca channels from bovine cardiac sarcolemmal membranes. Ca channel activity was recorded in the absence of nucleotides or soluble enzymes, over a range of membrane potentials and ionic conditions that cannot be achieved in intact cells. The dihydropyridine-sensitive L-type Ca channel, studied in the presence of Bay K 8644, was identified by a detailed comparison of its properties in artificial membranes and in intact cells. L-type Ca channels in bilayers showed voltage dependence of channel activation and inactivation, open and closed times, and single-channel conductances in Ba2+ and Ca2+ very similar to those found in cell-attached patch recordings. Open channels were blocked by micromolar concentrations of external Cd2+. In this cell-free system, channel activity tended to decrease during the course of an experiment, reminiscent of Ca2+ channel "rundown" in whole-cell and excised-patch recordings. A purely voltage-dependent component of inactivation was observed in the absence of Ca2+ stores or changes in intracellular Ca2+. Millimolar internal Ca2+ reduced unitary Ba2+ influx but did not greatly increase the rate or extent of inactivation or the rate of channel rundown. In symmetrical Ba2+ solutions, unitary conductance saturated as the Ba2+ concentration was increased up to 500 mM. The bilayer recordings also revealed activity of a novel Ca2+-permeable channel, termed "B-type" because it may contribute a steady background current at negative membrane potentials, which is distinct from L-type or T-type Ca channels previously reported. Unlike L-type channels, B-type channels have a small unitary Ba2+ conductance (7 pS), but do not discriminate between Ba2+ and Ca2+, show no obvious sensitivity to Bay K 8644, and do not run down. Unlike either L- or T-type channels, B-type channels did not require a depolarization for activation and displayed mean open times of greater than 100 ms.

Calcium and barium permeable channels from Aplysia nervous system reconstituted in lipid bilayers

Ion channels permeable to barium and calcium were reconstituted from the Aplysia nervous system into phospholipid bilayers formed on the tips of patch electrodes. With asymmetrical concentrations of barium or calcium on the two sides of the bilayer, the single-channel currents reversed at the calculated barium or calcium reversal potentials, indicating that the channels were cation selective. Channels with conductances of 10, 25 and 36 pS were routinely observed. Calcium and barium were equally effective as charge carriers for the 36-pS channel, whereas magnesium was at least fifteen-fold less effective. The gating of all three channels was independent of the voltage across the bilayer, but was affected by the dihydropyridine calcium channel agonist Bay K 8644 (Bay K). In the presence of Bay K but not in its absence, long discrete gating events were routinely observed, suggesting that the dihydropyridine increased the probability of long open states as it does for calcium channels in other systems. Bilayers invariably contained more than a single channel (or conductance state). This was observed even when the Aplysia nervous system membranes were prepared in the presence of cytoskeleton disrupting agents, or when the membrane proteins were diluted extensively with exogenous phospholipid. Furthermore, transitions between conductance levels were observed with high frequency. These findings, together with the fact that all of the conductance states share certain properties including voltage-independence and sensitivity to Bay K, suggest that the apparent multiple channel types may in fact represent subconductance states of a single ion channel.

Ca2+ channels from brain microsomal membranes reconstituted in patch-clamped bilayers

Single Ca2+ channels from brain microsomal membranes were reconstituted in bilayers made at the tips of patch-clamp micropipettes. The single-channel conductance was defined to be 107 pS in 50 mM Ca2+. The channel activity was stimulated by nucleotides and inositol 1,4,5-trisphosphate (Ins-P3), and was inhibited by ruthenium red. Na+ added asymmetrically to the membrane bilayer induced an increase in the Ca2+-channel activity. The described characteristics of these Ca2+ channels suggest that they may be responsible for the Ca2+ transport across the membranes of the endoplasmic reticulum system triggering and modulating various neurosecretory and excitatory processes in nerve cells.

Channels produced by spider venoms in bilayer lipid membrane: mechanisms of ion transport and toxic action

The selectivity of ion channels produced by latrotoxin obtained from a black widow spider venom and by venom from the spider Steatoda paykulliana in bilayer phospholipid membrane was studied. Experimental current-voltage curves of these channels were used for the estimation of parameters of a two barrier model of their energy profiles. Selectivities of both types of channels are similar. Alkaline earth cations are permeable, the permeability increasing in the order Mg2+ less than Ca2+ less than Sr2+ less than Ba2+. In contrast transition metal cations block the channel, their efficiency decreases in the order: Cd2+ greater than or equal to Ni2+ greater than Zn2+ greater than Co2+ greater than Mn2+ (Steatoda paykulliana spider venom) and Cd2+ greater than Co2+ greater than Ni2+ greater than Zn2+ greater than Mn2+ (latrotoxin). Amplitudes of current carried by corresponding ions are mainly determined by the depth of the potential well for this ion, i.e., by its affinity to the cation binding site in the channel. The channels are also permeable to monovalent cations but they do not bind them. Selectivity for monovalent cations depends on Ca2+ concentration at the cis-side of membrane in the micromolar range. However, the addition of Ca2+ to the trans-side up to 10 mM does not affect currents carried by monovalent ions. It is suggested that venom-induced calcium channels have two conformational states with different selectivities which interconvert upon binding one calcium ion. Possible general schemes for the organisation of calcium channels in excitable membranes are also discussed. Finally, using a mathematical model of synaptic transmission, possible mechanisms of toxic action of spider venoms are considered.

Calcium channels in planar lipid bilayers: insights into mechanisms of ion permeation and gating

Electrophysiological recordings were used to analyze single calcium channels in planar lipid bilayers after membranes from bovine cardiac sarcolemmal vesicles had been incorporated into the bilayer. In these cell-free conditions, channels in the bilayer showed unitary barium or calcium conductances, gating kinetics, and pharmacological responses that were similar to dihydropyridine-sensitive calcium channels in intact cells. The open channel current varied in a nonlinear manner with voltage under asymmetric (that is, physiological) ionic conditions. However, with identical solutions on both sides of the bilayer, the current-voltage relation was linear. In matched experiments, calcium channels from skeletal muscle T-tubules differed significantly from cardiac calcium channels in their conductance properties and gating kinetics.

Interactions of divalent cations with single calcium channels from rat brain synaptosomes

Voltage-dependent calcium channels from a rat brain membrane preparation ("synaptosomes") were incorporated into planar lipid bilayers. The effects of calcium, barium, strontium, manganese, and cadmium ions on the amplitudes and kinetics of single channel currents were examined. The order of single channel conductances was gBa greater than gSr greater than gMn, which was the inverse of the order of the mean channel open times: TMn greater than TCa = TSr greater than TBa. In contrast, the identity of the charge carrier had little or no effect on the mean closed times of the channel. Manganese, in the absence of other permeant ions, can pass through single channels (gMn = 4 pS). However, when added to a solution that contained another type of permeant divalent cation, manganese reduced the single channel current in a voltage-dependent manner. Cadmium, a potent blocker of macroscopic "ensemble" calcium currents in many preparations, reduced the current through an open channel in a manner consistent with Cd ions both not being measurably permeant and interacting with a single site. The permeant ions competed with cadmium for this site with the following order: Mn greater than Sr = Ca greater than Ba. These results are consistent with the existence of no less than one divalent cation binding site in the channel that regulates ion permeation.

Incorporation of calcium channels from cardiac sarcolemmal membrane vesicles into planar lipid bilayers

When purified porcine cardiac sarcolemmal membrane vesicles are incorporated into planar lipid bilayers formed at the tip of patch electrode pipettes, individual divalent cation channels can be monitored. Channel activity is increased in the presence of the Ca2+ channel agonist Bay K 8644, is voltage dependent, and selects for divalent cations over anions. The activity does not inactivate because it is maintained during prolonged depolarizations. Determination of divalent cation selectivity from the reversal potential of single-channel currents indicates a relative permeability ratio for Ba/Ca/Mg of 1:0.45:0.08. Mean channel conductance in 0.1 M Ba2+/0.01 M Mg2+ is 8 pS. Channels are reversibly blocked by the Ca2+ channel inhibitor nitrendipine, and inhibition can be competitively antagonized by Bay K 8644. Binding studies with 3H-labeled D-600 demonstrate the presence of high-affinity receptors for D-600 in sarcolemmal membranes (Kd = 6.4 X 10(-9) M; Bmax = 3 pmol per mg of protein). In addition, experiments with resolved D-600 stereoisomers indicate that (-)D-600 is at least 25-fold more potent than (+)D-600 in competing for this aralkyl amine receptor. Consistent with this, (-)D-600 is much more effective than the (+) isomer in inhibiting bilayer-incorporated channels. These results demonstrate that the divalent cation channel that has been reconstituted in planar lipid bilayers possesses many of the characteristics of voltage-regulated Ca2+ channels in heart and suggest that receptors for Ca2+ entry blockers are functionally associated with this channel.

Kinetic properties and selectivity of calcium-permeable single channels in Aplysia neurones

Two kinds of single channels, carrying inward currents even above the Na and Cl ion equilibrium potentials, were observed in outside-out patches from Aplysia neurones bathed in K-free internal and external solutions. The channel carrying the larger elementary current has been studied in detail. When the internal solution contained mainly CsCl, this channel usually inactivated during the first minutes following isolation of the membrane patch. However, when the internal solution contained NaCl instead of CsCl, the channel remained functional during several hours, thus allowing the present study. Na-Tris, NaCl-mannitol and Ca-Ba external substitution experiments showed that the channel studied is much more permeable to divalent cations than to sodium ions. Mono-exponential open-time distributions obtained under identical conditions from different membrane patches indicated either slow (in the order of 100 ms at 0 mV) or rapid (a few milliseconds at 0 mV) mean open-times. Biphasic open-time distributions could be obtained from other membrane patches under the same conditions. These results suggest the existence of two different gating modes. Both the open-time distribution and the closed-time distribution are voltage sensitive: membrane depolarization activates the channel by lengthening the openings and shortening the closures. The threshold of activation if any, is very low and the inactivation, if present, is never complete. Ca-Ba, Ca-Sr and Ca-Mg external substitution experiments showed that the elementary current amplitude is not very sensitive to the nature of the external divalent cation. The elementary current can be slightly larger when carried by Ba ions rather than by Ca ions, but is nearly identical whether carried by Ca, Sr or even Mg ions, which leads to the elementary conductance sequence: Ba greater than or equal to Ca = Sr congruent to Mg. In contrast, the mean open-time of the channel is very sensitive to the nature of the external permeant ion. The longest mean open-time is observed in the presence of Ca ions, and the mean open-time sequence is: Ca greater than Sr greater than Ba greater than Mg. The closed-time distribution is also affected by the nature of the external divalent cation. The above results show that the nature of the permeant ion affects the kinetic properties of the channel much more than its elementary current.(ABSTRACT TRUNCATED AT 400 WORDS)

Slow inactivation of the calcium current of Paramecium is dependent on voltage and not internal calcium

The isolated Ca2+ current from Paramecium caudatum was examined under voltage clamp with long conditioning depolarizations lasting for up to 5 min. The isolated transient Ca2+ current inactivates with tens of milliseconds due to Ca2+ -dependent Ca2+ -channel inactivation (Brehm & Eckert, 1978). When this fast inactivation was blocked by internally delivered EGTA, a much slower inactivation of the Ca2+ current was discovered. This slow inactivation had time constants of tens of seconds, depending on voltage. The development of this slow inactivation was further examined by following the Ca2+ transient after 1 s interruptions of the long depolarization. This development is voltage dependent; the rate of inactivation is higher with a larger depolarization. After a long depolarization, the Ca2+ current returns in two clearly separable steps. A portion of the current returns rapidly along an exponential time course with time constants of tens to hundreds of milliseconds. The remainder of the current returns slowly with time constants of tens of seconds. A longer conditioning depolarization generates a larger portion that recovers slowly. Internally delivered EGTA, sufficient to prevent most of the fast inactivation, did not change the time course or the extent of either the onset or the removal of the slow inactivation. The compound W-7, which inhibits the Ca2+ current itself, does not block the onset of this slow inactivation during depolarization. We conclude that the slow inactivation of the Ca2+ channel is a mechanistically different phenomenon from the fast Ca2+ -dependent Ca2+ -channel inactivation. The possible physiological and behavioural roles of this slow inactivation are discussed.

Agonists Bay-K8644 and CGP-28392 open calcium channels reconstituted from skeletal muscle transverse tubules

The recently described calcium channel agonists Bay-K8644 and CGP-28392 have been used to induce long-term opening of calcium channels from purified rat muscle transverse tubules (t-tubules) incorporated into planar phospholipid bilayers. Agonist-open channels are selective for divalent cations (except Mg++), display voltage-dependent kinetics, and are blocked by the calcium channel antagonist, nitrendipine. The sensitivity to dihydropyridine agonists and antagonists indicate that a pool of t-tubule calcium channels remain functional after membrane fractionation and purification.

Cation channels from Tetrahymena cilia incorporated into planar lipid bilayers

A single cation channel from Tetrahymena cilia was incorporated into planar lipid bilayers. This channel selected for K+, Na+, and Li+ over Cl- and gluconate-, and its single channel conductance (at +25 mV) was 211 +/- 8 pS (mean +/- SE) in 100 mM K+-gluconate. The channel was not voltage dependent and may contribute to the resting K+ conductance of ciliary membrane.

Characterization and purification of a soluble protein controlling Ca-channel activity in paramecium

The analysis of the voltage-sensitive Ca++ channel of the unicellular eucaryote, Paramecium has been extended to a biochemical level based on recent observations that the transfer of cytoplasm from wild-type cells into mutants lacking Ca++-channel function ("pawn" in P. tetraurelia and "CNR" in P. caudatum) causes mutant cells to regain Ca++-channel function. We have microinjected various cytoplasmic fractions into mutant cells and measured the restored Ca++-channel function using a convenient behavioral assay. Following the "curing" activity, we characterized and purified the component from wild-type cytoplasm that can restore the function missing in cells carrying mutations in the cnrC gene. The curing factor is not an RNA, but a heat-labile, -SH-containing protein that appears to affect existing mutant channels on the ciliary membrane. We have purified this factor over 500-fold from the soluble cytoplasm using conventional techniques. The protein is of low apparent molecular weight (less than 30,000 daltons), acidic, soluble, and does not have the properties of calmodulin.