UNIPROT:P50583 - FACTA Search

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Query: UNIPROT:P50583 (asymmetrical)
12,197 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The cuticle of the gill lamina of the crayfish Astacus leptodactylus (E), mechanically isolated, was mounted in an Ussing chamber and examined for its electrical properties. The cuticle of the gill lamina obtained from exuviae had similar properties. When perfused with artificial fresh water (AFW) outside and Van Harreveld solution (VH) inside, the transcuticular potential Voi was negative with respect to the inside, and close to the equilibrium potential for Cl- (ECl-). CH3COO-, HCO3-, SO4(2-) and cations (Na+, K+, Ca2+, Mg2+ and NH4+) behaved as impermeant ions with respect to Cl-. A decrease of pH (brought about with CO2) from 8.5 to 6.0 in AFW, VH or both had no effect on the potential. The cuticle area specific conductance was 20-30 mS/cm2 when superfused with AFW outside and VH inside. The conductance decreased linearly with log [Cl-] when Cl- was replaced by CH3COO-. Rectification was obvious when internal Cl- was reduced to 5 mmol/l. The Cl- selectivity of the cuticle could also be demonstrated in perfusing the cuticle with a single salt (NaCl, KCl, CaCl2, MgCl2 or LaCl3) and in diluting that salt on one side of the preparation or in replacing Cl- by CH3COO-, SO4(2-) and HCO3-. The potential changed almost linearly with log [Cl-] and was close to ECl-. The inner face of the cuticle was found to be slightly less selective than the outer face. The relative permeabilities were calculated to be: PCl- = 1, PNa+ = 0.001, PHCO3- = 0.0006, PCH3COO- = 0.0002. The dilution of a Cl- -free salt resulted in a cationic potential. The relative permeabilities of cations (NH4+, K+, Na+, Ca2+ and Mg2+) were found to range within a factor 2. The permeability of the cuticle to HCO3-, CH3COO- and SO4(2-) was 2-5 times lower. The cuticle conductance was linearly related to the activity of the salt perfusing the two sides of the preparation at equal concentrations. The molar area specific conductance to chloride salts was 14 (mS/cm2)/(mmol/l). That of Cl- -free salts ranged from 1 to 20 (microS/cm2)/(mmol/l) depending on the salt used. It was deduced that PCl- is 2 X 10(-3) cm/s and that all the other ions tested have permeabilities of 10(-7)-10(-6) cm/s. With large intensity current pulses the cuticle exhibited rectifying properties and an asymmetrical behaviour. Increasing the pH of the perfusing solution reduced the transcuticular potential established with a Cl- gradient.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Ionic permeabilities of the gill lamina cuticle of the crayfish, Astacus leptodactylus (E). 241 Jun 7

In this study we approach the transport phenomena by second-order phenomenological equations. If the condition of Onsager is not accepted, then it results that in the living system there is asymmetrical transport. The experimental investigations proved that the exchange between Na+-Ca2+ is asymmetric, as there takes place the transport of three ions of Na+ to one ion of Ca2+. This is an instance of concordance between theory and experimental research.
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PMID:The study of asymmetric transport phenomena in living systems through second-order phenomenological equations. 244 65

Batrachotoxin-modified, voltage-dependent sodium channels from canine forebrain were incorporated into planar lipid bilayers. Single-channel conductances were studied for [Na+] ranging between 0.02 and 3.5 M. Typically, the single-channel currents exhibited a simple two-state behavior, with transitions between closed and fully open states. Two other conductance states were observed: a subconductance state, usually seen at [NaCl] greater than or equal to 0.5 M, and a flickery state, usually seen at [NaCl] less than or equal to 0.5 M. The flickery state became more frequent as [NaCl] was decreased below 0.5 M. The K+/Na+ permeability ratio was approximately 0.16 in 0.5 and 2.5 M salt, independent of the Na+ mole fraction, which indicates that there are no interactions among permeant ions in the channels. Impermeant and permeant blocking ions (tetraethylammonium, Ca++, Zn++, and K+) have different effects when added to the extracellular and intracellular solutions, which indicates that the channel is asymmetrical and has at least two cation-binding sites. The conductance vs. [Na+] relation saturated at high concentrations, but could not be described by a Langmuir isotherm, as the conductance at low [NaCl] is higher than predicted from the data at [NaCl] greater than or equal to 1.0 M. At low [NaCl] (less than or equal to 0.1 M), increasing the ionic strength by additions of impermeant monovalent and divalent cations reduced the conductance, as if the magnitude of negative electrostatic potentials at the channel entrances were reduced. The conductances were comparable for channels in bilayers that carry a net negative charge and bilayers that carry no net charge. Together, these results lead to the conclusion that negative charges on the channel protein near the channel entrances increase the conductance, while lipid surface charges are less important.
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PMID:Batrachotoxin-modified sodium channels in planar lipid bilayers. Ion permeation and block. 244 Sep 77

The density and diameter distributions of intramembranous particles (IMPs) within unmyelinated axolemma from rat cervical sympathetic trunk were examined with freeze-fracture electron microscopy. The axolemma displays a highly asymmetrical partitioning of IMPs with ca. 1200 IMPs microns-2 on P-faces and ca. 100 IMPs microns-2 on E-faces. Particle sizes (diameters) are unimodally distributed on both fracture faces, with a range from 2.4 nm to 15.6 nm. Approximately 16% of the particles on P-faces and 28% of particles on E-faces are of a large (greater than 9.6 nm) diameter. On both fracture faces, the IMPs appear to be randomly distributed; no aggregations of particles were observed. The results indicate that there are ca. 230 large IMPs microns-2 of unmyelinated axolemma from rat cervical sympathetic trunk. The density of these IMPs is similar to the density of saxitoxin binding sites on unmyelinated axolemma from rat cervical sympathetic trunk (Pellegrino et al. 1984 (Brain Res. 305, 357-360)), which suggests that many of the large diameter particles may be the morphological correlate of voltage-sensitive Na+ channels.
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PMID:Freeze-fracture studies on unmyelinated axolemma of rat cervical sympathetic trunk: correlation with saxitoxin binding. 245 31

The mucosa that lines the airways is covered with a fluid film forming a hypophase between mucus and cell surface. To study the function of this epithelium aims at describing the mechanisms by which fluid is normally produced. Another goal to be pursued consists in looking for the origin of pathological situations, such as cystic fibrosis, in which the functioning of epithelial cell is altered. The elucidation of transport mechanisms present in the apical and in the basolateral membrane results in a conceptual model that illustrates the asymmetrical functioning of epithelial cells. Recent discoveries enlarge our understanding of membrane transport processes; in particular, a concerted, reciprocal regulation of the activity of both membranes was shown to be exerted via the intracellular composition. The tracheal epithelium absorbs Na+ and secretes Cl-. These two transports are active and electrogenic; their sum corresponds approximately to the short-circuit current measured in vitro. Na+ absorption is sensitive to amiloride from the luminal side and also to ouabain added to the serosal compartment. The process is a primary active transport, analogous to that found in amphibian epithelia or in mammalian colon. Cl- secretion is abolished by furosemide (or bumetanide), by ouabain or by Na+ suppression in the serosal incubation solution. The mechanism is a secondary active transport: Cl- influx across the basolateral membrane is coupled to Na+ (probably through Na+, K+, Cl- symport); energy is dissipated by the Na+-K+-ATPase localised in the basolateral membrane. Thus, Na+ is recirculated across that membrane by the pump activity, which maintains a favorable gradient for influx via the symport. Cl- efflux takes place by diffusion through the luminal membrane. This model applies to other epithelia in which Na+-coupled Cl- secretion was shown to take place. It is confirmed by isotopic fluxes measurements and by electrophysiologic properties of the apical and the basolateral membrane. Various agents are known to influence ion transports. In particular Cl- secretion is stimulated by substances that increase the intracellular concentration of cyclic AMP. At the membrane level, the number of active Cl- channels in the apical membrane is primarily controlled, then the basolateral membrane K+ permeability. Yet, species differences are worth to note: the trachea of the cow is barely sensitive to agents that exert a marked action on dog trachea. The tracheal epithelium is used as an experimental model for studying cystic fibrosis, a disease in which the apical membrane is almost devoid of functional Cl- channels, so that Cl- permeability is quite low.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:[Physiology of the tracheal epithelium]. 246 15

The characteristics of L-lysine transport were investigated at brush-border (maternal) and basal (fetal) sides of the syncytiotrophoblast in the term guinea-pig placenta artificially perfused either through the umbilical vessels in situ or through both circulations simultaneously. Cellular uptake, efflux and transplacental transfer were determined using a single-circulation paired-tracer dilution technique. Unidirectional L-[3H]lysine uptake (%) (perfusate lysine 50 microM) was high on maternal (M = 87 +/- 1) and fetal (F = 73 +/- 2) sides. L-[3H]Lysine efflux back into the ipsilateral circulation was asymmetrical (F/M ratio = 2.3) and transplacental flux occurred in favour of the fetal circulation. Unidirectional lysine influx kinetics (0.05-8.00 mM) gave Km values of 1.75 +/- 0.70 mM and 0.90 +/- 0.25 mM at maternal and fetal sides, respectively; corresponding Vmax values were 1.95 +/- 0.38 and 0.87 +/- 0.10 mumol.min-1.g-1. At both sides, lysine influx (50 microM) could be inhibited (about 60-80%) by 4 mM L-lysine and L-ornithine and less effectively (about 10-40%) by L-citrulline, L-arginine, D-lysine and L-histidine. At the basal side: (i) lysine influx kinetics were greatly modified in the presence of 10 mM L-alanine (Km = 6.25 +/- 3.27 mM; Vmax = 2.62 +/- 0.94 mumol.min-1.g-1), but unchanged by equimolar L-phenylalanine or L-tryptophan; (ii) in the converse experiments, lysine (10 mM) did not affect the kinetic characteristics for either L-alanine or L-phenylalanine; (iii) L-lysine and L-alanine influx kinetics were not dependent on the sodium gradient; (iv) the inhibition of L-[3H]lysine uptake by 4 mM L-homoserine was partially (60%) Na+-dependent. At the maternal side the kinetic characteristics for alanine influx were highly Na+-dependent, while lysine influx was partially Na+-dependent only at low concentrations (0.05-0.5 mM). Bilateral perfusion with 2,4-dinitrophenol (1 mM) reduced L-[3H]lysine uptake into the trophoblast and abolished transplacental transfer. It is suggested that lysine transport in the guinea-pig placenta is mediated by a specific transport system (y+) for cationic amino-acids. The asymmetry in the degree of sodium-dependency at both trophoblast membranes may in part explain the maternal-to-foetal polarity of placental amino-acid transfer in vivo.
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PMID:Lysine and alanine transport in the perfused guinea-pig placenta. 249 34

The purpose of this investigation was to study the effects of a distinct type of phospholipase C on sarcolemmal Na+-Ca2+ exchange. With this phospholipase C (Staphylococcus aureus), treatment of cardiac sarcolemmal vesicles resulted in a specific hydrolysis of membrane phosphatidylinositol. This hydrolysis of phosphatidylinositol also released two proteins (110 and 36 kDa) from the sarcolemmal membrane. Phospholipase C pretreatment of the sarcolemma resulted in an unexpected stimulation of Na+-Ca2+ exchange. The Vmax of Na+-Ca2+ exchange was increased but the Km for Ca2+ was not altered. This stimulation was specific to the Na+-Ca2+ exchange pathway. ATP-dependent Ca2+ uptake was depressed after phospholipase C treatment, but passive membrane permeability to Ca2+ was unaffected. Sarcolemmal Na+,K+-ATPase activity was not altered, whereas passive Ca2+ binding was modestly decreased after phospholipase C pretreatment. The stimulation of Na+-Ca2+ exchange after phosphatidylinositol hydrolysis was greater in inside-out vesicles than in a total population of vesicles of mixed orientation. This finding suggests that the cardiac sarcolemmal Na+-Ca2+ exchanger is functionally asymmetrical. The results also suggest that membrane phosphatidylinositol is inhibitory to the Na+-Ca2+ exchanger or, alternatively, this phospholipid may anchor an endogenous inhibitory protein in the sarcolemmal membrane. The observation that a transsarcolemmal Ca2+ flux pathway may be stimulated solely by phosphatidylinositol hydrolysis independently of phosphoinositide metabolic products like inositol triphosphate is novel.
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PMID:Role of phosphatidylinositol in cardiac sarcolemmal membrane sodium-calcium exchange. 254 59

A large conductance Ca2+-activated K+ channel from rabbit aorta was incorporated into planar lipid bilayers. This channel had a conductance of 337 +/- 7 pS in symmetrical 250 mM KCl solutions and had a Na+/K+ permeability ratio of less than 0.04. In asymmetrical solutions containing 300 mM KCl cis (intracellular), 100 mM KCl trans (extracellular) or 100 mM KCl cis 500 mM KCl trans, the reversal potentials for the channel were -30 and +46 mV, respectively. This channel possessed voltage-dependent activation and cis (intracellular) Ca2+ sensitivity. Cromakalim (50 nM) added to the trans side of the bilayer significantly increased the Popen by 56% from 0.09 +/- 0.01 to 0.14 +/- 0.01 (P less than 0.01) at -40 mV without altering the single channel conductance. This effect was dose-dependent, increasing at higher cromakalim concentrations. The primary effect of cromakalim was to decrease the tau slow of the channel closed state from 266 +/- 32 to 147 +/- 17 ms which is sufficient to account for the increase in Popen of the channel in the presence of cromakalim.
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PMID:A Ca2+-activated K+ channel from rabbit aorta: modulation by cromakalim. 259 75

The mechanism of melibiose symport by the melibiose permease of Escherichia coli was investigated by further analyzing the Na+ (H+ or Li+)-coupled facilitated diffusion reactions catalyzed by the carrier in de-energized membrane vesicles, with particular emphasis on the reaction of sugar exchange at equilibrium. It is first shown that melibiose exchange at equilibrium proceeds without concomitant movement of Na+, i.e. the coupled cation is kinetically occluded during the melibiose exchange reaction. These results provide further experimental support for the model of Na+ sugar co-transport of the physiological substrate melibiose previously suggested (Bassilana, M., Pourcher, T., and Leblanc, G. (1987) J. Biol. Chem. 262, 16865-16870) in which: 1) the mechanisms of co-substrate binding to (or release from) the carrier are ordered processes on both the outer (Na+ first, sugar last) and inner membrane surfaces (sugar first, Na+ last) and give rise to a mirror-type model; 2) release of Na+ from the carrier on the inner membrane surface is very slow and rate-limiting for carrier cycling but is fast on the opposite side, contributing to the asymmetrical functioning of the permease. On the other hand, analysis of the exchange of identical sugars (homologous exchange) and different sugar analogs (heterologous exchange) indicates that the overall rate of sugar exchange reaction coupled to Na+ or Li+ is limited by the rate of one (or more) partial step(s) associated with the inflow of co-substrates and most probably by the rate of sugar release into the intravesicular medium. It is proposed that the variability of the facilitated diffusion reactions catalyzed by the carrier in the presence of different coupled cations and/or sugar analogs reflects variations in the rate of co-substrate release from the carrier on the inner membrane surface.
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PMID:Melibiose permease of Escherichia coli. Characteristics of co-substrates release during facilitated diffusion reactions. 283 75

The currents through single Na+ channels were recorded from dissociated cells of the flexor digitorum brevis muscle of the mouse. At 15 degrees C the prolonged bursts of Na+ channel openings produced by application of the drug DPI 201-106 had brief sojourns to subconductance levels. The subconductance events were relatively rare and brief, but could be identified using a new technique that sorts amplitude estimates based on their variance. The resulting "levels histogram" had a resolution of the conductance levels during channel activity that was superior to that of standard amplitude histograms. Cooling the preparation to 0 degrees C prolonged the subconductance events, and permitted further quantitative analysis of their amplitudes, as well as clear observations of single-channel subconductance events from untreated Na+ channels. In all cases the results were similar: a subconductance level, with an amplitude of roughly 35% of the fully open conductance and similar reversal potential, was present in both drug-treated and normal Na+ channels. Drug-treated channels spent approximately 3-6% of their total open time in the subconductance state over a range of potentials that caused the open probability to vary between 0.1 and 0.9. The summed levels histograms from many channels had a distinctive form, with broader, asymmetrical open and substate distributions compared with those of the closed state. Individual subconductance events to levels other than the most common 35% were also observed. I conclude that subconductance events are a normal subset of the open state of Na+ channels, whether or not they are drug treated. The subconductance events may represent a conformational alteration of the channel that occurs when it conducts ions.
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PMID:Sodium channel subconductance levels measured with a new variance-mean analysis. 284 27

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