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  • 1
    ISSN: 1615-6102
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1749-6632
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Natural Sciences in General
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 12 (1989), S. 0 
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract. The energetics of coated vesicle-mediated endocytosis in turgid plant cells are discussed in terms of the known ATP requirement for clathrin cage dissociation and on the basis of the elementary expression for the energy w = P · v, where P corresponds to the turgor and v to the vesicle volume. The authors' calculations indicate that the possibility for formation of coated vesicles by endocytosis is limited to a rather low turgor range (P below about 0.1 MPa). This view is consistent with reports of well-developed clathrin coats on the plasma membranes in those cells having very low turgor such as root hairs and naked flagellates.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    The journal of membrane biology 136 (1993), S. 327-332 
    ISSN: 1432-1424
    Keywords: Guard cells ; Ionic relations ; Nonlinear network ; Membrane voltage ; Oscillations ; Osmosis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract In the plasmalemma of plants, the major ion transporters are voltage gated. Hence, they are intrinsically coupled via the membrane voltage. Theoretical predictions and electrophysiological recordings on guard cells demonstrate nonlinear oscillations of a dynamic system which provides longterm osmotic adjustment by switching between periods of net uptake and net release of salt, rather than by a steady-state.
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  • 5
    ISSN: 1432-1424
    Keywords: Key words: K+ conductance — Voltage-dependent channel gating — K+-dependent/stochastic gating model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. The effect of extracellular cation concentration and membrane voltage on the current carried by outward-rectifying K+ channels was examined in stomatal guard cells of Vicia faba L. Intact guard cells were impaled with double-barrelled microelectrodes and the K+ current was monitored under voltage clamp in 0.1–30 mm K+ and in equivalent concentrations of Rb+, Cs+ and Na+. From a conditioning voltage of −200 mV, clamp steps to voltages between −150 and +50 mV in 0.1 mm K+ activated current through outward-rectifying K+ channels (I K, out) at the plasma membrane in a voltage-dependent fashion. Increasing [K+] o shifted the voltage-sensitivity of I K, out in parallel with the equilibrium potential for K+ across the membrane. A similar effect of [K+] o was evident in the kinetics of I K, out activation and deactivation, as well as the steady-state conductance- (g K −) voltage relations. Linear conductances, determined as a function of the conditioning voltage from instantaneous I-V curves, yielded voltages for half-maximal conductance near −130 mV in 0.1 mm K+, −80 mV in 1.0 mm K+, and −20 mV in 10 mm K+. Similar data were obtained with Rb+ and Cs+, but not with Na+, consistent with the relative efficacy of cation binding under equilibrium conditions (K+≥ Rb+ 〉 Cs+ 〉 〉 Na+). Changing Ca2+ or Mg2+ concentrations outside between 0.1 and 10 mm was without effect on the voltage-dependence of g K or on I K, out activation kinetics, although 10 mm [Ca2+] o accelerated current deactivation at voltages negative of −75 mV. At any one voltage, increasing [K+] o suppressed g K completely, an action that showed significant cooperativity with a Hill coefficient of 2. The apparent affinity for K+ was sensitive to voltage, varying from 0.5 to 20 mm with clamp voltages near −100 to 0 mV, respectively. These, and additional data indicate that extracellular K+ acts as a ligand and alters the voltage-dependence of I K, out gating; the results implicate K+-binding sites accessible from the external surface of the membrane, deep within the electrical field, but distinct from the channel pore; and they are consistent with a serial 4-state reaction-kinetic model for channel gating in which binding of two K+ ions outside affects the distribution between closed states of the channel.
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  • 6
    ISSN: 1432-1424
    Keywords: Key words: Calcium — Channel — Current-voltage curves — Selectivity filter — Rate theory — Kinetic model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. Current-voltage relationships of a cation channel in the tonoplast of Beta vulgaris, as recorded in solutions with different activities of Ca2+ and K+ (from Johannes & Sanders 1995, J. Membrane Biol. 146:211–224), have been reevaluated for Ca2+/K+ selectivity. Since conversion of reversal voltages to permeability ratios by constant field equations is expected to fail because different ions do not move independently through a channel, the data have been analyzed with kinetic channel models instead. Since recent structural information on K+ channels show one short and predominant constriction, selectivity models with only one binding site are assumed here to reflect this region kinetically. The rigid-pore model with a main binding site between two energy barriers (nine free parameters) had intrinsic problems to describe the observed current-saturation at large (negative) voltages. The alternative, dynamic-pore model uses a selectivity filter in which the binding site alternates its orientation (empty, or occupied by either Ca2+ or K+) between the cytoplasmic side and the luminal side within a fraction of the electrical distance and in a rate-limiting fashion. Fits with this model describe the data well. The fits yield about a 10% electrical distance of the selectivity filter, located about 5% more cytoplasmic than the electrical center. For K+ translocation, reorientation of the unoccupied binding site (with a preference of about 6:5 to face the lumenal side) is rate limiting. For Ca2+, the results show high affinity to the binding site and low translocation rates (〈1% of the K+ translocation rate). With the fitted model Ca2+ entry through the open channel has been calculated for physiological conditions. The model predicts a unitary open channel current of about 100 fA which is insensitive to cytoplasmic Ca2+ concentrations (between 0.1 and 1 μm) and which shows little sensitivity to the voltage across the tonoplast.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    The journal of membrane biology 102 (1988), S. 141-152 
    ISSN: 1432-1424
    Keywords: Acetabularia ; patch clamp ; potassium channel ; relaxation analysis ; channel kinetics ; channel statistics ; reaction kinetic model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The contribution of the major K+ channel to the slow electrodynamic properties of theAcetabularia membrane has been investigated by patch-clamp techniques in the cell-attached mode using physiologically intact protoplasmic drops. This study comprises recording and statistical analysis of opening and closing events over long periods of time at various membrane voltagesV m, as well as measurement and evaluation of individual or averaged current responses of single channels upon large voltage steps. Although detailed observations reveal a variety of different states (such as bursts or various levels of conductance or noise), a serial three-state reaction kinetic model was adequate for the description of the channel properties which are relevant to the macroscopic electrical behavior. This model consists of an open state (o) and two closed states (1 and 2) with the approximate rat constantsk ol=1 sec−1,k lo=1 sec−1 k 12=20 · exp(−Vm/2kT) sec−1 andk 21=4· exp (V m e/2KT) sec−1. The current-voltage relationship of the open channel and the equilibriumk ol/k lo are relatively constant, whereas the (voltage-dependent) equilibriumk 12/k 21 can spontaneously change within an order of magnitude at constant voltage.
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    The journal of membrane biology 25 (1975), S. 183-208 
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The high membrane potential ofAcetabularia (E m=−170 mV) is due to an electrogenic pump in parallel with the passive diffusion system (E d=−80 mV) which could be studied separately in the cold, when the pump is blocked. Electrical measurements under normal conditions show that the pump pathway consists of its electromotive forceE p with two elementsP 1 andP 2 in series;P 2 is shunted by a large capacitance (C p=3 mF cm−2). The nonlinear current-voltage relationship ofP 1 (light- and temperature-sensitive) could be determined separately; it reflects the properties of a carrier-mediated electrogenic pump. The value ofE p (−190 mV) indicates a stoichiometry of 2∶1 between electrogenically transported charges and ATP. The electrical energy, normally stored inC p, compares well with the metabolic energy, stored in the ATP pool. The nonlinear current-voltage relationship ofP 2 (attributed to phosphorylating reactions) is also sensitive to light and temperature and is responsible for the region of negative conductance of the overall current-voltage relationship. The power of the pump (1 μW cm−2) amounts to some percent of the total energy turnover. The high Cl− fluxes (1 nmol cm−2 sec−1) and the electrical properties of the plasmalemma are not as closely related as assumed previously. For kinetic reasons, a direct and specific Cl− pathway between the vacuole and outside is postulated to exist.
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  • 9
    ISSN: 1432-1424
    Keywords: Acetabularia ; Cl− flux ; current-voltage relationships ; electrogenic pump ; ion-transport model ; nonlinear kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The current-voltage relationship of carrier-mediated, passive and active ion transport systems with one charge-carrying pathway can exactly be described by a simple reaction kinetic model. This model consists of two carrier states (one inside, one outside) and two pairs (forwards and backwards) of rate constants: a voltage-dependent one, describing the transport of charge and a voltage-insensitive one, summarizing all the other (voltage-independent) reactions. For the electrogenic Cl− pump inAcetabularia these four rate constants have been determined from electrical measurements of the current-voltage relationship of the pump (Gradmann, Hansen & Slayman, 1981;in: Electrogenic Ion Pumps, Academic Press, New York). The unidirectional Cl− efflux through the pump can also be calculated by the availiable reaction kinetic parameters.36Cl− efflux experiments on singleAcetabularia cells with simultaneous electrical stimulation (action potentials) and recording, demonstrate the unidirectional Cl− efflux to depend on the membrane potential. After subtraction of an efflux portion which bypasses the pump, agreement is found between the measured flux-voltage relationship and the theoretical one as obtained from the reaction kinetic model and its parameters from the electrical data.
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  • 10
    ISSN: 1432-1424
    Keywords: Physarum polycephalum ; membrane potential ; electrogenic pump ; cotransport ; Class-I model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Microplasmodia ofPhysarum polycephalum have been investigated by conventional electrophysiological techniques. In standard medium (30mm K+, 4mm Ca++, 3mm Mg++, 18mm citrate buffer, pH 4.7, 22°C), the transmembrane potential differenceV m is around −100 mV and the membrane resistance about 0.25 Ω m2.V m is insensitive to light and changes of the Na+/K+ ratio in the medium. Without bivalent cations in the medium and/or in presence of metabolic inhibitors (CCCP, CN−, N 3 − ),V m drops to about 0 mV. Under normal conditions,V m is very sensitive to external pH (pH o ), displaying an almost Nernstian slope at pH o =3. However, when measured during metabolic inhibition,V m shows no sensitivity to pH o over the range 3 to 6, only rising (about 50 mV/pH) at pH o =6. Addition of glucose or sucrose (but not mannitol or sorbitol) causes rapid depolarization, which partially recovers over the next few minutes. Half-maximal peak depolarization (25 mV with glucose) was achieved with 1mm of the sugar. Sugar-induced depolarization was insensitive to pH o . The results are discussed on the basis of Class-I models of charge transport across biomembranes (Hansen, Gradmann, Sanders and Slayman, 1981,J. Membrane Biol. 63:165–190). Three transport systems are characterized: 1) An electrogenic H+ extrusion pump with a stoichiometry of 2 H+ per metabolic energy equivalent. The deprotonated form of the pump seems to be negatively charged. 2) In addition to the passive K+ pathways, there is a passive H+ transport system; here the protonated form seems to be positively charged. 3) A tentative H+-sugar cotransport system operates far from thermodynamic equilibrium, carrying negative charge in its deprotonated states.
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