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  • 1
    ISSN: 1432-1424
    Keywords: Pump ; Inward rectifiers ; Wheat ; K+ channels ; Plasma membrane
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract An electrogenic pump, a slowly activating K+ inward rectifier and an intermittent, “spiky,” K+ inward rectifier, have been identified in the plasmalemma of whole protoplasts from root cortical cells of wheat (Triticum) by the use of patch clamping techniques. Even with high external concentrations of K+ of 100 m m, the pump can maintain the membrane potential difference (PD) down to −180 mV, more negative than the electrochemical equilibrium potentials of the various ions in the system. The slowly activating K+ inward rectifier, apparent in about 23% of protoplasts, allows inward current flow when the membrane PD becomes more negative than the electrochemical equilibrium potential for K+ by about 50 mV. The current usually consists of two exponentially rising components, the time constant of one about 10 times greater than the other. The longer time constant is voltage dependent, while the smaller time constant shows little voltage dependence. The rectifier deactivates, on return of the PD to less negative levels, with a single exponential time course, whose time constant is strongly voltage dependent. The spiky K+ inward rectifier, present in about 68% of protoplasts, allows intermittent current, of considerable magnitude, through the plasmalemma at PDs usually more negative than about −140 mV. Patch clamp experiments on detached outside-out patches show that a possibly multi-state K+ channel, with maximum conductance greater than 400 pS, may constitute this rectifier. The paper also considers the role of the pump and the K+ inward rectifiers in physiological processes in the cell.
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  • 2
    ISSN: 1432-1238
    Keywords: Key words Ventilator-associated pneumonia ; Non-bronchoscopic lung lavage ; Quantitative culture ; Bacterial index
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Objectives: (1)To assess the diagnostic utility of quantitative cultures of non-bronchoscopic lung lavage (NBL) in ventilator-associated pneumonia and evaluate the role of the Bacterial Index; (2) To assess the predictive value of NBL surveillance quantitative cultures in ventilated patients; (3) To evaluate the Clinical Pulmonary Infection Score (CPIS) system in ventilated patients.¶Design: A prospective comparison of NBL with bronchoscopic bronchoalveolar lavage and protected specimen brush.¶Setting: Three intensive care units in academic tertiary care centres.¶Patients: 145 adults ventilated for 72 h, with and without clinical signs of pneumonia.¶Interventions: Sampling of lower airway secretions by NBL, bronchoalveolar lavage and protected specimen brush.¶Main outcome measures: Diagnostic reliability of quantitative cultures, Bacterial Index and CPIS.¶Results: 34 episodes of clinical ventilator-associated pneumonia were documented in 32 patients. 9 episodes were confirmed by concordant blood/pleural culture or post-mortem lung examination. Qualitative concordance of the predominant pathogen between sequential NBL: bronchoalveolar lavage and protected specimen brush was 83 %. Sensitivity and specificity of non-directed bronchial lavage at a threshold of 104 CFU/ml were 68 % and 70 % respectively (p = 0.003) and were comparable with the bronchoscopic methods. However, the low positive predictive value of NBL indicates that quantitation in the absence of clinical signs is unlikely to be useful. Bacterial Index did not improve discriminatory power of quantitation compared with bacterial load of predominant organism. Mean CPIS for confirmed pneumonia cases was 8.4 ± 1.01, significantly higher than for non-pneumonia observations (p = 〈 0.0001).¶Conclusion: NBL is a simple, safe, cheap, readily-available method of diagnosing ventilator-associated pneumonia with comparable diagnostic accuracy to bronchoscopic techniques. Quantitation of respiratory tract cultures can exclude pneumonia in patients with equivocal clinical signs. The diagnostic threshold should vary depending on the length of ventilation, likelihood of pneumonia and antibiotic administration. The Bacterial Index is a flawed mathematical device that has no contributory role in pneumonia diagnosis. The CPIS has some diagnostic role in selected cohorts of ventilated patients.
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  • 3
    Electronic Resource
    Electronic Resource
    [s.l.] : Macmillan Magazines Ltd.
    Nature 388 (1997), S. 222-222 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] SirI write on behalf of 27 scientists in the School of Biological Sciences at Flinders University who have signed this letter. Despite the widely held view that Letters to Naturepresent results of exceptionally high scientific achievement and outstanding originality, the ...
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  • 4
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 191 (1961), S. 812-814 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Recently, in this laboratory, it has been possible to examine in more detail the action potential process by applying a Voltage-clamp' technique to the ecorticate internodal cells of a species of Nitella. The action potential in these cells has been described previously by Findlay2. In the ...
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  • 5
    ISSN: 1432-1424
    Keywords: K+ channels ; Hydrodictyon ; membrane ; TEA effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The potential difference across the membrane ofHydrodictyon africanum was controlled by voltage clamping and positive and negative steps in the PD were applied. For positive steps in the PD to values less negative than a threshold value, there is a PD and time-dependent increase in the outward current which has an S-shaped time course. Following the cessation of these steps, the current reverses instantaneously and declines with a simple time course. These currents show a strong K+ dependence and are blocked by tetraethylammonium (TEA) and nonyltriethylammonium (C9) ions, suggesting that they arise from the opening and then the closing of K+ channels. There is also a PD and time-dependent increase in the inward currents in response to negative steps in the membrane PD. The membrane properties have been described by three current-voltage curves, for the instantaneous current, for the steady-state current and for the current flow when the K+ channels are open. The response of the unclamped or free-running membrane PD to steps of constant current can be accounted for by the observed kinetics of the opening and closing of the K+ channels.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    The journal of membrane biology 83 (1985), S. 109-118 
    ISSN: 1432-1424
    Keywords: K+ channels ; ion channels ; Chara inflata ; membranes ; TEA+ effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Voltage-clamped steps in the electric potential difference (PD) across the membrane in cells of the green alga,Chara inflata, cause voltage- and time-dependent current flows, interpreted to arise from opening and closing of various types of ion channel in the membrane. With cells in the light, these channels are normally closed, and the resting PD is probably determined by the operation of an H+ efflux pump. Positive steps in PD from the resting level often caused the opening of K+ channels with sigmoid kinetics. The channels began to show opening when the PD≃−120 mV for an external concentration of K+ of 1.0mm. Return of the PD to the resting level caused closing of the channels with complex kinetics. Various treatments of the cell could cause these K+ channels to open, and remain open continuously, with the PD then lying closer to the Nernst PD for K+. The K+ channels have been identified by the blocking effects of TEA+. Another group of channels, probably Cl− and Ca2+ associated with the action potential open when the PD is stepped to values less negative than ≃−50 mV. Negative steps from the resting PD cause the slow opening, with a time course of seconds, of yet another type of channel, probably Cl−.
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  • 7
    ISSN: 1432-1424
    Keywords: plant ion channels ; patch clamp ; plasma membrane ; anion channel ; cation channel ; multistate channel
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary This report details preliminary findings for ion channels in the plasma membrane of protoplasts derived from the cotyledons ofAmaranthus seedlings. The conductance properties of the membrane can be described almost entirely by the behavior of two types of ion channel observed as single channels in attached and detached patches. The first is a cation-selective outward rectifier, and the second a multistate anion-selective channel which, under physiological conditions, acts as an inward rectifier. The cation channel has unit conductance of approx. 30 pS (symmetrical 100 K+) and relative permeability sequence K+〉Na+〉Cl− (1∶0.16∶0.03); whole-cell currents activate in a time-dependent manner, and both activation and deactivation kinetics are voltage dependent. The anion channel opens for hyperpolarized membrane potentials, has a full-level conductance of approx. 200 pS and multiple subconductance states. The number of sub-conductances does not appear to be fixed. When activated the channel is open for long periods, though shuts if the membrane potential (V m ) is depolarized; at millimolar levels of [Ca2+]cyt this voltage dependency disappears. Inward current attributable to the anion channel is not observed in whole-cell recordings when MgATP (2mm) is present in the intracellular solution. By contrast the channel is active in most detached patches, whether MgATP is present or not on the cytoplasmic face of the membrane. The anion channel has a significant permeability to cations, the sequence being NO 3 − 〉Cl−〉K+〉Aspartate (2.04∶1∶0.18 to 0.09∶0.04). The relative permeability for K+ decreased at progressively lower conductance states. In the absence of permeant anions this channel could be mistaken for a cation inward rectifier. The anion and cation channels could serve to clampV m at a preferred value in the face of events which would otherwise perturbV m .
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  • 8
    ISSN: 1432-1424
    Keywords: electrogenic transport ; diffusive components ; Hydrodictyon ; membrane
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary In cells of the freshwater algaHydrodictyon africanum, in solutions where [K+]0=0.1mm and pH0〉7.0, the membrane in the light is hyperpolarized. The membrane potential difference {ie179-1} has values from −180 to −275 mV, more negative than any ion diffusion potential difference, and is predominantly a function of pH0, and independent of [K+]0. The hyperpolarization of the membrane appears to arise from an electrogenic efflux of H+, estimated from voltage-clamp data to be about 8 nmol m−2 sec−1 when pH0=8.5. In the light the membrane conductanceg m is about 0.084 S m−2. At light-off, {ie179-2} becomes less negative, with a halftime for change of 15 to 30 sec andg m decreases by about 0.052 S m−2. After dark periods of up to 300 sec, {ie179-3} is largely independent of pH0 for values greater than 6.0 and usually behaves as a combined K+ and Na+ diffusion potential with permeability ratioP Na/P K=0.05 to 0.2. The membrane potassium conductanceg K has either a low value of 2–6×10−2 Sm−2, or a high value of up to 18×10−2 S m−2 depending on [K+]0, the transition from low to high values occurring when {ie179-4} moves over a threshold value that is more negative than {ie179-5}, the electrochemical equilibrium potential for K+. The time for half-change of the transition is about 30 sec. The results are consistent with a model of the membrane in which the pump electromotive force and conductance are in parallel with diffusive electromotive forces and conductances. When the pump is operating its properties determine membrane properties, and when it is inoperative, or running at a diminished rate, the membrane properties are determined more by the diffusive pathways. Changes in both pump rate andg K can account for a variety of characteristic changes in membrane PD and conductance occurring in response to ligh-dark changes, changes in light intensity, pasage of externally applied electric current across the membrane and changes in ionic constituents of the external medium.
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  • 9
    ISSN: 1432-1424
    Keywords: Cl− channels ; ion channels ; Chara inflata ; membranes ; punchthrough
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary An inward current which increases in magnitude over a period of seconds is activated when the membrane ofChara inflata (a green alga) in a K+-conductive state is hyperpolarized by a voltage clamp. The peak current and the half-time of activation are exponentially dependent on membrane potential difference. It was found by using an external Cl− electrode that the component exponentially dependent on potential was due to an efflux of Cl−. The measured current-voltage curves and the kinetics of deactivation of the current showed that other time-dependent components contributed to the net inward current. The “punchthrough” theory of Coster (Biophys. J. 5:669–686, 1965) does not adequately explain the inward current since a “punchthrough potential” could not be obtained, and the inward current was distinctly time dependent. The voltage and time dependence of the inward current strongly suggests that the Cl− efflux activated by hyperpolarization is through voltage-gated channels which open more frequently as the membrane is hyperpolarized.
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  • 10
    ISSN: 1432-1424
    Keywords: Cl− channels ; Chara inflata ; pH ; channel blockers ; NH 4 +
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The Cl− component of the voltage- and time-dependent inward current activated by hyperpolarizing the membrane ofChara inflata increases exponentially as the external pH, pH o , is lowered from 7 with the membrane potential difference (PD) kept constant. Lanthanum and anthracene-9-carboxylic acid (A-9-C, a Cl− channel blocker) both blocked the Cl− component and removed the pH o sensitivity of the inward current. Lanthanum, however, also decreased the K+ conductance. The hyperpolarized membrane is depolarized by A-9-C in a manner similar to that caused by the removal of external Cl−. Low external concentrations of NH 4 + stimulated the Cl− component of the inward current probably as a result of a change in cytoplasmic pH rather than as a result of a change in cytoplasmic [Cl−], since the effect was observed in Cl−-free solutions. The results show that the membrane PD, at hyperpolarized levels, is most likely determined by two factors: the proton extrusion pump, provided it has a reversal PD more negative than about −300 mV, and a voltage-dependent Cl− leak.
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