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    ISSN: 1432-1424
    Keywords: Key words: Calcium (Ca2+) — Permeation — Planar lipid bilayer — Potassium (K+) —rca channel — Wheat (Triticum aestivum L.)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. Calcium channels in the plasma membrane of root cells fulfill both nutritional and signaling roles. The permeability of these channels to different cations determines the magnitude of their cation conductances, their effects on cell membrane potential and their contribution to cation toxicities. The selectivity of the rca channel, a Ca2+-permeable channel from the plasma membrane of wheat (Triticum aestivum L.) roots, was studied following its incorporation into planar lipid bilayers. The permeation of K+, Na+, Ca2+ and Mg2+ through the pore of the rca channel was modeled. It was assumed that cations permeated in single file through a pore with three energy barriers and two ion-binding sites. Differences in permeation between divalent and monovalent cations were attributed largely to the affinity of the ion binding sites. The model suggested that significant negative surface charge was present in the vestibules to the pore and that the pore could accommodate two cations simultaneously, which repelled each other strongly. The pore structure of the rca channel appeared to differ from that of L-type calcium channels from animal cell membranes since its ion binding sites had a lower affinity for divalent cations. The model adequately accounted for the diverse permeation phenomena observed for the rca channel. It described the apparent submillimolar K m for the relationship between unitary conductance and Ca2+ activity, the differences in selectivity sequences obtained from measurements of conductance and permeability ratios, the changes in relative cation permeabilities with solution ionic composition, and the complex effects of Ca2+ on K+ and Na+ currents through the channel. Having established the adequacy of the model, it was used to predict the unitary currents that would be observed under the ionic conditions employed in patch-clamp experiments and to demonstrate the high selectivity of the rca channel for Ca2+ influx under physiological conditions.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-1424
    Keywords: Key words: Calcium channel — Methoxyverapamil — Verapamil — Wheat roots
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. Despite biochemical evidence for the existence of high-affinity phenylalkylamine receptors in higher plants, their effects on channel activity have only been demonstrated at relatively high concentrations. We have performed a quantitative single-channel analysis of the changes induced by extracellular verapamil in the rca channel [a wheat root plasma membrane Ca2+-selective channel (Piñeros & Tester, 1995. Planta 195:478–488)]. Concentrations as low as 0.5 μm verapamil induced a blockade of the inward current, with no evident reduction of the single-channel current amplitude. Blockade by verapamil was concentration and voltage dependent. Preliminary analysis suggested the blockade was due to a reduction in the maximum open state probability rather than a change in V0.5. Further analysis of the association and dissociation rate constants revealed a binding site located 56 to 59% down the voltage drop from the extracellular face of the channel, with a K d (0) of 24 to 26 μm. This results in a K d at −100 mV of 2 μm. Methoxyverapamil had qualitatively the same effects. This intra-pore binding site can be accessed directly from the extracellular side of the rca channel, but apparently not from the cytosolic side.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Planta 195 (1995), S. 478-488 
    ISSN: 1432-2048
    Keywords: Aluminium ; Calcium channel ; Planar lipid bilayer ; Plasma membrane ; Voltage clamp ; Triticum (Ca2+ channel)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract A new mechanism for calcium flux in wheat (Triticum aestivum L.) root cells has been characterized. Membrane vesicles were enriched in plasma membrane using aqueous-polymer two-phase partitioning and incorporated into artificial lipid bilayers, allowing characterization of single channels under voltage-clamp conditions. Membrane marker activities showed 74% and 83% purity in plasma membrane when expressed in terms of membrane area and activity, respectively. Since membrane vesicles obtained by aqueous-polymer two-phase partitioning yield a population of membrane vesicles of regular orientation, and vesicle fusion into planar lipid bilayers occurs in a defined manner, the orientation of the channel upon vesicle incorporation could be determined. Thus ionic activities and potentials could be controlled appropriately on what we propose to be the cytosolic (trans) and extracellular (cis) faces of the channel. The unitary conductance in symmetrical 1 mM CaCl2 was 27±0.4 (pS). The correlation between the theoretical and observed reversal potentials in asymmetrical conditions showed that the channel was highly selective for Ca2+ over Cl−. Experiments simulating physiological ionic conditions showed a PCa 2+/PK + of 17–26, decreasing in this range as the extracellular CaCl2 concentration increased from 0.1 to 1 mM. The channel was also permeable to the essential nutrient ions, Mg2+ and Mn2+. The open probability of the channel was strongly dependent on the membrane potential. Inactivation with time was observed at more negative membrane potentials, and was immediately reversed as soon as the membrane potential was decreased. At membrane potentials more negative than -130mV, the channel remained mainly in the closed state, suggesting that in vivo the channel would remain largely closed and would open only upon membrane depolarization. The channel was blocked by micromolar concentrations of extracellular verapamil and trivalent cations, Al3+ being the most effective of those tested. Exposure of the cytosolic and extracellular sides of the channel to inositol 1,4,5-trisphosphate had no effect on the channel activity. We suggest a plasma-membrane origin for the channel as shown by biochemical and electrophysiological evidence, and discuss possible physiological roles of this channel, both in Ca2+ uptake into roots and in signal transduction.
    Type of Medium: Electronic Resource
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  • 5
    Publication Date: 2018-03-06
    Description: The growing burden of cancer among several major noncommunicable diseases (NCDs) requires national implementation of tailored public health surveillance. For many emerging economies where emphasis has traditionally been placed on the surveillance of communicable diseases, it is critical to understand the specificities of NCD surveillance and, within it, of cancer surveillance. We propose a general framework for cancer surveillance that permits monitoring the core components of cancer control. We examine communalities in approaches to the surveillance of other major NCDs as well as communicable diseases, illustrating key differences in the function, coverage, and reporting in each system. Although risk factor surveys and vital statistics registration are the foundation of surveillance of NCDs, population-based cancer registries play a unique fundamental role specific to cancer surveillance, providing indicators of population-based incidence and survival. With an onus now placed on governments to collect these data as part of the monitoring of NCD targets, the integration of cancer registries into existing and future NCD surveillance strategies is a vital requirement in all countries worldwide. The Global Initiative for Cancer Registry Development, endorsed by the World Health Organization, provides a means to enhance cancer surveillance capacity in low- and middle-income countries.
    Print ISSN: 0193-936X
    Electronic ISSN: 1478-6729
    Topics: Medicine
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