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  • 1
    Electronic Resource
    Electronic Resource
    The uptake and metabolism of urea by Chara australis: IV. Symport with sodium—A slip model for the high and low affinity systems (1993)
    Springer
    The journal of membrane biology 136 (1993), S. 263-271 
    Details
    ISSN: 1432-1424
    Keywords: Sodium ; Urea ; Symport ; Cotransport ; Charophytes ; Slip model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract We have previously investigated the electrogenic influx of urea in Chara, and the urea- and sodium-dependent membrane current. We have shown that there is a sodium-stimulated component of urea influx and a urea-stimulated component of sodium influx, and that these are of the same size. We conclude that the electrogenic inward transport of urea, and of its analogues acetamide and acrylamide, is by sodium symport, with a stoichiometric ratio of 1∶1. The kinetics of the fluxes and currents show two different K M values for sodium in different cells and two different kinds of kinetics for the effect of urea on membrane current, one of which fits the Michaelis-Menten equation, while the other shows a maximum and fits the difference of two Michaelis-Menten terms, suggesting a phenomenon like cis-inhibition. Similarities in kinetic characteristics between the inhibitory site and the electrically silent uptake site (System II) lead us to suggest that the same protein may be responsible for both the low-K M, electrogenic influx of urea (System I) and the high-K M, electrically silent influx by System II. We suggest a “slip” model for urea uptake in Chara.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00233665
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  • 2
    Electronic Resource
    Electronic Resource
    Effects of external pH, fusicoccin and butyrate on the cytoplasmic pH in barley root tips measured by 31P-nuclear magnetic resonance spectroscopy (1985)
    Springer
    Planta 166 (1985), S. 341-347 
    Details
    ISSN: 1432-2048
    Keywords: Butyrate ; Cytoplasmic pH ; Fusicoccin ; Hordeum (cytoplasmic pH) ; Nuclear magnetic resonance ; Root (cytoplasmic pH)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract 31P-Nuclear magnetic resonance spectroscopy was used to measure the cytoplasmic pH (pHc) in barley (Hordeum vulgare L.) root tips. As the external pH was raised from 4–10, pHc was found to increase from 7.44 to 7.75. The sensitivity of pHc to changes in external pH decreased with increasing external pH. Metabolic inhibition by sodium azide caused pHc to fall by 0.3 units. Addition of 10 mM butyrate resulted in a gradual decline in pHc, by approx. 0.3 units over 90 min. At a concentration of 1 mM, butyrate had no effect on pHc even after 2 h. Fusicoccin caused pHc to rise by 0.1–0.2 units. In maize (Zea mays L.) root tips, pHc was shown to have a similar sensitivity to fusicoccin. The results are discussed in relation to the regulation of pHc and the possible role of pHc in determining transmembrane electrical potential differences.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00401171
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