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  • 2000-2004  (2)
  • Amiloride cAMP ENaC Epithelial sodium channel Protein kinase A  (1)
  • Capacitance measurements Endogenous ion channels Maitotoxin Nonselective cation channel Stretch-activated cation channel Xenopus laevis oocyte  (1)
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  • 2000-2004  (2)
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  • 1
    Electronic Resource
    Electronic Resource
    cAMP sensitivity conferred to the epithelial Na+ channel by α-subunit cloned from guinea-pig colon (2000)
    Springer
    Pflügers Archiv 439 (2000), S. 579-587 
    Details
    ISSN: 1432-2013
    Keywords: Amiloride cAMP ENaC Epithelial sodium channel Protein kinase A
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract. The rate of Na+ (re)absorption across tight epithelia such as in distal kidney nephron and colon is to a large extent controlled at the level of the epithelial Na+ channel (ENaC). In kidney, antidiuretic hormone (ADH, vasopressin) stimulates the expression/activity of this channel by a cAMP/protein-kinase-A- (PKA-) mediated pathway. However, a clear upregulation of ENaC function by cAMP could not be reproduced with cloned channel subunits in the Xenopus oocyte expression system, suggesting the hypothesis that an additional factor is missing. In contrast, we show here that membrane-permeant cAMP can activate ENaC expressed in Xenopus oocytes (3.8-fold) upon replacement of the rat α-subunit by a new α-subunit cloned from guinea-pig colon (gpα). This α-subunit is 76% identical with its rat orthologue originating from ADH-insensitive rat colon. The biophysical fingerprints of the hybrid ENaC formed by this guinea-pig α-subunit together with rat β- and γ-subunits are indistinguishable from those of rat ENaC (rENaC). Injection of the PKA inhibitor PKI-(6–22)-amide into the oocyte had no effect on the basal activity of rat ENaC but inhibited the activity of gpα-containing hybrid ENaC and greatly decreased its stimulation by cAMP. This suggests that, unlike for rat ENaC, tonic PKA activity is required for basal function of gpα-containing ENaC and that PKA mediates its cAMP-induced activation. This regulatory behaviour is not common to all ENaCs containing an α-subunit cloned from an ADH-responsive tissue since xENaC, which was cloned from the ADH-sensitive Xenopus laevis A6 epithelia, is, when expressed in oocytes, resistant to cAMP, similar to rat ENaC. This study demonstrates that the PKA sensitivity of ENaC can depend on the nature of the ENaC α-subunit and raises the possibility that cAMP can stimulate ENaCs by different mechanisms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004249900213
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Maitotoxin induces insertion of different ion channels into the Xenopus oocyte plasma membrane via Ca2+-stimulated exocytosis (2000)
    Springer
    Pflügers Archiv 439 (2000), S. 363-369 
    Details
    ISSN: 1432-2013
    Keywords: Capacitance measurements Endogenous ion channels Maitotoxin Nonselective cation channel Stretch-activated cation channel Xenopus laevis oocyte
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract. The activation of cation channels in oocytes of Xenopus laevis by the marine poison maitotoxin (MTX) was monitored as membrane current (I m), conductance (G m) and membrane surface area determined by continuous measurements of membrane capacitance (C m). When MTX (25 pM) was added to the bathing solution there was an abrupt, large increase in inward membrane currents. Current/voltage relationships (I/V curves) were linear and suggested activation of voltage-independent nonselective cation channels (NSCC). MTX-induced Ca2+-sensitive currents were mainly carried by Na+ and were suppressed by low (0 mM) or high (40 mM) external Ca2+ concentrations and removal of Na+. Gadolinium (Gd3+, 10–500 µM) also had inhibitory effects, demonstrating the possible involvement of stretch-activated cation channels (SACC). In a high concentration (500 µM), amiloride substantially reduced the MTX-activated current while lower amiloride concentrations (50–100 µM) stimulated the current further. Continuous measurements of C m revealed that MTX induced exocytotic delivery and functional insertion of new channel proteins into the plasma membrane, indicated by a Ca2+-dependent increase in membrane surface area by around 28%. From these data we conclude that MTX activates NSCC that require relatively high concentrations of amiloride to be blocked. Furthermore, MTX possibly stimulates activation of Gd3+- and Ca2+-sensitive mechanosensitive cation channels. Stimulation of these channels is achieved by exocytotic delivery and functional insertion of new channels into the plasma membrane in a pathway that depends on the presence of extracellular Ca2+.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004249900150
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    Paper (German National Licenses)
    Fulltext
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