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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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  • 2
    Electronic Resource
    Electronic Resource
    Cytoskeletal disruption in A6 kidney cells: Impact on endo/exocytosis and NaCl transport regulation by antidiuretic hormone (1995)
    Springer
    The journal of membrane biology 145 (1995), S. 193-204 
    Details
    ISSN: 1432-1424
    Keywords: Epithelial cell polarity ; Microfilament ; Microtubule ; Epithelial Na channel ; Cl channel ; Fluid phase endocytosis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract Antidiuretic hormone (ADH; 2.5 × 10−8 m vasotocin) produces a stimulation of apical fluid phase endocytosis, protein secretion and NaCl reabsorption in Xenopus laevis A6 distal nephron cell epithelia pretreated with aldosterone (10−6 m). The increase of NaCl transport is mediated by a sequential opening of apical Cl and Na conductances. The aim of this study was to characterize the actin and tubulin cytoskeleton of A6 cells and to assess the impact of its disruption on baseline and ADH-induced apical vesicular membrane movements and ion transport to test for possible functional links. The microfilament (MF) and microtubule (MT) networks and their disruption were visualized by confocal laser microscopy. Conditions of depolimerization were selected, by cytochalasin D or cold and nocodazole, respectively. MF disruption produced an increase in baseline apical protein secretion (exocytic movements) (plus 18%) and a decrease of its induction by ADH (minus 35%). MF disruption also increased baseline horseradish peroxidase uptake (endocytic movements) (plus 21%), however, without affecting its ADH-induced increase. In the case of MT disruption, the ADH-induced stimulation of both protein secretion and fluid phase endocytosis was decreased by 70 and 44%, respectively. At the ion transport level, MF and MT disruption only insignificantly affected the ADH-induced Cl conductance, while they decreased the ADH-induced stimulation of Na transport (amiloride-sensitive short-circuit current and conductance) by a factor of 2 to 4. In conclusion, both MT and MF disruption decrease ADH-induced apical protein secretion and Na conductance, while the ADH-induced apical Cl conductance is not significantly affected. Taken together the data support the hypothesis that the modulation of Na channel expression by apical vesicular membrane movements plays a role in Na transport expression and its regulation by ADH.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00237377
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    Paper (German National Licenses)
    Fulltext
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