ISSN:
1432-2013
Schlagwort(e):
Amiloride cAMP ENaC Epithelial sodium channel Protein kinase A
Quelle:
Springer Online Journal Archives 1860-2000
Thema:
Medizin
Notizen:
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.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1007/s004249900213
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