Time-dependent regulation by aldosterone of the amiloride-sensitive Na⁺ channel in rabbit kidney

Abstract 

The epithelial Na⁺ channel (ENaC) functions as the rate-limiting factor in aldosterone-regulated transcellular Na⁺ transport. In the study described here, the effect of aldosterone on ENaC mRNA levels, protein synthesis and benzamil-sensitive Na⁺ transport was investigated using primary cultures of immunodissected rabbit kidney connecting tubule and cortical collecting duct cells (CNT and CCD, respectively). After a lag time of 3 h, aldosterone caused transepithelial Na⁺ transport to increase, reaching maximal level of 260±44% after 16 h of incubation. The α, β and γ rabbit ENaC (rbENaC) mRNA levels, measured by semi-quantitative reverse transcriptase-polymerase chain reaction, were not changed by aldosterone during the first 3 h, but a twofold increase was apparent after 6 h; levels remained elevated for up to 16 h of incubation. Immunoprecipitation of [³⁵S]methionine-labeled rbENaC revealed a rise in protein levels of the α and β subunits, but the protein level of the γ subunit remained constant. In conclusion, our data suggest that in rabbit CNT and CCD the early increase in Na⁺ transport caused by aldosterone is due to the activation or insertion of existing Na⁺ channels into the apical membrane, and that the late response is mediated by increased synthesis of the α and β rbENaC subunits.