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

The epithelial Na+ channel (ENaC) functions as the rate-limiting factor in aldosterone-regulated transcellular Na+ transport. In the study described h ere, the effect of aldosterone on ENaC mRNA levels, protein synthesis and b enzamil-sensitive Na+ transport was investigated using primary cultures of immunodissected rabbit kidney connecting tubule and cortical collecting duc t cells (CNT and CCD, respectively). After a lag time of 3 h, aldosterone c aused transepithelial Na+ transport to increase, reaching maximal level of 260+/-44% after 16 h of incubation. The alpha, beta and gamma rabbit ENaC ( rbENaC) mRNA levels, measured by semi-quantitative reverse transcriptase-po lymerase 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 [S-35]methionine-labe led rbENaC revealed a rise in protein levels of the alpha and beta subunits , but the protein level of the gamma subunit remained constant. In conclusi on, our data suggest that in rabbit CNT and CCD the early increase in Na+ t ransport caused by aldosterone is due to the activation or insertion of exi sting Na+ channels into the epical membrane, and that the late response is mediated by increased synthesis of the alpha and beta rbENaC subunits.