Intracellular Action of Aldosterone on Sodium Transport

Summary

Additional evidence has been presented in support of the inference that aldosterone regulates active sodium transport by an induction mechanism, initiated at the level of DNA-dependent RNA synthesis. Fanestil’s (7) recent results indicate that DNA synthesis is not involved in the process. The intermediate active product is an aldosterone-induced protein(s) (AIP) which is accumulated during the induction process. This conclusion is derived from the finding that cycloheximide (an inhibitor of ribosomal assembly of polypeptides) did not inhibit the substrate-dependent response of toad bladders pre-treated with aldosterone (Fig. 3). The induction hypothesis does not, of course, rule out the possibility that aldosterone might either directly or indirectly reduce the rate of degradation of an active intermediate protein. To assess this possibility we (10) pre-treated hemibladders with aldosterone and then inhibited protein synthesis in the continued presence and absence of aldosterone. A stabilizing effect of aldosterone on AIP should have shown up as a decreased rate of inhibition of sodium transport when aldosterone was added to the media (Fig. 5). No such effect was found. The alternative possibility was also considered that aldosterone might induce at the translational rather than the transcriptional level (10). If this were the case, the latent period should be taken up in accumulating threshold amounts of AIP and should depend on intact protein synthesis pathways. Contrary to this expectation, we found that the system was insensitive to cycloheximide during the latent period as revealed by the rapid response in sodium transport on removal of cycloheximide just before the end of the latent period (Fig. 4). These results led us to conclude that aldosterone initiates the induction mechanism at the level of DNA-dependent RNA synthesis.

As an approach to the problem of where and how aldosterone initiates the induction process a series of studies have been carried out on the binding of aldosterone to subcellular sites and binding proteins. Some information has been provided by radioautographs of the toad bladder. ³H-Aldosterone was localized over the nuclear areas of the target epithelial cells but the inactive steroid, ³H-progesterone, was distributed more evenly between the nuclear and cytoplasmic regions. The nuclear localization pattern also appeared to be specific for mineralo-corticoids as 9α-fluorocortisol displaced ³H-aldosterone from the nuclear areas significantly but estradiol-17β was without effect (Table 2). The possibility that the binding process is involved in the initiation of the induction process has been gaining credence. My colleagues and I (13, 18, 20) have extended the studies of binding of aldosterone in tissues to the adrenal-ectomized rat kidney. Differential and saturable binding to the nuclear fraction was observed. Recently, we succeeded in isolating and partially purifying aldosterone-binding proteins from renal nuclear and cytosol fractions. These tissue binding proteins are stereo-specific (Table 4) and show an impressive correlation between affinities for steroids and the mineralocorticoid potency of these steroids. Moreover, significantly higher aldosterone-binding activity was found in the epithelial tissues (kidney and duodenal mucosa) than in the parenchymal organs, spleen, liver and brain (Table 3). Further studies are underway on the relation of tissue steroid-binding proteins to physiological action.