Binding of aldosterone metabolites in isolated tubular segments

High-performance liquid chromatography (HPLC) analysis was performed on tissue extracts from whole kidney cortex (after in vivo injection or in vitro incubation with [3H]-aldosterone ([3H]A], from products bound to aldosterone receptors in cytoplasmic and nuclear fractions, and from the convoluted (PCT) and straight (PR) portion of the proximal tubule and cortical collecting tubule (CCT) isolated by micro-dissection after in vitro incubation with [3H]A (10 and 80 nM). After in vitro incubation of whole tissue, about 10% of the radioactivity corresponded to three groups of metabolites, two polar ones, which eluted earlier than aldosterone, and one less polar, which eluted later than aldosterone. When HPLC analysis was performed on bound products, after removal of free [3H]A, only one group of polar metabolites was detected (about 8% of the radioactivity) in addition to native [3H]A in the cytoplasmic and nuclear fractions. The binding of polar metabolites was nonspecific (nondisplaceable) and did not vary with incubation time and aldosterone concentration. In isolated tubules, in addition to [3H]A, an early peak of polar metabolites, corresponding to that observed in bound fractions, was present in PCT (10%) and in PR and CCT (20-25%). Other metabolites were not found in isolated tubules. We conclude the following: 1) The kidney is able to form at least three groups of aldosterone metabolites in small amounts. 2) Only one of them (more polar) was detected, as a nonspecifically bound product, in cellular fractions containing hormone-receptor complexes after removal of free [3H]A. Other metabolites were removed with free [3H]A. 3) These polar metabolites were found in both mineralocorticoid target segments such as CCT and in classically nontarget segments such as the proximal tubule. 4) The nonspecificity of the binding of polar metabolites and the absence of selective localization along the nephron suggest that they have no major mineralocorticoid action.