Renal Aldosterone Research Articles

Molecular modifications of anti-aldosterone compounds: Effects on affinity of spirolactones for renal aldosterone receptors

The currently accepted hypothesis of the anti-mineralocorticoid action of spirolactones is that of competition for specific aldosterone receptors in target tissues. Binding of aldosterone to cytoplasmic receptors was studied by incubating adrenalectomized rat kidney slices with 3H-aldosterone in the presence of 10-fold non-radioactive dexamethasone to prevent binding of tracer to glucocorticoid receptors. From the ability of a series of 24 spirolactone analogues to compete for 3H-aldosterone binding sites under these conditions, the relative affinities for this receptor have been determined. Affinity for the receptor is decreased by B ring unsaturation at the C-6/C-7 position, by γ-lactone unsaturation, or by γ-lactone ring opening with the formation of the water-soluble K + salt. Affinity is markedly increased by esterification, or thioesterification, at the C-7 position in the B ring. Various 19-nor spirolactones show greater, equivalent or lesser affinity vis-a-vis their parent compounds. These structure-affinity relationships should define one of the determinants of pharmacologic activity.

Renal aldosterone receptors: studies with (3H)aldosterone and the anti-mineralocorticoid (3H)spirolactone (SC-26304).

In vivo, a spirolactone (SC-26304) inhibited the effects of aldosterone on urinary K(+):Na(+) ratios and the binding of [(3)H]aldosterone to renal cytoplasmic and nuclear receptors. Cytoplasmic binding of [(3)H]aldosterone and [(3)H]spirolactone (SC-26304) was similar in magnitude and involved the same set of sites. Under three sets of conditions-(i) in the intact rat, (ii) in kidney slices, and (iii) in reconstitution studies (mixing prelabeled cytoplasm with either purified renal nuclei or chromatin), [(3)H]spirolactone (SC-26304) did not yield specific nuclear complexes in contrast to the reproducible generation of these complexes with [(3)H]aldosterone. In glycerol density gradients, cytoplasmic [(3)H]aldosterone receptor complexes sedimented at 8.5 S and 4 S in low concentrations of salt and at 4.5 S in high concentrations of salt. Cytoplasmic [(3)H]spirolactone (SC-26304) receptor complexes sedimented at 3 S in low concentrations of salt and 4 S in high concentrations of salt. These results are discussed in terms of an allosteric model of the receptor system.

The roles of plasma binding and receptor specificity in the mineralocorticoid action of aldosterone.

The current hypothesis of the mechanism of action of aldosterone includes an initiating step of binding of the steroid to stereospecific receptor proteins in target tissues. In view of the very low circulating levels of aldosterone relative to other adrenal steroids, there must exist potent specificity—conferring mechanisms to insure appropriate aldosterone occupancy of mineralocorticoid receptors. Two specificity—conferring mechanisms are described which are posited to allow aldosterone occupancy of its appropriate receptor.In vitro, renal aldosterone binding sites are shown to fall into two classes. Those with a higher affinity for aldosterone—Kdlss (37 C)≅ 5X10-10—are termed Type I; those with a lower affinity—Kdiss (37 C)≅ 2.5 X lO-8— Type II. Desoxycorticosterone (DOC) has ∼80% the affinity of aldosterone for both sites. Corticosterone (B) has a high affinity for Type II sites, but less than 2% of the affinity of aldosterone for Type I. From the relative affinities of the three steroids for Type I, t...