The most relevant biological action of aldosterone in epithelial tissues is the regulation of sodium reabsorption through binding to the mineralocorticoid receptor (MR). Glucocorticoids also bind with high affinity to MR, which is usually protected by the enzyme 11β-hydroxysteroid dehydrogenase. This activity prevents MR activation by cortisol despite the large prevalence of this steroid in plasma. Nonetheless, there are some aspects of the mechanism of action of MR that are not entirely explained by this competitive metabolic mechanism of protection. The picture is even more complicated in those tissues such as the nervous system where the enzyme is expressed at very low levels or is directly absent in various areas of the brain. Therefore, other cellular and molecular mechanisms must also intervene to allow specific aldosterone biological effects in the presence of overwhelming concentrations of glucocorticoids. In this article, we discuss some possible mechanisms that permit the specificity of action for each type of steroid, including those related to the recently discovered novel molecular mechanism of activation of corticosteroid receptors and the structural requirements of a given ligand to favor the mineralocorticoid action via MR. The relative contribution of these mechanisms may vary in different target cells allowing the fine tuning of cellular functions depending on the degree of cooperation between steroids, receptors, chaperones associated to receptors, and other factors. All these regulatory interactions can be altered in some pathophysiological situations, most of them related to stressing situations.
Keywords: Aldosterone, cortisol, glucocorticoid receptor, heat-shock protein of 90-kDa, immunophilins, mineralocorticoid receptor, stress disorders, steroid conformation.