Aldosterone and Mineralocorticoid Receptors—Physiology and Pathophysiology

Aldosterone is a uniquely terrestrial hormone, first appearing in lungfish, which have both gills and lungs. Mineralocorticoid receptors (MRs), on the other hand, evolved much earlier, and are found in cartilaginous and bony fish, presumptive ligand cortisol. MRs have equivalent high affinity for aldosterone, progesterone, and cortisol; in epithelia, despite much higher cortisol circulating levels, aldosterone selectively activates MRs by co-expression of the enzyme 11β-hydroxysteroid dehydrogenase, Type 11. In tissues in which the enzyme is not expressed, MRs are overwhelmingly occupied but not activated by cortisol, which normally thus acts as an MR antagonist; in tissue damage, however, cortisol mimics aldosterone and acts as an MR agonist. The risk profile for primary aldosteronism (PA) is much higher than that in age-, sex-, and blood pressure-matched essential hypertensives. High levels of aldosterone per se are not the problem: in chronic sodium deficiency, as seen in the monsoon season in the highlands of New Guinea, plasma aldosterone levels are extraordinarily high, but cause neither hypertension nor cardiovascular damage. Such damage occurs when aldosterone levels are out of the normal feedback control, and are inappropriately elevated for the salt status of the individual (or experimental animal). The question thus remains of how excess salt can synergize with elevated aldosterone levels to produce deleterious cardiovascular effects. One possible mechanism is through the agency of the elusive ouabain-like factors (OLFs). Such factors are secreted from the adrenal in response to ACTH (adrenalocortical tropic hormone), to angiotensin via AT2R, and-the polar opposite of aldosterone-to sodium loading. They act on blood vessels to cause vasoconstriction and thus elevate blood pressure to dump excess sodium through pressure natriuresis. Their levels are chronically elevated in PA in response to the continually elevated sodium status, and they thus act to constrict coronary and systemic arteries. In the context of the elevated blood volume and total body sodium in a PA patient, this raises blood pressure and acts as the proximate cause of cardiovascular damage. If this is the case, it would appear to offer new insights into therapy for PA. One would be the use of digibindin, or its more recent successors as antagonists of OLFs acting on Na/K ATPase at the vessel wall. A second would be to routinely combine a low dose MR antagonist, an ENaC inhibitor, and sodium restriction as first-line therapy for bilateral aldosterone overproduction. Finally, for unilateral cases post-surgery, there is good reason to include low-dose MRs in drug therapy if required, given the ability of cortisol in damaged blood vessels to mimic aldosterone vasoconstrictor action.