Introduction

It is 59 years since the steroid hormone, aldosterone, was isolated and characterized by a group led by Simpson and Tait [1]. Aldosterone performs critical actions in the renal distal convoluted tubules for fluid and electrolyte balance, but in addition, numerous experimental and clinical studies in the past decade have provided insight into its mechanism of action in other tissues and into the role of its receptor, the mineralocorticoid receptor (MR). Elevated levels of aldosterone double the risk of mortality from cardiovascular disease (which continues to be the leading cause of mortality) and are also an independent risk factor [2–4]. Elevated levels of aldosterone relative to salt intake promote inflammation and fibrosis, leading to cardiac, vascular, and renal target-organ damage by activating MRs (listed as “aldosterone receptors” by some investigations). This concept is now changing, with further studies confirming that, unlike other steroid receptors, the MR has two ligands, aldosterone and cortisol. Clinical trials show that MR blockade with spironolactone and eplerenone lower blood pressure [5–7], particularly in resistant hypertension, and substantially increase survival and decrease hospitalization in patients with chronic, severe systolic heart failure (RALES) [8], heart failure after myocardial infarction (EPHESUS) [9], and last year, in patients with systolic heart failure with mild symptoms (EMPHASIS-HF) [10••], highlighting the important role of aldosterone in cardiovascular morbidity and mortality. This review provides an overview of publications in the past year that provide new information into the action of aldosterone and its receptor (MR) in heart disease.

Aldosterone and Salt

During heart failure, there is activation of the adrenergic and the renin-angiotensin-aldosterone systems [11, 12], with an increased plasma level of aldosterone associated with increased mortality in patients with heart failure [13] and acute myocardial infarction (AMI) [2, 3]. However, dietary salt intake also must be measured, because high aldosterone levels alone, when appropriate to physiological conditions such as salt restriction, are not detrimental to the heart [14–16]. Both experimental reports [14, 15] and a recent clinical study [17••] confirm that elevated levels of aldosterone in combination with high salt intake promote remodelling in the heart by potentiating MR-mediated signalling. The experimental studies showed that this resulted in myocardial fibrosis, left ventricular (LV) dilatation, and hypertrophy [14, 15, 18], although the mechanism of how high salt intake augmented MR activity could not be defined.

The controlled, cross-sectional study by Pimenta and colleagues [17••], published last year, extends these experimental studies by investigating the relationship between aldosterone, dietary salt, and LV dimensions in patients with confirmed primary aldosteronism, comparing them with patients with essential hypertension who were matched for age, gender, duration of hypertension, and ambulatory blood pressure. By using patients with primary aldosteronism, this study translates the experimental studies to humans, as the levels of aldosterone in primary aldosteronism are elevated independently of the renin-angiotensin system and are not suppressed by sodium loading [19]. Compared with patients with essential hypertension, patients with primary aldosteronism had significantly greater mean LV thickness and LV mass, measured by echocardiography. The strong, independent, positive relationship between increased levels of urinary sodium and LV measures confirm that high levels of aldosterone require high dietary salt to trigger cardiac damage, although the mechanism by which aldosterone and salt synergistically damage the heart remained speculative until later the same year.

The recent experimental studies by Shibata and colleagues [20••] provide a mechanism for the interaction between high salt and aldosterone to produce hypertension and renal damage. These investigators have previously shown a ligand-independent pathway for activation of MR by Rac1, which is a member of the Rho family of small GTPases [21]. Using both salt-resistant and salt-sensitive Dahl rats in their most recent study [20••], they show that a high-salt diet activates renal Rac1 in salt-sensitive rats, leading to hypertension and renal injury by activating MR signalling pathways, whereas renal Rac1 was downregulated in salt-resistant rats. To confirm the role of aldosterone, salt-sensitive rats were adrenalectomized and then provided a high-salt diet. Following adrenalectomy, renal Rac1 activity was significantly suppressed, as were both hypertension and albuminuria in the adrenalectomized rats, confirming that aldosterone is essential for the salt-induced Rac1 activation. In separate studies, adrenalectomized salt-sensitive Dahl rats were supplemented with aldosterone. Rac1 activity was restored by aldosterone infusion in adrenalectomized rats, further confirming the interdependence between aldosterone and the Rac1-mediated pathway in salt-sensitive hypertension.

The studies by Shibata and colleagues [20••] conclusively provide a mechanism for the synergistic action between high salt and aldosterone to produce renal damage and hypertension. It is not known, however, whether a similar mechanism contributes to the cardiac damage triggered by high levels of aldosterone and dietary salt. Since there are increasing reports that Rac1 promotes cardiac injury, the same group [22•] recently examined whether Rac1 is present in cardiomyocytes, and they found Rac1-induced MR activation in cultured rat cardiomyocytes, contributing to reactive oxygen species–mediated MR activation and cardiac damage. The investigators do identify the limitations of cell-based studies and acknowledge that cultured cardiomyocyte cells derived from an embryonic rat heart may behave differently from adult hearts, so further studies are required to determine the role of the Rac1-MR pathway in heart disease.

A further complication for sustained, aldosterone-mediated MR activation is electrical remodelling, with cardiac arrhythmias a common complication of congestive heart failure. Cellular overload of sodium (Na+) and calcium (Ca2+) play an important role in cardiac arrhythmias [23]. Aldosterone and MR activation have been shown to have direct effects on cardiomyocyte Ca2+ handling that may predispose to arrhythmias. Targeted activation of MR by conditional, cardiomyocyte-specific MR overexpression [24] led to defects in heart function and early sudden death. Surviving animals had major ECG abnormalities, with prolonged ventricular repolarization and spontaneous and triggered ventricular arrhythmias. These changes were associated with an increase in action potential duration and Ca2+ transient amplitude. The early sudden death was prevented by spironolactone, which also translated clinically: both the RALES [8] and EPHESUS [9] trials showed significant reductions in sudden cardiac death by low doses of MR antagonists.

Because intracellular levels of Ca2+ are dependent on the transmembrane Na+ concentration gradient, it is also important to identify what happens to intracellular sodium. Intracellular ion concentrations are not routinely measured, so two studies should be highlighted. The first is by Pieske and colleagues [25], who measured elevated intracellular levels of Na+ in myocardial tissue from patients with heart failure, although serum aldosterone levels were not measured. In the second study, osmotic minipumps were implanted in healthy rabbits to provide aldosterone at levels comparable to those in patients with heart failure; intracellular levels of Na+ and cardiomyocyte electrogenic Na+-K+ pump activity were measured [26]. Chronic elevation of aldosterone levels inhibited cardiomyocyte Na+-K+ pump activity and increased intracellular Na+, measured by ion-selective microelectrodes. There was no significant difference between the rate of rise in intracellular Na+ activity in the tissue from aldosterone-treated and vehicle-treated animals, indicating that the aldosterone-induced increase in steady-state Na+ is due to a reduction in the extrusion of Na+ via the Na+-K+ pump rather than to an increase in Na+ influx. High intracellular Na+ levels in the myocardium of patients with heart failure [25] may be related to aldosterone-induced Na+-K+ pump inhibition. This may lead to increased Ca2 via the Na/Ca exchanger to promote sarcoplasmic reticulum (SR) Ca2+ loading in normal myocardium but Ca2+ overload in failing hearts, triggering arrhythmia. Aldosterone-induced Na+-K+ pump inhibition may also contribute to cardiac remodelling in heart failure, as activation of growth-related genes in cardiac myocytes [27] and myocyte hypertrophy [28, 29] have been reported following pump inhibition.

Aldosterone and the Mineralocorticoid Receptor

The relationship between elevated plasma levels of aldosterone and long-term mortality has been demonstrated in patients with congestive heart failure [11–13] and AMI [2, 3]. There is now interest in measuring aldosterone levels in patients with mild heart failure, and recently in patients with coronary artery disease with preserved LV function and no AMI [30•]. In this latter study, higher plasma levels of aldosterone continued to be associated with a higher risk of death, and aldosterone was associated with the risk of acute ischemic event. Interestingly, in both the RALES [8] and EPHESUS [9] studies, patients with New York Heart Association (NYHA) Class III–IV heart failure had initial circulating levels of aldosterone within the physiologic range (0.25–0.41 nmol/mL) [31].

The results of the RALES [8] and EPHESUS [9] trials, in which the addition of a low-dose MR antagonist improved survival despite normal circulating levels of aldosterone, shifted the focus away from aldosterone and towards identifying whether other mechanisms may be involved, including regulation of MR [32•]. Circulating levels of glucocorticoids are higher than aldosterone, and unlike other nuclear receptors, MRs have equal affinity for aldosterone and cortisol (corticosterone in rodents) [33, 34]. In many tissues, MR is protected from activation by cortisol by the enzyme 11β-hydroxysteroid dehydrogenase type 2 (11βHSD2), which converts cortisol to the inactive steroid cortisone. Cardiomyocytes lack 11βHSD2, so MR are normally occupied by cortisol as a cortisol-MR complex. During inflammation and hypoxia, reactive oxygen species are generated and changes in the redox state may interfere with the cortisol-MR complex and trigger cortisol-mediated activation of the MR, mimicking the effect of aldosterone [35–37••]. Weir and colleagues [4] have shown that both aldosterone and cortisol are associated with medium-term LV remodelling measured early after AMI.

Direct evidence that changes in the redox state will interfere with the cortisol-MR complex, leading to activation of MR by cortisol, has been shown in both isolated rabbit cardiomyocytes [36] and isolated rat hearts subjected to ischemia-reperfusion [37••]. Acute aldosterone exposure in isolated cardiomyocytes was previously shown to increase electrogenic Na+-K+ pump activity secondary to increased Na+ influx [38]. Under normal redox conditions, aldosterone (10 nM) acutely increases pump activity 10-fold, whereas cortisol (100 nM) alone did not alter Na+ influx [36]. When co-perfused with 10 nM aldosterone, cortisol blocked the aldosterone-induced increase—evidence that cortisol is an MR antagonist in cardiac myocytes. Perfusion with oxidized glutathione (GSSG), to mimic reactive oxygen species generation, had no effect alone; co-perfusion with cortisol, which resulted in a substantial increase in pump activity, mimicked the action of aldosterone.

Similarly, during experimental myocardial infarction, there is an increase in the redox state, and cortisol mimics the action of aldosterone to aggravate cardiac damage, further supporting the premise that cortisol behaves as an MR agonist during changes in the redox state [37••]. This action of cortisol was blocked by spironolactone but not by the glucocorticoid receptor/progesterone receptor antagonist RU486, further supporting the activation of the cortisol-MR complex during tissue damage. In the same experimental study [37••], low-dose spironolactone (10 nM) was also shown to be protective during ischemia-reperfusion, in the absence of additional steroids; infarct size and apoptosis were reduced below control, providing a possible explanation for the cardioprotective effects of low doses of spironolactone in the RALES study.

Conclusions

Strong evidence is now emerging from both clinical and experimental studies to demonstrate that elevated levels not only of aldosterone but also of cortisol have a pathophysiologic role in heart disease. Although regulation of MR has been the least well defined of the steroid receptors, this situation is now changing, with increased focus on identifying the mechanisms involved in MR activation during tissue damage. Transgenic models of cell type–specific overexpression and deletion of the MR gene have started to provide additional information about MR signalling. Interest also is increasing in identifying MR interactions with specific co-activators and co-repressors, and whether these interactions may differ during tissue damage. This is an exciting period of discovery for identifying the mechanisms involved in MR-mediated cardiac damage.