Central mineralocorticoid receptors, sympathetic activity, and hypertension

The multifaceted mineralocorticoid receptor

The primary adrenal cortical steroid hormones, aldosterone, and the glucocorticoids cortisol and corticosterone, act through the structurally similar mineralocorticoid (MR) and glucocorticoid receptors (GRs). Aldosterone is crucial for fluid, electrolyte, and hemodynamic homeostasis and tissue repair; the significantly more abundant glucocorticoids are indispensable for energy homeostasis, appropriate responses to stress, and limiting inflammation. Steroid receptors initiate gene transcription for proteins that effect their actions as well as rapid non-genomic effects through classical cell signaling pathways. GR and MR are expressed in many tissues types, often in the same cells, where they interact at molecular and functional levels, at times in synergy, others in opposition. Thus the appropriate balance of MR and GR activation is crucial for homeostasis. MR has the same binding affinity for aldosterone, cortisol, and corticosterone. Glucocorticoids activate MR in most tissues at basal levels and GR at stress levels. Inactivation of cortisol and corticosterone by 11β-HSD2 allows aldosterone to activate MR within aldosterone target cells and limits activation of the GR. Under most conditions, 11β-HSD1 acts as a reductase and activates cortisol/corticosterone, amplifying circulating levels. 11β-HSD1 and MR antagonists mitigate inappropriate activation of MR under conditions of oxidative stress that contributes to the pathophysiology of the cardiometabolic syndrome; however, MR antagonists decrease normal MR/GR functional interactions, a particular concern for neurons mediating cognition, memory, and affect.

Central regulation of blood pressure by the mineralocorticoid receptor

Addition of mineralocorticoid receptor (MR) antagonists to standard therapy for heart failure, kidney disease, metabolic syndrome, and diabetes is increasing steadily in response to clinical trials demonstrating clear benefits. In addition to blocking deleterious activity of MR within the heart, vessels and kidneys, MR antagonists target MR in hemodynamic regulatory centers in the brain, thereby decreasing excessive sympathetic nervous system drive, vasopressin release, abnormal baroreceptor function, and circulating and tissue pro-inflammatory cytokines. However, brain MR are also involved with cognition, memory, affect and functions yet to be determined. Understanding specific central mechanisms involved in blood pressure regulation by MR is necessary for the development of agents to target downstream events specific to central hemodynamic regulation, not only to avoid the hypokalemia caused by inhibition of renal tubular MR, but also to avoid untoward long term effects of inhibiting brain MR that are not involved in blood pressure control.

A randomized trial on mineralocorticoid receptor blockade in men: effects on stress responses, selective attention, and memory

Corticosteroids, released in high amounts after stress, exert their effects via two different receptors in the brain: glucocorticoid receptors (GRs) and mineralocorticoid receptors (MRs). GRs have a role in normalizing stress-induced effects and promoting consolidation, while MRs are thought to be important in determining the threshold for activation of the hypothalamic-pituitary-adrenal (HPA) axis. We investigated the effects of MR blockade on HPA axis responses to stress and stress-induced changes in cognitive function. In a double-blind, placebo-controlled study, 64 healthy young men received 400 mg of the MR antagonist spironolactone or placebo. After 1.5 h, they were exposed to either a Trier Social Stress Test or a non-stressful control task. Responses to stress were evaluated by hormonal, subjective, and physiological measurements. Afterwards, selective attention, working memory, and long-term memory performance were assessed. Spironolactone increased basal salivary cortisol levels as well as cortisol levels in response to stress. Furthermore, spironolactone significantly impaired selective attention, but only in the control group. The stress group receiving spironolactone showed impaired working memory performance. By contrast, long-term memory was enhanced in this group. These data support a role of MRs in the regulation of the HPA axis under basal conditions as well as in response to stress. The increased availability of cortisol after spironolactone treatment implies enhanced GR activation, which, in combination with MR blockade, presumably resulted in a decreased MR/GR activation ratio. This condition influences both selective attention and performance in various memory tasks.

Functional variability in corticosteroid receptors is a major component of strain differences in fat deposition and metabolic consequences of enriched diets in rat

We aimed to distinguish mineralocorticoid (MR) from glucocorticoid receptor (GR) actions in the nutritional differences between the Fischer 344 (F344) and LOU/C (LOU) rat strains. The decrease of urinary Na+/K+ ratio induced via MR activation by aldosterone and decrease of circulating lymphocyte counts exerted via GR activation by dexamethasone revealed a higher efficiency of corticosteroid receptor in LOU than in F344 rats. Afterward, we submitted F344 and LOU male rats to adrenalectomy and to substitution treatments with agonists of MR or GR under 3 successive diets--standard, free choice between chow and pork lard, and an imposed high-fat/high-sugar diet--to explore the involvement of the interactions between activation of corticosteroid receptors and diet on food intake, body composition, and metabolic blood parameters in these rats. Lastly, we measured energy expenditure and substrate oxidization in various experimental conditions in LOU and F344 rats by indirect calorimetry. In LOU rats, we showed greater basal and MR-induced energy expenditure, diet-induced thermogenesis, and lipid oxidization. We showed that the F344 rat strain constitutes a relevant model of the unfavorable effects exerted by glucocorticoids via GR on food preference for high-calorie diets, abdominal fat deposition, diabetes, and other deleterious consequences of visceral obesity. Contrary to F344 rats, the LOU rats did not exhibit the expected visceral fat deposition linked to GR activation. This strain is therefore a relevant model of resistance to diet-induced obesity and to the deleterious effects exerted by glucocorticoids on metabolism.

Mineralocorticoid receptors in the brain and cardiovascular regulation: minority rule?

A small proportion of brain mineralocorticoid receptors (MR) mediate control of blood pressure, water and electrolyte balance, sodium appetite, and sympathetic drive to the periphery. Circulating inflammatory cytokines modulate MR-mediated changes in sympathoexcitation. Aldosterone binding to MR in the brain occurs, despite concentrations that are 2-3 orders of magnitude less than those of cortisol and corticosterone, which have similar affinity for the MR. The possible mechanisms for selective MR activation by aldosterone, the cellular mechanisms of MR action and the effects of brain MR on hemodynamic homeostasis are considered in this review. MR antagonists are valuable adjuncts to the treatment of chronic cardiovascular and renal disease; the crucial need to discover targets for development of selective therapy for specific MR functions is also discussed.

Association between renin-angiotensin-aldosterone system-related genes and blood pressure in a Korean population

AIMS. Blood pressure control is influenced by various genetic and environmental factors, and genetic susceptibility is important in the development of essential hypertension. Because the renin-angiotensin-aldosterone system (RAAS) has a key role in vasoconstriction, vasodilation, and sodium and electrolyte balance, it is central in blood pressure control and so is an appropriate target in hypertension treatments. The present study assessed the association of RAAS-related genes with blood pressure and hypertension in a Korean population. Single nucleotide polymorphisms (SNPs, n = 114) in nine RAAS-related genes (AGT, REN, ACE, ACE2, AGTR1, CYP11B2, NR3C2, MAS1, and CMA1) were assessed for their correlation with blood pressure and hypertension using genotype data of 8842 individuals from the Korea Association Resource subject pool. MAJOR FINDINGS. Linear regression analysis revealed a statistically significant association with blood pressure of 10 SNPs in six genes (ACE, ACE2, CYP11B2, NR3C2, MAS1, and CMA1). An additional hypertension case-control study identified 10 SNPs in NR3C2 and ACE that were linked to hypertension. PRINCIPAL CONCLUSION. Three SNPs (rs11737660, rs6810951, and rs10519963) in NR3C2 correlate with both blood pressure and hypertension. Genetic polymorphisms in RAAS-related genes appear to be associated with hypertension in a Korean population.