Effect of central amiloride infusion on mineralocorticoid hypertension

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

Advances in the neurophysiology of magnocellular neuroendocrine cells

Hypothalamic magnocellular neuroendocrine cells have unique electrical properties and a remarkable capacity for morphological and synaptic plasticity. Their large somatic size, their relatively uniform and dense clustering in the supraoptic and paraventricular nuclei, and their large axon terminals in the neurohypophysis make them an attractive target for direct electrophysiological interrogation. Here, we provide a brief review of significant recent findings in the neuroplasticity and neurophysiological properties of these neurones that were presented at the symposium "Electrophysiology of Magnocellular Neurons" during the 13th World Congress on Neurohypophysial Hormones in Ein Gedi, Israel in April 2019. Magnocellular vasopressin (VP) neurones respond directly to hypertonic stimulation with membrane depolarisation, which is triggered by cell shrinkage-induced opening of an N-terminal-truncated variant of transient receptor potential vanilloid type-1 (TRPV1) channels. New findings indicate that this mechanotransduction depends on actin and microtubule cytoskeletal networks, and that direct coupling of the TRPV1 channels to microtubules is responsible for mechanical gating of the channels. Vasopressin neurones also respond to osmostimulation by activation of epithelial Na⁺ channels (ENaC). It was shown recently that changes in ENaC activity modulate magnocellular neurone basal firing by generating tonic changes in membrane potential. Both oxytocin and VP neurones also undergo robust excitatory synapse plasticity during chronic osmotic stimulation. Recent findings indicate that new glutamate synapses induced during chronic salt loading express highly labile Ca²⁺ -permeable GluA1 receptors requiring continuous dendritic protein synthesis for synapse maintenance. Finally, recordings from the uniquely tractable neurohypophysial terminals recently revealed an unexpected property of activity-dependent neuropeptide release. A significant fraction of the voltage-dependent neurohypophysial neurosecretion was found to be independent of Ca²⁺ influx through voltage-gated Ca²⁺ channels. Together, these findings provide a snapshot of significant new advances in the electrophysiological signalling mechanisms and neuroplasticity of the hypothalamic-neurohypophysial system, a system that continues to make important contributions to the field of neurophysiology.

Aldosterone Mediated Regulation of Epithelial Sodium Channel (ENaC) Subunits in the Rat Hypothalamus

Current evidence suggests that the epithelial Na⁺ channel (ENaC) in the brain plays a significant role in the development of hypertension. ENaC is present in vasopressin (VP) neurons in the hypothalamus, suggesting that ENaC in VP neurons is involved in the regulation of blood pressure. Our recent study demonstrated that high dietary salt intake caused an increase in the expression and activity of ENaC that were responsible for the more depolarized basal membrane potential in VP neurons. A known regulator of ENaC expression, the mineralocorticoid receptor (MR), is present in VP neurons, suggesting that ENaC expression in VP neurons is regulated by aldosterone. In this study, the effects of aldosterone and corticosterone on ENaC were examined in acute hypothalamic slices. Real-time PCR and Western blot analysis showed that aldosterone and corticosterone treatment resulted in a significant increase in the expression of γENaC, but not α- or βENaC, and that this expression was attenuated by MR and glucocorticoid receptor (GR) antagonists. Moreover, chromatin immunoprecipitation demonstrated that the aldosterone-MR complex directly interacts with the promoter region of the γENaC gene. However, the treatment with aldosterone did not cause subcellular translocation of ENaC toward the plasma membrane nor an increase in ENaC Na⁺-leak current. These results indicate that expression of γENaC in VP neurons is induced by aldosterone and corticosterone through their MR and GR, respectively; however, aldosterone or corticosterone alone is not sufficient enough to increase ENaC current when they are applied to hypothalamic slices in vitro.

Effect of dietary salt intake on epithelial Na+ channels (ENaCs) in the hypothalamus of Dahl salt-sensitive rats

All three epithelial Na+ channel (ENaC) subunits (α, β, and γ) and the mineralocorticoid receptor (MR), a known regulator of ENaC, are located in vasopressin (VP) synthesizing magnocellular neurons in the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei. Our previous study showed that ENaC mediates a Na+ leak current that affects the steady-state membrane potential of VP neurons. This study was conducted in Dahl salt-sensitive (Dahl-SS) rats to determine if any abnormal responses in the expression of ENaC subunits and MR occur in the hypothalamus and kidney in response to a high dietary salt intake. After 21 days of high salt consumption, Dahl-SS rat resulted in a significant increase in γENaC expression and exhibited proteolytic cleavage of this subunit compared to Sprague-Dawley (SD) rats. Additionally, Dahl-SS rats had dense somato-dendritic γENaC immunoreactivity in VP neurons, which was absent in SD rats. In contrast, SD rats fed a high salt diet had significantly decreased αENaC subunit expression in the kidney and MR expression in the hypothalamus. Plasma osmolality measured daily for 22 days demonstrated that Dahl-SS rats fed a high salt diet had a steady increase in plasma osmolality, whereas SD rats had an initial increase that decreased to baseline levels. Findings from this study demonstrate that Dahl-SS rats lack a compensatory mechanism to down regulate ENaC during high dietary salt consumption, which may contribute to the development of hypertension.

Effect of dietary salt intake on epithelial Na+ channels (ENaC) in vasopressin magnocellular neurosecretory neurons in the rat supraoptic nucleus

KEY POINTS

A growing body of evidence suggests that epithelial Na⁺ channels (ENaCs) in the brain play a significant role in the regulation of blood pressure; however, the brain structures that mediate the effect are not well understood. Because vasopressin (VP) neurons play a pivotal role in coordinating neuroendocrine and autonomic responses to maintain cardiovascular homeostasis, a basic understanding of the regulation and activity of ENaC in VP neurons is of great interest. We show that high dietary salt intake caused an increase in the expression and activity of ENaC which resulted in the steady state depolarization of VP neurons. The results help us understand one of the mechanisms underlying how dietary salt intake affects the activity of VP neurons via ENaC activity.

ABSTRACT

All three epithelial Na⁺ channel (ENaC) subunits (α, β and γ) are located in vasopressin (VP) magnocellular neurons in the hypothalamic supraoptic (SON) and paraventricular nuclei. Our previous study demonstrated that ENaC mediates a Na⁺ leak current that affects the steady state membrane potential in VP neurons. In the present study, we evaluated the effect of dietary salt intake on ENaC regulation and activity in VP neurons. High dietary salt intake for 7 days caused an increase in expression of β- and γENaC subunits in the SON and the translocation of αENaC immunoreactivity towards the plasma membrane. Patch clamp experiments on hypothalamic slices showed that the mean amplitude of the putative ENaC currents was significantly greater in VP neurons from animals that were fed a high salt diet compared with controls. The enhanced ENaC current contributed to the more depolarized basal membrane potential observed in VP neurons in the high salt diet group. These findings indicate that high dietary NaCl intake enhances the expression and activity of ENaCs, which augments synaptic drive by depolarizing the basal membrane potential close to the action potential threshold during hormonal demand. However, ENaCs appear to have only a minor role in the regulation of the firing activity of VP neurons in the absence of synaptic inputs as neither the mean intraburst frequency, burst duration, nor interspike interval variability of phasic bursting activity was affected. Moreover, ENaC activity did not affect the initiation, sustention, or termination of the phasic bursting generated in an intrinsic manner without synaptic inputs.

Localisation of 11β-Hydroxysteroid Dehydrogenase Type 2 in Mineralocorticoid Receptor Expressing Magnocellular Neurosecretory Neurones of the Rat Supraoptic and Paraventricular Nuclei

An accumulating body of evidence suggests that the activity of the mineralocorticoid, aldosterone, in the brain via the mineralocorticoid receptor (MR) plays an important role in the regulation of blood pressure. MR was recently found in vasopressin and oxytocin synthesising magnocellular neurosecretory cells (MNCs) in both the paraventricular (PVN) and supraoptic (SON) nuclei in the hypothalamus. Considering the physiological effects of these hormones, MR in these neurones may be an important site mediating the action of aldosterone in blood pressure regulation within the brain. However, aldosterone activation of MR in the hypothalamus remains controversial as a result of the high binding affinity of glucocorticoids to MR at substantially higher concentrations compared to aldosterone. In aldosterone-sensitive epithelia, the enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) prevents glucocorticoids from binding to MR by converting glucocorticoids into inactive metabolites. The present study aimed to determine whether 11β-HSD2, which increases aldosterone selectivity, is expressed in MNCs. Specific 11β-HSD2 immunoreactivity was found in the cytoplasm of the MNCs in both the SON and PVN. In addition, double-fluorescence confocal microscopy demonstrated that MR-immunoreactivity and 11β-HSD2-in situ hybridised products are colocalised in MNCs. Lastly, single-cell reverse transcriptase-polymerase chain reaction detected MR and 11β-HSD2 mRNAs from cDNA libraries derived from single identified MNCs. These findings strongly suggest that MNCs in the SON and PVN are aldosterone-sensitive neurones.

Brain mineralocorticoid receptors in cognition and cardiovascular homeostasis

Mineralocorticoid receptors (MR) mediate diverse functions supporting osmotic and hemodynamic homeostasis, response to injury and inflammation, and neuronal changes required for learning and memory. Inappropriate MR activation in kidneys, heart, vessels, and brain hemodynamic control centers results in cardiovascular and renal pathology and hypertension. MR binds aldosterone, cortisol and corticosterone with similar affinity, while the glucocorticoid receptor (GR) has less affinity for cortisol and corticosterone. As glucocorticoids are more abundant than aldosterone, aldosterone activates MR in cells co-expressing enzymes with 11β-hydroxydehydrogenase activity to inactivate them. MR and GR co-expressed in the same cell interact at the molecular and functional level and these functions may be complementary or opposing depending on the cell type. Thus the balance between MR and GR expression and activation is crucial for normal function. Where 11β-hydroxydehydrogenase 2 (11β-HSD2) that inactivates cortisol and corticosterone in aldosterone target cells of the kidney and nucleus tractus solitarius (NTS) is not expressed, as in most neurons, MR are activated at basal glucocorticoid concentrations, GR at stress concentrations. An exception may be pre-autonomic neurons of the PVN which express MR and 11β-HSD1 in the absence of hexose-6-phosphate dehydrogenase required to generate the requisite cofactor for reductase activity, thus it acts as a dehydrogenase. MR antagonists, valuable adjuncts to the treatment of cardiovascular disease, also inhibit MR in the brain that are crucial for memory formation and exacerbate detrimental effects of excessive GR activation on cognition and mood. 11β-HSD1 inhibitors combat metabolic and cognitive diseases related to glucocorticoid excess, but may exacerbate MR action where 11β-HSD1 acts as a dehydrogenase, while non-selective 11β-HSD1&2 inhibitors cause injurious disruption of MR hemodynamic control. MR functions in the brain are multifaceted and optimal MR:GR activity is crucial. Therefore selectively targeting down-stream effectors of MR specific actions may be a better therapeutic goal.

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.

ENaC-expressing neurons in the sensory circumventricular organs become c-Fos activated following systemic sodium changes

The sensory circumventricular organs (CVOs) are specialized collections of neurons and glia that lie in the midline of the third and fourth ventricles of the brain, lack a blood-brain barrier, and function as chemosensors, sampling both the cerebrospinal fluid and plasma. These structures, which include the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), are sensitive to changes in sodium concentration but the cellular mechanisms involved remain unknown. Epithelial sodium channel (ENaC)-expressing neurons of the CVOs may be involved in this process. Here we demonstrate with immunohistochemical and in situ hybridization methods that ENaC-expressing neurons are densely concentrated in the sensory CVOs. These neurons become c-Fos activated, a marker for neuronal activity, after various manipulations of peripheral levels of sodium including systemic injections with hypertonic saline, dietary sodium deprivation, and sodium repletion after prolonged sodium deprivation. The increases seen c-Fos activity in the CVOs were correlated with parallel increases in plasma sodium levels. Since ENaCs play a central role in sodium reabsorption in kidney and other epithelia, we present a hypothesis here suggesting that these channels may also serve a related function in the CVOs. ENaCs could be a significant factor in modulating CVO neuronal activity by controlling the magnitude of sodium permeability in neurons. Hence, some of the same circulating hormones controlling ENaC expression in kidney, such as angiotensin II and atrial natriuretic peptide, may coordinate ENaC expression in sensory CVO neurons and could potentially orchestrate sodium appetite, osmoregulation, and vasomotor sympathetic drive.

Role of mineralocorticoid action in the brain in salt-sensitive hypertension

1. The mechanisms by which excessive salt causes hypertension involve more than retention of sodium and water by the kidneys and are far from clear. Mineralocorticoids act centrally to increase salt appetite, sympathetic drive and vasopressin release, resulting in hypertension that is prevented by the central infusion of mineralocorticoid receptor (MR) antagonists. The MR has similar affinity for aldosterone and the glucocorticoids corticosterone or cortisol. Specificity is conferred in transport epithelia by the colocalization of the MR with 11β-hydroxysteroid dehydrogenase Type 2. Coexpression also occurs in some neurons, notably those of the nucleus tractus solitarius that are activated by sodium depletion and aldosterone and mediate salt-seeking behaviour. 2. The salt-induced hypertension of the Dahl salt-sensitive rat is mitigated by the central infusion of a mineralocorticoid antagonist even though circulating aldosterone is normal or reduced in salt-sensitive (SS). Contrary to reports that salt appetite in the Dahl salt-sensitive rat is depressed, we found that it is increased compared with that in Spraque-Dawley rats. 3. Extra-adrenal aldosterone synthesis in the brain occurs in minute amounts that could only be relevant locally. Expression of aldosterone synthase mRNA and aldosterone concentrations in the brain of Dahl salt-sensitive rats are increased compared with Spraque-Dawley rats. The central infusion of inhibitors of aldosterone synthesis lowers salt-induced hypertension in the Dahl salt-sensitive rat, suggesting a role for excessive Dahl salt-sensitive synthesis in the brain. Brain MR, particularly those in the paraventricular nuclei, regulate inflammatory processes that are exacerbated by sodium and lead to cardiovascular dysfunction.

Epithelial Na⁺ sodium channels in magnocellular cells of the rat supraoptic and paraventricular nuclei

The epithelial Na⁺ channels (ENaCs) are present in kidney and contribute to Na⁺ and water homeostasis. All three ENaC subunits (α, β, and γ) were demonstrated in the cardiovascular regulatory centers of the rat brain, including the magnocellular neurons (MNCs) in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN). However, the functional significance of ENaCs in vasopressin (VP) and oxytocin (OT) synthesizing MNCs is completely unknown. In this study, we show with immunocytochemical double-labeling that the α-ENaC is colocalized with either VP or OT in MNCs in the SON and PVN. In addition, parvocellular neurons in the dorsal, ventrolateral, and posterior subregions of the PVN (not immunoreactive to VP or OT) are also immunoreactive for α-ENaC. In contrast, immunoreactivity to β- and γ-ENaC is colocalized with VP alone within the MNCs. Furthermore, immunoreactivity for a known target for ENaC expression, the mineralcorticoid receptor (MR), is colocalized with both VP and OT in MNCs. Using single-cell RT-PCR, we detected mRNA for all three ENaC subunits and MR in cDNA libraries derived from single MNCs. In whole cell voltage clamp recordings, application of the ENaC blocker benzamil reversibly reduced a steady-state inward current and decreased cell membrane conductance approximately twofold. Finally, benzamil caused membrane hyperpolarization in a majority of VP and about one-half of OT neurons in both spontaneously firing and quiet cells. These results strongly suggest the presence of functional ENaCs that may affect the firing patterns of MNCs, which ultimately control the secretion of VP and OT.

The central mechanism underlying hypertension: a review of the roles of sodium ions, epithelial sodium channels, the renin-angiotensin-aldosterone system, oxidative stress and endogenous digitalis in the brain

The central nervous system has a key role in regulating the circulatory system by modulating the sympathetic and parasympathetic nervous systems, pituitary hormone release, and the baroreceptor reflex. Digoxin- and ouabain-like immunoreactive materials were found >20 years ago in the hypothalamic nuclei. These factors appeared to localize to the paraventricular and supraoptic nuclei and the nerve fibers at the circumventricular organs and supposed to affect electrolyte balance and blood pressure. The turnover rate of these materials increases with increasing sodium intake. As intracerebroventricular injection of ouabain increases blood pressure via sympathetic activation, an endogenous digitalis-like factor (EDLF) was thought to regulate cardiovascular system-related functions in the brain, particularly after sodium loading. Experiments conducted mainly in rats revealed that the mechanism of action of ouabain in the brain involves sodium ions, epithelial sodium channels (ENaCs) and the renin-angiotensin-aldosterone system (RAAS), all of which are affected by sodium loading. Rats fed a high-sodium diet develop elevated sodium levels in their cerebrospinal fluid, which activates ENaCs. Activated ENaCs and/or increased intracellular sodium in neurons activate the RAAS; this releases EDLF in the brain, activating the sympathetic nervous system. The RAAS promotes oxidative stress in the brain, further activating the RAAS and augmenting sympathetic outflow. Angiotensin II and aldosterone of peripheral origin act in the brain to activate this cascade, increasing sympathetic outflow and leading to hypertension. Thus, the brain Na(+)-ENaC-RAAS-EDLF axis activates sympathetic outflow and has a crucial role in essential and secondary hypertension. This report provides an overview of the central mechanism underlying hypertension and discusses the use of antihypertensive agents.

Effects of central sodium on epithelial sodium channels in rat brain

We evaluated the effects of intracerebroventricular (icv) infusion of Na+-rich artificial cerebrospinal fluid (aCSF), with or without the mineralocorticoid receptor (MR) blocker spironolactone, on epithelial Na+ channel (ENaC) subunits and regulators, such as MR, serum/glucocorticoid-inducible kinase 1, neural precursor cells expressed developmentally downregulated 4-like gene, 11β-hydroxylase, and aldosterone synthase, in brain regions of Wistar rats. The effects of icv infusion of the amiloride analog benzamil on brain tissue and CSF Na+ concentration ([Na+]) were also assessed. In the choroid plexus and ependyma of the anteroventral third ventricle, ENaC subunits are present in apical and basal membranes. Na+-rich aCSF increased β-ENaC mRNA and immunoreactivity in the choroid plexus and increased α- and β-ENaC immunoreactivities in the ependyma. Na+-rich aCSF increased α- and β-ENaC-gold-labeled particles in the microvilli of the choroid plexus and in basolateral membranes of the ependyma. Spironolactone only prevented the increase in β-ENaC immunoreactivity in the choroid plexus and ependyma. In the supraoptic nucleus, paraventricular nucleus, and subfornical organ, Na+-rich aCSF did not affect mRNA expression levels of the studied genes. Benzamil significantly increased CSF [Na+] in the control, but not Na+-rich, aCSF group. In contrast, benzamil prevented the increase in hypothalamic tissue [Na+] by Na+-rich aCSF. These results suggest that CSF Na+ upregulates ENaC expression in the brain epithelia, but not in the neurons of hypothalamic nuclei. ENaC in the choroid plexus and ependyma appear to contribute to regulation of Na+ homeostasis in the brain.

Aldosterone synthesis in the brain contributes to Dahl salt-sensitive rat hypertension

The enzymes required for aldosterone synthesis from cholesterol are expressed in rat and human brains. The hypertension of Dahl salt-sensitive (SS) rats is mitigated by the intracerebroventricular (i.c.v.) infusion of antagonists of the mineralocorticoid receptor (MR) and downstream effectors of mineralocorticoid action, as well as ablations of brain areas that also abrogate mineralocorticoid-salt excess hypertension in normotensive rats. We used real time RT-PCR to measure mRNA of aldosterone synthase and 11beta-hydroxylase, the requisite enzymes for the last step in the synthesis of aldosterone and corticosterone, respectively, MR and the determinants of MR ligand specificity, 11beta-hydroxysteroid dehydrogenase types 1 and 2 (11beta-HSD1&2) and hexose-6-phosphate dehydrogenase (H6PDH). A combination of extraction and ELISA was used to measure aldosterone concentrations in tissue and urine of SS and Sprague-Dawley (SD) rats. Aldosterone synthase mRNA expression was higher in the brains and lower in the adrenal glands of SS compared with SD rats. The amounts of mRNA for MR, 11beta-hydroxylase, 11beta-HSD1&2 and H6PD were similar. Aldosterone concentrations were greater in brains of SS than SD rats, yet, in keeping with the literature, the circulating and total aldosterone production of aldosterone in SS rats were not. The selective inhibitor of aldosterone synthase, FAD286, was infused i.c.v. or subcutaneously in a cross-over blood pressure study in hypertensive SS rats further challenged by a high-salt diet. The i.c.v. infusion of FAD286, at a dose that had no effect systemically, significantly and reversibly lowered blood pressure in SS rats. Aldosterone synthesis in brains of SS rats is greater than in SD rats and is important in the genesis of their salt-sensitive hypertension.

Chronic central infusion of aldosterone leads to sympathetic hyperreactivity and hypertension in Dahl S but not Dahl R rats

Six-week-old Dahl salt-sensitive (S) and -resistant (R) rats received for 2 wk an intracerebroventricular infusion of aldosterone (Aldo) (22.5 ng/h) or vehicle containing artificial cerebrospinal fluid (aCSF) with 0.15 M Na+. At 8 wk, mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) were recorded in conscious rats at rest, in response to air stress, and to an intracerebroventricular injection of the α2-adrenoceptor agonists guanabenz or ouabain. Baroreflex control of RSNA and HR was estimated by using intravenous phenylephrine and nitroprusside. In Dahl S but not Dahl R rats, Aldo raised resting MAP by 20–25 mmHg, doubled sympathoexcitatory and pressor responses to air stress and sympathoinhibitory and depressor responses to guanabenz, and impaired baroreflex function. In Dahl S but not Dahl R rats, Aldo significantly increased content of ouabain-like compounds (OLC) in the hypothalamus and attenuated excitatory responses to ouabain. Aldo did not affect water intake, plasma electrolytes, or OLC in plasma and adrenal glands. In another set of three groups of Dahl S rats, Aldo dissolved in aCSF containing 0.16, 0.15, or 0.14 M Na+was infused intracerebroventricularly for 2 wk. CSF Na+concentration ([Na+]) showed only a nonsignificant increase, but resting MAP increased from 111 ± 3 mmHg in rats with Aldo in 0.14 M Na+to 131 ± 3 and 147 ± 3 mmHg with Aldo in 0.15 and 0.16 M Na+, respectively ( P < 0.05 for both). These findings indicate that in Dahl S rats, intracerebroventricular infusion of Aldo causes similar central responses as high salt intake, i.e., increases in brain OLC content, sympathetic hyperreactivity, and hypertension. The extent of the increase in blood pressure (BP) by intracerebroventricular Aldo depends on the [Na+] in the vehicle. In Dahl R rats, intracerebroventricular Aldo did not increase brain OLC, sympathetic reactivity, and BP, suggesting that in this rat strain, a decrease in central responsiveness to mineralocorticoids may contribute to its salt-resistant nature.

Sodium transport antagonism reduces thrombotic microangiopathy in stroke-prone spontaneously hypertensive rats

We examined whether amiloride, an agent that possesses epithelial sodium channel (ENaC)- and sodium/hydrogen exchange (NHE)-inhibitory activities, would exhibit renal vascular protection in saline-drinking, stroke-prone spontaneously hypertensive rats (SHRSP). SHRSP received amiloride (1.0 mg·kg−1·day−1, n = 6) or deionized water (3 mg·kg−1·day−1, n = 6) for 5 wk starting at 61 days of age. Systolic blood pressure (SBP) did not differ among the groups, and there was no difference in the average daily urine output, sodium excretion, or potassium excretion. Terminal urinary protein excretion, blood urea nitrogen, and renal thrombotic microangiopathic lesions were markedly reduced in the amiloride group with no difference in plasma renin activity (PRA). In a survival protocol, SHRSP infused subcutaneously with benzamil (0.7 mg·kg−1·day−1, n = 8), a selective ENaC inhibitor, dimethylamiloride (0.7 mg·kg−1·day−1, n = 8), a selective NHE inhibitor, or vehicle ( n = 7) had comparable SBP. Dimethylamiloride nonetheless prolonged survival of SHRSP ( P < 0.005 vs. vehicle), and benzamil-treated SHRSP lived even longer ( P < 0.0001 vs. vehicle; P < 0.05 vs. dimethylamiloride). In a separate series, plasma potassium concentration was elevated by dimethylamiloride (3.4 ± 0.1 meq/l, n = 8) and benzamil (3.3 ± 0.1 meq/l, n = 8) relative to vehicle (3.0 ± 0.1 meq/l, n = 8) at 4 but not at 24 h after dosing. These findings suggest the involvement of a sodium transport mechanism in the development of thrombotic microangiopathy in SHRSP, unrelated to marked changes in arterial pressure, PRA, plasma potassium, or urinary water and electrolyte excretion.

Brain sodium channels and ouabainlike compounds mediate central aldosterone-induced hypertension

Central nervous system (CNS) effects of mineralocorticoids participate in the development of salt-sensitive hypertension. In the brain, mineralocorticoids activate amiloride-sensitive sodium channels, and we hypothesized that this would lead to increased release of ouabainlike compounds (OLC) and thereby sympathetic hyperactivity and hypertension. In conscious Wistar rats, intracerebroventricular infusion of aldosterone at 300 or 900 ng/h in artificial cerebrospinal fluid (aCSF) with 0.145 M Na+ for 2 h did not change baseline mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA), or heart rate (HR). Intracerebroventricular infusion of aCSF containing 0.16 M Na+ (versus 0.145 M Na+ in regular aCSF) did not change MAP or RSNA, but significant increases in MAP, RSNA, and HR were observed after intracerebroventricular infusion of aldosterone at 300 ng/h for 2 h. Intracerebroventricular infusion of aCSF containing 0.3 M Na+ increased MAP, RSNA, and HR significantly more after intracerebroventricular infusion of aldosterone versus vehicle. After intracerebroventricular infusion of aldosterone, the MAP, RSNA, and HR responses to intracerebroventricular infusion of aCSF containing 0.16 M Na+ were blocked by blockade of brain OLC with intracerebroventricular infusion of Fab fragments or of brain sodium channels with intracerebroventricular benzamil. Chronic intracerebroventricular infusion of aldosterone at 25 ng/h in aCSF with 0.15 M Na+ for 2 wk increased MAP by 15-20 mmHg and increased hypothalamic OLC by 30% and pituitary OLC by 60%. Benzamil blocked all these responses to aldosterone. These findings indicate that in the brain, mineralocorticoids activate brain sodium channels, with small increases in CSF Na+ leading to increases in brain OLC, sympathetic outflow, and blood pressure.

Stimulation of brain Na+ channels by FMRFamide in Dahl SS and SR rats

Stimulation of brain Na+ channels by Phe-Met-Arg-Phe-NH2 (FMRFamide) increases sympathetic nerve activity and blood pressure (BP) in Wistar rats. Blockade of brain ouabain-like compounds (OLC) by specific antibody Fab fragments prevents these responses to intracerebroventricular FMRFamide. In the present study, we evaluated the effects of high-salt intake on brain FMRFamide levels and the responses of BP and brain OLC to intracerebroventricular infusion of FMRFamide in Dahl salt-sensitive (SS) and salt-resistant (SR) rats. FMRFamide and OLC content was measured with the use of RIA and ELISA, respectively. A high-salt diet (1,370 micromol Na+/g) for 2 wk significantly increased BP in Dahl SS but not in SR rats. On a regular salt diet, Dahl SS and SR rats showed similar FMRFamide levels in the whole hypothalamus, pons and medulla, and spinal cord. A high-salt diet for 2 wk did not affect FMRFamide levels in these tissues in both Dahl SS and SR rats. In Dahl SS but not in SR rats, chronic intracerebroventricular infusion of FMRFamide (200 nmol. kg(-1).day(-1)) for 2 wk significantly increased BP (mean arterial pressure: 116 +/- 5 vs. 100 +/- 2 mmHg; P < 0.01). Chronic intracerebroventricular infusion of FMRFamide significantly increased hypothalamic and pituitary OLC in Dahl SS but not SR rats. These results indicate that Dahl SS rats exhibit enhanced central responses to FMRFamide. In Dahl SS but not in SR rats on a high-salt diet, enhanced Na+ entry through FMRFamide-activated brain Na+ channels may increase brain OLC release, thereby leading to hypertension.

Role of central mineralocorticoid receptors in cardiovascular disease

Mineralocorticoids act directly through their receptors in specific centers in the central nervous system, kidneys, heart, and vascular smooth muscle to mediate hemodynamic homeostasis. These steroids also modulate renal and cardiovascular function indirectly through the autonomic nervous system. Complex homeostatic mechanisms under normal hormonal control become pathogenic when there is an excess of regulatory hormone. Experiments in which mineralocorticoid receptor antagonists or antisense oligodeoxynucleotides were administered centrally have clearly shown that centrally mediated effects on salt appetite, baroreceptor function, and autonomic drive to the renal and cardiovascular systems are crucial to the pathogenesis of hypertension and cardiovascular disease of hyperaldosteronism, and certain forms of genetic hypertension.