Brain sodium channels and central sodium-induced increases in brain ouabain-like compound and blood pressure

Epithelial Na + Channels Function as Extracellular Sensors

The epithelial Na + channel (ENaC) resides on the apical surfaces of specific epithelia in vertebrates and plays a critical role in extracellular fluid homeostasis. Evidence that ENaC senses the external environment emerged well before the molecular identity of the channel was reported three decades ago. This article discusses progress toward elucidating the mechanisms through which specific external factors regulate ENaC function, highlighting insights gained from structural studies of ENaC and related family members. It also reviews our understanding of the role of ENaC regulation by the extracellular environment in physiology and disease. After familiarizing the reader with the channel's physiological roles and structure, we describe the central role protein allostery plays in ENaC's sensitivity to the external environment. We then discuss each of the extracellular factors that directly regulate the channel: proteases, cations and anions, shear stress, and other regulators specific to particular extracellular compartments. For each regulator, we discuss the initial observations that led to discovery, studies investigating molecular mechanism, and the physiological and pathophysiological implications of regulation. © 2024 American Physiological Society. Compr Physiol 14:5407-5447, 2024.

Epithelial Sodium Channel and Salt-Sensitive Hypertension

The development of high blood pressure is influenced by genetic and environmental factors, with high salt intake being a known environmental contributor. Humans display a spectrum of sodium-sensitivity, with some individuals displaying a significant blood pressure rise in response to increased sodium intake while others experience almost no change. These differences are, in part, attributable to genetic variation in pathways involved in sodium handling and excretion. ENaC (epithelial sodium channel) is one of the key transporters responsible for the reabsorption of sodium in the distal nephron. This channel has an important role in the regulation of extracellular fluid volume and consequently blood pressure. Herein, we review the role of ENaC in the development of salt-sensitive hypertension, and present mechanistic insights into the regulation of ENaC activity and how it may accelerate sodium-induced damage and dysfunction. We discuss the traditional role of ENaC in renal sodium reabsorption and review work addressing ENaC expression and function in the brain, vasculature, and immune cells, and how this has expanded the implications for its role in the initiation and progression of salt-sensitive hypertension.

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.

Blood pressure regulation via the epithelial sodium channel: from gene to kidney and beyond

The epithelial sodium channel (ENaC) has long been recognized as playing a vital role in blood pressure (BP) regulation due to its involvement in fluid balance. The genes encoding the three ENaC subunits are likewise important contributors to hypertension, both in rare monogenic diseases and in the general population. The unusually high numbers of genetic variants associated with complex traits, including BP, that are located in non-coding areas suggest an involvement of these variants in regulatory functions. This may involve differential regulation of expression in different tissues. Emerging evidence indicates that the ENaC plays an important role in BP determination not only via its actions in the kidney, but also in other tissues commonly involved in BP regulation. The ENaC in the central nervous system is proposed to regulate BP via sympathetic nervous system activity. Recent evidence suggests that the ENaC contributes to vascular function and the myogenic response. Additional roles potentially include initiation of the baroreceptor reflex via ENaC in the baroreceptors and driving high salt intake with a 'taste for salt' via ENaC in the tongue. The present review describes the involvement of the ENaC in the determination of BP at a genetic and physiological level, detailing recent evidence for its role in the kidney and in other pertinent tissues.

Regulation of central Na+ detection requires the cooperative action of the NaX channel and α1 Isoform of Na+/K+-ATPase in the Na+-sensor neuronal population

The median preoptic nucleus (MnPO) holds a strategic position in the hypothalamus. It is adjacent to the third ventricle; hence, it can directly access the ionic composition of the CSF. MnPO neurons play a critical role in hydromineral homeostasis regulation by acting as central sensors of extracellular Na(+) concentration ([Na(+)](ext)). The mechanism underlying Na(+) sensing involves the atypical Na(+) channel, Na(X). Here we sought to determine whether Na(+) influx in Na(+) sensors is actively regulated via interaction with other membrane proteins involved in cellular Na(+) homeostasis, such as Na(+)/K(+)-ATPase. The Na(+)/K(+)-ATPase role was investigated using patch-clamp recordings in rat MnPO dissociated neurons. Na(+) current evoked with hypernatriuric solution was diminished in the absence of ATP/GTP, indicating that Na(+)/K(+)-ATPase play a central role in [Na(+)](ext) detection. Specific blockers of α1 and α3 isoforms of Na(+)/K(+)-ATPase, ouabain or strophanthidin, inhibited this Na(+) current. However, strophanthidin, which selectively blocks the α1 isoform, was more effective in blocking Na(+) current, suggesting that the Na(+)/K(+)-ATPase-α1 isoform is specifically involved in [Na(+)](ext) detection. Although strophanthidin did not alter either the membrane resistance or the Na(+) reversal potential, the conductance and the permeability of the Na(X) channel decreased significantly. Our results suggest that Na(+)/K(+)-ATPase interacts with the Na(X) channel and regulates the high [Na(+)](ext)-evoked Na(+) current via influencing the Na(+) influx rate. This study describes a novel intracellular regulatory pathway of [Na(+)](ext) detection in MnPO neurons. The α1 isoform of Na(+)/K(+)-ATPase acts as a direct regulatory partner of the Na(X) channel and influences Na(+) influx via controlling the Na(+) permeability of the channel.

Salt-induced hypertension in a mouse model of Liddle syndrome is mediated by epithelial sodium channels in the brain

Neural precursor cell expressed and developmentally downregulated 4-2 protein (Nedd4-2) facilitates the endocytosis of epithelial Na channels (ENaCs). Both mice and humans with a loss of regulation of ENaC by Nedd4-2 have salt-induced hypertension. ENaC is also expressed in the brain, where it is critical for hypertension on a high-salt diet in salt-sensitive rats. In the present studies we assessed whether Nedd4-2 knockout (-/-) mice have the following: (1) increased brain ENaC; (2) elevated cerebrospinal fluid (CSF) sodium on a high-salt diet; and (3) enhanced pressor responses to CSF sodium and hypertension on a high-salt diet, both mediated by brain ENaC. Prominent choroid plexus and neuronal ENaC staining was present in -/- but not in wild-type mice. In chronically instrumented mice, ICV infusion of Na-rich artificial CSF increased mean arterial pressure 3-fold higher in -/- than in wild-type mice. ICV infusion of the ENaC blocker benzamil abolished this enhancement. In telemetered -/- mice on a high-salt diet (8% NaCl), CSF [Na(+)], mean arterial pressure, and heart rate increased significantly, mean arterial pressure by 30 to 35 mmHg. These mean arterial pressure and heart rate responses were largely prevented by ICV benzamil but only to a minor extent by SC benzamil at the ICV rate. We conclude that increased ENaC expression in the brain of Nedd4-2 -/- mice mediates their hypertensive response to a high-salt diet by causing increased sodium levels in the CSF, as well as hyperresponsiveness to CSF sodium. These findings highlight the possible causative contribution of central nervous system ENaC in the etiology of salt-induced hypertension.

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.

Regulation of hypothalamic renin-angiotensin system and oxidative stress by aldosterone

In rats with salt-induced hypertension or postmyocardial infarction, angiotensin II type 1 receptor (AT(1)R) densities and oxidative stress increase and neuronal NO synthase (nNOS) levels decrease in the paraventricular nucleus (PVN). The present study was designed to determine whether these changes may depend on activation of the aldosterone -'ouabain' neuromodulatory pathway. After intracerebroventricular (i.c.v.) infusion of aldosterone (20 ng h(-1)) for 14 days, blood pressure (BP) and heart rate (HR) were recorded in conscious Wistar rats, and mRNA and protein for nNOS, endothelial NO synthase (eNOS), AT(1)R and NADPH oxidase subunits were assessed in brain tissue. Blood pressure and HR were significantly increased by aldosterone. Aldosterone significantly increased mRNA and protein of AT(1)R, P22phox, P47phox, P67phox and Nox2, and decreased nNOS but not eNOS mRNA and protein in the PVN, as well as increased the angiotensin-converting enzyme and AT(1)R binding densities in the PVN and supraoptic nucleus. The increases in BP and HR, as well as the changes in mRNA, proteins and angiotensin-converting enzyme and AT(1)R binding densities were all largely prevented by concomitant i.c.v. infusion of Digibind (to bind 'ouabain') or benzamil (to block presumed epithelial sodium channels). These data indicate that aldosterone, via 'ouabain', increases in the PVN angiotensin-converting enzyme, AT(1)R and oxidative stress, but decreases nNOS, and suggest that endogenous aldosterone may cause the similar pattern of changes observed in salt-sensitive hypertension and heart failure postmyocardial infarction.

Role of the epithelial sodium channel in salt-sensitive hypertension

The epithelial sodium channel (ENaC) is a heteromeric channel composed of three similar but distinct subunits, α, β and γ. This channel is an end-effector in the rennin-angiotensin-aldosterone system and resides in the apical plasma membrane of the renal cortical collecting ducts, where reabsorption of Na(+) through ENaC is the final renal adjustment step for Na(+) balance. Because of its regulation and function, the ENaC plays a critical role in modulating the homeostasis of Na(+) and thus chronic blood pressure. The development of most forms of hypertension requires an increase in Na(+) and water retention. The role of ENaC in developing high blood pressure is exemplified in the gain-of-function mutations in ENaC that cause Liddle's syndrome, a severe but rare form of inheritable hypertension. The evidence obtained from studies using animal models and in human patients indicates that improper Na(+) retention by the kidney elevates blood pressure and induces salt-sensitive hypertension.

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.

Endogenous cardiotonic steroids and salt-sensitive hypertension

Endogenous cardiotonic steroids (CTS), also called digitalis like factors, have been postulated to play important roles in pathogenesis of hypertension for nearly half of a century. For the past 50 years biomedical scientists have been in quest of an unidentified factor or hormone that both increases blood pressure and renal sodium excretion; this "natriuretic hormone" was, in fact, postulated to interact with the Na/K-ATPase. Recent discoveries have led to the identification of steroid molecules which are present in humans, rodents and amphibians, and which, in a complex manner, interact with each other and with the other systems that regulate renal salt handling and contribute to the salt-sensitivity of blood pressure. Recent findings include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of mechanisms by which CTS can signal through the Na/K-ATPase. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the central regulation of blood pressure and regulation of cell growth, and development of cardiovascular and renal fibrosis in particular.

Hypothalamic paraventricular nucleus mediates sodium-induced changes in cardiovascular and renal function in conscious sheep

The contribution of the paraventricular nucleus of the hypothalamus (PVN) in mediating cardiovascular, renal, hormonal, and sympathetic nerve responses to increased cerebrospinal fluid (CSF) [Na+] was investigated in conscious sheep. Intracerebroventricular hypertonic NaCl (0.5 mol/l, 20 μl/min for 60 min) increased arterial blood pressure [AP; +13.4 (sd 2.0) mmHg, P < 0.001] and central venous pressure [CVP; +2.8 (sd 1.3) mmHg, P < 0.001], but did not significantly change heart rate or cardiac output ( n = 6). Elevated CSF [Na+] also lowered plasma ANG II levels [−3.3 (sd 1.6) pmol/l, P = 0.004] and increased creatinine clearance [+31.5 (sd 32.7) ml/min, P = 0.03] and renal sodium excretion [+9.2 (sd 9.2) mmol/h, P = 0.003]. Lidocaine injection (1 μl, 2%) into the PVN prior to the ICV infusion had no apparent effect per se, but it abolished the AP, CVP, creatinine clearance, and ANG II responses to hypertonic NaCl, as well as reducing the increase in renal sodium excretion ( n = 6). Subsequent studies were performed in conscious sheep with chronically implanted electrodes for measurement of renal sympathetic nerve activity (RSNA). The effects of ICV hypertonic NaCl on AP and RSNA were measured before and after PVN-injection of glycine (250 nmol in 500 nl artificial CSF). ICV NaCl increased AP and decreased RSNA ( P < 0.001). These effects were significantly reduced by glycine ( P = 0.02–0.001, n = 5). Saline injected into the PVN ( n = 5) or lidocaine injected outside the PVN ( n = 6) had no effect on the response to ICV hypertonic NaCl. These results indicate that the PVN is an important mediator of cerebrally induced homeostatic responses to elevated sodium concentration/hyperosmolality.

Endogenous cardiotonic steroids: physiology, pharmacology, and novel therapeutic targets

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na(+)/K(+)-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

Enhanced pressor response to increased CSF sodium concentration and to central ANG I in heterozygous alpha2 Na+ -K+ -ATPase knockout mice

Intracerebroventricular (ICV) infusion of NaCl mimics the effects of a high-salt diet in salt-sensitive hypertension, raising the sodium concentration in the cerebrospinal fluid (CSF [Na]) and subsequently increasing the concentration of an endogenous ouabain-like substance (OLS) in the brain. The OLS, in turn, inhibits the brain Na(+)-K(+)-ATPase, causing increases in the activity of the brain renin-angiotensin system (RAS) and blood pressure. The Na(+)-K(+)-ATPase alpha (catalytic)-isoform(s) that mediates the pressor response to increased CSF [Na] is unknown, but it is likely that one or more isoforms that bind ouabain with high affinity are involved (e.g., the Na(+)-K(+)-ATPase alpha(2)- and/or alpha(3)-subunits). We hypothesize that OLS-induced inhibition of the alpha(2)-subunit mediates this response. Therefore, a chronic reduction in alpha(2) expression via a heterozygous gene knockout (alpha(2) +/-) should enhance the pressor response to increased CSF [Na]. Intracerebroventricular (ICV) infusion of artificial CSF containing 0.225 M NaCl increased mean arterial pressure (MAP) in both wild-type (+/+) and alpha(2) +/- mice, but to a greater extent in alpha(2) +/-. Likewise, the pressor response to ICV ouabain was enhanced in alpha(2) +/- mice, demonstrating enhanced sensitivity to brain Na(+)-K(+)-ATPase inhibition per se. The pressor response to ICV ANG I but not ANG II was also enhanced in alpha(2) +/- vs. alpha(2)+/+ mice, suggesting an enhanced brain RAS activity that may be mediated by increased brain angiotensin converting enzyme (ACE). The latter hypothesis is supported by enhanced ACE ligand binding in the organum vasculosum laminae terminalis. These studies demonstrate that chronic downregulation of Na(+)-K(+)-ATPase alpha(2)-isoform expression by heterozygous knockout increases the pressor response to increased CSF [Na] and activates the brain RAS. Since these changes mimic those produced by the endogenous brain OLS, the brain alpha(2)-isoform may be a target for the brain OLS during increases in CSF [Na], such as in salt-dependent hypertension.

Mechanisms in the PVN mediating local and central sodium-induced hypertension in Wistar rats

Sympathoexcitatory and hypertensive responses to central infusion of Na(+)-rich artificial cerebrospinal fluid (aCSF) are enhanced by aldosterone and mediated by mineralocorticoid receptors (MRs) and benzamil-blockable Na(+) influx, leading to "ouabain" release and ANG II type 1 (AT(1)) receptor stimulation. The present study evaluated the functional role of these mechanisms in the paraventricular nucleus (PVN). In conscious Wistar rats, Na(+)-rich aCSF was infused either directly into the PVN or intracerebroventricularly preceded by aldosterone and blockers. Infusion of Na(+)-rich aCSF in the PVN caused gradual increases in blood pressure (BP) and heart rate (HR). Aldosterone and a subpressor dose of ouabain in the PVN alone did not affect BP and HR but enhanced responses to Na(+). Eplerenone, benzamil, and "ouabain"-binding Fab fragments only blocked the enhancement by aldosterone, whereas losartan blocked all responses to Na(+)-rich aCSF in the PVN. Increases in BP and HR by intracerebroventricular infusion of Na(+)-rich aCSF were enhanced by aldosterone infused intracerebroventricularly, but not in the PVN. Telmisartan in the PVN again blocked all responses. In contrast, both eplerenone and benzamil in the PVN did not change the pressor responses to intracerebroventricular infusion of aldosterone and Na(+)-rich aCSF. These findings indicate that AT(1) receptors in the PVN mediate the responses to Na(+)-rich aCSF and their enhancement by aldosterone, both locally in the PVN or in the general CSF. MRs, benzamil-blockable Na(+) channels or transporters, and "ouabain" can be functionally active in the PVN, but in Wistar rats appear not to contribute to the pressor responses to short-term increases in CSF [Na(+)].

A role for benzamil-sensitive proteins of the central nervous system in the pathogenesis of salt-dependent hypertension

1. Although increasing evidence suggests that salt-sensitive hypertension is a disorder of the central nervous system (CNS), little is known about the critical proteins (e.g. ion channels or exchangers) that play a role in the pathogenesis of the disease. 2. Central pathways involved in the regulation of arterial pressure have been investigated. In addition, systems such as the renin-angiotensin-aldosterone axis, initially characterized in the periphery, are present in the CNS and seem to play a role in the regulation of arterial pressure. 3. Central administration of amiloride, or its analogue benzamil hydrochloride, has been shown to attenuate several forms of salt-sensitive hypertension. In addition, intracerebroventricular (i.c.v.) benzamil effectively blocks pressor responses to acute osmotic stimuli, such as i.c.v. hypertonic saline. Amiloride or its analogues have been shown to interact with the brain renin-angiotensin-aldosterone system (RAAS) and to effect the expression of endogenous ouabain-like compounds. Alterations of brain RAAS function and/or endobain expression could play a role in the interaction between amiloride compounds and arterial pressure. Peripheral treatments with benzamil, even at higher doses than those given centrally, have little or no effect on arterial pressure. These data provide strong evidence that benzamil-sensitive proteins (BSPs) of the CNS play a role in cardiovascular responsiveness to sodium. 4. Mineralocorticoids have been linked to human hypertension; many patients with essential hypertension respond well to pharmacological agents antagonizing the mineralocorticoid receptor and certain genetic forms of hypertension are caused by chronically elevated levels of aldosterone. The deoxycorticosterone acetate (DOCA)-salt model of hypertension is a benzamil-sensitive model that incorporates several factors implicated in the aetiology of human disease, including mineralocorticoid action and increased dietary sodium. The DOCA-salt model is ideal for investigating the role of BSPs in the pathogenesis of hypertension, because mineralocorticoid action has been shown to modulate the activity of at least one benzamil-sensitive protein, namely the epithelial sodium channel. 5. Characterizing the BSPs involved in the pathogenesis of hypertension may provide a novel clinical target. Further studies are necessary to determine which BSPs are involved and where, in the nervous system, they are located.

Endogenous digitalis: pathophysiologic roles and therapeutic applications

Endogenous digitalis-like factors, also called cardiotonic steroids, have been thought for nearly half a century to have important roles in health and disease. The endogenous cardiotonic steroids ouabain and marinobufagenin have been identified in humans, and an effector mechanism has been delineated by which these hormones signal through the sodium/potassium-transporting ATPase. These findings have increased interest in this field substantially. Although cardiotonic steroids were first considered important in the regulation of renal sodium transport and arterial pressure, subsequent work has implicated these hormones in the control of cell growth, apoptosis and fibrosis, among other processes. This Review focuses on the role of endogenous cardiotonic steroids in the pathophysiology of essential hypertension, congestive heart failure, end-stage renal disease and pre-eclampsia. We also discuss potential therapeutic strategies that have emerged as a result of the increased understanding of the regulation and actions of cardiotonic steroids.

The central role of the brain in salt-sensitive hypertension

PURPOSE OF REVIEW

To integrate recent studies showing that abnormal Na transport in the central nervous system plays a pivotal role in genetic models of salt-sensitive hypertension.

RECENT FINDINGS

Na transport-regulating mechanisms classically considered to reflect renal control of the blood pressure, i.e. aldosterone-mineralocorticoid receptors-epithelial sodium channels-Na/K-ATPase, have now been demonstrated to be present in the central nervous system contributing to regulation of cerebrospinal fluid [Na] by the choroid plexus and to neuronal responsiveness to cerebrospinal fluid/brain [Na]. Dysfunction of either or both can activate central nervous system pathways involving 'ouabain' and angiotensin type 1 receptor stimulation. The latter causes sympathetic hyperactivity and adrenal release of marinobufagenin - a digitalis-like inhibitor of the alpha1 Na/K-ATPase isoform - both contributing to hypertension on high salt intake. Conversely, specific central nervous system blockade of mineralocorticoid receptors or epithelial sodium channels prevents the development of hypertension on high salt intake, irrespective of the presence of a 'salt-sensitive kidney'. Variants in the coding regions of some of the genes involved in Na transport have been identified, but sodium sensitivity may be mainly determined by abnormal regulation of expression, pointing to primary abnormalities in regulation of transcription.

SUMMARY

Looking beyond the kidney is providing new insights into mechanisms contributing to salt-sensitive hypertension, which will help to dissect the genetic factors involved and to discover novel strategies to prevent and treat salt-sensitive hypertension.

Endogenous brain Na pumps, brain ouabain-like substance and the alpha2 isoform in salt-dependent hypertension

An endogenous ouabain-like substance (OLS) plays a critical role in the etiology of experimental models of human hypertension induced by a high salt diet. Early on, evidence for a role of this Na, K-ATPase inhibitor in blood pressure regulation was provided mainly by correlations of blood pressure with the levels of circulating Na, K-ATPase inhibitor. However, over the past decade, numerous studies have shown that endogenous Na pump inhibitors in the brain mediate salt-dependent hypertension in a variety of experimental models, including Dahl salt-sensitive (Dahl-S) and spontaneously hypertensive (SHR) rats on a high-salt diet. Other forms of hypertension that are known to be mediated by endogenous ouabain-like substances include steroid/salt- (e.g., DOCA-salt) and ACTH-induced hypertension. Even when exogenous ouabain is peripherally administered and/or the plasma ouabain/OLS level is increased in rats, the resulting hypertension is of CNS origin. After peripheral ouabain administration, ouabain levels increase in the plasma and the inhibitor subsequently accumulates in the brain. The ensuing hypertension is abolished by the intracerebroventricular (icv) administration of an anti-ouabain antibody (but not by the same antibody dose given iv), by discrete excitotoxic lesions in the brain or by ganglionic blockade, demonstrating that the response is neurally mediated. The pressor response to stimuli that increase the brain OLS (high salt diet, icv sodium) or to icv ouabain is abolished by icv losartan, demonstrating that the brain OLS activates the brain renin-angiotensin system (RAS) downstream. There are three isoforms of the catalytic alpha subunit of the Na, K-ATPase in the brain and cardiovascular system (alpha1, alpha2 and alpha3), but it is not known which brain isoform(s) mediate the hypertensive effects of circulating/CNS ouabain. Preliminary studies in gene-targeted mice suggest that the alpha2 isoform plays a critical role.

Intrahippocampal microinjection of an exquisitely low dose of ouabain mimics NaCl loading and stimulates a bufadienolide Na/K-ATPase inhibitor

BACKGROUND

Brain endogenous ouabain (EOU) raises blood pressure (BP) via an angiotensin II (ATII)-sensitive pathway in NaCl-loaded Dahl salt-sensitive rats (DSS). Brain EOU activates central and adrenocortical renin-angiotensin systems, and stimulates marinobufagenin, a vasoconstrictor and natriuretic inhibitor of sodium pump.

METHODS

We studied effects of acute NaCl loading (17 mmol/kg NaCl, intraperitoneally) on levels of EOU and marinobufagenin in several brain areas in DSS. We then studied effects of intrahippocampal administration of very-low-dose ouabain (60 pg) on EOU, marinobufagenin, BP, sodium excretion, and sodium-pump activity in the aorta and renal medulla in the absence and presence of anti-marinobufagenin and anti-ouabain antibodies, and losartan.

RESULTS

NaCl loading of DSS induced transient increases of EOU in the hippocampus and amygdala (15 min; 300%), supraoptical nucleus of hypothalamus (SON) (30 min, 230%) and pituitary (30 min; 85%), and ATII elevation in the SON (30 min). Intrahippocampal administration of ouabain (60 pg) stimulated ATII in the SON, produced natriuresis, 40 mmHg rise in BP, inhibition of sodium-pump in the renal medulla (19.6%) and aorta (25%), and a two-fold increase in renal marinobufagenin excretion. Pretreatment of rats with anti-marinobufagenin antibody prevented ouabain-induced pressor and natriuretic responses and sodium-pump inhibition. Pressor responses to ouabain were also prevented by losartan (intravenously) and by administration of anti-ouabain antibody into the SON.

CONCLUSIONS

NaCl loading of DSS induces a cascade of events, triggered by brain EOU and ATII. Intrahippocampal administration of a low-dose ouabain mimics effects of NaCl loading and stimulates marinobufagenin, which produces natriuresis, and inhibits the vascular sodium-pump, inducing an increase in BP.

Sequence analysis of coding and 3' and 5' flanking regions of the epithelial sodium channel alpha, beta, and gamma genes in Dahl S versus R rats

BACKGROUND

To test whether epithelial sodium channel (ENaC) genes' variants contribute to salt sensitive hypertension in Dahl rats, we screened ENaC alpha, beta, and gamma genes entire coding regions, intron-exon junctions, and the 3' and 5' flanking regions in Dahl S, R and Wistar rats using both Denaturing High Performance Liquid Chromatography (DHPLC) and sequencing.

RESULTS

Our analysis revealed no sequence variability in the three genes encoding ENaC in Dahl S versus R rats. One homozygous sequence variation predicted to result in a D75E substitution was identified in Dahl and Wistar rat ENaC alpha compared to Brown Norway. Six and two previously reported polymorphic sites in Brown Norway sequences were lost in Dahl and Wistar rats, respectively. In the 5' flanking regions, we found a deletion of 5GCTs in Dahl and Wistar rat ENaC alpha gene, five new polymorphic sites in ENaC beta and gamma genes, one homozygous sequence variation in Dahl and Wistar rat ENaC gamma gene, as well as one Dahl rat specific homozygous insertion of -1118CCCCCA in ENaC gamma gene. This insertion created additional binding sites for Sp1 and Oct-1. Five and three Brown Norway polymorphic sites were lost in Dahl and Wistar rats, respectively. No sequence variability in ENaC 3' flanking regions was identified in Dahl compared to Brown Norway rats.

CONCLUSION

The first comprehensive sequence analysis of ENaC genes did not reveal any differences between Dahl S and R rats that were isogenic in the regions screened. Mutations in ENaC genes intronic sequence or in ENaC-regulatory genes might possibly account for increased ENaC activity in Dahl S versus R rats.

Neuronal Responsiveness to Central Na + in 2 Congenic Strains of Dahl Salt-Sensitive Rats

Dahl salt-sensitive rats show increased Na(+) entry into the brain on high salt intake and increased sympathetic and pressor responses to central Na(+). We examined C10QTL2 and C17QTL to test whether they contribute to these phenotypes. In Dahl salt-sensitive, Lewis, and C10S.L16, and C17S.L2 congenic rats on a high salt diet for 8 to 10 days, blood pressure and heart rate were higher in Dahl salt-sensitive versus others and in C10S.L16 and C17S.L2 versus Lewis rats. Cerebrospinal fluid [Na(+)] increased by approximately 5 mmol/L in Dahl salt-sensitive, C10S.L16, and C17S.L2 compared with Lewis rats. In rats on a regular salt diet, 8-minute intracerebroventricular infusions of artificial cerebrospinal fluid with increasing [Na(+)] caused increases in blood pressure, heart rate, and renal sympathetic nerve activity, which were approximately 90% larger in Dahl salt-sensitive and C17S.L2 versus Lewis rats and only 35% to 45% larger in C10S.L16 versus Lewis rats. In another set of rats on regular salt, blood pressure and heart rate were recorded by telemetry before and during intracerebroventricular infusion of Na(+)-rich cerebrospinal fluid for 14 days. Na(+)-rich cerebrospinal fluid caused significantly larger increases in blood pressure and heart rate, larger responses to air stress and more impairment of baroreflex in Dahl salt-sensitive and C17S.L2 rats versus Lewis rats. In contrast, responses in C10S.L16 rats were similar to those in Lewis rats. These data suggest that, in Dahl salt-sensitive rats, genetic variants in C10QTL2 but not C17QTL contribute to increased neuronal responsiveness to cerebrospinal fluid [Na(+)]. However, neither of them contributes to the increase in cerebrospinal fluid [Na(+)] induced by high salt.

Activation of brain renin-angiotensin-aldosterone system by central sodium in Wistar rats

Functional studies indicate that the sympathoexcitatory and pressor responses to an increase in cerebrospinal fluid (CSF) [Na+] by central infusion of Na+-rich artificial cerebrospinal fluid (aCSF) in Wistar rats are mediated in the brain by mineralocorticoid receptor (MR) activation, ouabain-like compounds (OLC), and AT1-receptor stimulation. In the present study, we examined whether increasing CSF [Na+] by intracerebroventricular infusion of Na+-rich aCSF activates MR and thereby increases OLC and components of the renin-angiotensin system in the brain. Male Wistar rats received via osmotic minipump an intracerebroventricular infusion of aCSF or Na+-rich aCSF, in some groups combined with intracerebroventricular infusion of spironolactone (100 ng/h), antibody Fab fragments (to bind OLC), or as control gamma-globulins. After 2 wk of infusion, resting blood pressure and heart rate were recorded, OLC and aldosterone content in the hypothalamus were assessed by a specific ELISA or radioimmunoassay, and angiotensin-converting enzyme (ACE) and AT1-receptor binding densities in various brain nuclei were measured by autoradiography using 125I-labeled 351 A and 125I-labeled ANG II. When compared with intracerebroventricular aCSF, intracerebroventricular Na+-rich aCSF increased CSF [Na+] by approximately 5 mmol/l, mean arterial pressure by approximately 20 mmHg, heart rate by approximately 65 beats/min, and hypothalamic content of OLC by 50% and of aldosterone by 33%. Intracerebroventricular spironolactone did not affect CSF [Na+] but blocked the Na+-rich aCSF-induced increases in blood pressure and heart rate and OLC content. Intracerebroventricular Na+-rich aCSF increased ACE and AT1-receptor-binding densities in several brain nuclei, and Fab fragments blocked these increases. These data indicate that in Wistar rats, a chronic increase in CSF [Na+] may increase hypothalamic aldosterone and activate CNS pathways involving MR, and OLC, leading to increases in AT1-receptor and ACE densities in brain areas involved in cardiovascular regulation and hypertension.

Brain ouabain stimulates peripheral marinobufagenin via angiotensin II signalling in NaCl-loaded Dahl-S rats

OBJECTIVE

In NaCl-loaded Dahl salt-sensitive (DS) rats the transient stimulation of brain endogenous ouabain (EO) precedes the increase in renal excretion of marinobufagenin (MBG), a vasoconstrictor and natriuretic. In hypertensive DS rats, EO raises blood pressure (BP) via an ATII-sensitive pathway. We hypothesized that an NaCl-induced increase in MBG is linked to the EO-stimulated ATII pathway.

METHODS

We studied the effects of 3 h of NaCl loading (17 mmol/kg, intraperitoneally) in male DS rats treated with antibodies to MBG or ouabain, or with losartan (25 mg/kg).

RESULTS

NaCl loading alone induced a transient stimulation of pituitary EO (22.4 +/- 1.8 versus 12.2 +/- 1.3 pmol/g) and ATII (39.4 +/- 2.8 versus 18.4 +/- 3.2 ng/g), a sustained increase in MBG excretion (5.2 +/- 0.6 versus 1.1 +/- 0.2 pmol/h), a 40% inhibition of the renal sodium pump, a natriuretic response, a 35 mmHg increase in systolic BP, and an increase in adrenocortical ATII and MBG levels and in plasma norepinephrine. The anti-MBG antibody reduced the natriuresis (36%) and BP (40 mmHg), and restored renal sodium pump activity. The anti-ouabain antibody prevented the increase in pituitary ATII, reduced MBG excretion, natriuresis and BP, increased sodium pump activity, and prevented increases in plasma norepinephrine, pituitary and adrenocortical ATII, and adrenocortical MBG. Losartan mimicked the effects of the anti-ouabain antibody, but did not affect the excretion of EO. In adrenocortical cells of DS rats, ATII stimulated MBG secretion, and losartan blocked this effect.

CONCLUSIONS

In response to NaCl loading, brain EO, via an AT1 receptor pathway and probably via sympathetic activation, stimulates adrenocortical MBG, which inhibits the renal sodium pump and elevates BP.

Distribution of epithelial sodium channels and mineralocorticoid receptors in cardiovascular regulatory centers in rat brain

Epithelial sodium channels (ENaC) are important for regulating sodium transport across epithelia. Functional studies indicate that neural mechanisms acting through mineralocorticoid receptors (MR) and sodium channels (presumably ENaC) are crucial to the development of sympathoexcitation and hypertension in experimental models of salt-sensitive hypertension. However, expression and localization of the ENaC in cardiovascular regulatory centers of the brain have not yet been studied. RT-PCR and immunohistochemistry were performed to study ENaC and MR expression at the mRNA and protein levels, respectively. Both mRNA and protein for alpha-, beta-, and gamma-ENaC subunits and MR were found to be expressed in the rat brain. All three ENaC subunits and MR were present in the supraoptic nucleus, magnocellular paraventricular nucleus, hippocampus, choroid plexus, ependyma, and brain blood vessels, suggesting the presence of multimeric channels and possible regulation by mineralocorticoids. In most cortical areas, thalamus, amygdala, and suprachiasmatic nucleus, notable expression of gamma-ENaC was undetectable, whereas alpha- and beta-ENaC were abundantly expressed pointing to the possibility of a heterogeneous population of channels. The findings suggest that stoichiometrically different populations of ENaC may be present in both epithelial and neural components in the brain, which may contribute to regulation of cerebrospinal fluid and interstitial Na+ concentration as well as neuronal excitation.

Pressor response to CSF sodium in mice: mediation by a ouabain-like substance and renin-angiotensin system in the brain

Intracerebroventricular (i.c.v.) infusion of sodium in rats increases cerebrospinal fluid (CSF) [Na], mimicking the effects of a high salt diet in salt-sensitive strains and causing sympathetic hyperactivity and a pressor response that are mediated via both an endogenous brain ouabainlike substance (OLS) and the brain renin-angiotensin system (RAS). However, the concept that CSF sodium activates both the brain OLS and brain RAS to increase blood pressure has not been tested in any other species besides the rat. In the current study, it was established that continuous i.c.v. infusion of NaCl causes sustained increases in blood pressure and heart rate in both outbred (Swiss Webster, SW) and inbred (C57Bl/6) mouse strains. Subsequently, the mechanisms of the pressor effects were explored. In both SW and C57Bl/6, the i.c.v. administration of Fab fragments of an antibody with high affinity for ouabain and the OLS (Fab) abolished the pressor and tachycardic responses to i.c.v. sodium, as did the angiotensin II AT1 receptor antagonist losartan given i.c.v. In contrast, doses of NaCl, Fab and losartan that were effective i.c.v. were ineffective when given i.v. I.c.v. ouabain also caused the pressor and tachycardic responses, which were abolished by losartan (i.c.v.). In the reciprocal study, i.c.v. Fab had no effect on similar responses to i.c.v. angiotensin II. These studies demonstrate that the sustained blood pressure and heart rate responses caused by increases in CSF [Na] are mediated via both a brain OLS and the brain RAS. The RAS activation occurs downstream of the OLS effect.

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.