Central and peripheral slow-pressor mechanisms contributing to Angiotensin II-salt hypertension in rats

Sensational site: the sodium pump ouabain-binding site and its ligands

Cardiotonic steroids (CTS), used by certain insects, toads, and rats for protection from predators, became, thanks to Withering's trailblazing 1785 monograph, the mainstay of heart failure (HF) therapy. In the 1950s and 1960s, we learned that the CTS receptor was part of the sodium pump (NKA) and that the Na+/Ca2+ exchanger was critical for the acute cardiotonic effect of digoxin- and ouabain-related CTS. This "settled" view was upended by seven revolutionary observations. First, subnanomolar ouabain sometimes stimulates NKA while higher concentrations are invariably inhibitory. Second, endogenous ouabain (EO) was discovered in the human circulation. Third, in the DIG clinical trial, digoxin only marginally improved outcomes in patients with HF. Fourth, cloning of NKA in 1985 revealed multiple NKA α and β subunit isoforms that, in the rodent, differ in their sensitivities to CTS. Fifth, the NKA is a cation pump and a hormone receptor/signal transducer. EO binding to NKA activates, in a ligand- and cell-specific manner, several protein kinase and Ca2+-dependent signaling cascades that have widespread physiological effects and can contribute to hypertension and HF pathogenesis. Sixth, all CTS are not equivalent, e.g., ouabain induces hypertension in rodents while digoxin is antihypertensinogenic ("biased signaling"). Seventh, most common rodent hypertension models require a highly ouabain-sensitive α2 NKA and the elevated blood pressure is alleviated by EO immunoneutralization. These numerous phenomena are enabled by NKA's intricate structure. We have just begun to understand the endocrine role of the endogenous ligands and the broad impact of the ouabain-binding site on physiology and pathophysiology.

Salt Taste and Salt Sensitive Hypertension in HIV

PURPOSE OF REVIEW

To provide a summary of current literature and propose potential mechanistic models to help us understand the role of HIV infection/antiretroviral therapy (ART), salt taste sensitivity (STS), and salt sensitivity of blood pressure (SSBP) in hypertension development.

RECENT FINDINGS

The epithelial sodium channel (ENaC) is the main protein/sodium channel for recognizing Na + in the tongue and mediates preference to low-medium salt concentrations in animals and humans. Considering the pressor response to oral salt in individuals with SSBP, poor STS may worsen blood pressure. Specific genetic variants in ENaC are linked to salt taste perception and hypertension. HIV infection, some ART, and specific antihypertensive drugs are associated with reduced STS and an increased liking for salty foods. Persons with HIV (PWH) on ART may have a decreased STS and are at a higher risk of developing salt-sensitive hypertension. Inflammation mediated by dietary salt is one of the drivers of poor STS and salt-sensitive hypertension among PWH.

Effects of renal denervation on endogenous ouabain in spontaneously hypertensive rats

PURPOSE

To investigate the role of renal denervation (RDN) on endogenous ouabain (EO) secretion in spontaneously hypertensive rats (SHR).

METHODS

Sixteen 12-week-old male SHR were randomly separated into the renal denervation group (RDNX group) and sham operation group (sham group), and eight age-matched Wistar Kyoto rats (WKY) were served as control group. EO concentrations, the Na+- K+-ATPaseactivity, and the expression of Na+-K+-ATPase were assessed.

RESULTS

EO levels in serum, kidneys and hypothalamus of sham group were higher than in RDNX group (p < 0.05). Renal Na+-K+-ATPase activity subjected to denervation surgery showed significantly reduction when compared with the sham groups (p < 0.05). A positive correlation existed between norepinephrine (NE) content and Na+-K+-ATPase activity in the kidney (r2 = 0.579). Renal Na+-K+-ATPase α1 subunit mRNA expression was down-regulated in the RDNX group compared with the sham group (P < 0.05), while renal Na+-K+-ATPase α1 subunit mRNA expression was no statistical significance between the groups (P = 0.63). Immunohistochemical analysis showed that there were significant differences in the renal expression of Na+-K+-ATPasebetween the three groups (P < 0.05).

CONCLUSIONS

These experiments demonstrate that RDN exerted an anti-hypertensive effect with reduction of EO levels and Na+-K+-ATPase activity and Na+-K+-ATPase α1 subunit expression of kidney in SHR.

Role of blood-borne factors in sympathoexcitation-mediated hypertension: Potential neurally mediated hypertension in preeclampsia

Hypertension remains a threat for society due to its unknown causes, preventing proper management, for the growing number of patients, for its state as a high-risk factor for stroke, cardiac and renal complication and as cause of disability. Data from clinical and animal researches have suggested the important role of many soluble factors in the pathophysiology of hypertension through their neuro-stimulating effects. Central targets of these factors are of molecular, cellular and structural nature. Preeclampsia (PE) is characterized by high level of soluble factors with strong pro-hypertensive activity and includes immune factors such as proinflammatory cytokines (PICs). The potential neural effect of those factors in PE is still poorly understood. Shedding light into the potential central effect of the soluble factors in PE may advance our current comprehension of the pathophysiology of hypertension in PE, which will contribute to better management of the disease. In this paper, we summarized existing data in respect of hypothesis of this review, that is, the existence of the neural component in the pathophysiology of the hypertension in PE. Future studies would address this hypothesis to broaden our understanding of the pathophysiology of hypertension in PE.

Rare Variants in Genes Encoding Subunits of the Epithelial Na+ Channel Are Associated With Blood Pressure and Kidney Function

BACKGROUND

The epithelial Na+ channel (ENaC) is intrinsically linked to fluid volume homeostasis and blood pressure. Specific rare mutations in SCNN1A, SCNN1B, and SCNN1G, genes encoding the α, β, and γ subunits of ENaC, respectively, are associated with extreme blood pressure phenotypes. No associations between blood pressure and SCNN1D, which encodes the δ subunit of ENaC, have been reported. A small number of sequence variants in ENaC subunits have been reported to affect functional transport in vitro or blood pressure. The effects of the vast majority of rare and low-frequency ENaC variants on blood pressure are not known.

METHODS

We explored the association of low frequency and rare variants in the genes encoding ENaC subunits, with systolic blood pressure, diastolic blood pressure, mean arterial pressure, and pulse pressure. Using whole-genome sequencing data from 14 studies participating in the Trans-Omics in Precision Medicine Whole-Genome Sequencing Program, and sequence kernel association tests.

RESULTS

We found that variants in SCNN1A and SCNN1B were associated with diastolic blood pressure and mean arterial pressure (P<0.00625). Although SCNN1D is poorly expressed in human kidney tissue, SCNN1D variants were associated with systolic blood pressure, diastolic blood pressure, mean arterial pressure, and pulse pressure (P<0.00625). ENaC variants in 2 of the 4 subunits (SCNN1B and SCNN1D) were also associated with estimated glomerular filtration rate (P<0.00625), but not with stroke.

CONCLUSIONS

Our results suggest that variants in extrarenal ENaCs, in addition to ENaCs expressed in kidneys, influence blood pressure and kidney function.

Inverse salt sensitivity: an independent risk factor for cardiovascular damage in essential hypertension

OBJECTIVE

Salt sensitivity is a powerful risk factor for cardiovascular (CV) disease and mortality in both normotensive and hypertensive patients. We investigated the predictive value of the salt sensitivity phenotype in the development of CV events and hypertensive target organ damage (TOD) among essential hypertensive patients.

METHODS

Eight hundred forty-four naive hypertensive patients were recruited and underwent an acute saline test during which blood pressure (BP) displayed either no substantial variation (salt-resistant, SR individuals), an increase (salt-sensitive, SS), or a paradoxical decrease (inverse salt-sensitive, ISS). Sixty-one patients with the longest monitored follow-up (median 16 years) for blood pressure and organ damage were selected for the present study. A clinical score for TOD development based on the severity and the age of onset was set up by considering hypertensive heart disease, cerebrovascular damage, microalbuminuria, and vascular events.

RESULTS

CV events were significantly higher among SS and ISS than in SR patients. The relative risk of developing CV events was 12.67 times higher in SS than SR and 5.94 times higher in ISS than SR patients. The development of moderate to severe TOD was 10-fold higher in SS and over 15-fold higher in ISS than in SR patients. Among the three phenotypes, changes in plasma endogenous ouabain were linked with the blood pressure effects of saline.

CONCLUSIONS

Salt sensitivity and inverse salt sensitivity appear to be equivalent risk factors for CV events. The response to an acute saline test is predictive of CV damage for newly identified ISS individuals.

Sleep deprivation is associated with increased circulating levels of endogenous ouabain: Potential role in bipolar disorder

Endogenously produced cardiac glycosides, like endogenous ouabain (EO), are putative hormones that have been implicated in the pathophysiology of bipolar disorder. Individuals with bipolar disorder appear to be unable to sufficiently upregulate production of EO in situations of increased need. This study was performed to determine the effect of sleep deprivation on the circulating levels of EO. Plasma EO concentrations were measured by ouabain-radioimmunoassay in heterozygote Na,K-ATPase a2 knockout (KO) mice, which have been used as an animal model of mania, and wildtype siblings at baseline and after sleep fragmentation utilizing the moving bar method. a2 KO animals had elevated endogenous ouabain concentrations compared to wild type controls (0.82 ± SD 0.22 nM vs 0.26 ± 0.02, P = 0.03). Sleep fragmentation increased ouabain concentrations in wild type mice (0.53 ± 0.08 nM sleep fragmentation vs 0.26 ± 0.02 nM baseline, P = 0.04), but not in a2 KO mice (0.60 ± 0.07 nM sleep fragmentation vs 0.82 ± 0.22 nM baseline, P > 0.05). These studies demonstrate that sleep disturbance can increase EO in control mice but animals that exhibit some manic behaviors are unable to increase EO production.

No evidence for brain renin-angiotensin system activation during DOCA-salt hypertension

Brain renin-angiotensin system (RAS) activation is thought to mediate deoxycorticosterone acetate (DOCA)-salt hypertension, an animal model for human primary hyperaldosteronism. Here, we determined whether brainstem angiotensin II is generated from locally synthesized angiotensinogen and mediates DOCA-salt hypertension. To this end, chronic DOCA-salt-hypertensive rats were treated with liver-directed siRNA targeted to angiotensinogen, the angiotensin II type 1 receptor antagonist valsartan, or the mineralocorticoid receptor antagonist spironolactone (n = 6-8/group). We quantified circulating angiotensinogen and renin by enzyme-kinetic assay, tissue angiotensinogen by Western blotting, and angiotensin metabolites by LC-MS/MS. In rats without DOCA-salt, circulating angiotensin II was detected in all rats, whereas brainstem angiotensin II was detected in 5 out of 7 rats. DOCA-salt increased mean arterial pressure by 19 ± 1 mmHg and suppressed circulating renin and angiotensin II by >90%, while brainstem angiotensin II became undetectable in 5 out of 7 rats (<6 fmol/g). Gene silencing of liver angiotensinogen using siRNA lowered circulating angiotensinogen by 97 ± 0.3%, and made brainstem angiotensin II undetectable in all rats (P<0.05 vs. non-DOCA-salt), although brainstem angiotensinogen remained intact. As expected for this model, neither siRNA nor valsartan attenuated the hypertensive response to DOCA-salt, whereas spironolactone normalized blood pressure and restored brain angiotensin II together with circulating renin and angiotensin II. In conclusion, despite local synthesis of angiotensinogen in the brain, brain angiotensin II depended on circulating angiotensinogen. That DOCA-salt suppressed circulating and brain angiotensin II in parallel, while spironolactone simultaneously increased brain angiotensin II and lowered blood pressure, indicates that DOCA-salt hypertension is not mediated by brain RAS activation.

NO-induced vasodilation correlates directly with BP in smooth muscle-Na/Ca exchanger-1-engineered mice: elevated BP does not attenuate endothelial function

Arterial smooth muscle Na+/Ca2+ exchanger-1 (SM-NCX1) promotes vasoconstriction or vasodilation by mediating, respectively, Ca2+ influx or efflux. In vivo, SM-NCX1 mediates net Ca2+ influx to help maintain myogenic tone (MT) and neuronally activated constriction. SM-NCX1-TG (overexpressing transgenic) mice have increased MT and mean blood pressure (MBP; +13.5 mmHg); SM-NCX1-KO (knockout) mice have reduced MT and MBP (-11.1 mmHg). Endothelium-dependent vasodilation (EDV) is often impaired in hypertension. We tested whether genetically engineered SM-NCX1 expression and consequent BP changes similarly alter EDV. Isolated, pressurized mesenteric resistance arteries with MT from SM-NCX1-TG and conditional SM-NCX1-KO mice, and femoral arteries in vivo from TG mice were studied. Acetylcholine (ACh)-dilated TG arteries with MT slightly more than control or KO arteries, implying that SM-NCX1 overexpression does not impair EDV. In preconstricted KO, but not TG mouse arteries, however, ACh- and bradykinin-triggered vasodilation was markedly attenuated. To circumvent the endothelium, phenylephrine-constricted resistance arteries were tested with Na-nitroprusside [SNP; nitric oxide (NO) donor] and cGMP. This endothelium-independent vasodilation was augmented in TG but attenuated in KO arteries that lack NCX1-mediated Ca2+ clearance. Baseline cytosolic Ca2+ ([Ca2+]cyt) was elevated in TG femoral arteries in vivo, supporting the high BP; furthermore, SNP-triggered [Ca2+]cyt decline and vasodilation were augmented as NO and cGMP promote myocyte polarization thereby enhancing NCX1-mediated Ca2+ efflux. The TG mouse data indicate that BP elevation does not attenuate endothelium-dependent vasodilation. Thus, in essential hypertension and many models the endothelial impairment that supports the hypertension apparently is not triggered by BP elevation but by extravascular mechanisms.NEW & NOTEWORTHY Endothelium-dependent, ACh-induced vasodilation (EDV) is attenuated, and arterial myocyte Na+/Ca2+ exchangers (NCX1) are upregulated in many forms of hypertension. Surprisingly, mildly hypertensive smooth muscle-specific (SM)-NCX1 transgenic mice exhibited modestly enhanced EDV and augmented endothelium-independent vasodilation (EIV). Conversely, mildly hypotensive SM-NCX1-knockout mice had greatly attenuated EIV. These adaptations help compensate for NCX1 expression-induced alterations in cytosolic Ca2+ and blood pressure (BP) and belie the view that elevated BP, itself, causes the endothelial dysregulation in hypertension.

Sympathetic Activation in Hypertension: Importance of the Central Nervous System

The sympathetic nervous system plays a critical role in the pathogenesis of hypertension. The central nervous system (CNS) organizes the sympathetic outflow and various inputs from the periphery. The brain renin-angiotensin system has been studied in various regions involved in controlling sympathetic outflow. Recent progress in cardiovascular research, particularly in vascular biology and neuroscience, as well as in traditional physiological approaches, has advanced the field of the neural control of hypertension in which the CNS plays a vital role. Cardiovascular research relating to hypertension has focused on the roles of nitric oxide, oxidative stress, inflammation, and immunity, and the network among various organs, including the heart, kidney, spleen, gut, and vasculature. The CNS mechanisms are similarly networked with these factors and are widely studied in neuroscience. In this review, I describe the development of the conceptual flow of this network in the field of hypertension on the basis of several important original research articles and discuss potential future breakthroughs leading to clinical precision medicine.

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.

Ouabain, endogenous ouabain and ouabain-like factors: The Na+ pump/ouabain receptor, its linkage to NCX, and its myriad functions

In this brief review we discuss some aspects of the Na⁺ pump and its roles in mediating the effects of ouabain and endogenous ouabain (EO): i) in regulating the cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) via Na/Ca exchange (NCX), and ii) in activating a number of protein kinase (PK) signaling cascades that control a myriad of cell functions. Importantly, [Ca²⁺]_CYT and the other signaling pathways intersect at numerous points because of the influence of Ca²⁺ and calmodulin in modulating some steps in those other pathways. While both mechanisms operate in virtually all cells and tissues, this article focuses primarily on their functions in the cardiovascular system, the central nervous system (CNS) and the kidneys.

Mineralocorticoid Receptor-Dependent Impairment of Baroreflex Contributes to Hypertension in a Mouse Model of Primary Aldosteronism

Primary aldosteronism (PA) is the most common cause of secondary hypertension. The paucity of good animal models hinders our understanding of the pathophysiology of PA and the hypertensive mechanism of PA remains incompletely known. It was recently reported that genetic deletion of TWIK-related acid-sensitive potassium-1 and potassium-3 channels from mice (TASK^−/−) generates aldosterone excess and mild hypertension. We addressed the hypertensive mechanism by assessing autonomic regulation of cardiovascular activity in this TASK^−/− mouse line that exhibits the hallmarks of PA. Here, we demonstrate that TASK^−/− mice were hypertensive with 24-h ambulatory arterial pressure. Either systemic or central blockade of the mineralocorticoid receptor (MR) markedly reduced elevated arterial pressure to normal level in TASK^−/− mice. The response of heart rate to the muscarinic cholinergic receptor blocker atropine was similar between TASK^−/− and wild-type mice. However, the responses of heart rate to the β-adrenergic receptor blocker propranolol and of arterial pressure to the ganglion blocker hexamethonium were enhanced in TASK^−/− mice relative to the counterparts. Moreover, the bradycardiac rather than tachycardiac gain of the arterial baroreflex was significantly attenuated and blockade of MRs to a large degree rescued the dysautonomia and baroreflex gain in TASK^−/− mice. Overall, the present study suggests that the MR-dependent dysautonomia and reduced baroreflex gain contribute to the development of hyperaldosteronism-related hypertension.

Novel molecular mechanisms in the inhibition of adrenal aldosterone synthesis: Action of tolvaptan via vasopressin V2 receptor-independent pathway

BACKGROUND AND PURPOSE

We investigated the inhibitory effect and associated molecular mechanisms of tolvaptan on angiotensin II (AngII)-induced aldosterone production in vitro and in vivo.

EXPERIMENTAL APPROACH

In vitro, H295R human adrenocarcinoma cells were incubated with 1 μmol·L^-1 arginine vasopressin (AVP) or dDAVP, or tolvaptan (0.1, 1, and 3 μmol·L^-1 ) in the presence and absence of 100 nmol·L^-1 of AngII. In vivo, Sprague-Dawley rats were treated with tolvaptan 0.05% in the diet for 6 days in the presence and absence of 200 pmol·min^-1 AngII.

KEY RESULTS

Tolvaptan suppressed AngII-induced aldosterone production in a dose-dependent manner in H295R cells, whereas neither AVP nor dDAVP in the presence or absence of AngII altered aldosterone production, suggesting the vasopressin V₂ receptor was not involved in the inhibitory effect of tolvaptan on aldosterone synthesis. In addition, tolvaptan inhibited the AngII-induced increase in aldosterone synthase (CYP11B2) protein levels without suppressing CYP11B2 mRNA expression. Notably, tolvaptan increased the levels of unfolded protein response (UPR) marker DDIT3 and eIF2α phosphorylation (a UPR-induced event), which could block the translation of CYP11B2 mRNA into protein and thereby inhibit aldosterone production. In vivo, tolvaptan significantly inhibited AngII-induced increases in serum and adrenal aldosterone levels and CYP11B2 protein levels. This anti-aldosterone effect was associated with a reduction in the elevated systolic and diastolic BP.

CONCLUSIONS AND IMPLICATIONS

Tolvaptan inhibited AngII-stimulated aldosterone production via a V₂ receptor-independent pathway, which can counteract or even surpass its potential activating effect of diuresis-induced aldosterone secretion in certain aldosterone-mediated pathological conditions.

Revisiting the Brain Renin-Angiotensin System-Focus on Novel Therapies

Purpose of Review

Although an independent brain renin-angiotensin system is often assumed to exist, evidence for this concept is weak. Most importantly, renin is lacking in the brain, and both brain angiotensinogen and angiotensin (Ang) II levels are exceptionally low. In fact, brain Ang II levels may well represent uptake of circulating Ang II via Ang II type 1 (AT₁) receptors.

Recent Findings

Nevertheless, novel drugs are now aimed at the brain RAS, i.e., aminopeptidase A inhibitors should block Ang III formation from Ang II, and hence diminish AT₁ receptor stimulation by Ang III, while AT₂ and Mas receptor agonists are reported to induce neuroprotection after stroke. The endogenous agonists of these receptors and their origin remain unknown.

Summary

This review addresses the questions whether independent angiotensin generation truly occurs in the brain, what its relationship with the kidney is, and how centrally acting RAS blockers/agonists might work.

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.