Mild DOCA-salt hypertension: sympathetic system and role of renal nerves

Renal denervation for hypertensive heart disease and atrial fibrillation

Accumulating evidence supports the efficacy of renal denervation (RDN) as an antihypertensive treatment. Additionally, several RDN clinical studies, including meta-analyses, have suggested that RDN may potentially have beneficial effects on left ventricular hypertrophy, diastolic function, and new-onset/recurrence of atrial fibrillation (AF), although most studies were not randomized sham-controlled. In particular, the effects of RDN on left ventricular hypertrophy and AF recurrence appear to be relatively evident. Sympathetic activation plays a critical role in the development of hypertension, hypertensive heart disease, and AF. Notably, several studies suggest the cardioprotective effects of RDN even in the absence of significant blood pressure reduction, probably due to its sympathoinhibitory effects. It is imperative to establish the efficacy of RDN in patients with hypertensive heart disease and/or AF, focusing on parameters of sympathetic activity in the clinical setting, including randomized sham-controlled trials. Moreover, further basic research is essential to elucidate the therapeutic mechanisms of RDN beyond blood pressure lowering and the renal nerves-linked pathophysiologies of hypertensive heart disease and AF. This review outlines the effects of renal denervation on hypertensive heart disease, particularly on left ventricular hypertrophy and diastolic function, and on atrial fibrillation. The sympathoinhibitory effect of renal denervation, an important potential mechanism of its beneficial effects on heart disease, is also discussed.

Physiological ischemic training improves cardiac function through the attenuation of cardiomyocyte apoptosis and the activation of the vagus nerve in chronic heart failure

Purpose

This study investigated the functional outcomes of patients with chronic heart failure (CHF) after physiological ischemic training (PIT), identified the optimal PIT protocol, evaluated its cardioprotective effects and explored the underlying neural mechanisms.

Methods

Patients with CHF were randomly divided into experimental group (n = 25, PIT intervention + regular treatment) and control group (n = 25, regular treatment). The outcomes included the left ventricular ejection fraction (LVEF), brain natriuretic peptide (BNP) and cardiopulmonary parameters. LVEF and cardiac biomarkers in CHF rats after various PIT treatments (different in intensity, frequency, and course of treatment) were measured to identify the optimal PIT protocol. The effect of PIT on cardiomyocyte programmed cell death was investigated by western blot, flow cytometry and fluorescent staining. The neural mechanism involved in PIT-induced cardioprotective effect was assessed by stimulation of the vagus nerve and muscarinic M2 receptor in CHF rats.

Results

LVEF and VO2max increased while BNP decreased in patients subjected to PIT. The optimal PIT protocol in CHF rats was composed of five cycles of 5 min ischemia followed by 5 min reperfusion on remote limbs for 8 weeks. LVEF and cardiac biomarker levels were significantly improved, and cardiomyocyte apoptosis was inhibited. However, these cardioprotective effects disappeared after subjecting CHF rats to vagotomy or muscarinic M2 receptor inhibition.

Conclusion

PIT improved functional outcomes in CHF patients. The optimal PIT protocol required appropriate intensity, reasonable frequency, and adequate treatment course. Under these conditions, improvement of cardiac function in CHF was confirmed through cardiomyocyte apoptosis reduction and vagus nerve activation.

Fibroblast Growth Factor 23 Exacerbates Cardiac Fibrosis in Deoxycorticosterone Acetate-Salt Mice With Hypertension

Fibroblast growth factor 23 (FGF23) is associated with cardiovascular disease in patients with chronic kidney disease; however, the mechanisms underlying the effect of FGF23 on cardiac function remain to be investigated. Herein, we studied the effect of continuous intravenous (CIV) FGF23 loading in a deoxycorticosterone acetate (DOCA)-salt mouse model with mild chronic kidney disease and hypertension as well as heart failure with a preserved ejection fraction. Wild-type male mice were randomly allocated to 4 groups: normal control, vehicle-treated DOCA-salt mice, FGF23-treated DOCA-salt mice, and FGF23- and calcitriol-treated DOCA-salt mice. The DOCA-salt mice received the agents via the CIV route for 10 days using an infusion minipump. DOCA-salt mice that received FGF23 showed a marked increase in the serum FGF23 level, and echocardiography in these mice revealed heart failure with a preserved ejection fraction. These mice also showed exacerbation of myocardial fibrosis, concomitant with an inverse and significant correlation with Cyp27b1 expression. Calcitriol treatment attenuated FGF23-induced cardiac fibrosis and improved diastolic function via inhibition of transforming growth factor-β signaling. This effect was independent of the systemic and local levels of FGF23. These results suggest that CIV FGF23 loading exacerbates cardiac fibrosis and that locally abnormal vitamin D metabolism is involved in this mechanism. Calcitriol attenuates this exacerbation by mediating transforming growth factor-β signaling independently of the FGF23 levels.

Zyxin protects from hypertension-induced cardiac dysfunction

Arterial hypertension causes left ventricular hypertrophy leading to dilated cardiomyopathy. Following compensatory cardiomyocyte hypertrophy, cardiac dysfunction develops due to loss of cardiomyocytes preceded or paralleled by cardiac fibrosis. Zyxin acts as a mechanotransducer in vascular cells that may promote cardiomyocyte survival. Here, we analyzed cardiac function during experimental hypertension in zyxin knockout (KO) mice. In zyxin KO mice, made hypertensive by way of deoxycorticosterone acetate (DOCA)-salt treatment telemetry recording showed an attenuated rise in systolic blood pressure. Echocardiography indicated a systolic dysfunction, and isolated working heart measurements showed a decrease in systolic elastance. Hearts from hypertensive zyxin KO mice revealed increased apoptosis, fibrosis and an upregulation of active focal adhesion kinase as well as of integrins α5 and β1. Both interstitial and perivascular fibrosis were even more pronounced in zyxin KO mice exposed to angiotensin II instead of DOCA-salt. Stretched microvascular endothelial cells may release collagen 1α2 and TGF-β, which is characteristic for the transition to an intermediate mesenchymal phenotype, and thus spur the transformation of cardiac fibroblasts to myofibroblasts resulting in excessive scar tissue formation in the heart of hypertensive zyxin KO mice. While zyxin KO mice per se do not reveal a cardiac phenotype, this is unmasked upon induction of hypertension and owing to enhanced cardiomyocyte apoptosis and excessive fibrosis causes cardiac dysfunction. Zyxin may thus be important for the maintenance of cardiac function in spite of hypertension.

Left ventricular geometry, tissue composition, and residual stress in High Fat Diet Dahl-Salt sensitive rats

BACKGROUND

Hypertension drives myocardial remodeling, leading to changes in structure, composition and mechanical behavior, including residual stress, which are linked to heart disease progression in a gender-specific manner. Emerging therapies are also targeting constituent-specific pathological features. All previous studies, however, have characterized remodeling in the intact tissue, rather than isolated tissue constituents, and did not include sex as a biological variable.

OBJECTIVE

In this study we first identified the contribution of collagen fiber network and myocytes to the myocardial residual stress/strain in Dahl-Salt sensitive rats fed with high fat diet. Then, we quantified the effect of hypertension on the remodeling of the left ventricle (LV), as well as the existence of sex-specific remodeling features.

METHODS

We performed mechanical tests (opening angle, ring-test) and histological analysis on isolated constituents and intact tissue of the LV. Based on the measurements from the tests, we performed a stress analysis to evaluate the residual stress distribution. Statistical analysis was performed to identify the effects of constituent isolation, elevated blood pressure, and sex of the animal on the output of both experimental measures and modeling results.

RESULTS

Hypertension leads to reduced residual stress/strain intact tissue, isolated collagen fibers, and isolated myocytes in male and female rats. Collagen remains the largest contributor to myocardial residual stress in both normotensive and hypertensive animals. We identified sex-differences in both hypertensive and normotensive animals.

CONCLUSIONS

We observed both constituent- and sex-specific remodeling features in the LV of an animal model of hypertension.

Excessive dietary salt promotes aortic stiffness in murine renovascular hypertension

Renovascular hypertension is characterized by activation of the renin-angiotensin-aldosterone system, blunted natriuretic responses, and elevated sympathetic nerve activity. Excess dietary salt intake exaggerates arterial blood pressure (ABP) in multiple models of experimental hypertension. The present study tested whether a high-salt diet exaggerated ABP and vascular dysfunction in a 2-kidney, 1-clip (2K1C) murine model. Male C57BL/6J mice (8-12 wk) were randomly assigned, and fed a 0.1% or 4.0% NaCl diet, and instrumented with telemetry units to measure ABP. Then, the 2K1C model was produced by placing a cuff around the right renal artery. Systolic, diastolic, and mean ABP were significantly higher in mice fed 4.0% vs. 0.1% NaCl at 1 wk but not after 3 wk. Interestingly, 2K1C hypertension progressively increased arterial pulse pressure in both groups; however, the magnitude was significantly greater in mice fed 4.0% vs. 0.1% NaCl at 3 wk. Moreover, pulse wave velocity was significantly greater in 2K1C mice fed 4.0% vs. 0.1% NaCl diet or sham-operated mice fed either diet. Histological assessment of aortas indicated no structural differences among groups. Finally, endothelium-dependent vasodilation was significantly and selectively attenuated in the aorta but not mesenteric arteries of 2K1C mice fed 4.0% NaCl vs. 0.1% NaCl or sham-operated control mice. The findings suggest that dietary salt loading transiently exaggerates 2K1C renovascular hypertension but promotes chronic aortic stiffness and selective aortic vascular dysfunction.NEW & NOTEWORTHY High dietary salt exaggerates hypertension in multiple experimental models. Here we demonstrate that a high-salt diet produces a greater increase in arterial blood pressure at 1 wk after induction of 2-kidney, 1-clip (2K1C) hypertension but not at 3 wk. Interestingly, 2K1C mice fed a high-salt diet displayed an exaggerated pulse pressure, elevated pulse wave velocity, and reduced endothelium-dependent vasodilation of the aorta but not mesenteric arteries. These findings suggest that dietary salt may interact with underlying cardiovascular disease to promote selective vascular dysfunction and aortic stiffness.

Adipose Mesenchymal Cells-Derived EVs Alleviate DOCA-Salt-Induced Hypertension by Promoting Cardio-Renal Protection

Hypertension is a long-term condition that can increase organ susceptibility to insults and lead to severe complications such as chronic kidney disease (CKD). Extracellular vesicles (EVs) are cell-derived membrane structures that participate in cell-cell communication by exporting encapsulated molecules to target cells, regulating physiological and pathological processes. We here demonstrate that multiple administration of EVs from adipose-derived mesenchymal stromal cells (ASC-EVs) in deoxycorticosterone acetate (DOCA)-salt hypertensive model can protect renal tissue by maintaining its filtration capacity. Indeed, ASC-EVs downregulated the pro-inflammatory molecules monocyte chemoattracting protein-1 (MCP-1) and plasminogen activating inhibitor-1 (PAI1) and reduced recruitment of macrophages in the kidney. Moreover, ASC-EVs prevented cardiac tissue fibrosis and maintained blood pressure within normal levels, thus demonstrating their multiple favorable effects in different organs. By applying microRNA (miRNA) microarray profile of the kidney of DOCA-salt rats, we identified a selective miRNA signature associated with epithelial-mesenchymal transition (EMT). One of the key pathways found was the axis miR-200-TGF-β, that was significantly altered by EV administration, thereby affecting the EMT signaling and preventing renal inflammatory response and fibrosis development. Our results indicate that EVs can be a potent therapeutic tool for the treatment of hypertension-induced CKD in cardio-renal syndrome.

IGF-1 deletion affects renal sympathetic nerve activity, left ventricular dysfunction, and renal function in DOCA-salt hypertensive mice

To determine the influence of IGF-1 deletion on renal sympathetic nerve activity (RSNA), left ventricular dysfunction, and renal function in deoxycorticosterone acetate (DOCA)-salt hypertensive mice. The DOCA-salt hypertensive mice models were constructed and the experiment was classified into WT (Wild-type mice) +sham, LID (Liver-specific IGF-1 deficient mice) + sham, WT + DOCA, and LID+ DOCA groups. Enzyme-linked immunosorbent assay (ELISA) was used to detect the serum IGF-1 levels in mice. The plasma norepinephrine (NE), urine protein, urea nitrogen and creatinine, as well as RSNA were measured. Echocardiography was performed to assess left ventricular dysfunction, and HE staining to observe the pathological changes in renal tissue of mice. DOCA-salt induction time-dependently increased the systolic blood pressure (SBP) of mice, especially in DOCA-salt LID mice. Besides, the serum IGF-1 levels in WT mice were decreased after DOCA-salt induction. In addition, the plasma NE concentration and NE spillover, urinary protein, urea nitrogen, creatinine and RSNA were remarkably elevated with severe left ventricular dysfunction, but the creatinine clearance was reduced in DOCA-salt mice, and these similar changes were obvious in DOCA-salt mice with IGF-1 deletion. Moreover, the DOCA-salt mice had tubular ectasia, glomerular fibrosis, interstitial cell infiltration, and increased arterial wall thickness, and the DOCA-salt LID mice were more serious in those aspects. Deletion of IGF-1 may lead to enhanced RSNA in DOCA-salt hypertensive mice, thereby further aggravating left ventricular dysfunction and renal damage.

Renal nerves contribute to hypertension in Schlager BPH/2J mice

Schlager mice (BPH/2J) are hypertensive due to a greater contribution of the sympathetic nervous system (SNS) and renin-angiotensin system (RAS). The kidneys of BPH/2J are hyper-innervated suggesting renal nerves may contribute to the hypertension. We therefore determined the effect of bilateral renal denervation (RD) on hypertension in BPH/2J. Mean arterial pressure (MAP) was measured by radiotelemetry before and for 3 weeks after RD in BPH/2J and BPN/3J. The effects of pentolinium and enalaprilat were examined to determine the contribution of the SNS and RAS, respectively. After 3 weeks, MAP was -10.9 ± 2.1 mmHg lower in RD BPH/2J compared to baseline and -2.1 ± 2.2 mmHg in sham BPH/2J (P < 0.001, n = 8-10). RD had no effect in BPN/3J (P > 0.1). The depressor response to pentolinium was greater in BPH/2J than BPN/3J, but in both cases the response in RD mice was similar to sham. Enalaprilat decreased MAP more in RD BPH/2J compared to sham (-12 vs -3 mmHg, P < 0.001) but had no effect in BPN/3J. RD reduced renal noradrenaline in both strains but more so in BPH/2J. RD reduced renin mRNA and protein, but not plasma renin in BPH/2J to levels comparable with BPN/3J mice. We conclude that renal nerves contribute to hypertension in BPH mice as RD induced a sustained fall in MAP, which was associated with a reduction of intrarenal renin expression. The lack of inhibition of the depressor effects of pentolinium and enalaprilat by RD suggests that vasoconstrictor effects of the SNS or RAS are not involved.

Targeted afferent renal denervation reduces arterial pressure but not renal inflammation in established DOCA-salt hypertension in the rat

Recent preclinical studies show renal denervation (RDNx) may be an effective treatment for hypertension; however, the mechanism remains unknown. We have recently reported total RDNx (TRDNx) and afferent-selective RDNx (ARDNx) similarly attenuated the development of deoxycorticosterone acetate (DOCA)-salt hypertension. Whereas TRDNx abolished renal inflammation, ARDNx had a minimal effect despite an identical antihypertensive effect. Although this study established that ARDNx attenuates the development of DOCA-salt hypertension, it is unknown whether this mechanism remains operative once hypertension is established. The current study tested the hypothesis that TRDNx and ARDNx would similarly decrease mean arterial pressure (MAP) in the DOCA-salt hypertensive rat, and only TRDNx would mitigate renal inflammation. After 21 days of DOCA-salt treatment, male Sprague-Dawley rats underwent TRDNx ( n = 16), ARDNx ( n = 16), or Sham ( n = 14) treatment and were monitored for 14 days. Compared with baseline, TRDNx and ARDNx decreased MAP similarly (TRDNx -14 ± 4 and ARDNx -15 ± 6 mmHg). After analysis of diurnal rhythm, rhythm-adjusted mean and amplitude of night/day cycle were also reduced in TRDNx and ARDNx groups compared with Sham. Notably, no change in renal inflammation, injury, or function was detected with either treatment. We conclude from these findings that: 1) RDNx mitigates established DOCA-salt hypertension; 2) the MAP responses to RDNx are primarily mediated by ablation of afferent renal nerves; and 3) renal nerves do not contribute to the maintenance of renal inflammation in DOCA-salt hypertension.

Reduced Renal Mass, Salt-Sensitive Hypertension Is Resistant to Renal Denervation

Aim: Activation of the sympathetic nervous system is common in resistant hypertension (RHT) and also in chronic kidney disease (CKD), a prevalent condition among resistant hypertensives. However, renal nerve ablation lowers blood pressure (BP) only in some patients with RHT. The influence of loss of nephrons per se on the antihypertensive response to renal denervation (RDNx) is unclear and was the focus of this study.

Methods: Systemic hemodynamics and sympathetically mediated low frequency oscillations of systolic BP were determined continuously from telemetrically acquired BP recordings in rats before and after surgical excision of ∼80% of renal mass and subsequent RDNx.

Results: After reduction of renal mass, rats fed a high salt (HS) diet showed sustained increases in mean arterial pressure (108 ± 3 mmHg to 128 ± 2 mmHg) and suppression of estimated sympathetic activity (∼15%), responses that did not occur with HS before renal ablation. After denervation of the remnant kidney, arterial pressure fell (to 104 ± 4 mmHg), estimated sympathetic activity and heart rate (HR) increased concomitantly, but these changes gradually returned to pre-denervation levels over 2 weeks of follow up. Subsequently, sympathoinhibition with clonidine did not alter arterial pressure while significantly suppressing estimated sympathetic activity and HR.

Conclusion: These results indicate that RDNx does not chronically lower arterial pressure in this model of salt-sensitive hypertension associated with substantial nephron loss, but without ischemia and increased sympathetic activity, thus providing further insight into conditions likely to impact the antihypertensive response to renal-specific sympathoinhibition in subjects with CKD.

DOCA-Salt Hypertension: an Update

Hypertension is a multifaceted disease that is involved in ∼40% of cardiovascular mortalities and is the result of both genetic and environmental factors. Because of its complexity, hypertension has been studied by using various models and approaches, each of which tends to focus on individual organs or tissues to isolate the most critical and treatable causes of hypertension and the related damage to end-organs. Animal models of hypertension have ranged from Goldblatt's kidney clip models in which the origin of the disease is clearly renal to animals that spontaneously develop hypertension either through targeted genetic manipulations, such as the TGR(mRen2)27, or selective breeding resulting in more enigmatic origins, as exemplified by the spontaneously hypertensive rat (SHR). These two genetically derived models simulate the less-common human primary hypertension in which research has been able to define a Mendelian linkage. Several models are more neurogenic or endocrine in nature and illustrate that crosstalk between the nervous system and hormones can cause a significant rise in blood pressure (BP). This review will examine one of these neurogenic models of hypertension, i.e., the deoxycorticosterone acetate (DOCA), reduced renal mass, and high-salt diet (DOCA-salt) rodent model, one of the most common experimental models used today. Although the DOCA-salt model is mainly believed to be neurogenic and has been shown to impact the central and peripheral nervous systems, it also significantly involves many other body organs.

Chronic Blockade of Brain Endothelin Receptor Type-A (ETA) Reduces Blood Pressure and Prevents Catecholaminergic Overactivity in the Right Olfactory Bulb of DOCA-Salt Hypertensive Rats

Overactivity of the sympathetic nervous system and central endothelins (ETs) are involved in the development of hypertension. Besides the well-known brain structures involved in the regulation of blood pressure like the hypothalamus or locus coeruleus, evidence suggests that the olfactory bulb (OB) also modulates cardiovascular function. In the present study, we evaluated the interaction between the endothelinergic and catecholaminergic systems in the OB of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Following brain ET receptor type A (ET_A) blockade by BQ610 (selective antagonist), transcriptional, traductional, and post-traductional changes in tyrosine hydroxylase (TH) were assessed in the OB of normotensive and DOCA-salt hypertensive rats. Time course variations in systolic blood pressure and heart rate were also registered. Results showed that ET_A blockade dose dependently reduced blood pressure in hypertensive rats, but it did not change heart rate. It also prevented the increase in TH activity and expression (mRNA and protein) in the right OB of hypertensive animals. However, ET_A blockade did not affect hemodynamics or TH in normotensive animals. Present results support that brain ET_A are not involved in blood pressure regulation in normal rats, but they significantly contribute to chronic blood pressure elevation in hypertensive animals. Changes in TH activity and expression were observed in the right but not in the left OB, supporting functional asymmetry, in line with previous studies regarding cardiovascular regulation. Present findings provide further evidence on the role of ETs in the regulation of catecholaminergic activity and the contribution of the right OB to DOCA-salt hypertension.

Renal Nerves and Long-Term Control of Arterial Pressure

The objective of this review is to provide an in-depth evaluation of how renal nerves regulate renal and cardiovascular function with a focus on long-term control of arterial pressure. We begin by reviewing the anatomy of renal nerves and then briefly discuss how the activity of renal nerves affects renal function. Current methods for measurement and quantification of efferent renal-nerve activity (ERNA) in animals and humans are discussed. Acute regulation of ERNA by classical neural reflexes as well and hormonal inputs to the brain is reviewed. The role of renal nerves in long-term control of arterial pressure in normotensive and hypertensive animals (and humans) is then reviewed with a focus on studies utilizing continuous long-term monitoring of arterial pressure. This includes a review of the effect of renal-nerve ablation on long-term control of arterial pressure in experimental animals as well as humans with drug-resistant hypertension. The extent to which changes in arterial pressure are due to ablation of renal afferent or efferent nerves are reviewed. We conclude by discussing the importance of renal nerves, relative to sympathetic activity to other vascular beds, in long-term control of arterial pressure and hypertension and propose directions for future research in this field. © 2017 American Physiological Society. Compr Physiol 7:263-320, 2017.

Effects of equol on deoxycorticosterone acetate salt-induced hypertension and associated vascular dementia in rats

Oxidative stress is the major cause of neuronal cell degeneration observed in neurodegenerative diseases including vascular dementia (VaD), and hypertension has been found to increase the probability of VaD. Here, we investigated the effects of equol in deoxycorticosterone acetate (DOCA)-salt-induced hypertensive rats (DHRs) and the associated VaD. The systolic blood pressure of rats treated with low- (10 mg per kg body weight) and high-dose (20 mg per kg body weight) equol for 4 weeks was lower than that of the control group by 12.18 and 17.48% in a dose-dependent manner, respectively (p < 0.05), which was regulated by inhibiting angiotensin-converting enzyme (ACE) activity and increasing the nitric oxide (NO) production. Equol-treated DHRs showed a significant decrease in both the swimming distance and time required to reach the escape platform (78.20 to 82.56%, p < 0.05). In addition, the probe trial session and working memory test indicated that equol improved the long- and short-term memory of the rats. Moreover, the brain antioxidant activity was increased by elevating the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) levels, and the malondialdehyde (MDA) content and acetylcholinesterase (AChE) activity were decreased, indicating that equol suppressed oxidative stress. In conclusion, we demonstrated that equol exhibited comparable blood pressure (BP)-lowering and VaD-improving effects with the clinically used drug, lisinopril in DHRs while there was a positive correlation between the doses. Therefore, this bioactive compound may be useful for developing functional foods, thereby extending the application of equol-containing crops.

Protective Role for Tissue Inhibitor of Metalloproteinase-4, a Novel Peroxisome Proliferator-Activated Receptor-γ Target Gene, in Smooth Muscle in Deoxycorticosterone Acetate-Salt Hypertension

Loss of peroxisome proliferator-activated receptor-γ (PPARγ) function causes hypertension, whereas its activation lowers blood pressure. Evidence suggests that these effects may be attributable to PPARγ activity in the vasculature. However, the specific transcriptional targets of PPARγ in vessels remain largely unidentified. In this study, we examined the role of smooth muscle PPARγ during salt-sensitive hypertension and investigated its transcriptional targets and functional effect. Transgenic mice expressing dominant-negative PPARγ (S-P467L) in smooth muscle cells were more prone to deoxycorticosterone acetate-salt-induced hypertension and mesenteric arterial dysfunction compared with nontransgenic controls. Despite similar morphometry at baseline, vascular remodeling in conduit and small arteries was enhanced in S-P467L after deoxycorticosterone acetate-salt treatment. Gene expression profiling in aorta and mesenteric arteries revealed significantly decreased expression of tissue inhibitor of metalloproteinase-4 (TIMP-4) in S-P467L. Expression of TIMP-4 was increased by deoxycorticosterone acetate-salt treatment, but this increase was ablated in S-P467L. Interference with PPARγ activity either by treatment with a PPARγ inhibitor, GW9662, or by expressing P467L PPARγ markedly suppressed TIMP-4 in primary smooth muscle cells. PPARγ binds to a PPAR response element (PPRE) in chromatin close to the TIMP-4 gene in smooth muscle cells, suggesting that TIMP-4 is a novel target of PPARγ. The interference with PPARγ and decrease in TIMP-4 were accompanied by an increase in total matrix metalloproteinase activity. PPARγ-mediated loss of TIMP-4 increased, whereas overexpression of TIMP-4 decreased smooth muscle cell migration in a scratch assay. Our findings highlight a protective mechanism induced by PPARγ in deoxycorticosterone acetate-salt treatment, establishing a novel mechanistic link between PPARγ and TIMP-4.

Cerebrospinal Fluid Hypernatremia Elevates Sympathetic Nerve Activity and Blood Pressure via the Rostral Ventrolateral Medulla

Elevated NaCl concentrations of the cerebrospinal fluid increase sympathetic nerve activity (SNA) in salt-sensitive hypertension. Neurons of the rostral ventrolateral medulla (RVLM) play a pivotal role in the regulation of SNA and receive mono- or polysynaptic inputs from several hypothalamic structures responsive to hypernatremia. Therefore, the present study investigated the contribution of RVLM neurons to the SNA and pressor response to cerebrospinal fluid hypernatremia. Lateral ventricle infusion of 0.15 mol/L, 0.6 mol/L, and 1.0 mol/L NaCl (5 µL/10 minutes) produced concentration-dependent increases in lumbar SNA, adrenal SNA, and arterial blood pressure, despite no change in splanchnic SNA and a decrease in renal SNA. Ganglionic blockade with chlorisondamine or acute lesion of the lamina terminalis blocked or significantly attenuated these responses, respectively. RVLM microinjection of the gamma-aminobutyric acid (GABAA) agonist muscimol abolished the sympathoexcitatory response to intracerebroventricular infusion of 1 mol/L NaCl. Furthermore, blockade of ionotropic glutamate, but not angiotensin II type 1, receptors significantly attenuated the increase in lumbar SNA, adrenal SNA, and arterial blood pressure. Finally, single-unit recordings of spinally projecting RVLM neurons revealed 3 distinct populations based on discharge responses to intracerebroventricular infusion of 1 mol/L NaCl: type I excited (46%; 11/24), type II inhibited (37%; 9/24), and type III no change (17%; 4/24). All neurons with slow conduction velocities were type I cells. Collectively, these findings suggest that acute increases in cerebrospinal fluid NaCl concentrations selectively activate a discrete population of RVLM neurons through glutamate receptor activation to increase SNA and arterial blood pressure.

Global- and renal-specific sympathoinhibition in aldosterone hypertension

Recent technology for chronic electric activation of the carotid baroreflex and renal nerve ablation provide global and renal-specific suppression of sympathetic activity, respectively, but the conditions for favorable antihypertensive responses in resistant hypertension are unclear. Because inappropriately high plasma levels of aldosterone are prevalent in these patients, we investigated the effects of baroreflex activation and surgical renal denervation in dogs with hypertension induced by chronic infusion of aldosterone (12 μg/kg per day). Under control conditions, basal values for mean arterial pressure and plasma norepinephrine concentration were 100±3 mm Hg and 134±26 pg/mL, respectively. By day 7 of baroreflex activation, plasma norepinephrine was reduced by ≈40% and arterial pressure by 16±2 mm Hg. All values returned to control levels during the recovery period. Arterial pressure increased to 122±5 mm Hg concomitant with a rise in plasma aldosterone concentration from 4.3±0.4 to 70.0±6.4 ng/dL after 14 days of aldosterone infusion, with no significant effect on plasma norepinephrine. After 7 days of baroreflex activation at control stimulation parameters, the reduction in plasma norepinephrine was similar but the fall in arterial pressure (7±1 mm Hg) was diminished (≈55%) during aldosterone hypertension when compared with control conditions. Despite sustained suppression of sympathetic activity, baroreflex activation did not have central actions to inhibit either the stimulation of vasopressin secretion or drinking induced by increased plasma osmolality during chronic aldosterone infusion. Finally, renal denervation did not attenuate aldosterone hypertension. These findings suggest that aldosterone excess may portend diminished blood pressure lowering to global and especially renal-specific sympathoinhibition during device-based therapy.

Neurogenic and sympathoexcitatory actions of NaCl in hypertension

Excess dietary salt intake is a major contributing factor to the pathogenesis of salt-sensitive hypertension. Strong evidence suggests that salt-sensitive hypertension is attributed to renal dysfunction, vascular abnormalities, and activation of the sympathetic nervous system. Indeed, sympathetic nerve transections or interruption of neurotransmission in various brain centers lowers arterial blood pressure (ABP) in many salt-sensitive models. The purpose of this article is to discuss recent evidence that supports a role of plasma or cerebrospinal fluid hypernatremia as a key mediator of sympathoexcitation and elevated ABP. Both experimental and clinical studies using time-controlled sampling have documented that a diet high in salt increases plasma and cerebrospinal fluid sodium concentration. To the extent it has been tested, acute and chronic elevations in sodium concentration activates the sympathetic nervous system in animals and humans. A further understanding of how the central nervous system detects changes in plasma or cerebrospinal fluid sodium concentration may lead to new therapeutic treatment strategies in salt-sensitive hypertension.

Role of cardiac sympathetic nerves in blood pressure regulation

Stellate ganglionectomy (SGx) was used to assess the contribution of cardiac sympathetic nerves to neurogenic hypertension in deoxycorticosterone (DOCA)-salt treated rats. Experiments were conducted in two substrains of Sprague-Dawley (SD) rats since previous studies reported bradycardia in Charles River-SD (CR-SD) rats and tachycardia in SASCO-SD (SA-SD) rats with DOCA treatment suggesting different underlying neural mechanisms. Uninephrectomized male rats underwent SGx or SHAM surgery and were instrumented for telemetric monitoring of mean arterial pressure (MAP) and heart rate (HR). After recovery, 0.9% saline solution and DOCA (50mg) were administered. Baseline MAP (Days 0-5 average) after SGx in CR-SD rats (96±2mmHg; n=7) was not significantly different (p=0.08) than CR-SD SHAM rats (103±3mmHg; n=9); however, there was a significantly lower HR during the baseline period (377±7 vs. 432±7bpm, p<0.05) in SGx rats. In SA-SD rats baseline MAP was not different between SGx and SHAM rats and HR was lower in SGx rats (428±8 vs. 371±5bpm, p<0.05). After DOCA treatment in both substrains, MAP and HR were elevated similarly in SHAM and SGx groups showing minimal impact in both groups of SGx on hypertension development. However, overall MAP in SA-SD SHAM rats reached a significantly higher level (155±10mmHg vs 135±5mmHg, p<0.05) than that observed in CR-SD SHAM rats demonstrating that the magnitude of hypertensive response to DOCA-salt treatment varies between substrains. In conclusion, removal of cardiac sympathetic nerves did not alter the development or maintenance of DOCA-salt hypertension in SD rats.

Intracarotid hypertonic sodium chloride differentially modulates sympathetic nerve activity to the heart and kidney

Hypertonic NaCl infused into the carotid arteries increases mean arterial pressure (MAP) and changes sympathetic nerve activity (SNA) via cerebral mechanisms. We hypothesized that elevated sodium levels in the blood supply to the brain would induce differential responses in renal and cardiac SNA via sensors located outside the blood-brain barrier. To investigate this hypothesis, we measured renal and cardiac SNA simultaneously in conscious sheep during intracarotid infusions of NaCl (1.2 M), sorbitol (2.4 M), or urea (2.4 M) at 1 ml/min for 4 min into each carotid. Intracarotid NaCl significantly increased MAP (91 ± 2 to 97 ± 3 mmHg, P < 0.05) without changing heart rate (HR). Intracarotid NaCl was associated with no change in cardiac SNA (11 ± 5.0%), but a significant inhibition of renal SNA (-32.5 ± 6.4%, P < 0.05). Neither intracarotid sorbitol nor urea changed MAP, HR, central venous pressure, cardiac SNA, and renal SNA. The changes in MAP and renal SNA were completely abolished by microinjection of the GABA agonist muscimol (5 mM, 500 nl each side) into the paraventricular nucleus of the hypothalamus (PVN). Infusion of intracarotid NaCl for 20 min stimulated a larger increase in water intake (1,100 ± 75 ml) than intracarotid sorbitol (683 ± 125 ml) or intracarotid urea (0 ml). These results demonstrate that acute increases in blood sodium levels cause a decrease in renal SNA, but no change in cardiac SNA in conscious sheep. These effects are mediated by cerebral sensors located outside the blood-brain barrier that are more responsive to changes in sodium concentration than osmolality. The renal sympathoinhibitory effects of sodium are mediated via a pathway that synapses in the PVN.

Defensive effect of natrium diethyldithiocarbamate trihydrate (NDDCT) and lisinopril in DOCA-salt hypertension-induced vascular dementia in rats

RATIONALE

Vascular dementia and hypertension are increasing day by day, with a high degree of co-occurrence. Tremendous amount of research work is required so that new pharmacological agents may be identified for their appropriate therapeutic utility to combat different dementing disorders.

OBJECTIVES

This study investigates the effect of natrium diethyldithiocarbamate trihydrate (NDDCT), a nuclear factor kappa-B (NF-κB) inhibitor, as well as lisinopril, an angiotensin converting enzyme (ACE) inhibitor, on deoxycorticosterone acetate (DOCA) hypertension-induced vascular dementia in rats.

METHODS

DOCA was used to induce hypertension and associated vascular dementia. Morris water maze (MWM) was used for testing learning and memory. Endothelial function was assessed by acetylcholine-induced endothelium-dependent relaxation of aortic strips. Different biochemical estimations were used to assess oxidative stress (aortic superoxide anion, serum and brain thiobarbituric acid reactive species, and brain glutathione), nitric oxide levels (serum nitrite/nitrate), and cholinergic activity (brain acetyl cholinesterase activity).

RESULTS

DOCA treatment significantly raised the mean arterial blood pressure of rats, and these hypertensive rats performed poorly on MWM, reflecting impairment of learning and memory. DOCA treatment also impaired vascular endothelial function and different biochemical parameters. Treatments of NDDCT as well as lisinopril significantly attenuated DOCA hypertension-induced impairment of learning and memory, endothelial dysfunction, and changes in various biochemical levels.

CONCLUSIONS

DOCA-salt hypertension induces vascular dementia in rats. NF-κB as well as ACE inhibitors may be considered as potential pharmacological agents for the management of hypertension-induced vascular dementia.

Chronic infusion of angiotensin-(1-7) into the lateral ventricle of the brain attenuates hypertension in DOCA-salt rats

Angiotensin-(ANG)-(1-7) is known by its central and peripheral actions, which mainly oppose the deleterious effects induced by accumulation of ANG II during pathophysiological conditions. In the present study we evaluated whether a chronic increase in ANG-(1-7) levels in the brain would modify the progression of hypertension. After DOCA-salt hypertension was induced for seven days, Sprague-Dawley rats were subjected to 14 days of intracerebroventricular (ICV) infusion of ANG-(1-7) (200 ng/h, DOCA-A7) or 0.9% sterile saline. As expected, on the 21st day, DOCA rats presented increased mean arterial pressure (MAP) (≈40%), and impaired baroreflex control of heart rate (HR) and baroreflex renal sympathetic nerve activity (RSNA) in comparison with that in normotensive control rats (CTL). These changes were followed by an overactivity of the cardiac sympathetic tone and reduction of the cardiac parasympathetic tone, and exaggerated mRNA expression of collagen type I (≈9-fold) in the left ventricle. In contrast, DOCA rats treated with ANG-(1-7) ICV had an improvement of baroreflex control of HR, which was even higher than that in CTL, and a restoration of the baroreflex control of RSNA, the balance of cardiac autonomic tone, and normalized mRNA expression of collagen type I in the left ventricle. Furthermore, DOCA-A7 had MAP lowered significantly. These effects were not accompanied by significant circulating or cardiac changes in angiotensin levels. Taken together, our data show that chronic increase in ANG-(1-7) in the brain attenuates the development of DOCA-salt hypertension, highlighting the importance of this peptide in the brain for the treatment of cardiovascular diseases.

How NaCl raises blood pressure: a new paradigm for the pathogenesis of salt-dependent hypertension

Excess dietary salt is a major cause of hypertension. Nevertheless, the specific mechanisms by which salt increases arterial constriction and peripheral vascular resistance, and thereby raises blood pressure (BP), are poorly understood. Here we summarize recent evidence that defines specific molecular links between Na(+) and the elevated vascular resistance that directly produces high BP. In this new paradigm, high dietary salt raises cerebrospinal fluid [Na(+)]. This leads, via the Na(+)-sensing circumventricular organs of the brain, to increased sympathetic nerve activity (SNA), a major trigger of vasoconstriction. Plasma levels of endogenous ouabain (EO), the Na(+) pump ligand, also become elevated. Remarkably, high cerebrospinal fluid [Na(+)]-evoked, locally secreted (hypothalamic) EO participates in a pathway that mediates the sustained increase in SNA. This hypothalamic signaling chain includes aldosterone, epithelial Na(+) channels, EO, ouabain-sensitive α(2) Na(+) pumps, and angiotensin II (ANG II). The EO increases (e.g.) hypothalamic ANG-II type-1 receptor and NADPH oxidase and decreases neuronal nitric oxide synthase protein expression. The aldosterone-epithelial Na(+) channel-EO-α(2) Na(+) pump-ANG-II pathway modulates the activity of brain cardiovascular control centers that regulate the BP set point and induce sustained changes in SNA. In the periphery, the EO secreted by the adrenal cortex directly enhances vasoconstriction via an EO-α(2) Na(+) pump-Na(+)/Ca(2+) exchanger-Ca(2+) signaling pathway. Circulating EO also activates an EO-α(2) Na(+) pump-Src kinase signaling cascade. This increases the expression of the Na(+)/Ca(2+) exchanger-transient receptor potential cation channel Ca(2+) signaling pathway in arterial smooth muscle but decreases the expression of endothelial vasodilator mechanisms. Additionally, EO is a growth factor and may directly participate in the arterial structural remodeling and lumen narrowing that is frequently observed in established hypertension. These several central and peripheral mechanisms are coordinated, in part by EO, to effect and maintain the salt-induced elevation of BP.

Splanchnic sympathetic nerves in the development of mild DOCA-salt hypertension

We previously reported that mild deoxycorticosterone acetate (DOCA)-salt hypertension develops in the absence of generalized sympathoexcitation. However, sympathetic nervous system activity (SNA) is regionally heterogeneous, so we began to investigate the role of sympathetic nerves to specific regions. Our first study on that possibility revealed no contribution of renal nerves to hypertension development. The splanchnic sympathetic nerves are implicated in blood pressure (BP) regulation because splanchnic denervation effectively lowers BP in human hypertension. Here we tested the hypothesis that splanchnic SNA contributes to the development of mild DOCA-salt hypertension. Splanchnic denervation was achieved by celiac ganglionectomy (CGX) in one group of rats while another group underwent sham surgery (SHAM-GX). After DOCA treatment (50 mg/kg) in rats with both kidneys intact, CGX rats exhibited a significantly attenuated increase in BP compared with SHAM-GX rats (15.6 ± 2.2 vs. 25.6 ± 2.2 mmHg, day 28 after DOCA treatment). In other rats, whole body norepinephrine (NE) spillover, measured to determine if CGX attenuated hypertension development by reducing global SNA, was not found to be different between SHAM-GX and CGX rats. In a third group, nonhepatic splanchnic NE spillover was measured as an index of splanchnic SNA, but this was not different between SHAM (non-DOCA-treated) and DOCA rats during hypertension development. In a final group, CGX effectively abolished nonhepatic splanchnic NE spillover. These data suggest that an intact splanchnic innervation is necessary for mild DOCA-salt hypertension development but not increased splanchnic SNA or NE release. Increased splanchnic vascular reactivity to NE during DOCA-salt treatment is one possible explanation.