Age-related changes in the renal dopaminergic system and expression of renal amino acid transporters in WKY and SHR rats

Interactions between the intrarenal dopaminergic and the renin-angiotensin systems in the control of systemic arterial pressure

Systemic arterial hypertension is one of the leading causes of morbidity and mortality in the general population, being a risk factor for many cardiovascular diseases. Although its pathogenesis is complex and still poorly understood, some systems appear to play major roles in its development. This review aims to update the current knowledge on the interaction of the intrarenal renin-angiotensin system (RAS) and dopaminergic system in the development of hypertension, focusing on recent scientific hallmarks in the field. The intrarenal RAS, composed of several peptides and receptors, has a critical role in the regulation of blood pressure (BP) and, consequently, the development of hypertension. The RAS is divided into two main intercommunicating axes: the classical axis, composed of angiotensin-converting enzyme, angiotensin II, and angiotensin type 1 receptor, and the ACE2/angiotensin-(1-7)/Mas axis, which appears to modulate the effects of the classical axis. Dopamine and its receptors are also increasingly showing an important role in the pathogenesis of hypertension, as abnormalities in the intrarenal dopaminergic system impair the regulation of renal sodium transport, regardless of the affected dopamine receptor subtype. There are five dopamine receptors, which are divided into two major subtypes: the D1-like (D1R and D5R) and D2-like (D2R, D3R, and D4R) receptors. Mice deficient in any of the five dopamine receptor subtypes have increased BP. Intrarenal RAS and the dopaminergic system have complex interactions. The balance between both systems is essential to regulate the BP homeostasis, as alterations in the control of both can lead to hypertension.

Sodium-hydrogen exchanger regulatory factor-1 (NHERF1) confers salt sensitivity in both male and female models of hypertension in aging

Hypertension is a risk factor for premature death and roughly 50% of hypertensive patients are salt-sensitive. The incidence of salt-sensitive hypertension increases with age. However, the mechanisms of salt-sensitive hypertension are not well understood. We had demonstrated decreased renal sodium‑hydrogen exchanger regulatory factor 1 (NHERF1) expression in old salt-resistant F344 rats. Based on those studies we hypothesized that NHERF1 expression is required for the development of some forms of salt-sensitive hypertension. To address this hypothesis, we measured blood pressure in NHERF1 expressing salt-sensitive 4-mo and 24-mo-old male and female Fischer Brown Norway (FBN) rats male and female 18-mo-old NHERF1 knock-out (NHERF1-/-) mice and wild-type (WT) littermates on C57BL/6J background after feeding high salt (8% NaCl) diet for 7 days. Our data demonstrate that 8% salt diet increased blood pressure in both male and female 24-mo-old FBN rats but not in 4-mo-old FBN rats and in 18-mo-old male and female WT mice but not in NHERF1-/- mice. Renal dopamine 1 receptor (D1R) expression was decreased in 24-mo-old rats, compared with 4-mo-old FBN rats. However, sodium chloride cotransporter (NCC) expression increased in 24-mo-old FBN rats. In FBN rats, age had no effect on NaK ATPase α1 and NKCC2 expression. By contrast, high salt diet increased the renal expressions of NKCC2, and NCC in 24-mo-old FBN rats. High salt diet also increased NKCC2 and NCC expression in WT mice but not NHERF1-/- mice. Our data suggest that renal NHERF1 expression confers salt sensitivity with aging, associated with increased expression of sodium transporters.

Cardiometabolic and Inflammatory Benefits of Sympathetic Down-Regulation with Zamicastat in Aged Spontaneously Hypertensive Rats

The hyperactivity of the sympathetic nervous system (SNS) plays a major role in the development and progression of several cardiovascular diseases. One strategy to mitigate the SNS overdrive is by restricting the biosynthesis of norepinephrine via the inhibition of dopamine β-hydroxylase (DBH). Zamicastat is a new DBH inhibitor that decreases norepinephrine and increases dopamine levels in peripherally sympathetic-innervated tissues. The cardiometabolic and inflammatory effects of sympathetic down-regulation were evaluated in 50 week old male spontaneously hypertensive rats (SHRs) receiving zamicastat (30 mg/kg/day) for 9 weeks. After 8 weeks of treatment, the blood pressure (BP) and heart rate (HR) were assessed by tail cuff plethysmography. At the end of the study, 24 h urine, plasma, heart, and kidney were collected for biochemical and morphometric analyses. Zamicastat-induced sympathetic down-regulation decreased the high BP in SHRs, with no observed effect on HR. The heart-to-body weight ratio was lower in SHRs treated with zamicastat, whereas the body weight and kidney-to-body weight ratio were similar between both SHR cohorts. Zamicastat-treated SHRs showed reduced 24 h urine output, but the urinary amount of protein excreted and creatinine clearance rate remained unchanged. Zamicastat treatment significantly decreased plasma triglycerides, free fatty acids, and aspartate aminotransferase levels. Aged SHRs showed higher plasma levels of inflammatory markers as compared with age-matched normotensive Wistar-Kyoto rats. The inflammatory benefits attained with DBH inhibition were expressed by a decrease in CRP, MCP-1, IL-5, IL-17α, GRO/KC, MIP-1α, and RANTES plasma levels as compared with untreated SHRs. In conclusion, DBH inhibition decreased norepinephrine levels, reduced end-organ damage, and improved cardiometabolic and inflammatory biomarkers in aged male SHRs.

Mechanisms of altered renal sodium handling in age-related hypertension

The prevalence of hypertension rises with age to approximately two out of three adults over the age of 60 in the United States. Although the mechanisms underlying age-related hypertension are incompletely understood, sodium homeostasis is critical to the long-term regulation of blood pressure and there is strong evidence that aging is associated with alterations in renal sodium handling. This minireview focuses on recent advancements in our understanding of the vascular, neurohumoral, and renal mechanisms that influence sodium homeostasis and promote age-related hypertension.

Sustained high blood pressure reduction with etamicastat, a peripheral selective dopamine β-hydroxylase inhibitor

The aim of the present study was to evaluate the influence of chronic inhibition of dopamine ß-hydroxylase by etamicastat on the development of hypertension in the spontaneously hypertensive rat (SHR) and the sustainability of effects on the systolic and diastolic blood pressure in the SHR and the normotensive Wistar-Kyoto rat (WKY). WKY and SHR received etamicastat (10 mg/kg/d) from 5 weeks of age for 35 weeks in drinking water, and cardiovascular assessments were performed on a weekly basis. Etamicastat reduced systolic and diastolic blood pressure when SHRs reached the age of 16 weeks with mean decreases of 37 and 32 mm Hg, respectively, for the subsequent for 24 weeks of treatment, but did not prevent the increase in blood pressure (BP) aged between 5 and 11 week. The BP lowering effect of etamicastat in SHR was reversible on discontinuation and quickly resumed after reinstatement of therapy and was not accompanied by changes in heart rate. Etamicastat affected neither BP nor heart rate in WKY during 36 weeks of treatment. Etamicastat reduced urinary excretion of norepinephrine to a similar extent in WKY and SHR, accompanied by significant increases in urinary dopamine in SHR. Chronic administration of etamicastat did not adversely affected development of animals. Chronic dopamine ß-hydroxylase inhibition with etamicastat effectively decreases BP, although does not prevent the development of hypertension in the SHR.

A novel hemp seed meal protein hydrolysate reduces oxidative stress factors in spontaneously hypertensive rats

This report shows the antioxidant effects of a hemp seed meal protein hydrolysate (HMH) in spontaneously hypertensive rats (SHR). Defatted hemp seed meal was hydrolyzed consecutively with pepsin and pancreatin to yield HMH, which was incorporated into rat feed as a source of antioxidant peptides. Young (8-week old) SHRs were divided into three groups (8 rats/group) and fed diets that contained 0.0%, 0.5% or 1.0% (w/w) HMH for eight weeks; half of the rats were sacrificed for blood collection. After a 4-week washout period, the remaining 20-week old SHRs were fed for an additional four weeks and sacrificed for blood collection. Plasma total antioxidant capacity (TAC) and superoxide dismutase (SOD), catalase (CAT) and total peroxides (TPx) levels were determined. Results showed that plasma TAC, CAT and SOD levels decreased in the older 20-week old SHRs when compared to the young SHRs. The presence of HMH in the diets led to significant (p < 0.05) increases in plasma SOD and CAT levels in both young and adult SHR groups; these increases were accompanied by decreases in TPx levels. The results suggest that HMH contained antioxidant peptides that reduced the rate of lipid peroxidation in SHRs with enhanced antioxidant enzyme levels and total antioxidant capacity.

Increased Arterial Blood Pressure and Vascular Remodeling in Mice Lacking Salt-Inducible Kinase 1 (SIK1)

Rationale:

In human genetic studies a single nucleotide polymorphism within the salt-inducible kinase 1 ( SIK1 ) gene was associated with hypertension. Lower SIK1 activity in vascular smooth muscle cells (VSMCs) leads to decreased sodium-potassium ATPase activity, which associates with increased vascular tone. Also, SIK1 participates in a negative feedback mechanism on the transforming growth factor-β1 signaling and downregulation of SIK1 induces the expression of extracellular matrix remodeling genes.

Objective:

To evaluate whether reduced expression/activity of SIK1 alone or in combination with elevated salt intake could modify the structure and function of the vasculature, leading to higher blood pressure.

Methods and Results:

SIK1 knockout ( sik1 −/− ) and wild-type ( sik1 +/+ ) mice were challenged to a normal- or chronic high-salt intake (1% NaCl). Under normal-salt conditions, the sik1 −/− mice showed increased collagen deposition in the aorta but similar blood pressure compared with the sik1 +/+ mice. During high-salt intake, the sik1 +/+ mice exhibited an increase in SIK1 expression in the VSMCs layer of the aorta, whereas the sik1 −/− mice exhibited upregulated transforming growth factor-β1 signaling and increased expression of endothelin-1 and genes involved in VSMC contraction, higher systolic blood pressure, and signs of cardiac hypertrophy. In vitro knockdown of SIK1 induced upregulation of collagen in aortic adventitial fibroblasts and enhanced the expression of contractile markers and of endothelin-1 in VSMCs.

Conclusions:

Vascular SIK1 activation might represent a novel mechanism involved in the prevention of high blood pressure development triggered by high-salt intake through the modulation of the contractile phenotype of VSMCs via transforming growth factor-β1-signaling inhibition.

Renal amino acid transport systems and essential hypertension

Several clinical and animal studies suggest that "blood pressure goes with the kidney," that is, a normotensive recipient of a kidney genetically programmed for hypertension will develop hypertension. Intrarenal dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport. The candidate transport systems for L-DOPA, the source for dopamine, include the sodium-dependent systems B(0), B(0,+), and y(+)L, and the sodium-independent systems L (LAT1 and LAT2) and b(0,+). Renal LAT2 is overexpressed in the prehypertensive spontaneously hypertensive rat (SHR), which might contribute to enhanced L-DOPA uptake in the proximal tubule and increased dopamine production, as an attempt to overcome the defect in D1 receptor function. On the other hand, it has been recently reported that impaired arginine transport contributes to low renal nitric oxide bioavailability observed in the SHR renal medulla. Here we review the importance of renal amino acid transporters in the kidney and highlight pathophysiological changes in the expression and regulation of these transporters in essential hypertension. The study of the regulation of renal amino acid transporters may help to define the underlying mechanisms predisposing individuals to an increased risk for development of hypertension.

Appetitive and consummative responding for liquid sucrose in the spontaneously hypertensive rat model of attention deficit hyperactivity disorder

The spontaneously hypertensive rat (SHR) is one proposed animal model of attention deficit hyperactivity disorder (ADHD) argued to show strong face validity on the basis of behavioural characteristics. However, SHR may have fundamental alterations to the sensitivity of fluid reward due to altered renal function that has the potential to affect performance in complex reinforced behavioural tests. This could particularly confound determination of operant motivational alterations in the SHR. We assessed baseline bodyweight, home cage lab chow and water intake in the SHR and their typical control strains: Wistar and Wistar Kyoto. We also assessed sucrose preference, and appetitive and consummative positive and negative contrast for sucrose (4% versus 20%) on a motivational runway. As expected, SHR showed enhanced water intake compared to Wistar and Wistar Kyotos but comparable lab chow intake at baseline. SHR exhibited sucrose preference for 4% and 20%, as did both control strains, but the preference for 4% was enhanced in the SHR. SHR showed significant negative and positive contrast in sucrose consumption on the runway, as did Wistar Kyotos. Wistars exhibited neither. Appetitive contrast was not measurable in the SHR due to a robust locomotor velocity increase at the age of testing. The enhanced fluid intake found in the SHR argues against using fluid reinforcers in behavioural tests. We suggest the presence of both forms of contrast in the SHR is unusual for rats tested in ad lib. food conditions while the contrast pattern in Wistars indicate abnormalities in reward sensitivity in this control strain.