REGULATION OF RENAL MEDULLARY CIRCULATION BY THE RENIN-ANGIOTENSIN SYSTEM IN GENETICALLY HYPERTENSIVE RATS

Resilience of Spontaneously Hypertensive Rats to Secondary Insults After Traumatic Brain Injury: Immediate Seizures, Survival, and Stress Response

Traumatic brain injury (TBI) is one of the primary causes of mortality and disability, with arterial blood pressure being an important factor in the clinical management of TBI. Spontaneously hypertensive rats (SHRs), widely used as a model of essential hypertension and vascular dementia, demonstrate dysfunction of the hypothalamic-pituitary-adrenal axis, which may contribute to glucocorticoid-mediated hippocampal damage. The aim of this study was to assess acute post-TBI seizures, delayed mortality, and hippocampal pathology in SHRs and normotensive Sprague Dawley rats (SDRs). Male adult SDRs and SHRs were subjected to lateral fluid-percussion injury. Immediate seizures were video recorded, corticosterone (CS) was measured in blood plasma throughout the study, and hippocampal morphology assessed 3 months post-TBI. Acute and remote survival rates were significantly higher in the SHRs compared to the SDRs (overall mortality 0% and 58%, respectively). Immediate seizure duration predicted acute but not remote mortality. TBI did not affect blood CS in the SHRs, while the CS level was transiently elevated in the SDRs, predicting remote mortality. Neuronal cell loss in the polymorph layer of ipsilateral dentate gyrus was found in both the SDRs and SHRs, while thinning of hippocampal pyramidal and granular cell layers were strain- and area-specific. No remote effects of TBI on the density of astrocytes or microglia were revealed. SHRs possess a unique resilience to TBI as compared with normotensive SDRs. SHRs show shorter immediate seizures and reduced CS response to the injury, suggesting the development of long-term adaptative mechanisms associated with chronic hypertension. Though remote post-traumatic hippocampal damage in ipsilateral dentate gyrus is obvious in both SHRs and SDRs, the data imply that physiological adaptations to high blood pressure in SHRs may be protective, preventing TBI-induced mortality but not hippocampal neurodegeneration. Understanding the mechanisms of resilience to TBI may also help improve clinical recommendations for patients with hypertension. Limitation: since more than a half of the SDRs with prolonged immediate seizures or elevated CS 3 days after TBI have died, survivorship bias might hamper correct interpretation of the data.

Renal Medulla in Hypertension

Studies have found that blood flow to the renal medulla is an important determinant of pressure-natriuresis and the long-term regulation of arterial pressure. First, a brief review of methods developed enabling the study of the medullary circulation is presented. Second, studies performed in rats are presented showing medullary blood flow plays a vital role in the pressure-natriuresis relationship and thereby in hypertension. Third, it is shown that chronic reduction of medullary blood flow results in hypertension and that enhancement of medullary blood flow reduces hypertension hereditary models of both salt-sensitive rats and salt-resistant forms of hypertension. The key role that medullary nitric oxide production plays in protecting this region from ischemic injury associated with circulating vasoconstrictor agents and reactive oxygen species is presented. The studies cited are largely the work of my students, research fellows, and colleagues with whom I have performed these studies dating from the late 1980s to more recent years.

Chicken Muscle-Derived ACE2-Upregulating Peptide VVHPKESF Reduces Blood Pressure Associated with the ACE2/Ang (1-7)/MasR Axis in Spontaneously Hypertensive Rats

SCOPE

This study aims to investigate the antihypertensive effect of four chicken muscle-derived angiotensin (Ang)-converting enzymes (ACE)-regulating peptides: Val-Arg-Pro (VRP, ACE inhibition), Leu-Lys-Tyr and Val-Arg-Tyr (LKY and VRY, ACE inhibition and ACE2 upregulation), and Val-Val-His-Pro-Lys-Glu-Ser-Phe (VVHPKESF [V-F], ACE2 upregulation) in spontaneously hypertensive rats.

METHODS AND RESULTS

Rats (12-14 weeks old) are grouped: 1) untreated, 2) VRP, 3) LKY, 4) VRY, and 5) V-F. Blood pressure (BP) is monitored using implantable telemetry technology. Over 18-day oral administration of 15 mg kg-1 body weight (BW) per day, only peptide V-F significantly (p < 0.05) reduces BP, decreases circulating Ang II, and increases ACE2 and Ang (1-7) levels, and enhances aortic expressions of ACE2 and Mas receptor (MasR). Peptide V-F also attenuates vascular inflammation (TNFα, MCP-1, IL-1α, IL-15, and cyclooxygenase 2 [COX2]) and vascular oxidative stress (nitrotyrosine). The gastrointestinal (GI)-degraded fragment of peptide V-F, Val-Val-His-Pro-Lys (VVHPK), is also an ACE2-upregulating peptide. Peptides VRP, LKY, and VRY do not reduce BP, possibly due to low bioavailability or other unknown reasons.

CONCLUSIONS

Peptide V-F is the first ACE2-upregulating peptide, purified and fractionated from food proteins based on in vitro ACE2 upregulation, that reduces BP associated with the activation of ACE2/Ang (1-7)/MasR axis; the N-terminal moiety VVHPK may be responsible for the antihypertensive effect of V-F.

Cutaneous and renal vasodilatory response to local pressure application: A comparative study in mice

BACKGROUND AND AIM

We have reported a novel relationship involving mechanical stimulation and vasodilation in rodent and human skin, referred to as pressure-induced vasodilation (PIV). It is unknown whether this mechanism exists in kidney and reflects the microcirculation in deep organs. Therefore, we compared the skin and kidney PIV to determine whether their changes were similar.

METHODS

In anesthetized mice fed a normal salt-diet, laser Doppler flux (LDF) signals were measured when an increase in local pressure was applied to the surface of the head skin with the rate of 2.2Pa/s (1mmHg/min) and to the left kidney with a rate of 4.4Pa/s (2mmHg/min). The mechanism underlying renal PIV was also investigated. The skin and kidney PIV were also compared during salt load (4% NaCl diet).

RESULTS

The kidney had higher baseline LDF and vascular conductance compared to those of the skin. Pressure application increased the LDF in the kidney as well as in the skin with a comparable maximal magnitude (about 25% from baseline value), despite different kinetics of PIV evolution. As we previously reported in the skin, the kidney PIV response was mediated by the activation of transient receptor potential vanilloid type 1 channels, the release of calcitonin gene-related peptide, and the participation of prostaglandins and nitric oxide. In the absence of hypertension, high salt intake abolished the cutaneous PIV response and markedly impaired the renal one.

CONCLUSION

PIV response in the mouse kidney results from a neuro-vascular interaction. Despite some differences between the skin and the kidney PIV, the similarities in their response and signaling mechanisms suggest that the cutaneous microcirculation could reflect, in part, the microcirculation of the renal cortex.

A vasoconstrictor response to COX-1-mediated prostacyclin synthesis in young rat renal arteries that increases in prehypertensive conditions

This study aimed to determine whether prostacyclin (PGI2) functions as an endothelium-derived contracting factor (EDCF) in young rat renal arteries, and, if so, we wanted to examine the underlying mechanism(s) and how it changes in prehypertensive conditions. Vessels from Wistar-Kyoto (WKY) and prehypertensive spontaneously hypertensive rats (SHRs) of 25-28 days of age were isolated for functional and biochemical analyses. Result showed that following NO synthase (NOS) inhibition PGI2 and the thromboxane-prostanoid (TP) receptor agonist U-46619 evoked contractions in young WKY renal arteries that were similar to those in prehypertensive SHRs. Meanwhile, the endothelial muscarinic receptor agonist ACh evoked an endothelium-dependent contraction under NOS-inhibited conditions and a production of the PGI2 metabolite 6-keto-PGF1α; both were sensitive to cyclooxygenase (COX) and/or COX-1 inhibition but higher in prehypertensive SHRs than in young WKYs. Interestingly, in WKY renal arteries PGI2 did not evoke relaxation even after TP receptor antagonism that diminished the contraction evoked by the agonist. Indeed, PGI2 (IP) receptors were not detected in the vessel with Western blot. Moreover, we noted that treatment with the nonselective COX inhibitor indomethacin, which was started at the prehypertensive stage, blunted the elevation of systolic blood pressure and reduced the heart-to-body ratio in SHR within 2 mo of treatment. These results demonstrate that due to scarcity of IP receptors, PGI2, which is derived mainly from COX-1-mediated metabolism, acts as an EDCF in young WKY renal arteries, and it increases in prehypertensive conditions. Also, our data revealed that COX inhibition starting from the prehypertensive stage has an antihypertensive effect in young SHRs.

Factors regulating the renal circulation in spontaneously hypertensive rats

Hypertension is one of the leading causes of health morbidity and mortality which are linked to many life threatening diseases such as stroke heart problems and renal dysfunction.

The integrity of renal microcirculation is crucial to maintaining the clearance and the excretory function in the normotensive and hypertensive conditions. Furthermore, any alteration in the renal function is involved in the pathophysiology of hypertension.

The aim of this review is to provide a brief discussion of some factors that regulate renal haemodynamics in spontaneously hypertensive rats, an animal model of hypertension, and how these factors are linked to the disease.

Molecular targets of antihypertensive peptides: understanding the mechanisms of action based on the pathophysiology of hypertension

There is growing interest in using functional foods or nutraceuticals for the prevention and treatment of hypertension or high blood pressure. Although numerous preventive and therapeutic pharmacological interventions are available on the market, unfortunately, many patients still suffer from poorly controlled hypertension. Furthermore, most pharmacological drugs, such as inhibitors of angiotensin-I converting enzyme (ACE), are often associated with significant adverse effects. Many bioactive food compounds have been characterized over the past decades that may contribute to the management of hypertension; for example, bioactive peptides derived from various food proteins with antihypertensive properties have gained a great deal of attention. Some of these peptides have exhibited potent in vivo antihypertensive activity in both animal models and human clinical trials. This review provides an overview about the complex pathophysiology of hypertension and demonstrates the potential roles of food derived bioactive peptides as viable interventions targeting specific pathways involved in this disease process. This review offers a comprehensive guide for understanding and utilizing the molecular mechanisms of antihypertensive actions of food protein derived peptides.

Egg-derived tri-peptide IRW exerts antihypertensive effects in spontaneously hypertensive rats

Background

There is a growing interest in using functional food components as therapy for cardiovascular diseases such as hypertension. We have previously characterized a tri-peptide IRW (Ile-Arg-Trp) from egg white protein ovotransferrin; this peptide showed anti-inflammatory, anti-oxidant and angiotensin converting enzyme (ACE) inhibitor properties invitro. Given the pathogenic roles played by angiotensin, oxidative stress and inflammation in the spontaneously hypertensive rat (SHR), we tested the therapeutic potential of IRW in this well-established model of hypertension.

Methods and Results

16–17 week old male SHRs were orally administered IRW at either a low dose (3 mg/Kg BW) or a high dose (15 mg/Kg BW) daily for 18 days. Blood pressure (BP) and heart rate were measured by telemetry. Animals were sacrificed at the end of the treatment for vascular function studies and measuring markers of inflammation. IRW treatment attenuated mean BP by ~10 mmHg and ~40 mmHg at the low- and high-dose groups respectively compared to untreated SHRs. Heart rate was not affected. Reduction in BP was accompanied by the restoration of diurnal variations in BP, preservation of nitric oxide dependent vasorelaxation, as well as reduction of plasma angiotensin II, other inflammatory markers and tissue fibrosis.

Conclusion

Our results demonstrate anti-hypertensive effects of IRW invivo likely mediated through ACE inhibition, endothelial nitric oxide synthase and anti-inflammatory properties.

Blood pressure and renal hemodynamic effects of angiotensin fragments

Angiotensin (Ang) II, the main effector peptide of the renin-Ang system, increases arterial blood pressure through Ang II type 1A (AT(1a)) receptor-dependent arterial vasoconstriction and by decreasing renal salt and water excretion through extrarenal and intrarenal mechanisms. AT(2) receptors are assumed to oppose these responses mediated by AT(1) receptors, thereby attenuating the pressor effects of Ang II. Nevertheless, a possible role of AT(2) receptors in the regulation of renal hemodynamics and sodium homeostasis remains to be unclear. Several other Ang fragments such as Ang III, Ang IV, Ang-(1-7) and Ang A have also been shown to display biological activity. In this review, we focus on the effects of these Ang on blood pressure, renal hemodynamics and sodium water handling, and discuss the receptors involved in these actions.

Reactive oxygen species and vascular biology: implications in human hypertension

Increased vascular production of reactive oxygen species (ROS; termed oxidative stress) has been implicated in various chronic diseases, including hypertension. Oxidative stress is both a cause and a consequence of hypertension. Although oxidative injury may not be the sole etiology, it amplifies blood pressure elevation in the presence of other pro-hypertensive factors. Oxidative stress is a multisystem phenomenon in hypertension and involves the heart, kidneys, nervous system, vessels and possibly the immune system. Compelling experimental and clinical evidence indicates the importance of the vasculature in the pathophysiology of hypertension and as such much emphasis has been placed on the (patho)biology of ROS in the vascular system. A major source for cardiovascular, renal and neural ROS is a family of non-phagocytic nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (Nox), including the prototypic Nox2 homolog-based NADPH oxidase, as well as other Noxes, such as Nox1 and Nox4. Nox-derived ROS is important in regulating endothelial function and vascular tone. Oxidative stress is implicated in endothelial dysfunction, inflammation, hypertrophy, apoptosis, migration, fibrosis, angiogenesis and rarefaction, important processes involved in vascular remodeling in hypertension. Despite a plethora of data implicating oxidative stress as a causative factor in experimental hypertension, findings in human hypertension are less conclusive. This review highlights the importance of ROS in vascular biology and focuses on the potential role of oxidative stress in human hypertension.

Renal cortical and medullary blood flow responses to altered NO availability in humans

The objective of this study was to quantify regional renal blood flow in humans. In nine young volunteers on a controlled diet, the lower abdomen was CT-scanned, and regional renal blood flow was determined by positron emission tomography (PET) scanning using H(2)(15)O as tracer. Measurements were performed at baseline, during constant intravenous infusion of nitric oxide (NO) donor glyceryl nitrate and after intravenous injection of NO synthase inhibitor N(ω)-monomethyl-L-arginine (L-NMMA). Using the CT image, the kidney pole areas were delineated as volumes of interest (VOI). In the data analysis, tissue layers with a thickness of one voxel were eliminated stepwise from the external surface of the VOI (voxel peeling), and the blood flow subsequently was determined in each new, reduced VOI. Blood flow in the shrinking VOIs decreased as the number of cycles of voxel peeling increased. After 4-5 cycles, blood flow was not reduced further by additional voxel peeling. This volume-insensitive flow was measured to be 2.30 ± 0.17 ml·g tissue(-1)·min(-1) during the control period; it increased during infusion of glyceryl nitrate to 2.97 ± 0.18 ml·g tissue(-1)·min(-1) (P < 0.05) and decreased after L-NMMA injection to 1.57 ± 0.17 ml·g tissue(-1)·min(-1) (P < 0.05). Cortical blood flow was 4.67 ± 0.31 ml·g tissue(-1)·min(-1) during control, unchanged by glyceryl nitrate, and decreased after L-NMMA [3.48 ± 0.23 ml·(g·min)(-1), P < 0.05]. PET/CT scanning allows identification of a renal medullary region in which the measured blood flow is 1) low, 2) independent of reduction in the VOI, and 3) reactive to changes in systemic NO supply. The technique seems to provide indices of renal medullary blood flow in humans.