Impaired endothelial function in transgenic mice expressing both human renin and human angiotensinogen

How Does SARS-CoV-2 Affect the Central Nervous System? A Working Hypothesis

Interstitial pneumonia was the first manifestation to be recognized as caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); however, in just a few weeks, it became clear that the coronavirus disease-2019 (COVID-19) overrun tissues and more body organs than just the lungs, so much so that it could be considered a systemic pathology. Several studies reported the involvement of the conjunctiva, the gut, the heart and its pace, and vascular injuries such as thromboembolic complications and Kawasaki disease in children and toddlers were also described. More recently, it was reported that in a sample of 214 SARS-CoV-2 positive patients, 36.4% complained of neurological symptoms ranging from non-specific manifestations (dizziness, headache, and seizures), to more specific symptoms such hyposmia or hypogeusia, and stroke. Older individuals, especially males with comorbidities, appear to be at the highest risk of developing such severe complications related to the Central Nervous System (CNS) involvement. Neuropsychiatric manifestations in COVID-19 appear to develop in patients with and without pre-existing neurological disorders. Growing evidence suggests that SARS-CoV-2 binds to the human Angiotensin-Converting Enzyme 2 (ACE2) for the attachment and entrance inside host cells. By describing ACE2 and the whole Renin Angiotensin Aldosterone System (RAAS) we may better understand whether specific cell types may be affected by SARS-CoV-2 and whether their functioning can be disrupted in case of an infection. Since clear evidences of neurological interest have already been shown, by clarifying the topographical distribution and density of ACE2, we will be able to speculate how SARS-CoV-2 may affect the CNS and what is the pathogenetic mechanism by which it contributes to the specific clinical manifestations of the disease. Based on such evidences, we finally hypothesize the process of SARS-CoV-2 invasion of the CNS and provide a possible explanation for the onset or the exacerbation of some common neuropsychiatric disorders in the elderly including cognitive impairment and Alzheimer disease.

ACE (Angiotensin-Converting Enzyme) Inhibition Reverses Vasoconstriction and Impaired Dilation of Pial Collaterals in Chronic Hypertension

Leptomeningeal anastomoses (LMAs) are pial collaterals that perfuse the penumbra and important for stroke outcome. We previously showed LMAs from SHRs (spontaneously hypertensive rats) were vasoconstricted compared with normotensive Wistar rats. Here, we investigated mechanisms by which hypertension causes LMA vasoconstriction. SHRs were treated with the ACE (angiotensin-converting enzyme) inhibitor captopril, an Ang II (angiotensin II)-independent antihypertensive agent hydralazine, or vehicle for 5 weeks in drinking water (n=8/group). A group of Wistar rats (n=8) had regular drinking water served as controls. Blood pressure was measured twice weekly by tail-cuff. LMAs were isolated and studied under pressurized conditions. Vasoreactivity of LMAs, including myogenic responses, reactivity to Rho-kinase inhibitor Y-27632, and nitric oxide were measured. Both captopril and hydralazine lowered blood pressure in SHRs similar to Wistar. However, only captopril normalized LMA increased tone compared with untreated SHRs (15±2% versus 50±3%; P<0.01) that was similar to Wistar (16±2%) but not hydralazine (38±6%; P>0.05). Vasodilatory response of LMAs to Y-27632 was impaired in SHRs compared with Wistar (28±3% versus 81±4%; P<0.01) that was restored by captopril (84±5%; P<0.01) and partially hydralazine (59±4%). LMAs from all groups constricted similarly to NOS (NO synthase) inhibition; however, the vasodilatory response of LMAs to the nitric oxide donor sodium nitroprusside was impaired in SHRs compared with Wistar rats (29±4% versus 80±2%; P<0.01) that was restored by captopril (84±4%; P<0.01), not hydralazine (38±8%; P>0.05). These results suggest that ACE inhibition during chronic hypertension reversed vascular dysfunction and hyperconstriction of LMAs that could improve stroke outcome by increasing collateral perfusion.

Nutrients in the Prevention of Alzheimer's Disease

Alzheimer's disease (AD) is a disease caused by the complex interaction of multiple mechanisms, some of which are still not fully understood. To date, pharmacological treatments and supplementation of individual nutrients have been poorly effective in terms of the prevention and treatment of AD, while alternative strategies based on multimodal approaches (diet, exercise, and cognitive training) seem to be more promising. In this context, the focus on dietary patterns rather than on single food components could be more useful in preventing or counteracting the pathological processes typical of AD, thanks to the potential synergistic effects of various nutrients (neuronutrients). The aim of this narrative review is to summarize the currently existing preclinical and clinical evidence regarding the Mediterranean diet (MeDi), the Dietary Approaches to Stop Hypertension (DASH) diet, and the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet, which are three dietary patterns with well-known anti-inflammatory and antioxidant properties. Recently, they have been related to brain protection and AD prevention, perhaps thanks to their high content of neuroprotective bioactive compounds. Similarly, intermittent fasting (IF) or calorie restriction (CR) is emerging as interesting approaches that seem to promote hippocampal neurogenesis, activate adaptive stress response systems, and enhance neuronal plasticity, thus leading to motor and cognitive improvements in animal models of AD and hopefully also in human beings.

Hypertension prevents a sensory stimulation-based collateral therapeutic from protecting the cortex from impending ischemic stroke damage in a spontaneously hypersensitive rat model

Assessing potential stroke treatments in the presence of risk factors can improve screening of treatments prior to clinical trials and is important in testing the efficacy of treatments in different patient populations. Here, we test our noninvasive, nonpharmacological sensory stimulation treatment in the presence of the main risk factor for ischemic stroke, hypertension. Utilizing functional imaging, blood flow imaging, and histology, we assessed spontaneously hypertensive rats (SHRs) pre- and post-permanent middle cerebral artery occlusion (pMCAO). Experimental groups included a treatment SHR group (sensory-stimulated group), control untreated SHR group (no sensory stimulation), and a treated (sensory-stimulated) Wistar-Kyoto normotensive group. Unlike our previous studies, which showed sensory-based complete protection from impending ischemic cortical stroke damage in rats as seen in the treated Wistar-Kyoto group, we found that SHRs at 24hr post-pMCAO lacked evoked cortical activation, had a significant reduction in blood flow within the MCA, and sustained very large infarcts regardless of whether they received stimulation treatment. If translatable, this work highlights a potential need for a combined treatment plan when delivering sensory stimulation treatment in this patient population.

Cerebral blood flow regulation and neurovascular dysfunction in Alzheimer disease

Cerebral blood flow (CBF) regulation is essential for normal brain function. The mammalian brain has evolved a unique mechanism for CBF control known as neurovascular coupling. This mechanism ensures a rapid increase in the rate of CBF and oxygen delivery to activated brain structures. The neurovascular unit is composed of astrocytes, mural vascular smooth muscle cells and pericytes, and endothelia, and regulates neurovascular coupling. This Review article examines the cellular and molecular mechanisms within the neurovascular unit that contribute to CBF control, and neurovascular dysfunction in neurodegenerative disorders such as Alzheimer disease.

Cerebrovascular Disease: Consequences of Obesity-Induced Endothelial Dysfunction

Despite the well-known global impact of overweight and obesity in the incidence of cerebrovascular disease, many aspects of this association are still inconsistently defined. In this chapter we aim to present a critical review on the links between obesity and both ischemic and hemorrhagic stroke and discuss its influence on functional outcomes, survival, and current treatments to acute and chronic stroke. The role of cerebrovascular endothelial function and respective modulation is also described as well as its laboratory and clinical assessment. In this context, the major contributing mechanisms underlying obesity-induced cerebral endothelial function (adipokine secretion, insulin resistance, inflammation, and hypertension) are discussed. A special emphasis is given to the participation of adipokines in the pathophysiology of stroke, namely adiponectin, leptin, resistin, apelin, and visfatin.

Cerebral Small Vessel Disease: Targeting Oxidative Stress as a Novel Therapeutic Strategy?

Cerebral small vessel disease (SVD) is a major contributor to stroke, and a leading cause of cognitive impairment and dementia. Despite the devastating effects of cerebral SVD, the pathogenesis of cerebral SVD is still not completely understood. Moreover, there are no specific pharmacological strategies for its prevention or treatment. Cerebral SVD is characterized by marked functional and structural abnormalities of the cerebral microcirculation. The clinical manifestations of these pathological changes include lacunar infarcts, white matter hyperintensities, and cerebral microbleeds. The main purpose of this review is to discuss evidence implicating oxidative stress in the arteriopathy of both non-amyloid and amyloid (cerebral amyloid angiopathy) forms of cerebral SVD and its most important risk factors (hypertension and aging), as well as its contribution to cerebral SVD-related brain injury and cognitive impairment. We also highlight current evidence of the involvement of the NADPH oxidases in the development of oxidative stress, enzymes that are a major source of reactive oxygen species in the cerebral vasculature. Lastly, we discuss potential pharmacological strategies for oxidative stress in cerebral SVD, including some of the historical and emerging NADPH oxidase inhibitors.

Vascular dysfunctions in the isolated aorta of double-transgenic hypertensive mice developing aortic aneurysm

Angiotensin-II and oxidative stress are involved in the genesis of aortic aneurysms, a phenomenon exacerbated by endothelial nitric oxide synthase (eNOS) deletion or uncoupling. The purpose of this work was to study the endothelial function in wild-type C57BL/6 (BL) and transgenic mice expressing the h-angiotensinogen and h-renin genes (AR) subjected to either a control, or a high-salt diet plus a treatment with a NO-synthase inhibitor, N-ω-nitro-L-arginine-methyl-ester (L-NAME; BLSL and ARSL). BLSL showed a moderate increase in blood pressure, while ARSL became severely hypertensive. Seventy-five percent of ARSL developed aortic aneurysms, characterized by major histo-morphological changes and associated with an increase in NADP(H) oxidase-2 (NOX2) expression. Contractile responses (KCl, norepinephrine, U-46619) were similar in the four groups of mice, and relaxations were not affected in BLSL and AR. However, in ARSL, endothelium-dependent relaxations (acetylcholine, UK-14304) were significantly reduced, and this dysfunction was similar in aortae without or with aneurysms. The endothelial impairment was unaffected by catalase, superoxide-dismutase mimetic, radical scavengers, cyclooxygenase inhibition, or TP-receptor blockade and could not be attributed to sGC oxidation. Thus, ARSL is a severe hypertension model developing aortic aneurysm. A vascular dysfunction, involving both endothelial (reduced role of NO) and smooth muscle cells, precedes aneurysms formation and, paradoxically, does not appear to involve oxidative stress.

Angiotensin II-induced endothelial dysfunction is temporally linked with increases in interleukin-6 and vascular macrophage accumulation

Angiotensin II (Ang II) is associated with vascular hypertrophy, endothelial dysfunction and activation of a number of inflammatory molecules, however the linear events involved in the development of hypertension and endothelial dysfunction produced in response to Ang II are not well defined. The goal of this study was to examine the dose- and temporal-dependent development of endothelial dysfunction in response to Ang II. Blood pressure and responses of carotid arteries were examined in control (C57Bl/6) mice and in mice infused with 50, 100, 200, 400, or 1000 ng/kg/min Ang II for either 14 or 28 Days. Infusion of Ang II was associated with graded and marked increases in systolic blood pressure and plasma Ang II concentrations. While low doses of Ang II (i.e., 50 and 100 ng/kg/min) had little to no effect on blood pressure or endothelial function, high doses of Ang II (e.g., 1000 ng/kg/min) were associated with large increases in arterial pressure and marked impairment of endothelial function. In contrast, intermediate doses of Ang II (200 and 400 ng/kg/min) while initially having no effect on systolic blood pressure were associated with significant increases in pressure over time. Despite increasing blood pressure, 200 ng/kg/min had no effect on endothelial function, whereas 400 ng/kg/min produced modest impairment on Day 14 and marked impairment of endothelial function on Day 28. The degree of endothelial dysfunction produced by 400 and 1000 ng/kg/min Ang II was reflective of parallel increases in plasma IL-6 levels and vascular macrophage content, suggesting that increases in arterial blood pressure precede the development of endothelial dysfunction. These findings are important as they demonstrate that along with increases in arterial pressure that increases in IL-6 and vascular macrophage accumulation correlate with the impairment of endothelial function produced by Ang II.

Endothelium-dependent relaxations in the aorta from K(2p)6.1 knockout mice

K2P6.1 or TWIK-2, a two-pore domain K channel, is an important regulator of cardiovascular function. K2P6.1 is highly expressed in vascular smooth muscle and endothelium. Mice (8-12 wk) lacking functional K2P6.1 (K2P6.1(-/-)) are hypertensive and have enhanced vascular contractility. It is not known whether the lack of functional K2P6.1 in endothelium has a role in the vascular dysfunction in K2P6.1(-/-) mice. We tested the hypothesis: K2P6.1(-/-) mice have impaired endothelium-dependent relaxations. K2P6.1(-/-) mice were ∼35 mmHg more hypertensive than WT mice at both 8-12 wk (young adult) and 20-24 wk (mature mice, P < 0.01; n = 8-10). Endothelium-dependent relaxations of the thoracic aorta were evaluated by isometric myography after contraction with phenylephrine (10(-6) M). Maximal ACh-dependent relaxations were increased from 65 ± 1% to 73 ± 1% in the aorta from young adult (P < 0.01; n = 6) and from 45 ± 1% to 74 ± 1% in the aorta from mature (P < 0.001; n = 5) K2P6.1(-/-) mice compared with K2P6.1(+/+) littermates. However, in the aorta from young adult and mature K2P6.1(+/+) mice, 10(-5) M indomethacin, a cyclooxygenase inhibitor, increased maximal ACh relaxations to knockout levels. Enhanced relaxation was also seen with ATP, a P2Y purinergic agonist, and A23187, a nonreceptor-based agonist in mature K2P6.1(-/-) mice. Mature adult aorta from K2P6.1(-/-) showed an attenuated ACh-mediated contraction in the presence of nitro-l-arginine methyl ester (l-NAME) and without precontraction of 0.97 mN vs. 7.5 mN in K2P6.1(-/-) and K2P6.1(+/+) (P < 0.001; n = 5). In summary, K2P6.1(-/-) mice, which are hypertensive, have enhanced endothelium-dependent relaxations in the aorta due to the suppression of an indomethacin-sensitive constrictor component.

Roles of Brain Angiotensin II in Cognitive Function and Dementia

The brain renin-angiotensin system (RAS) has been highlighted as having a pathological role in stroke, dementia, and neurodegenerative disease. Particularly, in dementia, epidemiological studies indicate a preventive effect of RAS blockade on cognitive impairment in Alzheimer disease (AD). Moreover, basic experiments suggest a role of brain angiotensin II in neural injury, neuroinflammation, and cognitive function and that RAS blockade attenuates cognitive impairment in rodent dementia models of AD. Therefore, RAS regulation is expected to have therapeutic potential for AD. Here, we discuss the role of angiotensin II in cognitive impairment and AD. Angiotensin II binds to the type 2 receptor (AT₂) and works mainly by binding with the type 1 receptor (AT₁). AT₂ receptor signaling plays a role in protection against multiple-organ damage. A direct AT₂ receptor agonist is now available and is expected to reduce inflammation and oxidative stress and enhance cell differentiation. We and other groups reported that AT₂ receptor activation enhances neuronal differentiation and neurite outgrowth in the brain. Here, we also review the effect of the AT₂ receptor on cognitive function. RAS modulation may be a new therapeutic option for dementia including AD in the future.

Uncoupling of eNOS causes superoxide anion production and impairs NO signaling in the cerebral microvessels of hph-1 mice

In this study, we used the GTP cyclohydrolase I-deficient mice, i.e., hyperphenylalaninemic (hph-1) mice, to test the hypothesis that the loss of tetrahydrobiopterin (BH(4)) in cerebral microvessels causes endothelial nitric oxide synthase (eNOS) uncoupling, resulting in increased superoxide anion production and inhibition of endothelial nitric oxide signaling. Both homozygous mutant (hph-1(-/-)) and heterozygous mutant (hph-1(+/-) mice) demonstrated reduction in GTP cyclohydrolase I activity and reduced bioavailability of BH(4). In the cerebral microvessels of hph-1(+/-) and hph-1(-/-) mice, increased superoxide anion production was inhibited by supplementation of BH(4) or NOS inhibitor- L- N(G) -nitro arginine-methyl ester, indicative of eNOS uncoupling. Expression of 3-nitrotyrosine was significantly increased, whereas NO production and cGMP levels were significantly reduced. Expressions of antioxidant enzymes namely copper and zinc superoxide dismutase, manganese superoxide dismutase, and catalase were not affected by uncoupling of eNOS. Reduced levels of BH(4), increased superoxide anion production, as well as inhibition of NO signaling were not different between the microvessels of male and female mice. The results of our study are the first to demonstrate that, regardless of gender, reduced BH(4) bioavailability causes eNOS uncoupling, increases superoxide anion production, inhibits eNOS/cGMP signaling, and imposes significant oxidative stress in the cerebral microvasculature.

Role of angiotensin II receptor subtype activation in cognitive function and ischaemic brain damage

Recent clinical studies have demonstrated that angiotensin II type 1 (AT(1) ) receptor blockers (ARBs) reduce the onset of stroke, stroke severity and the incidence and progression of Alzheimer's disease and dementia. We can expect that ARBs exert these effects by both AT(1) receptor blockade and angiotensin II type 2 (AT(2) ) receptor stimulation. Moreover, recent experimental results support the notion that AT(2) receptor stimulation with AT(1) receptor blockade could contribute to protection against ischaemic brain damage at least partly due to an increase in cerebral blood flow and decrease in oxidative stress, and prevent cognitive decline. Cellular therapy has been focused on as a new therapeutic approach to restore injured neurons. In this context, it has been reported that AT(2) receptor stimulation enhances neurite outgrowth and decreases neural damage, thereby enhancing neurogenesis. Moreover, additional beneficial effects of ARBs with an AT(1) receptor blocking action with a partial peroxisome proliferator-activated receptor (PPAR)-γ agonistic effect have been reported, and interaction of AT(2) receptor activation and PPAR-γ might be involved in these ARBs' effects. This article reviews the effects of regulation of activation of angiotensin II receptor subtypes on ischaemic brain damage and cognitive function, focusing on the effects of AT(2) receptor stimulation.

Protecting against vascular disease in brain

Endothelial cells exert an enormous influence on blood vessels throughout the circulation, but their impact is particularly pronounced in the brain. New concepts have emerged recently regarding the role of this cell type and mechanisms that contribute to endothelial dysfunction and vascular disease. Activation of the renin-angiotensin system plays a prominent role in producing these abnormalities. Both oxidative stress and local inflammation are key mechanisms that underlie vascular disease of diverse etiology. Endogenous mechanisms of vascular protection are also present, including antioxidants, anti-inflammatory molecules, and peroxisome proliferator-activated receptor-γ. Despite their clear importance, studies of mechanisms that underlie cerebrovascular disease continue to lag behind studies of vascular biology in general. Identification of endogenous molecules and pathways that protect the vasculature may result in targeted approaches to prevent or slow the progression of vascular disease that causes stroke and contributes to the vascular component of dementia and Alzheimer's disease.

The thromboxane/endoperoxide receptor (TP): the common villain

The stimulation of thromboxane/endoperoxide receptors (TP) elicits diverse physiological/pathophysiological reactions, including platelet aggregation and contraction of vascular smooth muscle. Furthermore, the activation of endothelial TP promotes the expression of adhesion molecules and favors adhesion and infiltration of monocytes/macrophages. In various cardiovascular diseases, endothelial dysfunction is predominantly the result of the release of endothelium-derived contracting factors that counteract the vasodilator effect of nitric oxide produced by the endothelial nitric oxide synthase. Endothelium-dependent contractions involve the activation of cyclooxygenases, the production of reactive oxygen species along with that of endothelium-derived contracting factors, which diffuse toward the vascular smooth muscle cells and activate their TP. TP antagonists curtail the endothelial dysfunction in diseases such as hypertension and diabetes, are potent antithrombotic agents, and reduce vascular inflammation. Therefore, TP antagonists, because of this triple activity, may have a unique potential for the treatment of cardiovascular disorders.

Inhibition of bilirubin metabolism induces moderate hyperbilirubinemia and attenuates ANG II-dependent hypertension in mice

Population studies indicate that moderate hyperbilirubinemia is associated with reduced incidence of cardiovascular diseases, including hypertension. Despite this correlative evidence, no studies have directly tested the hypothesis that moderate increases in plasma bilirubin levels can attenuate the development of hypertension. This hypothesis was tested by treating mice with Indinavir, a drug that competes with bilirubin for metabolism by UDP-glucuronosyltransferase 1A1 (UGT1A1). Treatment of mice with Indinavir (500 mg x kg(-1) x day(-1), gavage) resulted in a twofold increase in plasma unconjugated bilirubin levels. Next, we determined the effect of Indinavir-induced changes in plasma bilirubin on the development of ANG II-dependent hypertension. Moderate hyperbilirubinemia was induced 3 days before the implantation of an osmotic minipump that delivered ANG II at a rate of 1 microg x kg(-1) x min(-1). ANG II infusion increased mean arterial pressure (MAP) by 20 mmHg in control mice but by only 6 mmHg in mice treated with Indinavir (n = 6). Similar to Indinavir treatment, direct infusion of bilirubin (37.2 mg x kg(-1) x day(-1) i.v.) resulted in a twofold increase in plasma bilirubin levels and also attenuated the development of ANG II-dependent hypertension. Moderate hyperbilirubinemia resulted in an increase in plasma nitrate/nitrite levels, which averaged 36 +/- 2 vs. 50 +/- 7 microM in ANG II vehicle vs. Indinavir-treated mice (n = 5). Moderate hyperbilirubinemia resulted in attenuation of vascular oxidative stress as determined by dihydroethidium staining of aortic segments. These results indicate that moderate hyperbilirubinemia prevents ANG II-dependent hypertension by a mechanism that may involve decreases in vascular oxidative stress.

Hypertension and cerebrovascular dysfunction

Essential hypertension has devastating effects on the brain, being the major cause of stroke and a leading cause of dementia. Hypertension alters the structure of cerebral blood vessels and disrupts intricate vasoregulatory mechanisms that assure an adequate blood supply to the brain. These alterations threaten the cerebral blood supply and increase the susceptibility of the brain to ischemic injury as well as Alzheimer's disease. This review focuses on the mechanisms by which hypertension disrupts cerebral blood vessels, highlighting recent advances and outstanding issues.

Quercetin and isorhamnetin prevent endothelial dysfunction, superoxide production, and overexpression of p47phox induced by angiotensin II in rat aorta

The dietary flavonoid quercetin reduces blood pressure and improves endothelial function in several rat models of hypertension. We analyzed the effects of quercetin and its methylated metabolite isorhamnetin on the aortic endothelial dysfunction induced by incubation with angiotensin II (AngII) in vitro for 6 h. AngII diminished the relaxant responses to acetylcholine in phenylephrine-contracted aorta. Coincubation with quercetin or isorhamnetin, or addition of superoxide (O(2)(-)) dismutase or apocynin to the assay medium, prevented these inhibitory effects. At 6 h, AngII induced a marked increase in O(2)(-) production as measured by dihydroethidium fluorescence, which was prevented by quercetin and isorhamnetin. AngII also increased the expression of p47(phox), a regulatory subunit of the membrane NADPH oxidase. Immunohistochemical analysis revealed that overexpression of p47(phox) occurred mainly in the medial layer. p47(phox) overexpression was also prevented by quercetin and isorhamnetin. Taken together, these results show for the first time, to our knowledge, that quercetin and isorhamnetin prevent AngII-induced endothelial dysfunction by inhibiting the overexpression of p47(phox) and the subsequent increased O(2)(-) production, resulting in increased nitric oxide bioavailability.

Cerebral vascular effects of angiotensin II: new insights from genetic models

Very little is known regarding the mechanisms of action of angiotensin II (Ang II) or the consequences of Ang II-dependent hypertension in the cerebral circulation. We tested the hypothesis that Ang II produces constriction of cerebral arteries that is mediated by activation of AT1A receptors and Rho-kinase. Basilar arteries (baseline diameter approximately 130 microm) from mice were isolated, cannulated and pressurized to measure the vessel diameter. Angiotensin II was a potent constrictor in arteries from male, but not female, mice. Vasoconstriction in response to Ang II was prevented by an inhibitor of Rho-kinase (Y-27632) in control mice, and was reduced by approximately 85% in mice deficient in expression of AT1A receptors. We also examined the chronic effects of Ang II using a model of Ang II-dependent hypertension, mice which overexpress human renin (R+) and angiotensinogen (A+). Responses to the endothelium-dependent agonist acetylcholine were markedly impaired in R+A+ mice (P<0.01) compared with controls, but were restored to normal by a superoxide scavenger (PEG-SOD). A-23187 (another endothelium-dependent agonist) produced vasodilation in control mice, but no response or vasoconstriction in R+A+ mice. In contrast, dilation of the basilar artery in response to a NO donor (NONOate) was similar in R+A+ mice and controls. Thus, Ang II produces potent constriction of cerebral arteries via activation of AT1A receptors and Rho-kinase. There are marked gender differences in cerebral vascular responses to Ang II. Endothelial function is greatly impaired in a genetic model of Ang II-dependent hypertension via a mechanism that involves superoxide.

Endothelial influences on cerebrovascular tone

The cerebrovascular endothelium exerts a profound influence on cerebral vessels and cerebral blood flow. This review summarizes current knowledge of various dilator and constrictor mechanisms intrinsic to the cerebrovascular endothelium. The endothelium contributes to the resting tone of cerebral arteries and arterioles by tonically releasing nitric oxide (NO•). Dilations can occur by stimulated release of NO•, endothelium-derived hyperpolarization factor, or prostanoids. During pathological conditions, the dilator influence of the endothelium can turn to that of constriction by a variety of mechanisms, including decreased NO• bioavailability and release of endothelin-1. The endothelium may participate in neurovascular coupling by conducting local dilations to upstream arteries. Further study of the cerebrovascular endothelium is critical for understanding the pathogenesis of a number of pathological conditions, including stroke, traumatic brain injury, and subarachnoid hemorrhage.

The lord of the ring: mandatory role of the kidney in drug therapy of hypertension

Strong evidence supports the idea that total peripheral resistance (TPR) is increased in all forms of human and experimental hypertension. Although the etiological participation of TPR in the origin and long-term maintenance of hypertension has been extensively debated, it now seems clear that the renal, nonadaptive, infinite gain-working, pressure-sensitive natriuresis and diuresis is the main mechanism of blood pressure control in the long term. The tissue, cellular, biochemical, and genetic sensors and executors of this process have not been fully identified yet, but the role of the renal medulla has gained growing attention as the physiopathological scenario in which the key regulatory elements reside. Specifically, the functionality of the renomedullary vasculature seems to be highly responsible for blood pressure control. The vasculature of the renal medulla becomes a new and more specific target for the therapeutic intervention of hypertension. Recent data on the effect of baroreceptor-controlled renal sympathetic activity on the long-term regulation of blood pressure are integrated. The renomedullary effects of the main antihypertensive drugs are discussed, and new perspectives for the therapeutic intervention of hypertension are outlined. Comparison of the genetic program of the renal medulla before and after the development of hypertension in spontaneously hypertensive and experimentally induced animal models might provide a mechanism for identifying the key genes that become activated or suppressed in the development of high blood pressure. These genes, their encoded proteins, or other elements related to their signalling and genetic pathways might serve as new and more specific targets for the pharmacological treatment of abnormally elevated blood pressure. Besides, proteins specifically located to the luminal side of the renomedullary vascular endothelium may serve as potential targets for site-directed drug and gene therapy.

Transgenic mice for studies of the renin-angiotensin system in hypertension

Hypertension is a polygenic and multi-factorial disorder that is extremely prevalent in western societies, and thus has received a great deal of attention by the research community. The renin-angiotensin system has a strong impact on the control of blood pressure both in the short- and long-term, making it one of the most extensively studied physiological systems. Nevertheless, despite decades of research, the specific mechanisms implicated in its action on blood pressure and electrolyte balance, as well as its integration with other cardiovascular pathways remains incomplete. The production of transgenic models either over-expressing or knocking-out specific components of the renin-angiotensin system has given us a better understanding of its role in the pathogenesis of hypertension. Moreover, our attention has recently been refocused on local tissue renin-angiotensin systems and their physiological effect on blood pressure and end-organ damage. Herein, we will review studies using genetic manipulation of animals to determine the role of the endocrine and tissue renin-angiotensin system in hypertension. We will also discuss some untraditional approaches to target the renin-angiotensin system in the kidney.

Angiotensin II attenuates functional hyperemia in the mouse somatosensory cortex

We investigated whether angiotensin II (ANG II), a peptide that plays a central role in the genesis of hypertension, alters the coupling between synaptic activity and cerebral blood flow (CBF), a critical homeostatic mechanism that assures adequate cerebral perfusion to active brain regions. The somatosensory cortex was activated by stroking the facial whiskers in anesthetized C57BL/6J mice while local CBF was recorded by laser-Doppler flowmetry. Intravenous ANG II infusion (0.25 mug.kg-1.min-1) increased mean arterial pressure (MAP) from 82 +/- 2 to 102 +/- 3 mmHg (P < 0.05) without affecting resting CBF (P > 0.05). ANG II attenuated the CBF increase produced by whisker stimulation by 65% (P < 0.05) but did not affect the response to hypercapnia or to neocortical application of the nitric oxide donor S-nitroso-N-acetyl penicillamine (P > 0.05). The effect of ANG II on functional hyperemia persisted if the elevation in MAP was offset by controlled hemorrhage or prevented by topical application of the peptide to the activated cortex. ANG II did not reduce the amplitude of the P1 wave of the field potentials evoked by whisker stimulation (P > 0.05). Infusion of phenylephrine increased MAP (P > 0.05 from ANG II) but did not alter the functional hyperemic response (P > 0.05). The data suggest that ANG II alters the coupling between CBF and neural activity. The mechanisms of the effect are not related to the elevation in MAP and/or to inhibition of the synaptic activity evoked by whisker stimulation. The imbalance between CBF and neural activity induced by ANG II may alter the homeostasis of the neuronal microenvironment and contribute to brain dysfunction during ANG II-induced hypertension.

Superoxide contributes to vascular dysfunction in mice that express human renin and angiotensinogen

This study examined vascular function and the role of superoxide in mice that chronically express human renin (R+) and human angiotensinogen (A+). Responses of aortas from R+/A+ mice and from their normotensive littermates (RA- mice) were examined in vitro. Endothelium-dependent relaxation to acetylcholine was impaired in vessels from R+/A+ mice (e.g., maximal relaxation to 100 microM acetylcholine was 45 +/- 5% and 65 +/- 3% in R+/A+ and RA- mice, respectively; P < 0.05). Relaxation was also impaired to the endothelium-independent dilators authentic nitric oxide and nitroprusside in vessels from R+/A+ mice. Maximal vasorelaxation to the endothelium-independent, non-nitric oxide dilator papaverine was similar in R+/A+ and RA- mice. Incubation of vessels from R+/A+ mice with Tiron (1 mM), a superoxide scavenger, improved relaxation to acetylcholine, nitric oxide, and nitroprusside. In contrast, incubation with diethyldithiocarbamate (1 mM), an inhibitor of copper-containing SODs, reduced acetylcholine- and nitroprusside-induced relaxation in vessels from both R+/A+ and RA- mice. Basal superoxide levels, measured with lucigenin-enhanced chemiluminescence (5 microM lucigenin) and hydroethidine-based fluorescent confocal microscopy, were higher in vessels from R+/A+ mice and were Tiron and polyethylene glycol-SOD sensitive. These results suggest that increased superoxide contributes to impaired nitric oxide-mediated relaxation in this genetic model of chronic angiotensin II-dependent hypertension.

Effects of NADH and NADPH on superoxide levels and cerebral vascular tone

Reactive oxygen species are important modulators of cerebral vascular tone. Recent evidence, mainly from the aorta, suggests that NAD(P)H oxidase is a major source of vascular superoxide. The goal of the present study was to examine the effects of NADH and NADPH that are commonly used to stimulate NAD(P)H oxidase activity, on superoxide levels and cerebral vascular tone. Basilar arteries and cerebral arterioles from normal rabbits were studied in vitro using isolated tissue baths and in vivo using a cranial window, respectively. In the basilar artery, NADH produced a biphasic response; low concentrations (0.1-10 microM NADH) produced marked relaxation, whereas higher concentrations (30-100 microM NADH) produced contraction. Responses to NADH were significantly (P < 0.05) inhibited in the presence of 4,5-dihydroxy-1,3-benzene-disulfonic acid (Tiron; a scavenger of superoxide, 10 mM). In contrast, NADPH (10-100 microM) produced moderate contraction of the basilar artery, which was inhibited in the presence of Tiron. In vivo, NADH produced Tiron-sensitive dilatation of cerebral arterioles. NADH and NADPH dose dependently increased superoxide levels in the basilar artery, as detected by lucigenin (5 microM)-enhanced chemiluminescence, but increases in superoxide were significantly greater for NADPH than NADH. These increases in superoxide were markedly reduced in the presence of polyethylene glycol-superoxide dismutase (300 U/ml) or diphenylene iodonium [0.1 mM, an inhibitor of flavin-containing enzymes, including NAD(P)H oxidase] but were not affected by indomethacin, N(G)-nitro-L-arginine, or allopurinol. These data suggest that NADH- and NADPH-induced changes in cerebral vascular tone are mediated by superoxide, produced by a flavin-containing enzyme, most likely NAD(P)H oxidase, but not xanthine oxidase or nitric oxide synthase.

Effects of age, gender, and blood pressure on myogenic responses of mesenteric arteries from C57BL/6 mice

The myogenic response (MR) may represent an important physiological parameter underlying arterial blood pressure (BP). We studied the effects of age, gender, and BP on the MR of mesenteric arteries from 8- to 52-wk-old mice. Increasing age and BP are associated with an increase in the perfusion pressure at which tone develops (myogenic set point). An inverse correlation exists between age and extent (magnitude) of the MR in male (r(2) = 0.93, P = 0.0087) and female mice (r(2) = 0.90, P = 0.013) as well as between BP and extent of the MR in male (r(2) = 0.96, P = 0.0036) and female (r(2) = 0.90, P = 0.014) mice. In contrast, the strength of the MR (slope of active diameter-pressure relationship) and phenylephrine-mediated constriction did not differ among these groups. Although gender had no effect on MR at any perfusion pressure or age, only male mice showed significant salt-induced hypertension and an associated increase in the set point and reduction in the extent of the MR. The set point and extent of the MR is linked to the in vivo pressure during development and experimental hypertension.

Novel mechanisms contributing to cerebral vascular dysfunction during chronic hypertension

Chronic hypertension is a major risk factor for numerous cardiovascular disorders and is strongly associated with stroke. Hypertension alters cerebral vascular structure and may have profound deleterious effects on cerebral vascular function, the underlying mechanisms of which are still not well understood. Recent findings have led to important developments in our understanding of novel areas of cerebral vascular biology. This review briefly examines new evidence for physiologic and pathologic roles of K(+) channels, the renin-angiotensin system and reactive oxygen species, and Rho and Rho-kinase in regulation of cerebral vascular tone.

Genetic manipulation of the renin-angiotensin system in the kidney

Over the past 5 years, genetic manipulation has revolutionized the way we examine physiological processes by providing a targeted specificity that was not possible previously. The application of transgenesis and gene targeting has been applied to numerous physiological pathways; and both will remain important tools as we reach the completion of the human genome project and begin to assess the function of newly identified genes. The renin-angiotensin system (RAS) has been the target of numerous transgenic and gene targeting studies designed to help uncover its role in cardiovascular regulation and organ development. Each gene of the system has now been both over-expressed and knocked out. It will be discussed as to how new advances in tissue-specific gene targeting by both over-expression and gene ablation can be used as powerful tools to dissect the role of the RAS in individual tissues.

Transgenic animals as models for human disease

Transgenic animals, especially mice, have been used quite extensively as models for various human diseases. At first, the level of scientific inquiry was driven by the need to establish the model. In many cases, these models may be considered quite crude because of their limitations. More recently, transgenic models of disease have become more refined and are currently being used to study the pathological mechanisms behind the disease rather than to just provide a model of the disease. Using some examples from the recent literature, we will document the current level and complexity of inquiry using transgenic animals. New techniques and techniques that may prove promising will be discussed.