Reversal of microvascular rarefaction and reduced renal mass hypertension

Role of the Renal Microcirculation in Progression of Chronic Kidney Injury in Obesity

BACKGROUND

Obesity is largely responsible for the growing incidence and prevalence of diabetes, cardiovascular and renal diseases. Current strategies to prevent and treat obesity and its consequences have been insufficient to reverse the ongoing trends. Lifestyle modification or pharmacological therapies often produce modest weight loss which is not sustained and recurrence of obesity is frequently observed, leading to progression of target organ damage in many obese subjects. Therefore, research efforts have focused not only on the factors that regulate energy balance, but also on understanding mechanisms of target organ injury in obesity. Summary and Key Message: Microvascular (MV) disease plays a pivotal role in progressive kidney injury from different etiologies such as hypertension, diabetes, and atherosclerosis, which are all important consequences of chronic obesity. The MV networks are anatomical units that are closely adapted to specific functions of nutrition and removal of waste in every organ. Damage of the small vessels in several tissues and organs has been reported in obesity and may increase cardio-renal risk. However, the mechanisms by which obesity and its attendant cardiovascular and metabolic consequences interact to cause renal MV injury and chronic kidney disease are still unclear, although substantial progress has been made in recent years. This review addresses potential mechanisms and consequences of obesity-induced renal MV injury as well as current treatments that may provide protection of the renal microcirculation and slow progressive kidney injury in obesity.

Salt, Angiotensin II, Superoxide, and Endothelial Function

Proper function of the vascular endothelium is essential for cardiovascular health, in large part due to its antiproliferative, antihypertrophic, and anti-inflammatory properties. Crucial to the protective role of the endothelium is the production and liberation of nitric oxide (NO), which not only acts as a potent vasodilator, but also reduces levels of reactive oxygen species, including superoxide anion (O2•-). Superoxide anion is highly injurious to the vasculature because it not only scavenges NO molecules, but has other damaging effects, including direct oxidative disruption of normal signaling mechanisms in the endothelium and vascular smooth muscle cells. The renin-angiotensin system plays a crucial role in the maintenance of normal blood pressure. This function is mediated via the peptide hormone angiotensin II (ANG II), which maintains normal blood volume by regulating Na+ excretion. However, elevation of ANG II above normal levels increases O2•- production, promotes oxidative stress and endothelial dysfunction, and plays a major role in multiple disease conditions. Elevated dietary salt intake also leads to oxidant stress and endothelial dysfunction, but these occur in the face of salt-induced ANG II suppression and reduced levels of circulating ANG II. While the effects of abnormally high levels of ANG II have been extensively studied, far less is known regarding the mechanisms of oxidant stress and endothelial dysfunction occurring in response to chronic exposure to abnormally low levels of ANG II. The current article focuses on the mechanisms and consequences of this less well understood relationship among salt, superoxide, and endothelial function.

Renin knockout rat: control of adrenal aldosterone and corticosterone synthesis in vitro and adrenal gene expression

The classic renin-angiotensin system is partly responsible for controlling aldosterone secretion from the adrenal cortex via the peptide angiotensin II (ANG II). In addition, there is a local adrenocortical renin-angiotensin system that may be involved in the control of aldosterone synthesis in the zona glomerulosa (ZG). To characterize the long-term control of adrenal steroidogenesis, we utilized adrenal glands from renin knockout (KO) rats and compared steroidogenesis in vitro and steroidogenic enzyme expression to wild-type (WT) controls (Dahl S rat). Adrenal capsules (ZG; aldosterone production) and subcapsules [zona reticularis/fasciculata (ZFR); corticosterone production] were separately dispersed and studied in vitro. Plasma renin activity and ANG II concentrations were extremely low in the KO rats. Basal and cAMP-stimulated aldosterone production was significantly reduced in renin KO ZG cells, whereas corticosterone production was not different between WT and KO ZFR cells. As expected, adrenal renin mRNA expression was lower in the renin KO compared with the WT rat. Real-time PCR and immunohistochemical analysis showed a significant decrease in P450aldo (Cyp11b2) mRNA and protein expression in the ZG from the renin KO rat. The reduction in aldosterone synthesis in the ZG of the renin KO adrenal seems to be accounted for by a specific decrease in P450aldo and may be due to the absence of chronic stimulation of the ZG by circulating ANG II or to a reduction in locally released ANG II within the adrenal gland.

Microvascular rarefaction: the decline and fall of blood vessels

The goals of this presentation are two-fold: (1) to briefly sketch the field of vascular rarefaction as a key component of various fibrotic diseases and (2) to illustrate it with four vignettes depicting diverse mechanisms of microvascular rarefaction. Specifically, I shall describe migratory and angiogenic incompetence of endothelial cells under conditions of reduced bioavailability of nitric oxide, role of endothelial-to-mesenchymal cell and mesenchymal stem cell-to-endothelial reprogramming, and potential role of antiangiogenic peptides in the development of graft vascular disease as exemplified by chronic allograft nephropathy.

Stereological evaluation of the kidneys of anencephalic and normal fetuses

The aim of this study is to test the glomerular and other quantitative parameters of kidneys of anencephalic fetuses and comparing those to "normal" fetuses. In this study, 20 kidneys of human fetuses (5 boys and 5 girls of anencephalic fetus, and 5 boys and 5 girls of normal fetus), at gestational ages of 25-30 weeks, were examined. This study is based on two basic research methods: one is a conventional anatomical measurement at the macroscopical level; the other is a design-biased stereological method at the microscopical level. Physical dissector and Cavalieri principle were used to estimate the total and numerical density of glomerulus and the volume of kidney, respectively. The results of the two types of investigation were compared based on anencephalic/normal and boy/girl kidneys at both the macroscopical and microscopical levels. There was no significant difference found between the quantitative features of kidneys (volume of kidneys and mean number and/or height of glomerulus) belonging to anencephalic and normal fetuses. The results of this study suggest that anencephalic fetuses did not differ from normal fetuses in respect of kidneys.

Effect of animal facility construction on basal hypothalamic-pituitary-adrenal and renin-aldosterone activity in the rat

Although loud noise and intense vibration are known to alter the behavior and phenotype of laboratory animals, little is known about the effects of nearby construction. We studied the effect of a nearby construction project on the classic stress hormones ACTH, corticosterone, renin, and aldosterone in rats residing in a barrier animal facility before, for the first 3 months of a construction project, and at 1 month after all construction was completed. During some of the construction, noise and vibrations were not obvious to investigators inside the animal rooms. Body weight matched for age was not altered by nearby construction. During nearby construction, plasma ACTH, corticosterone, and aldosterone were approximately doubled compared with those of pre- and postconstruction levels. Expression of CRH mRNA in the paraventricular nucleus of the hypothalamus, CRH receptor and POMC mRNA in the anterior pituitary, and most mRNAs for steroidogenic genes in the adrenal gland were not significantly changed during construction. We conclude that nearby construction can cause a stress response without long-term effects on hypothalamic-pituitary-adrenal axis gene expression and body weight.

Creation and characterization of a renin knockout rat

The renin-angiotensin system plays an important role in the control of blood pressure (BP) and renal function. To illuminate the importance of renin in the context of a disease background in vivo, we used zinc-finger nucleases (ZFNs) designed to target the renin gene and create a renin knockout in the SS/JrHsdMcwi (SS) rat. ZFN against renin caused a 10-bp deletion in exon 5, resulting in a frameshift mutation. Plasma renin activity was undetectable in the Ren-/- rat, and renin protein was absent from the juxtaglomerular cells in the kidney. Body weight was lower in the Ren-/- rats (than in the Ren+/- or wild-type littermates), and conscious BP on low-salt diet (0.4% NaCl) was 58 ± 2 mm Hg in the Ren-/- male rats versus 117 mm Hg in the Ren+/- littermates, a reduction of almost 50 mm Hg. Blood urea nitrogen (BUN) and plasma creatinine levels were elevated in the Ren-/- strain (BUN 112 ± 7 versus 23 ± 2 mg/dL and creatinine 0.53 ± 0.02 versus 0.26 ± 0.02 mg/dL), and kidney morphology was abnormal with a rudimentary inner renal medulla, cortical interstitial fibrosis, thickening of arterial walls, and abnormally shaped glomeruli. The development of the first rat knockout in the renin-angiotensin system demonstrates the efficacy of the ZFN technology for creating knockout rats for cardiovascular disease on any genetic background and emphasizes the role of renin in BP regulation and kidney function even in the low-renin SS rat.

Evaluation of metalloprotease inhibitors on hypertension and diabetic nephropathy

This study examined the effects of two new selective metalloprotease (MMP) inhibitors, XL081 and XL784, on the development of renal injury in rat models of hypertension, Dahl salt-sensitive (Dahl S) and type 2 diabetic nephropathy (T2DN). Protein excretion rose from 20 to 120 mg/day in Dahl S rats fed a high-salt diet (8.0% NaCl) for 4 wk to induce hypertension. Chronic treatment with XL081 markedly reduced proteinuria and glomerulosclerosis, but it also attenuated the development of hypertension. To determine whether an MMP inhibitor could oppose the progression of renal damage in the absence of changes in blood pressure, Dahl S rats were fed a high-salt diet (4.0% NaCl) for 5 wks to induce renal injury and then were treated with the more potent and bioavailable MMP inhibitor XL784 either given alone or in combination with lisinopril and losartan. Treatment with XL784 or the ANG II blockers reduced proteinuria and glomerulosclerosis by ~30% and had no effect on blood pressure. Proteinuria fell from 150 to 30 mg/day in the rats receiving both XL784 and the ANG II blockers, and the degree of renal injury fell to levels seen in normotensive Dahl S rats maintained from birth on a low-salt diet. In other studies, albumin excretion rose from 125 to >200 mg/day over a 4-mo period in 12-mo-old uninephrectomized T2DN rats. In contrast, albumin excretion fell by >50% in T2DN rats treated with XL784, lisinopril, or combined therapy. XL784 reduced the degree of glomerulosclerosis in the T2DN rats to a greater extent than lisinopril, and combined therapy was more effective than either drug alone. These results indicate that chronic administration of a selective MMP inhibitor delays the progression, and may even reverse hypertension and diabetic nephropathy.

Effect of high-dose total body irradiation on ACTH, corticosterone, and catecholamines in the rat

Total body irradiation (TBI) or partial body irradiation is a distinct risk of accidental, wartime, or terrorist events. Total body irradiation is also used as conditioning therapy before hematopoietic stem cell transplantation. This therapy can result in injury to multiple tissues and might result in death as a result of multiorgan failure. The hypothalamic-pituitary-adrenal (HPA) axis could play a causative role in those injuries, in addition to being activated under conditions of stress. In a rat model of TBI, we have established that radiation nephropathy is a significant lethal complication, which is caused by hypertension and uremia. The current study assessed HPA axis function in rats undergoing TBI. Using a head-shielded model of TBI, we found an enhanced response to corticotropin-releasing hormone (CRH) in vitro in pituitaries from irradiated compared with nonirradiated rats at both 8 and 70 days after 10-Gy single fraction TBI. At 70, but not 8 days, plasma adrenocorticotrophic hormone (ACTH) and corticosterone levels were increased significantly in irradiated compared with nonirradiated rats. Plasma aldosterone was not affected by TBI at either time point, whereas plasma renin activity was decreased in irradiated rats at 8 days. Basal and stimulated adrenal steroid synthesis in vitro was not affected by TBI. In addition, plasma epinephrine was decreased at 70 days after TBI. The hypothalamic expression of CRH messenger RNA (mRNA) and hippocampal expression of glucocorticoid receptor mRNA were unchanged by irradiation. We conclude that the hypertension of radiation nephropathy is not aldosterone or catecholamine-dependent but that there is an abscopal activation of the HPA axis after 10 Gy TBI. This activation was attributable at least partially to enhanced pituitary ACTH production.

Experimental models of renal disease and the cardiovascular system

Cardiovascular disease is a leading cause of death among patients with end stage renal failure. Animal models have played a crucial role in teasing apart the complex pathological processes involved. This review discusses the principles of using animal models, the history of their use in the study of renal hypertension, the controversies arising from experimental models of non-hypertensive uraemic cardiomyopathy and the lessons learned from these models, and highlights important areas of future research in this field, including de novo cardiomyopathy secondary to renal transplantation.

Role of the renin angiotensin system on bone marrow-derived stem cell function and its impact on skeletal muscle angiogenesis

Autologous bone marrow cell (BMC) transplantation has been shown as a potential approach to treat various ischemic diseases. However, under many conditions BMC dysfunction has been reported, leading to poor cell engraftment and a failure of tissue revascularization. We have previously shown that skeletal muscle angiogenesis induced by electrical stimulation (ES) is impaired in the SS/Mcwi rats and that this effect is related to a dysregulation of the renin angiotensin system (RAS) that is normalized by the replacement of chromosome 13 derived from the Brown Norway rat (SS-13(BN)/Mcwi consomic rats). The present study explored bone marrow-derived endothelial cell (BM-EC) function in the SS/Mcwi rat and its impact on skeletal muscle angiogenesis induced by ES. SS/Mcwi rats were randomized to receive BMC from: SS/Mcwi; SS-13(BN)/Mcwi; SS/Mcwi rats infused with saline or ANG II (3 ng kg(-1) min(-1)). BMC were injected in the stimulated tibialis anterior muscle of SS/Mcwi rats. Vessel density was evaluated in unstimulated and stimulated muscles after 7 days of ES. BMC isolated from SS/Mcwi or SS/Mcwi rats infused with saline failed to restore angiogenesis induced by ES. However, BMC isolated from SS-13(BN)/Mcwi and SS/Mcwi rats infused with ANG II effectively restored the angiogenesis response in the SS/Mcwi recipient. Furthermore, ANG II infusion increased the capacity of BM-EC to induce endothelial cell tube formation in vitro and slightly increased VEGF protein expression. This study suggests that dysregulation of the RAS in the SS/Mcwi rat contributes to impaired BM-EC function and could impact the angiogenic therapeutic potential of BMC.

T lymphocytes mediate hypertension and kidney damage in Dahl salt-sensitive rats

This study examined mechanisms by which immune cells participate in the development of hypertension and renal disease in Dahl salt-sensitive (SS) rats. Increasing dietary salt from 0.4% to 4.0% NaCl significantly increased renal infiltration of T lymphocytes from 8.8 +/- 1.2 x 10(5) to 14.4 +/- 2.0 x 10(5) cells/2 kidneys, increased arterial blood pressure from 131 +/- 2 to 165 +/- 6 mmHg, increased albumin excretion rate from 17 +/- 3 to 129 +/- 20 mg/day, and resulted in renal glomerular and tubular damage. Furthermore, renal tissue ANG II was not suppressed in the kidneys of SS rats fed 4.0% NaCl. Administration of the immunosuppressive agent mycophenolate mofetil (MMF; 20 mg.kg(-1).day(-1)) prevented the infiltration of T lymphocytes and attenuated Dahl SS hypertension and renal disease. In contrast to vehicle-treated rats, Dahl SS rats administered MMF demonstrated a suppression of renal tissue ANG II from 163 +/- 26 to 88 +/- 9 pg/g of tissue when fed high salt. Finally, it was demonstrated that the T lymphocytes isolated from the kidney possess renin and angiotensin-converting enzyme activity. These data indicate that infiltrating T cells are capable of participating in the production of ANG II and are associated with increased intrarenal ANG II, hypertension, and renal disease. The suppression of T-cell infiltration decreased intrarenal ANG II and prevented Dahl SS hypertension and kidney damage. As such, infiltrating cells are capable of participating in the established phase of Dahl SS hypertension.

Congenic strains reveal the effect of the renin gene on skeletal muscle angiogenesis induced by electrical stimulation

Previous studies have indicated the importance of angiotensin II (ANG II) in skeletal muscle angiogenesis. The present study explored the effect of regulation of the renin gene on angiogenesis induced by electrical stimulation with the use of physiological, pharmacological, and genetic manipulations of the renin-angiotensin system (RAS). Transfer of the entire chromosome 13, containing the physiologically regulated renin gene, from the normotensive inbred Brown Norway (BN) rat into the background of an inbred substrain of the Dahl salt-sensitive (SS/Mcwi) rat restored renin levels and the angiogenic response after electrical stimulation. This restored response was significantly attenuated when SS-13BN/Mcwi consomic rats were treated with lisinopril or high-salt diet. The role of ANG II on this effect was confirmed by the complete restoration of skeletal muscle angiogenesis in SS/Mcwi rats infused with subpressor doses of ANG II. Congenic strains derived from the SS-13BN/Mcwi consomic were used to further verify the role of the renin gene in this response. Microvessel density was markedly increased after stimulation in congenic strains that contained the renin gene from the BN rat (congenic lines A and D). This angiogenic response was suppressed in control strains that carried regions of the BN genome just above (congenic line C) or just below (congenic line B) the renin gene. The present study emphasizes the importance of maintaining normal renin regulation as well as ANG II levels during the angiogenesis process with a combination of physiological, genetic, and pharmacological manipulation of the RAS.

Angiotensin II is a critical mediator of prazosin-induced angiogenesis in skeletal muscle

OBJECTIVE

The purpose of this study was to determine whether a high-salt diet modulates physiological angiogenesis in skeletal muscle by altering angiotensin II (ANGII) and vascular endothelial growth factor (VEGF) levels.

METHODS

Sprague-Dawley rats were placed on a control diet (0.4% NaCl by weight) or high-salt diet (4.0% NaCl) prior to treatment with the vasodilator prazosin in the drinking water. In addition, a group of animals fed high salt were infused intravenously with ANGII at a low dose to prevent ANGII suppression by high salt, and a group of rats fed control diet were treated with the angiotensin II type I (AT(1)) receptor blocker losartan and prazosin.

RESULTS

Prazosin induced significant angiogenesis in the tibialis anterior muscle after 1 week of treatment. High-salt-fed rats demonstrated a complete inhibition of this angiogenic response. Maintenance of ANGII levels restored prazosin-induced angiogenesis in animals fed a high-salt diet. In addition, losartan treatment blocked prazosin-induced angiogenesis in animals on a control diet. Western blot analysis indicated that prazosin-induced angiogenesis was independent of changes in muscle levels of VEGF.

CONCLUSIONS

This study demonstrates an inhibitory effect of high salt intake on prazosin-induced angiogenesis. Further, these results indicate that ANGII acting through the AT(1) receptor is a critical pathway in this model of angiogenesis.

Combined effects of low-dose spironolactone and captopril therapy in a rat model of genetic hypertrophic cardiomyopathy

For several years, the severe side effects associated with the use of high doses of the aldosterone antagonist, spironolactone, limited its clinical use. Studies have recently shown efficacy and minimal side effects of low-dose spironolactone combined with standard therapy in the treatment of heart failure and hypertensive patients. The authors evaluated the effects of low-dose spironolactone alone or in combination with angiotensin-converting enzyme (ACE) inhibitors on the progression of left ventricular dysfunction and remodeling in a congenic rat model of hypertrophic cardiomyopathy. The congenic SS-16/Mcwi rats developed severe cardiac hypertrophy despite being normotensive even on high-salt diet. SS-16/Mcwi and SS/Mcwi rats were fed a low-salt (0.4% NaCl) diet and were treated with vehicle (CON), spironolactone (20 mg/kg/d subcutaneously), captopril (100 mg/kg/d drinking water), or both spironolactone and captopril for 4 weeks. Blood pressure, plasma peptides, cardiac fibrosis, and echocardiography measurements were evaluated. Spironolactone at a low dose had no effect on blood pressure, cardiac hypertrophy, and fibrosis in either strain. However, in combination with captopril, spironolactone decreased the cardiac hypertrophy more than captopril treatment alone. In the SS-16/Mcwi rats, the combined therapy significantly preserved the cardiac index when compared with control. These data indicate that the addition of low-dose spironolactone to captopril treatment was more effective in preventing the progression of heart hypertrophy and ventricular dysfunction in the SS-16/Mcwi than captopril alone. This study suggests that combined spironolactone and captopril therapy may be useful in the treatment of hypertrophic cardiomyopathy.

EFFECT OF HIGH SALT INTAKE ON LOCAL RENIN-ANGIOTENSIN SYSTEM AND VENTRICULAR DYSFUNCTION FOLLOWING MYOCARDIAL INFARCTION IN RATS

1. This study was performed to evaluate the effect of chronic high salt intake on local cardiac and renal components of the renin-angiotensin system (RAS) and its impact on cardiac remodelling and function after myocardial infarction (MI). 2. Rats submitted to coronary artery ligation to produce MI or sham operation (SO) were randomized to receive 1% NaCl solution or tap water as drinking water for 4 weeks. Plasma renin activity (PRA) and angiotensin-converting enzyme (ACE) activity were quantified. Tissue angiotensin (Ang) II and ACE activity were determined by ELISA and a fluorimetric assay, respectively. Renal and cardiac AT(1) and AT(2) receptor protein levels were quantified by western blot. 3. Independent of the lower PRA levels, MI promoted a significant increase in the left ventricular/bodyweight ratio and impaired cardiac function. The cardiac RAS was activated after MI with a significant increase in ACE activity, AngII and AT(1) receptor levels. The RAS was slightly attenuated under high-salt conditions. 4. Interestingly, high salt intake increased the expression of the AT(2) receptor by approximately twofold in the kidney of MI rats compared with the SO control group. Because of its natriuretic effect, the AT(2) receptor may counterbalance the salt overload and prevent the additional impairment of cardiac function. 5. The present study indicates that 4 weeks after MI, high salt intake did not further increase cardiac hypertrophy or further impair cardiac function in MI rats. A chronic increase in salt intake significantly suppressed PRA, but did not prevent activation of the local RAS or the progression of cardiac remodelling and left ventricular dysfunction caused by MI. 6. The present results show that inhibition of systemic renin production with salt overload does not affect ventricular remodelling after MI in rats. This suggests that local activation of the RAS in the heart, which was not suppressed by salt overload, exerts a predominant role for local adaptations of the heart after MI.

Role of superoxide and angiotensin II suppression in salt-induced changes in endothelial Ca2+ signaling and NO production in rat aorta

Male Sprague-Dawley rats were maintained on a low-salt (LS) diet (0.4% NaCl) or changed to a high-salt (HS) diet (4% NaCl) for 3 days. Increases in intracellular Ca2+ ([Ca2+]i) in response to methacholine (10 μM) and histamine (10 μM) were significantly attenuated in aortic endothelial cells from rats fed a HS diet, whereas thapsigargin (10 μM)-induced increases in [Ca2+]i were unaffected. Methacholine-induced nitric oxide (NO) production was eliminated in endothelial cells of aortas from rats fed a HS diet. Low-dose ANG II infusion (5 ng·kg−1·min−1 iv) for 3 days prevented impaired [Ca2+]i signaling response to methacholine and histamine and restored methacholine-induced NO production in aortas from rats on a HS diet. Adding Tempol (500 μM) to the tissue bath to scavenge superoxide anions increased NO release and caused Nω-nitro-l-arginine methyl ester-sensitive vascular relaxation in aortas from rats fed a HS diet but had no effect on methacholine-induced Ca2+ responses. Chronic treatment with Tempol (1 mM) in the drinking water restored NO release, augmented vessel relaxation, and increased methacholine-induced Ca2+ responses significantly in aortas from rats on a HS diet but not in aortas from rats on a LS diet. These findings suggest that 1) agonist-induced Ca2+ responses and NO levels are reduced in aortas of rats on a HS diet; 2) increased vascular superoxide levels contribute to NO destruction, and, eventually, to impaired Ca2+ signaling in the vascular endothelial cells; and 3) reduced circulating ANG II levels during elevated dietary salt lead to elevated superoxide levels, impaired endothelial Ca2+ signaling, and reduced NO production in the endothelium.

Angiotensin II infusion restores stimulated angiogenesis in the skeletal muscle of rats on a high-salt diet

Elevated dietary salt intake has previously been demonstrated to have dramatic effects on microvascular structure and function. The purpose of this study was to determine whether a high-salt diet modulates physiological angiogenesis in skeletal muscle. Male Sprague-Dawley rats were placed on a control diet (0.4% NaCl by weight) or a high-salt diet (4.0% NaCl) before implantation of a chronic electrical stimulator. After seven consecutive days of unilateral hindlimb muscle stimulation, animals on control diets demonstrated a significant increase in microvessel density in the tibialis anterior muscle of the stimulated hindlimb relative to the contralateral control leg. High salt-fed rats demonstrated a complete inhibition of this angiogenic response, as well as a significant reduction in plasma ANG II levels compared with those of control animals. To investigate the role of ANG II suppression on the inhibitory effect of high-salt diets, a group of rats that were fed high salt were chronically infused with ANG II at a low dose. Maintenance of ANG II levels restored stimulated angiogenesis to control levels in animals fed a high-salt diet. Western blot analysis indicated that inhibition of angiogenesis in high salt-fed rats was not due to changes in VEGF or VEGF receptor type 1 protein expression in response to stimulation; however, the degree to which VEGF receptor 2 protein increased with stimulation was significantly lower in high salt-fed animals. This study demonstrates an inhibitory effect of high salt intake on stimulated angiogenesis and suggests a critical role for ANG II suppression in mediating this antiangiogenic effect.

Chronic angiotensin II AT1 receptor blockade increases cerebral cortical microvessel density

Angiotensin II is known to stimulate angiogenesis in the peripheral circulation through activation of the angiotensin II type 1 (AT1) receptor. This study investigated the effect of angiotensin receptor blockade on cerebral cortical microvessel density. Rats (6–7 wk old, n = 5–17) were instrumented with femoral arterial and venous indwelling catheters for arterial blood pressure measurement and drug administration. Rats were treated for 3 or 14 days with the AT1 receptor blocker losartan (50 mg/day in drinking water) or vehicle. Brains were sectioned and immunostained for CD31, and microvessel density was measured. Treatment with losartan for 3 or 14 days resulted in a slight decrease in mean arterial blood pressure (3 days, 92 ± 1 mmHg; and 14 days, 99 ± 2 mmHg) compared with vehicle (109 ± 3 and 125 ± 4 mmHg, respectively). A furosemide + captopril 14-day treatment group was added to control for the blood pressure change (96 ± 3 mmHg). Microvessel density increased in groups treated with losartan for 14 days (429 ± 13 vessels/mm2) compared with vehicle (383 ± 11 vessels/mm2) but did not change with furosemide + captopril (364 ± 7 vessels/mm2). Thus AT1 receptor blockade for 14 days resulted in increased cerebral microvessel density in a blood pressure-independent manner.

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.

Enhanced skeletal muscle arteriolar reactivity to ANG II after recovery from ischemic acute renal failure

In addition to the long-term renal complications, previous studies suggested that after acute renal failure (ARF), rats manifest an increased pressor response to an overnight infusion of ANG II. The present study tested whether recovery from ARF results in alterations in sensitivity to the peripheral vasculature. ARF was induced in Sprague-Dawley rats by 45 min of bilateral renal ischemia and reperfusion. Animals were allowed to recover renal structure and function for 5-8 wk, after which the acute pressor responses to ANG II were evaluated either in vivo in in situ skeletal muscle arterioles or in isolated gracilis muscle arteries in vitro. Baseline arterial pressure was not different in ARF rats vs. sham-operated controls, although ARF rats exhibited an enhanced pressor response to bolus ANG II infusion compared with control rats. Steady-state plasma ANG II concentration and plasma renin activity were similar between ARF and control rats. Constrictor reactivity of in situ cremasteric arterioles from ARF rats was enhanced in response to increasing concentrations of ANG II; however, no difference was observed in arteriolar responses to elevated PO2, norepinephrine, acetylcholine, or sodium nitroprusside. Isolated gracilis muscle arteries from ARF rats also showed increased vasoconstriction in response to ANG II but not norepinephrine. In conclusion, recovery from ischemic ARF is not associated with hypertension but is associated with increased arteriolar constrictor reactivity to ANG II. Although the mechanisms of this altered responsiveness are unclear, such changes may relate, in part, to cardiovascular complications in patients with ARF and/or after renal transplant.

Importance of the renin-angiotensin system in the regulation of arterial blood pressure in conscious mice and rats

AIM

The present experiments were designed to determine the mechanism(s) for increased sensitivity to blockade of the renin-angiotensin system in mice in comparison with rats.

METHODS

Mice and rats, with indwelling femoral arterial and venous catheters, were chronically administered angiotensin II or pharmacological inhibitors of the renin-angiotensin system as sodium intake was altered.

RESULTS

Increasing sodium intake led to suppression of circulating renin, angiotensin II, and aldosterone in rats and mice in the absence of alterations in arterial blood pressure. Additional experiments demonstrated that continuous intravenous infusion of angiotensin II (20 ng kg(-1) min(-1)) significantly increased arterial blood pressure by approximately 35 mmHg in conscious rats at all levels of sodium intake (n = 6). In contrast, arterial pressure was unaffected by angiotensin II infusion in conscious mice under conditions of low sodium intake, although arterial pressure was increased by 16 mmHg when mice were administered a high sodium intake while infused with angiotensin II (n = 6). In comparison, blockade of the endogenous renin-angiotensin system led to significantly greater effects on arterial pressure in mice than rats. Continuous infusion of captopril (30 microg kg(-1) min(-1)) or losartan (100 microg kg(-1) min(-1)) resulted in a 55-90% greater fall in blood pressure in conscious mice in comparison with conscious rats.

CONCLUSION

The present studies indicate that arterial pressure in mice is more dependent upon the endogenous renin-angiotensin system than it is in rats, but mice are more resistant to the hypertensive effects of exogenous angiotensin II.

Influence of elevated renin substrate on angiotensin II and arterial blood pressure in conscious mice

The present experiments were performed to determine the influence of intravenous administration of renin substrate on plasma angiotensin II levels and mean arterial blood pressure in conscious C57BL/6J mice. Mice with chronic indwelling femoral arterial and venous catheters were acutely or chronically administered intravenous doses of a synthetic peptide corresponding to the 14 amino acids on the N-terminal of angiotensinogen. A dose-dependent increase in arterial blood pressure was observed as the intravenous bolus dose of the renin substrate was increased from 0.18 to 180 nmol kg(-1) with a maximal increase in pressure of 40 +/- 3 mmHg achieved following administration of the 18 nmol kg(-1) bolus (n = 11). Additional experiments demonstrated that a sustained intravenous infusion of the renin substrate led to a long-term increase in arterial blood pressure. The continuous infusion of renin substrate at 0.05 nmol kg(-1) min(-1) for 3 days did not alter arterial blood pressure from the control level of 119 +/- 5 mmHg (n = 5); however, arterial blood pressure significantly increased to 129 +/- 6 mmHg with an infusion rate of 0.5 nmol kg(-1) min(-1) and further increased to 141 +/- 3 mmHg when the renin substrate infusion was increased to 5.0 nmol kg(-1) min(-1). Finally, the infusion of renin substrate at 5.0 nmol kg(-1) min(-1) resulted in a significant increase in plasma angiotensin II concentration from 34 +/- 6 pg ml(-1) in vehicle-infused mice to 288 +/- 109 pg ml(-1). These results demonstrate that modulation of the circulating level of angiotensinogen can alter the plasma angiotensin II level and arterial blood pressure in normal animals.

Localization of the Renin–Angiotensin System Components to the Skeletal Muscle Microcirculation

OBJECTIVES

Local renin-angiotensin systems have been found in various organs and tissues throughout the body. The studies described here tested the hypothesis that a fully functional renin-angiotensin system exists in the skeletal muscle microvasculature. The purpose of this study was to localize the components and products of the system to the skeletal muscle microvasculature.

METHODS

The presence of mRNA and protein for renin and angiotensinogen was shown by reverse transcriptase polymerase chain reaction analysis and immunohistochemistry, respectively. Angiotensin II concentration in isolated microvessels was measured by high-performance liquid chromatography separation and radioimmunoassay.

RESULTS

Renin and angiotensinogen mRNA was detected from isolated cremaster microvessels. Specific staining for renin and angiotensinogen protein was found throughout the walls of microvessels. The concentration of the angiotensin II in the microvessels (104.3 +/- 18.1 fmol/g) was found to be higher than the concentration of angiotensin II in the plasma (8.7 +/- 1.2 fmol/mL) of the same animals. Also, it was shown that the microvessel angiotensin II concentrations in spontaneously hypertensive rats were higher than in Sprague-Dawley rats.

CONCLUSIONS

The current study demonstrates that within the skeletal muscle microvessels, the components and products necessary for a local renin-angiotensin system are present, but does not rule out the possibility of interaction between circulating and tissue renin-angiotensin systems.

Conversion of renin substrate tetradecapeptide to angiotensin II by rat MAB elastase-2

A new approach for the purification of rat mesenteric arterial bed (MAB) elastase-2 has been developed using the chromogenic substrates N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide and N-succinyl-Ala-Ala-Pro-Leu-p-nitroanilide to monitor the enzymatic activity during various stages of purification. The purified enzyme was evaluated in the presence of various inhibitors and confirmed to have angiotensin (Ang) II-forming ability. The active site-directed inhibitor acetyl-Ala-Ala-Pro-Leu-chloromethylketone (100 µmol·L-1), described for human pancreatic elastase-2, abolished the enzymatic activity, confirming that the enzyme is an elastase-2. Chymostatin (100 µmol·L-1), an inhibitor regarded as selective for chymases, also showed a remarkable inhibitory effect (94%), whereas captopril (100 µmol·L-1) had no effect at all on the Ang II-forming activity. The Ang II precursor renin substrate tetradecapeptide (RS-14P) was converted into Ang II by the rat MAB elastase-2 with the following kinetic constants: Km= 124 ± 21 µmol·L-1; Kcat= 629 min-1; catalytic efficiency (Kcat/Km) = 5.1 min-1µ(mol/L)-1. In conclusion, the strategy for the purification of rat MAB elastase-2 with the chromogenic substrates proved to be simple, rapid, accurate, and highly reproducible; therefore, it can be reliably and conveniently used to routinely purify this enzyme. The kinetic parameters for the formation of Ang II from RS-14P by rat MAB elastase-2 emphasize differences in substrate specificity between this and other Ang II-forming enzymes.Key words: N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide, N-succinyl-Ala-Ala-Pro-Leu-p-nitroanilide, elastase-2, angiotensin II, renin substrate tetradecapeptide.

Basal and adrenocorticotropin-stimulated corticosterone in the neonatal rat exposed to hypoxia from birth: modulation by chemical sympathectomy

We previously demonstrated that 7-d-old rat pups exposed to hypoxia from birth exhibit ACTH-independent increases in corticosterone associated with an increase in steroidogenic acute regulatory (StAR) and peripheral-type benzodiazepine receptor (PBR) proteins. The purpose of the present study was to determine whether this increase in corticosterone could be attenuated by chemical sympathectomy induced with guanethidine treatment. Rat pups were exposed to normoxia or hypoxia from birth and treated with vehicle or guanethidine and studied at 7 d of age. Hypoxia per se resulted in an increase in plasma corticosterone without a change in plasma ACTH. Guanethidine treatment attenuated the increase in basal corticosterone in hypoxic pups but did not attenuate ACTH-stimulated corticosterone production. This effect was specific as basal and ACTH-stimulated aldosterone was not affected. Guanethidine also attenuated the increase in StAR protein induced by hypoxia. Neither the effect of hypoxia nor that of guanethidine could be explained by changes in the levels of adrenal tyrosine hydroxylase, StAR, or P450scc mRNA, adrenal tyrosine hydroxylase immunohistochemistry, or adrenal catecholamine content. We conclude that chemical sympathectomy normalizes basal corticosterone levels but has no effect on ACTH-stimulated corticosterone levels in 7-d-old rats exposed to hypoxia from birth. The mechanism of the effect of guanethidine to normalize hypoxia-stimulated basal corticosterone remains to be identified, although StAR protein may be an important mediator. This ACTH-independent increase in corticosterone may be a mechanism by which the neonate can increase circulating glucocorticoids necessary for survival while bypassing the hyporesponsiveness of the neonatal hypothalamic-pituitary-adrenal axis.

Functional role, cellular source, and tissue distribution of rat elastase-2, an angiotensin II-forming enzyme

We recently described a chymostatin-sensitive elastase-2 as the major angiotensin (ANG) II-forming enzyme in the perfusate of the rat mesenteric arterial bed (MAB) with the same cDNA sequence as rat pancreatic elastase-2. The role of this enzyme in generating ANG II was examined in the rat isolated and perfused MAB. The vasoconstrictor effect elicited by ANG I and the renin substrate tetradecapeptide was only partially inhibited by captopril but abolished by the combination of captopril and chymostatin or N-acetyl-Ala-Ala-Pro-Leu-chloromethylketone (Ac-AAPL-CK; inhibitor originally developed for human elastase-2). The effect induced by [Pro11,d-Ala12]-ANG I, an ANG I-converting enzyme (ACE)-resistant biologically inactive precursor of ANG II, was blocked by chymostatin or Ac-AAPL-CK. It was also demonstrated that cultured rat mesenteric endothelial cells synthesize elastase-2 and that mRNA for this enzyme can be detected in different rat tissues such as the pancreas, MAB, lung, heart, kidney, liver, and spleen. In conclusion, the demonstration of a functional alternative pathway to ACE for ANG II generation in the rat MAB and the fact that cultured MAB endothelial cells are capable of producing and secreting elastase-2 represent strong evidence of a physiological role for this enzyme in the rat vasculature.

Role of the renin-angiotensin system during alterations of sodium intake in conscious mice

The present studies were performed to quantify circulating components of the renin-angiotensin-aldosterone axis and to determine the functional importance of this system during alterations in sodium intake in conscious mice. Increasing sodium intake from approximately 200 to 1,000 microeq/day significantly decreased plasma renin concentration from 472 +/- 96 to 304 +/- 83 ng ANG I. ml(-1). h(-1) (n = 5) but did not alter plasma renin activity from the low-sodium level of 7.7 +/- 1.1 ng ANG I. ml(-1). h(-1). Despite the elevated plasma renin concentration, plasma ANG II in mice on low-sodium level averaged 14 +/- 3 pg/ml and was significantly suppressed to 6 +/- 1 pg/ml by high-sodium intake (n = 7). Consistent with the modulation of ANG II, plasma aldosterone significantly decreased from 41 +/- 8 to 8 +/- 3 ng/dl when sodium intake was elevated (n = 6). In a final set of experiments, the continuous infusion of ANG II (20 ng. kg(-1). min(-1)) led to a mild salt-sensitive increase in mean arterial pressure from 108 +/- 2 to 131 +/- 2 mmHg as sodium intake was varied from low to high (n = 7). In vehicle-infused mice, mean arterial pressure was unaltered from 109 +/- 2 mmHg when sodium intake was increased (n = 6). These studies indicate that the physiological suppression of circulating ANG II may be required to maintain a constancy of arterial pressure during alterations in sodium intake in normal mice.

Protection against ischemia: a physiological function of the renin-angiotensin system

The renin-angiotensin system (RAS) is involved in a complex mechanism that serves to preserve the blood supply to organs so that they can maintain cellular function. Angiotensin II exerts this effect, independently of the blood pressure generated, through two time-related events: a fast opening of the reserve collateral circulation and a much slower response of new vessel formation or angiogenesis. This effect is observed in rats with ligation of the abdominal aorta and in gerbils with abrupt or progressive unilateral carotid artery ligation. Inhibition of the angiotensin-converting enzyme (ACE) or the angiotensin II receptor represses this effect, and it appears that it is mediated through a non-AT1 receptor site of angiotensin II. Many tumors, both benign and malignant, express renin and angiotensin. It seems that the stimulating action of angiotensin II on angiogenesis could also be involved in preserving the blood supply to tumor cells. Administration of converting enzyme inhibitors increases survival and decreases tumor size in tumor-bearing rats. These observations support the hypothesis that the RAS, directly or indirectly, is involved in situations in which the restoration of blood supply is critical for the viability of cells and that it is present not only in normal but also in pathological conditions such as tumors. In view of the ubiquitous presence of renins and angiotensins, it is also likely to be involved in other conditions, such as inflammation, arthritis, diabetic retinopathy, and retrolental fibroplasia, among others in which angiogenesis is prominent. In addition, angiotensin II could be involved, through the counterbalance of the AT1 and AT2 receptors, in the rarefaction of blood vessels as an etiologic component of essential hypertension.

The effect of fetal hypoxia on adrenocortical function in the 7-day-old rat

Fetal hypoxia in late gestation is a common cause of postnatal morbidity. The purpose of the present study was to evaluate adrenal function in vivo and in vitro in 7-d-old rat pups previously exposed to normoxia or hypoxia (12% O2) during the last 2-3 d of gestation. Seven-day-old rats exposed to fetal hypoxia had a small, but significant decrease in plasma aldosterone despite no decreases in plasma ACTH or renin activity. There was a small (approx 20%) but significant decrease in the aldosterone and corticosterone response to cAMP in vitro in dispersed cells from 7-d-old pups exposed to fetal hypoxia. The aldosterone, corticosterone, and cAMP response to ACTH, however, was not altered by prior fetal hypoxia. There was also no effect of fetal hypoxia on steroidogenic enzyme expression or zonal dimension in 7-d-old rats. We conclude that fetal hypoxia in late gestation results in a subtle decrease in cAMP-stimulated steroidogenesis. Fetal hypoxia appears to have minimal effects on subsequent adrenal function in the neonatal rat.

Reduced renal mass hypertension, but not high salt diet, alters skeletal muscle arteriolar distensibility and myogenic responses

The effects of high salt diet and reduced renal mass hypertension (RRM-HT) on skeletal muscle arteriolar distensibility and myogenic responses were investigated in male Sprague-Dawley rats. Rats were enclosed in an air-tight box with the in situ cremaster muscle exteriorized and viewed via television microscopy. Normotensive rats were fed low salt (0.4% NaCl) or high salt (4.0% NaCl) diet and RRM-HT rats were fed high salt diet for 4-6 weeks. With the cremaster muscle superfused with either physiological salt solution (for myogenic responses) or Ca(2+)-free physiological salt solution (for arteriolar distensibility), box pressure (and therefore, intravascular pressure) was increased in 5 mm Hg increments to a maximum of +30 mm Hg. The myogenic responses of arterioles were not altered by high salt diet, but were enhanced with RRM-HT. Arteriolar distensibility was not affected by high salt diet, but was reduced in RRM-HT rats compared to either normotensive rat group. These data suggest that high salt diet does not significantly alter either myogenic responses or the distensibility of rat cremasteric arterioles. However, RRM-HT enhances myogenic responses of these vessels while reducing arteriolar distensibility. The impact of these effects must be taken into account when interpreting data describing alterations in skeletal muscle microvessel reactivity for animals on high salt diet or with RRM hypertension.

Selective potentiation of angiotensin-induced constriction of skeletal muscle resistance arteries by chronic elevations in dietary salt intake

Sprague-Dawley rats were fed either a high-salt (HS, 4.0% NaCl) or a low-salt (LS, 0.4% NaCl) diet for 3 days (short-term) or 4-8 weeks (chronic). Vasoconstrictor responses to angiotensin II and norepinephrine were determined in isolated skeletal muscle resistance arteries and in distal arterioles of the in situ cremaster muscle. Myogenic responses to increases in transmural pressure were also assessed in skeletal muscle resistance arteries of animals on high- or low-salt diets. Chronic (but not short-term) HS diet selectively potentiated angiotensin II-induced constriction of skeletal muscle resistance arteries relative to vessels from LS controls. Myogenic responses and norepinephrine-induced constriction of resistance arteries were unaffected by either chronic or short-term HS diet. Constriction of cremasteric arterioles in response to angiotensin II and norepinephrine was unaffected by chronic or short-term elevations in dietary salt intake. These data suggest that chronic elevations in dietary salt intake lead to a selective increase in the constriction of skeletal muscle resistance arteries to angiotensin II that may allow these vessels to continue to regulate their tone in response to this peptide, despite the suppression of angiotensin II that occurs with high-salt diet.

Acute elevations in salt intake and reduced renal mass hypertension compromise arteriolar dilation in rat cremaster muscle

Alterations in arteriolar reactivity to dilator agonists were assessed in the skeletal muscle microcirculation of normotensive male Sprague-Dawley rats fed either high- (4% NaCl; HS) or low- (0. 4% NaCl; LS) salt diets and in reduced renal mass hypertensive rats (RRM-HT) on a high-salt diet for 3 days. An in situ cremaster muscle preparation was superfused with physiological salt solution, transilluminated, and viewed via television microscopy. A videomicrometer was used to measure changes in diameter of distal arterioles in response to increasing concentrations of acetylcholine (ACH), iloprost (ILO), cholera toxin (CT), forskolin (FOR), and sodium nitroprusside (SNP). Arteriolar dilation in response to ACH, ILO, and CT was significantly reduced in both HS and RRM-HT rats, while responses to FOR and SNP were decreased in RRM-HT rats only. The maximum dilation of the arterioles (determined during superfusion of the muscle with Ca2+-free solution containing 10(-4) M adenosine) was similar in the normotensive control animals on LS and HS diets, but was reduced in the RRM-HT rats, suggesting that early anatomic remodeling of the vessel wall may be occurring with RRM-HT. We conclude that arteriolar reactivity to endothelium-dependent and -independent vasodilator agonists is impaired as early as 3 days after the development of RRM hypertension or commencement of a high-salt diet in normotensive rats. Structural remodeling of the arteriolar wall, although becoming evident in the hypertensive rats, takes longer to develop than the impaired vasodilator reactivity.

Reappraisal of the pathogenesis and consequences of hyperuricemia in hypertension, cardiovascular disease, and renal disease

An elevated uric acid level is associated with cardiovascular disease. Hyperuricemia is predictive for the development of both hypertension and coronary artery disease; it is increased in patients with hypertension, and, when present in hypertension, an elevated uric acid level is associated with increased cardiovascular morbidity and mortality. Serum uric acid level should be measured in patients at risk for coronary artery disease because it carries prognostic information. Hyperuricemia is caused by decreased renal excretion. In this article, we suggest that this may be mediated by intrarenal ischemia with lactate generation and the inhibition of the secretion of urate by the anion-exchange transport system. The possibility that hyperuricemia directly contributes to cardiovascular or renal disease needs to be reconsidered. Although hyperuricemia is associated with a number of cardiovascular or renal risk factors, several studies have found uric acid level to be independently associated with increased mortality by multivariate analysis. If hyperuricemia is directly toxic, the most likely site is the kidney. Chronic hyperuricemia is strongly associated with chronic tubulointerstitial disease, and many of these patients have decreased renal function. Although it is possible that the hyperuricemia could simply be the consequence of the renal disease, further studies are necessary to rule out a pathogenic role for uric acid in the development of renal disease and salt-dependent hypertension.

Effects of daily sodium intake and ANG II on cortical and medullary renal blood flow in conscious rats

Implanted optical fibers and laser-Doppler flow measurement techniques were used for the sequential measurement of regional renal blood flow in conscious rats to determine the effects of an increase of daily NaCl intake on the renal cortical blood flow and blood flow to the outer and inner medulla. Cortical blood flow was increased significantly (32%) by the second day when NaCl intake was increased from 1 to 7 meq/day and was increased further (50%) on the second day after a further elevation of NaCl intake to 13 meq/day. Blood flow to the outer and inner medulla was not changed as NaCl intake was elevated. The increase in renal cortical flow was closely associated with significant reductions in circulating concentrations of ANG II from 31 to 16 pg/ml. Rats given a continuous infusion of nonpressor does of ANG II (5.0 ng.kg(-1).min-1) to maintain constant plasma concentrations of ANG II as sodium intake was increased exhibited no increase of cortical flow. We conclude that reductions of plasma ANG II associated with incremental increases of daily sodium intake result in a rise of renal cortical flow. The elevated blood flow to the renal cortex may enhance sodium excretion and contribute to long-term sodium homeostasis.