Endothelial dysfunction in the aorta of transgenic rats harboring the mouse Ren-2 gene

Both central sympathoexcitation and peripheral angiotensin II-dependent vasoconstriction contribute to hypertension development in immature heterozygous Ren-2 transgenic rats

Recently, we demonstrated that chronic blockade of the renin-angiotensin system (RAS) lowered the blood pressure (BP) of adult Ren-2 transgenic rats (TGR) mainly through the attenuation of central sympathoexcitation. However, the participation of central and peripheral mechanisms in the development of high BP in immature TGR remains unclear. In the present study, 6-week-old heterozygous TGR males were chronically treated with intracerebroventricular (ICV) or intraperitoneal (IP) infusions of the AT₁ receptor inhibitor losartan (1 or 2 mg/kg/day) for 4 weeks. The influence of these treatments on sympathetic- and angiotensin II-dependent BP components (BP response to pentolinium or captopril, respectively) as well as on BP response to exogenous angiotensin II were determined to evaluate the participation of central and peripheral RAS in hypertension development. Chronic IP losartan administration (1 or 2 mg/kg/day) lowered the BP of immature TGR by reducing both sympathetic and angiotensin II-dependent BP components. The central action of IP-administered losartan was indicated by a reduced BP response to acute ICV angiotensin II injection. Chronic ICV administration of a lower losartan dose (1 mg/kg/day) reduced only the sympathetic BP component, whereas a higher ICV administered dose (2 mg/kg/day) was required to influence the angiotensin II-dependent BP component. Accordingly, chronic ICV losartan administration of 2 mg/kg/day (but not 1 mg/kg/day) attenuated the BP response to acute intravenous angiotensin II application. In conclusion, central sympathoexcitation seems to play an important role in hypertension development in immature TGR. Central sympathoexcitation is highly susceptible to inhibition by low doses of RAS-blocking agents, whereas higher doses also affect peripheral angiotensin II-dependent vasoconstriction.

Animal Models of Hypertension: A Scientific Statement From the American Heart Association

Hypertension is the most common chronic disease in the world, yet the precise cause of elevated blood pressure often cannot be determined. Animal models have been useful for unraveling the pathogenesis of hypertension and for testing novel therapeutic strategies. The utility of animal models for improving the understanding of the pathogenesis, prevention, and treatment of hypertension and its comorbidities depends on their validity for representing human forms of hypertension, including responses to therapy, and on the quality of studies in those models (such as reproducibility and experimental design). Important unmet needs in this field include the development of models that mimic the discrete hypertensive syndromes that now populate the clinic, resolution of ongoing controversies in the pathogenesis of hypertension, and the development of new avenues for preventing and treating hypertension and its complications. Animal models may indeed be useful for addressing these unmet needs.

Ultrastructure Study of Transgenic Ren2 Rat Aorta - Part 1: Endothelium and Intima

BACKGROUND: The renin-angiotensin-aldosterone system plays an important role in the development and progression of hypertension and accelerated atherosclerosis (atheroscleropathy) associated with the cardiorenal metabolic syndrome and type 2 diabetes mellitus. Additionally, the renin-angiotensin-aldosterone system plays an important role in vascular-endothelial-intimal cellular and extracellular remodeling. METHODS: Thoracic aortas of young male transgenic heterozygous (mRen2)27 (Ren2) rats were utilized for this ultrastructural study. This lean model of hypertension, insulin resistance and oxidative stress harbors the mouse renin gene with increased local tissue (aortic) levels of angiotensin II and angiotensin type 1 receptors and elevated plasma aldosterone levels. RESULTS: The ultrastructural observations included marked endothelial cell retraction, separation, terminal nuclear lifting, adjacent duplication, apoptosis and a suggestion of endothelial progenitor cell attachment. The endothelium demonstrated increased caveolae, microparticles, depletion of Weibel-Palade bodies, loss of cell-cell and basal adhesion hemidesmosome-like structures, platelet adhesion and genesis of subendothelial neointima. CONCLUSION: These observational ultrastructural studies of the transgenic Ren2 vasculature provide an in-depth evaluation of early abnormal remodeling changes within conduit-elastic arteries under conditions of increased local levels of angiotensin II, oxidative stress, insulin resistance and hypertension.

Rapid Endocytosis of Copper-Zinc Superoxide Dismutase into Human Endothelial Cells: Role for Its Vascular Activity

Cytosolic CuZn-SOD (SOD1) is a dimeric, carbohydrate-free enzyme with a molecular weight of about 32 kDa and also circulates in human blood plasma. Due to its molecular mass it has been believed that the enzyme cannot penetrate the cell membrane. Here we report that rapid endocytosis of FITC-CuZn-SOD into human endothelial cells occurs within 5 min. Moreover, relaxation of rat aortic rings in response to CuZn-SOD is associated with a lag time of 45-60 s and only observed in the presence of intact endothelial cells. The results indicate acute and rapid endothelial cell endocytosis of CuZn-SOD, possibly via activation of a receptor-mediated pathway. Intracellular uptake via endocytosis may contribute to the vascular effects of CuZn-SOD, including vasodilation, and is likely to play a role in regulation of vascular tone and diseases such as atherosclerosis.

Angiotensin-(1-7) is involved in the endothelium-dependent modulation of phenylephrine-induced contraction in the aorta of mRen-2 transgenic rats

1. The contribution of the local vascular production of angiotensin-(1-7) [Ang-(1-7)] to the control of alpha-adrenergic-induced contractions in the aorta of Sprague-Dawley (SD) and TGR(mRen-2)27 [mRen-2] rats was studied. 2. In mRen-2 rats, contractile responses to phenylephrine were diminished as compared to control SD rats in endothelium containing but not in endothelium-denuded vessels. L-NAME increased contractile responses to phenylephrine in mRen-2 rats and, after nitric oxide synthase blockade, responses to phenylephrine became comparable in both strains. 3. Inhibition of angiotensin-converting enzyme (ACE) by captopril potentiated contractile responses in mRen-2 rats and diminished contractile responses in SD rats, both effects being dependent on the presence of a functional endothelium. The effect of captopril in mRen-2 rats was abolished in vessels pre-incubated with Ang-(1-7). 4. Blockade of Ang-(1-7) and bradykinin (BK) receptors by A-779 and HOE 140 respectively, increased phenylephrine-induced contraction in mRen-2, but not in SD rats. This effect was seen only in endothelium-containing vessels. 5. Angiotensin II AT(1) and AT(2) receptor blockade by CV 11974 and PD 123319 did not affect the contractile responses to phenylephrine in aortas of transgenic animals but diminished the response in SD rats. This effect was only seen in the presence of a functional endothelium. 6. It is concluded that the decreased contractile responses to phenylephrine in aortas of mRen-2 rats was dependent on an intact endothelium, the local release and action of Ang-(1-7) and bradykinin.

Involvement of cysteinyl leukotrienes in angiotensin II-induced contraction in isolated aortas from transgenic (mRen-2)27 rats

OBJECTIVES

We have previously reported that 5-lipoxygenase-derived products, and particularly the cysteinyl leukotrienes (CysLTs), were involved in angiotensin II (Ang II)-induced contractions in isolated aortas from spontaneously hypertensive rats.

DESIGN

The aim of this study was to assess the role of CysLTs in the vascular response to Ang II in an Ang II-dependent model of hypertension, the (mRen-2)27 transgenic rats (TGs).

METHODS

Intact aortic rings from TG and normotensive Sprague-Dawley rats (SDs) were suspended in organ chambers for isometric tension development in response to Ang II. In addition, the release of CysLTs in response to Ang II (0.3 micromol/l) was measured by enzyme immunoassay.

RESULTS

In isolated aortas from TG rats, pretreatment with the 5-lipoxygenase inhibitor (AA861, 10 micromol/l) or the CysLT1 receptor antagonist (MK571, 1 micromol/l) significantly (P < 0.05) reduced Ang II-induced contractions by 52 and 42%, respectively. In addition, Ang II induced a 2.6-fold increase in CysLT release (pg/mg dry weight tissue: 58.3 +/- 17.9 (Ang II, n = 7) versus 22.5 +/- 5.9 (basal, n = 7) P < 0.05), which was inhibited by the AT1 receptor antagonist losartan (1 micromol/l). In contrast, in aortas from SD rats, pretreatment with AA861 or MK571 did not alter Ang II-induced contraction and CysLT production remained unchanged after exposure to Ang II.

CONCLUSION

These data suggest that CysLTs are involved in the contractile responses to Ang II in isolated aortas from TG but not from SD rats.

Day-night variation in the in vitro contractility of aorta and mesenteric and renal arteries in transgenic hypertensive rats

TGR(mREN2)27 (TGR) rats develop severe hypertension and an inverted circadian blood pressure profile with peak blood pressure in the day-time rest phase. The present study investigated the in vitro responsiveness of different arteries of TGR rats during day and night. Twelve-week-old TGR rats and normotensive Sprague-Dawley (SPRD) controls, synchronized to 12h light, 12h dark (LD 12:12) (light 07:00-19:00), were killed at 09:00 (during rest) and 21:00 (during activity), and endothelium-dependent relaxation by acetylcholine and vascular contraction by angiotensin II were studied by measuring isometric force in ring segments of abdominal aorta and mesenteric and renal arteries. In SPRD rats, consistent day-night variation was found, with greater responses to angiotensin II during the daytime rest span. In TGR rats, biological time-dependent differences were found in the renal vasculature, but not in the aorta and mesenteric artery. Relaxation of SPRD rat aorta and mesenteric artery by acetylcholine was greater at 09:00, whereas in TGR rats, day-night variation was absent (mesenteric artery) or inverted (aorta). In conclusion, based on the study of two time points, day-night variation in vascular contractility of aorta and mesenteric artery is blunted in TGR rats, whereas renal artery segments showed an unchanged day-night pattern compared to SPRD controls.

Endothelial dysfunction and xanthine oxidoreductase activity in rats with human renin and angiotensinogen genes

We examined whether xanthine oxidoreductase (XOR), a hypoxia-inducible enzyme capable of generating reactive oxygen species, is involved in the onset of angiotensin (Ang) II-induced vascular dysfunction in double-transgenic rats (dTGR) harboring human renin and human angiotensinogen genes. In 7-week-old hypertensive dTGR, the endothelium-mediated relaxation of noradrenaline (NA)-precontracted renal arterial rings to acetylcholine (ACh) in vitro was markedly impaired compared with Sprague Dawley rats. Preincubation with superoxide dismutase (SOD) improved the endothelium-dependent vascular relaxation, indicating that in dTGR, endothelial dysfunction is associated with increased superoxide formation. Preincubation with the XOR inhibitor oxypurinol also improved endothelium-dependent vascular relaxation. The endothelium-independent relaxation to sodium nitroprusside was similar in both strains. In dTGR, serum 8-isoprostaglandin F(2alpha), a vasoconstrictor and antinatriuretic arachidonic acid metabolite produced by oxidative stress, was increased by 100%, and the activity of XOR in the kidney was increased by 40%. Urinary nitrate plus nitrite (NO(x)) excretion, a marker of total body NO generation, was decreased by 85%. Contractile responses of renal arteries to Ang II, endothelin-1 (ET-1), and NA were decreased in dTGR, suggesting hypertension-associated generalized changes in the vascular function rather than a receptor-specific desensitization. Valsartan (30 mg/kg PO for 3 weeks) normalized blood pressure, endothelial dysfunction, and the contractile responses to ET-1 and NA. Valsartan also normalized serum 8-isoprostaglandin F(2alpha) levels, renal XOR activity, and, to a degree, NO(x) excretion. Thus, overproduction of Ang II in dTGR induces pronounced endothelial dysfunction, whereas the sensitivity of vascular smooth muscle cells to nitric oxide is unaltered. Ang II-induced endothelial dysfunction is associated with increased oxidative stress and vascular xanthine oxidase activity.