Abnormalities in the hypothalamo-pituitary-adrenal axis in spontaneously hypertensive rats during development of hypertension

Abnormalities in the hypothalamo-pituitary-adrenal axis in spontaneously hypertensive rats (SHR) during development of hypertension were investigated using in vivo and in vitro methods. Plasma ACTH responses to hemorrhage and ether stress were significantly smaller in 7-week-old SHR than in age-matched Wistar-Kyoto rats (WKY), while plasma corticosterone baseline levels and its response to stress were greater in SHR than in WKY. There was no significant difference in the plasma ACTH response to ether stress between bilaterally adrenalectomized SHR and WKY replaced with a 25% corticosterone pellet for 6 days. Adrenalectomy prevented the development of hypertension in SHR; however, corticosterone replacement restored hypertension. Plasma ACTH showed a smaller response to iv CRH injection in SHR than in WKY, while the ACTH response to arginine vasopressin was not different between SHR and WKY. CRH concentrations in the median eminence, posterior pituitary, and cerebral cortex were lower in SHR than in WKY, while the CRH concentration in the median eminence was not different in SHR and WKY when they were adrenalectomized with or without corticosterone replacement. Basal in vitro CRH release from hypothalamic tissue was reduced in SHR, while CRH release in response to 56 mM KCl was not different in SHR and WKY. These results suggest that adrenocortical function is enhanced in young SHR, that reduced ACTH response to stress and exogenous CRH in SHR may be ascribed to higher plasma corticosterone levels, and that corticosterone is essential for the development of hypertension in SHR.     

Angiotensin II type 1 receptor blocker reduces monocyte adhesion to endothelial cells in spontaneously hypertensive rats

Monocyte adhesion to arterial endothelial cells is the initial step in atherosclerosis. Whereas angiotensin II is known to elicit leukocyte adhesion, it is not clear whether blockade of the angiotensin II receptor signaling reduces monocyte adhesion to endothelial cells beyond its antihypertensive action. This study compared the effect of two different antihypertensive drugs on monocyte adhesion to thoracic aorta endothelium in spontaneously hypertensive rats (SHR): the angiotensin II receptor blocker, valsartan (20 mg . kg(-1) . day(-1)) and the vasodilator, hydralazine (0.75 mg . kg(-1) . day(-1)). The effects were quantitated in vivo using an enface method that optimizes the observation of endothelial surfaces after immunohistochemical staining for CD68. Both agents significantly and comparably reduced blood pressure over 4-week treatment course. Both valsartan and hydralazine profoundly reduced monocyte adhesion compared with nontreated controls, with valsartan having a modestly more reductive effect. Both agents also reduced the intima and medial thickening with valsartan reducing the mean thickness modestly more than hydralazine. Our data confirms that the reduction of blood pressure is effective method to reduce monocyte adhesion. Also, our date demonstrates that valsartan has a modest beneficial effect on monocyte adhesion to endothelial cells and arterial intima-medial vessel thickening beyond its action as an antihypertensive agent.     

Angiotensin converting enzyme inhibition improves cardiac neuronal uptake of noradrenaline in spontaneously hypertensive rats

OBJECTIVES: It has been shown that a diminished sympathetic activity contributes to the hypotensive and cardioprotective actions of angiotensin converting enzyme (ACE) inhibitors (ACEI). Besides an inhibition of central sympathetic tone and peripheral noradrenaline release, we hypothesized that the interactions of ACEI with the sympathetic system may include a modulation of neuronal catecholamine uptake by peripheral nerves. DESIGN: We investigated the influence of fosinopril on noradrenergic uptake into cardiac neurones in vitro and in vivo in acute and chronic models. METHODS AND RESULTS: Acute administration of fosinoprilat to isolated perfused rat hearts increased the extraction of [3H]-noradrenaline from the perfusate by 39%. Treatment (14 days) of spontaneously hypertensive rats (SHR) with fosinopril (20 mg/kg per day) enhanced the cardiac uptake of i.v. administered [3H]-noradrenaline by 28%. The endogenous left ventricular content of noradrenaline was increased by 49% after an antihypertensive treatment of SHR with fosinopril (20 mg/kg per day). Identical increases in cardiac noradrenaline stores (53%) were observed in SHR treated with a blood pressure ineffective dose of fosinopril (0.2 mg/kg per day). The myocardial content of adrenaline was increased in parallel to noradrenaline after both dose regimes. CONCLUSIONS: It is concluded that ACEI increases neuronal uptake of catecholamines in SHR in a blood pressure-independent manner. This effect occurs acutely and is independent of central sympathetic activity. Therefore, we hypothesize that ACEI modulate the activity of the cardiac noradrenaline transporter by direct activation. The improved uptake of noradrenaline may contribute to the antihypertensive and cardioprotective effects of ACEI.     

Arginase inhibition reduces endothelial dysfunction and blood pressure rising in spontaneously hypertensive rats

OBJECTIVE: A decrease in nitric oxide (NO) bioavailability has been proposed to contribute to endothelial dysfunction and increased peripheral resistances during essential arterial hypertension. Given that arginine is a substrate for both arginase and NO synthase, arginase activity may be a critical factor in NO bioavailability. To test this hypothesis, we evaluated the effects of the arginase inhibitor alpha-difluoromethylornithine (DFMO) in spontaneously hypertensive rats (SHR). METHODS: Vascular reactivity experiments were performed on thoracic aortic rings from 10-week-old SHR and their normotensive counterparts, Wistar-Kyoto (WKY) rats. Blood pressure was measured by the tail-cuff method. DFMO treatment (30 mg/kg daily in drinking water) was started in 5-week-old SHR and maintained for 5 weeks. Aortic arginase I and arginase II expression as well as arginase activity were evaluated by western blotting and the spectrophotometric method, respectively. RESULTS: DFMO (1.2 x 10 mol/l) enhanced the vascular response to acetylcholine both in SHR (+24%, P < 0.01) and WKY rats (+12%, P < 0.01), and reversed the effects of the NO synthase inhibitor N-nitro-L-arginine-methyl-ester. The vasorelaxant response to sodium nitroprusside on endothelium-denuded rings was not affected by DFMO, neither in SHR nor in WKY rats. In SHR, DFMO prevented the increase in blood pressure and improved the response of aortic rings to acetylcholine. Finally, as compared with WKY rats, SHR exhibited increased expression of vascular arginase I (+72%, P < 0.05) and arginase II (+91%, P < 0.05) as well as increased arginase activity (+26%, P < 0.05). CONCLUSIONS: Our results showed that arginase inhibition reduced endothelial dysfunction and blood pressure rising in SHR.     

血管紧张素Ⅱ1型受体拮抗剂氯沙坦降低自发性高血压大鼠血管的肌源性紧张度的作用

目的:观察血管紧张素Ⅱ(AngⅡ)1型受体阻断剂氯沙坦对自发性高血压(SH)大鼠大脑中动脉血管肌源性紧张度的影响。方法:16只SHWistar大鼠随机分为SH组(n=8)和30 mg/(kg.d)氯沙坦干预(LI)组(n=8),另以8只正常WKY大鼠作为对照组,10周后,采用压力型小动脉测量仪在含2.5 mmol/L Ca2+或无Ca2+的生理盐溶液中,测量不同管腔压力下血管的主动态管径(active diameter,Da)和被动态管径(passive diameter,Dp)。肌源性紧张度以(Dp-Da)/Dp×100%表示。结果:SH大鼠大脑中动脉血管的肌源性紧张度同对照组相比升高67.9%,每天通过灌胃给予30 mg/(kg.d)的氯沙坦能够使SH大鼠大脑中动脉血管的肌源性紧张度降低24.9%。结论:SH大鼠大脑中动脉血管的肌源性紧张度同对照组相比显著增加,每天给予30 mg/(kg.d)的氯沙坦能够防止这种改变,说明肾素-血管紧张素-醛固酮系统参加了SH大鼠血管功能的重建过程,AngⅡ1型受体是其中重要的信号转导通路。     

Angiotensin II-induced proliferation of aortic myocytes in spontaneously hypertensive rats

In order to determine whether the morphological modifications observed in arterial media of spontaneously hypertensive rats (SHR) could be induced by an abnormal response of the smooth muscle cells to vasoactive agents, we studied the action of angiotensin (Ang) II on cultured aortic smooth muscle cells from both SHR and Wistar-Kyoto rats (WKY). Under our experimental conditions, Ang II exerts a mitogenic action on SHR cells, whereas its effect is very weak on WKY cells. Phospholipase C activation and c-fos and c-myc proto-oncogene expressions induced by Ang II are considerably enhanced in SHR cells, and these abnormalities may be linked to an increased number of Ang II receptors.     

Atorvastatin reduces oxidative stress in the rostral ventrolateral medulla of stroke-prone spontaneously hypertensive rats

Previously, we demonstrated that atorvastatin has sympatho-inhibitory effects with the upregulation of nitric oxide synthase in the brain in stroke-prone spontaneously hypertensive rats (SHRSP), and that reactive oxygen species in the rostral ventrolateral medulla (RVLM), where the vasomotor center is located, mediate the sympatho-excitatory effect. The aim of the present study was to determine if atorvastatin reduces oxidative stress in the RVLM of SHRSP along with the sympatho-inhibitory effect. SHRSP and Wistar-Kyoto (WKY) rats received standard feed with atorvastatin (50mg/kg per day) or standard feed for 30 days. Systolic blood pressure and heart rate were evaluated using the tail-cuff method. Urinary norepinephrine excretion was measured for 24 hours. After 30 days in SHRSP, blood pressure and urinary norepinephrine excretion were significantly lower in the atorvastatin group than in the control group. Thiobarbituric acid-reactive substance (TBARS) levels in the RVLM tissue obtained using the micropunch technique were used as measures of oxidative stress. Prior to the treatment, TBARS levels in the RVLM of SHRSP were significantly higher than those of WKY. After 30 days, TBARS levels in the RVLM of SHRSP were significantly lower in the atorvastatin group than in the control group. After 30 days in WKY, however, there were no differences in blood pressure, urinary norepinephrine excretion, and TBARS levels between the atorvastatin and control groups. These results suggest that atorvastatin reduces oxidative stress in the RVLM of SHRSP, which might contribute to the sympatho-inhibitory effects of atorvastatin in SHRSP.     

Lovastatin reduces renal vascular reactivity in spontaneously hypertensive rats

We have reported that lovastatin attenuates the development of hypertension in spontaneously hypertensive rats (SHR). The fall in arterial pressure is associated with an elevation in renal medullary blood flow, normalization of the pressure-natriuresis relationship, and diminished hypertrophy of renal arterioles. However, the mechanism by which lovastatin alters renal vascular tone is unknown. The present study examined the effects of lovastatin on renal vascular tone and the expression of G proteins. Four-week–old SHR were chronically treated with lovastatin (20 mg/kg/day) or vehicle by gavage for 4 weeks. At the end of the study, mean arterial pressure averaged 131 ± 4 (n = 5) and 160 ± 4 mm Hg (n = 6) in lovastatin- and vehicle-treated SHR, respectively. Renal arterioles isolated from lovastatin-treated SHR were significantly less responsive to norepinephrine and vasopressin than those obtained from vehicle-treated rats (ED₅₀: 5.0 v 1.8 × 10⁻⁷ mol/L for norepinephrine, and 8.0 v 5.2 × 10⁻¹⁰ mol/L for vasopressin). The fall in renal vascular reactivity in lovastatin-treated SHR was associated with reduced levels of ras and rho proteins in renal arterioles, whereas the expressions of heterotrimeric G proteins (G_s, G_q, G_i) were similar in renal arterioles from vehicle- and lovastatin-treated SHR. Overnight culture of renal arterioles with media containing lovastatin also diminished the expression of ras and rho proteins and the response to vasoconstrictors. These findings indicate that lovastatin diminishes the response to vasoconstrictors and the expression of small G proteins in the renal vasculature of SHR and suggest that a fall in the levels of ras and rho proteins in these vessels may contribute to the antihypertensive effects of lovastatin.     

Angiotensin type 2 receptor agonist compound 21 reduces vascular injury and myocardial fibrosis in stroke-prone spontaneously hypertensive rats

-Angiotensin type 2 receptor-mediated effects of angiotensin II appear to counteract many of the effects mediated via the angiotensin type 1 receptor. Compound 21 (C21), a selective angiotensin type 2 receptor agonist, has demonstrated beneficial effects on cardiac function after myocardial infarction in rodents. We hypothesized that C21 alone or in combination with an angiotensin type 1 receptor antagonist would blunt the development of hypertension and vascular damage in stroke-prone spontaneously hypertensive rats. Six-week-old stroke-prone spontaneously hypertensive rats received C21 (1 mg/kg per day), the angiotensin type 1 receptor antagonist losartan (10 mg/kg per day), C21 plus losartan, or vehicle PO for 6 weeks. Systolic blood pressure was lower in losartan and C21-losartan combination groups (P<0.001). Endothelium-dependent relaxation was enhanced (P<0.001) in the C21-losartan combination group at lower acetylcholine concentrations. C21 or C21-losartan combination reduced vascular stiffness, aortic medial and myocardial interstitial collagen content, and aortic fibronectin (P<0.05). C21 and losartan decreased the expression of 2 genes associated with cardiac hypertrophy, myosin heavy chain-β (myh7) by 30 to 50%, and α-skeletal muscle actin by 30% to 35% (P<0.05). C21-losartan combination caused an additional 40% reduction in myh7 compared with C21 (P<0.01). Aortic superoxide generation was reduced equally by the 3 treatments (P<0.001). Monocyte/macrophage infiltration in the aorta and kidney (P<0.001) and T-lymphocyte infiltration in the renal cortex (P<0.05) were lowered similarly by the 3 treatments. These data suggest that C21 alone or in combination with losartan may improve endothelial function and vascular composition and mechanics by reducing oxidative stress, collagen content, fibronectin, and inflammatory cell infiltration in stroke-prone spontaneously hypertensive rats.     

Angiotensin II binding sites in the superior cervical ganglia of spontaneously hypertensive and Wistar-Kyoto rats after preganglionic denervation

In the superior cervical ganglia (SCG) binding site density of angiotensin II (ANG II) was higher in adult spontaneously hypertensive rats (SHR) (571 +/- 29 fmol/mg protein) compared to that in the adult Wistar-Kyoto rat (WKY) (375 +/- 9 fmol/mg protein, P less than 0.05). The ANG II binding density was significantly decreased in the SCG of SHR (-59%, P less than 0.01) and of WKY (-39%, P less than 0.05) after unilateral preganglionic denervation (operated v sham-operated ganglia). Part of the binding sites in the superior cervical ganglia may be present in or be associated to preganglionic nerves, and the number of these sites is higher in SHR.     

Angiotensin II type 1 receptor blocker irbesartan ameliorates vascular function in spontaneously hypertensive rats regardless of oestrogen status

BACKGROUND : Angiotensin II type 1 (AT1) receptor overexpression may play a decisive role in endothelial dysfunction during oestrogen deficiency in spontaneously hypertensive rats (SHRs). Similarly, exaggerated production of angiotensin II and enhanced expression of AT1 receptor have been reported in vessels of SHRs compared with normotensive rats. OBJECTIVE : To test the hypothesis that antihypertensive treatment with the AT1 receptor antagonist, irbesartan, could not only decrease blood pressure but also ameliorate endothelial dysfunction associated with both hypertension and oestrogen deficiency. METHODS : Ovariectomized and sham-ovariectomized SHRs were treated with 50 mg/kg irbesartan per day, administered with chow for 30 weeks. Sham-ovariectomized and ovariectomized rats receiving no treatment were used as control groups. At the end of the treatment period, the vascular reactivity of aortic rings was studied. RESULTS : In the irbesartan-treated groups, vasoconstriction induced by Nomega-nitro-l-arginine methyl ester (l-NAME) was increased and the response to phenylephrine exhibited greater potentiation in the presence of l-NAME, demonstrating a greater availability of basal nitric oxide in these groups. In addition, chronic treatment with irbesartan similarly enhanced the responsiveness of aortic rings from ovariectomized or sham-ovariectomized rats to acetylcholine and sodium nitroprusside. Incubation with indomethacin did not significantly alter acetylcholine- and sodium nitroprusside-induced relaxations in the irbesartan-treated rats. However, relaxations induced by acetylcholine and sodium nitroprusside in aortic rings from non-treated rats were significantly greater in the presence of indomethacin. CONCLUSION : Our data suggest that irbesartan enhances basal nitric oxide availability and ameliorates vascular relaxations in SHRs, by decreasing the production of cyclooxygenase-dependent contracting factors in smooth muscle cells, regardless of oestrogen status.     

Converting enzyme inhibition normalizes QT interval in spontaneously hypertensive rats

We quantified the repolarization time (so-called QT interval) in a rat, an animal species that does not show a well-characterized T wave on surface ECG. We used spontaneously hypertensive rats (SHR) and converting enzyme inhibition to demonstrate a reversible increase in QT interval in pressure-overloaded hearts in the absence of ischemia. An implanted telemetry system recording ECG data in freely moving rats was used to automatically calculate the RR interval. The QT duration was manually determined by use of a calibrated gauge, and a time-frequency domain analysis was used to evaluate heart rate variability. Left ventricular mass was sequentially assessed by echocardiography. Before treatment, 12-month-old SHR had higher left ventricular mass, QT and RR intervals, and unchanged heart rate variability compared with age-matched Wistar rats. A 2-month converting enzyme inhibition treatment with trandolapril reduces systolic blood pressure, left ventricular mass, and QT interval. The RR interval and heart rate variability remains unchanged. There is a positive correlation between the QT interval and left ventricular mass. The SHR is suitable for longitudinal studies on the QT interval. Thus, the detection of the QT interval reflects the phenotypic changes that occur during mechanical overload and, on the basis of these criteria, allows an in vivo determination of the adaptational process.     

Inhibition of oxidative stress in rostral ventrolateral medulla improves impaired baroreflex sensitivity in stroke-prone spontaneously hypertensive rats

Reactive oxygen species (ROS) in rostral ventrolateral medulla (RVLM) of brainstem contribute to sympathoexcitation and are critically involved in the pathogenesis of hypertension. Baroreflex sensitivity (BRS) is a valuable prognostic parameter of the autonomic nervous system, and is impaired in hypertension. The aim of the present study was to determine whether or not a chronic reduction of ROS in the RVLM improves impaired BRS in hypertensive rats. We transfected adenovirus vectors encoding either manganese superoxide dismutase (AdMnSOD) or β-galactosidase (AdLacZ) into the RVLM of stroke-prone spontaneously hypertensive rats (SHRSP). We measured BRS using the spontaneous sequence method. BRS was significantly lower in SHRSPs than in Wistar-Kyoto rats. In the AdMnSOD-transfected SHRSP, blood pressure, heart rate, and sympathetic nervous system activation were significantly decreased from day 5 after the gene transfer. BRS in the AdMnSOD-transfected SHRSP was significantly increased from day 4 after the gene transfer with the reduction of ROS in the RVLM. Furthermore, in the AdMnSOD-transfected SHRSP, intravenous infusion of atropine dramatically decreased BRS. In contrast, in the AdLacZ-transfected SHRSP, atropine did not decrease BRS. These results suggest that chronic reduction of ROS in the local RVLM improves the impaired BRS in SHRSP through inhibition of the sympathetic component.     

不可预见刺激对大鼠行为学及HPA轴功能的影响

目的 探讨慢性轻度不可预见刺激(CUMS)对大鼠行为学和下丘脑-垂体-肾上腺皮质(HPA)轴功能的影响及其机制.方法 20只SD雄性大鼠随机分为正常对照组与模型组.模型组大鼠给予孤养加慢性轻度不可预见性刺激,用糖水消耗实验和高架十字迷宫检测大鼠行为学改变;试剂盒测定血清丙二醛(MDA)水平与超氧化物歧化酶(SOD)活性,以观察机体氧化应激能力改变;放射免疫法分析地塞米松抑制实验中血清皮质酮(corticosterone,CORT)浓度;免疫组化SP法检测下丘脑促肾上腺皮质激素释放因子(corticotrophin-releasing factor,CRF)表达变化.结果 与正常对照组相比,模型组大鼠糖水消耗量明显下降,在高架迷宫开放臂内的活动及停留时间亦明显减少,血清MDA水平增加,SOD活力减弱,CORT水平显著升高,下丘脑CRF表达明显增强.结论 CUMS致大鼠产生抑郁行为,其发生机制至少部分与体内自由基水平升高及HPA轴功能受损有关.     

The variability of baroreflex sensitivity in juvenile, spontaneously hypertensive rats

In this study the baroreflex sensitivity of conscious, juvenile, spontaneously hypertensive rats (SHRs) was compared. The study population consisted of 19 eight-week-old male SHRs. The baroreflex sensitivity was quantified as the derivative of the variation in heart rate (HR) and the variation of mean arterial pressure (baroreflex sensitivity = ΔHR/ΔMAP). MAP was manipulated with sodium nitroprusside (SNP) and phenylephrine (PHE), administered via an inserted cannula in the right femoral vein. The SHRs were divided into four groups: (1) low bradycardic baroreflex (LB) where the baroreflex gain (BG) was between 0 and -1 bpm/mmHg with PHE; (2) high bradycardic baroreflex (HB), where the BG was < -1 bpm/mmHg with PHE; (3) low tachycardic baroreflex (LT) where the BG was between 0 and 3 bpm/mmHg with SNP; (4) high tachycardic baroreflex (HT) where the BG was > 3 bpm/mmHg with SNP. We noted that 36.8% of the rats presented with an increased bradycardic reflex, while 27.8% demonstrated an attenuated tachycardic reflex. No significant alterations were noted regarding the basal MAP and HR. There were significant differences in the baroreflex sensitivity between SHRs in the same laboratory. One should be careful when interpreting studies employing the SHR as a research model.