Blood pressure-related changes of endothelium-dependent relaxation in the aorta from SHRSP at developmental ages of hypertension

Impaired hemodynamics and endothelial vasomotor function via endoperoxide-mediated vasoconstriction in the carotid artery of spontaneously hypertensive rats

The fact that endothelium removal increases diameter and compliance in the common carotid artery (CCA) of spontaneously hypertensive rats (SHR) and that improving CCA endothelium-dependent vasorelaxation has been shown to normalize a reduced systolic blood flow through the SHR CCA compared with normotensive Wistar-Kyoto rats (WKY) suggests that endothelial vasomotor dysfunction may be linked to altered large artery hemodynamics in hypertension. The experiments herein were designed to further investigate WKY and SHR CCA hemodynamics and endothelium-dependent vasomotor functions. It was hypothesized that CCA blood flow and conductance would be reduced throughout the cardiac cycle in SHR and that endothelium-dependent contractile activity would impair SHR CCA vasorelaxation. We report that mean, maximal systolic, and diastolic blood flow was reduced in SHR vs. WKY CCA, as was vascular conductance. Pressure was augmented in SHR CCA and accompanied by late systolic flow augmentation so that total flow during systole was indeed no different between strains, possibly explained by earlier lower body wave reflection. While ACh stimulation in isolated precontracted WKY CCA caused a robust nitric oxide (NO)-mediated vasorelaxation, endothelium-dependent, cyclooxygenase (COX)-mediated contractile activity stimulated by high ACh concentration impaired NO- and non-NO/non-COX-mediated vasorelaxation in precontracted SHR CCA. In quiescent CCA, this endothelium-dependent contractile response was COX-1 and thromboxane-prostanoid receptor mediated and modulated by the availability of NO. These data collectively suggest that endothelium-dependent, COX-mediated endoperoxide signaling in the CCA of SHR may elicit vasoconstriction, which could shift the mechanical properties of this conduit artery and contribute to reduced CCA blood flow in vivo.

Endothelial dysfunction: a multifaceted disorder (The Wiggers Award Lecture)

Endothelial cells synthesize and release various factors that regulate angiogenesis, inflammatory responses, hemostasis, as well as vascular tone and permeability. Endothelial dysfunction has been associated with a number of pathophysiological processes. Oxidative stress appears to be a common denominator underlying endothelial dysfunction in cardiovascular diseases. However, depending on the pathology, the vascular bed studied, the stimulant, and additional factors such as age, sex, salt intake, cholesterolemia, glycemia, and hyperhomocysteinemia, the mechanisms underlying the endothelial dysfunction can be markedly different. A reduced bioavailability of nitric oxide (NO), an alteration in the production of prostanoids, including prostacyclin, thromboxane A2, and/or isoprostanes, an impairment of endothelium-dependent hyperpolarization, as well as an increased release of endothelin-1, can individually or in association contribute to endothelial dysfunction. Therapeutic interventions do not necessarily restore a proper endothelial function and, when they do, may improve only part of these variables.

Fluctuation of serum NO(x) concentration at stroke onset in a rat spontaneous stroke model (M-SHRSP). Peroxynitrite formation in brain lesions

Malignant stroke-prone spontaneously hypertensive rats (M-SHRSP) develop hypertension and stroke at earlier ages than do nonmalignant SHRSP. Our previous findings suggested that reactive oxygen species were involved in the development of stroke in this strain. Nitric oxide (NO) which is more released at ischemia, might play a crucial role in stroke development by producing peroxynitrite, a neurotoxic substance. This study investigated whether the development of cerebrovascular lesion in M-SHRSP could be assessed by the fluctuation of serum NO(x) concentration, and whether peroxynitrite is associated with brain damage. Serum NO(x) levels were examined using an automated NO detector. Stroke-onset was temporally assessed according to a known method: changes in body weight, water intake, and neurologic symptoms. Cerebral lesions were confirmed by magnetic resonance imaging (MRI), and Evans blue extravasation at autopsy. MRI taken just after estimated stroke onset disclosed brain lesions. The baseline serum NO(x) level remained at 15-18 micromol/l, but the level gradually increased prior to stroke, and significantly at stroke onset. A marked rise in serum NO(x) occurred subsequently at poststroke. Immunohistochemical staining of nitrotyrosine, a peroxynitrite marker, was detected around vessels, neuronal cells and parenchyma in cerebral lesions. Stroke occurred in 50% of male M-SHRSP at 80 days of age. In conclusion, this study provides the first evidence for fluctuation of serum NO(x) at the onset of spontaneous stroke accompanying the appearance of peroxynitrite in brain lesions. Monitoring serum NO(x) would serve to assess the development of brain lesions at least in spontaneous stroke model.

Association of renal injury with nitric oxide deficiency in aged SHR: prevention by hypertension control with AT1 blockade

BACKGROUND

Aged spontaneously hypertensive rats (SHR) develop end-stage renal disease resembling that of uncontrolled essential hypertension in humans. Nitric oxide (NO) and angiotensin II (Ang II) play an important role in the regulation of blood pressure and the growth of vascular smooth muscle and renal mesangial cells. The relationship between renal NO system, Ang II activity and renal injury in aged SHR is not fully understood.

METHODS

The 8-week-old SHR were randomized into losartan-treated (30 mg/kg/day for 55 weeks) and vehicle treated groups. The age-matched Wistar-Kyoto rats (WKY) served as controls. Renal histology and tissue expressions of endothelial and inducible NO synthases (eNOS and iNOS) and nitrotyrosine were examined at 63-weeks of age.

RESULTS

Compared to the WKY group, untreated SHR showed severe hypertension, proteinuria, renal insufficiency, a twofold decrease in renal tissue eNOS and iNOS expressions and massive nitrotyrosine accumulation. This was associated with severe glomerulosclerosis, tubular atrophy and interstitial fibrosis. Losartan therapy normalized blood pressure, prevented proteinuria and renal insufficiency, abrogated the fall in renal eNOS and iNOS protein contents, mitigated renal nitrotyrosine accumulation, and prevented the histological abnormalities found in the untreated SHR.

CONCLUSIONS

Aged SHR exhibit severe renal lesions with acquired NO deficiency that are prevented by hypertension control with AT1 blockade. These findings point to the possible role of NO deficiency in the pathogenesis of renal lesions in aged SHR.

Decreased basal despite enhanced agonist-stimulated effects of nitric oxide in 12-week-old stroke-prone spontaneously hypertensive rat

This study examined both basal and agonist-stimulated effects of nitric oxide in rings of thoracic aorta and carotid artery from 12-week-old stroke-prone spontaneously hypertensive rats (SHRSP) and compared them to those found in rings from normotensive Wistar Kyoto (WKY) controls. Acetylcholine-induced endothelium-dependent relaxation was found to be five-fold more sensitive in both male and female SHRSP when compared with those from age- and sex-matched WKY rats. In contrast, we found a reduction in the effects of basal nitric oxide in the SHRSP rat. Specifically, the ability of basal nitric oxide to depress contractile responses to phenylephrine was found to be reduced in vessels from SHRSP when compared with those from WKY rats. In addition, the endothelium-dependent depression of vasodilator responses to the nitric oxide donor, glyceryl trinitrate, was reduced in vessels from SHRSP when compared to those from WKY rats. Thus, we have shown that the effects of basal nitric oxide are impaired in the SHRSP rat at an age when the effects of agonist-stimulated nitric oxide are actually enhanced. This impairment may be related to the greater susceptibility of basal nitric oxide to destruction by superoxide anion which is known to be produced in excess in this model of hypertension.

Dietary calcium decreases blood pressure without decreasing renal vascular resistance or altering the response to NO blockade

Many vasoactive elements are involved in the elevation of blood pressure in spontaneously hypertensive rats (SHR). Elevated dietary calcium has been observed to reduce blood pressure in SHR. This study investigates interactions among dietary calcium, renal vascular resistance (RVR), elevation of blood pressure and effects of norepinephrine and nitric oxide synthesis. We completed a series of experiments on two groups each (fed low, 0.1% and high, 2.0% dietary calcium, respectively) of 9-week-old Wistar Kyoto (WKY), 9-week-old and 6-week-old SHR. Although 9-week-old SHR had elevated baseline blood pressure compared to 9-week-old WKY and also compared to 6 week-old SHR, there was no corresponding elevation in baseline RVR. All SHR fed high calcium diets had lower blood pressure compared to low calcium diets, and there was no corresponding reduction in RVR. WKY controls' blood pressure and RVR were unaffected by dietary calcium levels. In all hypertensive rats the blood pressure and renal vascular resistance were elevated by N(G)-nitro-L-arginine methylester (L-NAME), but the dietary differences were sustained. Blood pressure of WKY was unaffected by the low dose of L-NAME. The increase in RVR to L-NAME was greater in SHR than in controls. The renal vascular response to norepinephrine was related to diet in older SHR, but smaller and unrelated to diet in younger SHR. Following L-NAME, WKY had greater responses to norepinephrine than 9-week-old SHR. We conclude that noradrenergic vasoconstriction is enhanced in the adult SHR, especially in the absence of high calcium diet. Alterations in NO synthesis may effect the norepinephrine response.