Superoxide Anion Production Is Increased in a Model of Genetic Hypertension

Abstract

Abstract-The hypothesis that the decreased nitric oxide (NO) availability observed in spontaneously hypertensive stroke-prone rats (SHRSP) is due to excess superoxide (O 2 Ϫ ) was examined. O 2 Ϫ generation, measured by lucigenin chemiluminescence, was studied in 12-to 16-week male and female Wistar-Kyoto rats (WKY) Key Words: superoxide Ⅲ endothelium Ⅲ nitric oxide Ⅲ rats, inbred SHR Ⅲ nitric oxide synthase E ndothelial dysfunction and a relative deficiency in nitric oxide (NO) may be associated with hypertension in humans 1,2 and in some models of experimental hypertension. 3,4 In the spontaneously hypertensive stroke-prone rat (SHRSP), a model of genetic hypertension, we have shown an attenuation of functional basal NO despite increased eNOS enzymatic activity. 5 Although endothelial NO synthase (eNOS) enzymatic activity was greater in SHRSP than in Wistar-Kyoto rats (WKY) when examined in vitro the possibility that the actual amount of eNOS was reduced in SHRSP in vivo could not be excluded from these results. Alternatively, eNOS levels could be similar or elevated but NO availability decreased because of more rapid removal after synthesis. Superoxide anion (O 2 Ϫ ) is produced in the vasculature and can scavenge NO forming peroxynitrite. Increased scavenging of NO by O 2 Ϫ could lead to a decrease in NO availability despite increased synthesis. Raised O 2 Ϫ levels have been reported recently in a number of models of endothelial dysfunction including hypertension, induced by either angiotensin infusion 6 or aortic banding. 7 In the majority of cases the source of excess O 2 Ϫ is uncertain, although involvement of NADH/NADPH oxidases 8 and xanthine oxidase 9 have been suggested.The aim of this study was to examine the hypothesis that the decreased NO availability observed in SHRSP is due to excess O 2 Ϫ , to identify the source of this O 2 Ϫ , and to examine other molecular mechanisms involved such as the expression of the gene-encoding enzyme involved in NO generation in the endothelium (eNOS). Methods AnimalsThree-to 4-month-old male and female WKY and SHRSP were obtained from the colonies established in Glasgow by brother and sister mating as previously described. 9 Blood pressure was measured 1 week before study by tail plethysmography according to our published protocol. 10

Central Infusion of L-Arginine or Superoxide Dismutase Does Not Alter Arterial Pressure in SHR.

“…Although chronic systemic L-Arg supplementation does not change the arterial pressure in SHR (10), some favorable effects have been observed such as reduced left ventricular hypertrophy (12) or the improvement of nephrosclerosis (13). Superoxide production is reported to be increased in stroke-prone SHR (14), and the physiological action of NO might be counteracted by superoxide even if NOS activity is augmented (23). Inhibition of the production of superoxide by ascorbate or other antioxidant agents improves endothelial function (24).…”

Redox Control of Renal Function and Hypertension

“…The enzyme reaction uses tetrahydrobiopterin as a cofactor, deficiency of which also results in the production of O 2 иϪ instead of NO (23). In SHRs and DOCA-salt sensitive mice, aortas demonstrated increased O 2 иϪ , levels that were normalized by L-NAME or endothelium removal (23,157). Nitrate tolerance and increased O 2 иϪ production was postulated to be due to eNOS uncoupling in long-term nitroglycerin treatment of rat aortic rings (226).…”

Prevention of Hypertension, Insulin Resistance, and Oxidative Stress by α-Lipoic Acid

“…8 Moreover, Hamilton et al 9 have shown in vitro that the treatment with SOD potentiated the NO-dependent relaxation in human thoracic artery and saphenous vein. In animal studies, arterial tissue O 2 Ϫ levels were reported to be increased in spontaneously hypertensive rats (SHR) 10,11 and in insulin-resistant rats. 12 Other studies have demonstrated that antioxidant treatment with ␣-tocopherol or tempol reduces the blood pressure in SHR.…”