High-salt intake enhances superoxide activity in eNOS knockout mice leading to the development of salt sensitivity

Abstract

A deficiency in nitric oxide (NO) generation leads to salt-sensitive hypertension, but the role of increased superoxide (O₂⁻) in such salt sensitivity has not been delineated. We examined the hypothesis that an enhancement in O₂⁻ activity induced by high-salt (HS) intake under deficient NO production contributes to the development of salt-sensitive hypertension. Endothelial NO synthase knockout (eNOS KO; total n = 64) and wild-type (WT; total n = 58) mice were given diets containing either normal (NS; 0.4%) or high-salt (HS; 4%) for 2 wk. During this period, mice were chronically treated with a O₂⁻ scavenger, tempol (400 mg/l), or an inhibitor of NADPH oxidase, apocynin (1 g/l), in drinking water or left untreated (n = 6–8 per group). Blood pressure was measured by radiotelemetry and 24-h urine samples were collected in metabolic cages. Basal mean arterial pressure (MAP) in eNOS KO was higher (125 ± 4 vs. 106 ± 3 mmHg) compared with WT. Feeding HS diet did not alter MAP in WT but increased it in eNOS KO to 166 ± 9 mmHg. Both tempol and apocynin treatment significantly attenuated the MAP response to HS in eNOS KO (134 ± 3 and 139 ± 4 mmHg, respectively). Basal urinary 8-isoprostane excretion rates (U_IsoV), a marker for endogenous O₂⁻ activity, were similar (2.8 ± 0.2 and 2.4 ± 0.3 ng/day) in both eNOS KO and WT mice. However, HS increased U_IsoV more in eNOS KO than in WT (4.6 ± 0.3 vs. 3.8 ± 0.2 ng/day); these were significantly attenuated by both tempol and apocynin treatment. These data indicate that an enhancement in O₂⁻ activity contributes substantially to the development of salt-sensitive hypertension under NO-deficient conditions.