Oxidative stress induces BH₄ deficiency in male, but not female, SHR

We previously published that female spontaneously hypertensive rats (SHR) have significantly greater nitric oxide (NO) bioavailability and NO synthase (NOS) enzymatic activity in the renal inner medulla (IM) compared with age-matched males, although the mechanism responsible remains unknown. Tetrahydrobiopterin (BH₄) is a critical cofactor required for NO generation, and decreases in BH₄ as a result of increases in oxidative stress have been implicated in the pathogenesis of hypertension. As male SHR are known to have higher levels of oxidative stress compared with female SHR, we hypothesized that relative BH₄ deficiency induced by oxidative stress in male SHR results in lower levels of NOS activity in renal IM compared with females. Twelve-week-old male and female SHR were randomized to receive tempol (30 mg/kg/day via drinking water) or vehicle for 2 weeks. Tempol treatment did not affect blood pressure (BP) in either sex, but reduced peroxynitrite levels only in males. Females had more total biopterin, dihydrobiopterin (BH₂), and BH₄ levels in renal IMs than males, and tempol treatment eliminated these sex differences. Females had greater total NOS activity in the renal IM than males, and adding exogenous BH₄ to the assay increased NOS activity in both sexes. This sex difference in total NOS and the effect of exogenous BH₄ were abolished with tempol treatment. We conclude that higher oxidative stress in male SHR results in a relative deficiency of BH₄ compared with females, resulting in diminished renal NOS activity in the male.