Tetrahydrobiopterin Alters Superoxide and Nitric Oxide Release in Prehypertensive Rats

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Abstract

Constitutive nitric oxide synthase (cNOS) with insufficient cofactor (6R)-5,6,7,8-tetrahydrobiopterin (H4 B) may generate damaging superoxide (O2-). This study was designed to determine whether cNOS-dependent generation of O2- occurs in spontaneously hypertensive rats (SHR) before the onset of hypertension. Aortas from 4-wk-old SHR and Wistar-Kyoto rats were used. cNOS was stimulated by calcium ionophore A23187. In situ measurements of nitric oxide and hydrogen peroxide by electrochemical sensors and O2- production by chemiluminescence method were performed. Isometric tension was continuously recorded. H4 B by high performance liquid chromatography and [(3) H]citrulline assay were determined in homogenized tissue. The A23187-stimulated production of O2- and its superoxide dismutase product hydrogen peroxide were significantly higher, whereas nitric oxide release was reduced in SHR aortas, with opposite results in the presence of exogenous H4 B. Furthermore, NG -monomethyl-L-arginine inhibited the generation of cNOS-dependent O2- by [approximately] 70%. Natural H4 B levels were similar in both strains; however, equivalent cNOS activity required additional H4 B in SHR. The endothelium-dependent relaxations to A23187 were significantly inhibited by catalase, and enhanced by superoxide dismutase, only in SHR; however, these enzymes had no effect in the presence of H4 B. Thus, dysfunctional cNOS may be a source of O2- in prehypertensive SHR and contribute to the development of hypertension and its vascular complications. (J. Clin. Invest. 1998. 101:1530-1537.)

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