Characterization of cerebral microvasculature in transgenic mice with endothelium targeted over-expression of GTP-cyclohydrolase i

Tetrahydrobiopterin (BH₄) is a critical determinant of nitric oxide (NO) production by nitric oxide synthase (NOS) in the vascular endothelium and its biosynthesis is regulated by the enzymatic activity of GTP-cyclohydrolase I (GTPCH I). The present study was designed to determine the effects of endothelium-targeted overexpression of GTPCH I (eGCH-Tg) on murine cerebral vascular function. Endothelium targeted over-expression of GTPCH I was associated with a significant increase in levels of BH₄, as well as its oxidized product, 7,8-dihydrobiopterin (7,8-BH₂) in cerebral microvessels. Importantly, ratio of BH₄ to 7,8-BH₂, indicative of BH₄ available for eNOS activation, was significantly increased in eGCH-Tg mice. However, expression of endothelial NOS, levels of nitrate/nitrite - indicative of NO production - remained unchanged between cerebral microvessels of wild-type and eGCH-Tg mice. Furthermore, increased BH₄ biosynthesis neither affected production of superoxide anion nor expression of antioxidant proteins. Moreover, endothelium-specific GTPCH I overexpression did not alter intracellular levels of cGMP, reflective of NO signaling in cerebral microvessels. The obtained results suggest that, despite a significant increase in BH₄ bioavailability, generation of endothelial NO in cerebral microvessels remained unchanged in eGCH-Tg mice. We conclude that under physiological conditions the levels of BH₄ in the cerebral microvessels are optimal for activation of endothelial NOS and NO/cGMP signaling.