Tetrahydrobiopterin, a cofactor for nitric oxide synthase, produces endothelium-dependent dilation of mouse pial arterioles

Background and Purpose: Tetrahydrobiopterin (THBP) is an essential cofactor for nitric oxide synthase (NOS), which is responsible for the synthesis of the endothelium-derived relaxing factor (EDRF) responsible for mediating the vasorelaxation produced by acetylcholine (ACh). Previous publications suggest that EDRF (ACh) is continuously synthesized and released from the endothelium of mouse pial arterioles. If so, then one may predict that addition of THBP will increase the local production of EDRF (ACh) and produce an endothelium-dependent relaxation that can be blocked by application of a known inhibitor of NOS. This study tests the production. Methods: The pial vessels were observed at a continuously suffused craniotomy site by means of intravital television microscopy. The effects of topically applied THBP on diameter were monitored before and after endothelial damage and before and after local treatment with the NOS inhibitor N-guanidino-L-monomethyl arginine (LNMMA). The endothelial damage was produced by a helium-neon laser in the presence of Evans blue dye. Results: A dose-dependent relaxation was produced by 10^-3 and 10^-2 mol/L THBP. The response was virtually eliminated by endothelial injury. LNMMA 10^-6 mol/L also greatly inhibited dilation. Conclusions: The data are consistent with all reports that THBP is a cofactor for constitutive endothelial NOS. The data are constant with previous results suggesting that EDRF(ACh) is continually synthesized and released. It appears that THBP increases this synthesis and consequently the local level of released EDRF(ACh). The continuous spontaneous synthesis/release of EDRF(ACh) modulates basal tone and, according to other studies, helps maintain a platelet-free endothelial surface.