Tetrahydrobiopterin is a secretory product of murine vascular endothelial cells.

Tetrahydrobiopterin (BH4) is an obligatory cofactor of nitric oxide synthase and an essential regulator of its activity. The murine vascular endothelial cell line send1 constitutively secretes large amounts of BH4 as well as scant amounts of neopterin, an oxidized intermediate in the de novo biosynthesis of BH4. Further enhancement of BH4 and neopterin secretion is achieved by activation with endotoxin (LPS) and interferon-gamma. This finding is in accordance with previous described BH4 secretion by human endothelial cells. It supports the view that endothelial cells are the source of BH4 serving vascular needs in vivo. In septic conditions, BH4 released by endothelial cells in large amounts could serve induced nitric oxide synthase in smooth muscle cells, thereby acting as another endothelium-derived relaxing factor mediating vasodilatation.