NO is a simple molecule, consisting of a single oxygen bonded
to one nitrogen atom. 11 electrons in the outer shell of NO
make the net spin of the molecule S = 1/2, which is responsible for
NO high reactivity as a free radical, and its unique properties
in terms of its action as a heme ligand.
Its very small molecular size and lipophilic nature allow the rapid diffusion of NO through plasma membranes and enable NO to communicate with neighboring cells, however, a short half-life in biological fluids (2 - 30 seconds) permits the molecule only for local action.
A vasculature is in constant state of active dilation
mediated by NO. Endothelial cells, the most inner part
of blood vessels, continuously release small amounts of NO which migrates to
nearby muscle cells and binds to the heme group of guanylate cyclase (GC),
the major target for NO. Activated GC produces cyclo-GMP, which, through
a cascade of protein kinases, induces smooth muscle relaxation. This dilates
the vessel and lowers blood pressure.
Endothelial generation of NO not only regulates blood pressure but also reduces blood clotting by acting directly on platelets. It reduces clotting by inhibiting platelet aggregation and adhesion.
More information on NO you will find in NO Home Page