1) Neurotransmitter or hormone binds with receptors on endothelial cells lining the artery, which in response releases nitric oxide (NO).
2) NO molecules from the endothelium travel into smooth muscle cells, where they activate an enzyme, guanylyl cyclase (GC).
3) GC converts guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP).
4) cGMP causes calcium ions to enter storage area of the cell. The lowered concentrations of calcium ions (Ca++) set off a cascade of cellular reactions that cause the cell’s contractile filaments (myosin and actin) to slide apart.
5) Smooth muscle cells relax.
6) Blood vessel dilates.