About Isosorbide-5-Mononitrate Alkyl nitrate derivative, a direct vasodilator, Antianginal. Mechanism of Action of Isosorbide-5-Mononitrate It is a direct smooth muscle relaxant, which have a prominent action vascular smooth muscles preferentially veins than arteries. It is enzymatically denitrated in the smooth muscle cells to release the reactive free radical nitric oxide which activates cytosolic guanylyl cyclase thus increases cyclic GMP levels which causes dephosphorylation of myosin light chain kinase(MLCK) through a cyclic GMP dependent protein kinase. Reduced availability of phosphorylated (active) MLCK interferes with activation of myosin and myosin fails to interact with actin to cause contraction and consequently relaxation occurs. Elevated levels of cyclic GMP also reduce calcium entry which also contributing to relaxation. It dilates veins more than arteries because veins have greater amount of enzyme that generate nitric oxide from nitrates.