The biologic basis for inhaled nitric oxide.

Nitric oxide is produced by nitric oxide synthase enzymes, which cleave the amino acid L-arginine to form nitric oxide and the amino acid L-citrulline. Many of the biologic actions of nitric oxide occur because nitric oxide activates guanylate cyclase, which in turn synthesizes a second-messenger molecule, cyclic guanosine 3',5'-monophosphate (cGMP). The increased concentration of cGMP activates cGMP-dependent protein kinase, reducing intracellular concentrations of calcium and relaxing smooth muscle. Nitric oxide also has many important effects that may not be mediated through increases of pulmonary cGMP activity. These include the ability to scavenge oxygen free radicals, reduce oxygen toxicity, and inhibit platelet and leukocyte aggregation. Nitric oxide is metabolized and excreted via a number of diverse pathways that may modify the toxicity of the molecule.