DISCOVERY OF ENDOGENOUSLY GENERATED NITRIC OXIDE
Because NO is an environmental pollutant, the finding that NO is synthesized by cells and activates specific intracellular signaling pathways was unexpected. The first indication that NO is gener-ated in cells came from studies of cultured macrophages, which showed that treatment with inflammatory mediators, such as bac-terial endotoxin, resulted in the production of nitrate and nitrite, molecules that are byproducts of NO breakdown. Similarly, injec-tion of endotoxin in animals elevated urinary nitrite and nitrate.
The second indication came from studies of vascular regulation. Several molecules, such as acetylcholine, were known to cause relaxation of blood vessels. This effect occurred only when the ves-sels were prepared so that the luminal endothelial cells covering the smooth muscle of the vessel wall were retained. Subsequent studies showed that endothelial cells respond to these vasorelaxants by releasing a soluble endothelial-derived relaxing factor (EDRF). EDRF acts on vascular muscle to elicit relaxation. These findings prompted an intense search for the identity of EDRF.
At the same time, it was observed that exogenous application of NO or organic nitrates, which are metabolized to NO, elicit a variety of effects including inhibition of platelet aggregation and vasorelaxation. These effects were particularly intriguing, since they appeared to involve the activation of highly specific cellular responses, rather than more general cytotoxic responses. Comparison of the biochemical and pharmacological properties of EDRF and NO provided initial evidence that NO is the major bioactive component of EDRF. These findings also made it clear that exogenously applied NO and NO-releasing compounds (nitrates, nitrites, nitroprusside;) elicitedtheir effects by recruiting physiologic signaling pathways that normally mediate the actions of endogenously generated NO.