The second indication came from studies of vascular tissue, the well-known target of nitroglycerin. Several molecules, such as acetylcholine, were known to cause relaxation of blood vessels. This effect occurred only when the vessels were prepared so that the luminal endothelial cells covering the smooth muscle of the vessel wall were retained (see Figure 7–5). Subsequent studies showed that these endothelial cells respond to 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.
NO was suspected to be EDRF because they have similar vasorelaxation effects. Systematic comparison of the biochemical and pharmacologic 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; see Chapters 11 and 12) elicit their effects by recruiting physiologic signaling pathways that normally mediate the actions of endogenously generated NO.
■ NITRIC OXIDE SYNTHESIS, SIGNALING MECHANISMS, & INACTIVATION
NO, written as NO• to indicate an unpaired electron in its chemical structure, or simply NO, is a highly reactive signaling molecule that is synthesized in cells by any of three closely related NO synthase (NOS, EC 1.14.13.49) isoenzymes, each of which is encoded by a separate gene and named for the initial cell type from which it was isolated (Table 19–1). These enzymes, neuronal NOS (nNOS or NOS-1), macrophage or inducible NOS (iNOS or NOS-2), and endothelial NOS (eNOS or NOS-3), despite their names, are each expressed in a wide variety of cell types, often with an overlapping distribution.
These NOS isoforms generate NO from the amino acid l-arginine in an O2-and NADPH-dependent reaction (Figure 19–1). This enzymatic reaction involves enzyme-bound cofactors, including heme, tetrahydrobiopterin, and flavin adenine dinucleotide (FAD). In the case of nNOS and eNOS, NO synthesis is triggered by agents and signaling events that increase cytosolic calcium concentrations. Cytosolic calcium forms complexes with calmodulin, an abundant calcium-binding protein, which then binds and activates eNOS and nNOS. On the other hand, iNOS is not regulated by calcium, but after synthesis is constitutively active. In macrophages and several other cell types, iNOS is normally not readily detectable until inflammatory mediators induce
TABLE 19–1 Properties of the three isoforms of nitric oxide synthase (NOS).
the transcription of the iNOS gene, resulting in accumulation of iNOS and synthesis of large quantities of NO.