cDNA cloning and expression of inducible nitric oxide synthase from rat vascular smooth muscle cells

Nitric oxide (NO) is an important signal molecule. In blood vessels, nitric oxide produced by the endothelium modulates vascular tone by inducing cGMP formation in smooth muscle cells. The latter cell type does not express NO synthase normally but expression is induced by the cytokines, interferon-gamma, tumor necrosis factor, and interleukin-1. We have constructed a cDNA library from cytokine-stimulated rat aortic smooth muscle cells and isolated a cDNA clone that contains the full-length sequence of inducible NO synthase. It is 4119 bp and confers NO synthesizing activity when transfected into COS-7 cells. The nucleotide sequence is 92% identical with inducible NO synthase of murine macrophages and contains a 497 bp untranslated 3' sequence with five conserved A(U)nA motifs that may be important in the regulation of mRNA turnover. This is supported by the short half-life (2.6 h) of smooth muscle NO synthase mRNA, which contrasts with brain and endothelial NO synthase.