Adventitia as a source of inducible nitric oxide synthase in the rat aorta

In the current study we used in vitro and in vivo models to determine the s ites of nitric oxide production in rat aortic tissue following cytokine sti mulation In vitro studies in which intact rat aortic rings were incubated w ith endotoxin (1 mu g/ mL) or interferon-gamma (600 U/mL) indicated that th e expression of inducible nitric oxide synthase (iNOS) activity was increas ed as measured by Northern blot analysis or determination of nitrite produc tion. In situ hybridization showed iNOS mRNA in the endothelium and adventi tia of the incubated aortic rings but not in the media. Immunohistochemical staining showed a similar localization for iNOS protein in the incubated r ings. Additional studies were performed in which bacterial endotoxin (4 mg/ kg) was administered to rats, and iNOS expression was assayed using in situ hybridization and immunohistochemistry. Clear increases in iNOS mRNA and p rotein were found in aortic tissue. Endothelial and adventitial cells were the major source of iNOS, with relatively low amounts of iNOS mRNA present in medial smooth muscle, consistent with in vitro findings. These studies i ndicate that the aortic adventitia is a potential source of NO, and suggest that the adventitial fibroblast may have an important paracrine role in re gulating arterial structure and function. (C) 1999 American Journal of Hype rtension, Ltd.