Abstract
Besides the constitutively expressed nitric oxide synthase (NOS) in endothelial cells, vascular smooth muscle cells (SMC) have the potential to express the inducible isoform of NOS (iNOS). Given the important effects of nitric oxide and the high output of this enzyme, expression of iNOS in SMC and its pathophysiological significance were investigated in the present study.
Angioplastic injury to the rat carotid artery caused a rapid expression of iNOS in medial SMC, and later preferentially in intimal SMC. NOS was also found underneath an intact endothelium in SMC as well as in macrophages in the lesions of aortic grafts. In vitro, stimulation of intimal SMC with LPS plus IFN-[gamma] resulted in a 5- to 8-fold higher NOS activity than its counterpart medial SMC. These data indicate that intimal SMC are capable of overexpressing iNOS, and serve as an important source of iNOS. Induction of iNOS was also characterized in SMC derived from arteries of newborn, adult, and old rats. Our results show that the capability of NOS expression is dramatically upregulated with aging, indicative of a tendency from incompetent in the newborn toward overactive in the old SMC.
Stimulating intimal SMC with either TNF-[alpha] or LPS resulted in a strong and prolonged activation of NF-[kappa]B. This was accompanied by a similar pattern of degradation of cytoplasmic I[kappa]B[alpha]. However, the identical treatment led to a moderate and transient activation of NF-[kappa]B and degradation of l[kappa]B[alpha] in medial SMC. Moreover, our results show that TNF-[alpha], LPS and H2O2 are unable to sufficiently activate NF-[kappa]B in newborn SMC, but trigger a strong activation of NF-[kappa]B in old SMC. Local application of L-NAME, an inhibitor of NOS, significantly prolonged platelet adhesion, and reduced the blood flow of endothelium-denuded vessels. These data indicate that NOS plays a role in preventing platelet adhesion and modulating vascular tone. NOS derived NO coqld also inhibit proliferation and mitochondrial respiratory function of both intimal and medial SMC in vitro, particularly the latter cells.
These findings indicate that expression of iNOS in vascular SMC is phenotype-dependent and developmentally regulated, and depends on the capacity of NF-[kappa]B activation. As a consequence, expression of NOS provides vascular SMC with additional functions that may compensate for the dysfunction of eNOS.
