Role of shear stress on nitrite and NOS protein content in different size conduit arteries of swine.

Mechanical forces elicited by the flow of blood (shear stress) and pressure (strain) cause acute releases of nitric oxide (NO) (Sessa et al. 1994, Awolesi et al. 1995). NO is implicated in the regulation of numerous cellular functions throughout the systemic circulation including the maintenance of low arterial tone at rest, inhibition of leucocyte–endothelial interactions, attenuation of platelet aggregation and inhibition of smooth muscle cell proliferation (Celermajer 1997, Rudic et al. 1998). There is evidence suggesting that heterogeneity of function exists within different size vessels (Kuo et al. 1995). The vascular wall consists of three different functional and structural compartments. Smooth muscle cells constitute the media layer, whereas endothelial cells constitute the intima and physiologically perceive the wall shear stress (WSS) at the interface between the flowing blood and the arterial wall. It has been shown that both endothelium and vascular smooth muscle can produce NO (Pollock et al. 1993, Boulanger et al. 1998). Although the vasculature constantly produces NO, and endothelial-derived NO is now recognized as a major mechanism regulating vascular tone and local blood flow (Chandran et al. 1998), a direct relation between the size of vessels, blood flow and the WSS-induced NO production under physiological conditions is unavailable. Nitric oxide synthase (NOS) is a family of isozymes catalysing the production of NO. NO produced by endothelial NOS (eNOS) acts as a vasodilator in the cardiovascular system, NO produced by neuronal NOS (nNOS) is known to act as a neurotransmitter/neuromodulator in both the central and the peripheral nervous systems, and the NO produced by inducible NOS (iNOS) acts as a cytotoxic effector in the immune system (Chandran et al. 1998). The eNOS expression has been thought to be responsible for shear stress-induced NO generation in endothelial cells (Cooke et al. 1990, Nishida et al. 1992). A low-level expression of iNOS has been recently found to occur, however, constitutively in the blood vessels, heart and other tissues under normal conditions (Park et al. 1996). The expression of nNOS, on the other hand, was first observed in the brain (Bredt et al.1990) and has since been found in vascular smooth muscle cells (Boulanger et al. 1998). Although these studies demonstrate that NOS is expressed in blood vessels, the potential role of the different NOS isoforms in the regulation of blood flow-induced NO in normal vessel wall remains to be determined. Here, we quantified the relation between shear stress, nitrite contents and NOS protein contents in different sizes of arteries. We chose segments of five conduit arteries from the hind limb of pig: abdominal aorta, common iliac artery, femoral artery, superficial femoral artery and a small branch of the femoral artery. The main goal of the study was to: (1) determine the level of shear stress in different sizes of arteries in vivo, (2) quantify tissue contents of nitrite in relation to arterial diameter, (3) determine the correlation between protein contents of eNOS, iNOS and nNOS and NO production, and (4) evaluate the role of WSS on protein contents of eNOS, iNOS and nNOS and NO production. We found that nitrite contents increase with decrease in artery diameter under physiological conditions and the regional difference in nitrite largely associated with eNOS and eNOS content is largely determined by WSS. Our findings underscore the relation between NO, NOS and dimensions of arteries.