This study was designed to investigate the insulin alteration as a consequence of hypercholesterolemic diet in rabbit model of early atherosclerosis. The cholesterol-rich diet resulted in a significant increase in total cholesterol, LDL-cholesterol, and HDL-cholesterol as well as the development of early lesions that were representative of fatty streak initiation similar to those in humans. Thus, the model allowed us to examine some factor alteration during the initiation of atherosclerosis. In current experiment, CRP significantly increased in hypercholesterolemic rabbits while there was no significant change in control group.CRP has not only been proposed as risk factors of cardiovascular disease, but has also been associated with the variables of insulin resistance syndrome . Since hypercholesterolemia is a major participator in the inflammatory process of atherosclerosis, several in vitro and in vivo studies have been showed the increased level of CRP in hypercholesterolemic animals and patients [12, 13].
As the results of our study indicated, 30 days of cholesterol-feeding enhanced the content of total nitrite, as has been demonstrated previously [14–16]. It has been proposed that enhanced NO synthesis might be a defense mechanism to compensate for continuous inactivation of NO by oxygen-derived free radicals [17, 18].Another proposed mechanisms responsible for the elevation of nitrite may be NO production by other isoforms of Nitric Oxide Synthase (NOS)enzymes. Increased NOS mRNA and protein of atherosclerotic vessels reported in other experiments showing that aortas of hypercholesterolemic rabbits release larger quantities of nitrogen oxides than do normal vessels in early atherosclerosis .
Interestingly, in parallel to total NO metabolite alteration, the hypercholesterolemia induced insulin increment in rabbits. At the cellular level, it has been shown that exposure to insulin increases eNOS mRNA and protein synthesis [20, 21]. Furthermore, insulin increased eNOS production through increasing the activity of AP-1, a transcription factor that bind to the eNOS promoter .
Furthermore, in humans, infusion of insulin causes NO dependent vasodilatation and increased blood flow . In both humans  and in animal models of insulin resistance , there is a specific impairment of PI3K-dependent signaling pathways. Thus, insulin resistance would be associated with a decrease in eNOS phosphorylation and decreased endothelial NO production. So, it seems that early hyperinsulinemia in the beginning of the atherosclerosis may be a protective mechanism for endothelial function. However, it has been shown that prolonged exposure of endothelial cells to high insulin levels induces a downregulation of the PI3K/Akt/eNOS axis. Such impairment of insulin signaling in prolonged hyperinsulinemia may result in ED and promote atherogenesis .
In this study, there was a significant negative correlation between plasma levels of insulin and CRP concentration several experiments demonstrated the anti-inflammatory effects of insulin [26–30]. It has been showed that insulin suppressed the expression of the pro-inflammatory intracellular adhesion molecule (ICAM)-1, the chemokine, monocyte chemoattractant protein-1 (MCP-1), and the key pro-inflammatory transcription factor, nuclear factor [kappa]B (NF[kappa]B) in human aortic endothelial cells at physiologically relevant concentrations [26, 27]. In patients with acute myocardial infarction, insulin also suppressed C-reactive protein (CRP) and serum amyloid A (SAA) by 40% within 24 h of the start of the insulin infusion while glucose concentrations rendered unchanged . This effect of insulin was corroborated in patients with myocardial infarction as well as in patients undergoing coronary artery bypass grafts in two studies [28–30].
In summary, insulin may promotes endothelial function, through increased NO production, which may has anti-inflammatory effects and cause slower atherosclerotic progression.