Dietary l-arginine decreases myointimal cell proliferation and vascular monocyte accumulation in cholesterol-fed rabbits

Abstract

l-arginine, the precursor of endogenous nitric oxide (NO), has been shown to enhance endothelial function and to reduce the progression of atherosclerosis in cholesterol-fed rabbits. In the present study, we investigated whether myointimal cell proliferation is enhanced in hypercholesterolaemic rabbit aorta and whether chronic treatment of the rabbits with l-arginine or with the NO synthase inhibitor l-NAME influences this proliferative response and vascular monocyte accumulation. Rabbits were fed 1% cholesterol or normal rabbit chow for 12 weeks. Subgroups of cholesterol-fed rabbits were treated with oral l-arginine (2.25%) or l-NAME (3 mg/dl) in drinking water. Myointimal cell proliferation was quantified in aortic segments by immunohistochemical detection of bromodeoxyuridine (BrdU) incorporation into nuclear DNA; vascular monocyte accumulation was assessed by immunohistochemistry using a monoclonal anti-macrophage/monocyte antibody (RAM-11). Plasma levels of l-arginine and the endogenous NO synthase inhibitor, ADMA, were quantified by high-performance liquid chromatography (HPLC). Cholesterol feeding increased the aortic intima/media (I/M) ratio, which was not measurable in the control group, to 1.9±0.3. This was paralleled by enhanced cell proliferation (cholesterol, 2.4±0.2%; P<0.05; control, 0.02±0.001% BrdU-positive cells per 72 h) and vascular monocyte accumulation. Double immunostaining for BrdU and α-actin showed that about two thirds of the proliferating cells were smooth muscle cells. ADMA levels increased from 0.8±0.1 μmol/l to 2.2±0.2 μmol/l in cholesterol-fed rabbits, but were unchanged by l-arginine or l-NAME treatment. Myointimal proliferation and intima/media ratios were correlated with ADMA plasma levels. Dietary l-arginine reduced monocyte accumulation by 85±2% ( P<0.05 vs cholesterol), myointimal cell proliferation (1.8±0.3% per 72 h; P<0.05) and intimal thickening (I/M ratio: 0.7±0.2), whereas the inhibitor of NO synthase, l-NAME, further increased cell proliferation to 3.1±0.4% per 72 h ( P<0.05). No significant difference was observed in vascular monocyte infiltration between the cholesterol and l-NAME groups. We conclude that cell proliferation and vascular monocyte accumulation are enhanced in hypercholesterolaemic rabbit aorta. These atherogenic effects can be attenuated by dietary l-arginine. Decreased NO formation might underlie the enhanced monocyte accumulation and cell proliferation in hypercholesterolaemic rabbit aorta. The observed inhibition of cell proliferation adds to our understanding of the antiatherosclerotic effects of l-arginine in vivo.