Background—We have recently found that administration of l-arginine to hypercholesterolemic rabbits induces regression of preexisting lesions. Others have previously shown that activation of the l-arginine/nitric oxide (NO) synthase pathway can induce apoptosis of vascular cells in vitro. Accordingly, the current study was designed to determine if dietary supplementation of l-arginine induces apoptosis of intimal lesions and if this effect is mediated through the NO synthase pathway.

Methods and Results—Male New Zealand White rabbits were fed a 0.5% cholesterol diet for 10 weeks and subsequently placed on 2.5% l-arginine HCl in the drinking water, and the cholesterol diet was continued for 2 weeks, at which time the aortas were harvested for histological studies. l-Arginine treatment increased the number of apoptotic cells (largely macrophages) in the intimal lesions by 3-fold (11.9±3.9 vs 3.9±1.4 apoptotic cells/mm², P<0.01). In subsequent studies, aortas were harvested for ex vivo studies. Aortic segments were incubated in cell culture medium for 4 to 24 hours with modulators of the NO synthase pathway. The tissues were then collected for histological studies and the conditioned medium collected for measurement of nitrogen oxides by chemiluminescence. Addition of sodium nitroprusside (10⁻⁵ mol/L) to the medium caused a time-dependent increase in apoptosis of vascular cells (largely macrophages) in the intimal lesion. l-Arginine (10⁻³ mol/L) had an identical effect on apoptosis, which was associated with an increase in nitrogen oxides released into the medium. These effects were not mimicked by d-arginine, and they were antagonized by the NO synthase inhibitor l-nitro-arginine (10⁻⁴ mol/L). The effect of l-arginine was not influenced by an antagonist of cGMP-dependent protein kinase, nor was the effect mimicked by the agonist of protein kinase G or 8-BR cGMP.

Conclusions—These results indicate that supplemental l-arginine induces apoptosis of macrophages in intimal lesions by its metabolism to NO, which acts through a cGMP-independent pathway. These studies are consistent with our previous observation that supplementation of dietary arginine induces regression of atheroma in this animal model. These studies provide a rationale for further investigation of the therapeutic potential of manipulating the NO synthase pathway in atherosclerosis.