We have recently shown that inhibition of nitric oxide (NO) synthesis by asymmetrical dmethylarginine (ADMA) accelerated endothelial cell (EC) senescence which was prevented by coincubation With L-arginine: however the effect of long-term treatment of L-arginine alone on senescence of ECs have not been investigated. Human ECs were cultured in medium containing different concentrations Of L-arginine until senescence. L-Arginine paradoxically accelerated senescence indicated by inhibiting telomerase activity. Moreover, L-arginine decreased NO metabolites, increased peroxynitrite, and 8-iso-prostaglandin F2 alpha formation. In old cells. the mRNA expression of human amino acid transporter (hCAT)2B, the activity and protein expression of arginase II were upregulated indicated by enhanced urea, L-ornithine, and L-arginine consumption. Inhibition of arginase activity, or transfection with arginase II siRNA prevented L-arginine-accelerated senescence. The most possible explanation for the paradoxical acceleration of senescence by L-arginine so far may be the translational and posttranslational activation of arginase II. (C) 2009 Elsevier Inc. All rights reserved.