Effects of NO donors and synthase agonists on endothelial cell uptake of L-Arg and superoxide production

It is commonly believed that the activity of NO synthase (NOS) solely contr ols NO production from its substrates, L-Arg and O-2. The Michaelis-Menten constant (K-m) of NOS for L-Arg is in the micromolar range; cellular levels of L-Arg are much higher. However, evidence strongly suggests that cellula r supply of L-Arg may become limiting and lead to reduced NO and increased superoxide anion (O-2(-).) formation, promoting cardiovascular dysfunction. Uptake of L-Arg into cells occurs primarily (similar to 85%) through the a ctions of a Na+-independent, carrier-mediated transporter (system y(+)). We have examined the effects of NOS agonists (substance P, bradykinin, and AC h) and NO donors (S-nitroso-N-acetyl-penicillamine and dipropylenetriamine NONOate) on transport of L-Arg into bovine aortic endothelial cells (BAEC). Our results demonstrate that NOS agonists increase yi transporter activity . A rapidly acting NO donor initially increases L-Arg uptake; however, afte r longer exposure, L-Arg uptake is suppressed. Exposure of BAEC without L-A rg to substance P and a Ca2+ ionophore (A-23187) increased O-2(-l). formati on, which was blocked with concurrent presence of L-Arg or the NOS antagoni st N-omega-nitro-L-arginine methyl eater: We conclude that factors includin g NO itself control y(+) transport function and the production of NO and O- 2(-)..