NO Is Not Exclusively Generated by the Reaction of <i>L</i>-Arginine and NOS and Is Poorly Identified by the Griess Reaction or Clark-Type Electrodes

Sohji Nagase, MD, PhD, Department of Internal Medicine, Institute of Clinical Medicine, University of Tsukuba, 1-1-1 Ten-nodai, Tsukuba, Ibaraki 305 (Japan), Tel/Fax +81 (298) 53-3202, E-Mail sohji-n@md.tsukuba.ac.jp Dear Sir, We read with great interest the report by Dr. Rivas-Cabañero et al. [1] published as a letter to the editor in Nephron last year. The authors found that nitrite and nitrate, stable oxidative products of NO (N02 + N03 = NOx) actually increase during the administration of the nitric oxide synthase (NOS) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), in the longterm incubation of isolated glomeruli. In fact, the concentration of NOx in the incubation medium containing L-NAME is higher than that of controls after 8 h of incubation. The Griess reaction was employed to measure NOx in the study. The authors speculated that the stimulated NO production is derived from enzymatic synthesis of L-arginine from L-NAME. On the other hand, a recent publication by Dr. Heyman et al. [2] demonstrates, using a Clarktype electrode to measure NO, a paradoxical increase in renal medullary NO production. These authors report that outer medullary NO electrode current did not fall but rose following the addition of L-NAME to renal perfusate despite a rise in blood pressure. They hypothesized that the NO current may reciprocally increase during a reduction in tissue oxygenation, as less NO is being scavenged by oxygen and reactive oxygen species. The possibility of a renal substance that interferes with the electrode current has also been considered. Recently, we reported that NOx, detected by the Griess reaction, increases following the reaction of hydrogen peroxide and D-arginine, L-arginine, L-canavanine, or even L-NAME. However, when we used chemilu-minescence for the detection of NO, it was detected only from the reactions of hydrogen peroxide and Dor L-arginine. We conclude that there is a pathway for NO synthesis from Dand L-arginine not dependent upon the action of NOS and that the use of NOx as a marker of NO generation is unreliable because nitrite and/or nitrate rather than NO could be released from L-canavanine and L-NAME through the action of hydrogen peroxide [3]. In addition, in a study monitoring NO generation by Clark-type electrode, we found that the existence of hydrogen peroxide alone