cGMP) which promotes vascular smooth muscle relax-An automated method for the simultaneous determiation.
nation of nitrite and nitrate in biological samples by
Many investigators have used L-arginine analogs for using a multichannel flow injection analyzer has been competitive inhibition of NOS as well as changes in developed. The method was based on the reaction of vascular cGMP content to determine whether nitric oxnitrite with Greiss reagent. The sample solution was ide was involved in an agonist-induced relaxation. injected and equally divided into two channels; chan-However, measuring changes in cGMP by radioimmunel one (1) represented total nitrite obtained by cadnoassay is an indirect and expensive method for inmium reduction of nitrate to nitrite while channel two vestigating nitric oxide-mediated mechanisms. There-(2) represented only nitrite. The absorbance of the fore, recent attempts have been made to directly color product was measured by photometric detectors measure nitric oxide or its more stable products of niwith 540-nm filters. This method combines high reprotrite (NO 0
2 ) and nitrate (NO 0 3 ). Measurements of reacducibility of sample introduction via flow injection tive nitric oxide are made by chemiluminescence using and sensitivity of spectrophotometric detection. The the reaction of nitric oxide with ozone (2,4) or luminoldetection limit is 25 nM for both nitrite and nitrate. The hydrogen peroxide (5,6) and by spectrophotometric chemistry manifolds are constructed of Teflon tubing assays using the reaction of nitric oxide with oxyhemowhich, along with a low-pressure Flowfit connector globin (7) or iron-sulfur proteins (8). For measuring system, provides for low maintenance, ease of use, and NO 0 2 and NO 0 3 , a chromatographic method (7), a reachigh sample throughput. We demonstrated that the tion of nitrite with the Greiss reagent followed by specsystem can be used for the determination of both nitrophotometric detection (9,10), an HPLC system with trate and nitrite in a variety of biological samples as a nitrite-Greiss reagent reaction (11,12), and fluorowell as a comparison of the results from this system and the HPLC system.