Direct electrochemical measurement of nitric oxide in vascular endothelium

The endothelium plays a critical role in maintaining vascular tone by relea sing vasoconstrictor and vasodilator substances. Endothelium-derived nitric oxide (NO) is a vasodilator rapidly inactivated by superoxide and by Fe(II ) and Fe(III), all found in significant quantities in biological systems. T hus due to the short life of NO in tissue (t(1/2) = 3-6 s), in situ quantif ication of NO is a challenging problem. We designed the present study to pe rform direct measurements of nitric oxide using the electrochemical porphyr inic sensor. The most significant advantages of this sensor is small size ( 0.5-8 mu m), rapid response time (0.1-1 ms), and low detection limit (10(-9 ) mol l(-1)). The porphyrinic sensor was used for in vitro and in vivo meas urements of NO in an isolated single cell or tissue. Effects of hypertensio n, endotoxemia, and ischemia/reperfusion on the release of NO and/or its in teraction with superoxide (O-2(-)) were delineated. In the single endotheli al cell (rabbit endocardium), NO concentration was highest at the cell memb rane (950 +/- 50 nmol l(-1)), decreasing exponentially with distance from c ell, and becoming undetectable at distances beyond 50 mu m. The endothelium of spontaneously hypertensive rats (SHR) released 35% less NO (580 +/- 30 nmol l(-1)) than that of normotensive rats (920 +/- 50 nmol l(-1)), due to the higher production of O-2(-) in SHR rats. Endothelial NO synthase (eNOS) generated NO (140 +/- 20 nmol l(-1)) in lung during the acute phase (first 10-15 min) of endotoxemia, followed by production of NO by inducible NOS. High production of O-2(-) was observed during the entire period of endotoxe mia. Ischemia (lower limb of rabbit) caused a significant increase of NO pe aking at 15 min and decreasing thereafter, also due to O-2(-) production. ( C) 1999 Elsevier Science B.V. All rights reserved.