USE OF NITRONYL NITROXIDE TO DISCRIMINATE THE CONTRIBUTION OF NITRIC-OXIDE RADICAL IN ENDOTHELIUM-DEPENDENT RELAXATION OF CONTROL AND DIABETIC BLOOD-VESSELS

Nitronyl nitroxides react with nitric oxide radical (. NO) to form imi no nitroxides. We used a nitronyl nitroxide, rboxyphenyl)-4,4,5,5-tetr amethylimidazoline-1-oxyl 3 oxide] (CPTIO) to evaluate the contributio n of . NO to basal tone and acetylcholine-induced endothelium-dependen t relaxation in control vs. diabetic rat aortic rings. In rings precon tracted with phenylephrine, CPTIO produced an additional increment in tension that was greater in control vs. diabetic rings, Tension after CPTIO was similar to that observed in rings pretreated with the NO syn thase inhibitor, L-nitroarginine or in rings without endothelium. This increment was insensitive to indomethacin, cysteine, tetraethylammoni um or catalase, but was sensitive to inhibition by the soluble guanyla te cyclase inhibitor, 1H- [1,2,4]oxadiazolo[4,3,-a]quinoxaline-1-one. L-Nitroarginine blocked relaxation to ACH by 100 and 90% in control an d diabetic rings, respectively, In contrast, CPTIO produced a concentr ation-dependent inhibition of ACH-induced relaxation that was greater in control rings, The residual CPTIO-resistant component of relaxation was equivalent to 26 and 43% of initial precontraction in control vs, diabetic rings, respectively, and was not altered by indomethacin, ca talase, cysteine or tetraethylammonium but was significantly inhibited by 1H-[1,2,4]oxadiazolo [4,3,-a]quinoxaline-1-one. These data suggest the release of additional unknown factor(s) that cannot be discerned using NO synthase inhibitors only. This CPTIO-resistant dilator is lik ely not a cyclooxygenase product or a hyperpolarizing factor but a fac tor that acts, in part, by activation of guanylate cyclase, This subst ance is possibly . NO that is not available for reaction with CPTIO ei ther by its diffusibility and sequestration or molecular rearrangement to a redox active form (i.e., not free . NO) or is a completely diffe rent vasodilator, The use of a more lipid soluble nitronyl nitroxide d erivative suggests a portion of the CPTIO-resistant relaxation in diab etic (but not control) rings could be explained by . NO sequestered in the lipid phase.