In vitro and in vivo studies of derivatives of 1,2-diazetine and nitronylnitroxide as donors and acceptors of nitric oxide

Various nitronylnitroxyl radicals (NNR) were studied as paramagnetic scavengers of nitric oxide. The radicals react with NO with rate constants of (0.6-1.1)·10⁴M^-1·sec^-1, forming stable iminonitroxyl radicals. This can be used to measure nitric oxide in solutions by EPR spectroscopy, the sensitivity being 1 μM for detection of NO and 0.3 nM/sec for measuring the NO generation rate in 1 h in a 0.2-ml sample. To overcome fast reduction of the radicals in biological samples, an NNR with a charged functional group was incorporated into the inner volume of large unilamellar phosphatidylcholine liposomes, decreasing the rates of NNR reduction by about 1000-fold. The approach was used to measure the activity of NO synthase from rat cerebellum. NNR were used to study the decomposition of 3,4-dihydro-1,2-diazete 1,2-dioxides (DD), supporting a previously proposed mechanism for DD decomposition with liberation of two mo- lecules of nitric oxide per DD molecule. The rate constants for DD decomposition are in the range from 10^-8 to 6.5·10^-7 sec^-1 in water and from 3·10^-7 to 1.6·10^-5 sec^-1 in dimethylsulfoxide at 37°C. Experiments with perfused rat tail artery showed that some DD derivatives are highly effective vasodilators at concentrations from 5 to 80 μM. Significant (up to 30%) decrease in systolic arterial blood pressure was observed in genetically hypertensive rats (ISIAH-strain) when some DD were injected intraperitoneally (40-200 μg/kg body weight), while the same effect of trinitroglycerin (TNG) was found at much higher dose (900 μg/kg).