INHIBITION BY SINGLET MOLECULAR-OXYGEN OF THE VASCULAR REACTIVITY IN RABBIT MESENTERIC-ARTERY

1 The effects of reactive oxygen intermediates derived from photoactiv ated rose bengal on the vascular reactivity have been evaluated in rab bit mesenteric artery ring preparations. The artery rings were exposed to xanthene dye rose bengal (50 nM) illuminated (6,000 lux) at 560 nm for 30 min. Spin trapping studies with 2,2,6,6-tetramethylpiperidine (TEMP) and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) with electron spin resonance spectrometry were also conducted in solution (and not within tissues) to determine quantitatively the reactive oxygen species gene rated from photoactivated rose bengal. 2 Contraction of the ring prepa rations induced by noradrenaline (10(-8) to 10(-4) M) was attenuated b y previous exposure to photolysed rose bengal; the observation that th e pD(2) decreased without a significant reduction in maximum tension g eneration is consistent with the view that receptor dysfunction may be involved in the effect of photolysed rose bengal. 3 Prior exposure to photolysed rose bengal of the ring preparations inhibited the endothe lium-dependent relaxation evoked by acetylcholine (10(-6) M) and calci um ionophore A23187 (10(-7) M), but not the endothelium-independent re laxation evoked by nitroglycerin (10(-6) M). 4 A variety of scavengers , superoxide dismutase (33 units ml(-1)), catalase (32 units ml(-1)) a nd 1,3-dimethyl-2-thiourea (DMTU, 10 mM), which should eliminate the s uperoxide anion radical, H2O2 and the hydroxyl radical, had no effect on the attenuated responses to noradrenaline and acetylcholine induced by photolysed rose bengal. In contrast, the inhibition of the observe d effect of photolysed rose bengal was obtained with addition of histi dine (25 mM), a singlet molecular oxygen quencher. 5 It was found that photolysis of rose bengal from a 1:2:2:1 quartet, characteristic of t he hydroxyl radical-DMPO spin adduct, which was effectively blunted by DMTU, superoxide dismutase and catalase whereas histidine was ineffec tive. The results of the electron spin resonance study also showed tha t a singlet molecular oxygen was produced by photoactivation of rose b engal; this was detected as singlet oxygen-TEMP product (TEMPO; 2,2,6, 6-tetramethylpiperidine-N-oxyl). The formation of the TEMPO signal was strongly inhibited by histidine, but not by DMTU, superoxide dismutas e and catalase. 6 It is suggested that the superoxide anion radical, H 2O2 and hydroxyl radical are formed in addition to singlet molecular o xygen, and the data obtained from the present study indicate that sing let molecular oxygen is one of the most destructive oxygen species. En dothelium-dependent relaxation is quite vulnerable to singlet molecula r oxygen. Singlet oxygen also depresses noradrenaline-induced contract ion possibly via oc-adrenoceptor dysfunction. This, in turn, may lead to vascular incompetence.