EFFECTS OF REACTIVE OXYGEN METABOLITES ON NOREPINEPHRINE-INDUCED VASOCONSTRICTION

Aortic rings, 4 mm in length, were obtained from rats and placed on is ometric force transducers in oxygenated Krebs buffer. Following a peri od of stabilization, the cumulative dose response relationship to nore pinephrine was assessed. The vessels were washed and allowed to return to baseline in Krebs buffer containing xanthine (0.5 mM). Xanthine ox idase (0.1 U/ml) was then added to the bath and vessels incubated for 30 min. The vessels were resuspended in Krebs buffer and cumulative do se-response curves to norepinephrine reevaluated. The results indicate that generation of reactive oxygen metabolites by xanthine/xanthine o xidase decreases the pD(2) from 7.80 +/- 0.04 to 7.40 +/- 0.09 with th e endothelium intact. Removal of the endothelium did not attenuate the contractile dysfunction, indicating that endothelial-derived metaboli tes were not mediating the loss of vasoconstrictor effectiveness. Maxi mal tension development did not differ between normal and oxidized ves sel rings. Introduction of oxypurinol (0.2 mg/ml) to the bath prevente d the loss of constrictor responsiveness, thereby confirming that all of the oxidants were derived from the xanthine/xanthine oxidase reacti on. Superoxide dismutase (200 U/ml) partially prevented the loss of no repinephrine responsiveness produced by xanthine oxidase-derived radic als. The pD(2) in the SOD + xanthine/xanthine oxidase-treated vessels rings (7.19 +/- 0.11) was significantly lower than control vessel ring s (7.49 +/- 0.04) and significantly higher than the xanthine/xanthine oxidase-treated vessels (6.89 +/- 0.06). Catalase (1000 U/ml) also par tially attenuated the loss of vascular norepinephrine responsiveness. The pD(2) for the catalase + xanthine/xanthine oxidase-treated vessels (7.15 +/- 0.02) was significantly lower than control vessels (7.39 +/ - 0.07) and significantly higher than the xanthine/xanthine oxidase-tr eated vessels (6.82 +/- 0.11). The pD(2) of vessels treated with a com bination of SOD and catalase (7.40 +/- 0.10) did not differ from contr ol vessels (7.49 +/- 0.12). The results of this study indicate that re active oxygen species produced by the interaction of xanthine with xan thine oxidase depress norepinephrine-induced vasoconstriction. The los s of vasoconstrictor responsiveness appears to involve both superoxide and hydrogen peroxide.