REGULATION OF INTRACELLULAR XANTHINE-OXIDASE BY ENDOTHELIAL-DERIVED NITRIC-OXIDE

We have previously shown that nitric oxide (NO) donors, such as nitros oglutathione, inhibit endothelial cell (EC) xanthine dehydrogenase (XD )/xanthine oxidase (XO) activity. The purpose of this study was to ass ess whether endothelial-derived NO plays any role in the regulation of intracellular XD/XO. We exposed rat pulmonary microvascular EC to L-a rginine (precursor of NO) or inhibitors of nitric oxide synthase (NOS) , i.e., N-G-nitro-L-arginine methyl esther (L-NAME) and N-G-nitro-L-ar ginine, in conditions of normoxia, hypoxia, and hypoxia followed by re oxygenation. Hypoxia alone caused a 1.9- and a 6.6-fold increase in XO and a 5-fold increase in XO + XD activities after 24 and 48 h of expo sure, respectively. The combination of hypoxia and L-NAME (300 mu M) t reatment amounted at 48 h to a 10- and 7.5-fold increase in XO and XO + XD activities, respectively, compared with normoxic untreated cells. L-NAME also prevented the decline in XD/XO activity that occurred in untreated EC after hypoxia-reoxygenation. On the other hand, treatment with L-arginine caused a dose-dependent decrease in XD/XO activity in hypoxic EC compared with cells provided with L-arginine-free medium. In separate experiments, we assessed the role of L-arginine supplement ation on the in vivo regulation of lung XD/XO by exposing male adult S prague-Dawley rats for a period of 5 days to a hypoxic hypobaric atmos phere (0.5 atm). Exposure to hypoxia produced a significant increase i n lung tissue XO activity and an increase in the ratio of XO to XD. L- Arginine supplementation in the drinking water prevented the increase in lung XO and the XO-to-XD ratio in hypoxic rats and caused a signifi cant decrease in XO and XD in rats exposed to normoxia. In conclusion, this study suggests that endogenous NO has a significant role in the regulation of XD/XO both in vitro and in vivo. By inhibiting XD/XO act ivity, NO may have a modulating effect in conditions of hypoxia and hy poxia-reoxygenation, where this enzyme is thought to be important.