Production of
reactive oxygen species (ROS) may be increased during
hypoxia in pulmonary arteries. In this study, the role of ROS in the effect of
hypoxia on
endothelin (ET) type B (ETB) receptor-mediated vasocontraction in lungs was determined. In rat intrapulmonary (approximately 0.63 mm ID) arteries, contraction induced by
IRL-1620 (a selective ETB receptor agonist) was significantly attenuated after 4 h of
hypoxia (30 mmHg Po2) compared with normoxic control (140 mmHg Po2). The effect was abolished by
tiron, a scavenger of
superoxide anions, but not by
polyethylene glycol (PEG)-conjugated
catalase, which scavenges H2O2. The hypoxic effect on ETB receptor-mediated vasoconstriction was also abolished by endothelium denudation but not by nitro-
L-arginine and
indomethacin. Exposure for 4 h to exogenous
superoxide anions, but not H2O2, attenuated the vasoconstriction induced by
IRL-1620. Confocal study showed that
hypoxia increased ROS production in pulmonary arteries that were scavenged by PEG-conjugated SOD. In endothelium-intact pulmonary arteries, the ETB receptor
protein was reduced after 4 h of exposure to
hypoxia, exogenous
superoxide anions, or ET-1.
BQ-788, a selective ETB receptor antagonist, prevented these effects. ET-1 production was stimulated in endothelium-intact arteries after 4 h of exposure to
hypoxia or exogenous
superoxide anions. This effect was blunted by PEG-conjugated SOD. These results demonstrate that exposure to
hypoxia attenuates ETB receptor-mediated contraction of rat pulmonary arteries. A
hypoxia-induced production of
superoxide anions may increase ET-1 release from the endothelium and result in downregulation of ETB receptors on smooth muscle.