Chronic hypoxia augments endothelin-B receptor-mediated vasodilation in isolated perfused rat lungs

To investigate whether chronic hypoxia affects endothelin-B (ETB) receptor-mediated pulmonary vasodilation, we compared the vasodilator responses to IRL-1620, a selective ETB-receptor agonist, in isolated perfused lungs from normoxic and chronically hypoxic adult male rats. IRL-1620 caused a dose-dependent vasodilation that was greater in the hypertensive lungs than in the normotensive lungs. In normotensive lungs, a nitric oxide (NO) synthase inhibitor, N-omega-nitro-L-arginine (L-NNA; 300 mu M), and an ATP-sensitive potassium (K-ATP)-channel inhibitor, glibenclamide (Glib; 10 mu M), each reduced the vasodilator response to IRL-1620 (1 nM), but the combination of L-NNA and Glib inhibited it more effectively than either drug alone. In contrast, L-NNA alone, but not Glib alone, completely blocked IRL-1620-induced vasodilation in hypertensive lungs. The vasodilator response to a K-ATP- channel opener, NIP-121 (1 mu M), but not the response to sodium nitroprusside (1 mu M), was enhanced in hypertensive lungs. We also found increased expression of mRNA for the ETB receptor in lung tissue after hypoxic exposure. In addition, semiquantitative immunohistochemistry demonstrated higher expression levels of ETB receptors in the endothelium of distal segments of the pulmonary artery in hypoxic than in normoxic rats. These results suggest that ETB receptor-mediated pulmonary vasodilation is augmented after chronic hypoxic exposure and that release of NO may be the sole mechanism of this vasodilation in hypertensive lungs, whereas both release of NO and activation of K-ATP channels are involved in normotensive lungs. We speculate that the underlying mechanism responsible for this augmentation may partly be related to upregulation of ETB receptors in the endothelium of pulmonary resistance arteries in hypertensive lungs.