PULMONARY VASCULAR-RESPONSE TO ENDOTHELIN IN RATS

This study investigated the pulmonary vascular response to endothelin (ET) in rats. In conscious rats, an incremental intravenous bolus of ET-1 (100-1,000 pM) caused, after an initial drop in systemic arterial pressure (Psa), a secondary dose-dependent increase of Psa concomitant with a decrease of cardiac output (CO) and heart rate (HR). Pulmonary arterial pressure (Ppa) remained unchanged, and pulmonary vascular resistance (PVR) increased significantly only after 1,000 pM (+40.0 +/- 10.4 at 15 min). Meclofenamate (6 mg/kg iv) did not alter hemodynamic response to ET (300 pM). After autonomic blockade with hexamethonium (6 mg/kg iv) plus atropine (0.75 mg/kg iv), bradycardia response to ET (300 pM) was blocked, but CO decreased, systemic vascular resistance increased, and PVR remained unchanged as in controls. In anesthetized ventilated rats, bolus injections of ET (10-1,000 pM) induced a transient dose-related decrease in compliance (-10.9 +/- 1.8% after 1,000 pM) but no change of conductance. In isolated lungs, Ppa increased at doses > 100 pM, and edema developed in response to 1,000 pM ET. The rise of Ppa in response to 300 pM was not altered by meclofenamate (3.2 x 10(-6) M) but was potentiated by inhibitors of endothelium-derived relaxing factor(s) (EDRF), methylene blue (10(-4) M), pyrogallol (3 x 10(-5) M), and N(G)-monomethyl-L-arginine (6 x 10(-4) M) (3.9 +/- 0.3, 4.6 +/- 0.5, and 5.9 +/- 0.3 mmHg, respectively, compared with 1.5 +/- 0.5 mmHg in control lungs). These results suggest that circulating ET is a more potent constrictor of the systemic circulation than of the pulmonary vascular bed. The pulmonary vasopressor activity of ET appears to be attenuated by the release of EDRFs but not of prostanoids.