Impact of endothelin-1 in endotoxin-induced pulmonary vascular reactions.

OBJECTIVES: Elevated endothelin-1 (ET-1) levels have been detected during sepsis. The aim of the study was to examine the role of thromboxane A2 (TXA2) and ET-1 in pulmonary vascular reactions after endotoxin (LPS) challenge. DESIGN: Prospective experimental study in rabbits. SETTING: Experimental laboratory in a university teaching hospital. SUBJECTS: Twenty-four adult rabbits of either sex. INTERVENTIONS: Experiments were performed on 30 isolated and ventilated rabbit lungs, which were perfused with a saline solution containing 10% autologous blood. MEASUREMENTS AND MAIN RESULTS: Pulmonary arterial pressure and lung weight gain were continuously registered. Perfusate samples were drawn intermittently to determine ET-1, TXA2, and prostacyclin (PGI2) concentrations. LPS isolated from Escherichia coli (0.5 mg/mL; n = 6) was added to the perfusate. A marked pulmonary arterial pressure increase followed by massive edema formation after 60 mins was observed after LPS injection. At the same time, elevated TXA2 and PGI2 levels in the perfusate were measured. ET-1 was detected 30 mins after LPS infusion (13.4+/-2.6 fmol/L). Pretreatment with the ET(A) receptor antagonist LU135252 (10(-6) M; n = 6) almost completely suppressed the pressure reaction after endotoxin injection (p < .01 at 50 and 60 mins) and reduced edema formation (p < .05). The cyclooxygenase inhibitor diclofenac (10 microg/mL; n = 6) was as effective as LU135252 in preventing vascular reactions after LPS injection. CONCLUSIONS: Pretreatment with the ET(A) receptor antagonist LU135252 and the cyclooxygenase inhibitor diclofenac reduced pulmonary vascular reactions after LPS challenge. Based on the current data, we conclude that the pulmonary arterial pressure increase and edema formation after LPS injection are related to an ET-1- and TXA2-dependent mechanism.

Characterization and distribution of endothelin receptors in the pulmonary circulation: investigation of isolated, perfused, and ventilated rabbit lungs.

The aim of the study was to investigate the distribution of 2 subtypes of endothelin-receptors, mediating the effects of endothelin-1 (ET-1) in the pulmonary circulation. Until now, it is still unclear, whether ET(A) receptors or ET(B) receptors or even both are localized in pulmonary vessels. The experiments were performed on 72 isolated and ventilated rabbit lungs that were perfused with a cell- and plasma-free buffer solution. The arterial pressure and the lung weight gain were continuously registered. Intermittently perfusate samples were taken for determination of thromboxane A2 (TXA2) and prostacyclin (PGI2). The injection of ET-1 (10(-8) M, n = 6) resulted in a biphasic increase in pulmonary arterial pressure (PAP) that was accompanied by the generation of TXA2 and PGI2. Pretreatment with the ET(A)-receptor antagonist LU135252 (10(-6) M, n = 6) suppressed the pressure response after ET-1 application (P < 0.01 at 120 min) and reduced the generation of TXA2 (P < 0.05 at 120 min) and PGI2 (P < 0.05 at 120 min). Pretreatment with the cyclooxygenase inhibitor diclofenac (10 microg/mL; n = 6) also reduced the PAP increase after ET-1 injection. In contrast to this, the pulmonary vascular pressure reaction after ET-1 application was elevated, when ET(B)-receptor antagonist BQ788 (10(-6) M; n = 6) was given. Furthermore, the PGI2 to TXA2 ratio was shifted from 2.3 to 0.9, reflecting a predominance of vasoconstrictive TXA2. The simultaneous application of LU135252 and BQ788 significantly reduced the PAP increase after ET-1 application, but no beneficial effects were observed compared with the application of LU135252 solely. The injection of the ET(B)-receptor agonist sarafotoxin S6c (S6c; 10(-8) M, n = 6) also induced an increase in PAP that was not attenuated by pretreatment with the ET(B)-receptor antagonist BQ788 (10(-6) M, n = 6). LU135252 (n = 6) as well as the application of LU135252 in combination with BQ788 (n = 6) failed to suppress the pressure response after S6c, whereas the cyclooxygenase inhibitor diclofenac (10 microg/mL, n = 6) alone and in combination with LU135252 and BQ788 (n = 6) was able to prevent the PAP increase after S6c injection (P < 0.001). The results demonstrate that the ET-1-induced increase in pulmonary vascular resistance is mainly mediated via ET(A) receptors, whereas ET(B) receptors seem to mediate vasodilation, which was shown by an imbalance of TXA2 and PGI2 generation. On the other hand, the ET(B)-receptor agonist S6c induced vasoconstriction, which was only attenuated by the cyclooxygenase inhibitor diclofenac. From the current results we conclude that, apart from vasoconstrictor ET(A) receptors, at least 2 ET(B)-receptor subtypes are expressed in the pulmonary circulation, one mediating vasoconstriction, which was not blocked by BQ788, and one mediating vasodilation, which was influenced by BQ788.