Endothelin receptor blockade attenuates air embolization-induced pulmonary hypertension in sheep

Plasma levels of endothelin-1 after a pulmonary embolism of bone marrow fat

BACKGROUND

During orthopedic surgery, embolization of bone marrow fat can lead to potentially fatal, intra-operative cardiovascular deterioration. Vasoactive mediators may also be released from the bone marrow and contribute to these changes. Increased plasma levels of endothelin-1 (ET-1) have been observed after pulmonary air and thrombo-embolism. The role of ET-1 in the development of acute cardiovascular deterioration as a result of bone marrow fat embolization during vertebroplasty was therefore investigated.

METHODS

Bone cement was injected into three lumbar vertebrae of six sheep in order to force bone marrow fat into the circulation. Invasive blood pressures and heart rate were recorded continuously until 60 min after the last injection. Cardiac output, arterial and mixed venous blood gas parameters and plasma ET-1 concentrations were measured at selected time points. Post-mortem, lung biopsies were taken for analysis of intravascular fat.

RESULTS

Cement injections resulted in a sudden (within 1 min) and severe increase in pulmonary arterial pressure (>100%). Plasma concentrations of ET-1 started to increase after the second injection, but no significant changes were observed. Intravascular fat and bone marrow cells were present in all lung lobes.

CONCLUSION

Cement injections into vertebral bodies elicited fat embolism resulting in subsequent cardiovascular changes that were characterized by an increase in pulmonary arterial pressure. Cardiovascular complications as a result of bone marrow fat embolism should thus be considered in patients undergoing vertebroplasty. No significant changes in ET-1 plasma values were observed. Thus, ET-1 did not contribute to the acute cardiovascular changes after fat embolism.

Modulation and roles of the endothelins in the pathophysiology of pulmonary embolism

Recent research on the endothelins (ETs) and their pathways in acute pulmonary embolism (APE) has led to significant advances in the understanding of this disease. ETs are potent vasoconstrictors and bronchoconstrictors found abundantly in the lung and can be released by stimuli such as endothelial injury, hypoxia, or thrombin, a key product in the coagulation cascade. Many studies using different approaches and methods of inducing pulmonary embolization, both in vitro and in vivo in various species, have mostly shown that ETs play an important role in the pathophysiology of APE. These results were obtained by comparing the hemodynamic data in the presence or absence of various ETs inhibitors, but also by assessing the modulation of the ET-related elements of this system by molecular, cell biology, and pharmacological methods. Based on the current understanding, a mechanism involving the ET pathway in the pathophysiology of APE is proposed for the reader's considerations. We postulate that ETs are primary mediators in APE based on the following: (i) their source from pulmonary endothelial cells where the primary injury takes place; (ii) their direct vasconstrictive, bronchoconstrictive, and promitogenic effects via distinct ET receptors; and (iii) their indirect effects associated with the secondary release of thromboxane and other mediators, which are released from inflammatory cells and platelets, which together can potentiate the overall hemodynamic response, most specifically the pulmonary vascular bed. Such combined effects of ETs on bronchomotor and vasomotor tone in the lung can adversely affect ventilation perfusion matching and lead to severe hypoxemia without causing significant changes in the chest X-ray of these patients. Thus, we may consider ET inhibitors as future current therapeutic agents in patients with PE.

The endothelin A receptor antagonists PD 156707 (CI-1020) and PD 180988 (CI-1034) reverse the hypoxic pulmonary vasoconstriction in the perinatal lamb

Endothelin-1 (ET-1) is considered an intermediary in the constrictor response of the pulmonary vasculature to hypoxia and, by extension, is assigned a prime role in the pathogenesis of pulmonary hypertension. We report here the antihypertensive action in the conscious newborn lamb of two novel endothelin A receptor antagonists, sodium 2-benzo-[1,3]dioxol-5-yl-4- (4-methoxy-phenyl)-4-oxo-3-(3,4,5-trimethoxy-benzyl)-but-2- enoate (PD 156707) and 4-(7-ethyl-benzo[1,3]dioxol-5-yl)-1, 1-dioxo-2-(2-trifluoromethyl-phenyl)-1,2-dihydro-1l6-benzo-[e][1,2]thiazine-3-carboxylic acid potassium (PD 180988), differing in chemical properties and half-life within the body. PD 156707 and PD 180988, given in the right atrium as a bolus followed by infusion, had little or no effect on pulmonary and systemic hemodynamics under normoxia. Conversely, they both reversed the pulmonary hypertension due to alveolar hypoxia while producing minor changes, or no change at all, in systemic vascular resistance. Furthermore, their pulmonary vascular effect outlasted administration. Pulmonary hypertension being elicited by infusion of the thromboxane A(2) analog, 9,11-epithio-11,12-methano-thromboxane A(2) (ONO-11113) was instead not amenable to ET(A)R inhibition. Blood levels of ET-1, which rose with hypoxia but not ONO-11113 treatment, were not changed by either antagonist. Consistent with findings in vivo, when using isolated pulmonary resistance arteries from term fetal lamb, PD 156707 curtailed the hypoxia- but not the ONO-11113-induced constriction. We conclude that PD 156707 and PD 180988 are selective inhibitors of pulmonary vasoconstriction resulting from hypoxia. Our findings support the use of these or allied compounds in the management of pulmonary hypertension in the neonate.

Therapeutic potential of endothelin receptor antagonists in diabetes

Endothelins (ETs) are widely distributed in the body and perform several vascular and non-vascular functions. Experimental evidence indicates that abnormalities of the ET system occur in several organs affected in chronic diabetic complications. Furthermore, ET antagonists were found to prevent structural and functional changes in the target organs of chronic diabetic complications in animal models. Abnormalities of plasma ET levels have also been demonstrated in human diabetes. This review discusses the role of ET in the pathogenesis of chronic diabetic complications. The current experimental evidence suggests that ET antagonism may potentially represent an adjuvant therapeutic tool in the treatment of chronic diabetic complications.