Pulmonary Endothelial Cells / Metabolism of Vasoactive Substances

The lungs metabolize a variety of vasoactive substances, including bradykinin (BK), angiotensin I (AT I), PGE₂ and F₂α, norepinephrine, 5-HT, 5’-ATP and 5’-AMP. In contrast, the lungs od not metabolize angiotensin II (AT II), PGA₂, histamine and epinephrine. Of the substances metabolized, all (with the possible exceptions of the prostaglandins) are processed primarily by the pulmonary endothelial cells. Furthermore, the means by which the substances are processed suggest that endothelial cells determine the vasoactive substances allowed to enter the systemic arterial circulation. BK is inactivated while AT I is converted to its potent homolog, AT II. AT II enters the arterial circulation. The metabolism of BK and AT I may be effected by the same enzyme. Pulmonary endothelial cells are a rich source of thromboplastin, an enzyme capable of degrading BK and AT I. However, the relationship of thromboplastin to the fates of these hormones is not clear : The metabolic products produced are not those produced by intact lungs nor by endothelial cells in culture. In addition, thromboplastin degrades substances (e.g. AT II), which are not degraded by intact lungs. Possibly the extrinsic clotting system plays a role when activated but not under physiologic conditions.