Komissarov AA, Florova G, Azghani A, Karandashova S, Kurdowska AK, Idell S. Active ␣-macroglobulin is a reservoir for urokinase after fibrinolytic therapy in rabbits with tetracycline-induced pleural injury and in human pleural fluids. Am J Physiol Lung Cell Mol Physiol 305: L682-L692, 2013. First published August 30, 2013 doi:10.1152/ajplung.00102.2013.-Intrapleural processing of prourokinase (scuPA) in tetracycline (TCN)-induced pleural injury in rabbits was evaluated to better understand the mechanisms governing successful scuPA-based intrapleural fibrinolytic therapy (IPFT), capable of clearing pleural adhesions in this model. Pleural fluid (PF) was withdrawn 0 -80 min and 24 h after IPFT with scuPA (0 -0.5 mg/kg), and activities of free urokinase (uPA), plasminogen activator inhibitor-1 (PAI-1), and uPA complexed with ␣-macroglobulin (␣M) were assessed. Similar analyses were performed using PFs from patients with empyema, parapneumonic, and malignant pleural effusions. The peak of uPA activity (5-40 min) reciprocally correlated with the dose of intrapleural scuPA. Endogenous active PAI-1 (10 -20 nM) decreased the rate of intrapleural scuPA activation. The slow step of intrapleural inactivation of free uPA (t1/2  ϭ 40 Ϯ 10 min) was dose independent and 6.7-fold slower than in blood. Up to 260 Ϯ 70 nM of ␣M/uPA formed in vivo [second order association rate (kass) ϭ 580 Ϯ 60 M Ϫ1 ·s Ϫ1 ]. ␣M/uPA and products of its degradation contributed to durable intrapleural plasminogen activation up to 24 h after IPFT. Active PAI-1, active ␣2M, and ␣2M/uPA found in empyema, pneumonia, and malignant PFs demonstrate the capacity to support similar mechanisms in humans. Intrapleural scuPA processing differs from that in the bloodstream and includes 1) dose-dependent control of scuPA activation by endogenous active PAI-1; 2) two-step inactivation of free uPA with simultaneous formation of ␣M/uPA; and 3) slow intrapleural degradation of ␣M/uPA releasing active free uPA. This mechanism offers potential clinically relevant advantages that may enhance the bioavailability of intrapleural scuPA and may mitigate the risk of bleeding complications. fibrinolytic therapy; rabbit model; pleural injury; urokinase; ␣-macroglobulin; human FIBRINOLYSINS, INCLUDING tissue type (tPA) and urokinase (active two-chain enzyme; tcuPA), are plasminogen activators (PAs) that have long been used to treat a variety of thrombotic conditions including acute myocardial infarction (23; 44), deep vein thrombosis (33, 40), ischemic stroke (1, 3), acute respiratory distress syndrome (ARDS) (20, 21), pulmonary emboli, and organizing pleural effusions (8,9,11,31,37,43). Although intrapleural fibrinolytic therapy (IPFT) has been in use for over 60 years, it has recently undergone reassessment in light of the disparate results seen in clinical trials (9, 11, 37). The efficacy of IPFT in adults remains a subject of ongoing debate. Intrapleural streptokinase was ineffective in patients with complicated parapneumonic pleural effusions and empyema (EMP), whereas ...