Inactivation of plasminogen activator inhibitor-1 accelerates thrombolysis of a platelet-rich thrombus in rat mesenteric arterioles

To investigate the role of active plasminogen activator inhibitor I (PAI-1) in the evolution of a microthrombus generated in the arteriolar microcirculation. the monoclonal antibody, 33H1F7, which transforms active PAI-I to a tissue type plasminogen activator (t-PA) substrate. was evaluated in an arteriolar thrombosis model in the rat mesentery. Arterioles (200-300 mum) were stimulated electrically to create an endothelial lesion: ADP was then perfused for 2 min to induce the formation of a platelet-rich thrombus which lysed spontaneously in 140 +/- 24 s. Two successive ADP superfusions produced comparable thrombi which lysed in comparable times. Different doses of 33H1F7 were infused to rats for 30 min and the dose which inactivates rapidly and totally active rat PAI-1 (300 mug/kg/min) was selected to be tested on the thrombosis model. Infusion of 33H1F7 beginning 10 min before the ADP application significantly reduced the lysis time in comparison to the control (123 +/- 30 s versus 169 +/- 33 s. P < 0.05, paired Student's t-test) and the cumulative thrombus area during the lysis period was decreased by 56 +/- 7%. These results demonstrate that inactivation of PAI-1 is able to accelerate lysis of a platelet-rich clot in a mesenteric arteriole of the rat. Thus active PAI-1 most likely participates to the resistance to thrombolysis in the arteriolar microcirculation and its inactivation may shorten ischemic periods after microvascular obstruction such as e.g. during cerebral stroke.