The pathophysiology of ischemia-reperfusion injury and the role played by the interaction of plasma proteins, including complement, with reperfused endothelium remains incompletely understood. Venular endothelial changes due to hypoxia followed by reoxygenation (H-R) are vital because venules are the primary site of fluid accumulation and polymorphonuclear leukocyte deposition due to inflammation. This investigation focused on whether H-R potentiates the response to permeability inducing agents found in activated plasma. Activated complement was studied by using zymosan activated plasma (ZAP). Permeability changes were assessed by quantitating rate of clearance of albumin across the monolayers. H-R alone did not change permeability relative to the normoxic condition. ZAP at 2% in normoxic cells increased albumin clearance from 2 +/- 0.2 to 9 +/- 1. 0 microL/h, which increased significantly to 13.5 +/- 2.0 microL/h when given to hypoxia-reoxygenation challenged monolayers. The permeability response to ZAP was dose related and not present with heat inactivated ZAP. ZAP at 2% altered the structure of the cytoskeleton of the human umbilical vein endothelial cells (HUVEC). However, addition of monoclonal anti-complement antibodies or addition of soluble complement receptor-1 did not attenuate ZAP-induced HUVEC permeability. Addition of zymosan-activated serum did not alter the permeability and addition of heparin inhibited the ZAP-induced changes in permeability, suggesting that these changes were mediated via thrombin and not complement. The increase in monolayer permeability due to ZAP was prevented by increasing intracellular adenosine-3',5'-cyclic monophosphate. These findings suggest that HUVEC monolayers challenged with H-R are more susceptible to increases in permeability induced by activated plasma components.