Rat platelets adhere to human thrombin-treated rat lungs under flow conditions

Platelet attachment to thrombin-treated endothelium was examined under flow conditions in the vascular bed of isolated perfused rat lungs. The infused thrombin bound to the pulmonary endothelium in a specific, competitive manner as shown by displacement of 125I-alpha-thrombin with an excess of cold diisopropylfluoro-phosphate-alpha-thrombin. alpha-Thrombin significantly increased the attachment of isolated 51Cr-labelled rat platelets to the perfused lungs. Scanning and high voltage electron microscopy showed single platelets in various stages of activation attached to pulmonary endothelium. The receptor binding site and catalytic activity of thrombin were essential to the enhancement of platelet attachment. Aspirin given to rats before the lung isolation had no effect on thrombin-induced platelet attachment. Thus, endothelial bound thrombin initiates platelet activation and enhances subsequent platelet attachment.