ISOPROTERENOL ANTAGONIZES ENDOTHELIAL PERMEABILITY INDUCED BY THROMBIN AND THROMBIN RECEPTOR PEPTIDE

We determined whether 1) amino-terminal peptides of the thrombin receptor increase endothelial permeability to a comparable extent as a-thrombin does, 2) isoproterenol attenuates the thrombin-induced increase in endothelial permeability by an antagonistic action to that of thrombin or by lowering baseline permeability, and 3) isoproterenol decreases permeability via stimulation of the beta2-adrenergic receptor. Permeability across monolayers of bovine pulmonary artery endothelial cells (CCL 209) was assessed by the clearance of I-125-labeled albumin. Thrombin receptor peptides increased permeability at 1 muM but required a dose of between 10 and 100 muM to equal the permeability response of 1 muM alpha-thrombin. Dose-response experiments demonstrated that isoproterenol antagonized the action of alpha-thrombin and a thrombin receptor peptide on endothelial permeability and that it lowered baseline permeability. This permeability-decreasing action of isoproterenol occurred via stimulation of the beta2-adrenergic receptor. Terbutaline, a partial beta2-agonist, prevented the thrombin-induced permeability, but dobutamine, a partial beta1-agonist, did not. The active stereoisomer of terbutaline and the racemic form mimicked the action of isoproterenol, but the inactive stereoisomer had no effect. ICI-118,551, a specific beta2-receptor antagonist, prevented the permeability-decreasing action of isoproterenol,whereas ICI-89,406, a specific beta1-receptor antagonist, did not. Competitive binding studies of I-125-pindolol with ICI-118,551 or ICI-89,406 demonstrated the presence of beta-adrenergic receptors, predominately beta2-receptors, on cell membrane homogenates. We suggest that 1) alpha-thrombin increases endothelial permeability in part by enzymatically exposing a new amino-terminus that acts as a ligand to activate its receptor and 2) beta2-adrenergic stimulation antagonizes a cell signaling process that thrombin elicits to increase endothelial permeability.