Characterization of endothelial thromboxane receptors in rabbit aorta

An increased synthesis of thromboxane (TX) A2 is associated with a number of cardiovascular diseases including atherosclerosis, unstable angina and hypertension. We previously identified a subgroup of NZW rabbits in which isolated arteries failed to contract to the TX agonists, U46619 or I-BOP. In vascular smooth muscle membranes, there was a significant decrease in TX receptors, termed TP. These rabbits are referred to as vTP− and those with the TP receptor are called vTP+. Because TP receptors are expressed in some types of endothelial cells, the present study was designed to determine whether functional TP receptors are present in endothelial cells cultured from aortas of vTP+ and vTP− rabbits. Radioligand binding studies were performed with 125I-BOP. Aortic endothelial cells from vTP+ rabbits exhibited specific and saturable binding. In contrast, in endothelial preparations from vTP− rabbit aortas, no measurable binding to 125I-BOP was detected. Using an anti-TP receptor antibody, we compared the amount of receptor expressed in endothelial cell lysates obtained from vTP+ and vTP− rabbits. Consistent with the results observed radioligand binding assays, the expression of TP receptor protein was decreased in vTP− compared to vTP+ endothelial cells. An in vitro wound healing assay was used on confluent monolayers of endothelial cells. In the untreated vTP+ cells, the area of the scratch was completely closed by 30 h. In the vTP+ cells treated with U46619 (3 μM), the rate of closure of the scratch area was reduced with approximately 12% of the scratch area remaining at 30 h. Pretreatment with the TP receptor antagonist, SQ 29548 (10 μM) prevented the inhibitory effect of U46619. The rate of closure of the scratch in the vTP− was not altered by U46619. In a separate study, U46619 (3 μM) increased the release of 6-keto PGF1α, the stable metabolite of prostacyclin, in vTP+ but not vTP− endothelial cells. Pretreatment with SQ29548 (10 μM) or the cyclooxygenase inhibitor, indomethacin (10 μM) blocked the increase in vTP+ endothelial cells. In vascular reactivity studies in aortas from vTP+ rabbits, removal of the endothelium enhanced the vasoconstrictor response to U46619 indicating that activation of endothelial TP receptors may modulate vascular tone via the release of the vasodilator, prostacyclin. The results of this study suggest an important role for endothelial TP receptors in modulating vascular function.