IDENTIFICATION OF FUNCTIONAL PGH2 TXA2 RECEPTORS ON HUMAN ENDOTHELIAL-CELLS

Although functional receptors for thromboxane A2 and prostaglandin H-2 (TxA2/PGH2) have been identified in platelets and vascular smooth mus cle cells, receptor-mediated events in human endothelial cells stimula ted by these endoperoxides have not been shown. Using cultured endothe lial cells harvested from human umbilical or saphenous veins, we measu red the effect of the TxA2 mimetic U46619 on mobilization of cytoplasm ic calcium ([Ca2+]i), as well as release of prostacyclin and expressio n of the proto-oncogene c-fos, intracellular events that have been lin ked to [Ca2+]i rise in stimulated endothelial cells. Addition of U4661 9 to confluent fura 2-loaded endothelial cells caused a concentration- dependent rise in intracellular [Ca2+]i, with agonist concentrations o f 300 nM producing a maximal [Ca2+]i rise. This [Ca2+]i rise was a uni form response observed in all individual endothelial cells throughout the monolayer, as shown by microspectrofluorimetric visualization. Sim ilar effects were seen with a structurally dissimilar endoperoxide ana logue, I-BOP, and with the naturally occurring endoperoxide PGH2. The initial [Ca2+]i rise was not reduced when extracellular [Ca2+]i was ch elated with EGTA, but a later ''plateau'' phase was eliminated. An ant agonist of the receptor for TxA2/PGH2 (SQ29548) strongly inhibited [Ca 2+]i mobilization. Stimulation of endothelial cells with U46619 also t ransiently increased expression of the proto-oncogene c-fos, as determ ined by RNA hybridization, and induced a fivefold increase in prostacy clin release. Thus, endoperoxides can stimulate human venous endotheli al cells by means of TxA2/PGH2 receptors, whose occupancy can activate intracellular events associated with functional changes.