SYNTHESIS, STORAGE AND REGULATED SECRETION OF TISSUE-TYPE PLASMINOGEN-ACTIVATOR BY CULTURED RAT-HEART ENDOTHELIAL-CELLS

The synthesis, storage and regulated secretion (acute release) of tiss ue-type plasminogen activator (tPA) were studied in cultured rat heart endothelial (RHE) cells. In this study, special attention was paid to the correspondence between tPA metabolism in RHE cells in vitro and t PA metabolism in rats in vivo. RHE cells synthetized tPA, as shown by a spectrophotometric activity assay, by fibrin autography and by an EL ISA for tPA antigen. The synthesis of tPA was strongly enhanced by pho rbol myristate acetate, by all-trans-retinoic acid, by 8-bromoadenosin e 3':5' cyclic-monophosphate (8-br-cAMP) and by compounds that enhance cAMP synthesis, such as cholera toxin and forskolin. Urokinase-type p lasminogen activator and plasminogen activator inhibitor were not dete cted. tPA was constitutively secreted by RHE cells, but was also prese nt in an intracellular pool of small dense granules, from which it cou ld be acutely released by stimulating the cells with thrombin or the c alcium ionophore A-23187. This release was maximal during the first mi nutes after stimulation. In all these aspects of tPA metabolism, RHE c ells closely resembled rat endothelial cells in vivo. It is concluded that cultured RHE rat heart endothelial cells constitute a relevant in vitro model for studying endothelial tPA metabolism.