EXTRAVASCULAR SECRETION OF T-PA BY THE INTACT SUPERFUSED CHOROID

Purpose. Demonstrate the continuous extravascular secretion of enzymat ically active t-PA by the intact choroid. Methods. Enucleated rat eyes were dissected under microscopic control to prepare eyecups for super fusion in a small organ culture system. Individual preparations provid ed access to intact retinal, retinal pigment epithelial, or choroidal surfaces in situ. Cups were superfused with oxygenated Kreb's buffer ( 100 mu l/minute) at 37 degrees C and pH 7.4 while suspended in a micro incubator. Released t-PA activity was measured by an amidolytic assay in 1-minute over flow samples and in cup media after flow interruption s. Results. Minimal basal secretion was detected in the overflow with the retina in place. Interruption of flow (10 minutes) produced a stat ic accumulation of 0.10 +/- 0.01 IU/ml. With the retinal pigment epith elium in place after retinal removal, t-PA release was moderately acce lerated. Removal of the retinal pigment epithelium to expose the Bruch 's-choroidal surface produced the greatest acceleration. The 10-minute static level was 0.48 +/- 0.13 IU/ml. Release from the scleral shell after choroid removal was negligible. An acute burst release of t-PA a ctivity (1.3 +/- 0.18 IU/ml) followed the infusion of bradykinin (5 to 20 mu M) into choroidal cups. Conclusion. Choroid appears to be the d ominant source of a continuously secreted t-PA in superfused rat eyecu p organ cultures. This enzymatically active t-PA is able to pass throu gh an apparently intact Bruch's membrane but is largely impeded by an intact retinal pigment epithelium. It is also possible that retinal pi gment epithelium contains inhibitors that could reduce the released t- PA activity. The authors propose that this secretory function of the u veal tissue, which has thus far not been recognized, may be a principa l mechanism for the regulation of intraocular fluid circulation and ma trix remodeling by plasmin in vivo.