Immunological quantitation of rabbit plasminogen activator inhibitor-1 in biological samples - Evidence that rabbit platelets do not contain PAI-1

Two immunoassays for the specific quantitation of rabbit plasminogen activa tor inhibitor-1 (PAI-1) antigen and activity in biological samples were dev eloped and applied for the evaluation of PAI-1 in rabbits. Levels of PAI-1 antigen in rabbit plasma were 9.8 +/- 3.6 ng/ml (mean +/- SD, n = 6), with a corresponding value of 20.5 +/- 13.5 ng/ml for PAI-1 activity. In rabbit serum PAI-1 antigen was 11.8 +/- 4.9 ng/ml (n = 6) and PAI-1 activity was 2 .9 +/- 2.0 ng/ml (n = 6). Endotoxin injection (20 mu g/kg, iv) induced a ti me-dependent increase of both PAI-1 antigen and PAI-1 activity levels in ra bbit plasma, eventually resulting in a 40- to 90-fold increase (p < 0.0001 vs baseline). A linear correlation was found between PAI-1 antigen and acti vity levels in normal plasma (r = 0.90, n = 6. p < 0.05) and in plasma from endotoxin-treated rabbits (r = 0.98, n = 20, p < 0.001). Analysis of PAI-1 antigen and activity in lysates of washed rabbit platelets revealed the ab sence of PAI-1 (i.e. < 0.03 ng/10(8) platelets). In conclusion, development of specific immunological assays allowed the qua ntitation of PAI-1 in rabbit samples. In striking contrast to other species (human, rat, mouse, pig) rabbit platelets lack detectable amounts of PAI-1 (i.e. > 100-1000 fold lower vs other species studied). This observation ma y have important implications for the use of experimental rabbit models esp ecially in studies on the role of platelets in various pathological conditi ons including thrombosis and atherosclerosis.