Summary
The effect of the serum content of human clots on their sensitivity to lysis with
plasminogen activators was studied in a system composed of 125I-fibrin labeled clots immersed in buffer or in citrated plasma. The effect was studied
with plasma clots before or after mechanical compression and with whole blood clots
before or after retraction, using either the fibrin specific plasminogen activators
recombinant tissue-type plasminogen activator (rt-PA) or recombinant single chain
urokinase-type plasminogen activator (rscu-PA), and the non-fibrin specific activators
recombinant two chain urokinase-type plasminogen activator (rtcu-PA), or streptokinase
(SK).
In a buffer milieu, all plasminogen activators had a similar fibrinolytic potency
towards serum-rich clots (non-compressed plasma clots or non-retracted blood clots):
50% clot lysis in 4 h required 50 to 85 ng plasminogen activator per ml. Serum-poor
clots (compressed plasma clots or retracted blood clots) were resistant to lysis in
a buffer milieu but became sensitive to lysis following preincubation in plasma for
48 h. These findings indicate that plasma proteins entrapped in clots contribute significantly
to their sensitivity to lysis and suggest that the amount of bound or entrapped plasminogen
may be a limiting factor. In a plasma milieu, all plasminogen activators lysed serum-
rich plasma or blood clots, albeit at higher concentrations (3 to 40 times higher
than in the buffer milieu) and with different efficiencies: 50% clot lysis in 4 h
required approximately 600 ng/ ml of rtcu-PA but 1,500 to 2,000 ng/ml of rscu-PA.
These findings suggest that components of plasma are responsible for increased resistance
of clots towards lysis and that the effect is variable for different plasminogen activators.
Serum-poor plasma or blood clots were very resistant to lysis with non-fibrin specific
agents, but became more sensitive after preincubation in plasma. However, serum-poor
plasma or blood clots were sensitive to lysis with fibrin specific plasminogen activators,
suggesting that during clot lysis with fibrin specific agents, plasminogen recruited
from surrounding plasma may contribute significantly to clot lysis. The concentration
of plasminogen activator required to obtain 50% clot lysis in a plasma milieu of compressed
plasma clots or retracted blood clots was 390 and 1,600 ng/ml respectively for rt-PA
and 1,100 and 3,200 ng/ml respectively for rscu-PA. These data suggest that in a plasma
milieu retracted blood clots are more sensitive to lysis with fibrin specific plasminogen
activators than with non-fibrin specific agents.
Keywords
Clot retraction - Clot lysis - Plasminogen activators
