Evidence is presented, confirming the presence of an anticoagulant sys
tem in the plasma of Atlantic salmon (Salmo salar L) and rainbow trout
(Oncorhynchus mykiss Walbaum) (Order: Teleostei, Family: Salmonidae)
that bears striking similarities with the protein C anticoagulant syst
em in mammals; its vitamin K-dependence was documented through a warfa
rin feeding trial. A potent activator of this system is the protein C
activator from the venom of the Central American Moccasin, Agkistrodon
bilineatus. When activated, the system splits the tripeptide substrat
e glu-pro-arg-pNa, which is the substrate preferred for the in vitro a
ssay of human protein C. It also prolongates the plasma activated part
ial thromboplastin time, indicating that the system is of clinical rel
evance. A temperature dependence of the plasma protein C-like activity
was evident, the mean activity being 5- to 10-fold higher, but also m
ore variable, in both species in summer and early fall, than it was in
winter. There was also a species difference, with rainbow trout havin
g the higher levels. In man, subnormal values of protein C implies an
increased risk of thrombosis. Whether this applies to fish is not know
n. It is, however, a fact that microvascular thromboses are prevalent
in farmed Atlantic salmon in winter, whereas thrombotic disease is not
a problem in rainbow trout; in the present study plasma protein C-lik
e activity was 30% (of a human reference plasma) in salmon at 4 degree
s C compared to a level of 60% in rainbow trout. A complicating factor
for the assay of protein C-like activity in salmonid. plasma, is the
poor stability of the inhibitory system upon storage. Consequently, as
says have to be done with freshly prepared citrated plasma.