Cerebral arterial contractions induced by human and bovine thrombin

Purified human and bovine thrombin produced comparable tonic contractions in isolated canine basilar arteries. The magnitude of the contractions was closely related to the number of thrombin Units studied rather than to the amount of protein added to the isolation bath. Thrombin had a much slower onset of action, but was more potent in generating sustained contractions than either serotonin or prostaglandin F2 alpha. Moreover, in contrast to serotonin and prostaglandin F2 alpha, the contractions caused by thrombin were not terminated by equivalent washing. The thrombin-induced contractions were significantly inhibited by prostacyclin, meclofenamic acid, phenoxybenzamine and glycerol. Prostacyclin was the most potent of these inhibitors. The results suggest that thrombin in a "free" form may cause vasoconstriction, in addition to platelet aggregation, in hemostasis and could contribute to the genesis of cerebral vasospasm associated with subarachnoid hemorrhage.