Mechanisms of contractile response of cerebral artery to externally-applied fresh blood.

The authors report and analyze the in vitro contractions of fresh bovine cerebral artery on exposure to blood. The vessel does not contract significantly until blood clots on its surfaces; the contraction begins at the moment of clotting and is maintained as long as extracellular calcium is available. Comparative studies with vasoconstrictor amines suggest that serotonin, liberated from platelets by the clotting process, is responsible for this contraction and that the adherent clot itself concentrates the serotonin of the surface of the vessel. This contraction persists throughout the"viability" of the in vitro preparation, approximately 10 hours. Serotonin is capable of producing this sustained contraction by increasing smooth muscle cell membrane permeability to extracellular calcium, which in turn activates native actinomycin.