Release of a Thromboplastic Substance from Arterial Walls by Epinephrine

Aortic preparations of 70 rabbits including the vasa vasorum were perfused in vitro by oxygenated saline under a pressure of 100 mm Hg. The main branches of the aorta were ligated and the perfusate, which irrigates the aortic lumen, but not the vasa vasorum, was sampled every minute. The samples were assayed for thromboplastic activity by means of the calcium clotting time. The calcium clotting time was shortened significantly in samples taken one, two, and three minutes after injection of 0.005 μg epinephrine. The response was present in perfusing with Tyrode's solution as well as with 0.9% NaCl but was absent if oxygen was withheld from the solution. Larger dose of epinephrine (0.05 μg) caused an initial shortening of the calcium clotting time, followed by a prolongation in two out of five cases. Still larger doses (0.5 μg) or much smaller ones (0.0005 μg) were without effect. In the aortas of rabbits which were pretreated by 5 mg/kg of nialamide two hours before removing the aorta, the previously mentioned effect of epinephrine was prevented. However, direct perfusion of the aortic preparation by a nialamide mixture in vitro did not prevent the release of thromboplastic active substance by epinephrine. Norepinephrine in doses of 0.005, 0.05, and 0.5 μg was found to be ineffective. These observations may explain the effects of epinephrine on the clotting time of blood originally described by Cannon and Gray.5