Studies on Antithrombin III (AT III)

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Abstract
Changes in canine AT III metabolism after i. v. administration of bovine thrombin were studied using I-125-AT III. I-125-AT III was prepared by the method of McFarlane.(1) On the second day after intravenous injection of I-125-AT III, bovine thrombin (30u/kg/hr) was infused continuously for 3hrs. Plasma fibrinogen level was decreased to 60% of initial level, while plasma AT III concentration and AT III metabolism did not change markedly. AT III. metabolism in the dogs infused larger amount of thrombin was studied. Total amount of 5, 000-15, 000u of thrombin was administered intravenously with simultaneous injection of the same units of heparin. Plasma concentration of AT III and the radioactivity of I-125-AT III decreased to 70-80% of the initial levels, although plasma concentration of fibrinogen did not change significantly.(2) When, 50μg of I-125-AT III, 1, 000u of bovine thrombin and 1, 000u of heparin were injected simultaneously, the half life of 1st phase of the plasma disappearance curve of I-125-AT III was shortend to 1.6 hours compared to the normal value of 5.5 hours.From these data, it is suggested that AT III forms a complex with thrombin and thereafter the AT III-thrombin complex is cleared out rapidly from blood stream.

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