1. During major abdominal surgery there are increases in Factor VIII and plasminogen activator activity, associated with elevated plasma concentrations of vasopressin, of a magnitude shown to affect haemostasis.

2. To investigate the mechanisms involved in the haemostatic response to surgery, 12 patients undergoing fibre-optic colonoscopy were studied, of which six had a complete and six had an incomplete examination.

3. Venous blood samples were taken before, during and after the procedure for assay of plasma vasopressin, adrenaline and noradrenaline concentrations, Factor VIII coagulant activity, von Willebrand factor antigen level, euglobulin clot lysis time, tissue-type plasminogen activator activity and tissue-type plasminogen activator inhibition.

4. In the six patients who underwent a complete procedure the median plasma vasopressin concentration rose from 0.6 pg/ml to 153 pg/ml during colonoscopy. Factor VIII coagulant activity rose from 0.9 to 2.4 i.u./ml and von Willebrand factor antigen level rose from 139 to 224%. Plasminogen activator activity increased from 20 to 144 units and tissue-type plasminogen activator activity rose from 107 to 1338 m-i.u./ml, whereas tissue-type plasminogen activator inhibition fell from 4.8 to 1.0 i.u./ml.

5. In the six patients in whom a limited procedure was performed, there were no changes in haemostatic function or in plasma vasopressin concentration. Plasma concentrations of adrenaline and noradrenaline did not change in either group.

6. The results indicate that vasopressin regulates the intrinsic coagulation pathway and fibrinolytic system in the absence of adrenaline release.

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