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Acute physical stresses such as major surgery, insulin-induced hypoglycaemia and exercise are associated with acute increases in circulating concentrations of factor VIII and increases in fibrinolytic activity. The mechanisms involved in producing these responses are partly under hormonal control and there is evidence that the neurohormones adrenaline and arginine vasopressin mediate some of the changes. Adrenaline infusions in man produce increases in both factor VIII and fibrinolysis and the rise in factor VIII is blocked by pretreatment with propranolol. Receptor blockade with propranolol also prevents the rise in factor VIII associated with hypoglycaemia to support the view that adrenaline is an important mediator under these circumstances. The pituitary antidiuretic hormone, arginine vasopressin, produces similar changes in haemostasis at plasma concentrations above those required for its renal effects, but within the range commonly seen during certain physical stresses. However, studies in clinical models that produce increases in vasopressin without a concomitant increase in adrenaline concentrations show enhanced fibrinolysis but no change in factor VIII. Thus it seems that adrenaline and vasopressin have a role in the regulation of haemostasis associated with stress, although the role of vasopressin in the regulation of factor VIII is open to question.