Sex and hormonal influences on platelet sensitivity and coagulation in the rat

Platelet sensitivity to adenosine di-phosphate (ADP), thrombin, collagen, arachidonic acid and prostaglandin I2 (PGI2) and the activity of the coagulation system as measured by the activated partial thromboplastin time, prothrombin time, Russell's viper venom time and plasma fibrinogen have been examined in male and female rats, female rats during the oestrous cycle and female rats treated with oestrogen and a progestogen. Male rat platelets were less sensitive to thrombin and more sensitive to inhibition by PGI2 than those from females and fibrinogen levels in male rat plasma were approximately twice those seen in females. During the oestrous cycle, platelets were more sensitive to ADP and less sensitive to thrombin at dioestrus. Following 6 weeks treatment with 17 beta-oestradiol or ethynyl oestradiol, both platelet aggregation and release of granular ATP induced by collagen were significantly reduced. Platelet sensitivity to other agents, ADP, arachidonic acid, thrombin and PGI2 was, however, unchanged following oestrogen treatment. Activation of factor X by Russell's viper venom was accelerated in rats treated with ethynyl oestradiol, although this enhancement was not reflected in the overall clotting times.