Oral contraceptives, lanosterol, and platelet hyperactivity in rat.

Lanosterol, a cholesterol precursor that increases considerably in the platelets of rats treated with oral contraceptives, was incubated with either platelet-rich plasma or washed platelet suspension. After 2 minutes there was a remarkable dose-related increase in platelet activity. This platelet hyperactivity was measured by clotting time and platelet aggregation could not be reproduced by cholesterol or ethinylestradiol.