Older
oral contraceptive (OC) formulations containing high doses of potent
synthetic estrogens and
progestins are associated with increased risk of
thrombosis. To examine the effects of current low-dose OC and
hormone replacement therapy (HRT) regimens on arterial
thrombosis, premenopausal and surgically postmenopausal cynomolgus monkeys were divided into four treatment groups. Premenopausal monkeys were given either no OCs or
ethinyl estradiol and
levonorgestrel as an OC at a dose equivalent to that currently given to women. Postmenopausal monkeys were given either no HRT or
conjugated equine estrogens and
medroxyprogesterone as an HRT at a dose equivalent to that currently given to women. The monkeys were fed an atherogenic diet containing these treatments for 27 to 30 months. At the end of this time, arterial
thrombosis was evaluated with a standardized
stenosis/injury procedure in the left carotid artery. Blood flow velocity was monitored for cyclic or permanent occlusive
thrombosis. The current OC and HRT regimens did not increase the susceptibility of the artery wall to develop an occlusive
thrombus following injury and
stenosis. In fact, there was a reduction in the incidence of
thrombosis in the OC animals compared with untreated controls. Increased amounts of
atherosclerosis were associated with an increased incidence of occlusive arterial
thrombosis. Several selected coagulation parameters [
von Willebrand factor,
protein C,
lipoprotein(a), and platelet aggregation] did not appear to be associated with either the amount of
atherosclerosis or incidence of arterial
thrombosis.