Menopause is a natural event that generally occurs in women between the ages of 45 and 55. During menopause, the body starts producing less estrogen and progesterone until menstruation eventually stops. Estrogen and progesterone are involved in many important functions in a woman's body, and the drastic decline of these hormones in menopause leads to significant changes in the body. Along with such changes, postmenopausal women are at a higher risk than premenopausal women for certain health problems, such as CVD. Previous studies have revealed that alterations in the breakdown of fatty acids in cardiac muscle play a key role in a variety of cardiac disorders. In studies involving human skeletal muscle, estrogen has been shown to increase the breakdown of fatty acids, while progesterone lessens this effect. This study will determine in postmenopausal women whether estrogen increases the heart's ability to break down fats for energy use and whether progesterone decreases this effect. This study will also analyze ovariectomized mice to determine if the candidate specific estrogen receptor modulators (SERMs) raloxifene and tamoxifen increase the heart's ability to use fats as energy and whether the increase is similar to that seen with estrogen. Study investigators will also create estrogen receptor knock-out mice (mice with estrogen receptors removed) to further explore the roles of estrogen and SERMs in heart metabolism.

Participation in this double-blind study will last up to 1 month and will include three study visits. During Visit 1, participants will undergo three standard clinical evaluations. The first evaluation, a medical screening, will include a medical history exam and blood tests to measure estrogen and progesterone levels, liver and kidney function, cholesterol levels, and blood sugar and insulin levels. For the second evaluation, participants will undergo a body composition study to measure total body fat and muscle content using a dual-energy x-ray absorptiometry (DEXA) scan. During the third evaluation, participants will undergo an electrocardiogram (ECG) and an echocardiogram (ECHO), each performed immediately before and after walking on a treadmill. The ECG will measure electrical activity of the heart, and the ECHO will involve imaging the heart with an ultrasound.

Visits 2 and 3, occurring 3 days apart, will each include two imaging tests of the heart: a positron-emission tomographic (PET) scan and a resting ECHO. Throughout both tests an ECG and blood pressure cuff will be used to monitor heart rhythm and blood pressure, respectively. During the PET scan, participants will lie flat in an imaging machine for three 45- to 60- minute intervals. Blood will be drawn and radioactive tracers will be injected via intravenous lines placed in the arms. The ECHO test will also be done during the PET scan.

Between Visits 2 and 3, participants will be randomly assigned to one of two hormone replacement therapy (HRT) regimens: estrogen plus placebo or estrogen plus progesterone. All participants will wear a patch containing estrogen and take a pill of either placebo or progesterone for the 3 days leading up to Visit 3. All participants will be asked for permission to store a sample of their blood for up to 10 years to be used in future research studies.