Influence of Early Menopause on Sympathetic Activation and Cardiovascular Function in Older Women

The current study is recruiting two groups of women aged between 60-70 years old; 1) women who had early natural/non-surgical menopause (age 40-44 yrs) and 2) women who had normal natural/non-surgical menopause (age 50-54). The investigators are recruiting women who do not have any cardiovascular disease, hypertension, diabetes, and other major health issue. The main hypothesis of the project is that early menopause negatively impacts the heart, blood vessel and nerve system due to the increased length of time with reduced hormone levels; which could explain the relationship between early menopause and greater risk of cardiovascular disease in later life. Sympathetic nerve system and cardiovascular functions will be evaluated in both groups to compare the impact of prolonged hormone deficiency. In addition, current study will examine the impact of inorganic nitrate supplementation (beet root juice) to see if there are any favorable changes after 2 weeks of beet root juice intake. There are 2 main test visits in the current study. Visit 1. Nerve system test (maximally take about 5 hours): Participants will rest in the supine position for ≥ 20 mins. Resting heart rate, breathing rate, finger blood pressure will be monitored. Cardiac output will be assessed using the acetylene gas rebreathing technique (1 hour). IV will be inserted for blood draw. Nerve signal search will be performed using a thin needle behind knee to find a signal from a specific nerve. At the same time, transcranial Doppler probe will be placed on the head to look at brain blood flow using soft head gear (1-2 hour). After acceptable signals are obtained, baseline data collection will begin. It will be followed by handgrip exercise and hand immersion to cold water to examine neural sympathetic response to different stimuli (30 mins). After this, to evaluate nerve response to different degrees of passive standings, a bed will be tilted to 30° and 60° for 8 mins each. Blood samples will be obtained in the supine resting position and at the end of 60° tilt (45 mins). Visit 2. Heart and vessel function assessment (Proximately take 2 hours): All measurements are non-invasive for this visit. Participants will rest in the supine position for ≥ 20 mins. Resting heart function will be assessed by ultrasound echocardiography (30mins). Vascular stiffness measurement will be performed using a probe placed on wrist, neck and leg groin area (30 mins). Vascular endothelial function will be measured using flow-mediated dilation and followed by smooth muscle responsiveness test will be performed by measuring brachial artery diameter changes for 10 min following 0.4 mg sublingual nitroglycerin administration (1 hour). After Visit 1-2, participant will be asked to drink either dietary nitrate supplementation or placebo for 2 weeks (randomly assigned). After 2 weeks intervention, participant will come back for post-intervention tests which are identical to visit 1 and 2. After completion of the first intervention, participants will have at least 2 weeks of wash-out period. After 2 weeks participant will be back to the lab and repeat the visit 1 and 2, as well as 2 weeks of dietary nitrate supplementation or placebo. (For those who received 2 weeks of nitrate supplementation for the first intervention, it will be a placebo for this intervention. For those who received placebo for the first intervention, it will be a nitrate supplementation for this intervention). After intervention participants will repeat the visit 1 and 2 again.