Hormone Replacement and Neural Cardiovascular Control in Postmenopausal Women

Overview

Older women have an exaggerated increase in blood pressure during exercise. However, the reasons for this are unclear. It is important to investigate this phenomenon because a greater blood pressure response to exercise has been associated with an increased risk of stroke and mortality in otherwise healthy individuals. A unique aspect of aging in women is the profound change in hormone levels (i.e. estrogen and progesterone) associated with menopause. The influence of changes in estrogen and progesterone levels on the cardiovascular responses to exercise is poorly understood. However, it has been suggested that these hormones might change the responsiveness of the cardiovascular system. Possible mechanisms that could account for these changes are the arterial baroreflex and feedback from the exercising muscle (known as the exercise pressor reflex), both of which are known to powerfully modulate blood pressure during exercise. However, to date, few human studies have thoroughly examined the influence of changes in hormone levels on baroreflex function during exercise or the exercise pressor reflex in older women. As such, the purpose of this research project is to assess baroreflex function and the exercise pressor reflex in older women after transdermal estrogen and placebo.

Study Design Older postmenopausal women will be randomized, in a double-blinded crossover design, to receive (i) transdermal estradiol, delivery rate 100 µg day-1 plus oral placebo, or (ii) placebo patch plus oral placebo. The experimental measurements and procedures below will be performed before and 4 weeks after each treatment. Experimental measurements: Blood Pressure- A blood pressure cuff will be wrapped around the right arm to obtain blood pressure via a standard automated auscultometric device (Welch-Allyn). In addition, beat-by-beat blood pressure will be obtained via finger photoplethysmography (Finapress blood pressure monitor). Heart Rate- Standard limb lead electrodes will be used to obtain heart rate measurements. Respiration- An elastic band will be placed around the subject's abdomen to measure rate and depth of breathing. Blood Samples- A venous catheter will be placed in the subjects arm for blood samples. For screening purposes measurements of glucose, electrolytes, cholesterol and triglycerides will be made. In addition, progesterone, estrogen and testosterone, will be assessed to verify hormonal status. Sympathetic Nerve Activity- A tiny microelectrode will be placed in the peroneal nerve of the leg, located just below the knee on the outer part of the leg. Alternatively, the median nerve located at the inside of the elbow will be used. At these points the nerves are closest to the surface of the skin. The course of the nerve will be determined by electrically stimulating through the skin with a pencil shaped electrode. When the nerve is stimulated, involuntary twitching and/or tingling sensations of the foot or hand will occur. The twitching or tingling will disappear when the stimulation is stopped. Once the nerve is found, two tiny, sterile, microelectrodes will be inserted through the skin. One is a reference electrode placed just above the nerve site (2 cm) and the other is the recording electrode. The recording electrode is advanced into the nerve. When the tip of the electrode enters the nerve, the subject may briefly notice either pressure or tingling sensations. At this point, minor adjustments in the position of the electrode will be made until an optimal nerve signal is obtained. Limb Blood Flow Measurements- Blood flow to the arm or leg may also be measured by using duplex Doppler ultrasound to non-invasively measure mean arterial blood velocity and diameter. Femoral or brachial blood velocity can be obtained by placing a Doppler flow probe on the skin over the common femoral artery distal to the inguinal ligament or the brachial artery, respectively. Ultrasound imaging of femoral artery or brachial artery diameter will be performed at a site matching that at which velocity is measured. The following formula is used to calculate blood flow: blood flow = π \* radius2 \* velocity. Urine samples will be collected to measure specific gravity and pH using test strips. Experimental Procedures: Neck Pressure and Neck Suction- A padded neck collar will be fitted around the anterior portion of the subject's neck for the application of brief 5-second periods of neck pressure and neck suction. This allows the application of positive or negative pressure to the carotid sinus baroreceptors which are located in this region. The positive pressure will transiently cause these receptors to detect a decrease in blood pressure and thus cause an acute increase in heart rate, and blood pressure. The negative pressure will transiently cause these receptors to detect an increase in blood pressure and thus cause an acute decrease in heart rate, and blood pressure. This procedure has been used extensively to non-invasively assess baroreflex function in healthy subjects aged between 18-80 years. Nevertheless, in subjects over 50 years of age Doppler imaging will be performed to screen for atherosclerotic plaques within the carotid vessels to rule out carotid artery disease and determine the feasibility of performing neck pressure and neck suction. Handgrip Exercise and Post Exercise Ischemia- The subject will be asked to perform handgrip exercise for either two minutes or until they feel like they cannot maintain the exercise (i.e., fatigue). Five seconds before stopping the exercise, a cuff around the upper arm will be inflated to impede blood flow for 2 minutes. This traps the metabolites that were produced during the exercise in the area of the muscle, which maintains stimulation of skeletal muscle metaboreceptor afferents. Since these metaboreceptor afferents contribute to the rise in blood pressure during exercise, continued stimulation of these receptors maintains a blood pressure response.