Little information is available regarding the impact of cyclic hormonal changes during the menstrual cycle on cardiovascular control by the sympathetic nervous system in women. In animals, estrogen and progesterone are known to alter circulatory control by affecting cardiovascular reflex function, endothelial function, vasomotor responses, and plasma volume. In this context, there have been limited studies that directly investigate the impact that hormonal changes during the menstrual cycle have on the sympathetic nervous system and cardiovascular function in humans. Additionally, the impact of oral contraceptives on these relationships have not been examined. Lastly, there is limited information regarding how the menstrual cycle or use of oral contraceptives alters the overall autonomic and cardiovascular responses to an integrative challenge such as orthostatic stress. In this proposal we will test the general hypothesis that during the luteal phase of the menstrual cycle(or the high-hormone phase of oral contraceptive use) the muscle sympathetic nerve activity and vasoconstrictor responses to sympathoexcitatory maneuvers (i.e., baroreceptor unloading, handgrip exercise, or orthostatic challenge) will be augmented. The strength of this proposed experimental paradigm is that studies are designed to investigate sequentially baroreceptor sensitivity, record the resulting change in sympathetic activity, and measure the vascular resonses to changes in sympathetic traffic during the follicular and luteal phases of the menstrual cycle. The functional impact of these changes on orthostatic tolerance will also be examined. The role of long-term exogenous estrogen and progesterone administration (oral contraceptives) on autonomic-circulatory control will be evaluated.
