Cerebrovascular and ventilatory responses to autonomic reflex stimulation in supine and upright postures in women throughout the menstrual cycle and men

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Women are known to have a greater drop in blood pressure upon standing (i.e. orthostatic stress) compared to men leading to fainting or symptoms of lightheadedness and dizziness. Although the exact reasons for this are unclear, different cardiovascular responses to stressors between men and women, and throughout the menstrual cycle, likely play a role. Importantly, breathing, CO2, and O2 each can impact cardiovascular function yet the influence of the menstrual cycle or pharmaceutical estrogen and progesterone (such as oral contraceptives (OC)) on these measurements during upright posture has never been investigated.***The main objective of this research program is to determine the effects of sex and female sex hormones (natural cycling estrogen and progesterone or OC) on the cardiovascular, respiratory and autonomic nervous responses to exercise, upright posture, high or low O2, or high CO2. There will be a specific emphasis on measurements of breathing, brain blood flow, and changes in blood hormones as these have not previously been investigated. This work will investigate young women in two phases of the menstrual cycle, the early follicular phase (i.e. estrogen and progesterone are low) and the luteal phase (i.e. both estrogen and progesterone are high), and age-matched men. Different types of OC will also be investigated and women taking OC will be tested during the placebo week and the maximal dose week. OC can be either cyclic (i.e. high progesterone substitute in all pills with increasing levels of estrogen substitute) or non-cyclic (i.e. high progesterone substitute in all pills with high estrogen substitute throughout the cycle). ***Aim #1: To compare autonomic balance (i.e. balance between sympathetic (‘fight or flight') and parasympathetic (‘rest and digest') control of the cardiovascular system) and blood flow through the body during handgrip exercise, post-exercise circulatory occlusion, passive limb movement, or chemoreceptor stimulation/inhibition (i.e. high or low O2 or CO2) in the supine and upright postures.***Aim #2: To determine the role of sex and female sex hormones on vascular endothelial function via flow-mediated dilation (FMD) measurements between all groups of men and women. FMD will be conducted in the supine and upright positions. ***For both Aims, women will be studied in the early follicular/low hormone phase and the luteal/high hormone phase of the menstrual cycle.***This research will investigate autonomic, cardiovascular, and respiratory responses to physiological stressors and the changes in those responses due to sex and/or female sex hormones. Further, this work is an essential step to investigating the vascular mechanisms behind the responses to these stressors, and will provide previously unknown insight into the reasons for greater incidence of low blood pressure during standing in women. ***********