Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Dr. J. Kevin Shoemaker


The purpose of this thesis was to test the hypothesis that changes in circulating sex hormone levels are associated with changes in muscle sympathetic nerve activity. The hypothesis was tested through the comparison of low- (early follicular [EF]) and high-hormone (midluteal [ML]) phases of the menstrual cycle and of hormonal contraceptive use (low hormone [LH] versus high hormone [HH]). The microneurography technique was used to compare both the frequency and size of bursts in muscle sympathetic nerve activity (MSNA) at baseline and during two sympatho-excitatory maneuvers: baroreceptor unloading elicited through lower body negative pressure, and chemoreflex stimulation elicited through a hypoxic-hypercapnic end-inspiratory apnea. Sympathetic responses to chemoreflex stimulation were also compared between women and men. All associations between MSNA and hormone phases occurred similarly between users and non-users of hormonal contraceptives. At baseline, MSNA was relatively elevated during the high hormone phases (ML and HH), at which point baseline sympathetic activity was similar to that observed in men. However, stimulation of the chemoreflex resulted in greater sympathetic activation during the low hormone phases (EF and LH) relative to the high hormone phases. Further, this hormone phase effect was mediated largely by greater increases in burst size, rather than the burst frequency component. This may indicate that central integration sites for MSNA are affected by circulating sex hormone levels. Finally, the sympathetic responses to baroreceptor unloading were graded to reductions in stroke volume, which, in turn, were affected by hormone levels. However, no evidence was observed to suggest a change in the central integration of baroreceptor afferent input occurred across phases of the menstrual cycle or hormonal contraceptive use in terms of baroreflex function. Together, these studies confirm that sympathetic nerve activity at baseline and sympathetic recruitment during chemoreflex stimulation are affected by hormone phase, while baroreceptor-mediated responses are not affected by the transition from low (EF and LH) to high hormone phases (ML and HH).