Menstrual cycle alters sympathetic neural responses to orthostatic stress in young, eumenorrheic women

Abstract

Sympathetic baroreflex sensitivity (BRS) and muscle sympathetic nerve activity (MSNA) responses during early follicular (EF) and midluteal (ML) phases of the menstrual cycle are controversial. We hypothesize an augmented sympathetic BRS and MSNA response to orthostatic stress during the ML phase of the menstrual cycle. MSNA, mean arterial pressure (MAP), and heart rate (HR) were recorded during progressive lower body negative pressure (LBNP) (-5, -10, -15, -20, -30, and -40 mmHg; 3 min/stage) in 13 healthy, eumenorrheic women (age 21 +/- 1 yr). Sympathetic BRS was assessed by examining relations between spontaneous fluctuations of diastolic arterial pressure and MSNA at rest and during progressive LBNP. Plasma estradiol (42 +/- 6 vs. 112 +/- 12 pg/ml; P < 0.01) and progesterone (2 +/- 0 vs. 10 +/- 2 ng/ml; P < 0.04) were elevated during the ML phase. Resting MSNA (8 +/- 1 vs. 11 +/- 1 bursts/min), MAP (79 +/- 2 vs. 78 +/- 2 mmHg), and HR (58 +/- 2 vs. 60 +/- 2 beats/min) were not different during EF and ML phases. MSNA and HR increased during progressive LBNP (P < 0.001), and the increases in MSNA burst frequency (bursts/min) and HR were similar during both phases. In contrast, increases in total MSNA (arbitrary units) during progressive LBNP were augmented during the ML phase (P < 0.04), but this response does not appear to be linked to differences in sympathetic BRS. Progressive LBNP did not change MAP during either phase. Our results demonstrate an augmentation of the MSNA response to progressive LBNP during the ML phase of the menstrual cycle. These findings suggest that hormonal fluctuations of eumenorrheic women may influence sympathoexcitation during an orthostatic challenge, but not through sympathetic baroreflex-mediated pathways.