Menstrual cycle effects on sympathetic neural responses to upright tilt

Abstract

Young women are more susceptible to orthostatic intolerance than men, though the sex-specific pathophysiology remains unknown. As blood pressure (BP) is regulated through the baroreflex mechanism, we tested the hypothesis that baroreflex control of muscle sympathetic nerve activity (MSNA) during orthostasis is impaired in women and can be affected by the menstrual cycle. MSNA and haemodynamics were measured supine and during a graded upright tilt (30 deg for 6 min, 60 deg for 45 min or till presyncope) in 11 young men and 11 women during the early follicular (EFP) and mid-luteal phase (MLP) of the menstrual cycle. Sympathetic baroreflex sensitivity was quantified using the slope of the linear correlation between total activity and diastolic BP during spontaneous breathing. Baroreflex function was further assessed during a Valsalva manoeuvre (VM). Although MSNA burst frequency responses during tilting were similar between sexes and menstrual phases, increases in total activity were lower in women during EFP than MLP (P = 0.030), while total peripheral resistance and plasma noradrenaline were not similarly lower; upright total activity tended to be lower in women during EFP than men (P = 0.102). Sympathetic baroreflex sensitivity did not differ between sexes (P = 0.676) supine (−281 ± 46 (s.e.m.) units beat −1 mmHg −1 in men vs −252 ± 52 in EFP and −272 ± 40 in MLP in women), at 30 deg tilt (−648 ± 129 vs −611 ± 79 and −487 ± 94), and at 60 deg tilt (−792 ± 135 vs −831 ± 92 and −814 ± 142); this sensitivity was not affected by the menstrual cycle (P = 0.747). Similar sympathetic baroreflex sensitivity between sexes and phases was also observed during the VM. Cardiovagal baroreflex sensitivity assessed during decreasing BP (i.e. early phase II of the VM) was comparable between sexes, but it was greater in men than women during increasing BP (i.e. phase IV); the menstrual cycle had no influences on cardiovagal baroreflex sensitivity. We conclude that the menstrual cycle affects sympathetic neural responses but not sympathetic baroreflex sensitivity during orthostasis, though upright vasomotor sympathetic activity is not clearly different between men and women. Not only sympathetic but also cardiovagal baroreflex sensitivity is similar between sexes and menstrual phases during a hypotensive stimulus. However, cardiovagal baroreflex-mediated bradycardia during a hypertensive stimulus is different between sexes but not affected by the menstrual cycle. Thus, other factors rather than sympathetic baroreflex control mechanisms contribute to sex differences in orthostatic tolerance in young humans.

Próba Valsavy zwiększa czułość składowej przywspółczulnej odruchu spowodowanego odbarczeniem baroreceptorów u młodych zdrowych osób

“…A further advantage of our study was the involvement of a large group of female subjects, as the study by Smith et al only had 3 female participants. Therefore, we confirmed that the cardiovagal baroreflex sensitivity during the hypotensive stimulus is similar between sexes [6]. Variations in the menstrual cycle were not accommodated in our study; however, menstrual phase does not appear to affect the general pattern of cardiovagal unload responses [6].…”

“…Therefore, we confirmed that the cardiovagal baroreflex sensitivity during the hypotensive stimulus is similar between sexes [6]. Variations in the menstrual cycle were not accommodated in our study; however, menstrual phase does not appear to affect the general pattern of cardiovagal unload responses [6]. Increases in sympathetic muscle-nerve activity triggered by VM are proportional to the level of straining [1].…”

Ventilatory baroreflex sensitivity in humans is not modulated by chemoreflex activation

“…The phase of the menstrual cycle might alter ventilatory baroreflex mechanisms. However, recent work suggests that neither the cardiovagal nor sympathetic baroreflexes are affected by menstrual cycle (9,15).…”

Discordant Orthostatic Reflex Renin–Angiotensin and Sympathoneural Responses in Premenopausal Exercising-Hypoestrogenic Women

“…Simulated orthostatic stress augments MSNA more during the mid-luteal phase (high estrogen and progesterone) than in the early follicular phase (low estrogen and progesterone) of the menstrual cycle. 23,24 Combined oral hormonal contraceptives in premenopausal women 25 and transdermal estrogen therapy in postmenopausal women 26 are reported not to influence the magnitude of MSNA increases elicited by orthostatic stress. The reason for such conflicting findings has yet to be resolved but difference in the mode of orthostatic challenge, the comparison of endogenous versus exogenous estrogen preparations, and confounding effects of progesterone 20 are plausible candidates.…”