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.
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