Sex Differences in the Sympathetic Neural Recruitment and Hemodynamic Response to Head-Up Tilt in Older Hypertensives

This study tested the hypothesis that older hypertensive women display augmented pressor responses and aberrant sympathetic neural discharge patterning in response to orthostatic stress versus older hypertensive men. We evaluated, in older hypertensive and normotensive men and women (n=12 each group), blood pressure, heart rate, cardiac index (acetylene rebreathing), total peripheral resistance, and muscle sympathetic nerve activity (microneurography) at baseline (supine; 3 minutes) and during graded head-up tilt (30 degrees for 5 minutes and 60 degrees for 20 minutes). Sympathetic action potential discharge patterns were studied using wavelet-based methodology. In the upright posture, systolic and diastolic blood pressure responses were greater in hypertensive women versus hypertensive men and normotensive women (P<0.05). No differences existed in the heart rate, stroke index, or cardiac index response between groups; however, the total peripheral resistance response throughout graded head-up tilt was markedly greater in hypertensive women (P<0.01). Yet, the increase in integrated muscle sympathetic nerve activity burst frequency and burst incidence were similar between hypertensive women and men in the supine and upright postures. However, the increase in the mean action potential content per integrated burst and recruitment of previously dormant, larger-sized action potentials during 60 degrees head-up tilt was greater in hypertensive women versus hypertensive men and normotensive women (P<0.001). Therefore, total sympathetic action potential firing frequency was markedly greater in hypertensive women throughout 60 degrees head-up tilt (P<0.001). In conclusion, older hypertensive women displayed exaggerated pressor and peripheral vasoconstrictor responses to orthostasis versus hypertensive men, under conditions of augmented and aberrant sympathetic neural recruitment, rather than increased burst frequency, in the upright posture.