Neural control of the circulation: how sex and age differences interact in humans

The autonomic nervous system is a key regulator of the cardiovascular system. In this review, we focus on how sex and aging influence autonomic regulation of blood pressure in humans in an effort to understand general issues related to the cardiovascular system as a whole. Younger women generally have lower blood pressure and sympathetic activity than younger men. However, both sexes show marked interindividual variability across age groups with significant overlap seen. Additionally, while men across the lifespan show a clear relationship between markers of whole body sympathetic activity and vascular resistance, such a relationship is not seen in young women. In this context, the ability of the sympathetic nerves to evoke vasoconstriction is lower in young women likely as a result of concurrent β2-mediated vasodilation that offsets α-adrenergic vasoconstriction. These differences reflect both central sympatho-inhibitory effects of estrogen and also its influence on peripheral vasodilation at the level of the vascular smooth muscle and endothelium. By contrast postmenopausal women show a clear relationship between markers of whole body sympathetic traffic and vascular resistance, and sympathetic activity rises progressively in both sexes with aging. These major findings in humans are discussed in the context of differences in population-based trends in blood pressure and orthostatic intolerance. The many areas where there is little sex-specific data on how the autonomic nervous system participates in the regulation of the human cardiovascular system are highlighted.