Sympathetic vasoconstriction in skeletal muscle: modulatory effects of aging, exercise training, and sex.

The sympathetic nervous system (SNS) is a critically important regulator of the cardiovascular system. The SNS controls cardiac output and its distribution, as well as peripheral vascular resistance and blood pressure at rest and during exercise. Aging is associated with increased blood pressure and decreased skeletal muscle blood flow at rest and in response to exercise. The mechanisms responsible for the blunted skeletal muscle blood flow response to dynamic exercise with aging have not been fully elucidated; however, increased muscle sympathetic nerve activity (MSNA), elevated vascular resistance, and a decline in endothelium-dependent vasodilation are commonly reported in older adults. In contrast to aging, exercise training has been shown to reduce blood pressure and enhance skeletal muscle vascular function. Exercise training has been shown to enhance nitric oxide-dependent vascular function and may improve the vasodilatory capacity of the skeletal muscle vasculature; however, surprisingly little is known about the effect of exercise training on the neural control of circulation. The control of blood pressure and skeletal muscle blood flow also differs between men and women. Blood pressure and MSNA appear to be lower in young women than in men. However, females experience a larger increase in MSNA with aging compared with males. The mechanism(s) underlying the altered SNS control of vascular function in females remains to be determined. This review summarizes our current understanding of the effects of aging, exercise training, and sex on sympathetic vasoconstriction at rest and during exercise. Areas where additional research is needed are also identified.