Influences of female reproductive hormones on sympathetic control of the circulation in humans.

The significant increase in cardiovascular disease risk with the loss of estrogen and progesterone at menopause has lead to increasing interest in the cardiovascular influences of female reproductive hormones. In addition to direct influences of estrogen to promote endothelium-dependent vasodilation, recent evidence demonstrates important influences of both estrogen and progesterone on the neural control of the peripheral circulation. These influences have been studied in two general contexts. First, the effects of these hormones on the sympathetic control of the cutaneous circulation have received substantial attention. The control of neurogenic vasodilation in the skin in response to hyperthermia is shifted to higher and lower internal temperatures by progesterone and estrogen, respectively. Reflex vasoconstrictor control of skin blood flow is shifted to higher internal temperatures when the hormones are elevated. Second, reproductive hormones have recently been shown to significantly alter sympathetic neural control of the skeletal muscle circulation. Sympathetic neural control of the skeletal muscle circulation (measured directly as muscle sympathetic nerve activity [MSNA]) is altered by hormone status such that resting MSNA is decreased by estrogen, as is the MSNA response to exercise. Furthermore, the baroreflex control of MSNA is significantly modified by estrogen and progesterone. Therefore, female reproductive hormones have widespread influences on the sympathetic control of the circulation in humans. The individual influences of estrogen and progesterone often antagonize one another, and when both hormone concentrations are increased, the net effect probably depends on their relative concentrations and bioactivity. The mechanisms responsible for these influences and their health-related implications deserve further attention.