Collateral damage: cardiovascular consequences of chronic sympathetic activation with human aging.

Adult aging in humans is associated with marked and sustained increases in sympathetic nervous system (SNS) activity to several peripheral tissues, including the heart, the gut-liver circulation, and skeletal muscle. This chronic activation of the peripheral SNS likely is, at least in part, a primary response of the central nervous system to stimulate thermogenesis to prevent further fat storage in the face of increasing adiposity with aging. However, as has been proposed in obesity hypertension, this tonic activation of the peripheral SNS has a number of adverse secondary cardiovascular consequences. These include chronic reductions in leg blood flow and vascular conductance, increased tonic support of arterial blood pressure, reduced limb and systemic alpha-adrenergic vasoconstrictor responsiveness, impaired baroreflex buffering, large conduit artery hypertrophy, and decreased vascular and cardiac responsiveness to beta-adrenergic stimulation. These effects of chronic age-associated SNS activation on the structure and function of the cardiovascular system, in turn, may have important implications for the maintenance of physiological function and homeostasis, as well as the risk of developing clinical cardiovascular and metabolic diseases in middle-aged and older adults.