α-Adrenergic control of skeletal muscle circulation at rest and during exercise in aging humans

Aging is associated with many changes in autonomic nervous system function that often lead to impairments in the normal ability to respond to physiological stressors commonly encountered in daily life. In addition, many of these chronic age-related changes in autonomic-circulatory function can potentially predispose the older adult to elevated risk for acute and chronic cardiovascular complications. One of the most pronounced and repeatable findings with respect to changes in the autonomic nervous system with human aging is the progressive increase in basal muscle sympathetic nerve activity (MSNA) directed to skeletal muscle vascular beds. Although the mechanism(s) underlying this sustained age-associated increase in MSNA are not completely understood, several changes in sympathetic a-adrenergic function occur with age. In this review, the authors discuss how aging affects (1) α-adrenergic control of skeletal muscle vascular tone under resting conditions and the differences that exist in this control of the upper and lower limbs (forearm vs leg circulation); (2) vasoconstrictor responsiveness to endogenous norepinephrine release, as well as the specific responsiveness of postjunctional α₁ - and α₂ -adrenergic receptors; and (3) sympathetic α-adrenergic control of muscle blood flow and vascular tone during exercise in humans. Further, they discuss how these changes in sympathetic α-adrenergic control of skeletal muscle blood vessels have important physiological and clinical implications for the aging human.