Vasodilation and vascular control in contracting muscle of the aging human

There is accumulating evidence in humans that the control of blood flow to dynamically contracting skeletal muscle is altered with normal aging. Despite some inconsistencies in the literature, most healthy older subject groups studied to date (approximately 55-75 years) exhibited attenuated leg blood flow responses to moderate intensity leg exercise in comparison to younger (approximately 20-30 years) controls. In most cases, older subjects also have higher arterial perfusion pressures at rest and at any submaximal workload than their younger counterparts, suggesting a heightened level of vasoconstriction in the legs that could result from structural and/or functional alterations within the arterial vasculature. The influence of age on vasodilator responsiveness in the contracting limbs of humans remains an open question, with reports of blunted, similar, and augmented increases in local vascular conductance during graded exercise in older vs. younger adults. Recent results from the authors' laboratory suggest that the ability of vascular smooth muscle in the legs to dilate in response to physiological and pharmacological stimuli is relatively well preserved with advancing age in men; however, a sex difference may exist in that older estrogen-deficient women exhibit blunted leg vasodilator responses compared to younger women. Potential mechanisms underlying altered vascular control within the resistance vasculature of contracting muscles of older men and women remain to be determined, but could include (1) fewer, smaller, and/or stiffer vessels, (2) impaired endothelium-dependent vasodilation, (3) augmented sympathetic vasoconstriction, (4) alterations in metabolic or myogenic control, and (5) reduced effectiveness of the skeletal muscle pump. Systematic research involving both humans and animal models will be necessary to fully elucidate the mechanisms underlying compromised muscle blood flow in old age.