Evidence for reduced sympatholysis in leg resistance vasculature of healthy older women

Inhibition of a sympathetic stimulus (i.e., sympatholysis) during forearm exercise is reduced with age in women. This age-related alteration has not been characterized in the lower extremity vasculature of women, and the potential for blunting of the conduit artery dilatory response to a sudden increase in shear stress [flow-mediated dilation (FMD)] has not been examined in older adults of either sex. In the present study, we assessed popliteal artery diameter and velocity (Doppler ultrasound) in 16 young (23 ± 1 yr) and 14 older (69 ± 1 yr) women after 5 min of distal calf occlusion (FMD), 3 min of hand immersion in ice water [cold pressor test (CPT)], and 5 min of distal calf occlusion combined with hand immersion in ice water (FMD+CPT). Peak popliteal conductance after 5-min ischemia was not significantly different in young vs. older women. During the combined stimulus (FMD+CPT), the magnitude of vasoconstriction in the calf (reduction in peak popliteal artery conductance) was similar (5-8%), despite reduced resting adrenergic sensitivity to CPT [young (Y): -27.3 ± 3.8%; older (O): -15.8 ± 2.2%; P < 0.05] and blunted muscle sympathetic nerve activity responses to CPT (Y: 12.7 ± 3.6 bursts/min; O: 7.8 ± 2.5 bursts/min; P < 0.05) in older women. Popliteal FMD, normalized to the shear stimulus, was attenuated by 60-70% in older women. Peak popliteal diameter, measured during the combined stimulus (FMD+CPT), was blunted in young but not in older women (Y FMD: 5.5 ± 0.1 mm; Y FMD+CPT: 5.4 ± 0.1 mm; P = 0.03; O FMD: 5.8 ± 0.2 mm; O FMD+CPT: 5.8 ± 0.2 mm). These results confirm previous findings of diminished reactivity in the conduit arteries of older humans and provide the first evidence of reduced sympatholysis in the leg resistance vasculature of older women.