Effects of Age on Sympathetic Vasoconstrictor Responsiveness in Exercising Leg Muscles
Open Access
- Author:
- Koch, Dennis William
- Graduate Program:
- Physiology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 17, 2008
- Committee Members:
- David Nathan Proctor, Dissertation Advisor/Co-Advisor
David Nathan Proctor, Committee Chair/Co-Chair
William Lawrence Kenney Jr., Committee Member
Lawrence Isaac Sinoway, Committee Member
James Anthony Pawelczyk, Committee Member
Mosuk Chow, Committee Member - Keywords:
- vascular control
vasoconstriction
sympatholysis
sympathetic
blood flow
exercise
aging - Abstract:
- Vasoconstrictor responsiveness to acute sympathetic stimulation declines with advancing age in resting skeletal muscle. However, the effect of age on sympathetic vasoconstrictor responsiveness in exercising skeletal muscle has not been examined previously. Therefore, the purpose of these studies was to examine the balance between sympathetic vasoconstriction and metabolic vasodilation in the leg vasculature, both during large and small muscle exercise. The purpose of the first study was to determine if age-related reductions in sympathetic vasoconstrictor responsiveness also occur in exercising skeletal muscle. Thirteen younger (20-30 yr) and seven older (62-74 yr) healthy non-endurance trained men performed cycle ergometer exercise at 60% of maximal oxygen uptake while leg blood flow (femoral vein thermodilution), mean arterial blood pressure (radial artery catheter), and plasma adrenaline and noradrenaline concentrations were measured. After steady state was reached (i.e. ~4 minutes), acute sympathetic stimulation was achieved by immersing a hand in ice water for 2-4 minutes (cold pressor test, CPT). CPT tended to cause a larger increase in mean arterial blood pressure in older men (O:163 vs. Y:102 mmHg) during exercise, but increases in arterial noradrenaline were similar (O:2.560.96 vs. Y:1.980.40 nM). However, the older men demonstrated a larger percentage reduction in exercising leg vascular conductance (leg blood flow/mean arterial pressure) during CPT compared to younger men (O: -13.63.1% vs. Y: -1.54.3%; P=0.04). Leg blood flow tended to increase in the younger men, but not in the older men (P=0.10). These results suggest, in contrast to what has been observed in resting skeletal muscle, that vasoconstrictor responsiveness to sympathetic stimulation is not reduced, but may be augmented in exercising muscle of healthy older humans. The results of the first study raised the possibility that aging enhances sympathetic vasoconstrictor responsiveness in exercising legs. However, age-associated reductions in cardiac reserve or age group differences in the absolute workload performed may have contributed to the increased vasoconstriction observed in older men in that study. In the second study, then, the topic was revisited. In this study, small muscle leg exercise (single leg knee extension) exercise was used to minimize cardiac output limitations, and all subject groups were matched for absolute exercise intensity. Therefore, 10 young (20-30 yrs) and 9 older (60-79 yrs) healthy, recreationally active men performed both active and passive single leg knee extension exercise before, during, and after acute sympathetic stimulation (ischemic handgrip at 40% maximal voluntary contraction to exhaustion, followed by 2 minutes of post-handgrip circulatory arrest). Blood pressure (Finometer), femoral artery blood flow (Doppler ultrasound), femoral conductance, and plasma catecholamines were measured at baseline, during exercise, and during exercise with sympathetic stimulation. No age group differences were found in the femoral vascular conductance response to sympathetic stimulation during passive exercise (P=0.56), but smaller decreases in conductance were noted in older versus younger men during active exercise (Y: 143%; O: 43%, P=0.045). In a subset analysis of 4 younger and 4 older men, sympatholytic responses were augmented in the older men (P=0.002). These results suggest that there is not an age-associated impairment in functional sympatholysis in healthy men under these conditions. Collectively, the results of these studies suggest that primary aging is unlikely to be associated with enhanced vasoconstrictor responsiveness in exercising leg muscles in healthy, normally-active men. Briefly, the results of the first study were not duplicated in the second study in which younger and older subject groups were matched on absolute exercise intensity.