Role of Alpha‐Adrenergic Vasoconstriction in Regulating Skeletal Muscle Blood Flow during Single Leg Knee Extension Exercise with Advancing Age

Abstract

BackgroundAging is associated with augmented sympathetic nervous system activity, which promotes vasoconstriction in the peripheral circulation through the alpha‐adrenergic pathway. However, whether this age‐related sympathoexcitation restrains exercising skeletal muscle blood flow and vascular conductance remains poorly understood.MethodsIn nine young (4F/5M, 25 ± 5 yr, 178 ± 8 cm, 73 ± 12 kg) and seven old (2F/5M, 63 ± 11 yr, 175 ± 9 cm, 78 ± 15 kg) healthy volunteers, we evaluated changes in leg blood flow (LBF, Doppler ultrasound), mean arterial pressure (MAP) and leg vascular conductance (LVC) before and after femoral intra‐arterial infusion of phentolamine (a nonselective alpha‐adrenergic antagonist) at rest and during knee‐extensor exercise (0, 5, and 10W). Results are presented as the change from control to adrenergic blockade conditions.ResultsAt rest, similar increases in LBF (269 ± 45 vs. 250 ± 60 ml/min, young vs. older) and LVC (3.4 ± 0.6 vs. 2.9 ± 0.6 ml/min/mmHg, young vs. old) were observed between groups following the adrenergic blockade. During exercise, the adrenergic blockade increased LBF to a similar degree in both young (0W: 330 ± 116, 5W: 178 ± 130, and 10W: 239 ± 127ml/min) and old (0W: 209 ± 57, 5W: 251 ± 22, and 10W: 165 ± 75 ml/min), with no difference between work rates in either group. Similarly, LVC increased as a consequence of the adrenergic blockade in both young (0W: 4.6 ± 1.2, 5W: 3.0 ± 1.2, and 10W: 3.1 ± 1.2 ml/min/mmHg) and old (0W: 3.6 ± 1.1, 5W: 4.2 ± 0.9, and 10W: 3.8 ± 0.9 ml/min/mmHg), with no difference between work rates. Both at rest and during exercise, MAP did not differ between groups or as a consequence of drug administration.ConclusionsUsing a pharmacologic approach to locally block alpha adrenergic vasoconstriction, we observed a similar drug‐induced change in leg hemodynamics in both the young and old groups. These data suggest that sympathetic restraint of limb blood flow persists during small muscle mass exercise, and that this adrenergic response is not impacted by advancing age.Support or Funding InformationFunded in part by the National Institutes of Health (HL118313) and the U.S. Department of Veterans Affairs (RX001697, RX001418, E6910R).