Role of α1-adrenergic vasoconstriction in the regulation of skeletal muscle blood flow with advancing age

Abstract

α1-Adrenergic vasoconstriction during dynamic leg exercise is diminished in younger individuals, although the extent of this exercise-induced “sympatholysis” in the elderly remains uncertain. Thus, in nine young (25 ± 1 yr) and six older (72 ± 2 yr) healthy volunteers, we evaluated changes in leg blood flow (ultrasound Doppler) during blood flow-adjusted intra-arterial infusion of phenylephrine (PE; a selective α1-adrenergic agonist) at rest and during knee-extensor leg exercise at 20, 40, and 60% of maximal work rate (WRmax). To probe the potential contributors to exercise-induced changes in α1-adrenergic receptor sensitivity, exercising leg O2 consumption (V̇o2) and lactate efflux were also evaluated ( n = 10). At rest, the PE-induced vasoconstriction (i.e., decrease in leg blood flow) was diminished in older (−37 ± 3%) compared with young (−54 ± 4%) subjects. During exercise, the magnitude of α1-adrenergic vasoconstriction in the active leg decreased in both groups. However, compared with young, older subjects maintained a greater vasoconstrictor response to PE at 40% WRmax (−14 ± 3%, older; −7 ± 2%, young) and 60% WRmax (−11 ± 3%, older; −4 ± 3%, young). It is possible that this observation may be attributed to lower absolute work rates in the older group, because, for a similar absolute work rate (≈10 W) and leg V̇o2 (≈0.36 l/min), vasoconstriction to PE was not different between groups (−14 ± 3%; older; −17 ± 5%, young). Together, these data challenge the concept of reduced sympatholysis in the elderly, suggesting instead that the inhibition of α1-adrenergic vasoconstriction in the exercising leg is associated with work performed and, therefore, more closely related to the rate of oxidative metabolism than to age per se.