Pharmacological modulation of adrenergic tone alters the vasodilatory response to passive leg movement in young but not in old adults

The age-related increase in a-adrenergic tone may contribute to decreased leg vascular conductance (LVC) both at rest and dur-ing exercise in the old. However, the effect on passive leg movement (PLM)-induced LVC, a measure of vascular function, which is markedly attenuated in this population, is unknown. Thus, in eight young (25 +/- 5 yr) and seven old (65 +/- 7 yr) subjects, this investigation examined the impact of systemic I3-adrenergic blockade (propanalol, PROP) alone, and PROP combined with either a1-adrenergic stimulation (phenylephrine, PE) or a-adrenergic inhibition (phentolamine, PHEN), on PLM-induced vasodilation. LVC, calculated from femoral artery blood flow and pressure, was determined and PLM-induced A peak (LVCApeak) and total vasodila-tion (LVCAUC, area under curve) were documented. PROP decreased LVCApeak (PROP: 4.8 +/- 1.8, Saline: 7.7 +/- 2.7 mL center dot mmHg-1, P < 0.001) and LVCAUC (PROP: 1.1 +/- 0.7, Saline: 2.4 +/- 1.6 mL center dot mmHg-1, P = 0.002) in the young, but not in the old (LVCApeak, P = 0.931; LVCAUC, P = 0.999). PE reduced baseline LVC (PE: 1.6 +/- 0.4, PROP: 2.3 +/- 0.4 mL center dot min-1 center dot mmHg-1, P < 0.01), LVCApeak (PE: 3.2 +/- 1.3, PROP: 4.8 +/- 1.8 mL center dot min-1 center dot mmHg-1, P = 0.004), and LVCAUC (PE: 0.5 +/- 0.4, PROP: 1.1 +/- 0.7 mL center dot mmHg-1, P = 0.011) in the young, but not in the old (baseline LVC, P = 0.199; LVCApeak, P = 0.904; LVCAUC, P = 0.823). PHEN increased LVC at rest and throughout PLM in both groups (drug effect: P < 0.05), however LVCApeak was only improved in the young (PHEN: 6.4 +/- 3.1, PROP: 4.4 +/- 1.5 mL center dot min-1 center dot mmHg-1, P = 0.004), and not in the old (P = 0.904). Furthermore, the magnitude of a-adrenergic modulation (PHEN - PE) of LVCApeak was greater in the young compared with the old (Young: 3.35 +/- 2.32, Old: 0.40 +/- 1.59 mL center dot min-1 center dot mmHg-1, P = 0.019). Therefore, elevated a-adrenergic tone does not appear to contribute to the attenuated vascular function with age identi-fied by PLM.NEW & NOTEWORTHY Stimulation of a1-adrenergic receptors eliminated age-related differences in passive leg movement (PLM) by decreasing PLM-induced vasodilation in the young. Systemic I3-blockade attenuated the central hemodynamic component of the PLM response in young individuals. Inhibition of a-adrenergic receptors did not improve the PLM response in older individu-als, though withdrawal of a-adrenergic modulation augmented baseline and maximal vasodilation in both groups. Accordingly, a-adrenergic signaling plays a role in modulating the PLM vasodilatory response in young but not in old adults, and elevated a-adrenergic tone does not appear to contribute to the attenuated vascular function with age identified by PLM.

Automated summary

The age-related increase in a-adrenergic tone may contribute to decreased leg vascular conductance both at rest and during exercise in the old. However, the effect on passive leg movement-induced leg vascular conductance, which is markedly attenuated in this population, is unknown.