Aging and forearm postjunctional alpha-adrenergic vasoconstriction in healthy men

Area of Science:

• Cardiovascular Physiology
• Aging Research
• Neuroendocrinology

Background:

• Muscle sympathetic nerve activity increases with age in humans.
• However, this does not lead to increased forearm vasoconstriction.
• This suggests a potential reduction in postjunctional alpha-adrenergic responsiveness.

Purpose of the Study:

• To test if reduced alpha-adrenergic responsiveness to norepinephrine (NE) causes blunted vasoconstriction with age.
• To determine if this effect is specific to alpha1- or alpha2-adrenergic receptors.

Main Methods:

• Compared forearm blood flow (FBF) responses to tyramine, phenylephrine (alpha1-agonist), and clonidine (alpha2-agonist) in young and older men.
• Measured deep venous norepinephrine (NE) concentrations.
• Utilized local beta-adrenergic blockade and nonselective alpha-blockade.

Main Results:

• Tyramine-induced FBF reduction was blunted in older men despite higher NE release.
• Phenylephrine-induced FBF reduction was also blunted in older men, indicating reduced alpha1-adrenergic responsiveness.
• Clonidine responses were similar between age groups, suggesting alpha2-receptors were unaffected.
• Overall sympathetic alpha-adrenergic vasoconstrictor tone was lower in older men.

Conclusions:

• Human aging reduces forearm postjunctional alpha-adrenergic responsiveness to endogenous NE.
• This reduction appears specific to alpha1-adrenergic receptors.
• Sympathetic alpha-adrenergic vasoconstriction's contribution to basal forearm vascular tone is diminished in aging healthy men.