title: Aging and forearm postjunctional alpha-adrenergic vasoconstriction in healthy men.
abstract: BACKGROUND
  Muscle sympathetic vasoconstrictor nerve activity increases with age in healthy humans but does not result in an augmented forearm vasoconstrictor tone. We tested the hypothesis that this is due to a reduction in postjunctional alpha-adrenergic responsiveness to endogenous norepinephrine (NE) release and determined whether this was specific to alpha1- or alpha2-adrenergic receptors.
  
  
  RESULTS
  Forearm blood flow (FBF, by strain-gauge plethysmography) responses to local intra-arterial infusions of tyramine (which evokes endogenous NE release), phenylephrine (selective alpha1-agonist), and clonidine (alpha2-agonist) were determined in 10 young (aged 26+/-1 [mean+/-SEM] years) and 10 older (aged 65+/-1 years) healthy normotensive men after local beta-adrenergic blockade with propranolol. Basal forearm vascular tone was not different in young men and older men. The percentage reduction in FBF in response to the highest dose of tyramine was blunted in older men compared with young men (-37+/-3% versus -49+/-3%, respectively; P<0.01) despite a greater increase in deep venous NE concentration in older men (910+/-103 versus 565+/-69 pg/mL, respectively; P<0.001). Maximal reductions in FBF to phenylephrine were also blunted in older men (-47+/-2% versus -58+/-3% in young men, P<0.05). In contrast, the reductions in FBF (-36+/-7% versus -40+/-3% for older versus young men, respectively) and also in venous NE concentration (-79+/-24 versus -84+/-13 pg/mL for older versus young men, respectively) to clonidine were similar in the 2 groups. Finally, forearm sympathetic alpha-adrenergic vasoconstrictor tone (assessed via nonselective alpha-blockade with phentolamine) was significantly lower in older men.
  
  
  CONCLUSIONS
  Our results indicate that human aging is associated with a reduction in forearm postjunctional alpha-adrenergic responsiveness to endogenous NE release and that this might be specific to alpha1-adrenergic receptors. Furthermore, the contribution of sympathetic alpha-adrenergic vasoconstriction to basal forearm vascular tone is reduced with age in healthy men.
authors.affiliation: Department of Anesthesiology and General Clinical Research Center, Mayo Clinic and Foundation, Rochester, Minn 55905, USA.
authors.forename: Frank A
authors.initials: FA
authors.lastname: Dinenno
authors.forename: Niki M
authors.initials: NM
authors.lastname: Dietz
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Joyner
mesh_terms: D000322:Adrenergic Agonists / Q000494:pharmacology; D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology*; D005542:Forearm / Q000098:blood supply / Q000294:innervation; D006439:Hemodynamics; D006801:Humans; D008297:Male; D009638:Norepinephrine / Q000378:metabolism; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D012039:Regional Blood Flow; D013564:Sympathetic Nervous System / Q000502:physiology; D014661:Vasoconstriction; D014680:Veins / Q000378:metabolism
chemical_list: D000322:Adrenergic Agonists; D011942:Receptors, Adrenergic, alpha; D009638:Norepinephrine