ARTERIAL ALPHA-ADRENERGIC RESPONSIVENESS IS DECREASED AND SNS ACTIVITY IS INCREASED IN OLDER HUMANS

We tested the hypotheses that 1) there is an age-associated decrease i n arterial alpha-adrenergic responsiveness and 2) there is upregulatio n of this response during suppression of sympathetic nervous system (S NS) activity. We measured forearm blood flow (FABF) by plethysmography during brachial artery infusions of the alpha-adrenergic agonist nore pinephrine (NE) and the nonadrenergic agonist angiotensin II (ANG II) in 15 young and 14 older healthy human subjects. Among the old (O) rel ative to the young (Y) we identified greater plasma NE levels (Y: 1.29 +/- 0.07 nM vs. O: 2.14 +/- 0.17 nM; P = 0.0001) a decrease in NE-med iated reduction in FABF [analysis of variance (ANOVA) P = 0.04]; and, in contrast, no difference in ANG II-mediated reduction in FABF (ANOVA P = 0.43). In the nine older subjects studied during guanadrel (G) to suppress SNS activity, we identified decreased plasma NE levels [plac ebo (P): 2.11 +/- 0.24 nM vs. G: 1.09 +/- 0.09 nM; P = 0.002], increas ed NE-mediated FABF response (ANOVA P = 0.01), and no difference in FA BF response to ANG II (ANOVA: P = 0.69) compared with P. We conclude t hat there is appropriate desensitization of arterial alpha-adrenergic responsiveness among the older relative to the young subjects that is specific for the alpha-adrenergic system. Among the older subjects the re is homologous upregulation of this response when SNS activity is su ppressed.