Simplified measurement of norepinephrine kinetics: application to studies of aging and exercise training

We assessed simplified approaches to measurement of steady-state norepinephrine (NE) kinetics (short, nonprimed infusions of [3H]NE or of unlabeled NE and arterialized venous sampling), then reexamined the kinetic mechanism(s) of the age-associated increase in plasma NE, and tested the hypothesis that the latter is the result of a sedentary lifestyle. We studied 17 young (21-28 yr) and 21 elderly (60-76 yr) subjects and a subset (n = 8) of the latter again after 1 yr of physical training. NE appearance rates (Ra) and NE metabolic clearance rates (MCRs), calculated from arterialized venous data, were not significantly different from those calculated from arterial data, whereas those calculated from venous data were substantially (approximately 50%) higher. NE Ra and NE MCR, determined from infusions of unlabeled NE were approximately 20% higher than those determined with [3H]NE, a finding plausibly attributed to approximately 20% suppression of endogenous NE appearance. Arterialized venous plasma NE concentrations were significantly higher in the elderly as a result of significantly higher NE Ra and lower NE MCR. However, arterial NE Ra was not increased, and venous NE MCR was not decreased significantly in the elderly. In the subset of elderly subjects, 1 yr of physical training, which increased peak O2 consumption by 24%, did not decrease plasma NE or NE Ra or increase NE MCR. Therefore, 1) arterial sampling provides no practical advantage over arterialized venous sampling in the measurement of NE kinetics. 2) The use of unlabeled NE infusions to determine NE kinetics overestimates NE Ra and NE MCR by approximately 20%.(ABSTRACT TRUNCATED AT 250 WORDS)