Measurement and prediction of sympathetic nervous system activity in humans

The measurement of norepinephrine (NE) kinetics has broadened our understanding of sympathetic nervous system activity involvement in energy expenditure, blood pressure regulation and substrate utilization. This methodology, however, has several disadvantages that include administration of radioactivity to volunteers, its expense, and time consuming laboratory analyses. Thus, we developed an equation to predict plasma NE appearance rate (NEAP) derived from infusions of tritiated norepinephrine performed in 113 healthy men (18-78 yr.). The accuracy of the equation was tested using cross-validation procedures. Age, fat mass, plasma [NE], waist circumference, and body weight (BW) correlated significantly with plasma NE appearance rate. The resulting prediction equation in the validation group was: NEAP = 0.00003148 BW [NE], with R2 = 0.76 and was successfully cross-validated. Using pooled data from both groups the prediction equation was: NEAP (microg/min) = 0.00003108 x [NE (pg/ml)] x [Body weight (kg)] with an R2 = 0.70 and SEE = 0.096 microg/min. The magnitude of error associated with this equation allows for detection of age-associated, exercise-induced changes and blood pressure-related differences in plasma NE appearance rate published in the literature. We provide an equation that offers a relatively simple and accurate alternative to estimate plasma NE appearance from the measurement of arterialized plasma [NE] and body weight.