MEASUREMENT AND PREDICTION OF SYMPATHETIC NERVOUS-SYSTEM ACTIVITY IN HUMANS

The measurement of norepinephrine (NE) kinetics has broadened our unde rstanding of sympathetic nervous system activity involvement in energy expenditure, blood pressure regulation and substrate utilization. Thi s methodology, however, has several disadvantages that include adminis tration of radioactivity to volunteers, its expense, and time consumin g laboratory analyses. Thus, we developed an equation to predict plasm a NE appearance rate (NEAP) derived from infusions of tritiated norepi nephrine 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 sig nificantly with plasma NE appearance rate. The resulting prediction eq uation in the validation group was: NEAP = 0.00003148 BW [NE], with R- 2 = 0.76 and was successfully cross-validated. Using pooled data from both groups the prediction equation was: NEAP (mu g/min) = 0.00003108 x [NE (pg/ml)] x [Body weight (kg)] with an R-2 = 0.70 and SEE = 0.096 mu g/min. The magnitude of error associated with this equation allows for detection of age-associated, exercise-induced changes and blood p ressure-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 me asurement of arterialized plasma [NE] and body weight.