Plasma norepinephrine pharmacokinetics during mental challenge.

We simultaneously infused tracer-labeled norepinephrine (NE) and isoproterenol (ISO) intravenously into 14 subjects to measure forearm and total body NE pharmacokinetics at rest and in response to mental challenge (video game or cognitive task). Mental challenge was associated with significantly increased heart rate (24%), systolic blood pressure (13%), cardiac output (impedance cardiography, 9%), forearm blood flow (38%), and the rate of release of endogenous NE into arterial blood (total body NE spillover, 29%), but not with changes in cardiac output (r = 0.68) and systolic blood pressure (r = 0.60), whereas those of antecubital venous NE were not. Forearm extraction of NE was related inversely to forearm blood flow both at rest (r = -0.80) and during mental challenge (r = -0.81), and total body clearance of NE was positively related to cardiac output at rest (r = 0.78) and during mental challenge (r = 0.54). The results indicate that mental challenge is associated with generally increased sympathetically-mediated NE release that determines the hemodynamic responses. Because of regional changes in sympathetic activity and blood flow during psychological stress, changes in antecubital venous NE and even arterial NE may not reflect accurately sympathetic nerve activity. Measurement of total body and regional NE pharmacokinetics avoids these difficulties.