Biochemical quantification of sympathetic nervous activity in humans using radiotracer methodology: fallibility of plasma noradrenaline measurements.

We have developed radiotracer techniques for studying noradrenaline kinetics, to assess better sympathetic nervous system function in humans. Tritiated l-noradrenaline was infused intravenously (0.35 microCi/m2/min) to plateau plasma concentration. Noradrenaline plasma clearance was calculated from plasma tritiated noradrenaline concentration at steady state, and the rate of spillover of noradrenaline to plasma derived from plasma noradrenaline specific radioactivity. Mean noradrenaline spillover at rest in 34 normal subjects was 0.33 micrograms/m2/min (range 0.17-0.61 micrograms/m2/min). Predictably, noradrenaline spillover was reduced in patients with subnormal sympathetic nervous system activity, 0.16 +/- 0.09 micrograms/m2/min in eight patients with idiopathic peripheral autonomic insufficiency, and 0.11 +/- 0.07 micrograms/m2/min (mean +/- SD) in six patients with essential hypertension treated with clonidine (0.45 mg daily). Noradrenaline line plasma clearance in normal subjects was 1.32 +/- 0.28 L/m2/min. Clearance fell with age, causing the previously described rise in plasma noradrenaline concentration with aging. Unexpected effects of drugs were encountered, for example chronic beta-adrenergic blockade in patients with essential hypertension reduced noradrenaline clearance. Plasma noradrenaline concentration measurements were not in agreement with noradrenaline release rate values, and do not reliably indicate sympathetic nervous system activity, in instances such as these where noradrenaline clearance is abnormal.