1. An improved approach for the determination of plasma [3H]noradrenaline ([3H]NA) kinetics in man is described, incorporating the extraction of plasma [3H]catechols on alumina and separation of [3H]NA from [3H]dihydroxymetabolites by high-performance liquid chromatography (HPLC).
2. After a 30 min intravenous infusion, [3H]NA accounted for 57.2 ± 13.2% of the radioactivity recovered by the procedure, while the dihydroxymetabolites 3,4-[3H]dihydroxyphenylethylene glycol ([3H]DHPG) and 3,4-[3H]dihydroxymandelic acid ([3H]DOMA) accounted for 32.3 ± 11.5% and 4.9 ± 6.0% respectively. After 90 min of constant infusion the proportion due to [3H]NA fell to 44.4 ± 10.4%, while that due to [3H]DHPG rose to 45.9 ± 9.5% because of an increase in the amount of [3H]DHPG at the later time.
3. Plasma [3H]NA radioactivity rose rapidly during the constant infusion and usually reached a plateau by 30 min. However, in individual subjects large variations in plasma [3H]NA radioactivity occurred during the course of the infusion, implying rapid and variable changes in plasma [3H]NA clearance.
4. The inclusion of a step to separate [3H]NA from [3H]dihydroxymetabolites is necessary if the aim is to determine plasma [3H]NA kinetics, as a large proportion of the radioactivity recovered from plasma on alumina is due to the presence of these metabolites.