Changes in blood levels of noradrenaline in man following short and long term exogenous infusion]

Norepinephrine (NE) was infused in four normal men in doses of 1.5, 6.0 and 9.0 microgram/min for 60 min. The rise in plasma NE was observed only after 5 min and steady state plasma NE and infusion rates. The calculated turnover time averaged 35 sec for 1.5 microgram/min infusion and 20 sec for both 6.0 and 9.0 microgram/min infusion experiments. These differences may be explained by adjustments occurring in inactivation processes in relation to infused doses. A basal endogenous overflow rate of 15 ng kg-1 min-1 was calculated for NE. In the month following NE infusion plasma concentrations decreased exponentially but remained above preinfusion values during about 10 days. The above results confirm the important role of inactivation mechanisms to remove NE from plasma and show that the amine taken up by sympathetic neurons may be released over a long period following the end of infusion.