Neuronal uptake, metabolism, and release of tritium-labeled norepinephrine during assessment of its plasma kinetics.

The extent to which intravenously infused [3H]norepinephrine ([3H]NE) is stored within and released by sympathetic nerves (tracer recycling) was examined by assessment of its plasma concentrations and by those of its intraneuronal metabolite dihydroxyphenylglycol (DHPG). Tracer recycling, as assessed by perturbations to steady-state plasma [3H]NE, was not apparent in humans during sympathetic activation by orthostasis or exercise. During intense electrical stimulation of cardiac sympathetic nerves after an [3H]NE infusion in dogs, plasma [3H]NE was higher in coronary sinus than arterial plasma, consistent with tracer recycling by cardiac sympathetic nerves. The specific activity of released [3H]NE was similar to that of [3H]NE and [3H]DHPG in cardiac tissue, indicating that both released [3H]NE and [3H]DHPG were derived from the same pool of neuronally stored [3H]NE. During intravenous infusion of [3H]NE in humans, plasma [3H]NE reached a steady state within 12 min and remained constant, whereas plasma [3H]DHPG increased progressively, reflecting metabolism of an increasing amount of [3H]NE leaking from sympathetic vesicles. Plasma concentrations of [3H]DHPG approached or exceeded those of [3H]NE after the end of radiotracer infusions and in the venous drainage of the heart and liver. Thus, to avoid error during assessment of plasma [3H]NE kinetics, an analytical step should be employed to separate plasma [3H]DHPG from [3H]NE. Because [3H]DHPG is derived from [3H]NE within the neuron, the specific activity of neuronally stored [3H]NE could be assumed to be no higher than the specific activity of plasma [3H]DHPG. This provided a means to estimate the maximum extent of tracer recycling, thereby indicating that, during intravenous infusion of [3H]NE, tracer recycling contributed negligibly (less than 5%) to steady-state plasma [3H]NE at normal levels of sympathetic activity.