Effect of norepinephrine uptake blockers on norepinephrine kinetics

We studied the effect of a single oral dose of the neuronal norepinephrine uptake blocker, desipramine 125 mg, on norepinephrine kinetics. Desipramine reduced the plasma norepinephrine clearance by approximately 20%, from 1.33 +/- 0.22 to 1.08 +/- 0.19 l/m2/min (p less than 0.01). Similarly, plasma norepinephrine clearance was slowed in patients with sympathetic nerves damaged by disease (idiopathic peripheral autonomic insufficiency). Desipramine also reduced the rate of spillover of norepinephrine to plasma, 0.27 +/- 0.07 to 0.15 +/- 0.04 micrograms/m2/min, leaving the plasma norepinephrine concentration unchanged. Disappearance of tritiated norepinephrine from plasma, after infusion to steady state, was biexponential, with half-time of the rapid-removal phase (t1 1/2) = 2.0 +/- 0.4 min and half-time of the second exponential (t2 1/2) = 34 +/- 10 min. The rapid-removal phase was sensitive to disturbances in the neuronal uptake of norepinephrine, the t1 1/2 being prolonged by desipramine and lengthened in the patients with peripheral autonomic insufficiency. In contrast, the selective extraneuronal norepinephrine uptake blocker, cortisol, 500 mg intravenously, had no effect in normal subjects on either plasma norepinephrine clearance or the t1 1/2 value. Neuronal uptake of norepinephrine contributes to the overall removal of norepinephrine from plasma. Extraneuronal uptake of norepinephrine could not be demonstrated at existing plasma norepinephrine concentrations.