Assessment of norepinephrine secretion and production

A noninvasive method is described for assessing the disposition of NE produced by peripheral sympathetic neurons. Total NE production is estimated from integrated NE metabolite excretion. The rate at which NE enters the bloodstream (apparent NE secretion) is estimated by isotope dilution analysis. The rate at which NE is metabolized locally is calculated by subtracting apparent NE secretion from NE production. In 10 normal volunteers, basal NE production by the peripheral sympathetic nervous system was 11.1 +/- 1.0 nmol/m2/min. Apparent NE secretion, by contrast, was only 2.4 +/- 0.18 nmol/m2/min. Our data thus demonstrate that in normal individuals under basal conditions, at least 78% of the NE produced by the sympathetic nervous system undergoes local metabolism and that no more than 22% of NE diffuses into the circulation.