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A sensitive and specific double-isotope dilution method for assaying norepinephrine and epinephrine separately is described. The method is based on enzymatic conversion of these two catecholamines to their respective metanephrines and was applied successfully to measurement of resting plasma concentrations of norepinephrine and epinephrine in normal subjects with the finding that norepinephrine (0.20 ± 0.08 µg/liter) normally constitutes approximately 80% of the plasma catecholamine content and epinephrine (0.05 ± 0.03 µg/liter) 20%.
Engelman et al. (Thu,) studied this question.