Measurement of norepinephrine and epinephrine in small volumes of human plasma by a single isotope derivative method: Response to the upright posture

A single-isotope modification of the double-isotope derivative method for the measurement of norepinephrine and epinephrine, employing S-adenosylmethionine (³H-methyl) rather than S-andenosylmethionine (¹⁴C-methyl), was found to be sufficiently sensitive (to less than 10 pg/ml) and reproducible (interassay coefficient of variation of 3.5% for norepinephrine and 8.5% for epinephrine) for the measurement of these catecholamines in 1.0 ml of human plasma obtained from supine, resting subjects. In 8 normal subjects the mean (±SE) plasma norepinephrine concentration rose from 182 ± 21 pg/ml in the resting, supine position to 425 ± 28 pg/ml (p < 0.001) after 5 mins in the standing position. Corresponding supine and standing mean plasma epinephrine concentrations were 44 ± 11 pg/ml and 73 ± 17 pg/ml (p < 0.02) respectively. Supine plasma norepinephrine and epinephrine concentrations measured in 18 subjects with this method were quite comparable to those reported with the parent double-isotope method.