Catecholamine clearance is not enhanced by increasing levels within the physiological range

Effect of differently induced plasma norepinephrine increments on norepinephrine sulfate formation

We investigated whether similar increments in venous plasma norepinephrine (NE) concentration caused by exercise and by intravenous NE infusion will elevate plasma norepinephrine sulfate (NES) to similar concentrations. In randomized order venous plasma NE concentration was elevated to similar concentrations by bicycle exercise (BE; 65% VO(2)max) and by intravenous NE infusion at rest (INF; 0.14 microg/min/kg). N = 11 subjects participated in the study. Increments in plasma NE and the area under curve of plasma NE were similar during BE (11.2 +/- 1.3 nM; 411 +/- 23 nM/min; means +/- S.E.) and INF (12.6 +/- 1.9 nM; 429 +/- 27 nM/min). Plasma NES was significantly elevated to similar concentrations with BE (from 5.7 +/- 1.0 to 8.5 +/- 1.3 nM) and with INF (from 5.6 +/- 0.9 to 8.9 +/- 1.0 nM). Plasma NE and NES concentration during control conditions remained unchanged. Heart rate decreased significantly to 43 +/- 1 beats/min with INF and increased significantly to 162 +/- 3 beats/min with BE. Systolic blood pressure increased with both, INF and BE (155 +/- 3 mmHg; 179 +/- 6 mmHg, respectively). Present findings firstly show that intravenously infused NE is sulfoconjugated in humans, indicating that a major part of NE is sulfoconjugated in blood or at sites easily accessible from blood. Secondly, plasma NE may be a useful additional marker for NES release.